Experimental Thermal and Fluid Science xxx (2013) xxx–xxx

Contents lists available at ScienceDirect

Experimental Thermal and Fluid Science

journal homepage: www.elsevier .com/locate /et fs

Experimental analysis of Closed Loop Two Phase Thermosyphon (CLTPT)for energy systems

0894-1777/$ - see front matter � 2013 Elsevier Inc. All rights reserved.http://dx.doi.org/10.1016/j.expthermflusci.2013.08.013

⇑ Corresponding author. Tel.: +39 0502217154; fax: +39 0502217160.E-mail address: alessandro.franco@ing.unipi.it (A. Franco).

Please cite this article in press as: A. Franco, S. Filippeschi, Experimental analysis of Closed Loop Two Phase Thermosyphon (CLTPT) for energy systemTherm. Fluid Sci. (2013), http://dx.doi.org/10.1016/j.expthermflusci.2013.08.013

Alessandro Franco ⇑, Sauro FilippeschiDepartment of Energy, Systems, Territory and Constructions Engineering (DESTEC), University of Pisa, Largo Lucio Lazzarino, 2, 56126 PISA, Italy

a r t i c l e i n f o

Article history:Received 18 June 2013Received in revised form 17 August 2013Accepted 17 August 2013Available online xxxx

Keywords:Two phase flowClosed Loop Two Phase ThermosyphonsHeat pipe solar collectorsConfined boilingFlow patternInstabilities

a b s t r a c t

The authors have designed and realized an experimental test rig for the analysis of Closed Loop TwoPhase Thermosyphons (CLTPT) of small dimensions where heat flow rate up to 1.7 kW can be furnished.The experimental test rig consists of an evaporator and a water cooled horizontal condenser placed about1 m over the evaporator. The main characteristic of this apparatus is the presence of three different waysof measuring the mass flow rate: a continuous mode, an integral mode and an indirect mode.

The purpose of this analysis is to investigate the correlation between mass flow rate and heat flow rate.The results of an experimental analysis by using water and ethanol as tested fluids at different operatingconditions are shown discussed and analyzed.

The influence of several parameters on the performances was studied experimentally: in particularheat load, operating pressure and fluid filling. The limits in the heat and mass transport are evidencedtogether with the unstable behavior at the high heat input.

From this work, understanding and useful information are provided for designing and building a two-phase thermosyphon for systems like solar heating.

� 2013 Elsevier Inc. All rights reserved.

1. Introduction

Two-phase natural circulation loops or Closed Loop Two PhaseTherrmosyphons (CLTPT) represents a particular version of heatpipes [1]. They have a lot of technical applications due to their sim-plicity, high heat transfer capability, and passive operation. Thesesystems work under gravity with the condenser above theevaporator and do not require pump or capillary action of a wickstructure to allow the heat transfer fluid to be returned to theevaporator.

The authors of the present paper have analyzed the operatingmode and reviewed the experimental activities on these devicesin a recent paper [2]. Several engineering applications are possiblefor CLTPT such as solar water heaters, geothermal systems, emer-gency cooling systems in nuclear reactor cores, electrical machinerotor cooling, gas turbine blade cooling, and thermoelectric refrig-eration systems together with conventional applications of ther-mal control [3–7]. These applications cover a wide range ofpower from ten of MWs of Nuclear Reactors down to few Ws incase of electronic equipment applications.

The two-phase loop thermosyphons, especially for low powerapplications (below 1 kW of heat input), are of particular interestin connection with the development of environmental systems in

which the correlation of mass flow rate and heat flow rate isimportant: among the others solar collectors, photovoltaic mod-ules cooling systems, heat exchangers for geothermal systemsand thermal control system devices [8].

In the last years it is of particular interest the connection ofCLTPT and solar collectors. In particular glass heat pipe solar collec-tors are becoming very popular for heating water production be-cause overall performance is higher than that in conventionalsingle-phase systems. The use of this particular devices allows tominimize heat dispersion to the environment, and to reach poten-tially temperature levels in competition with much more expen-sive concentrating solar collectors [9].

The performance of two-phase solar thermosyphon systems de-pends on several system parameters such as solar heat flux that re-ceived by collector, circulating mass flow rate, driving pressure,pressure drop and heat transfer characteristics in the system. Toget stable operation and best performance, previously mentionedparameters must be balanced in the system. In turn, these param-eters also depend on physical structural of the systems and theyhave interrelation to each other [10].

A two-phase loop heat transfer device is able to transfer largeamounts of heat with heat transfer rate of 103–105 W/m2 K duringboiling/evaporation and condensation. A CLTPT consists of an evap-orator, a condenser and two adiabatic sections (the riser and thedowncomer). The heat transfer rate, which can be transferred fromevaporator to condenser, mainly depends on the diameter of the

s, Exp.

Nomenclature

Bo Bond numberd diameter of lift tube (m)Fr Froude numberg acceleration of gravity (m s�2)h specific enthalpy (J kg�1)H height of liquid in the reservoir (m)H/L ratio defining the fill of the apparatusI current intensity (A)L length of lift tube (m)m mass flow rate (kg s�1 or kg min�1)p pressure (kPa)pr relative pressure (Pa)q heat flux (W/m2)Q input thermal power and heat flow rate (W)T temperature (K)t time (s)V velocity (m s�1)x steam quality

We Weber numberDT10 temperature difference (T1–T0) at the condenser (�C)DT12 temperature difference (T1–T2) between evaporator and

condenser (�C)DV voltage (V)e void fractionr surface tension (N m�1)q density (kg m�3)

Subscriptscond at the condensereV at the evaporatorL of liquidm of the mixturesat saturationV of the saturated steamw of the refrigeration water at the condenser

2 A. Franco, S. Filippeschi / Experimental Thermal and Fluid Science xxx (2013) xxx–xxx

pipes, the working fluid, the adiabatic section length and the oper-ative orientation with respect to the gravity.

All these loop can operate in two different modes: at constantdifference of temperatures like in the case of solar collectors, orat fixed heat flux, like as in the case of thermal control systems.In particular cases, as the mass flow rate is confined, the mass flowrate can approach a maximum value and it is larger than the massflow rate due to the latent heat of vaporization.

For the design of such a kind of device different models havebeen developed based on the solution of conservation equationsof mass, momentum and energy in two phase systems. Howeverthey must be joined with an experimental characterization of theheat and mass transfer mechanisms at the evaporator section likeas the boiling phenomena inside the flow channels, because thelink between fluid-dynamic, thermal problems and gravitationaleffects are not completely known. The authors in [8], have consid-ered the systems from a methodological and from an experimentalpoint of view focusing attention on the behavior of the completesystem which is strongly influenced by the type of flow, and onthe influence of the diameter of the pipes. They observed in partic-ular that the size of the pipe mainly in the evaporation zone is ofparticular importance so that the devices could be divided in highBond number loops, where the capillary force can be neglected andlow Bond number loops, where the capillary forces cannot be ne-glected at all. The Bond number however is not the only parameterwhich characterizes the thermal and fluid dynamic behavior ofthese device and the theoretical prediction of the behavior of smalldimensions CLTPT is quite difficult.

Different experimental devices are reported in the literature onCLTPT (with pipes having a hydraulic diameter in the range 1–10 mm). They are critically analyzed in [2] in order to highlightthe main operating characteristics, the measurement method andthe main results obtained in terms of heat and mass transfer andthe influence of the various experimental conditions analyzed.The authors observed that for channels of small dimensions(approximately less than 10 mm) it is evident that the distributionof the mass flow against the heat flux approaches to a maximumand then slowly decreases. In such a way two regimes can be iden-tified: a gravity dominant regime where the mass flow rate in-creases as the heat flux increases and a friction dominant regimewhere the mass flow rate decreases as the heat flux increases.

Several papers studied the thermal performance of CLTPT if ap-plied to a particular application, but these data are difficulty to be

Please cite this article in press as: A. Franco, S. Filippeschi, Experimental analysisTherm. Fluid Sci. (2013), http://dx.doi.org/10.1016/j.expthermflusci.2013.08.01

used for general cases. The tested fluids are water, new refrigerantsand hydrocarbons (butane, methanol) [11–19]. Some papers studythe effect of the filling ratios, other the effect of the sub-cooling butno attempt to give general predictions of their behavior has beenmade. Other papers correlate the mass flow rate with the heat fluxand analyzes the trend of mass flow rate against the heat load.

The heat transfer rate in these loop are deeply influenced by thecirculation. A complete comprehension of the phenomenon can beobtained by measuring the heat flow rate at the condenser, and themass flow rate connected with the flow patterns. It clearly appearsthat good performance of those devices can be obtained only iden-tifying a range of optimal operating conditions.

Another important topic which affects the performances ofthese loops is their instabilities. In a CTPLT the intermittent regimeand the two-phase instabilities are connected with heat flux, massflow rate, two-phase flow pattern, pressure drops, operating pres-sure and liquid head.

A detailed analysis of the two-phase flow structures during theintermittent boiling region at different operative sub-atmosphericpressure in a loop thermosyphon operating with water as workingfluid is reported in [19]. It is shown as the flow pattern, the ampli-tude and frequency of these self-sustained oscillations are affectedby the heat transfer rate in the evaporator and in the condenser.

Referring to boiling conditions in small and narrow channelsTadrist in [20] affirms that the bubble growth and departing timecould be not in phase with the pressure drop and the mass flow in-let leading to self-sustained oscillations. An intermittent boilingperiod is observed where the waiting time for new bubbles is muchlonger than the growth time.

The idea pursued in this paper is to conduct a quantitative andqualitative analysis on the operating mode of CLTPT in order to beable to identify criteria and guidelines for the analysis of thermal-fluid-dynamics performances of such systems in configurationstypical of concentrating solar collectors (heat flux higher than1000 W/m2). Attention is particularly dedicated to the analysis ofthe circulation of the working fluid, mainly controlled by gravityand by the surface tension effects taking place on the glass/liquidinterface and on the connection between heat and mass flow rate.

In particular in this paper shows experimental data obtainedusing a new experimental device in which it is possible to measurethe correlation between mass flow rate and heat input and be-tween heat input and condensing rate. The experimental deviceis designed with the objective of reproducing the qualitative

of Closed Loop Two Phase Thermosyphon (CLTPT) for energy systems, Exp.3

A. Franco, S. Filippeschi / Experimental Thermal and Fluid Science xxx (2013) xxx–xxx 3

operating mode of a solar collector in typical quasi steady condi-tions. The mass flow rate has been measured in two different wayswith a good repeatability and low errors. The experimental analy-sis aims to understand the presence of a maximum of mass flowrate with the heat flow rate in different conditions. Moreover theeffects of filling ratio and of pressure are analyzed. The workingfluids used during the tests are water and ethanol.

2. Closed Loop Two Phase Thermosyphons: operating principleand design variables

The CLTPT analyzed in this work is a self acting flow loops. Inthese applications the loop thermosyphons usually operate atlow pressure (0.1–10 bar) and temperatures in the range 20 �C–120 �C.

The link between fluid-dynamic, thermal problems and gravityis evident mainly in the low power applications (below 500 W).But the operating mode of the devices in case of pipes of smalldiameter is also influenced by capillary forces, gravitational forces,inertia forces. Moreover those devices are often characterized byoperational instability. The operating principle with respect tothe single tube thermosyphons is to separate the pathways of thereplenishment liquid from that of the vapor escaping from evapo-ration zone. In such a way a meaningful increase of heat transportwith respect to the conventional two phases closed thermosy-phons could be obtained. These effects are more remarkable asthe size of the devices decrease. The confinement of boiling andcondensation generate two-phase flow structure which can en-hance or in some cases deteriorate the heat and mass transferThe variables and the parameters that influences the performancesof the CLTPT are a lot: the heat load (Q), the geometrical parame-ters of the loop like the distance between evaporator and con-denser (L), the length of the heat input zone, the pressure dropsand thermal resistances at different parts of the thermosyphon,the thermo-physical properties of the working fluid, the operatingpressure and other operational variables like the subcooling of thefluid.

According to the exhaustive analysis of Khandekhar [21] differ-ent types of loop thermosyphons can be analyzed, depending onthe operating mode, on the presence of wick structure, on thegeometry of evaporator with respect to the diameter of the con-necting pipes (high Bond number and low Bond number).

A detailed discussion of the working principles of CLTPT is be-yond the purpose of this paper; the reader is referred to the liter-ature [1,2,8,21]. The specific interest of the authors is related toCLTPT with internal diameter of the evaporator equal to that ofthe upward connecting pipes (low Bond number).

Experimental results for large diameter (high Bond number)Closed Loop Two Phase Thermosyphons demonstrates some inter-esting elements. First the heat transfer coefficients are more or lessindependent on vapor quality and mass flux, but strongly depen-dent on heat flux and saturation pressure. The use of macro-scaleboiling heat transfer correlations and models based on theabovementioned schematization does not correctly predict theconnections between heat transfer and mass flow rate. It is clearthat further systematic studies are required to generate a sufficientbody of knowledge of the transport mechanism responsible forthe variation of the flow structure and heat transfer. Besides thedistinction between small diameter channels and mini-channelsis not clearly established in the literature, as discussed in [16,22].

It is, therefore, not clear when the natural circulation can bestudied as a flow boiling regime or a pool boiling regime. Someauthors individuate an intermittent boiling regime which dependssome much by the operative pressure in case of sub-atmosphericcondition above all. Two different behaviors, connected with

Please cite this article in press as: A. Franco, S. Filippeschi, Experimental analysisTherm. Fluid Sci. (2013), http://dx.doi.org/10.1016/j.expthermflusci.2013.08.01

instabilities are observed at low heat load and at high heat load.The increasing heat load affects the inertia forces of the mass flowrate, so that in low heat load the inertia force are negligible and thegravity effect are dominant. At the high heat load opposite condi-tions are often observed.

Attempts to manage the experimental data in a dimensionlessmode have been made giving some interesting informations. Therelative importance of inertial and capillary forces is indicated bythe Weber number:

We ¼ qV2dr

ð1Þ

where L is a characteristic length of the system (identified in theparticular case of cylindrical tubes with the pipe diameter d and Vis a characteristic velocity. If We� 1, inertia forces are dominant.Otherwise the relative importance of inertia (related to fluid veloc-ity) and gravity (related to density variations) forces are indicatedby the Froude number defined as the ratio of velocity and pipediameter:

Fr ¼ V2

gdð2Þ

If Fr� 1, inertia forces are dominant compared to gravityforces. The Froude number is the ratio of Weber and Bond number,Fr = We/Bo. The Bond number (Bo) is defined as:

Bo ¼ qgd2

rð3Þ

The Bond number is the ratio between the gravitational forceand the surface tension force exerted on a bubble and givesindication of the confinement of the bubbles and of the differentbehavior observed by the various authors in similar experimentaldevices. The Bond number allows defining a range of pipediameters to obtain bubble development and slug flow in the pipe.According to the combination pipe diameter and fluid differentoperating modes can be identified, as discussed by the authors in[8] and in [23].

Using water as working fluid this range is approximately be-tween 5 mm and 40 mm, while for refrigerant fluids and otherlow boiling fluids like ethanol, the minimum value of the diametercan be below 5 mm.

Different combination of We and Bo number identifies differentoperating regimes of the devices. In general three possible operat-ing zone can be evidenced, but some experimental data available inthe literature permits to understand in the majority of the cases,pipes of geometries in the range between some mm and somecm, and natural circulation (without external pumping), the typicaloperating zone is in a zone at the boundaries of Inertia dominantand Gravity dominant. In the small dimensions natural circulationloops the effect of buoyancy forces is dominant. In this regimegravity dominant regime for a small change in quality there is alarge change in the void fraction and therefore density and buoy-ancy force. The increased buoyancy force can be balanced by a sig-nificant increase in the corresponding frictional force which ispossible only at higher flow rates. The gravity dominant regimeis characterized by an increase of the flow rate with heat flux. Ifthe heat input increase over a certain value (different in depen-dence of the combination pipe diameter – refrigerant fluid) it canbe identified as friction dominant regime.

The analysis of devices like those described can be carried outby means of a theoretical analysis or otherwise referring to previ-ous experimental activities. The theoretical analysis based on theconstruction of a numerical model gives only general indicationabout the operation of the device. On the other hand a lot of exper-imental analysis available are limited only to quite reduced range

of Closed Loop Two Phase Thermosyphon (CLTPT) for energy systems, Exp.3

4 A. Franco, S. Filippeschi / Experimental Thermal and Fluid Science xxx (2013) xxx–xxx

of the input power. Unfortunately the great part of the results areonly of qualitative value, due to the extreme simplicity of theapparatus. For this reason, in order to better investigate the oper-ating mode of those devices, a specific experimental apparatushas been designed and constructed: this is schematically reportedin Fig. 1.

The experimental apparatus permits a sensitivity analysis withrespect to the main input variables of those devices, in particular ofthe filling ratio and of the operating pressure. A device like thoseanalyzed is correctly defined if for the particular fluid analyzed,the link between the mass flow rate (m) and the heat flow rate(Q) is defined as a function of the main operating variables of thedevices. In particular the interest is in defining the dependenceof mass flow rate on the main variables of the CLTPT, like the heat

(a

(bFig. 1. Design of the CLTPT experimental system established in this study. (a) S

Please cite this article in press as: A. Franco, S. Filippeschi, Experimental analysisTherm. Fluid Sci. (2013), http://dx.doi.org/10.1016/j.expthermflusci.2013.08.01

flow rate, Q, the pipe diameter, d, the height of the fluid withrespect to the heat input level, H, the distance between evaporatorand condenser, L. The performances of this device are influenced bya combination of many processes such as heat input, nucleation,bubble growth and pumping action, two-phase flow pattern, oper-ating pressure, geometric parameters of the device, circulationvelocity, pressure drop and filling ratio.

All above processes are interconnected, but for a given geome-try and a particular working fluid, the main controlling parametersappears to be the heat flow rate at the evaporator, the operatingpressure and the volume of fluid in the apparatus. The combinationof those parameters and variables determine operation stability,circulation velocity, void fraction, flow pattern and the heated wallsuperheat.

)

)chematic system design; and (b) a picture of the experimental apparatus.

of Closed Loop Two Phase Thermosyphon (CLTPT) for energy systems, Exp.3

A. Franco, S. Filippeschi / Experimental Thermal and Fluid Science xxx (2013) xxx–xxx 5

Since the circulation velocity is not a controlling parameter, theprediction of the operation conditions in the natural-circulationloop is a rather complex task. Franco in [15] describes the con-struction and the experimental results obtained on a simple appa-ratus, using water and four different refrigerants: R11, R113,HCFC141b and FC72. The analysis is limited to a maximum valueof the input power of 500 W. Though if the analysis was not partic-ularly accurate, however it permitted to define some interestingcharacteristics of the operating mode of those devices and inparticular:

– the occurrence of a maximum of mass flow rate as a function ofthe heat input;

– the important influence of the pipe diameter of the filling ratio,of the operating pressure and of the subcooling of the fluid atthe evaporator inlet;

– the great differences between the various fluids tested.

But this operating range appears to be quite reduced for thepurpose of investigating the various possible engineering applica-tions. For this reason, in order to investigate more general operat-ing conditions a new experimental apparatus has been constructedwith the particular aim of increasing the range of investigation upto input power over 1000 W, in order to make this systems suitablenot only for cooling and thermal control devices but also for appli-cations in the field of renewable energy systems application con-nected in particular with solar energy and geothermal energy,like the above mentioned wickless heat pipe solar collectors. Fromthe analysis described in [15] even if limited to a reduced numberof cases, it appears clear that a successful operation of a two-phaseloop thermosyphon can be obtained with a compromise betweenheat and mass transport efficiency. The first is obtained for quitehigh void fractions (annular flow pattern) while the second wouldrequire quite low values of void fractions (typical of slug flow andannular flow).

3. Experimental facility and tests

Basing on the previous discussion and considering the results ofthe main experimental activities available in the literature theauthors has designed an experimental test rig in which it will bepossible to analyze the thermo-fluid dynamic behavior of CLTPTand in particular the link between heat flow rate and mass flowrate growing input power ranging approximately from 0 to1800 W.

3.1. Experimental facility: description, sizing and definition of therange of the various parameters

The main loop consists of an evaporator, which is placed on theupcomer (riser), a condenser and two different downcomers. Theevaporator is a copper cylinder 155 mm long with an internaldiameter of 10 mm. It is heated by two silicon type thermoheaters(model MINCO HK5488R17.2L12A) of 750 W nominal power(17.2 Ohm of electrical resistance) which can generate heat flowrate up to 1500 W at the high temperature in the experimentaltests it was possible to increase the input power up to 1700 W.The input power has been furnished by a DC Power supply (AgilentDC6575A) with output rate (0–120 V and 0–18 A).

The thermal resistances operate in parallel, corresponding to aheat flux of about 300 kW/m2. The condenser, which is placed1200 mm over the evaporator, is made of a copper tube(ID = 10 mm, OD = 20 mm) water cooled. In the cooling loop anelectromagnetic flow meter (Siemens SITRANS F M MAGFLO5000)allows measurement the mass flow rate of the cooling water with

Please cite this article in press as: A. Franco, S. Filippeschi, Experimental analysisTherm. Fluid Sci. (2013), http://dx.doi.org/10.1016/j.expthermflusci.2013.08.01

an accuracy of about 1%: so this permit to control quite well thecondenser.

The condenser is realized with a tube–tube heat exchanger inwhich the cooling liquid in maintained at the imposed temperatureby means of an external cooler (ENCO Thermoflex 1400): the con-denser is 400 mm long. The upcomer is made of a glass pipe(ID = 10 mm) 750 mm long. All the other connections are realizedwith a copper tube (ID = 10 mm). During the thermal performancetests the temperature of the cooling bath has can be maintained atan imposed temperature and the mass flow rate can be exactlycontrolled.

The main characteristic of this apparatus are three differentways of measuring the mass flow rate: a continuous mode, an inte-gral mode and a volumetric mode. The first one is given by themass flow rate placed on the primary line in the single phase tubeafter the condenser.

This mass flow meter is a Coriolis type (Siemens SITRANS F CMASS2100). The periodic and reverse fluid passing through theCoriolis flow meter provide a measure affected by high errorsand it is used only as comparative integral measurement. As thesecondary line is activated, an integral measurement of the massflow rate is given analyzing the volume liquid collected in a fixedtime inside a metering glass tank. In addition a high speed videocamera is used to evaluate the velocity and the volume of the slugand the plug in the riser.

Six T-type thermocouples and three pressure transmitters WIKAUnitrans UT-10 (Pressure range 0–6 bar) are inserted into the loopaccording to the positions that can be evidenced in Fig. 1.

In particular two thermocouples measure the temperaturebefore and after the evaporator and the condenser and the sixththermocouple, not represented in the scheme acquire the envi-ronmental temperature. The position of the three pressure trans-ducer is evidenced in Fig. 1: two are in the left side of the loop,after the condenser and the third is after the Coriolis flow inthe bottoming part of the loop. All the signals are acquired bythe Agilent HP32790 acquisitions system and stored in a com-puter. The various flow patterns and the motion of the plugsand the slugs has been observed with a high-speed acquisitionvideo-camera FASTEC Trouble Shooter 1000ME. It has been usedwith recording rates up to 2000 fps with the sensitivity of1280 � 1024 pixels.

3.2. Definition of the experimental values of filling ratio

The experiments consisted in measuring the liquid mass flowrate as a function of the input thermal power, for different valuesof the filling ration. The filling ratio is another important operatingparameter of devices based on the CLTPT principle. Considering thegeometry of the experimental test rig described in Fig. 1, this canbe summarized by the height H of the fluid in the compensationchamber or alternatively by the ratio between H and the totallength of the riser L.

The value of this parameter must be in the range between 0.20and 0.30. The very narrow range of submergence ratio variationmeans very narrow range of available void fraction in the lift tube.Really, for stable operation one needs to have a positive value of apressure head defined

Dp ¼ ðqLH � qmLÞ � g ð4Þ

where qL is the density of the liquid phase and qm is the density oftwo phase mixture. Because an average density of the two-phasemixture is determined starting from the value of the density ofthe liquid qL and of the saturated steam, qV via void fraction, e

qm ¼ ð1� eÞ � qL þ eqV ð5Þ

of Closed Loop Two Phase Thermosyphon (CLTPT) for energy systems, Exp.3

Table 2Saturation properties of ethanol at the various pressure tested.

P(bar)

Tsat

(�C)qL (kg/m3)

qV (m3/kg)

hL (kJ/kg)

hV (kJ/kg)

r (N/m)

0.16 39.23 773.40 0.30992 298.37 1204.2 0.020080.32 52.45 761.03 0.57491 335.75 1224.8 0.018340.37 55.45 758.73 0.65611 344.50 1229.5 0.017990.72 70.25 744.60 1.21210 389.23 1252.2 0.016191.01 78.24 736.38 1.64730 414.35 1264.3 0.01524

Table 3Correspondence between heat load, specific heat input and electrical parameters of

6 A. Franco, S. Filippeschi / Experimental Thermal and Fluid Science xxx (2013) xxx–xxx

thus

Dp ¼ qLL � ðH=L� qm=qLÞ � g ð6Þ

qm=qL ¼ 1� e � Dq=qL ð7aÞ

with

Dq ¼ qL � qV ð7bÞ

From Eqs. (4)-(7) it is clear that to obtain stable conditions thefollowing inequalities must be satisfied:

qm=qL 6 S ð8Þ

e Pð1� SÞDq=qL

ð9Þ

To obtain stable conditions, it in necessary that S is below0.2 � 0.3

e � Dq=qL > 0:7� 0:8 ð10Þ

To satisfy this condition a quite high void fraction is necessary;at these void fractions the most probable flow regime is the annu-lar one, as really appears in a lot of the cases analyzed.

3.3. Experimental procedure

The first experimental analysis has been referred to the use ofwater and ethanol as working fluid. The two fluids are selected be-cause they are representative of different behavior.

The tests are carried out with the two fluids varying two mainparameters: the filling ratio of the device (ranging from a lower va-lue H/L = 0.2 to H/L = 0.32) and the operating pressure in the device(ranging from 0.1 bar to 1 bar). The reduced pressure can be ob-tained by means of a vacuum pump directly connected to the loopthrough a channel in the upper part of the loop (Fig. 1). For eachtested fluid the liquid level in the apparatus was at the beginningfixed at H/L = 0.32. After the complete cycle of texts the apparatushas been discharged at H/L = 0.28, then at H/L = 0.25 and finally atH/L = 0.2.

For the various H/L values each test has been repeated at differ-ent saturation pressure from atmospheric pressure down to0.1 bars. The saturation properties of the fluids at the varioustested conditions are provided in Tables 1 and 2 for water and eth-anol respectively.

For each pressure value (p) tested the saturation temperature(Tsat) and the density and specific enthalpy of liquid (qL and hL)and of saturated vapor (qV and hV) calculated with REFPROP are re-ported in the Tables together with the surface tension (r). A spe-cific diameter of the lift tube (d = 10 mm) has been tested. All thetests has been carried out at a quite stable value of the room tem-perature (between 19 and 20 �C) maintained by an air conditioner,while the temperature of the Cold Bath is maintained at 18 �C.

The experimental procedure is described below. Before tocharging the CLTPT with the working fluid (water or pure ethanol),the loop was evacuated using a high vacuum pump. Then the fluidwas heated in order to minimize the amount of noncondensablegases that was dissolved inside. The detailed temperature and

Table 1Saturation properties of water at the various pressure tested.

P(bar)

Tsat

(�C)qL (kg/m3)

qV (m3/kg)

hL (kJ/kg)

hV (kJ/kg)

r (N/m)

0.10 47.0 989.03 0.07456 199.65 2587.2 0.068230.25 65.0 980.54 0.16121 271.96 2617.4 0.065220.33 70.5 977.40 0.20319 295.52 2627.1 0.064110.60 85.5 968.27 0.36036 358.10 2652.1 0.061471.01 100.0 950.66 0.59772 419.06 2706.0 0.05891

Please cite this article in press as: A. Franco, S. Filippeschi, Experimental analysisTherm. Fluid Sci. (2013), http://dx.doi.org/10.1016/j.expthermflusci.2013.08.01

pressure values were recorded for the steady stage after injectionfor the measurement of injected mass load. After the injectionand safety check, the cooler water circulation system can bestarted; after the circulation of water soon reached steady statethe DC heating system can be initiated.

After the preliminary phase, the input thermal power was var-ied in the range between 0–1700 W with a step of 100 W, corre-sponding to heat flux in the evaporator section ranging from 0 to360 kW/m2.

Table 3 provides the details of the various experimentalconditions.

3.4. Uncertainty analysis

The voltage and amperage of DC power supply have an accuracyof 0.1 V and 0.01 A, respectively. The error on the heat flow rate isprimarily due to the difficulty in maintaining a completely stablevoltage and current output and to the variation of the resistancewith the increase of temperature. However, according to theorycontained in Taylor [24], the uncertainty associated with the mea-surement of heat flow can be estimated to be below 3%.

The employed T-type thermocouples have an accuracy of 2%reading. The uncertainty associated to the measurement ofpressure was of the order of 0.01 bar, so depending on the variousoperating conditions can be estimated to be variable in the range1–10%. A more critical point is represented by the measurementof mass flow rate. Considering the Coriolis flow meter, the uncer-tainty is quite high, mainly in case of quite low values of mass flowrate, for this reason only the values obtained with the indirectmethod, obtained with the volumetric method are considered. Inthis particular case the error on the mass flow rate is due to the factthat the mass flow rate continually fluctuates, mainly for heat flowrate over 500–600 W. In this case, mainly using the indirect meth-od is important to consider the value as the measure obtained forthree different time steps: 20 s, 40 s and 60 s. Considering the pos-sible error on the time measurement and on the volume collectedthe uncertainty on the mass flow rate is estimated to be of 8%.

4. Experimental results

In the present analysis the results obtained using water andethanol as operating fluid are reported. The two fluids has been

the in some particular conditions.

Q (W) DV (V) I (A) q (kW/m2)

100 29.88 3.40 21.2300 51.60 5.86 63.6500 66.50 7.33 106.1800 84.30 9.48 169.8

1000 94.57 10.60 212.31200 104.04 11.60 254.71500 114.80 12.80 318.41700 123.00 13.80 360.9

of Closed Loop Two Phase Thermosyphon (CLTPT) for energy systems, Exp.3

A. Franco, S. Filippeschi / Experimental Thermal and Fluid Science xxx (2013) xxx–xxx 7

selected because they are quite different and their thermophysicalproperties are known with great accuracy. Mass transport and theseveral flow patterns were observed in the various tests. At verylow heat input (below 100 W) bubbly flow is observed but no masspass through the condenser. As the heat input increases, but thedriving force is still not sufficient for pumping action, a bubble for-mation and growth in the lift tube is observed. The fluids in theevaporator simply oscillates inside the rises around the evaporatorsection without being lifted to the top and it did not pump the li-quid up the tube at all. A minimum heat input (100–200 W) is re-quired to obtain slug flow and a mass transport flow from lower toupper reservoir. No critical conditions in the evaporator are ob-served and this was expected because the maximum heat fluxwas below 360 kW/m2 (Table 3).

The data are organized as follows: the mass flow rate as a func-tion of heat input for different values of filling ratio, represented bythe ratio H/L at atmospheric pressure and at 0.1 bar is reported.After that the effect of pressure variation is analyzed with refer-ence to some specific values of H/L and the analysis of heat flowat evaporator and condenser is finally reported. Concerning thevalues of pressure, it is important to underline that pressure valuesare only reference values referred to the start of the experimentaltest because pressure oscillates in a sensible manner. This effectwill be discussed in detail in the final part of the paper.

4.1. Results obtained with water

Fig. 2 shows the link between heat input and mass flow rate atatmospheric pressure for four different values of the filling ratio. Asclearly appears a maximum of the mass flow rate is obtained andthis maximum (0.3 kg/min) appears to be not particularly depen-dant on the filling ratio. For High H/L ratios the mass flow rate in-creases more quickly with the power even if the maximum value isthe same. Over 700 W an annular flow is observed up to the max-imum (1400 W). After this maximum a intermittent boiling is ob-served where the flow pattern periodically pass through slug flow,churn flow, annular and slug again. Similar results have been de-scribed in [18].

When the maximum flow rate is observed, the heat flow ratesupplied to the evaporator is not simultaneously removed by thecondenser. The compensation chamber (Fig. 1) allows this unbal-ance heat transfer. This condition however shows that the uppervalues of mass flow rate are connected with the maximum heattransfer rate removing from the condenser. If the mass flow rateis too much for the condenser so that a part of vapor pass directlythrough the condenser: the temperature and the operating pres-sure in the downcomer increases. The mass flow rate start to de-creases and the condenser is able to condenser again this massflow rate. The temperature and pressure slightly falls again andthe mass flow rate assume periodic characteristics.

0

0,05

0,1

0,15

0,2

0,25

0,3

0,35

0 200 400 600 800 1000 1200 1400 1600 1800

Q [W]

m [k

g/m

in]

H/L = 0.20H/L = 0.25H/L = 0.28H/L = 0.32

Fig. 2. Mass flow rate as a function of the heat input for water at different values offilling ratio represented by the parameter H/L at atmospheric pressure.

Please cite this article in press as: A. Franco, S. Filippeschi, Experimental analysisTherm. Fluid Sci. (2013), http://dx.doi.org/10.1016/j.expthermflusci.2013.08.01

The effect of pressure is surely more important. As shown inFig. 3 a lower operating pressure (0.1–0.6 bar) determines a sensi-ble shift of the maximum of the mass flow rate a quite low value ofinput power: in particular for high vacuum level (pressure 0.1 bar),a value of the mass flow rate close to the maximum (0.52–0.53 kg/min) can be obtained at Q = 700 W. In the meantime an increase inthe maximum mass flow rate is observed: the maximum value,quite higher than 0.4 kg/min, is shifted at heat input below1000 W. This particular behavior can be justified also by the influ-ence of an important subcooling effect, not specifically investigatedin the present analysis.

Fig. 4 provides the mass flow rate as a function of the heat inputfor a given operating pressure (p = 0.1 bar) at different filling ratio(from 0.2 to 0.32). In this case it clearly appears that the increase ofthe ratio H/L cause variation of the maximum of the mass trans-port, from 0.3 kg/min at atmospheric pressure up to 0.6 kg/minat 0.1 bar. This means that the operating conditions of those devicecan be shifted from the maximum of the mass flow rate (obtainedwith reduced operating pressure) to the maximum of heat transfer,that requires higher pressure level. Anyway the experiments con-firms the good performance of water in the evaporation sectionfor heat transport (no crisis is observed in the evaporator) andthe great importance of operating pressure (see for example thedifferent mass transport at a pressure level of 0.1 bar with two dif-ferent values of H/L, respectively 0.2 and 0.32.). The major effect ofthe operating pressure and subcooling can be also represented inFig. 5, where the input power supplied to the evaporator is plottedagainst the mass flow rate removed at the condenser. Consideringthe results of Fig. 5, it is interesting to observe the growing differ-ence between the heat flow at the evaporator and those removedat the condenser. The difference can be related to different ele-ments: first of all the size of the condenser does not allow the massflow rate to be completely condensed and in addition the possibil-ity that a part of the heat given in the evaporation zone will be di-rectly collected to the compensation chamber (reverse flow).

The improving of the mass transport at low pressure (0.1 bar)lead to higher condensing rate than those obtained in the case ofatmospheric operating pressure equal to 1 bar. The differencecould be of about 200 W at heat flux about 1500 W. However theenhancement of the power transferred to the condenser is approx-imately 15% at high input powers (1500 W) and negligible at lowinput power (100–300 W).

4.2. Results obtained with ethanol

A second group of test has been carried out by using ethanol asoperating fluid. In Fig. 6 the results obtained at atmospheric pres-sure at different values of the filling ratio (H/L) are presented. Inthis case it is clearly evident the occurrence of the maximum ofthe mass transport (that is obtained for a value of the heat input

0

0,1

0,2

0,3

0,4

0,5

0,6

Q [W]

m [k

g/m

in]

p = 0.1 barp = 0.25 barp = 0.6 barp = 0.33 bar

0 200 400 600 800 1000 1200 1400 1600 1800

Fig. 3. Mass flow rate as a function of the heat input for water at a given filling ratio(H/L = 0.32) at different operating pressure ranging from p = 0.1 bar to 0.6 bar.

of Closed Loop Two Phase Thermosyphon (CLTPT) for energy systems, Exp.3

00,050,1

0,150,2

0,250,3

0,350,4

0,45

0 200 400 600 800 1000 1200 1400 1600

Q [W]

m [k

g/m

in]

H/L = 0.2H/L = 0.25H/L = 0.32

Fig. 4. Mass flow rate as a function of the heat input for water at a reducedoperating pressure (p = 0.1 bar) at different filling ratios.

0200400600800

10001200140016001800

Qco

nd [W

]

Qevap = 0.1 barp = 0.6 barp = 1 bar

0 200 400 600 800 1000 1200 1400 1600 1800

Q [W]

Fig. 5. Comparison between heat input and heat output at the condenser for waterat given filling ratio (H/L = 0.32) at different operating pressure.

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0 200 400 600 800 1000 1200 1400 1600 1800

Q [W]

m [k

g/m

in]

H/L=0.32H/L=0.25H/L=0.20

Fig. 6. Mass flow rate as a function of the heat input for ethanol at different valuesof filling ratio represented by the parameter H/L at atmospheric pressure.

0

0,1

0,2

0,3

0,4

0,5

0,6

0 200 400 600 800 1000 1200 1400Q [W]

m [k

g/m

in]

p= 1 barp=0.72 bar

p=0.37 bar

Fig. 7. Mass flow rate as a function of the heat input for ethanol at a given fillingratio (H/L = 0.25) at different operating pressure.

0200400600800

1000120014001600

0 200 400 600 800 1000 1200 1400 1600

Q [W]

Qco

nd [W

]

Qevap = 1 barp = 0.32 barp = 0.16 bar

Fig. 8. Comparison between heat input and heat output at the condenser forethanol at given filling ratio (H/L = 0.32) at different operating pressure.

10 15 20 25 30 35 40 450

5

10

15

20

25

30

35

40

45

Bond number

Web

er n

umbe

r

Water P=1 bar Water P=1 barWater P=1barWater P=1 barWater P=0.11 barWater P=0.25 barWater P=0.32 barWater P=0.6 barEthanol P=1 barEthanol P=0.72 barEthanol P=0.37 bar

Gravitydominant

Inertiadominant

Mass quality<0.9

Fr=1

Fig. 9. Hydrodynamic regimes in pipes: connection among heat and mass flow rateand the various operating variables through dimensionless numbers and identifi-cation of typical regimes.

8 A. Franco, S. Filippeschi / Experimental Thermal and Fluid Science xxx (2013) xxx–xxx

ranging from 1000 to 1200 W). In this case the critical operatingconditions appear to be connected to the occurrence of critical heattransfer conditions in the evaporator section.

While the variation of filling ratio cause a reduction of the max-imum of mass transfer but has a reduced effect on the qualitativebehavior of the device, the operating pressure has a stronger influ-ence, mainly for reduced values of filling ratio (Fig. 7). The globalperformance of the heat transfer device are reported in Fig. 8where the heat transferred at the condenser is represented withreference to the heat input at the condenser. It is clear that in thisparticular case of ethanol as working fluid, the maximum of theheat transport is limited by the occurrence of critical heat transferconditions in the evaporator section.

4.3. Discussion of the results: consideration about the operating modeof the device and on the instability

The data collected on the device show clearly that the operatingmode is clearly a gravity dominant regime. As shown in Fig. 9, thegreat part of the data collected during the experiments, are

Please cite this article in press as: A. Franco, S. Filippeschi, Experimental analysisTherm. Fluid Sci. (2013), http://dx.doi.org/10.1016/j.expthermflusci.2013.08.01

contained in the bottom part of the plain of We number as a func-tion of Bo number. The values are obtained considering quality val-ues in the range between 0.3 and 0.8, estimated by directelaboration of the images acquired with the high speed video-cam-era and confirmed by the analysis of the literature concerning flowpatterns [25]. This shows that the behavior of such a kind of de-vices is quite far from those characterized by flow boiling.

Considering the performance of the two fluids, a differentbehavior can be clearly evidenced. While in case of water the crisisof the systems is at the condenser, in case of ethanol the crisis canbe referred to the evaporator in particular at the occurrence of theCritical Heat Flux (CHF).

A last consideration concerns the operating mode of the devicesabout the instability. According to [13][14] two different kind ofinstabilities characterize the operation of those devices: type Iinstability is given by the counterflow of the upstream driven bythe evaporator and the backward liquid condensed in the riser,whereas type II instability is due to dryness of the evaporator.

of Closed Loop Two Phase Thermosyphon (CLTPT) for energy systems, Exp.3

-20

-10

0

10

20

30

40

50

60

70

0 200 400 600 800 1000 1200 1400 1600 1800

Q [W]

ΔT

[°C

]

DT10 (p = 0.1 bar)DT12 (p = 0.1 bar)

DT10 (p = 0.6 bar)DT12 (p = 0.6 bar)DT10 (p = 1 bar)

DT12 (p = 1 bar)

ΔT10ΔT12ΔT10ΔT12ΔT10ΔT12

Fig. 10. Temperature difference at the evaporator and between evaporator and condenser for three different operating pressures (water at H/L = 0.32).

p r[b

ar]

t (sec)

t (sec)

(a)

(b)

p3

p3

p1

p1

p r[b

ar]

Fig. 11. Operating pressure in two different points of the loop for water for twodifferent heat flow rate (rating pressure p = 0.25 bar).

A. Franco, S. Filippeschi / Experimental Thermal and Fluid Science xxx (2013) xxx–xxx 9

The two conditions described in [13][14] can be effectively ob-served. Before the maximum of the heat flux the operating mode isquite stable and reduced pressure oscillation can be observed. Thefirst instability can be observed close to the maximum of the massflow rate (with water just after power input of 800–1000 W). Theinstability is clearly evidenced by the strong oscillations of massflow rate and by the changes of temperature difference at the evap-orator (DT10 = T1–T0) and between evaporator and condenser(DT12 = T1–T2). Fig. 10 provides the various data relative to waterat three different operating pressures.

Moreover it is observed that approaching the maximum of themass flow rate the flow pattern became strongly annular and thepressure oscillates in a more sensible way.

Similar observation characterize the behavior of ethanol. More-over with ethanol the second type of instability can be observedtoo because for input power just over 1200 W, critical conditionsin the evaporation zone can be evidenced in addition to the sameproblem referred to the first type of instability.

The first type of instability can be experimentally evidenced bythe remarkable pressure difference in the various points of the loopand by the strong variation of pressure with time in the condensa-tion zone.

Fig. 10 represents two particular operating conditions referredto the experimental analysis of water at operating pressure of

Please cite this article in press as: A. Franco, S. Filippeschi, Experimental analysisTherm. Fluid Sci. (2013), http://dx.doi.org/10.1016/j.expthermflusci.2013.08.01

0.25 bar. In Fig. 11 the relative pressure in two different operatingpoint of the loop are reported using the same time scale.

According to the schematization reported in Fig. 1 the operatingpressure is measured before the evaporator (p3) and after the con-denser (p1).

In the upper part of the figure (Fig. 11a) the pressure oscilla-tions in a stable operating condition is reported (P = 500 W). Inthe bottoming part (Fig 11b) is reported the pressure oscillationsafter the reaching of the maximum of the mass flow rate(P = 1200 W). It is possible to observe how in a particular pointof the loop a pressure variation of the order of 0.03 bar can be evi-denced, this causing not meaningless temperature oscillations inthe loop while the pressure drop in the loop remains quite thesame (about 0.05 bar).

5. Conclusions

The study proposed in this paper presents experimental resultsabout the heat and mass transfer performances of a small dimen-sion CLTPT operating at atmospheric and sub-atmospheric condi-tions with the perspective of defining design criteria of systemsbased on this principle as solar collectors.

The experimental apparatus has been designed to overcome thelimit of other experimental devices available in the technical liter-ature. The experimental apparatus has been set up and tested fordifferent operating conditions. In this device particular attentionis dedicated to the connection between heat and mass flow rateand different measurement methods are implemented: in particu-lar the mass flow rate is measured with two different methods: aCoriolis flow meter and an indirect method. Water and ethanolare used as working fluid representing two typical range of ther-mophysical properties. The effect on receivable heat flux, operatingpressure and filling ratio are observed.

The tests have been carried out varying the heat input (in therange 0–1700 W) the operating pressure (in the range0.1 bar–1 bar), the level of the liquid in the evaporator zone(variable between 0.2 and 0.32 m). From the experimental analysisthe following results can be evidenced:

– the operation of devices like those analyzed appears to be reallydominated by gravity forces and the friction dominant regime isreduced;

– a maximum of the mass flow rate is always present both withwater and ethanol and it varies in the range between 500 and1500 W: it identifies the transition from a typical annular flowpattern to an instable regime and critical conditions at the con-denser due to the fixed geometrical dimension;

of Closed Loop Two Phase Thermosyphon (CLTPT) for energy systems, Exp.3

10 A. Franco, S. Filippeschi / Experimental Thermal and Fluid Science xxx (2013) xxx–xxx

– the maximum of mass flow rate appears to be strongly influ-enced by the operating pressure and on the combinationbetween operating pressure and filling ratio;

– using water as working fluid no remarkable influence in theoperating mode is caused by the variation of filling ratio, whileoperating at sub-atmospheric conditions, mainly at pressureapproaching 0.1 bar, the filling ratio appears as an importantvariable. Major differences can be evidenced with ethanol;

– the difference between the heat flow rate at the evaporator andthe heat removed at the condenser can be remarkable, mainlyfor water at heat flow rate higher than 1000 W;

– the operating mode of the system appear quite stable and con-tinuous up to input power about 800 W; for higher heat inputthe operating mode appears to characterized by instabilityand periodicity and instabilities grow for heat input over1000–1200 W;

– the influence of the fluid used appears to be important: usingethanol as working fluid, the systems could reach critical condi-tions in the evaporation zone, causing the presence of a secondtype of instability.

From the results we conclude that the quantitative prediction ofthe performance of CLTPT appears to be quite difficult: the individ-ual and the inter-correlative effects of the various variables arevery difficult to be exactly identified even if it possible to establisha hierarchical level of the various variables. It is possible to recom-mend using water in thermosyphons for use at heat flow rate high-er than 1000 W, while for heat flow rate below 1000 W the use ofethanol could be advantageous.

After this first results, that authors in the future analysis willconcentrate their attention on further experimental analysis ofwater with different diameters of the pipes (e.g. 5 mm) and onexperimental analysis of different operating fluids like the refriger-ant FC72.

Acknowledgments

Authors wishing to acknowledge the financial support of MIUR(Ministry of Instruction, University and Research) in the field ofPRIN2008 Programme and the student Federico Belfi for his sup-port during the experimental analysis.

References

[1] D.A. Reay, P.E. Kew, Heat Pipes, Fifth ed., Butterworth-Heinemann, Burlington,USA, 2006.

[2] A. Franco, S. Filippeschi, Closed Loop Two-Phase Thermosyphon of smalldimensions: a review of the experimental results, Microgravity Science andTechnology 24 (2012) 165–179.

Please cite this article in press as: A. Franco, S. Filippeschi, Experimental analysisTherm. Fluid Sci. (2013), http://dx.doi.org/10.1016/j.expthermflusci.2013.08.01

[3] J.S. Lee, S.H. Rhi, C.N. Kim, Y. Lee, Use of two-phase loop thermosyphons forthermoelectric refrigeration: experiment and analysis, Applied ThermalEngineering 23 (2003) 1167–1176.

[4] M. Esen, H. Esen, Experimental investigation of a two-phase closedthermosyphon solar water heater, Solar Energy 79 (2005) 459–468.

[5] P. Garrity, G. Klausner, R. Mei, A flow boiling microchannel evaporator plate forfuel cell thermal management, Heat Transfer Engineering 28 (2007) 877–884.

[6] B.R. Chen, Y.W. Chang, W.S. Lee, S.L. Chen, Long-term thermal performance of atwo-phase thermosyphon solar water heater, Solar Energy 83 (2009) 1048–1055.

[7] A. Ordaz-Flores, O. García-Valladares, V.H. Gómez, Experimentalcharacterisation and technical feasibility of a closed two-phase vs aconventional solar water heating thermosyphon, Applied ThermalEngineering 31 (2011) 1313–1322.

[8] A. Franco, S. Filippeschi, Experimental analysis of heat and mass transfer insmall dimension, two phase loop thermosyphons, Heat Pipe Science andTechnology 2 (2010) 163–182.

[9] D. Fiaschi, G. Manfrida, Model to predict design parameters and performancecurves of vacuum glass heat pipe solar collectors, Energy 58 (2013) 2–35.

[10] N.Z. Aung, S. Li, Numerical investigation on effect of riser diameter andinclination on system parameters in a two-phase closed loop thermosyphonsolar water heater, Energy Conversion and Management 75 (2013) 25–35.

[11] M.K. Na, J.S. Jeon, H.Y. Kwak, S.S. Nam, Experimental study on Closed-LoopTwo-Phase thermosyphon devices for cooling MCMs, Heat TransferEngineering 22 (2001) 29–39.

[12] S. Mukherjee, I. Mudawar, Pump less loop for narrow channel and micro-channel boiling, ASME Journal of Electronic Packaging 125 (2003) 431–441.

[13] H. Honda, Z.G. Zhang, N. Takata, Flow and heat transfer characteristics of anatural circulation evaporative cooling system for electronic components,ASME Journal of Electronic Packaging 126 (2004) 317–324.

[14] R. Khodabandeh, R. Furberg, Instability, heat transfer and flow regime in a two-phase flow thermosyphon loop at different diameter evaporator channel,Applied Thermal Engineering 30 (2010) 1107–1114.

[15] A. Franco, Heat transfer and flow pattern in two-phase loops: an experimentalinvestigation, in: Proc. of 5th Eurotherm Conference, Eindhoven, TheNetherlands, 2008.

[16] C.L. Ong, J.R. Thome, Macro-to-microchannel transition in two-phase flow:Part 1 – Two-phase flow patterns and film thickness measurements,Experimental Thermal and Fluid Science 35 (2011) 37–47.

[17] H. Jouhara, A.J. Robinson, Experimental investigation of small diameter two-phase closed thermosyphons charged with water, FC-84, FC-77 and FC-3283,Applied Thermal Engineering 30 (2010) 201–211.

[18] S.W. Chang, D.C. Lo, K.F. Chiang, C.Y. Lin, Sub-atmospheric boiling heat transferand thermal performance of two-phase loop thermosyphon, ExperimentalThermal and Fluid Science 39 (2012) 134–147.

[19] S.A. Brusly, A. Mathew, K. Ramachandran, B.C. Pillai, V.K. Karthikeyan, Thermalperformance of anodized two phase closed thermosyphon (TPCT),Experimental Thermal and Fluid Science 48 (2013) 49–57.

[20] L. Tadrist, Review on two-phase flow instabilities in narrow space,International Journal of Heat and fluid flow 28 (2007) 54–62.

[21] S. Khandekhar, Thermo-Hydrodynamics of Closed Loop Pulsating Heat Pipes,Doctoral dissertation, Universitaet Stuttgart, Germany, 2004. (at – <http://elib.unistuttgart.de/opus/volltexte/2004/1939/>).

[22] J.G. Collier, J.R. Thome, Convective Boiling and Condensation, Third ed.,Clarendon Press, Oxford, 1996.

[23] S. Filippeschi, Comparison between miniature periodic two-phasethermosyphons and miniature LHP applied to electronic cooling equipment,Applied Thermal Engineering 31 (2011) 785–792.

[24] J.R. Taylor, An Introduction to Error Analysis: The Study of Uncertainties inPhysical Measurements, University Science Books, Sausalito, 1997.

[25] S.G. Kandlikar, M. Shoji, V.K. Dhir, Handbook of Phase Change, Boiling andCondensation, Taylor and Francis, Philadelphia, USA, 1999.

of Closed Loop Two Phase Thermosyphon (CLTPT) for energy systems, Exp.3