Ingo Stotz∗, Grazia Lamanna† and Bernhard Weigand‡
Institut fur Thermodynamik der Luft- und Raumfahrt, Universitat Stuttgart, 70569 Stuttgart, Germany
Johan Steelant§
ESTEC-ESA, 2200 AG Noordwijk, The Netherlands
For gaining deeper insight in the process of fuel evaporation in dense fuel sprays, under-standing the fluid disintegration mechanisms at high chamber pressures and tempera-
tures is of particular importance.This paper describes a newly commissioned double-diaphragm shock tube (DDST) facility,developed at the Institute of Aerospace Thermodynamics (ITLR) of the Universitat Stutt-gart. A shock tube is an ideal tool to investigate disintegration and evaporation processes,as it can provide a uniform and well-defined thermodynamic state over a wide range ofpressures and temperatures.The double-diaphragm shock tube at ITLR consists of five major components: driver, buf-fer, driven section, test section, and dump tank. Driver, buffer, and driven section have acylindrical inner diameter. The introduction of the short buffer between the driver and thedriven section reduces the initial pressure load on each of the two diaphragms and improvesthe controllability and triggering of the experiments. The test section has a square crosssection to allow the application of flat fused-silica windows for optical access. The obser-vable length within the test section is 100 mm. In order to partially dispose the boundarylayer and to accomplish the transition from the cylindrical driven section to the square testsection, a skimmer is mounted at the beginning of the test section realising a semi-directconnection between these two parts. The driver section is 3 m long, the driven section isabout 9.5 m long leading to an overall facility length of about 12.5 m.The new shock tube facility was characterised for several representative conditions, usinga helium/argon mixture as the driver gas and argon as the test gas. Pressure levels of upto 50 bar have been attained and temperatures greater than 2000 K were achieved. Thesevalues are suitable to investigate most hydrocarbon fuels of interest (e.g. n-dodecane as akerosene substitute) over a wide range of different thermodynamic regimes (sub,- trans-and supercritical).In the endwall of the shock tube, a fast-acting high-pressure injection system is mounted torapidly introduce the fuel into the test section, as test-times are limited to about 4–5 ms.First spray characterisation experiments are currently ongoing.
Nomenclature
D Inner diameter of the injector [µm] SubscriptsMS Incident shock Mach number 0 Reference valuep Pressure [bar] 1 Driven section (initial)T Temperature [K] 4 Driver (initial)γ Adiabatic exponent 5 Driven section (after shock reflection)
∗Research Assistant, Institut fur Thermodynamik der Luft- und Raumfahrt (ITLR), Pfaffenwaldring 31, 70569 Stuttgart,Germany.
†Senior Research Assistant, Institut fur Thermodynamik der Luft- und Raumfahrt (ITLR), Pfaffenwaldring 31, 70569 Stutt-gart, Germany.
‡Professor, Institut fur Thermodynamik der Luft- und Raumfahrt (ITLR), Pfaffenwaldring 31, 70569 Stuttgart, Germany.§Senior Researcher, Propulsion and Aerothermodynamics Division, Keplerlaan 1, 2200 AG Noordwijk, The Netherlands.
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Comprehensive knowledge on the thermodynamics and fluid mechanics of liquid droplets in dense fuelsprays is of importance to understand the process of liquid jet disintegration in many high-pressure,
high-temperature applications such as aerospace propulsion systems, air-breathing engines, gas turbines ordiesel engines. The need to burn the fuel more efficiently and thereby increasing the overall performance ofthe propulsion system leads to chamber pressure and temperature levels above the thermodynamic criticalpoint of many fuels. For the design of modern combustors it is therefore essential to develop a soundcharacterisation of the fluid disintegration and vaporisation processes in the critical and supercritical regime.Increasing the chamber pressures and temperatures above the thermodynamical critical point opens upsome unresolved issues that need to be investigated in more detail as the fluid behaviour differs from whatconventionally occurs at low-pressure/low-temperature conditions (e.g. disappearance of surface tension,vanishing enthalpy of vaporisation among other property singularities near the critical point).
A shock tube is an ideal experimental tool to obtain such fundamental information, as it can providea uniform and well-defined thermodynamic state over a wide range of pressure and temperature levels ina controlled and highly repeatable manner. For this account a double-diaphragm shock tube (DDST) wasdeveloped at the Institute of Aerospace Thermodynamics (ITLR) of the Universitat Stuttgart in order toprovide basic information on liquid-hydrocarbon-spray disintegration and vaporisation processes over a widerange of thermodynamic regimes under controlled laboratory conditions. Shock tubes have been applied toliquid-spray breakup, vaporisation, and combustion measurements before1–3 a thorough review can be foundin Petersen4 .
This paper describes the shock tube hardware including the data acquisition equipment as well as ex-perimental procedures. Furthermore the gas dynamic performance of the facility is characterised and someresults for preliminary spray injection experiments are presented.
II. Apparatus Design
Invented by Paul Vieille5 who first reported on their application over 100 years ago, shock tubes have beenused for decades in laboratories for heterogenous fundamental research. In a shock tube, the desired ther-
modynamic state is generated through the compression and heating of the test-gas by shock waves. Typicalapplications of shock tubes are e.g. super- and hypersonic flow studies, liquid-spray breakup investigationsor chemical kinetic and combustion experiments.
A. Shock Tube
Having the aforementioned applications in mind a double-diaphragm shock tube (DDST) was developed,built, and characterised over a range of typical thermodynamic conditions. A special feature of this shocktube is its square test section (50mm x50mm), although the driver and the driven section have a cylindricalcross section (72mm). A semi-direct connection between the cylindrical and the square part is realised byintroducing a skimmer at the beginning of the test section which punches out a quadratic portion from theincident shock wave. The skimmer also disposes some part of the boundary layer which is subsequently fedinto the dump tank. This technique was first proposed by Schardin6 . The driver has a length of 3000mmfollowed by the buffer which is 59mm long. The cylindrical part of the driven section is 8359mm long, thesquared cross-section portion consisting of skimmer and test section is 1000mm long leading to an overallfacility length of about 12.5m. A schematic sketch of the shock tube is given in Figure 1.
Before an experiment driver, buffer, and driven section are separated through aluminium diaphragmswith burst pressures ranging from 10 bar to 50 bar. Diaphragm burst pressures are varied by altering theirthickness, their composition and the score depth. The diaphragms are force fitted between driver and buffer,buffer and driven section respectively. Driver, buffer and driven section have three sharp concentrical sealingedges, which bite into the aluminium diaphragms by tightening six tension screws thus sealing the differentshock tube sections. Between the sections two spacer disks are introduced in order not to overtighten thediaphragm section and hence forcing the sealing edges too deeply into the diaphragms. This has to be
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DriverBuffer/Diaphragms
Driven SectionDump TankTest Section
1000mm ( 50mm) 8359mm ( 72mm) 3059mm ( 72mm)
Vacuum-Valve
Injection System
Figure 1: Schematic of the DDST.
necessarily avoided as the diaphragms may fail at the sealing edges and the centre of a diaphragm may betorn out and driven down the driven section, thus severely damaging the test section windows. A sketch ofthe diaphragm setup is shown in Figure 2. The shock tube can be run in either single or double diaphragmmode, the latter having the advantage of reducing the pressure load on the diaphragms therefore enablingthe application of thinner diaphragms7,8 . If run in single diaphragm mode only one diaphragm is used whilethe other one is replaced by an annulus made of the same material as the diaphragm. In the endwall of thetest-section a fast-acting fuel injector is mounted in order to study the liquid-spray injection and breakupbehind the reflected shock wave.
Driver BufferDriven Section
Spacer Disks
Diaphragms
Figure 2: Mounting of the diaphragms between driver, buffer and driven section.
B. Test Section
The test section for the spray injection studies was eroded from a block of stainless steel. This manufacturingtechnique provides very high contouring accuracy as well as a high-precision surface. The skimmer whichconnects the cylindrical part with the square test section was built in an identical manner. A photographof this semi-direct connection is given in Figure 3a. The square cross section allows the introduction of flatfused-silica windows on the side walls of the test section hence providing good optical access to the regionof interest. The flush-mounted fused-silica windows in the side walls cover the full height of the duct anda length of 60mm. They are designed in a manner so that they can be flipped by 180, providing a field ofview of 100mm in total. A picture of the test section is shown in Figure 3b.
C. Auxiliary and Data Acquisition Equipment
Shock tube operation requires additional equipment. Before a facility run the shock tube is evacuated topressures smaller 10−5 bar. A Leybold Trivac D8B vacuum pump with a throughput of 8.5 m3
/h is used toevacuate the driver and the buffer. Driven section, test section and dump tank are pumped down with
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(a) Skimmer (b) Test Section
Figure 3: Photographs of the skimmer (a), and the test section (b) of the DDST.
an Alcatel 2063 vacuum pump (throughput 65 m3
/h). The vacuum pressures are controlled using LeyboldThermovac TR201 and TR211 vacuum gauges with a Leybold Combivac CM31 control unit.
Transient pressure traces are monitored with fast-response piezoelectric pressure transducers (Kistler603B) and the corresponding charge amplifiers (Kistler 5007 and 5011 ) and recorded on a Nicolet transientrecorder. These transducers are flush-mounted in 5 ports upstream the shock tube’s endwall as they areused to monitor the pressure behind the incident and the reflected shock wave within the test section. Thepressure transducers are shielded against heat transfer with a thin layer of red RTV silicone. From thesepressure profiles and the known spacing between the transducers shock velocities and Mach numbers can bederived.
In the qualification phase of the DDST, Schlieren images of the incident and reflected shock waveshave been acquired in addition to the pressure recordings. The Schlieren system was arranged in a typicalz-type manner9 . Due to the fast processes involved in the experiments a high-speed spark flash lamp(Nanolite KL - L) was used resulting in an exposure time of about 20 ns. The Schlieren pictures wereacquired with a 1280(H)x1024(V) pixels PCO sensicam CCD camera. If slightly modified, this setup canalso be used to acquire shadowgraph images of the spray injection. The Schlieren setup is given in Figure 4a.
The transducers located in the test section can be either triggered from the tunnel recoil which is detectedby a light barrier or from an additional (Kistler 701A) pressure transducer located in the buffer which sensesthe pressure rise due to the rupture of the first diaphragm. This manually triggered transducer is also used togauge the diaphragm burst pressure especially when new diaphragms are applied. It is necessary to keep theoperating time of the piezoelectric transducers as short as possible in order to avoid natural drift. All othercomponents (CCD camera, Nanolite and injection system, etc.) are triggered from one of the transducerlocated in the test section, the timing is controlled by a purpose-built Labsmith LC880 trigger unit.
D. Injection System
The shock tube is designed for hydrocarbon spray disintegration experiments. In these studies, the liquid-fuel is introduced into the test section after the shock wave has been reflected at the endwall. As test timesare of the order of a few milliseconds, a fast-response injection system is installed which rapidly introducesa defined amount of fuel into the test section. The fuel pressure can be varied from 100 bar up to 1400 barwith injection times ranging from a couple of hundreds of microseconds up to 10ms and delay times of about400µs after the trigger pulse. The fuel supplying the injector is pressurised by a Bosch CP-3 high-pressurepump, the injection system is controlled by a BSG ECU-CR.6.2 control unit. Two pinhole injectors withan inner-diameter of D = 150µm and D = 236µm are available at the moment. The functional principle ofthe fuel injection technique is shown in the conceptual sketch given in Figure 4b.
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Nanolite
Test Section
Injection
System
CCD
Camera
Aperture
Achromatic
Lens
Condenser
Lens
Knife
Edge
Parabolic
Mirror
Parabolic
Mirror
(a) Schlieren setup
Driven SectionSkimmer
Fused-SilicaWindows
Injector
Test Section
Precompressed Test Gas
High-Pressure/High-Temperature
Test Gas
Reflected Shock Wave
Dump Tank
FuelSpray
(b) Concept of fuel injection
Figure 4: Schematic of the Z-type Schlieren setup (a), topview of the endwall fuel injection system showingliquid-spray injection behind the reflected shock wave (b).
III. Shock Tube Performance
Aseries of qualification experiments was performed to characterise the shock tube performance for typicaltest-pressures and test-temperatures. As the test gas argon was used, the driver gas consisted of a
helium/argon-mixture. These tests provided information of the overall shock tube performance, resulting incalibration diagrams (as a function of filling conditions) and achievable test times. The experiments werealso conducted to characterise the behaviour of the skimmer and its interaction with the shock waves.
A. Pressure Profiles and Performance Diagramms
The shock tube was qualified for initial fill pressure ratios p4/p1 ranging from 5 to 200 for argon test gas.The driver gas consisted of a mixture of helium and argon (He/Ar: 80 Vol.%/20 Vol.%) in order to increasethe test time. Dynamic pressure traces were recorded on several locations in the sidewall of the test section.The transducers are also used to sense the passage of the shock wave and hence to determine the timeintervals between the transducers. From these intervals and the known distances between the transducersthe Mach number of the shock waves can be derived. The temperature T0 right after the reflected shock canbe calculated from the measured initial shock speed (Mach number) using one-dimensional shock relations.Transient temperature profiles can be calculated from the pressure traces assuming an isentropic relationbetween pressure and temperature for an ideal gas T = T0 · (p/p0)
(γ−1)/γ . The isentropic assumption holdsfor shock wave experiments with negligible energy release10,11 which is the case for these experiments.
A typical pressure trace and the derived temperature plot for argon test gas and an initial fill pressureratio of p4/p1 = 18 (driver gas, He/Ar: 80%/20%) are plotted in Figure 5 (transducer location 2.5mm fromthe endwall). The test time is defined as the time between passage of the reflected shock wave and the timeof the first substantial pressure drop, which is due to an expansion wave most likely caused by the interactionbetween the reflected shock wave and the contact surface. Pressure levels up to 50 bar and temperaturesgreater 2000K were obtained. The test time for a typical experiment is about 4 − 5ms.
Finally performance diagrams can be derived from the experimental data, plotting the fill pressure ratiosp4/p1 against the initial shock Mach numbers MS , the reflected temperature T5 respectively. The perfor-mance diagram for argon test gas (driver gas, He/Ar: 80%/20%) is given in Figure 6. The experimentalresults are compared with ideal one-dimensional shock theory12 for the same incident shock Mach numbersand similar gas compositions. The experimental p4/p1 values are by a factor of about 2 greater than thetheoretically calculated ones, which is a typical value found in literature. The discrepancy between measuredand ideal shock Mach numbers is caused by boundary layer growth, non-ideal shock formation and non-idealdiaphragm rupture. These influences deteriorate with increasing Mach numbers, therefore the measured andideal curves diverge for higher Mach numbers.
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15 16 17 18 19 200
10
20
30
40
Time [ms]
Pre
ssure
[bar]
(a) Pressure
15 16 17 18 19 200
300
600
900
1200
Time [ms]
Tem
pera
ture
[K
]
(b) Temperature
Figure 5: Typical sidewall pressure trace for argon test gas (a), temperature plot derived from pressure traceusing 1D shock relations and an isentropic assumption (b) (transducer location 2.5mm from the endwall,p4
p1
= 18, driver gas He/Ar: 80%/20%).
1 1.5 2 2.5 3 3.50
50
100
150
200
250
M S
p4/p
1
Experimental Data
Curve Fit
Ideal Theory
Figure 6: Performance diagram for argon test gas, driver gas He/Ar: 80%/20%.
B. Schlieren Photographs
Besides the pressure measurements, Schlieren images of the shock waves and the flow-field within the testsection were acquired at different times. The time increment quoted for the Schlieren photographs givenin Figure 7 refers to the incident shock wave passing a pressure transducer located 60mm upstream of theendwall, from which the CCD camera and the flashlight were triggered. In the images this transducer issitting directly at the righthand edge of the window. The images were all taken at equal conditions (argontest gas, driver gas He/Ar: 80%/20%, p4/p1 = 18) and similar optical settings.
As can bee seen in Figures 7a - 7c which all show the incident shock wave at different times, there arealso oblique shocks visible. Theses waves originate from the flow behind the incident shock wave whichinteracts with the skimmer. Compared to the primary shock these other waves are much weaker, as canfirstly be concluded from the much lower intensity level and secondly from the pressure plots where nounexpected pressure fluctuations occur. After shock reflection (Figures 7d - 7f) there are, in addition tothe main (reflected) shock wave, several shock interactions visible which emerge from the corners of the testsection endwall and the fuel injector. Although clearly visible in the Schlieren pictures, there again is noindication of these disturbances in the pressure plots, leading to the conclusion that these secondary shockwaves are very faint. This assumption is further confirmed by looking at the Schlieren image taken at 235µs(see Figure 7f) on which these waves already faded as the test gas is slowed down.
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(a) 10 µs (b) 45 µs
(c) 80 µs (d) 113 µs
(e) 180 µs (f) 235 µs
Figure 7: Sample Schlieren images of the incident and reflected shock wave acquired at different times(p4
p1
= 18, argon test gas, driver gas He/Ar: 80%/20%).
C. Spray Injection
As the injection system has been installed just recently only few very premature results are presented here.The emphasis of the scheduled hydrocarbon disintegration experiments lies on the injection of the fluid
into a supercritical environment. Therefore the general feasibility of such an experiment is demonstrated. Forthe spray injection experiments it is important that the conditions within the test section are as undisturbed
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as possible. It was shown in the previous section that the secondary shock waves decay shortly after shockreflection.
This can be assured by the shadowgraph image shown in Figure 8a acquired at a time typical for fuelinjection (at 2000µs after shock reflection). Figure 8b gives a shadowgraph image of n-dodecane fuel injectedbehind the reflected shock wave (nozzle diameter D = 150µm) into argon test gas acquired at equal conditionsand at the same time. The test conditions chosen for both experiments were p5 = 26bar and T5 = 900Kwhich is supercritical for n-dodecane. N-dodecane was chosen as the fuel for its similar properties to kerosene.The spray visualised in Figure 8b appears very symmetric thus confirming the previously made conclusionthat the conditions in the test section are uniform during the test time. At the moment liquid-spray injectionexperiments at different chamber conditions are ongoing.
(a) No injection (b) n-dodecane injection
Figure 8: Shadowgraph images acquired at 2000µs after shock reflection (p4
p1
= 18, argon test gas, driver gas
He/Ar: 80%/20%).
IV. Conclusion
A new double-diaphragm shock tube for hydrocarbon disintegration studies was developed and charac-terised at the Institute of Aerospace Thermodynamics (ITLR) of the Universitat Stuttgart. The outstandingfeature of this shock tube is the introduction of a skimmer which realises a semi-direct connection betweenthe cylindrical driven section and the square test section. The facility was qualified over a range of pre-sumable operational conditions, with argon as the test gas and a helium/argon mixture as the driver gas.Performance diagrams were derived from transient pressure measurements and the feasibility of the design,particularly of the skimmer was shown. Additionally Schlieren images were acquired to visualise the shockwaves and the test gas flow for different times. A fast-acting fuel injection has been installed, its practicabi-lity was demonstrated. First hydrocarbon-spray experiments with emphasis on supercritical disintegrationare currently ongoing.
Acknowledgments
This work was performed within the Long-Term Advanced Propulsion Concepts and Technologies (LAP-CAT) project investigating high-speed airbreathing propulsion. LAPCAT, coordinated by ESA-ESTEC,
is supported by the EU within the 6th Framework Programme Priority 1.4, Aeronautic and Space, Contractno.: AST4-CT-2005-012282.Further information on LAPCAT can be found on http://www.esa.int/techresources/lapcat.
The financial support of the EU for the present study is highly appreciated. We also kindly acknowledgethe help of Mr. Harald Hettrich for designing the shock tube facility. Furthermore the helpful discussionswith Dr. Stefan Arndt and Dr. Dietmar Zeh from Bosch regarding various aspects of the injection systemare highly acknowledged.
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1Mullaney, G., “Shock Tube Technique for Study of Autoignition of Liquid Fuel Sprays,” Industrial and Engineering
Chemistry, Vol. 50, No. 1, January 1958, pp. 53–58.2Cadman, P., “Shock Tube Combustion of Liquid Hydrocarbon Sprays at High Temperatures,” Shock Waves @ Mar-
seille II , edited by R. Brun and L. Dumitrescu, 1995, pp. 179–184.3Eric L. Petersen, “A Shock-Tube Facilty for Spray-Combustion and Reacting-Flow Visualization,” 33rd AIAA Fluid
Dynamics Conference and Exhibit , AIAA, Orlando, USA, June 23–26 2003, AIAA 2003-3881.4Eric L. Petersen, “Solid- and Liquid-Phase Combustion Measurements Using a Shock Tube: A Review,” JANNAF 37th
Combustion Subcommittee Meeting, JANNAF, 13–17 November 2000, pp. 567–588.5Paul Vieille, Memorial des Poudres et Salpetres, Vol. 10, Gauthier-Villars, Quai des Grands-Augustins 55, Paris, 1899.6Schardin, H., “Uber das Stosswellenrohr,” Tech. Rep. 14m/51, Laboratoire de Recherches de Saint-Louis (LRSL), 1955,
pp. 231–252.7Gaydon, A. G. and Hurle, I. R., The Shock Tube in High-Temperature Chemical Physics, Chap. & H, 1963.8Oertel, H., Stossrohre, Springer-Verlag, 1966.9Gary S. Settles, Schlieren and Shadowgraph Techniques, Springer-Verlag, 2001.
10Eric L. Petersen, “Nonideal Effects Behind Reflected Shock Waves in a High-Pressure Shock Tube,” Shock Waves,Vol. 10, 2001, pp. 405–420.
11Matthew A. Oehlschlaeger, David F. Davidson, and Jay B. Jeffries, “Temperature Measurement Using Ultraviolet LaserAbsorption of Carbon Dioxide Behind Shock Waves,” Applied Optics, Vol. 44, No. 31, November 2005, pp. 6599–6605.
12John D. Anderson, Modern Compressible Flow with Historical Perspective, McGraw-Hill, 2nd ed., 1990.
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