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An investigation of Air Curtains Flows Effects on heat and

mass transfer Characteristics in a cold room

Dr.Gamal ElHariry, Eng. Ahmed AbuZeid, Prof.Dr.Essam E. Khalil*

Faculty of Engineering, Cairo University, Egypt, *khalile1@asme,org

The main purpose of the present paper is to demonstrate the importance of air curtains on the

likely performance of cold rooms .The paper explores the benefits of using computational fluid

dynamics (CFD) as a tool for simulation of the flow and heat transfer characteristics of air curtains

in a vertical display case and cold rooms .This can be achieved by using computational fluid

dynamics software to simulate the air flow pattern and the temperature distribution in a frozen

food vertical display cabinet .Previous research work is analyzed and critically viewed in respect

of previous literature reported to predict the sensible heat transfer across the air curtain and using

it to investigate the effect of various design parameters such as jet angles and initial velocities on

heat transfer . The present work emphasized the importance of the three dimensional modelling of

the flow regimes downstream of an air curtain used to restrict cold room infiltration. The present

work highlights the significance of using a full three dimensional procedure, using a commercial

CFD code "Fluent 6.2." and gives examples of comparisons with relevant experimentations. The

effect of operating parameters (outlet velocity, temperature and humidity ratio) on the performance

of the air curtain is investigated together with the external wind velocity and curtain outlet angle

.Few published test cases were used to access the validity of the model assumptions and good

agreements with the experiments were generally shown.

Keywords: air curtain, plane free jet, velocity profile, temperature profile, humidity ratio profile

I. Introduction

When opened a door separating a cold storage area from a warm room permits a substantial loss of refrigerated air.

Warm air flows into the cold room through the lower part. This results in energy losses, safety hazards in the form of

condensation and icing on the floor and fog in the doorway; and possibly food spoilage. Strip doors used on coolers

and freezers to reduce these effects impair visibility and are unpleasant to pass through due to condensation and

frosting and accumulate dirt and possible bacterial growth. Studies have proven that Air Curtains, when properly

sized and adjusted, are up to 85% efficient in controlling the flow of air through cooler and freezer doorways. If the

cold storage door is open over one hour per day the Air Curtain is a cost effective way to save refrigeration costs1-10

.

Installed on the warm side of the doorway the Air Curtain emits an air stream which reaches the floor at an angle

and splits into two separate air streams. By properly adjusting the volume of the air and the angle of the nozzle, one

air stream would balance against the other which is trying to leave the cooled room, while the other counteracts the

warm air trying to enter. The correct Air Curtain sizing and adjustments must be made for each specific application

so that a narrow, high velocity, low volume stream of air is projected over the entire opening creating a sufficiently

stiff curtain of air. Built-in adjustments in the Air Curtain must include fully adjustable mounting brackets, variable

volume controls and individually adjustable louvers in the nozzle. The narrow nozzle limits the amount of air in the

doorway area and hence the turbulence, thus increasing the efficiency of the unit. In addition to providing a

substantial energy savings and increased safety, Cold Storage Air Curtains can increase the time between defrosting

by a factor of four, depending on the particular freezer or cooler4. Also, their ability to maintain the cold room

temperature right up to the doorway improves product quality and increases the useful floor space.

46th AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit25 - 28 July 2010, Nashville, TN

AIAA 2010-6931

Copyright © 2010 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

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The benefits of air curtains can be listed as

▪ Increased employee comfort.

▪ Energy savings through control of air transfer

▪ Faster and safer traffic flow and increased production due to clear and open doorways. Door maintenance cost savings due to decreased breakdowns. ▪ Increased usable space near door areas.

▪ Elimination of ice and fog in cold storage areas.

▪ Increased safety in door areas due to better visibility

II. Mathematical Modelling and Assumptions

The present numerical investigation was based on solving the governing equations that described airflow inside the

cold room by a CFD program FLUENT 6.2 5(commercially available CFD program). This numerical approach

solves the partial differential equations governing the transport of mass, three momentum, energy and species in a

fully turbulent three dimensional domain under steady state conditions in addition to standard k – ε model equations

for turbulence closure6-7

.

Computational Fluid Dynamics Models

The different governing partial differential equations are typically expressed in a general form as:

ΦΦΦΦ +

∂

Φ∂Γ

∂

∂+

∂

Φ∂Γ

∂

∂+

∂

Φ∂Γ

∂

∂=Φ

∂

∂+Φ

∂

∂+Φ

∂

∂S

zzyyxxW

zV

yU

xeffeffeff ,,,ρρρ

(1)

Where ρ is the air density and Φ is the dependent variable, SΦ = Source term ofΦ, and U, V, W are the velocity

vectors, and ΓΦ,eff is the effective diffusion coefficient. The effective diffusion coefficients and source terms for the

various differential equations8 are listed in the table 1.

Table 1. Terms of Partial Differential Equations (PDE) equation 1.

Φ ΓΦ,eff SΦ

Continuity 1 0 0

X-momentum U µeff -∂P/∂x +ρg+ SU

Y-momentum V µeff -∂P/∂y+ρg (1+β∆t) + SV

Z-momentum W µeff -∂P/∂z+ρg+ SW

H-equation H µeff/σH SH

RH-Equation RH µeff/σRH SRH

τ-age equation τ µeff/στ ρ k-equation k µeff/σk G - ρ ε ε-equation ε µeff/σε C1 ε G/k – C2 ρ ε2

/k

µeff = µlam + µ t µ t = ρ Cµ k2 / ε

G = µt [2{(∂U/∂x)2 +(∂V/∂y)

2 +(∂W/∂z)

2}+(∂U/∂y + ∂V/∂x)

2 +(∂V/∂z + ∂W/∂y)

2 +(∂U/∂z + ∂W/∂x)

2]

SU = ∂/∂x(µeff ∂Φ/∂x)+∂/∂y(µeff ∂Φ/∂x)+∂/∂z(µeff ∂Φ/∂x)

SV = ∂/∂x(µeff ∂Φ/∂y)+∂/∂y(µeff ∂Φ/∂y)+∂/∂z(µeff ∂Φ/∂y)

SW = ∂/∂x(µeff ∂Φ/∂z)+∂/∂y(µeff ∂Φ/∂z)+∂/∂z(µeff ∂Φ/∂z)

SH is the source of Energy at nodal points

Turbulence model constants C1 = 1.44, C2 = 1.92, Cµ = 0.09

σH = 0.9, σRH = 0.9, στ = 0.9, σk = 0.9, σε = 1.225

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Boundary Conditions

The solution of the governing equations can be realized through the specifications of appropriate boundary

conditions. The values of velocity, temperature, kinetic energy, and its dissipation rate should be specified at all

boundaries. A non-slip condition at all solid walls is applied to the velocities. The logarithmic law of the wall (wall

function) of Launder and Spalding6

was used here, for the near wall boundary layer. At inlets, the air velocity was

assumed to have a uniform distribution; inlet values of the temperature were assumed to be of a constant value and

uniform distribution. All velocity components were set as zeros initially, and temperatures were assumed to be equal

to the steady state value of the comfort condition.

Numerical Procedure

The Computer Program, FLUENT 5 was used to solve the time-independent (steady state) conservation

equations together with the standard k-ε model as Launder and Spalding 6

and the corresponding boundary

conditions. The numerical solution grid divided the space of the cold room into discretized computational cells of

the order of 500,000 tetrahedral cells. The discrete finite difference equations were solved with the SIMPLE

algorithm, Khalil11

. Solution convergence criteria, was applied at each iteration and ensured the summations of

normalized residuals were less than 0.001 for flow, 0.001 for k andε, and 10-6

for energy. The predictions of flow

and turbulence characteristics are in general qualitative agreement with the corresponding experiments and

numerical simulations published by others, Neilsen4. Nevertheless discrepancies exist and particularly in the vicinity

of recirculation zone boundaries. More discrepancies were also observed in situation with heating flows than those

of ventilation or cooling.

Convergence and Stability

The simultaneous and non-linear characteristics of the finite difference equations necessitate that special measures

are employed to procure numerical stability (convergence); these include under relaxation of the solution of the

momentum and turbulence equations by under relaxation factors which relate the old and the new values of Φ as

follows,

( ) oldnew 1 Φγ−+Φγ=Φ

(2)

Where γ is the under-relaxation factor. It was varied between 0.2 and 0.3 for the three velocity components as the

number of iteration increases. For the turbulence quantities, γ was taken between 0.2 and 0.4 and for other variables

between 0.2 and 0.6. The required iterations for convergence are based on the nature of the problem and the

numerical conditions (grid nodes, under-relaxation factor, initial guess, etc.). So the time (on the computer

processor) required to obtain the results is based on many factors. The computational number of iterative steps is

selected according to space cell (spatial difference) to yield converged solutions11

. The validity of the present

computational technique was assessed previously in the open literature, for example Khalil 3, 7

; where reference

should be made for more detailed readings.

III. Results and Discussions

A typical orthogonal three dimensional grid composed of 1070775 mesh cells was used to obtain grid independent

results .The Random Access Memory (RAM) required for the numerical grid depends mainly on the selected CFD

code, grids between 100,000 and 1000,000 mesh cells will require between 40 and 300 Mb of RAM.

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Cold store Model description

Mini type constructional cold store was chosen as a study room and is schematically shown in figure 1. Its exterior

dimension is 4.8 m (L) x 5.8 m (W) x 3.8 m (H). The thickness of the wall of cold store is 150 mm, which is made

up of polystyrene foam. This outside-cold room was modelled by entrance zone that have the same dimensions of

cold room 4.8 m (L) x 5.8 m (W) x 3.8 m (H). The room had an entrance (W x H = 1.36 m x 3.2 m) with a 0.15 m

doorframe .These are typical dimensions of commercial cold rooms commonly used for food storage.

Figure 1. Cold Room Arrangement

A typical industrial air curtain was considered here designed to be installed on doors of cold stores and freezers;

which has low noise centrifugal double inlet fans driven by an external rotor motor with built in thermal protection

contact and is provided with five speed selection device. The unit had perforated inlet grille with large absorbing

surface to minimize the air pressure drop. It does not need filter. The air curtain simulated here, shown in Figure 2,

had the following Specifications:

Airflow: 1000 m3/hr

Fans power input 230v-50Hz: 0.372 KW

Noise level: 53 dB

Weight: 29 kg

Figure2. Air Curtain Design, L=1.0 m

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Boundary Conditions:

1. Inlet Air Conditions:

The inlet air conditions to the air curtain are taken as the average day maximum of 35ºC and 56% relative humidity,

representing July conditions in Cairo. The inlet air condition to the air curtain is modelled as pressure outlet with

target mass flow rate equals to 0.4424 kg/s in negative x-direction since pressure outlet boundary conditions are

used to define the static pressure at flow outlets (and also other scalar variables, in case of backflow). The selected

flow rates and velocities refer to local practice in Egyptian cold rooms designs. The use of a pressure outlet

boundary condition instead of an outflow condition often results in a better rate of convergence when backflow

occurs during iteration.

2. Outlet air conditions

The outlet air condition from the air curtain is modelled as velocity inlet boundary condition with air velocity that

equal to 6 m/s

3. Walls

The walls of cold rooms are to be set at – 20°C which correspond to the inside condition of the cold room. Also it is

assumed that the walls have relative humidity of 90 % which correspond to the inside condition of the cold room as

most of foodstuffs storage conditions lie between 85 and 95 % relative humidity 12

.The no slip condition is enabled

for all walls, while using the standard wall function for near wall treatment. The walls of modelled entrance hall are

to be set at 35°C which correspond to the outside condition of the cold room. Also it is assumed that in the wall

vicinity the relative humidity is 56 % which correspond to the outside condition of the cold room12

.

Modelled Case studies:- Case 1: air curtain discharge is considered as planer jet with axial velocity of 6 m/s in the negative y – direction.

Case 2: air curtain discharge is considered as planer jet with axial velocity of 6 m/s with 15º toward the cold room

Figure 3 and 4 show the air temperature contours as well as the velocity contours identifying some dead zones where

no significant circulation velocity was observed for case 1 along X-axis at X= 1.18 m; at the middle of the air

curtain.

Figure 3. (Case 1) Temperature contours (K), vertical X-Y plane at Z = 1.18 m.

Cold Room

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Figure 4. (Case 1) velocity contours (m/s), vertical X-Y plane at Z = 1.18 m.

Figure 5 shows the humidity ratio contours for case 1 along X-axis at z = 1.18 m; at the middle of the air curtain.

Food stuff should be ideally preserved at low moisture content as explained in boundary conditions; this was not

achieved completely as shown in Figure 5; it can be seen that some zones in the cold room have higher moisture

content which occur due to moisture migration from the outside of cold room to the inside of the room due to the

difference in vapour pressure. This pressure difference draws outside ambient air into the store, with its associated

moisture. Air also infiltrates into the store from door openings where high velocity air currents can be created.

Figure 5. (Case 1) Humidity ratio distribution (kgv/kga), Y-X plane at Z = 1.18 m.

Different velocity, temperatures and water vapour content contours at Y-Z plane at X=5.07 m are shown hereafter in

Figures 6, 7 and 8.The predictions clearly identified the air curtain effects on the velocity contours in door vicinity

as well as the effects on the air thermal pattern and relative humidity values near the door where the air curtain is

located. Such effect would have strong influence on the validity of the food stuff stored in the cold room.

Dead zones

Cold Room

Cold Room

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Figure 6. (Case 1) velocity contours (m/s), vertical plane at X = 5.07 m.

Figure 7. (Case 1) Temperature contours (K), vertical plane at X = 5.07 m.

Figure 8. (Case 1) Humidity ratio distribution (kgv/kga), vertical plane at X = 5.07 m.

Cold Room

Cold Room

Cold Room

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For Case 2, figures 9 and 10 show the air temperature contours as well as the velocity contours identifying some

dead zones where no significant circulation velocity was observed for case 2 along X-axis at X= 1.18 m. At the

middle plane of the air curtain; the tilting the air jet towards the cold room by an angle 15 o resulted in directing the

jet through (pass) into the cold room.

Figure 9. (Case 2) Temperature contours, vertical plane at z = 1.18 m.

Figure 10. (Case 2) velocity contours along the X-axis, vertical plane at z = 1.18 m.

Dead zones

Cold Room

Cold Room

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Figure 11 shows the humidity ratio contours for case 2 along X-axis at z = 1.18 m. The tilting of the air curtain jet

resulted in better preservation of the cold room product as no cold air is allowed to exit. Food stuff should be ideally

preserved at low moisture content as explained in boundary conditions; this was better achieved here than earlier in

Figure 5.

Figure 11. (Case 2) Humidity ratio distribution (kgv/kga), vertical plane at z = 1.18 m.

Different velocity, temperatures and water vapour content contours at Y-Z plane at X=5.07 m are shown hereafter in

Figures 12, 13 and 14.The predictions clearly identified the air curtain effects on the velocity contours in door

vicinity as well as the effects on the air thermal pattern and relative humidity values near the door where the air

curtain is located. Such effect would have strong influence on the validity of the food stuff stored in the cold room.

Figure 12. (Case 2) Temperature contours (K), vertical plane at x = 5.07 m.

Cold Room

Cold Room

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Figure 13. (Case 2) velocity contours (m/s), vertical plane at x = 5.07 m.

Figure 14. (Case 2) Humidity ratio distribution (kgv/kga), vertical plane at x = 5.07 m.

IV.Concluding Remarks

This paper is a preliminary attempt to investigate the flow patterns, thermal characteristics and humidity variation in

a cold room with the use of air curtain. This work is based on the finite difference technique to predict flow

parameters of a single air curtain unit. The present model made use of a commercially available CFD program that

was found to adequately predict the air curtain effects on room air flow patterns. On the basis of the results

demonstrated here, one can recommend the use of such numerical tool to design a cold room air flow pattern to

Cold Room

Cold Room

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control the flow regimes, thermal distribution and relative humidity; these are major factors in efficient cold room

design and operation.

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2Khalil, E. E., 2008, Efficient Energy Utilization in Air Conditioned Buildings in Egypt: New Directive”

THETACONF II, PP.527-532, Cairo, December 2008, EGYPT

3Khalil, E. E., 2000, Computer aided design for comfort in healthy air conditioned spaces, Proceedings of Healthy

Buildings 2000, Finland, Vol. 2, pp 461-466. 4Nielsen, P. V., 1989, Numerical Prediction of air distribution in rooms, ASHRAE, Building Systems: room air and

air contaminant distribution, 1989. 5FLUENT 6.2 Documentation,

© Fluent Inc. 2005.

6Launder, B. E., and Spalding D. B., 1974, The Numerical Computation of Turbulent Flows, Computer Methods

App. Mech., pp. 269-275.

7Khalil.E.E. 2009, Air Conditioning and Refrigeration Designs for Environmental Sustainability, Proceedings of

ICGSI, AC01, Thailand, December 2009 8Spalding, D. B., and Patankar, S. V., 1974, A Calculation Procedure for Heat, Mass and Momentum Transfer in

Three Dimensional Parabolic Flows, Int. J. Heat & Mass Transfer, 15, pp. 1787. 9Elasfouri, A.S. , Abou El-Kassem,E.K and Taha.S.R.,2009, Flow Field Characteristics of a Single Plane Jet Air

Curtain Directed Vertically Downward at Opening between two Non-Isothermal Zones,IECEC Paper number:

AIAA-2009-4512. 10

Elasfouri, A.S. , Abou El-Kassem,E.K and Taha.S.R.,2009Heat Transfer Rates of Vertically Downward, Single

Plane Jet Air Curtains,IECEC Paper number: AIAA-2009-4610 11

Khalil, E.E., 1983, Modeling of Furnaces and Combustors, Abacus Press, UK. 12

Egyptian HVAC Code, HBRC, 2004