1、2009 ASHRAE 617ABSTRACT Tilted air jet planes are used as barriers between two envi-ronments of different temperature, humidity and quality. Entrainment of one environment Fluid (air) into the curtain by shear layer mixing contributes to both the sensible and the latent heat load on the other enviro
2、nment and the impingement of the air curtain formed. Perturbation of one side affects the shape of the air jet and might endanger its integrity. Protrusion present in the direction of the flow impacts the performance of the air curtain and defeats its purpose of existence.Computational Fluid Dynamic
3、s (CFD) method is used to evaluate the performance of the air curtain formed by the tilted jet plane and is also validated by comparing the CFD calcu-lations results with experimental results.Qualitative design combination of various parameters and various levels of obstruction in the direction of t
4、he flow(s) are proposed in order to achieve optimal performance of the air curtain.INTRODUCTIONTilted air plane jets can be used as dynamic barriers to control and ensure invisible separation between two environ-ments. Quality control and conditions of the environments including temperature, contami
5、nants, pressure and humidity can be maintained independently upon the provision of the specific parameters that will allow the maintenance of the integrity of the air plane jets.The performance of this dynamic non-physical barrier will be under continuous threat of fluctuating due to the easy existe
6、nce of perturbation that could result from a change in the various conditions of thermal and pressure conditions on one side of the environment, or from the intrusions of personnel, hands or insects resulting in breaking the integrity of the barrier and requirement of further periods of time to rebu
7、ild the jet coupled with consequences of such broken integrity. The complexity of maintaining the quality and condi-tion of one environment from changes due to influence of another adjacent environment necessitates identification of the combination of factors and in specific the angle causing the ti
8、lt of the air jet, that affect the optimal performance of the tilted air jet separating the two environments. Such mainte-nance is managed upon the consideration of the dynamic nature, the balance between the jet momentum flux and the pressure difference between the two environments, surface stresse
9、s, infiltration, entrainment, turbulence generation, internal interactions, physical obstruction by objects and people and nature of work space.A typical application of the tilted air jet plane is being used in refrigerated display cabinets that exist practically in any commercial outlet, supermarke
10、t or mall. Air curtain is used, as a replacement of a see- through glass door. Surveys indicated that such rigid glass door may affect the propensity of the consumers to pick goods from the display cabinet. With the continuous increase of energy cost, the generation of suitable refrigerated environm
11、ent became a concern for owners in specific when such environment is not protected well against the infiltration of the opposite side of the curtain environment which is usually at different temperature and humidity condi-tions. If infiltration rate accounts for 70% -80% of a typical case cooling lo
12、ad and if the refrigeration accounts for 50% of typical store electric load, then efforts should be undertaken to mini-mize the infiltration rate aiming at reducing the energy cost.Having identified the need for the tools which enable identification of the performance of a tilted air jet, the object
13、ive Effects of Jet Inclination Angle and Geometrical Parameters on Air Curtain PerformanceSamir R. Traboulsi, PEng Ali Hammoud, PhD M. Farid Khalil, PhDMember ASHRAESamir R. Traboulsi is a senior lecturer in the Department of Engineering Management, American University of Beirut, Lebanon. Ali Hammou
14、d is a professor of fluid mechanics, Department of Mechanical Engineering, Beirut Arab University, Beirut, Lebanon. M. Farid Khalil is a professor in the Department of Mechanical Engineering, Alexandria University, Alexandria, Egypt.LO-09-058 2009, American Society of Heating, Refrigerating and Air-
15、Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions 2009, vol. 115, part 2. For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAEs prior written permission.618 ASHRAE Transactionsof th
16、is present work is to develop a model-based design meth-odology for the establishment of tilted air jet plane, that will facilitate the design and or evaluate existing designs and in particular the effects of the inclination angle of the jet on the performance of the air curtain. Air jet planes can
17、be vertical, horizontal or tilted and were introduced for the first time in year 1916. Function and tightness studies were performed in the last 40 years and mainly concentrated on the vertical and horizontal types. Under several titles, air jet planes were immensely considered in research work in b
18、oth domains: experimental and compu-tational. Some were successful in addressing those parameters that have significant impact on the performance of the jet. Identification of those parameters and quantification allowed some how the determination of certain rates like infiltration and or entrainment
19、 expressions.Many have contributed in developing a number of math-ematical models to aid in the design and performance predic-tion of the air jets. Explicit method was employed to solve the differential equations describing the flow and to prove that the performance of the air jet can be simulated e
20、ffectively using the finite difference technique; Hetsroni and Hayes, 1. Finite element method as well as other several patents were taken out on open protection devices with few investiga-tions have been reported; M. Havet et al. 2, who made the study on an air curtain used as a dynamic barrier to
21、separate two environments indicated that the curtain is strongly sensitive to perturbations such as draughts. Studies taking into consider-ation all major parameters affecting the air curtain flow field by the utilization of modern analytical, computational and exper-imental techniques, were done by
22、 H. Navaz et al. 3, by Bran-don S. Field, Eric Loth,4 and by M. Amin, et al. 5, on the entrainment of ambient air on vertical air curtain upon varying the Reynolds numbers (4200-8000) and the Richardson Number (013-0.58) which again showed that the entrainment of the ambient air was governed by vari
23、ety of eddy engulfing structures. Also, a numerical simulation was utilized on the two dimensional solution of a vertical down ward-blowing plane jet, J.J Costa et al.6, and on the flow and heat transfer char-acteristics of vertical air curtain in a vertical display cabinet with a two fluid turbulen
24、ce model; K-Z Yuet al.7. However, many experimental works were done on the air jet with little on tilted angle in comparison with the hori-zontal and vertical air barriers. Works indicated that a breaking point for air curtains occurs if the deflection modulus is below the minimum value for the part
25、icular air curtain configuration and the initial turbulence intensity has a moderate effect on the rate of heat transfer through the curtain; Howel reports, 8, 9. Another experiment showed that the mass entrainment rate, dominated by eddy engulfment of ambient air, was directly proportional to the a
26、ir speed of a down ward vertical blowing isothermal wall jet at moderate Reynolds Numbers (1500-8500) with significant inflow turbulence; B. S. Field Y-G Chen, X-L Yuan,11. Also, H. K. Navaz et al. 12, carried an investigation on the Jet entrainment in air curtain of open refrigerated display cases
27、where certain parameters like turbulent intensity, shape of the mean velocity profile at the discharge air grille, and the Reynolds Number were identified, quantified and the amount of entrained air was computed and showed that the shape of the vertical velocity profile and the turbulence intensity
28、present at the supply air grille controlled the entrainment rate and at different stages. Plane air jets were experimentally studied as well by K. Loubier, M. Pavageau, 13using PIV with an emphasis put on the flow structure in the impingement region of jet systems. Experimental results were not alwa
29、ys in conformity with previous works as it was the case with the findings of I. Gray, P. Luscombe et al. 14 when describing that need of having only 70% of total air delivery in circulation needs through air curtain and the balance through side discharge. The correlation of the numerical solutions w
30、ith the experimental works results were limited and in specific when using the Computational Fluid Dynamics (CFD) technique. An apparent conflict was demonstrated upon lowering the Reynolds Number aiming at minimizing the air entrainment in a vertical air curtain with the risk of loos-ing the integr
31、ity of the air curtain structure; H. K. Navaz, et al. 15. CFD predictions on infiltration were shown to vary with time limiting the possibility of utilizing the analytical models; A.M. Foster, et al. 16.CFD modeling which was used to aid the design of retail display cabinets provided a rapid means t
32、o understand air flows, optimum jet velocity and their effect on surrounding temperatures; A. M. Foster et al. 17.From the above review, it was concluded that the effect of jet inclination on the performance of air curtain was not well considered, thus opening areas for attracting further investigat
33、ion.METHODOLOGYTo overcome the difficulty in getting the unreliable results in data collected from an experimental set up, this research describes the several experimental and numerical tools that are used in analyzing and assessing the performance of the tilted air jet plane. Making use of Computer
34、 Fluid Dynamics (CFD) and its code 18,will contribute to savings in terms of both time and money, and such experiments can be performed for a final check of the correctness of numerical results.Experimental Set-Up ModelExperimental studies were carried out on a refrigerated display vertical cabinet
35、with internal dimension of D x W x H = 0.6 m x 1.8 m x 1.9 m. This cabinet was located in laboratory ASHRAE Transactions 619facilities at a Universitys research laboratory in a room of 10 m x 10 m x 4 m with its back side to one of the walls. The modular simple display case composed of supply air gr
36、ille with minimum width of 4 cms, and return air grille posi-tioned at variable angles and the air in the room is allowed to mix with the supply incoming air from the jet along the length of the Discharge Air Grille, DAG and along its height. The domain is bounded by two surfaces on the width of the
37、 Discharge Air grille, DAG and the Return Air Grille, RAG. The back panel of the display cabinet was perforated to allow theoretically, when goods stored on shelves are not in close proximity of the back panel, cooling for rear products and also to support the flow of vertical air curtain to ensure
38、better performance. Many tests were carried with practically sealed perforation (0%-5%) when noting that most of the display cabinets when used, are fully packed on their shelves and obstructing the flow of air from the back panels.In a display cabinet, air is drawn through a linear grille at the ba
39、se of the opening and fans then force it through the cool-ing coil situated underneath the load volume. The cooled air is forced to a supply plenum located behind the compartment. A fraction of the air is sometimes fed into the unit through perfo-rated plate at the back of the cabinet, while the rem
40、aining quantity of the cold air is blown through the Discharge Air Grille, DAG forming the tilted jet plane.Figure 1 shows a typical vertical display cabinet where the spillage and entrained air portions determine the mass of air returning to the Return Air Grille, RAG.Where in Figure 2, the Tilt An
41、gle ( ) and the discharge angle ( ) are shown with variation to either the positive or negative sides. Data ObtainedThe main aim of experiments was to detect the velocity uniformity at the start of the curtain flow and how the integrity of the curtain is affected by entrainment. The experimental res
42、ults not only provide necessary boundary conditions for later calculation, but also supply data that will be later compared with the results of simulation to assess the accuracy and viability of the established CFD model.Due to the irregularities of the flow, a more or less signif-icant amount of am
43、bient air is always entrained, reducing the temperature control capabilities and increasing the energy consumption. The data acquisition systems included temperature acqui-sition system equipped with special grade T thermocouples Figure 1 Schematic refrigerated display cabinet.620 ASHRAE Transaction
44、saccurate +/- 0.1C, a relative humidity reader accurate to +/- 3%, flow meters. The set up of thermocouples in the experiment were varied in positions to allow more coverage in readings. The sampling interval is 2 seconds. A continuous colored smoke-wire technique has been developed for flow visuali
45、zation. The improved contrast of colored smoke sheet facilitates flow visualization where a vertical smoke-wire is used for introduc-ing controlled sheets of smoke streaklines while running the cabinet fan. Regulated drops of a mixture of paraffin oil with colored dye are allowed to fall along the s
46、tainless steel wire. The wire is thus coated with a thin film of falling paraffin or with minute colored droplets along the length. The oil mixture subsequently evaporates through the resistive heating of the wire thus producing a sheet of colored streaklines. The visu-alization provided a good insi
47、ght over the shape of the steady Figure 2 Schematic layout and description of the tilt angles (25 16 deg.Tilt Negative Angle () VariationsWhen changing the tilt angle to negative values, by extend-ing the discharge grille to the direction of the room, the minimum Figure 3 Pressure profiles developme
48、nt between the discharge air and room boundaries.Figure 4 Velocity contours at different flow rates: (a) 0.13 kg/s, (b) 0.02 kg/s, (c) 0.05 kg/s, and (d) 0.08 kg/s.(a)(b) (d)(c)624 ASHRAE Transactionsvalues of infiltration rates are found as shown in Figure 7 for a flow rate of 0.04 Kg/s and H/WDAG=
49、 16.Vectors of velocity are shown in Figure 8 pertinent to the negative tilt angle variations of -10 and -2.5 degrees with more linear shape at () = -10, and upon closing all the back panel slots openings (leakage assumption of 5% only). The minimum infiltration rate is found at tilt angle of -11 () -10.5 With infiltration rate C of 0.360 and for a flow rate of 0.05 Kg/s.This infiltration rate at negative tilt angle is found to be less than the rate of the positive angle when taken about 10 degrees. (C= 0. 47019), resulting in prefe
