1、2009 ASHRAE 255ABSTRACTOpen Refrigerated Vertical Display Cases greatly contribute to the energy consumption of supermarkets. In the recent decades air curtains have significantly improved the efficiency of the systems; nevertheless, it is possible to change the design specifications of display case
2、s to achieve higher degrees of efficiency. A simulator apparatus was constructed to easily vary several geometrical and fluid dynamic variables that affect the air curtain performances, and then a series of experimental tests were performed by a noble tracer gas tech-nique to measure the infiltratio
3、n rate in each test case scenario. The study focuses only on the cases that do not benefit from back panel flow and therefore the ratio is 1. In addition to the infiltration rate measurement, a new method using tracer gas is used to measure the total flow rate of the display case. It was found that
4、the infiltration rate is strongly dependent on the investigated variables.INTRODUCTIONIn supermarkets, Open Refrigerated Vertical Display Cases (ORVDC) are commonly used to store and maintain food products at prescribed temperatures. A large number of such systems are constructed without doors (Figu
5、re 1a) allow-ing easy access to the food products. These systems also prevent the fogging problem that occurs on the interior side of the glass of the refrigerators with doors. As a result, a lot of ambient air laden with higher contents of energy than the cold air within the system, can penetrate o
6、nto the shelves of the system and undesirably increase the temperature of the food products. When this occurs, the temperature sensors placed close to the food prod-ucts operate the compressor until the lower prescribed temper-ature is attained. In 2003, a study by California Energy Commission, and
7、Southern California Edison Co. estimated that about 70,000 ORVDCs were operating in the supermar-kets of the United States. The projected energy consumption associated with these systems was a tremendous amount, about 1,000 GW-hr (3.4 1012Btu/hr) per year. In several studies such as the work of Howe
8、ll and Adams (1991), up to 80% of the cooling load of the systems were attributed to the infiltrated warm air. It is also estimated that up to 50% of the total energy costs of supermarkets is due to the operation of the display cases. Thus, one may conclude that the source of up to 40% of the total
9、energy costs of a supermarket is from entrain-ment of room air by ORVDCs, which will end up with 400 GW-hr (1.37 1012Btu/hr) per year. Given the enormous amount of energy, designing the systems with a more efficient air curtain is crucial. Efficiency of the systems can be partially characterized by
10、the amount of infiltrated mass. In the first step, this mass should be measured accurately, which is done by tracer gas in this study.In HVAC applications tracer gases are used to measure the infiltration or leakage of air between the interior and exterior of a building. Losses can occur through clo
11、sed windows and doors or any other holes and cracks in the building construc-tion. In the same fashion, tracer gases are applicable in measuring the leakage from air distribution units. Moreover, effectiveness of a ventilation system in distributing air within a room or zone can be evaluated by rele
12、asing a tracer gas and tracking it spatially and temporally. A review on the tracer gas techniques in HVAC applications can be found in the work of McWilliams (2002).Amin et al. (2008a) and Faramarzi et al. (2008), for the first time analytically and experimentally tailored the tracer Experimental I
13、nvestigation of the Effect of Various Parameters on the Infiltration Rates of Single Band Open Vertical Refrigerated Display Cases with Zero Back Panel FlowMazyar Amin Dana Dabiri, PhD Homayun K. Navaz, PhDMember ASHRAEMazyar Amin is a PhD candidate and Dana Dabiri is an assistant professor in the A
14、eronautics and Astronautics Department, University of Washington, Seattle, WA. Homayun K. Navaz is a professor in the Mechanical Engineering Department, Kettering University, Flint, MI. LO-09-022 2009, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org).
15、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.256 ASHRAE Transactionsgas application to the flow rate measurement of infi
16、ltrated air into the display case, as well as measured the operating flow rate of the system. In addition, they demonstrated that the entrained air and infiltrated air are not essentially identical since not all the entrained air can infiltrate into the RAG (Return Air Grille) and/or onto the shelve
17、s after the initial mixing in the air curtain. In this approach they showed (Equa-tion 1) that the amount of infiltrated air is directly proportional to the total flow rate of the display case, i.e. the flow rate through the RAG (Figure 1b). It is also a function of concen-trations of the tracer gas
18、 at the RAG, Discharge Air Grille (DAG), and the entrained ambient air. Therefore, an accurate measurement of the infiltration is contingent to the accuracy of total flow rate measurement at the RAG.(1)This flow rate is also referred to as absolute infiltration rate.A similar temperature-based relat
19、ionship (Equation 2) has been previously proposed by Rigot (1990) and Navaz et al. (2005b).(2)The thermal method (Equation 2) has in fact several disadvantages over the tracer gas technique. Firstly, many numerical and experimental studies such as the work of Navaz et al. (2002, 2005b), and Faramarz
20、i et al. (2008) have shown that the velocity profiles in the DAG and RAG are not neces-sarily uniform; so, the average temperatures that will be used in this equation must be measured with good resolution in the cross section and must be mass-weighted average quantities. Therefore, an auxiliary expe
21、rimental method is required for the velocity measurements. Secondly, the velocity measure-ments should be performed with good resolution across the width (small dimension) of the DAG and RAG to result in accurate average temperature. Also, during the measurements the variations in the total flow rat
22、e of the display case during and between frosting and defrosting stages in a full cycle of refrigeration should be taken into account. Some conventional velocity measurement techniques such as Hot-Wire Anemom-etry (HWA) and Pitot-tube velocimetry, although not as expensive, are not very practical in
23、 such an application since they cannot provide the required accuracy and resolution when several of them compacted in a narrow cross section area along the smaller dimension. For example, there is a slight variation in the velocity profile at different longitudinal sections (in z-direction shown in
24、Figure 2) of a display case due to imperfections in design and also presence of wires, etc. Tracer gas method, samples from the flow at least at three different longitudinal sections and at each section the samples are drawn by probes that have several holes on them. This creates better averaging al
25、ong across the DAG or RAG widths. Insertion of the probes is done neatly, which causes the least possible flow distraction during the operation of the system. In HWA, however, for an accurate measurement, several measurements should be done at each section, which may require insertion of several hot
26、 wires. This is in addition to the difficulty associated with insertion of hot-wire within at sections of the duct that should be distant from the inlet or discharge of the duct and are not easily accessible.Figure 1 (a) A typical vertical ORVDC (courtesy of Hill Phoenix Co.) and (b) schematic of si
27、de view with flow streamlines.mInfmRAGCDAGCRAGCDAGCAmb-=mInfmRAGTDAGTRAGTDAGTAmb-=ASHRAE Transactions 257In the tracer gas technique, the concentration of the tracer gas can be sampled simultaneously and continuously at several points along the lengths of the zones, DAG and RAG. Then at each zone th
28、e concentrations are averaged along the length, resulting in a more accurate, practical, engineering flow rate and infiltration rate. This technique does not require any knowledge about the cross-section area of the measure-ment region but a good mixing between tracer gas and air is essential.The la
29、ser based visualization techniques, such as Particle Image Velocimetry (PIV) or Laser Doppler Velocimetry (LDV) are more accurate but are time-consuming, cumber-some and cannot keep up with the pace of infiltration data collection. Thirdly, in cases that buoyancy effect is negligible (small Richards
30、on numbers), the systems can be tested under isothermal condition. As a result, Equation 2 is not applicable, but tracer gas can be conveniently used. Fourthly, this equa-tion, does not take into consideration the effect of humidity and humidity ratio, and it is more suited for a dry air. None-thele
31、ss, the comparison between the infiltration rate of tracer gas and the thermal method (Amin et al., 2008a) has pointed out a difference of up to 7% with the former method taking larger values (with identical in Equations 1 Re is the Reynolds number based on the average velocity of the DAG and width
32、of the DAG as its characteristics length. The last term on the right side accounts for the ratio of flow discharged from the DAG to the total operating flow rate. This study only focuses on the cases where the flow rate through the DAG and RAG are the same, i.e. the last term is one.Overall, this wo
33、rk attempts to present and discuss the results of tracer gas technique used for measuring the mass flow rate and infiltration flow rate of open display cases that lack the flow from the back side. However, even if zero back panel is not popular, the authors after several studies (includ-ing future w
34、orks on non-zero back panels) would like to show why and under what circumstances zero back panel flow is not very popular. However, this judgment is put off for after completing the future studies.PROBLEM DESCRIPTIONThe main goal of the current work is to experimentally investigate the dependency o
35、f the infiltration rate on variables on the right side of Equation 3. In order to investigate the effect of the variables with the minimum resolution to form infiltra-tion vs. variable curves, each variable should take at least three values, resulting a total number of 243 tests for all the permu-ta
36、tions. Obviously, constructing one display case to accom-modate each geometrical configuration is not feasible and requires multitudes of display cases that may cost hundreds of thousands of dollars. Therefore, to alter each variable, a modu-lar, versatile, flexible experimental equipment referred t
37、o as the Proof-Of-Concept Air Curtain (POCAC), or simulator, was designed and constructed for conducting the infiltration and mass flow rate tests by tracer gas. An injection system to release the tracer gas, and a sampling and analyzing system to measure the amount of tracer gas concentration at ea
38、ch desired location were employed.Simulator ApparatusFigure 3 depicts the simulator. This simulator is a system by which all the variables in Equation 3 can be easily varied. The system excludes a refrigeration system Figure 2 Schematic of relative geometrical positions of DAG and RAG.mRAGmInfmtot-
39、fHwDAG- , , Re, mDAGmtot-,=mtotmRAGmBPmDAG+=258 ASHRAE Transactionsas it was previously indicated by some researchers such as Navaz et al. (2002) and Faramarzi et al. (2008) that at the range of Reynolds numbers, which are typical in this application, the difference between the DAG and ambient tempe
40、ratures is not very significant and determinative to the amount of infiltrated air. In typical display cases the air curtain is momentum-driven and buoyancy has negli-gible effect. Nevertheless, one may anticipate that at smaller Reynolds numbers the effect of Buoyancy plays more important role in t
41、he entrainment. In the tracer gas technique, the measurement is based on the change in the concentration of tracer gas from the DAG to RAG; thus it is not required to have information about the thermal quantities and relative humidity. A wooden Display Case Prototype was inserted adjacent to the DAG
42、 and RAG to mimic an actual display case. The prototype consists of four shelves, which are fixed, horizontal (i.e. with no tilt angle) and equally distanced. The simulator creates a velocity profile close to a uniform distribution with relatively low turbulence intensity ( 2.5% to 3%) at the discha
43、rge point. To obtain a closer velocity profile to a uniform one with low turbulence intensity at the DAG, the design of the simulator was inspired by the principles of wind tunnel design. The simulator consists of several air manipulators to improve the uniformity of the flow (especially to improve
44、the homogeneity of the tracer gas in the air flow within the passages of the simulator), and to reduce the turbulence and agitation in the flow. The length of the duct in the upstream of the DAG is suffi-ciently long and the DAG is distant from the last bend (bend #2) in order to insert several air
45、manipulators to eventually reduce the turbulence at the DAG discharge. Within the simulator there are 6 perforated plates, 2 sets of turning blades, 5 fine screens, and 3 honeycombs. Immediately at the upstream of the DAG, a contrac-tion nozzle assists further attenuating the turbulence. Variation o
46、f the height of the opening is made possible by a neoprene bellows; due to the flexibility of the bellows it can provide the desired throw angle ( ) as well. Similarly, the horizontal movement is made by another bellows in the lower part of the apparatus. The expansion-to-contraction ratio of the be
47、llows is about 10-to-1. The flow is supplied by a crossflow fan that spans almost the entire length of the simulator; this can give more uniformity to the flow in z-direction than the popular axial fans. The highest point of the system from the floor is 118.1 in. (3 m), the maximum and minimum width
48、s of the simulator in y-direction are 108.7 in. (2.76 m) and 82.3 in. (2.09 m), respectively, and the length of the system in z-direction is 46 in. (1.17m). The opening height (the Figure 3 Side view of the simulator.ASHRAE Transactions 259vertical distance between the DAG and RAG) can vary between
49、2.75 in. (0.07 m) to a maximum of 26 in. (0.66 m). The widths and lengths of both DAG and RAG are identical and equal to 1.6 in. (0.0406 m) and 45 in. (1.143 m), respectively. It should be mentioned that since non-dimensional vari-ables are used in this study, the non-dimensionalized results of the work can be generalized for any real display case that its non-dimensional variables falls within the range of the current study. However, the dimensions of the simula
