Modeling & Experiments On A Vertical Open Display Cabinet With Central Air Supply

Vertical open display cabinets are the important refrigerated food supply chain and widely used in supermarkets. They have the advantages of good display and easy getting of products stored. In a vertical open display cabinet, the only barriers between the refrigerated food inside the cabinet and the ambient air in the store are one or more forced-air curtains. However, these air curtains are easily disturbed by the ambient air, which leads to much power consumption of display cabinet. The temperature of display cabinet periodically fluctuates according to the frosting and defrosting cycle of the evaporator in the display cabinet, which may result in serious damage of the quality of the freezing food.In order to solve problems mentioned above, a new type display cabinet, so-called"vertical display cabinet with central air supply (VDCCAS)"is developed and the temperature and energy-saving performance of VDCCAS are studied both by modeling and by experimental method in this paper. Based on above purposes, this dissertation realized the following work:1. The experimental rig for a VDCCAS was established and the operation characteristic of VDCCAS was investigated. In the experimental rig, the options of refrigeration system (VDCCAS or conventional vertical display cabinet–CVDC), air supply method (vertical display cabinet with refrigerating machine air supply–VDCRMAS or vertical display cabinet with environment air supply– VDCEAS) and working cabinets could be easily switched in the experimental rig. For VDCRMAS, the suitable defrost cycle & defrost time, temperature fluctuation characteristic, the optimal air curtain velocity and the strategy of air distribution for the temperature equilibrium between two cabinets were presented. For VDCEAS, the power consumption and the influence of air curtain velocity on the cabinet temperature were investigated.2. A modified two-fluid turbulence model was established to simulate the flow and heat transfer characteristics of air curtains in an open vertical display cabinet. The air exiting the back panel, the inner air curtain and the outer air curtain were taken as the first fluid and described with the standard k-εturbulence model. The air outside the display cabinet was considered as the second fluid and calculated by the laminar model. Different from the existing two-fluid model, the mass transfer rate equation between the turbulent and the non-turbulent fluids was modified, and the face coefficient of honeycomb (the ratio of the effective area to the total area of honeycomb) was fetched with the volume fraction of air curtain.3. CFD simulation of a VDCCAS. Five parts including air supply duct, air supply structure, air curtain, air return structure and air return duct were integrated and the modified two fluid model was employed to simulate the air flow and heat transfer in a VDCCAS. The velocity and temperature characteristics were analyzed and the predicted temperatures were compared with measurements by experiments.4. A correlation model of thermal entrainment factor (TEF) for air curtain was developed. According to the dimensional analysis and theoretical deduction, the TEF for air curtain could be simplified as a correlation function of six dimensionless numbers: Reynolds number indicating effect of air curtain supply parameters, Richardson number indicating effect of ambient parameter, two dimensionless numbers indicating effect of air curtain geometry parameters, back panel airflow ratio and the TEF for back panel air. Based on CFD simulation results, the TEF for air curtain could be simplified as a correlation function of six dimensionless numbers mentioned above.5. The quick temperature simulation model of VDCCAS was established. The simulation model of air supply duct, air supply structure, air curtain, air return structure, and air return duct were distributed parameters model, lumped parameter model, correlation model, single-node model and distributed parameters model, respectively. These models were integrated as the quick temperature simulation model of VDCCAS. The algorithm of quick temperature simulation model of VDCCAS was developed and the simulation results were compared with both the predicted temperatures based on CFD and the measurements.6. The air supply strategy of VDCCAS was developed. The advantages and disadvantages of refrigerating machine and environment air supply method were analyzed. Based on these air supply methods, a new air supply method-- union air supply was introduced and the air supply strategy of VDCCAS was presented. The quick temperature simulation model was also adopted to analyze the air supply strategy of VDCCAS.Based on the above works, the main results are summarized as follows:1. The experimental findings show that VDCRMAS has better temperature performance than that of CVDC. Comparing to CVDC, VDCRMAS increase the defrost cycle from 6h to 9h, and reduce the maximum product temperature rise in the defrost cycle from 3.0oC to 2.0oC, which shows great advantages. To reach the lowest temperature distribution in the cabinet, the optimum Reynolds number is 4350-5000. When two cabinets work together, the airflow ratio of the two cabinets should be 1.1~1.2 in order to keep the temperature balance between the two cabinets.2. The experimental findings show that has better energy-saving performance than that of CVDC. VDCEAS can introduce the outdoor cold air to the display cabinet to implement refrigeration, which can save much energy consumption. However, the application of VDCEAS is limited by environment temperature.3. The modified two-fluid model for air curtain can both reflect the proper mass transfer rate between the air curtains and ambient air and depict the honeycombs perfectly. The comparisons between the simulated temperatures and the experimental ones indicated that the modified two-fluid model can give better agreement with the measurements than k-εmodel and two original two-fluid models.4. The correlation model for TEF for air curtain also shows good accuracy. By comparing with the published experimental data, the average deviation of TEF and return temperature predicted by the correlation model was 0.9% and 0.1oC, respectively. The deviations indicated that the correlation model is enough accurate for engineering application.5. The tendency of predicted temperatures by CFD is close to that of measurements. When one cabinet works, the average difference and maximum difference are -0.9 oC and -1.8 oC, respectively. While two cabinets work together, the average difference and maximum difference are -2.0 oC and -4.0 oC, respectively. Therefore, the CFD modeling is successful.6 The accuracy of quick temperature simulation model is a little lower than that of CFD modeling. However, the quick temperature simulation model spends less time (2 minutes) than CFD modeling (about 2 hours). Therefore, the quick temperature simulation model shows potential for engineering application.7. The quick temperature simulation model was adopted to analyze the air supply strategy of VDCCAS. The results show that the air supply strategy can increase the energy efficiency and extend the application range of VDCCAS.