Simulation of the performance of single jet air curtains for vertical refrigerated display cabinets

Air curtains are widely used in refrigerated display cabinets as well as doorways of cold rooms and retail premises. The main purpose of the air curtain is to reduce the air exchange and hence heat and moisture transfer between the conditioned environment and the surrounding ambient. This paper presents a comprehensive model, based on the finite difference technique, which can be used to predict and optimise the performance of air curtains. Based on results obtained from the model, correlations for the heat transfer across refrigerated display cabinet air curtains have been developed to enable quick calculations and parametric analyses for design and refrigeration equipment sizing purposes. Both models have been validated against results from tests on a vertical refrigerated display cabinet air curtain.     

Parametric evaluation of refrigerated air curtains for thermal insulation

The refrigerated air curtain used for cavity insulation in the present study is an idealization of the refrigerated air curtain used in supermarket food display cabinets. The thermal insulation performance of refrigerated air curtains is very important for perishable food storage and energy saving for refrigerated display cabinets. Thermal insulation ability of recirculated refrigerated air curtains was numerically studied in the present work. The results show that the refrigerated air curtains are negatively buoyant jets and tend to flow toward the inside cabinet due to stack pressure, so the initial momentum must be sufficiently large to sustain the pressure difference across the air curtain and assure the thermal insulation. The length–width ratio and the discharge angle of the air curtains, the height–depth ratio of the cavity and the dimension and position of the inside shelves would greatly influence the thermal insulation performance of air curtains, therefore were extensively discussed. The maximum Richardson numbers or the optimum parameter selections in each situation were presented for practical design of refrigerated air curtains used in multi-deck display cabinets.     

Simulation of air curtains for vertical display cases with a two-fluid model

In a vertical open display case, air curtains are used to weaken the influence of ambient air on the store. The flow and heat transfer characteristics of air curtains in a vertical display case are simulated with a two-fluid turbulence model in this paper, which takes the fluid in the space to be simulated as a mixture of turbulent fluid and non-turbulent fluid. The air curtains are taken as a turbulent fluid and described by the conventional k–ε turbulence model. The ambient air outside the display case is considered as a non-turbulent fluid and calculated by a laminar model. The exchanges of mass, momentum and energy between the turbulent and the non-turbulent fluids are expressed by empirical relations. The simulation results based on the two-fluid model are compared not only with experimental data, but also with the simulation results when the k–ε model is used for the whole simulated space. The comparisons indicate that the two-fluid model enables to predict thermal stratification phenomenon more accurately and shows better agreement with the measured values than the k–ε model.     

A correlation model of thermal entrainment factor for air curtain in a vertical open display cabinet

Thermal entrainment factor (TEF) is an important parameter to describe the cold preservation performance of a display cabinet. In order to calculate the TEF of a display cabinet rapidly and accurately, the method of establishing a correlation model based on computational fluid dynamics (CFD) simulation results is presented and the correlation model of TEF for typical single and double air curtain in a vertical open display cabinet is developed in this paper. The TEF of display cabinet predicted by the correlation model gives a maximum deviation of ?9.4% and a mean deviation of ?0.1% compared with the published experimental data. In addition, the correlation model can be used to calculate the air curtain return temperature, showing a maximum deviation of 0.9 °C and an average deviation of 0.1 °C compared with the published experimental data.     

Performance simulation of refrigerated display cabinets operating with refrigerants R22 and R404A

This paper describes the analysis and performance comparison of a display cabinet system using refrigerant R404A and its substitute refrigerant R22. The model of the display cabinet is developed at steady state and integrated from three main component sub-models, air-cooling finned-tube evaporator, air curtain and display cabinet body. The evaporator model is built up based on the distributed method, which can simulate the heat exchangers with different circuit structures. The frost effect on the performance of the evaporator is included in the model. The correlations for the heat transfer and pressure drop calculations of both air and refrigerant sides are purposely selected in the evaporator model. In addition, the evaporator model has been validated with experimental results at steady states from published literature. Several correlated functions from the detailed numerical solution are used for the model of the air curtain. Some simplifications are also utilized for the model of display cabinet body. The performance simulation and comparison of the display cabinet using refrigerants R404A and R22 are carried out at different indoor ambient conditions especially at varied ambient air humidity to mimic the actual indoor space conditions in super stores. Some significant results such as the comparison of cooling load requirement for different refrigerant display cases have been obtained from the simulation, which can significantly contribute to the optimal cabinet design and operating analysis.     

Simulation of a cavity insulated by a vertical single band cold air curtain

The cold air curtain used for cavity insulation in the present study is a fundamental idealization of a refrigerated air curtain in display cases. Flow and heat transfer characteristics of vertical single band cold air curtains are studied numerically to investigate the effects of mixed convection and height/width ratios. The results show that for a given Grashof number, the Richardson number must be less than a critical value to assure thermal insulation. From studies of the effect of height/width ratios, it is found that small height/width ratios yield an increased critical Richardson number (thus a reduced critical Reynolds number) and a decreased volumetric infiltration rate. For the cavity whose height/depth ratio equals 2 in the present study, the critical Richardson numbers are 0.62, 0.28 and 0.20 for the height/width ratios of 10, 15 and 20, respectively.     

Multiple Natural Convection Solution in Porous Media Under Cross Temperature and Concentration Gradients

This article is devoted to the study of air curtains applied to reduce the refrigerated chambers heat gains. The proposed strategy is based on the numerical simulation of air curtains by means of computational fluid dynamics (CFD) using RANS modeling and their corresponding experimental validation. Further work on the reduction of the detailed numerical results into overall energetic parameters is also presented. Unsteady three-dimensional numerical parametric studies are carried out, simulating the process of refrigerated chamber sudden door opening. The numerical solutions are verified and the influence of the turbulence model used is also investigated. The studies are centered on the influence of air curtain location, the air suction combination, and both the air discharge velocity and the discharge angle.     

MEPR versus EEPR valves in open supermarket refrigerated display cabinets

This paper presents the comparative experimental field performance of mechanical evaporator pressure regulating valves (MEPR) and electronic evaporator pressure regulating valves (EEPR) under the identical operating conditions of supermarket open multi-deck refrigerated display cabinets. The main goal of the supermarket refrigeration system design is to keep the displayed product at the required constant temperature, while minimising the cooling load to increase the overall energy efficiency of the system. Field tests have shown that the electronic evaporator pressure valve has a significant effect on improving the cabinet temperature and reducing the rate of frost formation on the evaporator coils with subsequent improvements in the air curtain strength.     

导流格栅对冷藏陈列柜性能影响的试验研究

在M3工况下对敞开立式食品冷藏陈列柜安装导流格栅前后的冷藏性能进行试验研究,分析了开式食品冷藏陈列柜加装导流格栅前后陈列柜内温度及能耗的变化规律。试验结果表明:在搁架前端加装导流格栅能够显著改善风幕性能,达到节能的效果。在陈列柜搁架安装导流格栅后的后排食品包平均温度都在不同程度上高于安装之前的食品包平均温度。且搁架末端设置空气导流格栅比未设置导流格栅的陈列柜的电能消耗节省约23%。     

An experimental and analytical evaluation of the insulative properties of double glazing and thick curtains

Theoretical and experimental evaluations are made of thermal curtains applied to double-glazed greenhouse structures. The reflectance and emittance of the curtain is permitted to be different on the two surfaces: It may be partially transparent or opaque to thermal radiation, and it may have thick blanket insulative properties. The effect of cloud cover upon the night-sky radiation temperature is included in the model. Convection coefficients are prescribed for the curtain and glazed surfaces, and the effect of air leakage into the space between the curtain and inner layer of glazing is included as an empirically defined parameter. The thermal balance for the system is obtained in terms of the radiative, convective and air-leakage parameters relevant to each component. The resulting set of thermal balance equations are solved iteratively using existing matrix-solution programs at the University of Waterloo. An experimental rig was constructed to determine nighttime heat loss from a conventional double-walled polycarbonate glazing material which is partially transparent to thermal radiation. The rig was operated with and without an insulating curtain. An opaque multilayer curtain and a semitransparent bubble film curtain were tested. The agreement between the thermal model and the test rig was within ±5% for all cases. Cloud cover was found to be a significant parameter that alters heat loss upwards by 11% from fully cloudy to clear-sky conditions. Intermediate cloud cover conditions were investigated as well and were found to be well predicted by an existing correlation for sky temperature vs cloud cover. The analysis is extended to provide a simple resistance coefficient for heat loss from the glazed structure based upon inside and outside temperatures for a specified cloud cover. It is found that the semiopaque curtain and double-glazed combination has 3.74 times the thermal resistance of a single glass structure as compared to a value of 3.81 for the opaque curtain and double-glazed arrangement. In addition the semiopaque curtain can be used for solar shading during the summer time, thereby serving a dual purpose that significantly increases its economic advantage.     

非均匀热负荷条件下的平板冲击冷却优化

冲击冷却是涡轮冷却中常见的方式，其优化设计涉及多种几何参数，是典型的高维问题。在冲击冷却结构的设计过程中，需要根据涡轮的热负荷情况适应性地设计冷却结构，以提高综合冷却效率和表面温度的均匀性。实验或数值模拟耗时长且成本高，而代理模型可以快速预测结果，配合计算机自动寻优算法可显著提高设计效率和效果。为了降低优化设计的成本、提高优化过程的效率，以平板冲击冷却为研究对象，同时考虑非均匀热负荷的影响，通过数值模拟构建数据集，建立了基于迭代算子神经网络的代理模型，并使用遗传算法对斑状非均匀热载荷条件下孔位置排布进行了优化。优化结果显示：对于优化潜力较低的结构，优化策略保持了靶板平均温度水平不变；对于优化潜力较高的结构，可以降低靶板平均温度约2.6 K;所研究各结构的表面温度标准差普遍降低70%以上。     

Experimental study of effect of an inner thermal curtain in evaporative cooling system of a cascade greenhouse

In this paper experimental study has been carried out in a cascade greenhouse with inner thermal curtain to see the effect of thermal curtain. A thermal model has also been developed to predict the air temperature in a cascade greenhouse. The fan-pad system has been used for evaporative cooling and an inner thermal curtain has been used to divide the greenhouse in two zones. Experiments have been conducted in hot summer conditions at Solar Energy Park, IIT Delhi, New Delhi, India for empty greenhouse. Statistical analysis has been carried out to validate the agreement of experimental observations with predicted values. The values of the root mean square percent deviation and coefficient of correlation has been found out 9.0%, 0.90; 5.0%, 0.95 and 7.0%, 0.97 for April, May and June in case of evaporative cooling without curtain in greenhouse-2. The degree of freedom for the experimental work is 10.0. It is found that the use of evaporative cooling with a thermal curtain reduces the temperature of greenhouse by 5 °C and 8 °C in the second zone of greenhouse-1 and 2 in comparison to greenhouse without curtain in May.     

Numerical evaluation of various thermal management strategies for polymer electrolyte fuel cell stacks

Thermal management is one of the key factors required to ensure good performance polymer electrolyte fuel cell (PEFC) stacks. The choice of the thermal management strategy depends on the specific application, size, weight, design, complexity, and cost. In this work, we investigate various alternative thermal management strategies for PEFC stacks, e.g., forced convection in specially design cooling plate/channel with either (i) liquid or (ii) air as the coolant; (iii) edge-air cooling with fins and; combine oxidant and coolant flow (open-cathode) with (iv) forced and (v) natural convection air cooling. A three-dimensional two-phase model, comprising of the equations of conservation of mass, momentum, species, energy and charge, is employed to quantify the performance of various cooling strategies. The results demonstrate that thermal management is essential to ensure good stack performance. Liquid cooling, as expected, performs the best compared to air cooling, whereas natural convection cooling is just marginally able to maintain a stack with large number of cells from steep drop in performance. Finally, results presented in this paper can provide useful design guidelines for selection of a suitable thermal management strategy for a PEFC stack and its near-to- or optimum cooling condition.     

遗传算法和复合形法确定坝基帷幕灌浆优化方案

以一般岩基上的混凝土重力坝为例,根据已有灌浆帷幕参数的经验公式,在考虑帷幕的渗流量、坝基渗透压力和幕体本身的水力坡降不超过允许值为约束条件的情况下,进行简单的公式推导;最后以帷幕的工程造价最小为目标函数建立优化设计数学模型,并分别采用遗传算法和复合形法对坝基帷幕灌浆进行优化研究,寻找一般情况下的最优帷幕参数的组合。     

Simulation of multi-deck medium temperature display cabinets with the integration of CFD and cooling coil models

In this paper, the model for the multi-deck medium temperature display cabinets is developed with the integration of CFD and cooling coil sub-models. The distributed method is used to develop the cooling coil model with the airside inputs from the outputs of the CFD model. Inversely, the airside outputs from the cooling coil model are used to update the boundary conditions of the CFD model. To validate this cabinet model, a multi-deck medium temperature display cabinet refrigerated with a secondary refrigerant cooling coil was selected as a prototype and mounted in an air conditioned chamber. Extensive tests were conducted at constant space air temperature and varied relative humilities. The cabinet model has been validated by comparing with the test results for the parameters of air at different locations of the flow path, and temperatures of refrigerant and food product, etc. The validated model is therefore used to explore and analyse the cabinet performance and control strategies at various operating and design conditions.     

Combined photovoltaic and solar thermal systems for facade integration and building insulation

Most photovoltaic (PV) facades are built as curtain facades in front of thermally insulated buildings, with air ducts in between. This causes additional costs for support structure and installation, while heat dissipation from the solar cells is often not optimal. Measurements carried out are facing both concerns: integration of a thermal insulating layer (which meets the latest German heat-preserving regulation, WSV 95) into the PV facade, plus additional cooling by active ventilation or water flow. Active ventilation at conventional curtain PV facades allows a reduction of cell operating temperatures of 18 K, resulting in an 8% increase in electrical energy output at an airspeed of about 2 m/s. Cell temperatures increase by 20.7 K at thermal insulating PV facade elements (TIPVE) without cooling, which causes a 9.3% loss of electrical yield, but installation costs can be reduced by 20% (all related to a conventional PV curtain plus a heat-insulating facade at a building). HYTIPVE, a hybrid thermal insulating PV facade element combined with a water cooling system, which could also serve for heating up water, lowers the operating cell temperature by 20 K and increases electrical yield by 9% (compared with conventional curtain PV facades). Further economic investigations of such a HYTIPVE, including its operational costs and substitution effect, related to the electrical and thermal yield, are in progress.     

Performance enhancement of gas turbines by inlet air‐cooling in hot and humid climates

In this paper, a model to study the effect of inlet air‐cooling on gas turbines power and efficiency is developed for two different cooling techniques, direct mechanical refrigeration and an evaporative water spray cooler. Energy analysis is used to present the performance improvement in terms of power gain ratio and thermal efficiency change factors. Relationships are derived for an open gas turbine cycle with irreversible compression and expansion processes coupled to air‐cooling systems. The obtained results show that the power and efficiency improvements are functions of the ambient conditions and the gas turbine pressure ratio. The performance improvement is calculated for, ambient temperatures from 30 to 50°C, the whole range of humidity ratio (10–100%) and pressure ratio from 8 to 12. For direct mechanical refrigeration air‐cooling, the power improvement is associated with appreciable drop in the thermal efficiency. The maximum power gain can be obtained if the air temperature is reduced to its lowest limit that is the refrigerant evaporation temperature plus the evaporator design temperature difference. Water spray cooling process is sensitive to the ambient relative humidity and is suitable for dry air conditions. The power gain and efficiency enhancement are limited by the wet bulb temperature. The performance of spray evaporative cooler is presented in a dimensionless working graph. The daily performance of the cooling methods is examined for an ABB‐11D5 gas turbine operating under the hot humid conditions of Jeddah, Saudi Arabia. The results indicate that the direct mechanical refrigeration increased the daily power output by 6.77% versus 2.57% for the spray air‐cooling. Copyright © 2005 John Wiley & Sons, Ltd.     

Innovative cathode flow-field design for passive air-cooled polymer electrolyte membrane (PEM) fuel cell stacks

A novel cathode flow-field design suitable for a passive air-cooled polymer electrolyte membrane (PEM) fuel cell stack is proposed to enhance the water-retaining capability under excess dry air supply conditions. The innovative cathode flow-field is designed to supply more air to the cooling channels and further enables deceleration of the reactant air in the gas channels and acceleration of the coolant air in the cooling channels simultaneously along the air flow path. Therefore, the design facilitates the waste heat removal through the cooling channels while the water removal by the reactant air is minimized. The conceptual cathode flow-field design is validated using a three-dimensional PEM fuel cell model. The detailed simulation results clearly demonstrate that the new cathode flow-field design exhibits superior water-retaining capability compared with a conventional cathode flow-field design (parallel flow channel configuration) under typical air-cooled fuel cell operating conditions. This study provides a new strategy to design cathode flow-fields to alleviate notorious membrane dehydration and unstable performance issues in a passive air-cooled PEM fuel cell stack.     

某型气冷涡轮级的三维优化设计

1引言随着叶轮机械设计技术的不断进步,对叶片造型理论和设计方法提出了更高要求,叶片设计往往决定着效率、压比、重量等诸多性能参数,涉及到来源于不同准则的许多目标和约束。与叶轮机械设计相关联的优化问题通常涉及到许多约束和大量参数,一般导致目标函数有许多极值点。目前     

Heating and cooling potential of an earth-to-air heat exchanger using artificial neural network

In this article, we use the concept of artificial neural network and goal oriented design to propose a computer design tool that can help the designer to evaluate any aspect of earth-to-air heat exchanger and behavior of the final configuration. The present study focuses mostly on those aspects related to the passive heating or cooling performance of the building. Two models have been developed for this purpose, namely deterministic and intelligent. The deterministic model is developed by analyzing simultaneously coupled heat and mass transfer in ground whereas the intelligent model is a development of data driven artificial neural network model. Six variables influencing the thermal performance of the earth-to-air heat exchangers which were taken into account are length, humidity, ambient air temperature, ground surface temperature, ground temperature at burial depth and air mass flow rate. Furthermore, a sensitivity analysis was carried out in order to evaluate the impact of various factors involved in the energy balance equation at the burial depth. The model was validated against experimental data sets. Moreover, the developed algorithm is suitable for the calculation of the outlet air temperature and therefore of the heating and cooling potential of the earth-to-air heat exchanger system. The Intelligent model predicts earth-to-air heat exchanger outlet air temperature with an accuracy of ±2.6%, whereas, the deterministic model shows an accuracy of ±5.3%.