Two- and three-dimensional CFD applied to vertical display cabinets simulation

A commercial CFD code has been employed to simulate the air flow pattern and the temperature distribution in a frozen food vertical display cabinet. At first the choice of solver parameters has been investigated in a 2D modelisation. 3D simulations have been then performed, and the effects of the cabinet length, of the warm air curtain and of longitudinal ambient air movement have been investigated. The results show that, in short cabinets, 3D secondary vortices at the side walls provide the most important mechanism for hot air entrainment. Comparison with experimental results shows that a 2D simulation is totally inadequate for such configurations, while 3D computations predict refrigeration power within engineering accuracy. Furthermore, the computed refrigerating power shows that even low room air velocity, due to its interaction with the end-wall vortices, has a significant impact on cabinet performance.     

Design guidelines for the forced-air cooling process of strawberries

The complex structure of the package systems currently used by the strawberry industry has prevented an efficient design of the forced-air cooling process. In this study, we investigated the mechanisms through which different design parameters affect the rate and uniformity of cooling, using a previously validated computational fluid dynamics model. The results indicated that the vent area has a significant effect on the cooling rate, but not on its uniformity. A design that reduces bypassing will not necessarily increase the cooling rate, because there is less bypass air to cool down the air exiting from each clamshell, so that the air entering the next clamshell becomes warmer. Periodic airflow reversal improves the rate and homogeneity of the cooling process.     

Effectiveness and optimum jet velocity for a plane jet air curtain used to restrict cold room infiltration

The effectiveness of a 1.0 m wide air curtain fitted over a 1.36 m wide entrance to a cold store has been evaluated. It was shown that careful setting up of the air curtain (adjusting the jet velocity and angle) was needed, this achieved an effectiveness of 0.77 compared to the initial value of only 0.31 as set by the installer. An analytical model to predict the optimum jet velocity was compared to measured data. It is important to choose the correct safety factor (an increase in the jet velocity) for this model, as an effectiveness of between 0.37 and 0.70 could be produced using the range of safety factors found in the literature. A 2D computational fluid dynamics (CFD) model predicted a maximum effectiveness of 0.84 and showed how the effectiveness of the curtain was related to the shape of the jet and how this varied with jet velocity and door open duration.     

Airflow blockage and guide technology on energy saving for spiral quick-freezer

The effect of airflow blockage and guide technology on energy saving for spiral quick-freezers were investigated by simulating and analyzing the airflow field and measuring of the velocity distribution in the freezing zone for different designs. The k– turbulence model was used. The velocities and temperatures of the air in the freezing zone for different designs of airflow blockage and guide boards were measured. The study shows that the airflow pattern plays a key role on energy efficiency, freezing time, and production rate. In the study case, through the optimization of the airflow blocking boards and the guide boards, the average air velocity in the freezing zone would be enhanced to 2.5–2.7 times compared with the original design. Correspondingly for bean curds in a stationary condition, the freezing time would be shortened by 78–85%, energy efficiency and the production rate would be increased by approximately 18–28% individually.     

Improvement of air distribution in refrigerated vertical open front remote supermarket display cases

This paper presents some of the results derived from extensive experimentation on display cases for supermarkets and derives some possible improvements to augment temperature uniformity and energy performance. The effect of the perforation pattern of the rear duct on the distribution of airflow between these and the front curtain is brought out. The critical component is air infiltration across the curtain, which is governed by internal air distribution and curtain characteristics. It is observed that a 70–30 distribution of flow between the curtain and the rear duct perforations yields a performance that satisfies the standards. Further, a judicious distribution of perforations on the rear duct at various levels and across is necessary. Correlations available in the literature are useful in making a qualitative assessment of the results.     

Thermal uniformity and rapid cooling inside refrigerators

The authors developed a new air-supply system for improving the thermal uniformity and the cooling rate inside a fresh food cabinet of a household refrigerator. For these purposes, we added a blower and jet slots to a conventional cooled air supply system. The jet slots circulate the air inside the cabinet at a higher velocity to optimize airflow velocity and its distribution. The jet stirs the air inside the cabinet and improves thermal uniformity, which resulted in half the temperature deviation in the cabinet as that of the conventional systems. The jet also improves the heat transfer on the surface of foods. We achieved a four times higher cooling rate with the new system than that with the conventional ones. In order to cut down the development period, we applied computational fluid dynamics to study air distribution inside the cabinet with the new system. We also derived the model of the cooling process by the jet using theoretical and empirical equations and applied it to decide the jet velocity for the rapid cooling.     

陈列柜风幕的数值研究及其影响因素分析

本文根据流体力学和数值传热学的基本方程，建立了陈列柜风幕及柜内空气流动和换热的二维数学模型；并采用湍流模型和SIMPLER算法进行了数值计算。文中重点分析了风幕送风速度、环境空气温度和送回风口尺寸三个因素对风幕性能的影响。本文的研究结果为陈列柜的优化设计和实验研究提供了依据。     

Measurements of the air flow field in the freezer compartment of a top-mount no-frost refrigerator: the effect of temperature

The energy consumption of a household refrigerator is closely related to the distributions of temperature inside its compartments. Since, such distributions are dependent on the chilled air circulating inside the refrigerator, its energy consumption can be reduced and its efficiency improved through optimization of the air flow fields inside the compartments. In this work, particle image velocimetry (PIV) was used to measure velocity fields inside the freezer compartment of a commercial top-mount no-frost refrigerator at two critical vertical sections at three different temperatures levels. To perform the tests, a commercial unit was equipped with two specially constructed insulated windows which enabled clear visualization of the flow field under real operating conditions. The results show that significant changes in the air flow distribution occur when the temperature is reduced.     

旋风分离器减阻杆减阻的PIV实验研究

采用先进的PIV实验技术对Stairmand型旋风分离器中安装减阻杆前后的强湍流场进行了测量。通过速度场、湍流强度、Reynolds应力等物理量的对比分析,表明减阻杆降低了中心涡核区的旋转动能和湍流强度。对减阻杆减阻机理进行了更深入的探讨。     

风幕特性对立式低温陈列柜运行性能影响的实验研究

风幕特性的好坏对敞开式制冷陈列柜性能影响很大.通过改变风幕的层数和各层风幕的送风速度,实验测试陈列柜运行期间柜内各层搁架上的瞬态温度值,来研究低温陈列柜内温度的变化和分布情况.研究表明,对于立式敞开式低温陈列柜采用三层风幕的结构是必要的,合理选配各层风幕的送风速度可使柜性能得到提高.     

Computation of the airflow in a pilot scale clean room using K- turbulence models

This work deals with the assessment of the airflow in a food-processing clean room. The flow pattern inside the working area of a pilot scale clean room was numerically investigated using a computational fluid dynamics code based on a finite volume formulation. Two versions of the k- turbulence model were tested: the standard and the RNG version. The analysis of the velocity magnitude does not reveal sensitive differences between them. Moreover, both models well predict the main features of the flow and numerical results agree with experimental measurements. However, a further examination shows that the RNG k- turbulence model predicts more swirls and more complex trajectories. As the standard k- model overestimates the turbulent diffusion, the RNG version seems to be more suitable to calculate the airflow in clean rooms. The influence of initial turbulence intensity is also pointed out. Finally, the study of the airflow below a laminar flow unit confirms that the design of clean rooms can benefit from the numerical approach.     

立式开式陈列柜风幕送风速度对其温度分布影响的实验研究

实验研究立式开式陈列柜风幕送风速度对其温度分布的影响。实验结果表明，降低其内风幕的送风速度或提高其外风幕的送风速度，均能使风幕向柜内侧偏移，减少冷风幕的外溢，提高风幕封闭敞口的能力，改善陈列柜的性能。     

CFD simulation of refrigerated display cabinets

The finite element method is employed for the analysis of velocity and temperature distributions in refrigerated open display cabinets. The CFD code is based on the streamfunction-vorticity formulation, and incorporates a LES turbulence model. As an example of application, a vertical multi-deck cabinet is investigated under different operating conditions. The numerical results have been validated by comparison with experimental tests performed in accordance with the EN441 Standard. The influence of various design parameters has been investigated.     

Modified two-fluid model for air curtains in open vertical display cabinets

A modified two-fluid turbulence model is established to simulate the flow and heat transfer characteristics of air curtains in an open vertical display cabinet. The air exiting the back panel (AEBP), the inner air curtain and the outer air curtain are taken as the first fluid and described with the standard k− turbulence model (KE). The air outside the display cabinet is 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 is modified, and the face coefficient of honeycomb (the ratio of the effective area to the total area of honeycomb) is fetched with the volume fraction of air curtain in order to conveniently depict the blow characteristics of the honeycomb. The comparisons between the simulated temperatures and the experimental ones indicate that the modified two-fluid model (MTF) can give better agreement with the measurements than KE and two original two-fluid models (TF1 and TF2).     

Development of a design tool for display case evaporators

A model for simulating a fin-and-tube display case evaporator has been developed to serve as a design tool for improving performance in frosting conditions. It is capable of simulating cross-counterflow evaporators with multiple modules having completely different geometries. Quasi-steady heat and mass transfer calculations provide local values of all relevant variables, including heat and mass transfer coefficients, air and refrigerant-side pressure drops and fin and tube frost thicknesses. A multi-lump method enables the model to simulate both tube and fin frost thicknesses and surface temperatures. The results presented in this paper reflect interactions with the display case and its air curtains and predict local and overall effects of frost accumulation, to facilitate optimization of the larger display case system.     

基于华铸CAE的铸造充型过程卷气缺陷定量预测研究

目的 针对目前单向流数值模拟软件无法模拟卷入金属液中的气体及其演变情况的问题,开展铸造充型过程卷气缺陷定量预测研究,开发基于华铸CAE的卷气定量预测系统,深入理解铸造卷气缺陷的形成原理与运动机理,准确预测金属充型过程中的卷气量和卷气造成的气孔缺陷,为生产实践提供指导。方法 提出了一种金属充型过程卷入气体搜寻与追踪算法,将卷入金属液中未经考虑的气体进行保留,从而修正单向流系统,并基于此开发了卷气定量预测系统,并对该系统进行了验证。结果 应用该系统后,原本未经考虑而消失的孤立气泡被保留下来以便进行后续计算,对液体体积进行了再分配,保证修改前后液体体积守恒,并修正了金属液速度,实现了卷气定量预测系统的基本功能。结论 开发的卷气定量预测系统能够考虑到卷入金属液中的气体,并对气体做出定量预测,实现金属充型过程中气孔缺陷的定量预测,对生产实践中的工艺优化有一定的指导作用。     

卧式陈列柜风幕的数值分析

利用CFD软件phoenics,通过对卧式陈列柜传热过程的分析,分别在忽略辐射和不忽略辐射2种情况下建立陈列柜风幕的k-ε数学模型,模拟计算不同送风速度下2种模型的风幕性能以及柜体热负荷,并与试验数据进行对比.结果表明,考虑辐射的模型的模拟结果更符合试验数据.同时对比不同送风速度对风幕性能的影响,风幕送风速度过小,冷射流不能到达回风口,不能形成完整的风幕;风幕送风速度的过大,增加了风幕射流与环境热空气的热质交换,使得柜内温度升高,所以必须选择合理的送风速度.     

Airflow patterns in an enclosure loaded with slotted pallets

A reduced scale model and CFD predictions are used to investigate experimentally and numerically the airflow patterns within a refrigerated truck loaded with slotted but empty pallets. Air velocity measurements are carried out on a reduced scale model with a Laser Doppler Anemometer above and inside the pallets. The numerical predictions obtained with Computational Fluid Dynamics package using the RSM turbulence model showed a satisfactory agreement with experimental data in high velocity zones. An approach was developed to evaluate the local ventilation efficiency and the fresh air within the pallets. The flow rate through the last pallet is about 35 times smaller than for the five first pallets. In terms of fresh air the difference is not so high; the equivalent fresh airflow rate through the last pallet is about six times smaller than for the first pallets.     

立式敞开式冷藏陈列柜蒸发器结霜特性的实验研究

实验测试不同环境温湿度工况下立式敞开式冷藏陈列柜蒸发器的蒸发温度、冷风幕送回风温度、蒸发器的凝水量及结霜水量,以研究环境温湿度变化对柜性能的影响.研究表明,当柜在较高环境相对湿度运行时,柜内蒸发器表面结霜量很少,冷空气中80%以上的析水以凝露形式形成.此时停机融霜没有必要,并使柜温上升过高.     

Refrigerant flow instability as a means to predict the need for defrosting the evaporator in a retail display freezer cabinet

For refrigerated display cabinets to perform their function of keeping food cold, there must be free movement of air through the evaporator. The moisture in the ambient air entrained in the cabinet forms frost on the evaporator. It is traditional for heat to be applied to the evaporator at regular intervals to melt this frost. The frequency, typically 3–4 times per day, is enough to avoid the frost becoming excessive even in extreme conditions. For much of the time defrosting is not always necessary. A large portion of the energy used during a defrost is an overhead – heating and then cooling the metal and the food rather than melting the frost. The effect of this is examined in the paper along with the results from testing an algorithm that detects the need for a defrost from the pattern of refrigerant flow (or evaporator exit superheat). The algorithm allows the number of defrosts to be reduced without excessively raising the temperature of food stored in the cabinet. The reduction in energy and carbon dioxide emission were examined and were shown to be substantial.