Modelling and Thermal Analysis of Tray‐layered Fruits inside Ventilated Packages during Forced‐air Precooling

Ventilated packaging is widely used in the forced‐air precooling practice for horticultural produce. Fresh fruits are living organisms which are sensitive to temperature in turn related to airflow and heat transfer inside package. In this study, a transient mathematical model considering heat of respiration and evaporation is developed to predict the thermal response of tray‐layered fruits in ventilated packages during forced‐air precooling. Specifically, the heat source is combined with the energy conservation equation and loaded into numerical solution by User Defined Function (UDF). Temperature profiles of three variously distributed circular and oblong vents in three different patterns (spaced, paralleled and crossed stacking) are simulated, separately. The results show that the heat source affects fruit cooling process, and the layered fruit in paralleled stacking pattern tends to be cooled better than others. Furthermore, the results indicate that vertical oblong vent could improve the longitudinal and lateral airflow, while non‐central vent design could greatly improve the overall cooling performance. Definitely a triangular distribution of three circular vents was superior to laterally distributed centre vents with 66.5% higher uniformity and 2.5°C lower of the highest temperature. Compared with the three identical vertical oblong vent conditions, vent design with one hand hole and two side vertical oblong vents can be cooled more uniformly with an increase of 6.5%. It is revealed that vents with large major‐to‐minor axis ratio could be applied to balance airflow and ease cooling differences for a rapid but uniform cooling. Experimental validations were performed for Sim2, Sim4 and Sim6, Sim8, Sim9, and good agreement was obtained considering the five vent conditions with the error less than 3.5°C but coordinated later (within the limits of the experimental uncertainty).Thus the numerical model can be used to predict and optimize temperature distribution within precooling packages. Copyright © 2016 John Wiley & Sons, Ltd.     

Air velocity characteristics within vented pallets loaded in a refrigerated vehicle with and without air ducts

During transport using refrigerated vehicles, this being a vital link in the cold chain, the maintaining of even temperature throughout the cargo is essential in order to preserve the quality, safety and shelf life of perishable food. Within the refrigerated container, the temperature level and its homogeneity are directly governed by airflow patterns. The design of the air-distribution system should allow these airflows to compensate heat fluxes exchanged through the insulated walls or generated by the products.In this paper, a reduced-scale model and CFD predictions were used to investigate experimentally and numerically the airflow patterns within a typical refrigerated truck configuration loaded with vented pallets filled with spherical objects. The experiments were carried out using a laser Doppler velocimetry and thermal sphere-shaped probes located inside the pallets. The aim was to investigate air velocity characteristics above and within pallets. The performance of ventilation was characterized with and without supply air duct systems. Both configurations are extensively used in refrigerated transport. Full-scale measurements were also performed within a load of fruit during transport in a refrigerated truck in order to evaluate the temperature distribution under given operating conditions.The numerical modelling of airflow was performed using the computational fluid dynamics (CFD) fluent code and the Reynolds Stress Model (RSM) turbulence model. Numerical and experimental data make it possible to evaluate the air ventilation levels and their heterogeneity between the pallets. The numerical predictions show reasonable agreement with experimental data. The studied supply air duct system improves significantly the homogeneity of ventilation in the vehicle.     

气流场ESPI载频条纹图的消噪处理与相位提取方法

为拓展ESPI方法在流动可视化测量技术中的应用,对气流场ESPI载频条纹图进行了傅立叶变换、数字全低通滤波,和傅立叶逆变换的处理以消除散斑噪声,并直接将其替换为原散斑相关条纹图,进而成为保留了流场信息的高对比度、低噪声的灰度条纹图,便于提取原散斑图中流动相位信息。根据模拟数据确定了由这两种条纹图互换所引起的空间相移量和相应流场测量中的修正量。提出了用灰度扫描法对互换后的条纹图进行一维灰度扫描,由灰度计算得出气流扰动的一维相位差分布和流场一维温度分布;同时也用FTP法对该图进行了二维相位展开和温度分布计算,亦可对多帧条纹图进行时间轴上的逐点相位提取以计算其他流场参数。结果表明,计算数据与实测符合,方法简单易行,为在气流场参数测量中应用动态ESPI方法提供了一种实用有效的方法。     

热金属表面多喷嘴风冷过程气-固耦合传热模拟

目的 获得多喷嘴风冷过程的界面换热系数,并研究风冷工艺参数对界面换热的影响规律.方法 基于Fluent软件对三喷嘴强制风冷传热过程进行"气-固"耦合分析,获得高压气流的流速场和钢板表面温度场.基于"气-固"耦合分析得到钢板表面平均温度曲线,利用自开发的反传热软件计算得到"气-固"耦合界面换热系数,并将界面换热系数以第三类边界条件施加在钢板表面进行瞬态传热分析.结果 对于直径为4 mm的喷嘴,当喷嘴间距为10～16 mm时,喷嘴间距对高压气体的流速场影响较大,气流的卷吸效应随着喷嘴距离的增大而增强;喷嘴间距对界面换热系数影响较小,喷嘴至钢板表面的距离对界面换热系数影响较大;随着喷嘴至钢板表面距离的增大,各股气流逐渐汇合为一股,各股气流的滞止区也逐渐汇合,钢板表面温度更加均匀;将界面换热系数以第三类边界条件施加在钢板表面进行瞬态传热分析,得到的钢板表面温度与"气-固"耦合分析得到的钢板表面平均温度曲线吻合得较好.结论 获得的界面换热系数可为多喷嘴风冷过程数值模拟提供可靠的数据,保证温度场的求解精度.     

Improved airflow method and packaging system for forced-air cooling of strawberries

The package system designs currently used by the strawberry industry are largely based on empirical techniques and have limited the efficiency of forced-air cooling applications. In this study, a new system capable of promoting a more uniform and energy-efficient cooling was developed. The design of this new system was based on a series of numerically based guidelines (previously developed by using a validated CFD model of the process) and involved not only the design of individual clamshells and trays, but also the overall circulation of the airflow across the palletized structure. The improved performance of the system was experimentally assessed. For the same airflow conditions, the new design significantly improved the uniformity and energy-efficiency of the process, while replicating of the cooling rate of commercial designs. In particular, no significant differences were found among the cooling rates of individual clamshells, and the pressure drop across the system was decreased by 70%.     

Comparative analysis of the natural convection process between hollow concrete brick layer and crushed rock layer

On the basis of cooling mechanisms and its factors of crushed rock layer widely used in the Qinghai–Tibet railway, a new hollow concrete brick layer is proposed for the Qinghai–Tibet freeway. A high-precision macro wind velocity detector was employed to study the natural convective characteristics of both types of rock layers and their differences. The natural convection velocity in both types of rock layers resulting from the temperature difference was investigated for the first time through modeling experiment. Natural convection velocity in the hollow concrete brick layer with weak convection resistance was higher than that in the crushed rock layer. Their maximum velocities were 0.10 m/s and 0.08 m/s, respectively. The different heat transfer processes and their intensities resulting from the boundary temperature fluctuations were the primary reasons for inner asymmetric temperature fluctuation and decline. The cooling process closely related to convection time and convection velocity. The temperature difference was the key factor in the occurrence of convection and its intensity change, although the air temperature changes also had an effect on the airflow process. The occurrence of natural convection required a start-up temperature difference. Moreover, a linear relationship was observed between natural convection velocity and temperature difference.     

旅客列车硬座车厢内气流模拟与浓度场分析

采用稳态不可压缩雷诺时均N-S方程、k-ε湍流模型,对旅客列车空调硬座车厢内气流场和浓度场进行了数值计算。采用立方体代表旅客,以人体呼出的CO2作为代表性污染物,研究了非空载下车厢内气流和浓度分布。计算结果表明：现有的送风方式除车厢两端外,车厢内沿长度方向气流分布比较均匀;人体散热和太阳辐射对车厢内流场温度场影响较大,非空载时车厢内流场分布与空载时有较大差别,太阳照射和人体产生的热气流使车厢内存在较大的温度梯度;车厢内过道区浓度较低,但座位区由于人员集中,人体呼吸区污染物积聚,浓度偏高,且车厢中部断面污染     

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.     

空调房间四种通风方式对污染物分布影响数值模拟与分析

采用RNG k-ε模型对4种侧送风方式下的空调房间进行数值模拟,对室内污染物衰减结果及污染物浓度场进行分析,并通过示踪气体衰减方法计算室内通风效率.结果表明当送回风口位于同侧且在同一垂直面上时室内空气品质最好.     

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.     

大空间建筑下送风分层空调系统区域间换热特性研究

本文采用实验和计算流体力学(CFD)方法,研究了带有柱状下送风系统的大空间建筑中,由气流换热和温差导热组成的对流转移热的计算方法.在缩尺模型实验室中,研究了三种不同排风比的热环境和负荷特性,根据实验得到的中心线垂直温度分布和空调冷负荷验证了CFD模拟结果,并基于相似理论,将缩尺数值模型拓展至全尺寸工况模拟.结果表明:在...     

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.     

航空活塞发动机高空模拟试验台温度系统研究

在航空活塞发动机高空模拟试验中,模拟温度低、温度范围跨度大,高空模拟试验台（简称高空台）上应用了空气制冷和电加热两种调温措施,控制难度较大。将模拟温度划分为高温、常温、低温三个温度段,分别应用三种不同的调节方式进行温度控制;在系统数学建模和温度变化动态特性分析基础上,对高空台温度系统的调节过程进行了仿真研究,结果表明该方法能够满足温度的调节速度和控制精度要求。为了满足带涡轮增压器的航空活塞发动机对进气温度的要求,建立了涡轮增压器与中冷器之间的数学换热关系,以稳态时的发动机进气温度为例进行了仿真计算,结果与实际情况相符。     

大温差空气处理机组表冷器换热能力优化设计及实验验证

冷冻水系统大温差设计对空调系统节能具有重要意义。配套使用空气处理机组也需采用大温差设计,达到节能效果,本文在空气处理机组换热机理的理论基础上,分析了冷冻水大温差对空气处理设备换热性能的影响。通过优化表冷器管形、回路等方法,采用焓差法实验对比了大温差工况下不同管形、不同回路、不同翅片片距等对表冷器性能的影响,在相同换热截面下,大温差空气处理机组较常规机组能耗降低约20%,并在此基础上,提出了大温差表冷器优选结构方案,采用高效内齿形换热管,4排高效换热管可与6排光管表冷器达到相同冷风比,且整机能耗低约5%,为大温差设计的应用和实践提供借鉴和参考。     

On the Method of Air Jet Cooling in Green Manufacturing

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Entropy generation in a channel resembling gas turbine cooling passage: Effect of rotation number and density ratio on entropy generation

Flow into a passage resembling a gas turbine blade cooling passage is considered and entropy generation rate in the passage is examined for unique rotation number and density ratios. In the simulations, leading and trailing walls of the passage are assumed to be at constant temperature. A control volume approach is introduced to discretize the governing equations of flow, heat transfer, and entropy generation. Reynolds stress turbulence model is accommodated in the simulation to account for the turbulence. The study is extended to include two rotational speeds and three density ratios. The passage aspect ratio is kept 10:1. It is found that volumetric entropy generation rate attains high values at passage inlet due to attainment of high temperature gradient in this region. Increasing rotation number and density ratio enhances volumetric entropy generation rate in the passage.     

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.     

Conceptual containment venting strategy for Susquehanna steam electric station

To mitigate the consequences of postulated BWR reactor accidents, generic industry guidance for venting the primary containment has been proposed to prevent catastrophic containment failure, maintain low pressure vessel injection capability, and/or remove combustible gas which may cause containment failure if ignited. Detailed analysis of the generic guidance shows that such venting must not be performed irrespective of off-site consequences. The decision to vent must be made with an evaluation of the off-site consequences. The conclusion of this paper is that venting without a source term or with a limited source term is permissible, either to maintain core cooling or to preserve the containment function for the future. However, with a large potential source term, such as would result from severe core melt and relocation, venting is not an effective risk management policy and maintenance of "containment" is paramount. This paper reviews the generic venting approach, the reasoning behind the generic approach, and presents a conceptual venting strategy for the Susquehanna Steam Electric Station (SSES) in light of PP and L's (formerly Pennsylvania Power and Light, Co.) own severe accident analysis experience.     

Modeling the forced-air cooling process of fresh strawberry packages,Part III: Experimental validation of the energy model

The aim of this study was to validate a mathematical model previously developed for predicting the cooling rate of individual packages of strawberries (clamshells) during an industrial forced-air cooling application. The differences between the predicted and experimental profiles of the average-fruit temperature per clamshell were less than 0.7 °C (within the limits of the experimental uncertainty). The 7/8th cooling time of individual clamshells was predicted within less than 3% of the experimental value. In addition, the local performance of the model and its capability to predict the strawberry moisture loss were qualitatively analyzed. The moisture loss was predicted between 73% and 88% of the experimental value. The predicted temperature profile of individual fruits and airflow within clamshells followed the general trends experimentally determined. Finally, the results corroborated that the transport phenomena during force-air cooling applications can be modeled by decoupling the momentum transport from the transport of energy and mass.     

An experimental study on frosting of laminar air flow on a cold surface with local cooling

This paper presents the heat and mass transfer characteristics of the humid airflow in frosting conditions. A flat plate of aluminum with cooling modules at the central region was used for the simulation of flat surface part of the fin of the heat exchanger. The local surface temperature of the plate and the local thickness and total mass of the frost on the plate were measured to analyze the heat and mass transfer characteristics. In order to analyze the frosting characteristics, an analysis algorithm was developed, which can provide the local air temperature, the frost surface temperature, the sensible and the latent heat flux distributions at the test plate. Also, by integrating the local heat flux distribution, the average heat flux characteristics were analyzed. The present experiment and analysis found that the characteristic of the upstream airflow was very different from that of the downstream airflow.