Heat Transfer and Moisture Loss of Spherical Fresh Produce

A computerized procedure was developed for simulating the post harvest cooling and moisture loss of produce by using a reliable mathematical model. This model was derived by assuming temperature and time variable respiration heat generation and temperature variable thermophysical property values. The developed procedure was employed to examine influence of five key dimensionless parameters on the cooling rate and moisture loss. These parameters are related to the rates of changes in density and thermal conductivity with changes in produce temperature, surface heat transfer conductance, transpiration rate, and environmental relative humidity. Theoretical results obtained from our examination were verified through heat transfer and moisture loss experiments by using fresh potatoes and tomatoes.     

Heat and mass transfer in combined convective and far-infrared drying of fruit leather

Combined convective and far-infrared drying is a challenging assignment due to complex relationship between heat and mass transfer. In this paper, heat and mass transfer of fruit leather drying with combination of hot air and far-infrared has been carried out. The heat and the mass transfer coefficients were analyzed by heat–mass analogy. It could be found that the ratio between heat and mass transfer coefficients for the combination technique could not be obtained from the heat–mass analogy classical model and a modification is needed. The modified correlations for predicting ratio of heat and mass transfer coefficients and the heat transfer coefficient in term of heat transfer Nusselt number are developed. The model could fit the experimental data quite well within ±10% deviation.     

Heat and mass transfer modeling in air drying of foods

Heat and mass transfer modeling in the air drying of solids is considered as a two stage procedure. The former aims to discover the dominant heat and mass transfer phenomena (heat versus mass, internal versus external), the latter aims to formulate empirical equations for the calculation of the corresponding heat and mass transport properties (mass diffusivity, thermal conductivity, boundary heat and mass transfer coefficients). A model discrimination procedure was proposed in the first stage, and a model building procedure was examined in the second stage. The proposed procedures were applied to a wide set of experimental data in potato drying. The resulting model takes into account moisture diffusion and convective heat and mass transfer in the air boundary layer. Moisture diffusivity is dependent on material moisture content and temperature, while heat and mass transfer coefficients can be considered as constants.     

食品微波加热过程中的传热模型

微波加热是一种快速简便的食品复热手段,但此种方法通常会导致受热不均匀。为了更好的将微波加热技术应用于食品的复热过程,解决受热过程"冷点"的出现,作者综述了食品微波加热过程传热模型的研究进展,并针对数值计算方法、模型理论和功率吸收进行了详细阐述,探讨了微波能在食品中转化成热能的机制,并分析了研究过程中存在的问题和发展方向。     

Modeling Flow and Heat Transfer During Freezing of Foods in Forced Airstreams

ABSTRACT: Mathematical modeling of food freezing has been limited to the modeling of the internal heat transfer where the external convective heat-transfer coefficients are assumed or empirically estimated. Previous procedures followed to solve the external boundary layer in tandem with the internal heat transfer were constrained by numerical complexities due to the transient nature of the heat transfer, requiring unsteady formulation for the flow. In this article, attempts have been made to decouple the flow and heat transfer equations for the external boundary layer flow over a food product being frozen. The flow equations have been solved as a steady-state problem using Falker-Skan transformations of the boundary layer equation. The heat-transfer equation for fluid flow is solved as an unsteady-state problem in conjunction with the internal heat transfer and phase change inside the product undergoing freezing. The model is validated for a case of air-impingement freezing.     

织物层热湿传递过程仿真研究

对服装织物层基本几何单元内空气层与纱线热湿传递机理模型进行了数值模拟分析 ,建立了空气层与纱线交界处的热湿传递动态过程 ,通过计算机仿真分析了织物内纱线、空气层的温度场和湿度场 ,并探讨了纱线材料特性、织物的组织结构对织物层热湿传递过程的影响规律     

微管道阵列流体流动及其耦合传热的数值模拟

针对一种阵列式微管道换热器,利用FLUENT软件对其进行了数值模拟。将恒壁温、恒热流和流固耦合计算方法的模拟结果进行对比,结果表明:采用流固耦合传热边界条件更有助于揭示换热器局部温度场变化的实际情况,模拟结果与实际情况吻合较好,能够为微管道换热器结构优化设计提供参考依据。     

Heat transfer in a tunnel pasteuriser. Part II: Heat transfer coefficients

The effect of heat transfer resistances on the overall heat transfer between water spray and liquid heated in a glass container has been studied. Values for convective and overall heat transfer coefficients were determined. The overall heat transfer coefficient was affected by hot water spray intensity, location of bottle in relation to the spray nozzle axis and the shape of the bottle. The effect of the viscosity of the heated liquid on the overall heat transfer coefficient was very pronounced. Variation in convective and overall heat transfer coefficients with height on the bottle wall was also investigated.It was found that the largest heat transfer resistance occurs in the glass when water is the heated liquid. With liquids of viscosity higher than 1·1 × 10^?3 Pa s the main resistance is in the heated liquid. Resistance to heat transfer from the hot water spray to the bottle wall is around eight times smaller than the resistance of the glass wall. The physical properties of the liquid being heated greatly affect heat transfer under tunnel pasteuriser conditions.     

基于FLUENT的管壳换热器壳程流场数值模拟与分析

通过简化管壳式换热器模型,采用非结构网格划分,选用κ-ε湍流模型,应用CFD软件FLUENT对壳程流体流动和传热过程进行了数值模拟,得到了不同折流板间距情况下壳程流体温度场、压力场以及速度场的分布情况。分析了折流板间距对壳程流体流场分布、换热器传热速率以及压力损失的影响,并得出了进口流速与传热量和压力损失之间的关系。模拟结果与理论研究结果相符合,对管壳式换热器的设计和改进有一定的参考价值。     

矩形肋槽烘缸的导热模型

对矩形肋槽烘缸壁壳内的传热作了分析,考虑肋壁各处传热系数的不同,采用有限差分法建立了导热模型,通过对不同工况的计算,对肋槽尺寸对烘缸传热特性的影响进行了探讨。结果表明,增大肋宽有利于提高传热速率;在肋壁表面上的传热系数较低时,增大肋高可以提高传热速率;传热系数较高时,增大肋高,有可能使肋壁表面上的对流热阻的减小慢于导热热阻的增大,反而降低传热速率;肋效率和外壁面的温度均匀性也呈现出类似的表现。     

Heat transfer through glassfiber fabrics coated with silicone resin

Coatings can improve the performance of textiles, including flame retardants, water repellents and for heat resistance and weather resistance. Experimental methods are used normally to prepare heat insulation fabrics using coating technology. But there are many problems such as long processing time, high energy consumption, and the production of fumes. As computer technology develops, it is important to use numerical methods to predict the heat transfer performance of the coated fabrics. In this work, based on the yarn paths and their cross-sectional shapes, geometry models of the coated fabrics have been established. The finite element method was used to solve the heat transfer equations, and the numerical model was established to predict heat transfer performance. It was found that: (1) when comparing with the SEM images, the geometry model can represent well the structure and appearance of the coated fabric; (2) validation of the experimental data indicates that the numerical simulation model of heat transfer through the coated fabric is reliable and useful to predict the heat transfer performance, and the average relative deviation between the numerical data and experimental data was less than 7.0%; (3) when the thickness of the silicone coating layer is less than 0.50 mm, the temperature of the back face of fabric rapidly increased between 1 and 10 s, and it remained more than 510 °C as the heating time was extended. When the coating thickness is between 0.75 and 2 mm, the rising tendency of the temperature becomes gradually slower, and their temperatures remained 489, 455, 369 and 281 °C when the thickness of the coating layer is 0.75, 1.00, 1.50 and 2.00 mm, respectively.     

室外高温环境下通风服装的传热模型与实验研究

为提高夏季室外高温日晒环境下作业人员的热舒适性,对人体、服装与环境之间传热过程进行分析,以此作为针对性降温方式的理论基础建立了通风服装在室外高温日晒环境下的传热模型,并采用真人实验的方法验证了该传热模型;分析了送风后衣下空间空气参数及人体主观反映的改变。结果表明:采用该模型计算得到的人体得热量与失热量误差仅为9.1%,人体表面温度在10 min左右趋于稳定;给衣下空间送入处理后的冷空气时,衣下空间内空气温湿度均比未通风时低,强迫对流的存在大大地提高了体表汗液蒸发速率,人体主观热感觉与皮肤湿润感得到明显改善。     

仓储粮堆通风干燥湿热传递模型的研究进展

从古到今,治国理政的首要之务是粮食安全。粮食仓储安全尤为重要,关系到一个国家的国家安全。由于粮食干燥储存方法原始、干燥技术落后等原因,我国每年收获后会损失大量粮食。粮食中湿热分布不均不仅会降低粮食的品质,而且会因霉变、虫害等问题严重影响食品安全。因此,如何快速、均匀而又低能耗地降低粮堆含水率或者温度,保证粮食品质,同时节约能源,是粮食干燥储藏研究的一个重要方向,具有重要的理论及现实意义。通风干燥是保证储粮中湿热均匀分布的重要措施,建立干燥模型预测粮堆的湿热分布是研究粮堆干燥过程中湿热传递的重要手段。该研究综述了粮堆湿热传递的经验模型、连续模型和离散模型,其中经验模型容易获得,但是应用较少;连续模型应用较多,但不能反映粮堆的孔隙结构;离散模型可以很好的反应粮堆的孔隙结构,但其计算量大,并且难以理解和构建。最后该研究指出可以根据不同的湿热传递过程,结合先进的计算方法和处理技术,建立相应的模型,以满足不同的模拟要求,并对粮堆通风干燥湿热传递模型未来的研究方向进行了展望。     

Heat and Mass Transfer During the Warm Water Blanching of Potatoes

A model of heat and mass transfer with simultaneous chemical reaction is proposed for analyzing the influence of operating variables on the reducing sugar content at the surface of blanched potatoes. This content is partially responsible for the color of the finished product. The involved parameters (potato thermal conductivity, heat transfer coefficient of the system and kinetic constants for overall reaction of reducing sugar generation) were evaluated in separate experiments. The apparent diffusion coefficient of reducing sugars in potatoes was the only parameter obtained from blanching experiments. Temperature and concentration profiles and effect of blanching temperature on surface reducing sugar content are analyzed using the developed model. The possible underestimation of the diffusion coefficient when simultaneous starch hydrolysis is not taken into account is also considered.     

Prediction of thermal resistance of woven fabrics. Part II: Heat transfer in natural and forced convective environments

The aim of the paper is to observe the effects of convection on the total heat transfer of the fabric. Convective heat transfer through textiles can be simulated with the help of computational fluid dynamics (CFD). The first part of this paper has dealt with the mathematical model for predicting the conductive and radiative heat transfer. The second part of the paper describes the use of CFD to simulate the wind tunnel of the instrument. The fabric has been subjected to natural and forced convection. The convective heat transfer coefficients obtained from the simulation were used to find the resistance due to convection. The final values obtained from the model were compared with the values obtained from the thermal resistance measuring instrument fabricated to measure the thermal resistance in natural and forced convective modes. The mathematical model gave excellent prediction in forced as well as natural convective mode when coupled with CFD results.     

三相循环流化床麦草浆黑液蒸发器传热模型

对三相循环流化床蒸发器流动沸腾传热机理进行了分析.建立了适用于烧碱法麦草浆黑液系统的三相循环流化床传热模型.验证实验表明:模型计算值和实验数据吻合较好,最大偏差在30%以内.     

Heat Transfer Rates to Liquid Foods During Flame-Sterilization

The influences of headspace, rotational speed of the can, and product viscosity on the rates of heat transfer during flame-sterilization of carboxy-methyl-cellulose (CMC) model solutions, were studied. It was observed that more than 90% of the heat transfer resistance was external, and so the headspace volume and product viscosity had almost no effect on the rates of heat transfer, whilst the can rotational speed was the important parameter that needed to be controlled. An empirical correlation between the Nusselt, Reynolds and Prandtl dimensionless numbers for the external and internal conditions was obtained, in order to describe the heat transfer as a function of the hot air and product thermophysical properties, the can r.p.m. and the diameter. The results will be useful for improving flame-sterilizer utilization.     

热疗纺织品热性能测试评价及传热机制研究进展

针对目前热疗纺织品的实际应用、疗效评价、使用安全性及热舒适性等问题,总结了具有热敷、热疗及其他保健作用的可穿戴产品的研究、应用及发展现状。首先,将热疗产品分为电加热、红外线发热两类,分析现阶段热疗纺织品的热性能测试及治疗疗效评估方法,分别从人体热生理实验、动物实验优缺点来阐述不同实验方法对热性能预测的影响。然后,通过可穿戴式热疗产品传热机制的研究,阐述了皮肤传热模型与纺织品传热模型的研究现状,总结了有限元模拟在人体皮肤组织及纺织品热传递研究中的应用。最后,指出在未来的研究中,应全面准确地模拟人体生物组织与纺织品的热传递系统,建立可穿戴热疗纺织品的热舒适性及治疗安全性测评标准。     

换热管内置直齿扭带的阻力与传热特性研究

为了研究直齿扭带的阻力与传热特性,将12根不同结构参数的扭带分别置入换热管内进行了冷态与热态实验。实验结果表明,直齿扭带的宽度、扭距、齿间距是影响换热系数和阻力系数的主要因素,并由此建立了实验状况下包含各影响因子的阻力系数关联式和换热系数关联式。直齿扭带比光滑扭带的阻力系数增加了约8.3%~35.4%,而努塞尔系数提高13.1%~74.1%。