Study on heat transmission through multilayer clothing assemblies under different convective modes

The present work reports a detailed study on dry thermal resistance of multilayered clothing assemblies under different modes of heat transmission (i.e. non-convective mode, natural convection and forced convection). A series of multilayered fabric assemblies have been created with different combinations of fabric layers (like plain woven, nonwoven wadding, warp-knitted spacer fabric, weft-knitted breathable coated and weft-knitted aluminium coated) and air gaps of different thicknesses between fabric layers. Significant impacts of thickness of air gaps and mode of heat transmission on thermal resistance of multilayered clothing assemblies have been observed. The thermal resistance of the fabric assemblies increases with the increase in the thickness of air gap in between fabric layers. The thermal resistance of all the fabric assemblies at any given air layer thickness is highest under non-convective mode of heat transmission, followed by natural convection mode and the forced convection mode shows least thermal resistance. Incorporation of coated fabric in the outer layer reduces the effect of forced convection. ANOVA has been prepared and F value has been calculated to find the effect of modes of convection, type of fabric and thickness of air layer on thermal transmission properties. The results obtained from this detailed study will help in designing multilayered clothing assemblies suitable for different climatic conditions.     

Quantifying Enhancement in Heat Transfer Due to Natural Convection During Canned Food Thermal Sterilization in a Still Retort

Thermal sterilization of canned viscous liquid foods using saturated steam is enabled by natural convective heat transfer. However, the governing equations for two-dimensional convective heat transfer may be only rigorously solved by numerical calculations. On the other hand, if conduction is assumed to be the only mode of heat transfer, the thermal sterilization problem has analytical solutions for simple boundary conditions. However, the conduction model may not be appropriate in describing thermal sterilization of even viscous liquid foods and may cause considerable error in the prediction of the important parameters such as slowest heating zone (SHZ) temperature and lethality. The longer time for sterilization recommended by the conduction model may lead to overprocessing and an unacceptable food product. The objective of this work is to quantify the faster temperature rise in the food can due to natural convection when compared to the temperature rise obtained by only conductive heating. The consequent enhancement in lethalities is also reported. In addition, this work’s objective is to investigate how quickly the natural convective heat transfer effects begin to dominate over the solely conduction heating mode. The volume-averaged temperature as well as the SHZ temperature variations with time was calculated for the convection-augmented mode using computational fluid dynamics (CFD) simulations. Lethality values were then calculated based on volume-averaged temperature as well as the SHZ temperature. Food cans of different aspect ratios and food medium thermal conductivities are considered in this analysis. For the food system investigated, the critical Fourier number at which the transition to convection-augmented mode of heat transfer occurred is identified and explained from scaling considerations. In the conduction-dominated mode, it was possible to use analytical solutions to predict the volume-averaged and SHZ temperatures of the liquid food undergoing thermal sterilization. The Nusselt number correlation developed by Kannan and Gourisankar (2008) was used in the lumped parameter transient heat transfer model to predict the volume-averaged temperatures in the convection-dominated region. The volume-averaged temperatures from this approach were found to be in good agreement with the CFD simulation results. The time predicted for the SHZ to reach the minimum sterilization temperature was significantly lower when convective heating was also considered. The volume-averaged temperature and SHZ temperature enabled an estimation of overall sterility levels attained and minimum sterility levels prevalent inside the can, respectively. Even though the volume-averaged temperature increase due to convection was only about 10 K, the resulting accumulated lethality values were higher by an order of magnitude. The increase in SHZ temperatures was much higher in the convection-augmented mode, and consequently greater integrated lethalities were attained. The simple conduction model that is amenable to analytical solution cannot be used to approximate the heat-transfer-related phenomena even for “quick estimation” purposes when convection effects are significant. This precaution is found necessary even for the reasonably high viscous carboxy methyl cellulose system, whose average viscosity values ranged between 13 and 3 Pa s during the course of the sterilization process.     

非稳态自然对流换热系数计算方法及其在防护服隔热预报中的运用

为得到相对精确的热防护服非稳态隔热性能预测数值,基于传热系统的非稳态传热模型,在防护服材料外表面分别为恒温边界条件和Howard模型对流边界条件下,利用有限差分法分别计算了2种边界条件下模拟皮肤处温度的时程曲线,所得结果均与实验测量值差别较大。在细致分析传热过程的基础上,综合运用傅里叶定律、牛顿冷却定律、以及Howard模型对流边界条件,在线化假设下提出了一种自然对流换热系数计算方法,代入非稳态模型后利用有限差分法进行了求解,随着时间步长的加密,计算结果迅速向实验测量值收敛,当时间步长为0.001 s时,计算结果与测量结果误差小于0.1 ℃。     

考虑内部辐射下织物热湿传递现象的数值研究

人体在着装情况下的热湿传递现象对人体的热舒适性的研究有着很重要的地位。考虑了由于温度差而引起的织物内部的辐射热传递，对织物中的热湿传递现象进行了数值研究，建立了考虑辐射现象下织物内部热湿传递的简单模型，并讨论了计算结果。     

仓储粮堆内部自然对流和热湿传递的数学分析及验证

本文基于仓储粮堆内部自然对流、热湿耦合传递的数学模型,采用数学分析的方法对模型中各个方程中的各项的物理意义和数量级大小进行了分析,探讨了仓储粮堆内部自然对流、热量传递和水分迁移过程的相互关系。提出了判断粮堆内部自然对流强弱的瑞利数及其影响因素,分析了仓型结构、粮种及仓外大气温度对粮堆内部自然对流、热量传递和水分迁移的影响,并通过数值模拟对数学分析结果进行了验证。结果表明,数学分析方法是分析仓储粮堆内部自然对流、热量传递和水分迁移过程的一种有效途径,数学分析的结果可以为仓型设计、储粮生态系统的模拟、仓储技术管理提供借鉴。     

木棉系列絮料的保暖性

木棉是一种天然纤维素纤维,中空度85%以上,非常适合制作保暖材料,但是由于传统木棉絮料的强力低,持久回弹性差,使其用量越来越小,所开发的木棉絮料制作新技术克服了上述缺陷。为进一步探明木棉絮料的综合性能,比较研究了木棉絮料和其他现有絮料的静态热阻和对流散热量,发现木棉絮料的保暖性仅次于羽绒,明显优于四孔、七孔涤纶棉絮料和喷胶棉,与化纤絮料相比木棉纤维将絮料中的空隙分割的很小,使得絮料的对流散热量大幅度降低,而且木棉絮料的传导热阻也明显高于现有化纤絮料。     

THERMAL PROCESS SIMULATION of CANNED FOODS UNDER MECHANICAL AGITATION1

This paper describes how the distributed parameter conduction-heating numerical model was modified and tested to accurately predict the can center temperature for canned foods which exhibit combined heat transfer of pure conduction with added forced convection caused by mechanical agitation. This innovation will open the door to more widespread use of such mathematical models as part of the control system logic for making accurate adjustments to process conditions in response to unexpected retort temperature deviations on line during thermal processing of canned foods in agitating retorts.     

衣下空气层厚度对着装人体热传递的影响

位于人体皮肤与服装内表面之间的空气层是着装人体向外环境传热的重要途径。通过在人工气候室中重构热平板仪,测量了不同衣下空气层厚度时的面料表面温度,研究空气层厚度与面料表面温度之间的相互关系。建立了模拟显热量从皮肤表面经过衣下空气层、面料层向外环境传递的数学模型,分析衣下空气层厚度及环境条件对着装人体热传递的影响。实验及模拟结果表明,衣表温度随着衣下空气层厚度的增加而下降,但当空气层厚度达到16 mm时,自然对流的出现加大了皮肤向面料内表面的传热量,导致衣表温度升高。环境温度及风速与衣下空气层组合起来,共同影响人体通过服装系统向环境的散热量。     

Experimental and numerical investigation of heat and mass transfer during drying of Hayward kiwi fruits (Actinidia Deliciosa Planch)

In this paper we undertake an experimental and numerical study on heat and mass transfer analysis during drying of kiwi fruits. In the experimental part, the effects of various drying conditions in terms of air velocity, temperature and relative humidity on drying characteristics of kiwi fruits are investigated. In the numerical part, the external flow and temperature fields are studied using a commercial CFD package. From these fields, the local distributions of the surface convective heat transfer coefficients for the fruits are determined to predict the local convective mass transfer coefficients through the analogy between the thermal and concentration boundary layers (known as the Chilton–Colburn analogy). In addition, the time-dependent temperature and moisture distributions for different cases are obtained using the code developed to investigate heat and mass transfer aspects inside the fruits. Numerical results are then compared with experimental data and a considerably high agreement is obtained.     

Modelling and simulation of heat transfer by convection in aerogel treated nonwovens

Simulation and numerical modeling are becoming increasingly popular due to the ability to seek solutions for a problem without undertaking real-life experiments. For the problems of heat transfer, these techniques to generate relevant data by incorporating different changes to the input parameters. Heat transfer property of textile materials is a major concern since it influences comfort properties of clothing. In this paper, numerical simulation was applied to evaluate the heat flux, temperature distributions, and convective heat transfer coefficients of the fibrous insulating materials treated with aerogel. The computational model simulated the insulation behavior of nonwoven fabrics without and with aerogel. Ansys and Comsol were used to model and simulate heat transfer. The simulation was performed assuming laminar flow and since the Mach number was < 0.3, the compressible flow model with Mach number < 0.3 was used. The results of simulation were correlated to experimental measurements for validation. Furthermore, aerogel-treated fabric samples showed better thermal performance. Using this model, the heat transfer properties of the nonwoven fabrics treated with aerogel can be optimized further.     

计算流体力学技术在粮食储藏中的应用

计算流体动力学(computational Fluid Dynamics,简称CFD)是通过计算机数值计算和图像显示,对包含有流体流动、热量和质量传递等相关物理现象所做的分析、计算和优化的数值模拟工具。通过CFD技术在储粮通风工程中的应用实例,对CFD技术进行了介绍,并对采用CFD技术模拟储粮中的通风过程中流动、热湿传递过程和生态系统的研究现状进行了综述,探讨了采用CFD对储粮中的流动、传热传质过程进行数值模拟的优势以及未来发展趋势。     

Impact of Overall and Particle Surface Heat Transfer Coefficients on Thermal Process Optimization in Rotary Retorts

ABSTRACT: This study attempts to examine the significance of recent research that has focused on efforts to estimate values for global and surface heat transfer coefficients under forced convection heating induced by end‐over‐end rotation in retorting of canned peas in brine. The study confirms the accuracy of regression analysis used to predict values for heat transfer coefficients as a function of rotating speed and headspace, and uses them to predict values over a range of process conditions, which make up the search domain for process optimization. These coefficients were used in a convective heat transfer model to establish a range of lethality‐equivalent retort temperature–time processes for various conditions of retort temperature, rotating speed, and headspace. Then, they were coupled with quality factor kinetics to predict the final volume average and surface quality retention resulting from each process and to find the optimal thermal process conditions for canned fresh green peas. Results showed that maximum quality retention (surface and volume average retention) was achieved with the shortest possible process time (made possible with highest retort temperature), and reached the similar level in all cases with small difference between surface and volume average quality retention. The highest heat transfer coefficients (associated with maximum rotating speed and headspace) showed a 10% reduction in process time over that required with minimum rotating speed and headspace. The study concludes with a discussion of the significance of these findings and degree to which they were expected.     

应用ABAQUS的织物热传递有限元分析

为拓展机织物热舒适性设计、评估和优化的思路,提供一种有效预测织物热阻的方法,基于织物的三维模型及传热学理论,对织物系统进行一维热传递的数值模拟,得到织物传热过程中的温度分布特征。利用显微图像及切片技术获取织物几何结构参数,建立平纹织物的几何模型,并与纱线周围静止空气装配,构成织物系统的三维模型,借助有限元分析软件ABAQUS,根据模拟环境设置边界条件与相互作用,计算模型的数值解。最后通过模拟皮肤散热的恒温平板实验,对数值模拟结果进行验证,结果显示：织物外表面温度的模拟值与实验值随时间变化的一致性较好,理论热阻值与实验热阻值相对误差为3.9％,二者吻合度较高。     

Natural convective cooling of cheese: Predictive model and validation of heat exchange simulation

The proposed FEM model describes natural convective air cooling of cheese curd and cheeses with different sizes, chemical composition and initial temperature, including temperature-dependent functions accounting for the variation of specific heat capacity and thermal conductivity of cheeses. Both the calculated convective heat transfer coefficients (from 3.58 to 15.15 W/m² K) and the ratio between Grashof and square Reynolds numbers confirmed that heat exchange was natural convective. The model permitted to accurately predict the transient temperature change into the cheese, as shown by the mean RMSEs values (from 0.34 to 2.29 °C). Higher RMSEs values (up to 3.29 °C) were obtained for cheese curds, because some deviations from the assumptions of the model occurred. These higher RMSEs values for cheese curds quantify the importance of compliance with the model’s assumptions to ensure a best fit between the simulated and experimental data.     

Thermal Sterilization of Conduction-Heated Foods in Plastic Cylindrical Cans Using Convective Boundary Condition

A mathematical model was developed to evaluate thermal processing of foods in cylindrical plastic cans. This model included convective heat transfer coefficients for heating and cooling media, thermal diffusivities of plastic can wall and the canned food, and contact conductance between the plastic wall and the canned food. Temperatures estimated by the model at the coldest point in a can agreed closely with those determined experimentally during thermal processing. Thermal diffusivity of can wall and heat transfer coefficients of heating and cooling media considerably influenced the sterilizing values of the processed food.     

木材热压过程热量与含湿量传输数学模型初探

木材在热压过程中其内部热量和含湿量的交化,对木材加工质量的影晌很大。本文在理论分析的基础上,建立了合理简化的数学模型,预测热压过程中木材内部的瞬态温度分布,并开发了相应的计算机软件;在其它条件相同的情况下,用该模型对不同含水率的木材求解,得到与实验测量值基本一致的结果。     

EXPERIMENTAL COMPARISON OF NATURAL CONVECTION AND CONDUCTION HEAT TRANSFER

Magnitude of fluid motion is significant in convective heat transfer (the faster the fluid motion, the greater the heat transfer), while in conductive heat transfer, there is no physical movement of objects undergoing heat transfer. Due to this statement, it is a real fact that convection can be many times faster than conduction. The objective of this research was to compare natural convection and conduction by creating both heat transfer mechanisms in the same product. For this purpose, canned water and 1% agar-gelled water were used in the experimental and further computational fluid dynamics studies. Experiments were conducted at 70C and in boiling water, and ANSYS V.10 (Ansys Inc., Canonsburg, PA) was used in the numerical simulations. The results showed that addition of agar prevented the natural convection phenomena in the gels resulting in pure conduction while the effect of natural convection, which occurred due to thermal buoyancy effects in the given gravitational force field, was obvious in the case of water. Creation of both natural convection and conduction heat transfer mechanisms in the same medium is an important contribution as the effect of natural convection over the conduction heat transfer can directly be emphasized.

PRACTICAL APPLICATIONS

Creation of both natural convection and conduction heat transfer mechanisms in the same medium would be an important contribution as the effect of natural convection over the conduction heat transfer can experimentally be emphasized. The results of this study are significant to show the significant difference between these heat transfer modes as both mechanisms were created in the same medium, and these results will be useful especially for teaching heat transfer purposes.     

Improved 3-D temperature uniformity in a laboratory drying oven based on experimentally validated CFD computations

The present paper discusses a experimentally validated 3-D CFD analysis of the flow and thermal processes in a laboratory drying oven with a forced air circulation. The device is mainly used in the food and chemical industry to store products at a constant, spatially uniform temperature. The existing device has been assessed according to a certification procedure and then numerically simulated on the basis of the mathematical model developed, including all heat transfer modes, temperature-dependent air properties and local heat transfer coefficients on the external walls, etc. Furthermore, parameters such as the rotational speed of the device fan, the effectiveness of the distribution gaps and the rate of heat generated in the electrical heaters have been tested to improve the temperature uniformity within the chamber. Finally, several changes of the device configurations, such as the location of the heaters, the fan and the fan baffle, have been considered. As a result, the temperature uniformity has been significantly improved, which was also confirmed in the experimental test of the modified prototype.     

化工热力学在测定织物热阻中的应用

对润湿状态下织物热阻的研究，能更好的了解人体显性出汗后服装热阻的变化。根据化工热力学中平壁热传导理论，研究了润湿棉织物热阻的变化。通过理论得知，局部润湿织物的总热阻的倒数等于润湿部分和未润湿部分热阻倒数之和，这与实测结果吻合较好。对着装后润湿服装热阻进行了预测。