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。     

CONVECTIVE AND CONDUCTIVE EFFECTS OF HEAT TRANSFER IN POROUS MEDIA

An experimental study was made of heat transfer by natural convection inside packed vertical cylinders whose wall temperatures varied sinusoidally with time. The packings were small glass or plastic spheres with water as the interstitial fluid. The purpose of this research was to stimulate, in a controlled laboratory model, the heat transfer to grain stored in bins. Temperature distributions throughout the bed were determined for various sizes of packing, several height-to-diameter ratios, and for a number of frequencies of sinusoidal forcings. The results are presented graphically in terms of the amplitude ratios of the interior temperatures to the wall temperature and the phase lag between the interior and wall temperatures. The operating conditions ranged from essentially pure conduction to moderately strong natural convection. The temperature distributions near the wall of the cylindrical container were found to be predicted by a simple one-dimensional model. Temperature distributions throughout an entire packed bed were quite complex when there was significant natural convection and, although general overall predictions were possible, accurate calculation of the temperature at a specific location and time was impossible.     

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.     

Numerical simulation of thermodynamic and fluid-dynamic processes during the high-pressure treatment of fluid food systems

In the present contribution, thermodynamic and fluid-dynamic effects of high-pressure treatment are analyzed by means of numerical simulation. The following process is considered: pure water is compressed up to 500 MPa in a 4-ml chamber at different compression rates by enforced mass inflow. After the compression phase, the inflow is stopped for 200 s. The spatial and temporal evolution of temperature fields and fluid velocity fields are analyzed. It can be shown that the fluid motion is dominated by forced convection at the beginning of the pressurization. At the beginning of temperature increase, heat transfer into the surrounding housing develops. Due to density differences, free convection sets in and dominates the fluid motion for a few seconds (0–8.0 s) after the beginning of pressurization. Free convection subsists during the pressure holding (approx. 500 MPa) until 225 s. The temperature differences occurring in the high-pressure volume depend strongly on the pressure ramp during pressurization. For the highest examined compression rate of 500 MPa for 24.5 s, the maximum instantaneous temperature variation in the fluid volume is approximately 8 K. It can be observed that due to a vortex motion, food particles or micro-organisms suspended inside the vortex undergo a periodic temperature treatment with a variation of 6 K. A comparison of the numerical results with experimental results of pressure and local temperature obtained in the same configuration shows an excellent agreement.     

Numerical simulation of a steam-injection pilot study for a PCP-contaminated aquifer

The following study was focused on the simulation of a steam-injection field pilottest conducted in our past research. The scope of research contained two main subjects: heat transfer and contaminant transport when steam was injected into a pentachlorophenol (PCP)-contaminated aquifer. Numerical simulation of the heat transfer during the field test showed that vertical permeability is more influential to the distribution of water temperature than the horizontal permeability. If the vertical permeability is relatively high, the steam in the aquifer has a higher tendency to migrate upward and cause the aquifer temperature to rise faster. The simulation results also showed that heat convection is very sensitive to the soil permeability. Therefore, high permeability media makes the effect of heat convection more important on applying the steam-injection method. Heat conduction dominates the heat transfer within the hot aqueous zone. However, the hot aqueous zone is relatively smaller than the steam zone when steam is injected into the aquifer. Therefore, heat conduction is not as important as heat convection within the steam zone, which is the same result observed in the field test. Specific heat of soil media is also a sensitive factor. A numerical simulator, T2VOC, was utilized to simulate the PCP transport in the aquifer when steam was injected into the aquifer. The results showed that the shape of PCP distribution was identical to that of steam. It illustrated thatthe steam carried PCP upward and laterally. The high vertical soil permeability causes the steam to migrate upward with PCP easily. A low partitioning coefficient allows PCP to be desorbed easier, also an important factor. A majority of the PCP in the soil was transferred to the aqueous phase as the water temperature increased, showing similar results to those observed in the field test. According to the sensitivity analysis, PCP transport is more sensitive to the vertical permeability than the partitioning coefficient. The steam-injection rate is also an important operation parameter and may determine the success of the remediation work.     

Toroid cans – An experimental and computational study for process innovation

Use of toroid cans might be an innovative approach to increase heat transfer rates in canning. Therefore, the objectives of this study were to design a toroid can and to determine its possible use. For these objectives, a toroid can was designed, and water filled and pineapple slices placed cans were thermally processed. Tomato paste filled cans were used to demonstrate the improvements in heating rates. In convection heating, recirculation in both side walls was observed computationally to lead to increased heat transfer rate while the cold point shifted from center towards to outer wall in conduction heating due to the effect of heat transfer from inside surface. Enhancement in heat transfer leads to shorter processing times to achieve required sterility with better quality attributes, and toroid cans might be a possible process innovation in canning with this respective.     

PARTICLE MOTION AND HEAT TRANSFER IN CANS DURING END-OVER-END ROTATION: INFLUENCE OF PHYSICAL PROPERTIES AND ROTATIONAL SPEED

The effect of particle density and fluid viscosity on particle motion and fluid-to-particle heat transfer coefficient, h_fp, was investigated experimentally, at different rotational speeds (0 to 20 rpm) during end-over-end rotation of cans. Cans filled with water or oil, and fitted with a spherical particle (diameter = 19 mm) suspended using a flexible fine-wire thermocouple were processed in a water immersion rotary retort (Stock PR-900) at 120C. Temperatures at the center of the particle and the surrounding fluid were measured during end-over-end rotation. The h_fp was evaluated by matching the accumulated lethalities computed from experimental time-temperature data with those obtained by solving the governing partial differential equations of conduction heat transfer in spherical geometry with appropriate initial and boundary conditions, using a finite difference computer program. Overall heat transfer coefficient, U, was calculated from thermal energy balance. U values varied from 118 to 800 W/m². K and h_fp values varied from 34 to 825 W/m². K depending on the operating conditions. U values increased with decreasing fluid viscosity and increasing rotational speed and the results were comparable to those reported in the literature. The h_fp values were also affected in a similar fashion increasing with rotational speed and particle density and decreasing with fluid viscosity. The effect of particle density on h_fp was more significant than those of fluid viscosity and rotational speed. The particle motion video taped under simulated end-over-end rotation was used to explain the variations in heat transfer between the different runs.     

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.     

Computational Fluid Dynamics Modeling of the Thermal Processing of Canned Pineapple Slices and Titbits

Thermal processing of canned fruits is an important preservation technique used to increase the shelf life of canned foods through the inactivation of spoilage microorganisms and enzymes. The objective of this study was to develop a computational fluid dynamics model to investigate the temperature profiles during the thermal processing of canned pineapple products. Two different kinds of products such as canned pineapple slices and titbits were analyzed to investigate the effect of size reduction of the product on the efficacy of heat transfer during thermal processing. The simulation results were validated with the experimental measurements of temperatures. The temperature profile, slowest heating zone (SHZ), and the effects of natural convection and conduction heating on canned pineapple slices and titbits were studied. In the canned pineapple slices, the SHZ was found to lie inside the pineapple slices. In contrast, for the pineapple titbits, the SHZ was present at the bottom of the can. The pineapple titbits were found to achieve a rapid temperature increase owing to the combined effects of buoyancy-induced natural convection and increased surface area available for higher heat transfer. This finding signifies the retention of the nutritive properties of pineapple by preventing the loss of heat-labile nutrients like vitamins without compromising the commercial sterility of the product.     

织物热传导机理的分析和探讨

分析了纺织品传热机理。从宏观角度看,纺织品作为纺织纤维和空气的集合体,其传热途径包括传导、对流、辐射和伴随着水汽传输而发生的潜热传递;从微观角度看,纺织品是声子作为载体实现热传导的。最后还介绍了影响织物导热系数的各个因素。     

织物热传导机理的分析和探讨

分析了纺织品传热机理。从宏观角度看，纺织品作为纺织纤维和空气的集合体，其传热途径包括传导、对流、辐射和伴随着水汽传输而发生的潜热传递；从微观角度看，纺织品是声子作为载体实现热传导的。最后还介绍了影响织物导热系数的各个因素。     

DIMENSIONLESS CORRELATIONS FOR MIXED and FORCED CONVECTION HEAT TRANSFER to SPHERICAL and FINITE CYLINDRICAL PARTICLES IN an ASEPTIC PROCESSING HOLDING TUBE SIMULATOR

Dimensionless correlations for estimating heat transfer coefficients for spherical and finite cylinders under mixed and forced convection heat transfer regimes were individually investigated using multiple regression of statistically significant dimensionless groups. Food and model particles were subjected to heat treatments in an aseptic processing holding tube simulator (carrier fluid: 0, 0.5, and 1.0%, carboxymethyl cellulose solutions, w/w basis; temperature: 90 to 110C; flow rate 1.0 to 1.9 × 10⁻⁴ m³/s). the type of test material was found to have a significant effect on the Nusselt number, and hence developed correlations. Introducing a diffusivity ratio defined as the ratio of particle-to-fluid thermal diffusivities was found to consistently improve developed models. Excellent correlations (R²≥ 0.97) were obtained between the Nusselt number and dimensionless groups for all models. Natural convection in heat transfer correlations in situations where forced convection is expected to be the dominant mechanism was accommodated.     

Numerical Simulation of Natural Convection Heating of Canned Thick Viscous Liquid Food Products

Sterilization of a thick viscous liquid food in a metal can sitting in an upright position and heated from the side wall (T_w= 394 K) only in a still retort was simulated. The liquid had temperature dependent viscosity but constant specific heat and thermal conductivity. Equations of mass, motion and energy conservation for an axisymmetric case were solved and plots of temperature, velocity and streamlines were provided for natural convection heating and isotherms compared with pure conduction contour plots. Results indicated that the natural convection moved the slowest heating point to the bottom center. The bottom of the can heated up slower than predicted by pure conduction heating. The magnitude of the axial velocity was found to be of the order of 10^?5 m/sec which varied with time and position in the can.     

INDIVIDUAL HEAT TRANSFER MODES IN BAND OVEN BISCUIT BAKING

The individual modes of heat transfer, e.g., conduction, radiation and convection, are considered for the processing of products in conventional band ovens. A specific theoretical model is considered for the baking of biscuits in an indirect fired oven. Values of individual heat transfer constants in the theoretical model and major effects of the individual modes of heat transfer were determined using lab scale heating devices. Extrapolating these results to a band oven baking process, the model indicated a heat transfer profile of about 20% heat transferred by conduction, about 45% by radiation and about 35% by forced convection in the band oven, with about half the heat being absorbed as sensible heat, and about half as latent heat.     

纳米流体在液态食品杀菌中的应用及其作用机制研究进展

纳米流体是一种能替代热交换器中传统介质的新兴流体,其具有稳定性好、可重复利用、节约能耗等优点,以纳米流体为传热介质的热交换器有传热效率高、杀菌时间短的特点,更好地保持了食品感官和营养特性.该流体已成功应用于牛奶、果汁等液态食品的杀菌.本文概述了不同类型的纳米流体,如多壁碳纳米管(Multi-wall carbon na...     

Simulation of a forced-air-twin-chamber for measuring heat treatment uniformity in harvested tomatoes

A heat transfer simulation model combining convection and conduction was developed to predict accurately the temperatures of air and produce (tomatoes) placed within a custom designed experimental set-up used for the measurement of the uniformity of hot air treatment. The thermal conductivity of the produce and air humidity have significant effects on the uniformity of heat treatment, particularly on the temperature gradients generated in the design. Tomato position, signifying the length of tunnel, is another factor to be considered when designing such experimental device. While air velocity is a factor affecting heat treatment uniformity, the tomato orientation did not show any significant effect. The simulation was experimentally validated and found to be accurate. This method could also be used for other fruits.     

Study of water evaporation and condensation in a domestic refrigerator loaded by wet product

This study was carried out to gain a better insight into evaporation and condensation phenomena due to natural convection in a domestic refrigerator. An experiment was undertaken in a refrigerator loaded with 40 humid plaster cylinders (50 mm diameter). The weight variation of the cylinders was followed over 12 days and the rate of water evaporation (or condensation for some cylinders) was calculated. This measured experimental rate was compared with one obtained from a CFD simulation which took into account air flow by free convection, heat transfer (convection, conduction and radiation) and mass transfer (water evaporation and condensation). The position of high evaporation or condensation rate was well predicted despite the fact that the simulation most often underestimated the experimental values. Near the cold wall where condensation was observed (or near the warm wall where evaporation was observed), the cylinder temperature was lower (or higher) than that of air. This demonstrates the importance of the radiative exchanges between the walls and the products inside a refrigerator which in turn can explain condensation or dehydration phenomena.     

用于非冷藏生鲜食品运输过程的多层瓦楞纸盒热性能模拟

通过建立并验证食品非冷藏运输过程中多层保温盒传热的三维模型,揭示了包装盒内辐射传热是除了传导外的关键作用因素。通过减少辐射,使食品在贮藏的前30h内表面的铝箔添加量降低10%以上。将最初冷冻的凝胶包放在盒的中心能够明显延迟其融化,使食物的保存时间加倍。结果分析表明:盒壁的导电性和盒内表面的发射率和凝胶袋的发射率是影响性能的关键参数,而对流换热系数的影响则不太显著。     

MATHEMATICAL MODELING of TRANSIENT HEAT and MASS TRANSPORT IN A BAKING BISCUIT

Drying behavior of a single baking biscuit was modeled using unsteady state, anisotropic, two dimensional, simultaneous heat and mass balances. Solutions of these equations agreed well with the experimentally determined temperature and the moisture data. Modeling revealed that in the outer sections of the baking biscuit conduction and diffusion were the dominant heat and mass transfer mechanisms, respectively. In the central section of the biscuit the gas cells cracked with the increased vapor pressure and the upward volume expansion, then air/vapor enclaves were formed among the horizontal dough layers in the radial direction. the dominant heat and mass transfer mechanisms in the central section of the biscuit were convection. Presence of two different regime zones in a baking biscuit may have important consequences concerning the strength of the commercial products against crumbling during marketing and consumption.