CFD studies on natural convective heating of canned food in conical and cylindrical containers

Enhancing natural convective heat transfer in canned food sterilization is explored through modifications to container geometry and its orientation. A conical geometry, of equal volume and height as that of the cylinder, pointing either vertically up or down was considered. The non-Newtonian fluid, 0.85% w/w sodium carboxy-methyl cellulose (CMC), was taken as the test food material and its laminar flow behavior was investigated using computational fluid dynamics (CFD). The movement of the slowest heating zone (SHZ) temperature was tracked and compared for the three geometries. The SHZ temperature was observed to attain the final sterilization fluid temperature of 100 °C fastest for an upright conical geometry followed by the cylinder and the downward pointing cone. There is scope for enhancing the thermal sterilization process through geometry modifications but without mechanical agitation or rotation. Further, the geometry modification if non-uniform must be complemented by its suitable orientation.     

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.     

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.     

The Finite Element Method in Thermal Processing of Foods

A variational finite element approach using triangular simplex and 2nd order quadrilateral elements was employed to analyze several problem areas that are of practical importance in the thermal processing of conduction heating products: (1) A method was developed to calculate conversion factors for thermal process design applicable to glass jars filled with conduction heating products. (2) The overshooting of temperatures after steam-off was studied, indicating that not including the contribution of overshooting to sterilization values in process design, can lead to gross overprocessing. (3) A method of correcting sterilization values to account for harmonious fluctuations in retort temperature was developed. (4) The air cooling of cans by natural convection was analyzed.     

A finite element model for conduction errors in thermocouples during thermal sterilization of conduction-heating foods

Errors in temperature measurement due to heat conduction along thermocouples employed in heat penetration studies of food which is heated by conduction have been quantified, using a finite element numerical solution of the unsteady state heat transfer equation. For a 0.56-mm (24s.w.g.) Type T thermocouple, temperature at the centre of a can may be overestimated by over 2°C in the heating phase of a typical process. The errors are much larger for thicker thermocouples. The orthodox conservative design strategy for thermal processes, relying on the lethal effect of the heating phase and treating the cooling phase as a safety margin, may not be a 'safe' strategy. The conduction errors result in an overestimate of the heating-phase lethality and an underestimate of the cooling-phase lethality. The application of a correction to the lag factor is not adequate to compensate for the derestimation of the lethality during the cooling stage. The errors in the determination of the thermal diffusivity from heat-penetration data are much lower than the errors in the determination of the sterilization effect delivered by a particular process. The silicon elastomer used to experimentally validate the model employed in this study, is shown to be an ideal material to model food which is heated by conduction.     

Influence of Selected Thermal Processing Conditions on Steam Consumption and on Mass Average Sterilizing Values

The influence of can size, type of food (conductive or convective), retort temperatures, initial temperature of food and target sterilizing value on steam consumption was studied. A 2⁵ factorial design of experiments was used. Steam consumption was measured by using steam flow meters. A mass average sterilizing value was computed for each process by using experimentally determined heat penetration parameters. Steam consumption was significantly high for processing larger cans compared to smaller cans both containing equal quantities of conduction heating food simulant. The can size had no significant influence on steam consumptions with the convection heating food simulant. Steam consumption and mass average sterilizing values were reduced significantly by employing a high retort temperature to obtain a high target F_p value. With a low target F_p value, the type of food simulant did not affect significantly mass average sterilizing value.     

Optimal Sterilization Temperatures for Conduction Heating Foods Considering Finite Surface Heat Transfer Coefficients

Optimal sterilization temperatures are defined as the processing temperatures which result in a minimum surface cook-value after achieving the desired degree of sterility. They were calculated as a function of product heating rate, surface heat transfer coefficient, initial food temperature, heating medium come-up-time, z-value for the quality factor and target F_o-value. Different one-dimensional heat transfer shapes were considered. Compared to the other variables, initial temperature and heating medium come-up-time had little influence on optimal processing temperature. Regression equations were developed relating optimal temperatures with all relevant variables.     

The geometry of shadows: effects of inhomogeneities in electrical field processing

Ohmic heating uses the inherent electrical resistance of a material to generate heat. To produce safe, high quality, food, it is important to investigate any inhomogeneities which may occur in the process. The presence of any inclusions of significantly lower electrical conductivity in a solid–liquid mixture can cause a cold shadow within the liquid; internal heating differences can arise in a particle caught in this region. Experimental measurements and computational modelling of these problems have been carried out. When the particle was an electrical insulator, differences in heating rate due to thermal conduction gave significant temperature differences, convective mixing within the liquid had a marked effect on thermal response. For electrically conducting particles, heating rates are due to internal resistance heating as well as thermal conduction effects. In both cases, the size of the shadow is of the order of magnitude of the inclusion size. Simulations predicted the type and magnitude of the effects seen: it is difficult to model temperatures without accurate data on convective mixing.     

Apparent Heat Transfer in a Forced Convection Oven and Properties of Baked Food

Parameters for expressing the heating performance and baking results of sponge cakes dependent on heating performance in a forced convection oven were studied. The heating performance of a forced convection oven may be expressed by the apparent heat transfer coefficient which was measured at various air temperatures and velocities. Both the air velocity and temperature of the circulating air affected the apparent heat transfer coefficient in a forced convection oven and determined the final properties of the baked food. The effects of these parameters on sponge cakes baked in the forced convection oven were observed.     

THE EFFECT OF THERMOCOUPLE AND RECEPTACLE TYPE ON OBSERVED HEATING CHARACTERISTICS OF CONDUCTION-HEATING FOODS PACKAGED IN SMALL METAL CONTAINERS

Three different types of thermocouples and receptacles were studied to determine the effect on the observed heat penetration rate of a conduction‐heating food packaged in 202 × 204 and 211 × 300 cans. The studies were conducted at two product initial temperatures (70 and 140F) and three retort temperatures (240, 250 and 260F) using an Allpax R&D retort. The results indicated that the Ecklund stainless steel receptacles and needle type thermocouples in 202 × 204 cans increased the apparent heat penetration rates significantly, which resulted in underestimated Ball Formula process times (≈5%) and overestimated process lethalities (≈27%). However, in the 211 × 300 cans only minimal effects were seen on the apparent heat penetration rates, Ball process times and resulting lethalities. The use of plastic Delrin receptacles and thermocouples in 202 × 204 and 211 × 300 cans produced results in close agreement with those obtained from thin‐gauge flexible thermocouples having minimum conduction error.     

Estimation of confidence interval of pasteurizing values of conduction-heated Sous Vide food in a combination oven

The thermal diffusivity of potato and the apparent heat transfer coefficients (h) in a conduction chiller and in a combination oven at 85, 90 and 95°C and vario-steaming mode were determined experimentally. Based on K-S tests and assumption, the variabilities of all six parameters (thermal diffusivity, h values for heating and cooling, initial food temperature, process medium temperatures and z value) were represented by normal distribution only (model A) or by both normal and gamma distributions (model B). The mean P₇₀ values and their 95% confidence intervals for three oven temperatures, estimated from experimental temperature histories, were 1510 ± 101, 1590 ± 131 and 1590 ± 170 min (for each oven temperature, n = 100) respectively. Good agreement was obtained between these estimations and those predicted using the Monte Carlo procedure and models A and B.     

Determination of convective heat transfer coefficient and specific energy consumption of potato using an ingenious self tracking solar dryer

In the present work, an attempt has been made to experimentally determine the heat transfer properties of potato in terms of convective heat transfer coefficient, specific energy consumption and specific heating rate. Drying experiments with potato cylinders have been performed in an in-house fabricated laboratory scale natural convection indirect solar dryer with self tracking mechanism. The convective heat transfer coefficient of cylindrical potato samples was evaluated by considering the combined effects of heat capacities of food product as well as radiative heat transfer from drying chamber to the food product. This study revealed that the convective heat transfer coefficient for potato cylinders was varying from 11.73 to 16.23 W/m² °C with an experimental error of 7.86 %. The specific energy consumption was decreasing exponentially with drying time, and the average value was estimated to be 3,491 kJ/kg. It was also observed that the specific heating rate for potato cylinders decrease with dimensionless moisture content.     

午餐肉罐头杀菌条件的探讨

397克午餐肉传热曲线斜率f_h为66,j值为1.32。采用鲍尔公式计算的结果,若加热温度为112℃,118℃,121℃,125℃,130℃时,要使罐内中心点F值达到3,在离罐壁15mm处的F值分别为12.99,36.38,52.46,130.01,334.3,通过实罐试验,采用130℃高温短时杀菌的罐头因受热过度,有脂肪析出,弹性也差。本试验说明,午餐肉罐头采用116—118℃的杀菌条件为宜。     

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.     

Experimental and Analytical Temperature Distributions during Oven-Based Convection Heating

ABSTRACT:  Mathematical models, combined with experimental evaluation, provide an approach to understand, design, and optimize food process operations. Magnetic resonance imaging (MRI), as an experimental technique, is used extensively in both medical and engineering applications to measure and quantify transport processes. Magnetic resonance (MR) was used in this study to assess a mathematical model based on Fourier's second law. The objective was to compare analytical solutions for the prediction of internal temperature distributions in foods during oven-based convective heating to experimental temperature measurements and determine at what point during the heating process a coupled heat and mass transport process should be considered. Cylindrical samples of a model food gel, Russet potato and rehydrated mashed potato were heated in a convection oven for specified times. Experimentally measured internal temperatures were compared to the internal temperatures predicted by the analytical model. Temperatures distributions in the axial direction compared favorably for the gel and acceptably for the Russet and mashed potato samples. The MR-acquired temperatures in the radial direction for the gel resulted in a shallower gradient than predicted but followed the expected trend. For the potato samples, the MR-acquired temperatures in the radial direction were not qualitatively similar to the analytical predictions due to moisture loss during heating. If temperature resolution is required in the radial direction, moisture losses merit the use of transport models that couple heat and mass transfer.     

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.     

Thermal Properties of Beef Loaf Produced in Foodservice Systems

There is a lack of information on thermal and other properties of foods, especially food mixtures. Physical properties of beef loaf prepared following formula and procedures used in alternate foodservice systems were determined: heat capacity (0.88 cal/g°C uncooked, 0.91 heated to 60°C) moisture content (72.1% uncooked, 66.2% heated), thermal conductivity (0.40 w/cm°C uncooked, 0.47 heated), fat (17.6% uncooked, 13.0% heated) and density (1.00 g/cm³ uncooked and 0.70 heated). The surface heat transfer coefficient for a forced convection oven was also determined (62 w/ m²°C.) Times to heat beef loaves to specified ending temperatures in a forced convection oven at 163°C and 176°C were calculated. There was less than 1 min difference in calculated heating time required to reach the desired end temperature. The actual times to reach ending temperature show very good agreement with the calculated times.     

A SEMI-EMPIRICAL APPROACH TO HANDLE BROKEN-LINE HEATING: DETERMINATION OF EMPIRICAL PARAMETERS AND EVALUATION OF PROCESS DEVIATIONS

An existing semi-empirical model for simulating product temperature profiles during thermal processing of conduction or convection heating foods under time varying boundary conditions (variable retort temperatures) was extended for the case of broken-line heating products. The use of the method for determination of the empirical heat penetration parameters for broken-line heating curves as defined by Ball (j_h, f_hl, f_h2, x_bh) was evaluated. Starch solutions, showing broken-line heating behavior, were used as a food simulant.
To investigate the consistency of the determined broken-line heating parameters, and to test the applicability of the method when boundary conditions are time dependent, process deviations consisting of drops on the heating medium temperature during the holding phase of a process were evaluated. The model is a promising approach, if the correct empirical parameters are used.     

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.     

Coupled 3D heat and mass transfer model for numerical analysis of drying process in papaya slices

An experimental and numerical study for the drying process of a solid food, Chilean papaya slices, was carried out in a range of air temperatures from 40 to 80 °C. The unsteady temperature and moisture distributions results inside the sample were predicted by using an unsteady tri-dimensional coupled heat conduction and mass diffusion mathematical model. The validation procedure includes a comparison with experimental and numerical temperature and moisture content results obtained from experimental data. The samples thermophysical properties as density, specific heat, and thermal conductivity are assumed to vary non-linearly with temperature. The convective heat and mass transfer coefficients were found by the analytical model. The water effective diffusion coefficient, the drying curves and the center temperature were measured by physical experiments. It was found from the experimental results that slices of papaya present an isotropic behavior with an uncertainty between 6.0% and 9.0%. According to statistical test results (RE%), the finite volume method based calculations gave a very good fit quality.