Computational fluid dynamics (CFD) analyses provide insight on the natural convective processes occurring during the sterilization of canned liquid food. The definition and estimation of heat transfer coefficients pertaining to the transient heat transfer occurring in cylindrical food cans is presented. The heat transfer coefficients defined on the basis of absolute mass flow averaged and volume-averaged temperatures are compared. The contributions to the overall heat transfer rate from different surfaces of the uniformly heated cylindrical can are evaluated. The influence of the temperature difference to viscosity ratio on the peculiar Nusselt number trends is discussed. Unified correlations are provided for the Nusselt number as functions of Fourier number, food can aspect ratio and thermal conductivity of the food medium in both the conduction and convection dominated heat transfer regimes. © 2008 Elsevier Ltd. All rights reserved.

Aravamudan Kannan

Department Of Chemical Engineering

Aspect ratio

Computational fluid dynamics

Heat transfer

Natural convection

STERILIZATION (CLEANING)

Thermal conductivity

CYLINDRICAL FOOD CANS

Fourier number

THERMAL FOOD STERILIZATION

Canning

IIT Madras is a public technical and research university located in Chennai, Tamil Nadu. Founded in 1959, it is recognised as an Institute of National Importance.

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IIT Madras

Journal of Food Engineering

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. © 2009 Springer Science + Business Media, LLC.

Food and Bioprocess Technology

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

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. © 2005 Elsevier Ltd. All rights reserved.

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

Mass transfer from solids, which has important applications in a number of chemical and pharmaceutical industries, has been studied experimentally and semiempirically under turbulent flow conditions, and correlations are available in the literature to calculate the mass-transfer coefficients from pellets, rotating cylinders and disks etc. However, mass transfer under laminar flow has not been sufficiently addressed. One of the difficulties here is the strong Reynolds number dependence of the flow pattern, for example, due to the onset of Taylor vortices for the case of a rotating cylinder. This problem is circumvented by using a computational fluid dynamics (CFD)-based solution of the governing equations for the case of a cylinder rotating inside a stationary cylindrical outer vessel filled with liquid. The parameters cover a range of Reynolds number (based on the cylinder diameter, and the tangential speed of the cylinder), Schmidt number and the ratio of the outer to inner cylinder diameters. The results confirm that the circumferential velocity profile is a strong function of the Reynolds number and varies from a nearly Couette-type flow at very low Reynolds numbers to a boundary layer-like profile at high Reynolds numbers. The onset of Taylor vortices has a strong effect on the flow field and the mass-transfer mode. The calculations show that the Sherwood number has a linear dependence on the Reynolds number in the Couette-flow regime, and roughly square-root dependence after the onset of Taylor vortices. Correlations have been proposed to calculate the Sherwood number taking account of these effects. © 2005 American Institute of Chemical Engineers.

AIChE Journal

Effect of Taylor Vortices on mass transfer from a rotating cylinder

It is desirable to reduce the time taken for sterilization without compromising on the criterion of the slowest heating zone (SHZ) in the container reaching the minimum prescribed temperature. In this study, the required sterilization times for food cans of equal volumes but different shapes ranging from full cone through truncated cones to full cylinder were investigated. Further, the effect of orientation of the can was also investigated by inclining these geometries from 0-180°. The non-Newtonian fluid (0.85% w/w CMC) was taken as the test food material and its laminar flow behavior analysed using computational fluid dynamics. It was observed that among these geometries the lowest sterilization time was observed for a vertically oriented upright full conical can. The cylinder gave lower sterilization time when oriented horizontally than vertically. The sterilization times of truncated conical cans were intermediate to those determined for full cone and cylinder. Even a small inclination in the orientation of the cans may lead to significant increase in the sterilization times due to decrease in the convective velocities. The slowest heating zone movement in both axial and radial directions was tracked for various geometries and typical results are presented. The convective velocities, volume of region above and the surface area below the slowest heating zone determine the time taken for sterilization. When geometry modifications are made for aesthetic reasons to the food container, the utilization of known sterilization times for the conventional cylindrical can of the same volume may be inappropriate for suitable thermal treatment. © 2005 Elsevier Ltd. All rights reserved.

International Journal of Heat and Mass Transfer

Enhanced food sterilization through inclination of the container walls and geometry modifications

Convection Heat Transfer

Free-Convective Heat Transfer

Heat transfer analysis of canned food sterilization in a still retort

Journal	Journal of Food Engineering
ISSN	02608774
Open Access	No