A.G. Abdul Ghani

Numerical simulation of natural convection heating of canned food by computational fluid dynamics

Abstract Natural convection heating within a can of liquid food during sterilization is simulated by solving the governing equations for continuity, momentum and energy conservation for an axisymmetric case using a commercial Computational Fluid Dynamics (CFD) package ( PHOENICS ). Transient flow patterns and temperature profiles within model liquids (sodium carboxy-methyl cellulose (CMC) and water) have been predicted. The model liquids, CMC and water, were assumed to have constant properties except for the viscosity (temperature dependent) and density (Boussinesq approximation). It has been shown that the action of natural convection forces the slowest heating zone (SHZ) to migrate towards the bottom of the can as expected. The shape and the size of the SHZ area are different for CMC and water. The magnitude of the axial velocity was found to be in the range of 10 −5 –10 −4 m/s for CMS and of 10 −2 –10 −1 m/s for water, these magnitudes of course vary with time and position in the can. The time required for the SHZ to reach the sterilization temperature of 100°C was 1800 s for CMC and only, 150 s for water.

Numerical simulation of transient temperature and velocity profiles in a horizontal can during sterilization using computational fluid dynamics

Abstract In this work, sterilization of a canned liquid food (carrot–orange soup) in a metal can lying horizontally and heated at 121°C from all sides is simulated for a 3-D geometry. Transient temperature, flow pattern, and shapes of the slowest heating zone (SHZ) during natural convection heating of canned liquid foods are predicted. The partial differential equations describing the conservation of mass, momentum and energy conservation are solved numerically using a commercial computational fluid dynamics (CFD) software (PHOENICS), which is based on a finite-volume method of analysis. The simulation shows the influence of natural convection on the liquid-flow pattern and on the movement of the SHZ. The action of natural convection forces the SHZ to migrate towards the bottom of the can as expected. The SHZ eventually stays in a region that is about 20–25% of the can height from the bottom. The secondary flow formation and its effect on the shape of the SHZ are evident. The results of this work are compared with those for vertical can. It shows faster heating in the vertical can, which is expected due to the enhancement of natural convection caused by its longer height.

Thermal sterilization of canned food in a 3-D pouch using computational fluid dynamics

Abstract Sterilization of food in cans has been well studied both experimentally and theoretically, but little or no work has been done on sterilization of food in pouches. The food pouches have only been recently introduced to the market. In this study, transient temperature, velocity profiles and the shape of the slowest heating zone (SHZ) have been established for a uniformly heated three-dimensional pouch containing carrot–orange soup, using saturated steam at 121°C. The computational fluid dynamics (CFD) code PHOENICS was used for this purpose. The liquid food used in the simulation has temperature-dependent viscosity and density. From the simulations, the maximum axial velocity of the soup was found to be 10 −2 −10 −4 mm s −1 , which was due to the small height of the pouch and high viscosity of the soup. The SHZ was found to migrate into a region within 30–40% of the pouch height above the bottom and at a distance approximately 20–30% of the pouch length from its widest end. The experimental measurements were conducted at Heinz Watties Australasia based in New Zealand. The measured temperature at different locations in the pouch was compared with that predicted. Both results were found to be in good agreement. The results of a simulation done for the same pouch geometry and material considering pure conduction mechanism were also presented for the purpose of comparison.

An investigation of deactivation of bacteria in a canned liquid food during sterilization using computational fluid dynamics (CFD)

Abstract Thermal processing of a liquid food always results in important biochemical changes such as bacteria deactivation and nutrient concentration changes. To estimate these changes the liquid food needs to be tagged and followed, which is a difficult task for most flow conditions. In this study, the computational fluid dynamics (CFD) code PHOENICS is used to predict temperature distribution and concentration of the live bacteria in a can filled with liquid food. The governing equations for continuity, momentum and energy are solved numerically together with bacteria concentration, using a finite volume method. Arrhenius equation was used to describe bacteria deactivation kinetics, and it was introduced to the existing software package using a FORTRAN code. The diffusion of bacteria was modelled using the modified Brownian diffusion equation. Natural convection that occurs during thermal sterilization of viscous liquid (the aqueous solution of sodium carboxy-methyl cellulose (CMC)) in a cylindrical can heated from all sides, has been studied. Saturated steam at 121°C was used as the heating medium, and the model liquid was assumed to have constant properties except for the viscosity (temperature dependent) and density (Boussinesq approximation). The simulations have provided transient flow pattern, live bacteria concentration and temperature profiles, which highlight the slowest heating zone (SHZ) resulted from different periods of heating. The results show that the action of natural convection forces the SHZ to migrate towards the bottom of the can, and eventually stay in a region that is about 10–15% of the can height from the bottom. The secondary flow formation and its effect on the shape of the SHZ were evident. The simulations also show how the concentration of the live bacteria depends on both temperature distribution and flow pattern. The effect of diffusion on the rate of sterilization has been found to be negligible in the cases simulated in this study.

The effect of can rotation on sterilization of liquid food using computational fluid dynamics

Abstract In this work, sterilization of a viscous liquid food (carrot–orange soup) in a metal can lying horizontally and rotated axially in a still retort was simulated. The rotating can at 10 rpm was assumed to be heated by steam at 121 °C. The governing equations of mass, momentum and energy conservation for the three-dimensional can were solved using a commercial computational fluid dynamics package (PHOENICS), which is based on a finite volume method of solution. Transient temperature and velocity profiles caused by natural and forced convection heating were presented and compared with those for a stationary can. The results indicated that the combined effect of natural and forced convection splits the slowest heating zone (SHZ) into two distinct regions, unlike what has been previously observed in the stationary can. The volume of the SHZ was found to cover less than 5% of the total volume of the rotated can at the end of heating, which is due to the effect of rotation. The magnitude of the maximum axial velocity of the fluid after 1000 s of heating was found ranging from 2.3×10 −5 to 3.2×10 −4 ms −1 , compared with 2.2×10 −7 to 2.1×10 −4 ms −1 for the stationary can. The localized high velocity near the two ends of the can spread gradually throughout the whole length of the can as heating progresses.

Theoretical and experimental investigation of the thermal inactivation of Bacillus stearothermophilus in food pouches

Abstract Numerical simulations are used to profile temperature, velocity vector and viable bacteria (Bacillus stearothermophilus spores) concentration in a three-dimensional pouch filled with beef–vegetable soup during thermal sterilization. The partial differential equations of the continuity, momentum and energy equations are solved numerically, together with an equation defining viable bacteria concentration, based on finite volume method of analysis. Saturated steam at 121°C is used as a heating medium, and the model liquid is assumed to have constant properties except for the viscosity (temperature dependent) and density (Boussinesq approximation). The simulations show clearly the dependency of the concentration of live bacteria on both the temperature distribution and the flow pattern as sterilization proceeds. Experimental measurements are done to validate the bacteria concentration profiles. The experimental measurements are conducted at the Biotechnology Laboratory located at the Chemical and Materials Engineering Department, University of Auckland in New Zealand. The measured concentrations of B. stearothermophilus spores at different periods of heating are compared with those obtained from simulations, showing good agreements.

Theoretical and experimental investigation of the thermal destruction of Vitamin C in food pouches

Abstract In this study, theoretical analysis and experimental measurements for the determination of Vitamin C (ascorbic acid) concentration during thermal sterilization of a three-dimensional pouch filled with carrot-orange soup, are presented. Experimental validation was performed by measuring the concentration of ascorbic acid in the soup, using high performance liquid chromatography (HPLC) and a titration method. Transient temperature, velocity profiles and the shape of the slowest heating zone (SHZ) during heating were also studied. The simulation covered a heating cycle of 3000 s. The computational fluid dynamics (CFD) code PHOENICS was used for this purpose. Saturated steam at a temperature of 121 °C (250 °F) was assumed to be the heating medium, which is the sterilization temperature commonly used in the canning industry. Partial differential equations describing the conservation of mass, momentum and energy were solved numerically, together with the concentration equation for Vitamin C, using a finite volume method. The temperature dependence of the carrot-orange soups viscosity and density was incorporated in the simulation. The results of the simulations showed that natural convection plays an important role in the heat transfer within the liquid food in the pouch. The SHZ was found to migrate towards the bottom of the pouch into a region within 30–40% of the pouch height, closest to its deepest end. These results also showed that the vitamin profiles depend not only on the temperature distribution, but on the velocity profiles in the pouch. The measured values were compared with those predicted, showing reasonable agreement with an average deviation of 4.5%.

A computational and experimental study of heating and cooling cycles during thermal sterilization of liquid foods in pouches using CFD

Abstract In this study, a theoretical analysis of a heating and cooling cycle during sterilization of a three-dimensional pouch filled with carrot-orange soup was presented and analysed. Transient temperature, the shape of the slowest heating zone (SHZ) during heating and the slowest cooling zone (SCZ) during cooling were presented and studied. The simulation covered the whole heating and cooling cycles of 3600s and 1200s durations, respectively. The computational fluid dynamics (CFD) code PHOENICS was used for this purpose. Saturated steam at 121°C and water at 20°C were assumed to be the heating and cooling media, respectively. The partial differential equations describing the conservation of mass, momentum, and energy were solved numerically using the finite volume method. The liquid food used in the simulation has a temperature-dependent viscosity and density. At the end of heating, the SHZ was found to have settled into a region within 30–40 per cent of the pouch height above the bottom and at a distance approximately 20–30 per cent of the pouch length from its deepest end. In the cooling cycle, the slowest cooling zone (SCZ) was found to develop in the core of the pouch and gradually migrate toward the widest end. The vertical location of this slowest cooling zone was about 60–70 per cent of the pouch height. Experimental validation has been performed by measuring the temperature distribution in the pouch during heating and cooling, using thermocouples fixed at different locations. The predicted results were in good agreement with those obtained from the experiments.

A computational fluid dynamics study on the effect of sterilization temperatures on bacteria deactivation and vitamin destruction

Abstract The optimization of thermal processes such as sterilization relies on the accuracy of relevant kinetic data for bacterial inactivation and quality evolution. It is also dependent on the geometry and heating mechanism involved in the process. In these processes or systems, profiles of temperature distribution, bacteria concentration and concentrations of vitamins C (ascorbic acid), B1 (thiamin) and B2 (riboflavin) in a can filled with cherry juice during thermal sterilization have been obtained through numerical simulations. Different heating medium temperatures of 121, 130 and 140°C were tested. In order to generate these profiles, the continuity, momentum and energy equations are solved numerically, together with those of bacteria and vitamins concentrations, using the computational fluid dynamics code PHOENICS, combined with reaction kinetics models. Natural convection that occurs during thermal sterilization of viscous liquid (concentrated cherry juice, 74 °Brix) in a cylindrical can heated from all sides has been studied in this work. The simulations show clearly the dependences of the concentration of live bacteria and different vitamins on both the temperature distribution and the flow pattern as sterilization proceeds. The results also show that the best sterilization temperature may not always be 121 °C, depending on the quality requirements imposed on individual food material of concern.