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In this work, the heat and mass transfer characteristics of kiwifruit samples during vacuum freeze-drying process have been investigated numerically and experimentally. The numerical simulation with moving grid method is adopted to simulate the radial and axial moving sublimation interfaces during vacuum freeze-drying. The simulated average moisture content and temperature of kiwifruit during drying are validated with the measured data, with the mean average error (MAE) of 14.7%. The findings demonstrate that the sublimation interface shifts toward the center during the drying process and the sublimation rate and pressure gradient are significant in the early stage of drying. After 10 h of drying process, the sublimation rate dropped by 80% due to the increase of the thickness of the dried region and the decrease of the shelf plate heat transfer. In addition, it is noted that the increasing shelf plate temperature and chamber pressure would cause an increase of sublimation rate, thereby reducing the drying time. Although increasing the sample thickness can improve the sublimation rate, the drying time becomes longer due to the presence of more ice crystals in thicker kiwifruit samples. [ABSTRACT FROM AUTHOR]

Abstract: The study of mixed convection heat transfer in horizontal ducts with radiation effects has been numerically examined in detail. This work is primarily focused on the interaction of the thermal radiation with mixed convection for a gray fluid in rectangular horizontal ducts. The vorticity–velocity method is employed to solve the three-dimensional Navier–Stokes equations and energy equation simultaneously. The integro-differential radiative transfer equation was solved by the discrete ordinates method. The attention of the results is focused on the effects of thermal buoyancy and radiative transfer on the development of temperature, the friction factor and the Nusselt number. Results reveal that radiation effects have a considerable impact on the heat transfer and would reduce the thermal buoyancy effects. Besides, the development of temperature is accelerated by the radiation effects. [Copyright &y& Elsevier]

Abstract: In this numerical study, a channel flow of turbulent mixed convection of heat and mass transfer with film evaporation has been conducted. The turbulent hot air flows downward of the vertical channel and is cooled by the laminar liquid film on both sides of the channel with thermally insulated walls. The effect of gas–liquid phase coupling, variable thermophysical properties and film vaporization are considered in the analysis. In the air stream, the k–ε turbulent model has been utilized to formulate the turbulent flow. Parameters used in this study are the mass flow rate of the liquid film B, Reynolds number Re, and the free stream temperature of the hot air T o. Results show that the heat flux was dramatically increases due to the evaporation of liquid water film. The heat transfer increases as the mass flow rate of the liquid film decreases, while the Reynolds number and inlet temperature increase, and the influences of the Re and T o are more significant than that of the liquid flow rate. It is also found that liquid film helps lowering the heat and mass transfer rate from the hot gas in the turbulent channel, especially at the downstream. [Copyright &y& Elsevier]

Abstract: A two-dimensional numerical model has been established to investigate the performance of the PEM fuel cells. Parameters used in the analysis include the porosity and thickness of the gas diffuser layer (GDL). Results show that increasing the porosity of gas diffusion layer causes the increasing of mass transfer of fuel and air and results in a higher reaction rate. Therefore, a better performance of the fuel cell and more fuel consumption rate are observed. It is also demonstrated that the performance of the fuel cell increases with a decrease in the thickness of gas diffusion layer. The effects of liquid water condensation and flow directions of fuel and air are also considered in this analysis. Predicted results show that the performance of the PEM fuel cell without consideration of liquid water effect is always higher than that with consideration of liquid water effect. In addition, the performance of fuel cell with co-flow pattern of fuel and air is larger than that with counter flow. [Copyright &y& Elsevier]

The transient behaviors of natural convection heat and mass transfer along a vertical wavy surface subjected to step changes of wall temperature and wall concentration have been investigated numerically by using a simple coordinate transformation to transform the complex wavy surface into a flat plate. An implicit marching finite-difference scheme is employed for the analysis. It is well know that the buoyancy ratio N and Schmidt number Sc are the parameters of importance for heat and mass transfer. The wavy geometry is also an important factor for this problem. Numerical results show that the flow field takes more time to reach steady condition for a high amplitude-wavelength ratio α, small buoyancy ratio N, and large Schmidt number Sc. The effects of wavy geometry, buoyancy ratio N and Schmidt number Sc on the transient local skin-friction, local Nusselt number and local Sherwood number have been discussed in detail. [Copyright &y& Elsevier]

A numerical study of mixed convection heat and mass transfer along a vertical wavy surface has been carried out numerically. The wavy surface is maintained at uniform wall temperature and constant wall concentration that is higher than that of the ambient. A simple coordinate transformation is employed to transform the complex wavy surface to a flat plate. A marching finite-difference scheme is used for present analysis. The buoyancy ratio N, amplitude–wavelength ratio α, and Richardson number (Gr/Re2) are important parameters for this problem. The numerical results, including the developments of skin-friction coefficient, velocity, temperature, concentration, Nusselt number as well as Sherwood number along the wavy surface are presented. The effects of the buoyancy ratio N and the dimensionless amplitude of wavy surface on the local Nusselt number and the local Sherwood number have been examined in detail. [Copyright &y& Elsevier]

This paper experimentally investigates the heat transfer performance and characteristics of a heat sink with bio-mimetic oscillating foil. The oscillating foil is driven by a novel mechanism of grooved cam with linkage. The air flow is drawn into a rectangular duct through the motion of the bio-mimetic oscillating foil. Various heat sinks were set at the outlet of the duct to study the thermal performance. Two types of grooved cams were designed to obtain different angular velocity profiles for the oscillating foils. The angular velocity was specified as fast forward/slow backward and sinusoidal patterns. Foils with three flexural stiffness values were utilized in the study. The averaged velocity at the inlet was measured to estimate the Reynolds number and Strouhal number. The effects of configuration of heat sink design with oscillating foil were also under investigation. The junction temperature of the heat sink was measured to obtain the effective thermal resistance. Results show that the angular velocity profile and flexural stiffness of the oscillating foil have a significant effect on the thermal performance of the heat sink. [ABSTRACT FROM AUTHOR]