Jin-Sheng Leu

Heat Transfer and Fluid Flow Analysis for Plate-Fin and Oval Tube Heat Exchangers With Vortex Generators

This article investigates the effectiveness of embedded vortex generators in enhancing the heat transfer performance of a plate-fin heat exchanger with a four-row staggered oval tube bundle. Two different types of vortex generator are considered, namely annular and inclined block. Numerical simulations are performed to analyze the effects of the three-dimensional turbulence induced by the vortex generators on the heat transfer and fluid flow characteristics of the heat exchanger. The results indicate that compared to a plate-fin heat exchanger with circular tubes, the use of oval tube fins and vortex generators increases the heat transfer rate by 3 to 16% and reduces the pressure drop by 17 to 35% for inlet velocities in the range of 1 to 8 m/s. Furthermore, the vortex generators make possible an average area reduction ratio of 14 to 18%. Overall, the results show that the inclined block shape vortex generators yield the greatest improvement in the heat transfer performance at medium to high inlet velocities.

Variable viscosity effects on the vortex instability of free convection boundary layer flow over a horizontal surface in a porous medium

Abstract The role of temperature-dependent viscosity is studied in the flow and vortex instability of a heated horizontal free convection boundary layer flow in a saturated porous medium. For an isothermal surface, similarity solutions are found to exist for viscosity variation expressed as a general function of temperature. For exponential variation of viscosity with temperature, the numerical results for Nusselt number, critical Rayleigh number and associated wave number at the onset of vortex instability are presented over a wide range of wall to ambient viscosity ratio parameters. It is found that the variable viscosity effect enhances the heat transfer rate and destabilizes the flow for liquid heating, while the opposite trend is true for gas heating.

The natural convection from a point heat source embedded in a non-Darcian porous medium

Abstract The natural convection flow from a heat point source embedded in a non-Darcian porous medium is investigated by employing local similarity and modified Kellers Box methods. The non-Darcian effects of convective, inertia and thermal dispersion are all considered. The results indicate that the non-Darcian effects decrease the centerline velocity and temperature and also increase the velocity and temperature boundary layer thicknesses. In addition, solutions using the local similarity method over-estimate the centerline velocity and temperature.

TECHNICAL NOTE: VARIABLE VISCOSITY EFFECTS ON THE VORTEX INSTABILITY OF FREE CONVECTION BOUNDARY LAYER FLOW OVER A HORIZONTAL SURFACE

The role of temperature dependent viscosity is studied in the flow and vortex instability of a heated horizontal free convection boundary layer flow. Numerical results for the Nusselt, shear stress, critical Grashof, and wave numbers are presented for Prandtl numbers Pr∞ = 0.7, 7, 50, 100, and 500. It is shown that, for liquid heating, variable viscosity effect enhances the heat transfer rate and destabilizes the flow, while for gas heating, the opposite trend is true.

The wall and free plumes above a horizontal line source in non-Darcian porous media

Abstract The natural convection flows from the wall and free plumes above a horizontal heat line source in a non-Darcian porous medium are investigated. The non-Darcian convective, boundary viscous and inertia effects are all considered. The results indicate that the non-Darcian effects decrease the peak velocities and increase the maximum temperatures, and thicken the temperature boundary layer. In addition, the wall plume has a lower peak velocity and a higher maximum temperature than the corresponding free plume. Moreover, solutions by using local similarity and local non-similarity methods overestimate the maximum temperature and peak velocity for both wall and free plume cases.

Analysis of Heat Transfer and Flow Characteristics for the Arrangement of Piezoelectric Fan on Heated Cylinder

Piezoelectric fans are an effective device for heat transfer enhancement in low convective regions due to their low power consumption, low noise, and operational simplicity. This study performs a numerical and experimental investigation into the effects of a piezoelectric fan on the velocity and temperature fields near a cylindrical heat source with a constant heat flux on the sidewalls and upper flat surface. In performing the investigation, the blade is placed at various locations along the cylindrical axis and is arranged such that it vibrates in either the vertical direction or the horizontal direction. The results show that the airstream induced by the vibrating fan covers a broader area of the heated surface when the blade vibrates in the vertical direction, and therefore yields a better heat transfer performance than that achieved when the fan vibrates in the horizontal direction. In addition, it is found that the maximum heat transfer augmentation ratio is obtained at dimensionless fan positions ranging from 0.6 to 0.75, and has values of 1.94, 1.53, and 1.44 given dimensionless fan tip clearances of 0.5, 1.0, and 1.5, respectively. Finally, it is shown that while the numerical results underestimate the effect of the vibrating piezoelectric fan in enhancing the heat transfer performance by around 1–14%, a good qualitative agreement is observed between the numerical and experimental results.

The Computer Aided Design of Steam Surface Condensers

Steam surface condenser is an important component in a power plant. It condenses the latent heat of the turbine exhaust steam. Therefore, the turbine efficiency is directly influenced by the performance of the steam surface condenser. This work is to develop a computer-aided condenser design software, written in Quick Basica language. This software can be run in a personal computer(PC/XT, PC/AT). The design methodology is based a method proposed by Heat Exchange Institute for Steam Surface Condensers[1]. This program has the following four functions: (1)Rating the condenser (2)Sizing the condenser (3) Calculating the cleanliness factor and (4) Determining the condenser absolute pressure. The features of this program are powerful interactive ability and high accuracy. Calculation results are available within seconds. The program has been checked against controlled performance test and operating data on installed steam surface condensers.