The objective of the present work is to investigate the laminar mixed convection flow and heat tr... more The objective of the present work is to investigate the laminar mixed convection flow and heat transfer in the entrance region of heated rectangular ducts rotating about a parallel axis. Heating conditions of isothermal and iso-flux are both considered. Boussinesq approximation is invoked to take into account buoyancy effect induced by centrifugal force. Navier-Stokes/Boussinesq system can be cast into a dimensionless form, in which five governing parameters, the Prandtl number Pr, rotational Reynolds number J, rotational Grashof number Gr{sub {Omega}}, aspect ratio {gamma} and the eccentricity E, are involved. Mechanisms of secondary vortex development in the ducts are explored by a theoretical analysis on vorticity transport equation. The values of Pr and E are fixed as 0.7 and 10, respectively. For various combinations of the other thee parameters, a vorticity-velocity method implemented with a marching technique is employed to solve the resultant three-dimensional system for simultaneously developing flow and temperature fields. The emphasis is placed on the rotational effects, including both coriolis force and centrifugal buoyancy; on the non-isothermal flow and the related heat transfer. The results reveal that the friction factors and heat transfer rates can be enhanced by Coriolis and rotation-induced buoyancy effects; and the variations of themore » local values are closely related to the evolution of the secondary vortices in ducts. The differences in flow behaviors and thermal characteristics for UWT and UHF are also investigated by the present theoretical analysis on secondary flow mechanism as well as the computational results.« less
The reasons for deviations between micro and macro thermal-flow characteristics still remain uncl... more The reasons for deviations between micro and macro thermal-flow characteristics still remain unclear and the experimental evidence of high confidence is not available. The electrokinetic effect is probably a significant point in manifestation of the discrepancy between micro and macro liquid flows. The objective of this study is to develop an analysis for the thermal field in microchannels at constant wall temperature (CWT) and, by comparing with the results at constant heat flux (CHF), to explore effects of wall-heating conditions on pressure-driven forced convection characteristics with the presence of electrokinetic effects. A theoretical analysis is developed on a model of the simultaneously hydrodynamically and thermally developed liquid convection in a flat microchannel of two parallel plates at prescribed CWT or CHF boundary conditions. Linearized Poisson-Boltzmann equation and fully-developed momentum equation are employed to obtain the electrical potential, the flow velocity, and the streaming potential. By using the electro-hydrodynamic solutions, temperature distributions and heat transfer rates characterized by Nusselt numbers are developed. The present results reveal that the zeta potentials of the two walls have pronounced impact on the electro-hydrodynamic natures of the flow and, therefore, significantly influence the thermal characteristics in microchannels. The electrokinetic effects degrade Nusselt numbers in general. With the data at symmetric wall-heating conditions, it is also demonstrated that the heat transfer deterioration in CHF is relatively more pronounced than that in CWT cases.
Numerical Heat Transfer Part A-applications, Nov 10, 2000
Page 1. HEAT AND FLOW EVALUATION IN ROTATING CIRCULAR DISKS PASSAGE WITH A CFD APPROACH ... 2, pp... more Page 1. HEAT AND FLOW EVALUATION IN ROTATING CIRCULAR DISKS PASSAGE WITH A CFD APPROACH ... 2, pp. 39^46, 1996. 8. CY Soong, Theoretal Analysis for Axisymmetric Mixed Convection between Rotating Coaxial Disks, Int. J. Heat Mass Transfer, vol. ...
International Journal of Heat and Mass Transfer, Sep 1, 1990
... linear in x and, for convenience, it is specified as T~(x) = T,w(0)+AT~x. The characteristic ... more ... linear in x and, for convenience, it is specified as T~(x) = T,w(0)+AT~x. The characteristic temper-ature difference can now be determined at AT~ = zH, in which z ... ki= -v(cosh K+cos K)-2(1 -y)]+Cffsinh Ky k~ = -y-sinhK)+Cs(sinKy-y'sinK)} (30) a= 1 ~[Sa"`5(k2-k3)Z-SaSkl(k2+k,) - 8a ...
International Journal of Computational Engineering Science, Mar 1, 2001
Convective heat transport affects the thermal field and drag coefficient in a head-disk assembly ... more Convective heat transport affects the thermal field and drag coefficient in a head-disk assembly significantly. The flow structure and the temperature distribution in ventilated disk passage are being investigated thoroughly in the present work. Two computational methods are used namely: finite volume method (FVM) and finite element method (FEM). Both are based on the Navier–Stokes partial differential equations but with different algorithms and meshes. With different settings of operating conditions on disk, the following important parameters are considered: inlet Reynolds number, rotational speed, shroud clearance and wall temperature. Both co-rotating and counter-rotating disks are also being studied. The former is based on FORTRAN code and the latter on FASTFLO PDE calculator. All the codes are run on a SGI UNIX workstation. It is found that both the Corolis force and centrifugal-buoyancy have important effects on the flow structure and heat transfer due to the rotatinal speed and inlet velocity. Nusselt number decreases along the radius, especially near the disk outer edge. It decreases more rapidly along the radius and the absolute value is much greater for nonventilated disks. The predicted friction coefficients (Cf) are also very different in both cases. For the ventilated case, Cf varies rapidly. While in the nonventilated case, Cf is rather constant when rotational speed (Ω) is low ( 800 rpm), Cf varies appreciably along the radius. Finally, the shroud clearance has the pronunced effects only in the high-Re flow. The results are compared, discussed and their agreement is good.
International Journal of Heat and Mass Transfer, Oct 1, 2001
In the present study, thermal-fluid behaviors in a rectangular enclosure of width-to-height aspec... more In the present study, thermal-fluid behaviors in a rectangular enclosure of width-to-height aspect ratio 4:1 heated from below with wall-temperature varied sinusoidally in time is investigated numerically. The main concerns are the effects of the wall-heating modulation on the threshold of thermal instability and the oscillatory cellular convection heat transfer performance at the post-critical conditions. For investigation of heating modulation
The objective of the present work is to investigate the laminar mixed convection flow and heat tr... more The objective of the present work is to investigate the laminar mixed convection flow and heat transfer in the entrance region of heated rectangular ducts rotating about a parallel axis. Heating conditions of isothermal and iso-flux are both considered. Boussinesq approximation is invoked to take into account buoyancy effect induced by centrifugal force. Navier-Stokes/Boussinesq system can be cast into a dimensionless form, in which five governing parameters, the Prandtl number Pr, rotational Reynolds number J, rotational Grashof number Gr{sub {Omega}}, aspect ratio {gamma} and the eccentricity E, are involved. Mechanisms of secondary vortex development in the ducts are explored by a theoretical analysis on vorticity transport equation. The values of Pr and E are fixed as 0.7 and 10, respectively. For various combinations of the other thee parameters, a vorticity-velocity method implemented with a marching technique is employed to solve the resultant three-dimensional system for simultaneously developing flow and temperature fields. The emphasis is placed on the rotational effects, including both coriolis force and centrifugal buoyancy; on the non-isothermal flow and the related heat transfer. The results reveal that the friction factors and heat transfer rates can be enhanced by Coriolis and rotation-induced buoyancy effects; and the variations of themore » local values are closely related to the evolution of the secondary vortices in ducts. The differences in flow behaviors and thermal characteristics for UWT and UHF are also investigated by the present theoretical analysis on secondary flow mechanism as well as the computational results.« less
The reasons for deviations between micro and macro thermal-flow characteristics still remain uncl... more The reasons for deviations between micro and macro thermal-flow characteristics still remain unclear and the experimental evidence of high confidence is not available. The electrokinetic effect is probably a significant point in manifestation of the discrepancy between micro and macro liquid flows. The objective of this study is to develop an analysis for the thermal field in microchannels at constant wall temperature (CWT) and, by comparing with the results at constant heat flux (CHF), to explore effects of wall-heating conditions on pressure-driven forced convection characteristics with the presence of electrokinetic effects. A theoretical analysis is developed on a model of the simultaneously hydrodynamically and thermally developed liquid convection in a flat microchannel of two parallel plates at prescribed CWT or CHF boundary conditions. Linearized Poisson-Boltzmann equation and fully-developed momentum equation are employed to obtain the electrical potential, the flow velocity, and the streaming potential. By using the electro-hydrodynamic solutions, temperature distributions and heat transfer rates characterized by Nusselt numbers are developed. The present results reveal that the zeta potentials of the two walls have pronounced impact on the electro-hydrodynamic natures of the flow and, therefore, significantly influence the thermal characteristics in microchannels. The electrokinetic effects degrade Nusselt numbers in general. With the data at symmetric wall-heating conditions, it is also demonstrated that the heat transfer deterioration in CHF is relatively more pronounced than that in CWT cases.
Numerical Heat Transfer Part A-applications, Nov 10, 2000
Page 1. HEAT AND FLOW EVALUATION IN ROTATING CIRCULAR DISKS PASSAGE WITH A CFD APPROACH ... 2, pp... more Page 1. HEAT AND FLOW EVALUATION IN ROTATING CIRCULAR DISKS PASSAGE WITH A CFD APPROACH ... 2, pp. 39^46, 1996. 8. CY Soong, Theoretal Analysis for Axisymmetric Mixed Convection between Rotating Coaxial Disks, Int. J. Heat Mass Transfer, vol. ...
International Journal of Heat and Mass Transfer, Sep 1, 1990
... linear in x and, for convenience, it is specified as T~(x) = T,w(0)+AT~x. The characteristic ... more ... linear in x and, for convenience, it is specified as T~(x) = T,w(0)+AT~x. The characteristic temper-ature difference can now be determined at AT~ = zH, in which z ... ki= -v(cosh K+cos K)-2(1 -y)]+Cffsinh Ky k~ = -y-sinhK)+Cs(sinKy-y'sinK)} (30) a= 1 ~[Sa"`5(k2-k3)Z-SaSkl(k2+k,) - 8a ...
International Journal of Computational Engineering Science, Mar 1, 2001
Convective heat transport affects the thermal field and drag coefficient in a head-disk assembly ... more Convective heat transport affects the thermal field and drag coefficient in a head-disk assembly significantly. The flow structure and the temperature distribution in ventilated disk passage are being investigated thoroughly in the present work. Two computational methods are used namely: finite volume method (FVM) and finite element method (FEM). Both are based on the Navier–Stokes partial differential equations but with different algorithms and meshes. With different settings of operating conditions on disk, the following important parameters are considered: inlet Reynolds number, rotational speed, shroud clearance and wall temperature. Both co-rotating and counter-rotating disks are also being studied. The former is based on FORTRAN code and the latter on FASTFLO PDE calculator. All the codes are run on a SGI UNIX workstation. It is found that both the Corolis force and centrifugal-buoyancy have important effects on the flow structure and heat transfer due to the rotatinal speed and inlet velocity. Nusselt number decreases along the radius, especially near the disk outer edge. It decreases more rapidly along the radius and the absolute value is much greater for nonventilated disks. The predicted friction coefficients (Cf) are also very different in both cases. For the ventilated case, Cf varies rapidly. While in the nonventilated case, Cf is rather constant when rotational speed (Ω) is low ( 800 rpm), Cf varies appreciably along the radius. Finally, the shroud clearance has the pronunced effects only in the high-Re flow. The results are compared, discussed and their agreement is good.
International Journal of Heat and Mass Transfer, Oct 1, 2001
In the present study, thermal-fluid behaviors in a rectangular enclosure of width-to-height aspec... more In the present study, thermal-fluid behaviors in a rectangular enclosure of width-to-height aspect ratio 4:1 heated from below with wall-temperature varied sinusoidally in time is investigated numerically. The main concerns are the effects of the wall-heating modulation on the threshold of thermal instability and the oscillatory cellular convection heat transfer performance at the post-critical conditions. For investigation of heating modulation
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