Heat Transfer Deviation of Laminar Free Convection Caused by Boussinesq Approximation

We assess the effects of free convection on the boundary layer formed along a flat surface stretching vertically in a quiescent fluid. The flow is laminar and incompressible, the buoyancy forces conform to the Boussinesq approximation and the surface temperature is variable. The two-point boundary value problem of the coupled momentum and energy equations is solved using a simple and accurate relaxation method that provides the general nonsimilar solution to the flow. The effect of free-convection currents on velocity and temperature profiles, skin friction, and heat transfer is studied by varying the flow Grashof and Prandtl numbers. Zero shear stress and heat-transfer rate are predicted at some axial coordinate on a surface with decreasing wall temperature. Also the skin friction is markedly modified by the buoyancy while the heat transfer at the surface is correspondingly only moderately influenced.

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Numerical study on laminar free convection heat transfer between sphere particle and high pressure water in pseudo-critical zone

Thermal Science ◽

10.2298/tsci121113039w ◽

2014 ◽

Vol 18 (4) ◽

pp. 1293-1303 ◽

Cited By ~ 8

Author(s):

Liping Wei ◽

Youjun Lu ◽

Jinjia Wei

Keyword(s):

Heat Transfer ◽

High Pressure ◽

Free Convection ◽

Fluidized Bed ◽

Numerical Study ◽

Convection Heat Transfer ◽

Boussinesq Approximation ◽

Convection Heat ◽

Pressure Water ◽

Free Convection Heat

Supercritical water fluidized bed reactor (SCWFBR) is a promising new reaction vessel which can effectively gasify wet biomass and efficiently produce hydrogen. Free convection heat transfer from particle in supercritical water (SCW) is a major basic heat transfer mechanism in a fixed bed or fluidized bed with low superficial velocity. In this paper, numerical study on the steady free convection heat transfer around single sphere particle in high pressure water of pseudo-critical zone was carried out. Both the Boussinesq approximation method and real properties model (considering variable specific heat, density, viscosity, and conductivity of SCW) were incorporated to simulate the flow and temperature field. With respect to Boussinesq approximation, real properties model shows higher vorticity and temperature gradients in the vicinity of the sphere surface, which shows variation of thermo-physical property has remarkable effect on the free convection heat transfer process. High local Nusselt number and high heat transfer rate were observed with real properties model.

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On Gaseous Free-Convection Heat Transfer With Well-Defined Boundary Conditions

Journal of Heat Transfer ◽

10.1115/1.1318213 ◽

2000 ◽

Vol 122 (4) ◽

pp. 661-668 ◽

Cited By ~ 3

Author(s):

M. R. D. Davies ◽

D. T. Newport ◽

T. M. Dalton

Keyword(s):

Heat Transfer ◽

Nusselt Number ◽

Free Convection ◽

Boundary Conditions ◽

Convection Heat Transfer ◽

Boussinesq Approximation ◽

Horizontal Cylinder ◽

Convection Heat ◽

Variable Property ◽

Free Convection Heat

The scaling of free convection heat transfer is investigated. The nondimensional groups for Boussinesq and fully compressible variable property free convection, driven by isothermal surfaces, are derived using a previously published novel method of dimensional analysis. Both flows are described by a different set of groups. The applicability of each flow description is experimentally investigated for the case of the isothermal horizontal cylinder in an air-filled isothermal enclosure. The approach taken to the boundary conditions differs from that of previous investigations. Here, it is argued that the best definition of the boundary conditions is achieved for heat exchange between the cylinder and the enclosure rather than the cylinder and an arbitrarily chosen fluid region. The enclosure temperature is shown both analytically and experimentally to affect the Nusselt number. The previously published view that the Boussinesq approximation has only a limited range of application is confirmed, and the groups derived for variable property compressible free convection are demonstrated to be correct experimentally. A new correlation for horizontal cylinder Nusselt number prediction is presented. [S0022-1481(00)01604-2]

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