Natural convection from upper and lower surfaces of an inclined isothermal plate

Using interferometric techniques the phenomemon of natural convection from an inclined isothermal plate in air was investigated. The results show that for plate inclinations up to 60 degrees from the vertical, the plate can be treated as vertical. For angles between 60 and 75 degrees a gradual change is noted. For inclination of 75° the heat transfer exhibits definite changes from vertical plate predictions. The upper and lower surfaces can be treated as similar for all angles below 60 degrees, and only at 75° or greater a distinction between them becomes apparent/ The assumption that the parallel component of acceleration due to gravity, when used for any inclination angle, gives correct heat transfer rates is not valid.     

Natural convection from an isothermal downward facing horizontal plate

Natural convection from an isothermal horizontal surface facing downward, has been investigated using the interferometric technique. Mach-Zehnder interferograms were obtained for a rectangular plate in air, and the experimentally determined local Nusselt numbers showed a surprising dependance on the temperature difference, between the plate and the ambient, a fact that has not been reported by previous investigations of the phenomenon.     

Natural convection heat transfer from isothermal horizontal plates of different shapes

Experiments were carried out on natural convection heat transfer from isothermal plates facing upwards in air in the range of Gr·Pr from , Both the average and local heat transfer were determined. Plates of different shapes (square, rectangular and circular) were used and “corner” and “edge” effects were investigated. Dimensionless equations are suggested for the laminar and turbulent regions.     

Natural convection in triangular enclosures with protruding isothermal heater

Natural convection heat transfer from a protruding heater located in a triangular enclosure has been analyzed numerically. Temperature of inclined boundary of the triangle is lower than the temperature of the heater, which has constant temperature boundary condition. The remaining walls are insulated. The study is formulated in terms of the vorticity-stream function procedure and numerical solution was performed using the finite difference method. Air was chosen as working fluid with Pr = 0.71. Governing parameters, which are effective on flow field and temperature distribution, are; Rayleigh number, aspect ratio of triangle enclosure, dimensionless height of heater, dimensionless location of heater and dimensionless width of heater. Streamlines, isotherms, velocity profiles, local and mean Nusselt numbers are presented. It is found that all parameters related with geometrical dimensions of the heater are effective on temperature distribution, flow field and heat transfer.     

Natural convection heat transfer from a horizontal isothermal elliptical cylinder with internal heat generation

This work examines the natural convection heat transfer from a horizontal isothermal cylinder of elliptic cross section in a Newtonian fluid with temperature dependent internal heat generation. The governing boundary layer equations are transformed into a non-dimensional form and the resulting nonlinear systems of partial differential equations are solved numerically applying cubic spline collocation method. Results for the local Nusselt number and the local skin-friction coefficient are presented as functions of eccentric angle for various values of heat generation parameters, Prandtl numbers and aspect ratios. Results show that both the heat transfer rate and skin friction of the elliptical cylinder with slender orientation are higher than the elliptical cylinder with blunt orientation. Moreover, an increase in the heat generation parameter for natural convection flow over an isothermal horizontal elliptic cylinder leads to a decrease in the heat transfer rate from the elliptical cylinder and an increase in the skin friction of the elliptical cylinder.     

Natural convection heat transfer below downward facing horizontal surfaces

The laminar steady-state convection below a horizontal infinite strip and below a horizontal circular plate is determined analytically for the limiting case of high Prandtl numbers. The plate surface is assumed to be either isothermal or uniformly heated. This convection is caused by the non-uniform temperature distribution at the edges of the plate. Therefore, this problem has to be described by an elliptic differential equation which was not taken into account in earlier integral methods. Applying the method of matched asymptotic expansions, similarity solutions are deduced for a region around the stagnation point. Moreover, an interpolation with the limiting case of low Prandtl numbers yields heat transfer correlations for arbitrary Prandtl numbers.     

An analysis on the thermal instability of forced convection flow over isothermal horizontal flat plate

Thermal instability of forced convection flow over an isothermal horizontal flat plate in the form of longitudinal vortices is examined by introducing three-dimensional spatial dependence of the perturbation quantities. The system of stability equations has been simplified significantly by considering the limiting case of very large Prandtl numbers and by seeking similarity solutions for the amplitude functions of the perturbation quantities. The effect of x-dependent temperature perturbation is shown to stabilize the flow as compared with x-independent temperature perturbation, which explains very well the extant theoretical results and experimental observations.     

Natural convection heat transfer from an isothermal vertical surface to a fluid saturated thermally stratified porous medium

Presented here are the results of a numerical study of natural convection heat transfer in a stably stratified, fluid saturated low porosity medium, in which Darcy flow prevails. In this investigation, the boundary layer approximations are discarded and a wide range of ambient thermal stratification levels is considered. The results indicate that the ambient thermal stratification has a significant effect on the flow and temperature fields, and that this effect differs considerably at higher levels of stratification. The flow reversal and temperature defects are significantly smaller in the porous media than in a viscous fluid, due to the stabilization of the flow by the solid matrix. To generalize the result, the Nusselt number data are correlated with the thermal stratification parameter to yield a functional relationship.     

Free convection heat transfer from isothermal spheres in water

An experimental investigation of free convection heat transfer from heated spheres to water is reported. The experimental data extend over a wide range of Rayleigh number, thus covering the laminar, transition, and beginning of the turbulent regimes. The temperature and velocity fields around a heated isothermal sphere have been measured, with the velocity profiles determined from the use of hot-film anemometry techniques. When possible, the comparison of the experimental results with the prediction of theory shows good agreement.     

Natural convective heat transfer from isothermal cuboids

The paper presents results of theoretical and experimental investigations of the convective heat transfer from isothermal cuboid. The analytical solution was performed taking into account complete boundary layer length and the manner of its propagation around isothermal cuboid. It arises at horizontal bottom surface and grows on vertical lateral surface of the block. After changing its direction, the boundary layer occurs above horizontal surface faced up and next it is transformed into buoyant convective plume. To verify obtained theoretical solution the experimental study has been performed. The experiment was carried out for three possible positions of the same tested cuboid.As the characteristic linear dimension in Nusselt-Rayleigh theoretical and experimental correlations we proposed the ratio of six volumes to the cuboids surface area, for the analogy to the same ratio using as the characteristic dimension for the sphere, which is equal to the sphere’s diameter. It allowed performing the experimental results independently from the orientation of the block. The Rayleigh numbers based on this characteristic length ranged from 10⁵ to 10⁷. The Nusselt number describing intensity of convective heat transfer from the cuboid can be expressed by: Nu=XRa^1/5+YRa^1/4, where X and Y are coefficients dependent on the cuboid’s dimensions. For the range of provided experiment the experimental Nusselt-Rayleigh relation can be presented in the form:

     

Natural convection heat transfer from a hot rectangular and a square corrugated plate to a cold flat plate

IntroductionReduction of heat loss from the absorber plate of asolar collector through the cover plates improvescollector efficiency. Therefore, the natllral convectionheat loss across air layers bounded by tWo parallel platesis of special interest to the designers of solar collectors.Most of the investigations on heat transfer in aconfined space have been cAned out with parallel platesin horizontal and inclined positions. Hollands, et al.[l]experimentally investigated the heat trallsferchara…     

Natural convection boundary-layer adjacent to an inclined flat plate subject to sudden and ramp heating

The natural convection thermal boundary-layer adjacent to an inclined flat plate subject to sudden heating and a temperature boundary condition which follows a ramp function up until a specified time and then remains constant is investigated. The development of the flow from start-up to a steady state has been described based on scaling analyses and verified by numerical simulations. Different flow regimes based on the Rayleigh number are discussed with numerical results for both boundary conditions. For ramp heating, the boundary-layer flow depends on the comparison of the time at which the ramp heating is completed and the time at which the boundary layer completes its growth. If the ramp time is long compared with the steady-state time, the layer reaches a quasi-steady mode in which the growth of the layer is governed solely by the thermal balance between convection and conduction. On the other hand, if the ramp is completed before the layer becomes steady; the subsequent growth is governed by the balance between buoyancy and inertia, as for the case of instantaneous heating.     

Natural convective heat-transfers from an isothermal horizontal hemispherical cavity

A simplified analytical solution, numerical calculations with the use of the FLUENT/UNS code and experimental studies of laminar free-convection heat transfer from an isothermal hemispherical cavity in unlimited space are presented. The analytical solution is based on an adaptation of the methods used for inclined isothermal plates. In the proposed solution, the control surface of the hemisphere was considered as a small inclined surface. Inclination of this surface was a function of azimuth angle. The results of theoretical analysis, numerical calculation and experimental procedure are presented in the Nusselt and Rayleigh number: relations Nu_D=0.296 Ra_D^1/4, Nu_D=0.340 Ra_D^1/4 and Nu_D=0.316 Ra_D^1/4 respectively. The comparison of theoretical and numerical solutions with experimental results presented in this paper shows good agreement.     

Natural convection heat transfer along a vertical flat plate with a projection in the turbulent region

In the present study, the heat transfer coefficients occurring with a projection in the turbulent region of a vertical flat plate were measured experimentally for various projection heights in the range of 0 to 20 mm. The wall temperature and fluid flow fields were also visualized using a liquid crystal sheet and nylon 12 powder, respectively. The average and local Nusselt numbers reach 1.07 to 1.22 and 1.2 to 1.7 times those for pure turbulent natural convection, respectively. The maximum enhancement rates of heat transfer are attained at a position of 2.3 to 3.3 times the projection height from the upper projection surface toward the downstream, and these positions are in good agreement with those of the reattachment of the fluid flow and with centers of dark red regions in the liquid crystal. On the other hand, the heat transfer coefficients in the just upstream and downstream regions of the projection are small compared with those for no projection. By introducing the nondimensional parameter R, the present experimental results are rearranged quantitatively and effectively. © 2001 Scripta Technica, Heat Trans Asian Res, 30(3): 222–233, 2001     

Combined convection from an isothermal horizontal rod buried in a porous medium

This paper reports the results of a numerical investigation of mixed convection heat transfer from a horizontal rod of circular cross-section that is embedded in a porous medium. The rod temperature is first assumed to be the same as that of the medium and then suddenly increased to a higher constant value. The steady-state problem has been solved by the method of series truncation in combination with a finite-difference scheme for the two flow configurations of parallel and counter-flow regimes. The flow and thermal fields as well as the variations of the average and local heat transfer rates with a wide range of Reynolds number, Grashof number and buoyancy parameter have been examined in detail for a Prandtl number of 0.7. One of the interesting features found is the occurrence of a recirculating flow zone near the upper half surface of the rod in the case of the counter-flow regime. The numerical method and the results presented fill in a gap in the literature on one of the most fundamental problems in the field of mixed convection in porous media.