522 Modelling of Combined Forced-and Natural-Convection Heat Trasfer along Bottom Surface of LNG Tank
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Combined forced and natural convection heat transfer from upward-faceing horizontal heated flat plate is nodeled by phenomenological method modifying natural convection heat transfer model at high layer. In this model, mixed or pure natural convection heat transfer coefficient is estimated as averaged one. In the model new boundary layer is assumed to start its growth from a position that previous layer finishes by releasing hot fluid plume "Thermal", and vanishes again at a point where a certain critical condition is satisfied. This theory can explain the transitional phenomena from pure forced convection to pure natural convection through mixed convection. The varidity of the theory is confirmed by experiment using upward-faceing horizontal heated flat plate.Keywords:
Forced convection
Film temperature
Rayleigh–Bénard convection
This paper completes the description of geometry optimization in stacks of parallel plates that generate heat. The spacing between plates, or the number of plates in a fixed volume, has been maximized in two limits: pure natural convection and pure forced convection. In this paper, the in-between regime of mixed convection is modeled numerically. After simulating the flow and temperature fields in configurations with a variety of spacings, this paper reports the optimal spacings and the dimensionless groups that govern them (Rayleigh number, pressure drop number, mixed convection ratio). It shows that the numerical results match the results in the limits of natural convection and forced convection. The paper constructs a correlation that bridges the gap between the two limits, and provides a single formula for optimal spacings covering the entire domain, from natural convection to forced convection.
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Convection phenomena in the floating zone melting for the purification of the materials and single crystal growth were studied by ring heater method. Two kinds of NaNO₃ rods, 4㎜ and 6㎜ in diameter, were used in order to simulate the convection phenomena under the micro-gravity condition. The experiments on the effect of coupling between natural convection and forced convection, and between Marangoni convection and forced convection were conducted to find the effect of Marangoni convection in the floating zone. The forced convection induced by rotating the rod of 13rpm suppressed the natural convection and the shape of solid/liquid interface was changed from convex to planar when the counter-rotation was applied in the experiment of coupling between natural convection and forced convection. But it was found that the forced convection induced by single rotation of 73 rpm could not suppress the Marangoni convection completely in the experiment of coupling between Marangoni convection and forced convection. It can be suggested that the flow and the temperature fields in the floating zone method by electron beam where large temperature gradient is expected should be controlled by the application of forced convection.
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This paper presents experimental and numerical studies for the case of turbulent forced and mixed convection flow of water through narrow vertical rectangular channel. The channel is composed of two parallel plates which are heated at a uniform heat flux, whereas, the other two sides of the channel are thermally insulated. The plates are of 64 mm in width, 800 mm in height, and separated from each other at a narrow gap of 2.7 mm. Qualitative results are presented for the normalized temperature and velocity profiles in the transverse direction with a comparison between the forced and mixed convection flow for both the cases of upward and downward flow directions. The effect of the axial locations and the parameter Gr/Re on the variation of the normalized temperature profiles in the transverse direction for both the regions of forced and mixed convection and for both of the upward and downward flow directions are obtained. The normalized velocity profiles in the transverse directions are also determined at different inlet velocity and heat fluxes for the previous cases. It is found that the normalized Nusselt number is greater than one in the mixed convection region for both the cases of upward and downward flow and correlated well with the dimension-less parameter Z for both of the forced and mixed convection regions. The temperature profiles increase with increasing the axial location along the flow direction or the parameter Gr/Re for both of the forced and mixed convection regions, but this increase is more pronounced in the case of the mixed convection flow. For the forced convection region, the velocity profile depends only on Re with no difference between the upward and downward flow directions. Whereas, for the case of mixed convection flow, the velocity profile depends on the parameter Gr/Re with a main difference between upward and downward flow. These results are of great importance for any research reactor using plate type fuel elements or for any engineering application in which mixed convection flow through rectangular channel is encountered.
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Effects of buoyancy forces on a forced-convection laminar flow as well as the forced-convection effects on a purely free-convection laminar flow about a vertical flat plate are analyzed. Solutions are obtained by expanding the stream function and the temperature function into series in terms of the parameter Grx/Rex2 when approaching from the forced-convection side, whereas for the free-convection approach in terms of the parameter (Grx/Rex2)−1/2. Velocity and temperature profiles, shear stress, and heat transfer are given for various values of the governing parameters.
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Heat transfer by combined free and forced convection from a heated tube to a transverse air stream has been experimentally studied over a wide range of Grashof and Reynolds numbers. The data obtained have been correlated with Gr/Re2.5 as the correlating parameter. Criteria for transition from free convection to combined convection and from combined convection to forced convection have also been obtained.
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This paper deals with the effects of a forced convection on the heat transfer by turbulent natural convection, when both convections occur in the same direction on a vertical flat plate, and the relations among Nusselt's, Grashoff's and Reynolds' number are theoretically analyzed. As the result of this study, we found that, in some cases, the coefficient of heat transfer by combined natural and forced convections is several times as large as that by the natural convection or the forced convection alone, and the natural convection can not be neglected when the velocity of forced convection is comparatively small and the temperature differences between the plate and the fluid is large.
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Forced convection
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