Coefficient of convection heat transfer

CS = total-exchange area between gas and surface subscript R indicates allowance for radiatively adiabatic surfaces. h = coefficient of convective heat transfer. 

On introducing the coefficient of convective heat transfer by the formula a = x/6, we finally obtain... 

Film condensation in a vertical tube. To present a theoretical description of film condensation in tubes is much more difficult, since there may arise a strong dynamic interaction between the moving vapor and the flowing condensate film. If the direction of the vapor motion coincides with the direction of the condensate motion due to gravity, then, owing to viscous friction on the phase boundary, the velocity of the film flow increases, its thickness decreases, and the coefficient of convective heat transfer also increases. If the direction of the vapor motion is opposite to that of the condensate flow, then we have the opposite situation. If the vapor velocity increases, then the film may partially separate from the wall and convective heat transfer can increase sharply. 

For the calculation of the local coefficient of convective heat transfer a when the condensation takes place in tubes provided that flow in the condensate film is laminar, the following relation based on experimental data was proposed in [200] ... 

Obviously the coefficient of convective heat transfer at the outer surface would have to amount to oia = 26.69 W/(m K) in order to fulfil the boundary conditions... 

Results of a comparative calculation are shown in Fig. 4.24. It is based on Eq. (4.80) using the boundary condition of Eq. (4.81) with a = 6.9 W/(m K) and shows that the surface temperature becomes much higher, viz. 138.3 °C, if the heat is to be removed with realistic coefficients of convective heat transfer. This is problematic with respect to the protection of employees and efficient use of energy. 

Thus the effect of the two main parameters of interest, e.g., the thickness of the film and the value of the coefficient of convective heat transfer, is examined intensively for the process of heat transfer and consequently for the process of mass transfer of the diffusing substance. 

Simultaneous Effect of the Thickness of the Film and the Coefficient of Convective Heat Transfer... 

The coefficient of convection heat transfer, h, depends on the Nusselt number, that is, properties of the coolant, geometry of the passages, and flow characteristics. 

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