Coefficient of convection

Figure 9.21. Film coefficients of convection for flow of water through a tube at 289 K...

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] ... 

Realistic boundary conditions are used either during the cure or after cure. Especially during the cooling period of the rubber, the coefficient of convection at the surface of the rubber is calculated for natural convection to vertical surfaces, leading to ... 

The value of the coefficient of convection h depends on the nature of the convection. 

FIGURE 2.3 Scheme of heat transferred by conduction through the rubber sheet of thickness 2L (-L [Pg.31]

Calculation of this heat loss can be made by evaluating the Grashof and Prandtl numbers, and then the Nusselt number, leading to the value of the coefficient of convective transfer h. Then, using the method described in Chapter 2, Section 2.6, it is possible to obtain the heat loss by convective effect. 

Solution of Diffusion in Sheets with Infinite Coefficient of Convection... 

The slope at the beginning of the stage of absorption is oblique in Figures 7.6 and 7.7. This is due to the finite coefficient of convective transfer on the surface of the rubber discs [4-6,19]. It should be recalled that a vertical tangent is associated with an infinite value of the coefficient h in... 

In Equation 9.1, we recall that h is the coefficient of convection in the fluid that bounds the rubber surface. Depending on the value of the stirring rate of the fluid, the convection may be either forced or free. 

When the convection is free, the value of the coefficient of convection h in the fluid is finite, and it depends on the dimensions of the rubber. The convection may be either turbulent or laminar. In both cases, a new dimensionless number is obtained... 

In Equations 10.1 through 10.3, the coefficients of convective heat and mass transfer, h and ho, are evaluated in terms of the appropriate Nusselt and Sherwood numbers for evaporating droplet [22]. 

Blasco and Alvarez [28] and Alvarez and Blasco [29] considered the application of flash drying to moisture removal of fish and soya meals. Heat, momentum, aud mass balauce equations were formulated. The model was solved numerically with appropriate coefficients of convective heat and mass transfer. Dilute phase transport of homogeneous radial mono-size particle distribution was considered. The conveying superheated steam was assumed to be an ideal gas. The initial period for heating the particles, during which condensation takes place, was neglected. Using the film theory [30], the effect of the mass transfer on the heat transfer coefficient... 

Coefficient of convective heat exchange [ W/m K] Ratio of vapor chemical potential to that of liquid [1]... 

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. 

Here i and 2 re the coefficients of convective heat emission in the surrounding medium, Ai and Aj are the coefficients of heat conductivity... 

Thus from this recall of heat transfer, with the similarity between the two processes of heat transfer and mass transfer controlled by diffusion, the necessity of admitting without ambiguity that the course for the mass transfer should emerge as follows diffusion through the thickness of the sheet associated with the convection into the liquid. Finally, the parameters of main importance for a polymer package in contact with a liquid food are the diffusivity and the coefficient of convection. The diffusivity is concentration-dependent, as for example the case of highly plasticised polyvinylchloride where the plasticiser concentration may reach up to 50% of the polymer, but in the present case of the low concentration of the additives distributed in the polymer of the packaging -which are necessary to provide its qualities - the diffusivity can be considered as constant. 

On the other hand, the value of the coefficient of convection largely depends on the rate of stirring of the liquid. Both these parameters, e.g., the diffusivity and coefficient of convection, increase with temperature. The solubility of the diffusing substance in the liquid may appear of interest, in spite of the fact that the concentration of the additives in the polymer package is very low. When the solubility of this substance in the food is much lower than its initial concentration in the polymer, a partition factor intervenes, as its concentration in the food is limited by the solubility. 

The value of the coefficient of convection, h, largely depends on the type of convection being rather low in motionless liquid, it increases notably with the rate of stirring of the liquid up to a very large value when the liquid is strongly stirred. 

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