Surface heat flux

Under normal operating conditions the first wall must handle high plasma surface heat fluxes (Table 1), as well as volumetric heat loadings due to the penetrating neutron and electromagnetic radiation. The volumetric heat loading is dependent... 

The heat flux, E, from BLEVEs is in the range 200 to 350 kW/m is much higher than in pool fires because the flame is not smoky. Roberts (1981) and Hymes (1983) estimate the surface heat flux as the radiative fraction of the total heat of combustion according to equation 9.1-32, where E is the surface emitted flux (kW/m ), M is the mass of LPG in the BLEVE (kg) h, is the heat of combustion (kJ/kg), is the maximum fireball diameter (m) f is the radiation fraction, (typically 0.25-0.4). t is the fireball duration (s). The view factor is approximated by equation 9.1-34. where D is the fireball diameter (m), and x is the distance from the sphere center to the target (m). At this point the radiation flux may be calculated (equation 9.1-30). 

Heat transfer for streamline flow over a plane surface—constant surface heat flux... 

The value of the integral in the energy balance (equation 11.55) is again given by equation 11.60 [substituting (6S - 8o) for 0 ]. The heat flux q0 at the surface is now constant, and the right-hand side of equation 11.55 may be expressed as (—qa/Cf,p). Thus, for constant surface heat flux, equation 11.55 becomes ... 

By comparing equations 11.61 and 11.66, it is seen that the local Nusselt number and the heat transfer coefficient are both some 36 per cent higher for a constant surface heat flux as compared with a constant surface temperature. 

In the buffer zone the value of d +/dy+ is twice this value. Obtain an expression for the eddy kinematic viscosity E in terms of the kinematic viscosity (pt/p) and y+. On the assumption that the eddy thermal diffusivity Eh and the eddy kinematic viscosity E are equal, calculate the value of the temperature gradient in a liquid flowing over the surface at y =15 (which lies within the buffer layer) for a surface heat flux of 1000 W/m The liquid has a Prandtl number of 7 and a thermal conductivity of 0.62 W/m K. 

Reactor wall thermal boundary conditions can have a strong effect on the gas flow and thus the deposition. Here, for example, we indicate how cooling the reactor walls can enhance deposition uniformity. We consider the results of three simulations comparing the effects of two different wall boundary conditions. Figure 4 shows how the ratio of the computed susceptor heat flux to the onedimensional heat flux varies with the disk radius for the different conditions (the Nusselt number Nu is a dimensionless surface heat flux). In two cases the reactor walls are held at 300 K (0 = 0), and in one case the walls are insulated ( 0/ r —... 

Investigations Boiling Fluid Heating surface Heat flux 10VBtu/h ft2 Pressure (in. Hg) r (mm) 8. (mm)... 

Gaertner (1965) studied nucleate pool boiling on a horizontal surface in a water pool under atmospheric pressure. He increased the surface heat flux gradually. The vapor structures on the surface progressed from discrete bubbles to vapor columns and vapor mushrooms, and finally to vapor patches (dryout). The observed pictures of vapor mushroom and vapor patch are also sketched in Figure 5.3. 

The critical water flow rate required for extinguishment can be found as well. To match the data more appropriately, select 5.7 g/m2 s for nip corresponding to m v and q l equal to 0 from Figure 9.17. This requires the total flame net surface heat flux to be, for L = 1.6 kJ / g,... 

For arbitrary surface heat fluxes the problem must be solved numerically. However, if the desired quantity is the surface heat flux necessary to produce a specified free-boundary motion, Eqs. (120)-(124) become linear. By a judicious choice of X a closed-form solution can be obtained. This procedure was previously followed by Baer and Ambrosio (Bl) for the semi-infinite slab. With the choice... 

Finally, an over-all energy balance requires that for constant surface heat flux H ... 

Stefan gave an exact solution for the constant-velocity melting of a semi-infinite slab initially at the fusion temperature. This was extended by Pekeris and Slichter (P2) to freezing on a cylinder of arbitrary surface temperature and Kreith and Romie (K6) to constant-velocity melting of cylinders and spheres by a perturbation method, in which the temperature is assumed to be expressible in terms of a convergent series of unknown functions. To make the method clear, consider the freezing of an infinite cylinder of liquid, of radius r0, at constant surface heat flux. For this geometry the heat equation is... 

D Mass diffusivity or thermal H(t), Had) Surface heat flux at front... 

An infinite plate with a temperature Kstrl and radiant efficiency j will transmit, independent of the distance, to a similar plate of frozen product with a temperature Kstr2 and a radiant efficiency e2 an amount of radiation energy. The surface heat flux q is ... 

Priestley C.H.B. and Taylor R.J. (1972). On the assessment of surface heat flux and evapo-transpiration using large scale parameters. Monthly Weather Rev., 100, 81-92. 

Yet another boundary condition encountered in polymer processing is prescribed heat flux. Surface-heat generation via solid-solid friction, as in frictional welding and conveying of solids in screw extmders, is an example. Moreover, certain types of intensive radiation or convective heating that are weak functions of surface temperature can also be treated as a prescribed surface heat-flux boundary condition. Finally, we occasionally encounter the highly nonlinear boundary condition of prescribed surface radiation. The exposure of the surface of an opaque substance to a radiation source at temperature 7 ,-leads to the following heat flux ... 

It can be seen that the surface heat flux is proportional to the ratio of volume to surface area, and to the strength of the internal heat generation. 

A. Two-dimensional flow over a series of geometrically similar bodies having a specified surface temperature was discussed in this chapter. If the surface heat flux rather than the temperature is specified, a dimensional temperature of the form (T - T )l(qwrUk) should be used. Derive the parameters on which the mean surface temperature will depend in this situation. Viscous dissipation effects can be ignored. 

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