Forced convection defined

Heat transfer (e.g., free or forced convection) Defined as boundary condition Correct heat transfer coefficients must be used (analytically calculated)... 

First the dimensionless characteristics such as Re and Pr in forced convection, or Gr and Pr in free convection, have to be determined. Depending on the range of validity of the equations, an appropriate correlation is chosen and the Nu value calculated. The equation defining the Nusselt number is... 

The kind of convective heat transfer—forced convection or natural (at floor, wall, or ceiling)—must be considered and taken into account by selecting appropriate values for the convective heat transfer coefficient see Eq. (11.14)). Thus, the heat transfer coefficient implicitly assumes the flow situation at the surface. Normally, coefficients for convective heat transfer are considered as a preset constant parameter (the coefficient may be defined as variable, however, depending on other parameters). Therefore, the selection of appropriate values is crucial. Values for heat transfer coefficients can be found in several references a comprehensive summary is given in Daskalaki. ... 

The wall heat flux qmi cannot be evaluated as in Section II,B. Numerous experimental studies on heat transfer in this two-phase forced-convection region have been carried out, and the results of these investigations are usually presented in the form of a correlation for the wall heat-transfer coefficient hmi, which is defined as in Eq. (32b). Most of these correlations fit one of two generalized forms. The first is... 

There are very many situations in which well-defined patterns of convection can be established, and analytical expressions for vf derived. Such situations usually involve forced convection, in which the movement of the liquid is determined by rotation, agitation, forced flow over a flat surface, etc. Once the functional form of vf is known, solutions for c as a function of x are sought so that values of the current can be found and compared with those obtained experimentally. 

The problem of burn-out prediction is a difficult one, and one on which a great deal of experimental work is being carried out, particularly in connection with nuclear-reactor development. Much of the earlier literature is rather confused, due to the fact that the mechanics of the burn-out were not carefully defined. Silvestri (S8) has discussed the definitions applicable to burn-out heat flux. It appears possible to define two distinctly different kinds of burn-out, one due to a transition from nucleate to film boiling, and one occurring at the liquid deficient point of the forced-convection region. The present discussion treats only the latter type of burn-out fluxes. The burn-out point in this instance is usually determined by the sudden rise in wall temperature and the corresponding drop in heat flux and heat-transfer coefficient which occur at high qualities. 

In a hydrodynamically free system the flow of solution may be induced by the boundary conditions, as for example when a solution is fed forcibly into an electrodialysis (ED) cell. This type of flow is known as forced convection. The flow may also result from the action of the volume force entering the right-hand side of (1.6a). This is the so-called natural convection, either gravitational, if it results from the component defined by (1.6c), or electroconvection, if it results from the action of the electric force defined by (1.6d). In most practical situations the dimensionless Peclet number Pe, defined by (1.11b), is large. Accordingly, we distinguish between the bulk of the fluid where the solute transport is entirely dominated by convection, and the boundary diffusion layer, where the transport is electro-diffusion-dominated. Sometimes, as a crude qualitative model, the diffusion layer is replaced by a motionless unstirred layer (the Nemst film) with electrodiffusion assumed to be the only transport mechanism in it. The thickness of the unstirred layer is evaluated as the Peclet number-dependent thickness of the diffusion boundary layer. 

For the prediction of the Nusselt number in ducts of non-circular-cross section (like concentric annular ducts) the same equations can be used for forced convection in the turbulent regime. In this case, the inside diameter should be replaced in evaluating Nu, Re, and (D/L) by the hydraulic diameter defined as,... 

To demonstrate Pawlowski s matrix transformation technique, an example will be used in which a forced convection problem, where a fluid with a viscosity p, a density p, a specific heat Cp and a thermal conductivity k, is forced past a surface with a characteristic size D at an average speed u. The temperature difference between the fluid and the surface is described by AT = Tf — Ts and the resulting heat transfer coefficient is defined by h. 

The last of these is the impedance which has been considered throughout this chapter. We now consider forced convection. For low frequencies the diffusion layer thickness due to the a.c. perturbation is similar to that of the d.c. diffusion layer in these cases convection effects will be apparent in the impedance expressions. For the rotating disc electrode these frequencies are lower than 40 Hz33. For higher frequencies where the two diffusion layers are of quite different thicknesses, the advantage of hydrodynamic electrodes is that transport is well defined with time, as occurs with linear sweep voltammetry. 

This defines die conditions under which the flow can be assumed to be purely forced convective. 

When a plate on which condensation occurs is sufficiently large or there is a sufficient amount of condensate flow, turbulence may appear in the condensate film. This turbulence results in higher heat-transfer rates. As in forced-convection flow problems, the criterion for determining whether the flow is laminar or turbulent is the Reynolds number, and for the condensation system it is defined as... 

Tubular electrode — Working electrode design employing a tube of the electrode material to be studied with (i.e. secondary battery) the electrolyte solution flowing through the orifice of the tube. This way well-defined forced convection of solution can be established. -> Mass transport processes can be treated mathematically. 

Flow In Round Tubes In addition to the Nusselt (NuD = hD/k) and Prandtl (Pr = v/a) numbers introduced above, the key dimensionless parameter for forced convection in round tubes of diameter D is the Reynolds number Re = (.7 ) u where G is the mass velocity G = m/Ac and Ac is the cross-sectional area Ac = kD2I4. For internal flow in a tube or duct, the heat-transfer coefficient is defined as... 

As was the case in forced convection involving a single phase, Iteat transfer ill condensation also depends on whether the condensate flow is laminar or turbulent. Again the criterion for the flow regime is provided by the Reynolds number, which is defined as... 

Experimental gas-solid mass transfer data are presented for the well defined supercritical CO 2-naphthalene system at 10-200 atm and 35 C. These data are compared with low pressure gas-solid and liquid-solid systems. It has been found that both natural and forced convection are important under these conditions and that mass transfer rates at near-critical conditions are higher than at lower or higher pressure. 

Figure 11-9. Forced convection for large Sc b is defined by Eq. 11-128 (source Ref. 4).

On the basis of data available up to 1964, Metais and Eckert [190] established the forced convection boundary of the mixed convection regime, and their results are presented in Fig. 4.50. The line was drawn where natural convection was thought to alter the heat transfer from that for pure forced convection by 10 percent. Figure 4.49 defines the nomenclature for this problem. 

In convective heat transport heat transfer takes place via a fluid flowing on a wall. If the fluid flows solely as the result of buoyancy forces, then this is known as free convection, as opposed to forced convection, for example, by pumps or compressors. Since effective heat exchange is only possible in the case of turbulent flow in or around the tube, the heat transfer can be described by a two-film model, in which the wall is regarded as a hydrodynamic boundary layer of thickness 8 and a heat-transfer coefficient a is defined (Equation 2.3.1-10) ... 

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