Forced convection flow

Dry Running Pump Again, no lubrication or dissipation of hear. Remove the heat with a double seal and barrier tank with forced convective flow. 

Gases and Liquids Tending to Gas Gases cannot lubricate the seal fltees. No dissipation of heat. Use a dual seal with forced convective flow. 

J6. Jiji, L. M Incipient boiling and the bubble boundary layer formation over a heated plate for forced convection flow in a pressurized rectangular channel, Ph.D. Thesis, Univ. of Michigan, Ann Arbor, 1962. 

L7. Levy, S., Prediction of the critical heat flux in forced convection flow, GEAP-3961... 

Davis EJ, Anderson GH (1966) The incipience of nucleate boiling in forced convection flow. AlChE 12 774-780... 

Forced-Convection Flow. Heat transfer in pol3rmer processing is often dominated by the uVT flow advectlon terms the "Peclet Number" Pe - pcUL/k can be on the order of 10 -10 due to the polymer s low thermal conductivity. However, the inclusion of the first-order advective term tends to cause instabilities in numerical simulations, and the reader is directed to Reference (7) for a valuable treatment of this subject. Our flow code uses a method known as "streamline upwindlng" to avoid these Instabilities, and this example is intended to illustrate the performance of this feature. 

Heating Forced convection flow Reaction channel (laser-LIGA) width depth length 500 pm 50 pm 9.5 mm... 

Heating Forced convection flow oven electrical heating Number of reaction channels per platelet 34... 

Green, S. J., G. W. Mauer, and A. Weiss, 1962, Burnout and Pressure Drop Studies for Forced Convection Flow of Water Parallel to Rod Bundles, ASME Paper 62-HT-43, Natl. Heat Transfer Conf., Houston, TX. (3)... 

Maulbetsch, J. S., and P. Griffith, 1965, A Study of System-Induced Instabilities in Forced Convection Flows with Subcooled Boiling, MIT Engineering Projects Lab. Rep. 5382-35, Massachusetts Institute of Technology, Cambridge, MA. (5)... 

Forced convection Flow rates, geometry Important... 

Forced Convection. In forced convection, flow is generated by some external means, e.g. a pump, and heat transfer coefficients would be expected to be a function of... 

In all flows involving heat transfer and, therefore, temperature changes, the buoyancy forces arising from the gravitational field will, of course, exist. The term forced convection is only applied to flows in which the effects of these buoyancy forces are negligible. In some flows in which a forced velocity exists, the effects of these buoyancy forces will, however, not be negligible and such flows are termed combined- or mixed free and forced convective flows. The various types of convective heat transfer are illustrated in Fig. 1.5. 

In some forced convective flows it has been found that the Nusselt number is approximately proportional to the square root of the Reynolds number. If, in such a flow, it is found that h has a value of 15 W/m2K when the forced velocity has a magnitude of 5 m/s, find the heat transfer coefficient if the forced velocity is increased to 40 m/s. 

Consider laminar forced convective flow over a flat plate at whose surface the heat transfer rate per unit area, qw is constant. Assuming a Prandtl number of 1, use the integral equation method to derive an expression for the variation of surface temperature. Assume two-dimensional flow. 

As explained in Chapter 1, natural or free convective heat transfer is heat transfer between a surface and a fluid moving over it with the fluid motion caused entirely by the buoyancy forces that arise due to the density changes that result from the temperature variations in the flow, [1] to [5]. Natural convective flows, like all viscous flows, can be either laminar or turbulent as indicated in Fig. 8.1. However, because of the low velocities that usually exist in natural convective flows, laminar natural convective flows occur more frequently in practice than laminar forced convective flows. In this chapter attention will therefore be initially focused on laminar natural convective flows. 

The continuity equation (8.9) and the energy equation (8.12) are identical to those for forced convective flow. The x- and y-momentum equations, i.e., Eqs. (8.10) and (8.11), differ, however, from those for forced convective flow due to the presence of the buoyancy terms. The way in which these terms are derived was discussed in Chapter 1 when considering the application of dimensional analysis to convective heat transfer. In these buoyancy terms, is the angle that the x-axis makes to the vertical as shown in Fig. 8.3. 

It should be noted that, in contrast to forced convective flows, in natural convective flows, due to the temperature-dependent buoyancy forces in the momentum equations, the velocity and temperature fields are interrelated even though the fluid properties are assumed to be constant except for the density change with temperature. 

As in the case with forced convective flows, there are many free convective flows that can be analyzed with sufficient accuracy by adopting the boundary layer assumption. Essentially this boundary layer assumption is that the flow consists of two regions ... 

The boundary layer equations for free convective flow will be deduced using essentially the same approach as was adopted in forced convective flow. Attention will, as discussed above, be restricted to the case of two-dimensional laminar boundary layer flow. Attention will initially be focused on a plane surface that is at an angle, 4>, the vertical as shown in Fig. 8.4. The x-axis is chosen to be parallel to this surface as shown in Fig. 8.4. 

As was done in dealing with forced convective flow over a uniform temperature plate, it is assumed that the velocity and temperature profiles are similar at all values of x, i.e., that ... 

Therefore, the parameter GrIRe2 will be important in determining whether a boundary layer flow can be treated as a forced convective flow or as a free convective flow or as a combined convective flow. 

A consideration of the results given in Fig. 9.7 in conjunction with Eqs. (9.35) and (9.36) shows that the buoyancy forces increase the heat transfer rate and wall shear stress in assisting flow and decrease these quantities in opposing flow. If it is assumed that the effect of the buoyancy forces on the heat transfer rate can be neglected, i.e., that the flow can be assumed to be a purely forced convective flow, when ... 

Because, for flow over a heated surface. r>ulc>x is positive and ST/ y is negative. S will normally be a negative. Hence, in assisting flow, the buoyancy forces will tend to decrease e and e, i.e., to damp the turbulence, and thus to decrease the heat transfer rate below the purely forced convective flow value. However, the buoyancy force in the momentum equation tends to increase thle mean velocity and, therefore, to increase the heat transfer rate. In turbulent assisting flow over a flat plate, this can lead to a Nusselt number variation with Reynolds number that resembles that shown in Fig. 9.22. 

Solution. In forced convective flow the turbulent shear stress is given by ... 

Sparrow, E.M. and Gregg, J.L., Buoyancy Effects in Forced Convection Flow and Heat Transfer , Trans. ASME, J. Appl. Mech.. Sect. E, Vol. 81. pp. 133-135, 1959. 

Eq. (10.34) together with Eqs. (10.11) to (10.14) constitutes the set of equations governing forced convective flow through a porous medium. As discussed in the previous section, the distribution of the velocity components is the same as would exist with potential flow in the same geometrical situation. This potential flow solution gives the values of u, v, and w which can then be used in Eq. (10.34) to give the temperature distribution. 

If the Darcy assumptions are used then with forced convective flow over a surface in a porous medium, because the velocity is not assumed to be 0 at the surface, there is no velocity change induced by viscosity near the surface and there is therefore no velocity boundary layer in the flow over the surface. There will, however, be a region adjacent to the surface in which heat transfer is important and in which there are significant temperature changes in the direction normal to the surface. Under many circumstances, the normal distance over which such significant temperature changes occur is relatively small, i.e., a thermal boundary layer can be assumed to exist around the surface as shown in Fig. 10.9, the ratio of the boundary layer thickness, 67, to the size of the body as measured by some dimension, L, being small [15],[16]. 

In order to illustrate the use of the boundary layer equations, consider, first, two-dimensional forced convection flow over a flat plate that is buried in a porous material in such a way that it is aligned with the fluid flow. The situation being considered is thus as shown in Fig. 10.10. 

Our development in this chapter is primarily analytical in character and is concerned only with forced-convection flow systems. Subsequent chapters will present empirical relations for calculating forced-convection heat transfer and will also treat the subjects of natural convection and boiling and condensation heat transfer. 

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... 

Levy, S.t Prediction of the Critical Heat Flux in Forced Convection Flow, USAEC Rep. 3961, 1962. 

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