Convection vertical surfaces

For subcooling, a liquid inventory may be maintained in the bottom end of the shell by means of a weir or a hquid-level-controUer. The subcoohng heat-transfer coefficient is given by the correlations for natural convection on a vertical surface [Eqs. (5-33 ), (5-33Z )], with the pool assumed to be well mixed (isothermal) at the subcooled condensate exit temperature. Pressure drop may be estimated by the shell-side procedure. 

Free convection flows along heated and cooled vertical surfaces and above heat sources, covered in Section 7.5 ... 

FIGUftE 7.58 Convection flows along vertical surfaces. 

Convection flow along vertical surfaces (Fig. 7.63) is also of major interest in industrial ventilation, where large production units with a vertical extension are often present. When the vertical extension of the surface is small, the convection flow is mainly laminar, but at larger extensions the flow is tiir-... 

TABLE 7.20 Characteristics of Convection Flovn along Vertical Surfaces... 

Convection is the heat transfer in the fluid from or to a surface (Fig. 11.28) or within the fluid itself. Convective heat transport from a solid is combined with a conductive heat transfer in the solid itself. We distinguish between free and forced convection. If the fluid flow is generated internally by density differences (buoyancy forces), the heat transfer is termed free convection. Typical examples are the cold down-draft along a cold wall or the thermal plume upward along a warm vertical surface. Forced convection takes place when fluid movement is produced by applied pressure differences due to external means such as a pump. A typical example is the flow in a duct or a pipe. 

Models and Correlations. A multitude of different models and correlations have been proposed for prediction of the heat transfer coefficient at vertical surfaces in fast fluidized beds. To organize the various models in some context, it is helpful to consider the total heat transfer coefficient as comprised of convective contributions from the lean-gas... 

Examine data taken under a natural convection lateral vertical surface spread for a particle board as shown in Figures 8.12(a) and (b) [17]. The correlation of these data based on Equation (8.29) suggests that the critical flux for ignition is... 

The corresponding laminar natural convection burning rate on a vertical surface was... 

E. Combined Forced and Free Laminar Convection from Vertical Surfaces When Both Are in the Same Direction (Assisting Flows)... 

For natural convection along a vertical surface, the following correlations can be used [3] ... 

Laminar and turbulent free convective boundary layers on a vertical surface. 

It will be seen from the results given by the similarity solution that the velocities are very low in natural convective boundary layers in fluids with high Prandtl numbers. In such circumstances, the inertia terms (i.e., the convective terms) in the momentum equation are negligible and the boundary layer momentum equation for a vertical surface effectively is ... 

Martin, B.W., An Appreciation of Advances in Natural Convection Along an Isothermal Vertical Surface , Int. J. Heat Mass Transfer, Vol. 27, pp. 1583-1586, 1984... 

Touloukian, Y.S., Hawkins, G.A., and Jakob, M., Heat Transfer by Free Convection from Heated Vertical Surfaces to Liquids, Trans. ASME, Vol. 70, pp. 13-23. 1948. 

Clausing, A.M., Natural Convection Correlations for Vertical Surfaces Including Influences of Variable Properties", J. Heat Transfer, Vol. 105, No. 1, pp. 138-143, 1983. 

Bejaln, A. and Lage, J.L., The Prandtl Number Effect on the Transition in Natural Convection Along a Vertical Surface , / Heat Transfer. Vol. 112. pp. 787-790, 1990. 

Ramachandran, N., Armaly, B.F., and Chen, T.S., Measurements and Predictions of Laminar Mixed Convection Flow Adjacent to a Vertical Surface . J. Heat Transfer, Vol. 

Siebers, D.L., Schwind, R.G., and Moffat, R.J., Experimental Mixed Convection Heat Transfer from a Large, Vertical Surface in Horizontal Row , Sandia Rept. SAND 83-8225, Sandia National Laboratories, Albuquerque. NM, 1983. 

Hsieh, J.C., Chen, T.S., and Armaly, B.F., Nonsimilarity Solutions for Mixed Convection from Vertical Surfaces in Porous Media Variable Surface Temperature or Heat Rux , Int. J. of Heat and Mass Transfer, Vol. 36, pp. 1485-1493, 1993. 

An enclosure contains helium at a pressure of 1.3 atm and has two vertical heating surfaces, which are maintained at 80 and 20JC, respectively. The vertical surfaces are 40 by 40 cm and are separated by a gap of 2.0 cm. Calculate the free-convection heat transfer between the vertical surfaces. 

Related Calculations. The equations presented above can be simplified for natural-convection heat transfer in air. The usual cases yield the following equations For vertical surfaces,... 

As reported in Ref. , the spread rate of a flame moving up a vertical surface of a sufficiently thick PMMA sheet increases under the effect of an external heat radiation. Depending on the heat radiation intensity and exposure time, various effects on the flame spread rate are observed. Additional heating of the polymer surface by a radiative flux results, first of all, in a decrease of the temperature dilTerence (T — Tp) and, in accordance with Eq. (2.19), in an increase of v. The experimental relationship v (T — To)" at T = 363 °C is close to that predicted by theory. According to Femandez-Pello , an increase of the initial polymer surface temperature, Tp, cause a parallel enhancement of the natural convection in the boundary heat layer and heat radiation by the surface, leading to its partial cooling. Therefore, when the intensity of the external radiative heat flux is low, the flame spread rate increases with time, but only up to a certain constant value. 

Natural Convection over Surfaces 510 Vertical Plates (Fj = constant) 512 Verbeal Plates (4 = constant) 512 Vertical Cylinders 512 Inclined Plates 512 Horizontal Plates 513 Horizontal Cylinders and Spheres 513... 

For the heated vertical plate and horizontal cylinder, the flow results from natural convection. The stagnation configuration is a forced flow. In each case the flow is of the boimdai7 Kiyer type. Simple analytical solutions can be obtained when the thickness of the du.st-free space is much smaller than that of the boundary layer. In this case the gas velocity distribution can be approximated by the first term in an expansion in the distance norroal to the surface. Expressions for the thickness of the dust-free space for a heated vertical surface and a plane stagnation flow are derived below. 

By what factor would the heat loss be reduced by covering the inside wall with aluminum foil Would it be better to put the aluminum foil halfway between the two walls (Correlations for natural convection at vertical surfaces give a film coefficient of 3.9 W/m - C for each wall.)... 

C. C. Chen and R. Eichhom, Natural Convection From a Vertical Surface to a Thermally Stratified Fluid, J. Heat Transfer (98) 446-451,1976. 

A. M. Clausing and S. N. Kempka, The Influences of Property Variations on Natural Convection From Vertical Surfaces, J. Heat Transfer (103) 609-612,1981. 

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