Convection flow along vertical surfaces

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-... 

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

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. 

Use the computer program for two-dimensional laminar free convective flow to And the Nusselt number variation along a vertical plate whose surface temperature varies in such a way that Tw - r. is equal to 10°C over the lower half of the plate and equal to 30°C over the upper half of the plate. 

Consider a vertical hot flat plate immersed in a quiescent fluid body. We assume the natural convection flow to be steady, laminar, and two-dimensional, and the fluid to be Newtonian with constant properties, including density, with one exception the density difference p — is to be considered since it is this density difference between the inside and the outside of the boundary layer that gives rise to buoyancy force and sustains flow. (This is known as the Boussines.q approximation.) We take the upward direction along the plate to be X, and the direction normal to surface to be y, as shown in Fig. 9-6. Therefore, gravelly acts in the —.t-direclion. Noting that the flow is steady and two-dimensional, the.t- andy-compoijents of velocity within boundary layer are II - u(x, y) and v — t/(.Y, y), respectively. 

Plot the free-convection boundary-layer thickness along a 0.3-m high vertical plate which is maintained at a uniform surface temperature of 50CC and exposed to stagnant air at ambient pressure and a temperature of 10°C. Assume the flow remains laminar. 

Air at a temperature of 10°C flows upward at a velocity of 0.8 m/s over a wide vertical 15-cm high flat plate which is maintained at a uniform surface temperature of 50°C. Plot the variation of the local heat transfer rate with distance along the plate from the leading edge. Also show the variations that would exist in purely forced and purely free convective now. 

Air at a temperature of 10°C flows vertically upwards over a 0.1 m high vertical plate whose surface temperature increases linearly from 10°C to 40CC with distance along the plate. Numerically determine how the heat transfer rate varies along the plate for various forced velocities between that which gives effectively forced convection and that which gives effectively free convection. Assume two-dimensional flow. 

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