Convection flow from horizontal surfaces

Convection flows from horizontal surfaces are very difficult to determine in the same basic way as for point, line, or vertical sources. The reason is that the flows behave in a very unstable way and leave the fiat surface from different positions at different times, partly depending on the total air movement in... 

It was proposed by the author (Stralmann et al., 1988) that thermophores is could be used to suppress particle deposition on wafers during clean room operations in the microelectronics industry. To estimate the effect of an applied temperature gradient on particle deposition, the flow of filtered air over the surface of a horizontal wafer can be approximated by a stagnation flow (Fig. 3.12), For both the plane and axially symmetric stagnation flows, the gas velocity component normal to the surface and the temperature fields depend only on the distance from the surface. In the absence of natural convection, the gas velocity normal to the surface in the neighborhood of the plane stagnation flow is... 

Introduction. For the problem depicted in Fig. 4.44, the heat transfer by pure forced convection would increase monotonically with Reynolds number along the curve shown. The heat transfer by pure natural convection from the same surface for various Ra is denoted by the horizontal lines in the figure. If Re is slowly increased from zero in the real problem, the measured values of Nu would at first follow the natural convection curve, since the superimposed forced convection velocities are too feeble to affect the heat transfer. If the forced convection assists the natural convection, the Nu curve in Fig. 4.44 will break upward along path A at larger Re and approach the pure forced convection curve from above. If the flows are opposed, Nu passes through a minimum along path B in Fig. 4.44 and approaches the forced convection curve from below. Mixed convection occurs when the heat transfer is significantly different from that for either pure natural convection or pure forced convection. 

Vertical motions in the atmosphere result from (1) convection from solar heating of the Earth s surface, (2) convergence or divergence of horizontal flows, (3) horizontal flow over topographic features at the Earth s surface, and (4) buoyancy caused by the release of... 

After the flue gas leaves the combustion chamber, most furnace designs extract further heat from the flue gas in horizontal banks of tubes in a convection section, before the flue gas is vented to the atmosphere. The temperature of the flue gases at the exit of the radiant section is usually in the range 700 to 900°C. The first few rows of tubes at the exit of the radiant section are plain tubes, known as shock tubes or shield tubes. These tubes need to be robust enough to be able to withstand high temperatures and receive significant radiant heat from the radiant section. Heat transfer to the shock tubes is both by radiation and by convection. After the shock tubes, the hot flue gases flow across banks of tubes that usually have extended surfaces to increase the rate of heat transfer to the flue gas. The heat transferred in the radiant section will usually be between 50 and 70% of the total heat transferred. 

A solar collector consists of a horizontal copper tube of outer diameter 5 cm enclosed in a concentric thin glass tube of 9 cm diameter. Water is heated as it flows through the tube, and the annular space between the copper and glass lube is filled with air at 1 atm pressure. During a clear day, the temperatures of the tube surface and the glass cover are measured to be bO C and 32°C, respectively. Determine the rale of heat loss from the collector by natural convection per meter length of the tube. Answer 17.4 W... 

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. 

Horizontal Fluid Layers. A uniform volumetric heat production q " in a horizontal layer bounded above by an isothermal surface and on the sides and bottom by adiabatic surfaces is depicted in Fig. 4.40. For a stationary fluid, the Nusselt number defined in the figure is Nu = 2, and the temperature difference used to construct the Rayleigh number is T0 - 7] = q" L l2k. As Ra increases from zero, the layer remains stable and heat flow is by conduction until a critical Rayleigh number of 1386 is reached [167]. Thereafter convection promotes a monotonic increase in Nu with Ra. For water (2.5 < Pr < 7), and for Ra < 10 2, the heat transfer data of Kulacki et al. [166-168] are accurately represented by... 

Horizontal Flow. For laminar flow over the upper surface of a horizontal heated plate (or over the bottom surface of a cooled plate), the center of the mixed convection regime can again be estimated by equating the forced convection Nusselt number from Eq. 4.154 to that for natural convection from Eq. 4.39c (for detached turbulent convection). This results in... 

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