Plates natural convection over

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... 

The velocity and temperature profiles for natural convection over a vertical hot plate are also shown in Fig. 9 -6. Note lhat as in forced convection, the thickness df the boundary layer increases in the flow direction. Unlike forced convection, however, the fluid velocity is zero at the outer edge of the velocity boundary layer as well as at the surface of the plate. This is expected since the fluid beyond the boundary layer is motionless. Thus, the fluid velocity increases with distance from the surface, reaches a maximum, and gradually decreases to zero at a distance sufflciently far from (be surface. At the. surface, the fluid temperature is equal to the plate temperature, and gradually decreases to the temperature of the surrounding fluid at a distance sufficiently far from the surface, as shown in the figure. In the case of cold surfaces, the shape of the velocity and temperature profiles remains the same but their direction is reversed. 

Answer by Author Subsequent to pressurization the initial turbulence caused by the boiling decayed, as was evidenced by a time-wise decrease in the heat transfer coefficient at all longitudinal positions. These heat transfer coefficients were for nonboiling conditions and they decreased from a maximum value to a minimum, as indicated on our curves. The ratio of this minimum value of the nonboiling heat transfer coefficient to that which one would predict from a Nusselt — Grashof — Prandtl number relationship for natural convection over vertical flat plates for turbulent flow has the following values 2 in. above the bottom the ratio is 0.49 6 in. above the bottom the ratio is 0.59 10 in. above the bottom the ratio is 0.77. 

The details of natural convective flows over surfaces other than flat plates have only recently been studied experimentally (A7, Jl, P3, SI2). We consider a heated sphere in an infinite, stagnant medium. Flow is directed toward the surface over the bottom hemisphere and away from the surface over the top hemisphere with a stagnation point at each pole (P3, S12). The lower pole is considered the forward stagnation point. 

Transition to turbulence in the natural convective flow over a vertical plate. 

This analysis is based on the use of the momentum and energy integral equations which for natural convective flow over a vertical plate are ... 

Solution. The following integrals arise in the approximate solution for turbulent natural convective boundary layer flow over a flat plate discussed above ... 

A 0.3-m vertical plate is maintained at a surface temperature of 65°C imd is exposed to stagnant air at a temperature of 15°C and standard ambient pressure. Compare the natural convective heat transfer rate from this plate w ith that which would result from forcing air over the plate at a velocity equal to the maximum velocity that occurs in the natural convective boundary layer. 

The side of i small laboratory furnace can be idealized as a vertical plate 0.6 m high and 2.5 m wide. The furnace sides are at 40°C and the surrounding air is at 25°C. If air is blown vertically over the side of the fur.iace, estimate the lowest forced air velocity that would cause the heat-transfer coefficient to depart noticeable from its natural convection value. 

Natural convective flow over a vertical plate. 

A heat pump system utilizes a heat exchanger buried in water-saturated soil as a heat source. The heat exchanger basically consists of a series of vertical plates with height of 30 cm and a width of 10 cm. These plates are effective Is at a uniform temperature of 5°C. The soil can be assumed to have a permeability of 10 0 nr and apparent thermal conductivity of 0.1 W/m-K. The temperature of the saturated soil far from the heat exchanger is 30°C. Assuming natural convective flow and that there is no interference between the flows over the individual plates, find the mean heat transfer rate to a plate. 

Discuss how the analysis of natural convective flow over a vertical flat plate in a saturated porous medium must be modified if there is a uniform heat flux rather than a uniform temperature at the surface. 

The set of three partial differential equations (the continuity, momentum, and the energy equations) iliai govern natural convection flow over vertical isothermal plates can be reduced to a set of two ordinarj nonlinear differential equations by the introduction of a similarity variable. But the resulting equations must still be solved numerically [Ostrach (1953)]. Interested readers arc referred to advanced books on the topic for detailed discussions [e.g., Kays and Crawford (1993)],... 

In Chapter 5, we learned the foundations of convection. Integrating the governing equations for laminar boundary layers, we obtained expressions for the heat transfer associated with forced convection over a horizontal plate and natural convection about a vertical plate. We also found analytically, as well as by the analogy between heat and momentum, that the thermal and momentum characteristics of laminar flow over a flat plate are related 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... 

A difference which might be anticipated, so far as size is concerned, is the response of the system as the function of height above the bottom. It appears from our results that in our experimental system we still are experiencing the influence of the bottom corner. In tanks of larger size, for heights to radius ratios equivalent to ours, it is entirely possible that one would be removed from the influence of the bottom corner, which would result in a free-convection process more similar to that of turbulent flow over vertical flat plates in natural convection. 

It is not certain whether h2 is significant at this point. Using Eqs. 5.123 and 5.124 for forced convection heat transfer involving flow over a fiat plate and the conditions given in the problem, we estimate h2 = 2.8 W/m K, which is quite small. For free or natural convection from a vertical flat plate we use Eq. 5.132 to find = 3.87 W/m K, which again is small. Hence, we consider only the heat transfer at the film-drum surface and Eq. 5.190 becomes... 

A booth should be of sufficient size to contain any naturally occurring emissions and so minimize escape via the open face. An air velocity of 0.56 m/s is required over the whole open face a higher velocity is needed if there is significant air movement within the booth or to cope with convection currents. Booths should be deep enough to contain eddies at the rear corners baffle plates or multiple offtakes may be necessary with shallow booths. 

Numerically predicted variation of Nusselt number variation with Reynolds number in turbulent assisting mixed convective flow over a vertical plate. (Based on results obtained by Patel K., Armaly B.F., and Chen T.S., Transition from Turbulent Natural to Turbulent Forced Convection Adjacent to an Isothermal Vertical Plate , ASME HTD, Vol. 324, pp. 51-56, 1996. With permission.)... 

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