Horizontal surfaces, natural convection

Da.skalaki E. Natural convection heat transfer coefficients from vertical and horizontal surfaces for building applications. Energy and Buddings, vol. 20, no.. T, 1994. 

Estimate the heat transfer coefficient for natural convection from a horizontal pipe 0.15 m diameter, with a surface temperature of 400 K to air at 294 K... 

Pera, L. and Gebhart, B., "Natural Convection Boundary Layer Flow Over Horizontal and Slightly Inclined Surfaces , Int. J. Heat Mass Transfer. Vol. 16, p. 1131. 1973. 

A horizontal cylinder having a diameter of 5 cm and an emissivity of 0.5 is placed in a large room, the walls of which are maintained at 35°C. The cylinder loses heat by natural convection with an h of 6.5 W/m2 °C. A sensitive thermocouple placed on the surface of the cylinder measures the temperature as 30°C. What is the temperature of the air in the room ... 

Rotern, Z., and L. Claassen Natural Convection above Unconfined Horizontal Surfaces, J. Fluid Mech., vol. 39, pt. 1, p. 173, 1969. 

Llyod, J. R., and W. R. Moran Natural Convection Adjacent to Horizontal Surface... 

D. R. TURNER hi the case of a vertical electrode, this is certainly true because of the streaming of either more dense or less dense anode products across the face of the electrode. This was one reason why we used horizontal electrodes. In this case anode films leave the surface by a normal diffusion process. You, of course, still have a natural convection effect, but it is reduced considerably. The potential current curve is different for the two cases the horizontal electrode gives a sharp change in the curve at the critical current density while die vertical electrode does not give a sharp change. 

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... 

FIGURE 9-11. Natural convection flows on the upper and lower surfaces of a horizontal hot plale. 

Consider a 0.6 m X 0.6-m thin square plate in a room at 30°C. One side of the plate is maintained at a temperature of 90°C, while the other side is insulated, as shown in Fig. 9-15. Determine the rate of heat transfer from the plate by natural convection if the ptate is (a) vertical, (W horizontal with hot surface facing Up, and (c) horizontal with hot surface facing down. 

A 10-m-long section of a 6-cm diameter horizontal hot-water pipe passes through a large room whose temperature is 2TC. If the temperature and the emissivity of Ihe outer surface of the pipe are 73°C and 0,8, respectively, determine the rale of heal loss from the pipe by (a) natural convection and (b) radiation. 

In a production facility, thin square plates 2 m X 2 m in siie coming out of the oven at 270°C are cooled by blowing ambient air at I8°C horizontally parallel to their surfaces. Determine the air velocity above which the natural convection effects on heat transfer are less than 10 percent and thus are negligible. 

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... 

The components of an electronic system dissipating 180 W are located in a 1.2-m-long horizontal duct whose cross section is 15 cm X 15 cm. Ibe components in llie duct are cooled by forced air, wliich enters at 30°C at a rate of 0.62 m /min and leaves al 38°C. The surfaces of the sheet metal duct are not painted, and thus radiation heat transfer from the outer surfaces is negligible. If the ambient air temperature is 27°C, determine (a) the heat tran.sfer from the outer surfaces of the duct to the ambient air by natural convection and (6) the average temperature of the duct. 

Consider a flat-plate solar collector placed horizontally on the flat roof of a house. The collector is 1.5 m wide and 6 m long, and the average temperature of ihe exposed surface of Ihe collector is 42 C, Determine ihe rale of heal loss from the collector by natural convection duiing a calm day when Ihe ambient air temperature is 8 C. Also, determine the heat loss by radiation by taking Ihe eniissivily of the collector surface to be 0.9 and the effective sky temperature to be - 15 C. /l/iswers 1750 W, 2490 W... 

Recognizing that this is a natural convection problem with hot horizontal surface facing up, the Nussell number and the convection heat transfer coefficients are determined to be... 

Note that the distributions are cardinally different from those over smooth surfaces like in the previous cases. Again, two parts of the profiles have to be discussed. Over the top SCS s level z = h = 6 m, the wind velocity distributions grow monotonically in the case of a strong wind the temperature diminishes, as a rule. Few cases where the wind velocity diminishes over the SCS are characterized by a weak external wind so that the horizontal forced convection is perhaps comparative with the intense natural convective motion rising up from the heated and wetted air layer within SCS. 

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. 

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... 

We have already noted that the general class of flows driven by buoyancy forces that are created because the density is nonuniform is known as natural convection. If we examine the Boussinesq approximation of the Navier-Stokes equations, (12-170), we can see that there are actually two types of natural convection problems. In the first, we assume that a fluid of ambient temperature 71, is heated at a bounding surface to a higher temperature I. This will produce a nonuniform temperature distribution in the contiguous fluid, and thus a nonuniform density distribution too. Let us suppose that the heated surface is everywhere horizontal. Then there is a steady-state solution of (12-170) with u = 0, and the body-force terms balanced by a modification to the hydrostatic pressure distribution, such that... 

Consider the natural convection from a horizontal cylinder rotating with an angular frequency to (Fig. 5P-9). The peripheral surface temperature of the cylinder is Tm and the ambient temperature is To,. The diameter of the cylinder is D. Assuming that the natural convection resulting from rotation and that from gravity can be superimposed, express the Nusselt number in terras of the appropriate dimensionless numbers. 

What heat-transfer coefficient would be expected for natural convection to water at 212°F and 1 atm, outside a 1-in. horizontal pipe with a surface temperature of 213°F Compare with Fig. 13.5 and comment on the difference. 

FIGURE 4.10 Definition sketch for natural convection on a horizontal upward-facing plate of arbitrary planform. Only the top heated surface of area A is heated. 

Circular Isothermal Fins on a Horizontal Tube. Tsubouchi and Masuda [269] measured the heat transfer by natural convection in air from circular fins attached to circular tubes, as in the configuration shown in Fig. 4.23/ Correlations for the heat transfer from the tips of the fins (see the figure for definition), and from the cylinder plus vertical fin surfaces, were reported separately. 

Hauf and Grigull [133-135] precisely measured the natural convection heat transfer inside a tube following a step change in the temperature of a fluid in forced convection over the outside of the tube. In this case the heat transfer coefficient on the outer surface is constant throughout the transient, and the heat capacity of the wall plays an important role. Cheng et al. [50] have studied conditions leading to the formation of ice inside horizontal tubes (without throughflow), also with uniform heat transfer coefficient between the outside boundary and a cold environment. 

When both bottom and top surfaces are maintained at constant temperatures and there is internal generation, there is a superposition of the horizontal layer problem discussed in the section on natural convection within enclosures and the internal generation problem previously described. These are characterized by the external Rayleigh number defined in the section on natural convection within enclosures and the internal Rayleigh number defined in Fig. 4.40a. The dependence of the layer stability on these parameters has been discussed by Ning et al. [208]. The heat transfer at the top and bottom surfaces has been estimated for these conditions by Baker et al. [13], Suo-Anttila and Catton [276], and Cheung [51],... 

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. 

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... 

R. C. Birkebak and A. Abdulkadir, Heat Transfer by Natural Convection From the Lower Side of Finite Horizontal, Heated Surface, Proc. Int. Heat Trans. Conf, Elsevier Publishing, Amsterdam, paper NC 2.2,1970. 

T. Fujii, H. Honda, and I. Morioka, A Theoretical Study of Natural Convection Heat Transfer From Downward-Facing Horizontal Surfaces With Uniform Heat Flux, Int. J. Heat Mass Transfer (16) 611-627,1973. 

J. H. Lienhard V, J. R. Lloyd, and W. R. Moran, Natural Convection Adjacent to Horizontal Surface of Various Planforms, J. Heat Transfer (96) 443-447,1974. 

At the turn of the century, Henri Benard, a young French physicist, published the first truly systematic study of natural convection in a horizontal fluid layer (B4, B5, B6). In a horizontal liquid layer heated from below B6nard, sought to measure and to define the most stable steady-state convection currents prevailing under given conditions. He utilized liquid layers only a few millimeters in thickness, initially in an apparatus giving a free upper surface, and of considerable horizontal extent (about 20 cm) so that edge effects could not influence the form of the convection pattern. For these studies. 

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