Document 3NdQG42XDmJ7emYe0p8wownLa

34 CHAPTER 5 ; L952 Guide ;Tablr 2^-Approximate Unit Conductances fob Thermal Convection < fob Several Flow Systems {Concluded) Case 1 Heat Transfer Equation* and itb'Limits of Application Refer* encb ' Forced Convection NOMENCLATURE AND DIMENSIONS FOR TABLE 2 cp = heat capacity at constant pressure, Btu per (pound) (Fahrenheit degree). D = cylinder diameter, feet. / = subscript denoting film. Heat Transfer 95 g = body force per unit mass, feet per hour per hour. (For static system on earth, g = 32.2 X 3600* feet per hour per hour.) G = 3600 Ump = mass flow per unit'cross-sectional area normal to flow, pounds per (hour) (square foot of flow cross-section). Not = Grashof modulus, dimensionless (Nat = D'p'flCiig/p1). he = average unit thermal convective conductance from the leading edge of surface to the position i, Btu per (hour) (square foot) (Fahrenheit degree). AC1 = local unit thermal convective conductance, at the position x from the leading edge of surface, Btu per (hour) (square foot) (Fahrenheit degree). k = thermal conductivity, Btu per (hour) (square foot) (Fahrenheit degree per foot thickness). - ........ I = a dimension of the system, feet. . m = a subscript denoting mean. P = pressure, atmospheres. Pa = pressure (atmospheric) atmospheres. t = temperature, Fahrenheit. T = temperature, Fahrenheit, absolute. i u -- fluid, velocity, feet per second. ! : ' 1\ V = volume, cubic feet. x = a dimension of the system, feet. p = coefficient of cubical expansion (p =: 1 dV for perfect gases fi = 1/T. At'= 'difference between wall and .fluid, temperatures, Fahrenheit degrees. It = fluid viscosity, pounds,per (hour) (foot). ... .. p = density, pounds per cubic foot. . > = infinity, referring the quantity to a point not directly affected by the phenomenon in question. Table 3. . Radiation Factors or Emissivities, c* For the determination of factor Fe in Equation 3" ' Class Surfaces Fraction of Black-Body . ,Radiation ; r .; At 50-100 F At 1000 F Absorptivity . (FOR : Solar"' Radiation 1 . A 8mall hole in a large beat, sphere, furnace, or enclosure............... ........................................ 0.97 to 6.99 2 -Black non-metallic surfaces such as* asphalt, car- ' 3 Red brick and tile, concrete and stone, rusty steel 4 and iron, dark paints (red. brown, green, etc.).. -0.85 to 0.95 Yellow and buff brick and stone*, firebrick, fire .5 clay.................................................................. 0.85 to 0.95 White or light-cream brick, tile, paintor paper. plaster, whitewash::.......... v.......... ........ 0.85 to 0.95 6 0.90 to 0.95 7 Bright aluminum paint; gilt or bronze paint__ 0.40 to 0.60 8 Dull brass, copper, or aluminum; galvanized steel; polished iron........................................... 0.20 to 0.30 9 10 Polished brass, copper, monel metal................... 0.02 to 0.05 Highly polished aluminum, tin plate, nickel, enromium.................................................. 0.02 to 0.04 - ,, . 0.97 to 0.99. 0.97 to 0.99 ... 0.90 to 6.98 . 0-.85to0.98 :*t 0.75 to 0.90 0.65 to 0.80 0.70 to 0.85 0.50 to 0.70 0.60 to 0.75 0.30 toO.50 Transparent* 0.30 to 0.50 0.30 to 0.50 0.40 to 0.65, 0.05 to 0.15 ;0.30 to 0.50' 0.05 to 0.10 0.10 to 0.40 * Emissivities of other materials may be found in Reference 4. s Reflects about 8 percent. fourth powers of the absolute surface temperatures (7Y -- 7V). The proportionahty factor (oFaFe) may be conveniently separated into three parts (excepting in some problems involving interreflections, where it is not possible to'divide,the product (FaFe) into separate terms): v =?. the Stefan-Boltzmann radiation constant = 1730 X 10-11 Btu per (hour) ' (square foot) (Fahrenheit degree absolute temperature to the fourth power). Fa =' the geometrical factor which is dimensionless and SI. This factor accounts' for the shape and relative position of the two surfaces. The value of Fa = 1 may be used in the cases of large parallel planes, long concentric cylinders or smaller bodies in large enclosures. ,= the emissivity factor which is also' dimensionless and S 1. This factor ac counts for the absorption add emission characteristics of the surfaces for the