Document XOw6XwEp0yjqBN1O7rz2x83oK

94 CHAPTER S 1958: Gmd Table 2. Approximate Unit Conductances fob Thermal Convection, fob^ Y Several Flow Systems (Concluded) Heat Transfer Equation* and its Limits oi Application Fobced Convection 9 Same as Case 7. General Equation (Laminar Flow) -"(?)"()" For (y) < 500.000 irmf mT Same as Case 7. he (*) " (up)wEquationfor Air (Laminar Flow) Ip, = 0.0562(70" For (2?) < 500.000 3"|jg i i 1 Free Convection ii Equation for Air /AI\0M 10* < JVGr < 10* ' 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). jVGr =, Grashof modulus, dimensionless (Nar = D'ptp&lg/fft). hc -- average unit thermal convective conductance from the leading edge of surface to the position x, Btu per (hour) (square foot) (Fahrenheit degree). hex = 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. Pe = pressure (atmospheric) atmospheres. t = temperature, Fahrenheit. T = temperature, Fahrenheit, absolute. u -- fluid velocity, feet per second. V = volume, cubic feet. y x = a dimension of the system, feet. P = coefficient of cubical expansion (ft = y1 (^dfV)p)i for perfect gases p = 1/T. M = difference between wall and fluid temperatures, Fahrenheit degrees.. p = 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. I Free convection past a heated horizontal cylinder. T1 hj I Free convection past a single vertical sur-| face. 7777-7777-' U--- i | Free convection past a heated horizontal surface (face up). (...,, 'V/' Equation for Air * om= ()"" (*y- !0> < WGr < 10> >'=om(B'u(7y'' 10* < VGr < 10 Equation for Air C/ P\#M *- 8 k) \t) 10* < Nqt < 10' Equation for Air * - -239 (/?P;)\,M /vAtI)V-" HP < NGi < 10' Free convection past a heated horizontal I:surfaoe (face do.wn). * Fluid properties should be evaluated at the arithmetic mean fluid temperature, tt -- (t surface -f- tfloig divided by 2. -Si* b These expressions are suitable approximations to longitudinal flow in other than right circular cylinders speroctvioidnesd, tthheehhyyddrraauulilcicddiaiammeeteterrisiseeqmuapllotoyefdouarsttihmeescothneducirtodeism-eeencstiioonnalpaarreaamdeitveidr.edFboyr nthoen-wciercttueldarpeHgB eter. ' S 0 For low rates of heat transfer by free convection the exponent decreases towards zero, and for higjgB rates, increases towards 0.33. The above equations employing an exponent equal to 0.25 are applicable ins* intermediate range indicated. 2n NOMENCLATURE AND DIMENSIONS FOR TABLE 2 cp = heat capacity at constant pressure, Btu per (pound) (Fahrenheit degrCsS D = cylinder diameter, feet. g| / = subscript denoting film. Table 3. Radiation Factors or Emissivities, t* For the determination of factor Fe in Equation S Class Surfaces Fraction of Black-Body Radiation At 50-100F At 1000 F Absorptivity FOB Solar Radiation I A small hole in a large box, sphere, furnace, or I enclosure.................................................................... | Black non-metallic surfaces such as asphalt, car 0.97 to 0.99 0.97 to 0.99 0.97 to 0.99 bon, slate, paint, paper....................................... I Red brick and tile, concrete and stone, rusty steel 0.90 to 0.98 0.90 to 0.98 0.85 to 0.98 and iron, dark paints (red, brown, green,etc.).. Yellow and buff brick and stone,' firebrick, fire 0.85 to 0.95 clay............................................................................. | White or light-cream brick, tile, paint or paper, 0.85 to 0.95 0.75 to 0.90 0.70 to 0.85 0.65 to 0.80 0.50 to 0.70 plaster, whitewash...................................f............... Window glass................................................................ I Bright aluminum paint; gilt or bronze paint.... 1 Dull brass, copper, or aluminum; galvanized 0.85 to 0.95 0.90 to 0.95 0.40 to 0.60 0.60 to 0.75 0.30 to 0.50 Transparent* 0.30 to 0.50 9 10 steel; polished iron.................................................. Polished brass, copper, monel metal....................... 0.20 to 0.30 0.30 to 0.50 i Highly polished aluminum, tin plate, nickel, 0.02 to 0.05 0.05 to 0.15 | chromium............... 0.02 to 0.04 0.05 to 0.10 0.40 to 0.65 0.30 to 0.50 0.10 to 0.40 ' Emissivities of other materials may be found in Reference 4. * Reflects about 8 percent. ifoouurrtthn powers of the absolute surface temperatures (7Y -- 2Y)- The pro portionality 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): a = the Stefan-Boltzmann radiation constant = 1730 X 10-'* Btu per (hour) (square foot) (Fahrenheit degree absolute temperature to the fourth power). Fa = the geometrical factor which is dimensionless and S 1. 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. , fE - the emissivity factor which is also dimensionless and 1. this lactor ac counts for the absorption and emission characteristics of the surfaces for the