Document JJwqbXG1LKoggqKrVpJEb0yLB

94 Table 2. Approximate^ nit CHAPTER 5 1954 Guide Thermal Convection fob (Concluded) Case System Heat Transfer Equation* its Limits op Appucation Forced Convection Refer ence Same as Case 7. General Equation (Laminar Flow) '*?--(?)"()*" Same as Case 7. For (y) < 500.000 be leverage) ~ 2A* Jar Air (Laminar Flow) hez = 0.0562(7-1)(-U^-Pf" For (~) < 000.000 fQttotton for Air /R y. /ATA"*5 0o2n (jr) 10* < #Gr < 10' Free convection past a heated horizontal cylinder. ________ T1 1*3 Free con^o^pSa a single vertical sur face.- Jk13 r///`/'7`/T U-- i --m Free convection past a heated horizontal 1 --*isurface (face up). f r f f" Equation for Air K/ pTJv" (Vori)\` a I0> < We, < 10' / p \* M he * 0.420 uy v*y 10* < ^ Gr < l0,D Equation for Air . =8 (rS * (t) 10' < ^Gr < 10? Equation for Air 10* < ^Gr < l0? Free convection past a heated horizontal * F|luidsuprrfaocpeer(tfieasceshdoouwldn)b.e evaluated at the arithI metic mean fluid temperature, fl = (t mrfo* + t fluid) b These expressions are suitable approximations to longitudinal flow in other than rmht circular cylinders, dprivoivdideeddbtyhe2.hydraulic diameter is employed as the conduit dimension parameter. For non-circular crosssections, the hydraulic diameter is equal to four times the cross-sectional area divided by tbe wetted perim equal to four times tne crow-w^v*^.---------- For low rates of heat transfer by free convec*ti:o--n ttVhiea ex*npoonneenntt ddeeccrreeaasseess ttoowwaarrddss zzeerroo,, aanndd ffoorr higher eter. ..j.rt v* T,h* above equations employing an exponentequal to 0**.2rt5e -a-re- -a"p*-p.)l*i/cabK)le in the c For low rates o rates, increases towards0.33. me intermediaite range mindicatead. NOMENCLATURE AND DIMENSIONS FOR TABLE 2 cP = heat capacity at constant pressure, Btu per (pound) (Fahrenheit degree). D = cylinder diameter, feet. f = 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 Uap = mass flow per unit cross-sectional area normal to flow, pounds per (hour) (square foot of flow cross-section). No, = Grashof modulus, dimensionless. (Nq, = D*/>*f>&ig/i.*). ho = 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. P,, = pressure (atmospheric) atmospheres. t = temperature, Fahrenheit. T = temperature, Fahrenheit, absolute. u = fluid velocity, feet per second. V = volume, cubic feet. x = a dimension of the system, feet. P = coefficient of cubical expansion (0 = 1 dV for perfect gases 0 = 1/T. &t = difference between wall and fluid temperatures, Fahrenheit degrees. Ii = 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, e* For the determination of factor Ft: in Equation 3 Class Surfaces Fraction or Black-Body Radiation At 50-100 F At 1000 F Absobptivitt - YOB SOUAB . Radiation 1 A small bole in a large box, sphere, furnace, or enclosure................................................................... 0.97 to 0.99 0.97 to 0.99 0.97 to 0.99 2 Black non-metailic surfaces such.as asphalt, car bon, slate, paint, paper.......................................... 0.90 to 0.98 0.90 to 0.98 0.85 to 0.98 3 Red brick and tile, concrete and stone, rusty steel and iron, dark paints (red, brown, green, etc.).. 0.85 to 0.95 0.75 to 0.90 0.65 to 0.80 4 Yellow and buff brick and stone, firebrick, fire clay.............................................................................. 0.85 to 0.95 0.70 to 0.85 0.50 to 0.70 5 White or light-cream brick, tile, paint or paper, 6 plaster, whitewash................................................... 0.85 to 0.95 0.60 to 0.75 0.30 to 0.50 Transparent* 7 Bright aluminum paint; gilt or bronze paint___ 0.40 to 0.60 0.30 to 0.50 8 Dull brass, copper, or aluminum; galvanized steel; polished iron................................................... 0.20 to 0.30 0.30 to0.50 0.40 to 0.65 10 Polished brass, copper, monel metal...................... 0.02 to 0.05 0.05 to.0.15 Highly polished aluminum, tin plate, nickel, 0.30 to 0.50 chromium.................................................... ........... 0.02 to 0.04 0.05 to 0.10 0.10 to 0.40 Emissivities o! other materials may be found in Reference 4. * Reflects about 8 percent. fourth powers of the absolute surface temperatures (TV -- TV). The pro portionality factor (gFaFe) 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): 9 -- 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 1. This factor accounts for tne 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 faqtor which is also dimensionless aiid ^ j. This factor ac counts for the absorption and emission characteristics of the surfaces for the