Document 4JLdmGd2RVKZevYO6v3Ogoj3x

jjpmp. as Case 7. Free Convection0 /PV* /A*\** 10* < ^Qr < 10' I Free oonvection past a heated horizontal cylinder. ___________________ Equationfor Air ^(xr (?r \ -- , Free convection past a single vertical sur-j face. t/Jrr/'rr U- * t. ~-~*l | Free convection past a heated horizontal surface (face up;. 10' < NGi < 107 J'"Mr(rr 108 < ^Gr < MM* Equation for Air ( ) \r)p V- /Af\#J* k 10* < tfGr < 107 Equation for Air / p\I.H /At\*M *. = o-239 (k) \t) 10* < Nqt < 107 I Free oonvection past a heated horizontal surface (faoe down). * Fluid properties should be evaluated at to arithmetic mean fluid temperature, U --------,.operuessuouiu ucoiuu.^..______ (( surface + t fluid) divbidTehdesbeye2x.pressions are suitable approximations to longitudinal flow in other than right circular cylinders, provided the nydraulic diameter is employed as the conduit dimension parameter. For non-circular cross' sections, the hydraulic diameter is equal to four times the cross-sectional area divided by the wetted perim- uons, the hydraulic aiamewt ia e^uai w .---------erateterc.sF, ionrcrleoawsersatteoswaorfdhse0a.3t 3tr.anTshfeerabbyovfereeeqcuoantivoencsteiomn pthloeyienxgpaonneexnpt odnecernetaesqe-s-ua*tol--tw-o--a-0-r.d-2as5waz,erernoan,pHapnflnidcrafhboilrgehhineigrhtheer intermediate range indicated. 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 = mass flow per unit cross-sectional area normal to flow, pounds Sit (hour) (square foot of flow cross-section), rashof modulus, dimensionless (Ngt = DWP&tg/p*). h0 = average unit thermal convective conductance from the leading edge of surface to the position x, Btu per (hour) (square foot) (Fahrenheit degree). = 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. P0 = pressure (atmospheric) , atmospheres. t = temperature, Fanrenheit. T = temperature, Fahrenheit, absolute. u -- fluid velocity, feet per second. V = volume, cubic feet, x = a dimension of the system, feet. 0 -- coefficient of cubical expansion (0 = y1 ^dfV)p) y fr perfect gases 0 = X/T. At = 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. Table 3. Radiation Factors or Emissivities, e* For the determination of factor Fe in Equation 3 Class Surfaces Fraction of Black-Bodt Radiation At 50-100 F At 1000 F Solas Radiation 1 A small hole in a large box, sphere, furnace, or enclosure......................................................................... 2 Black noQ-metallic surfaces such as asphalt, car bon, slate, paint, paper............................................... Red brick and tile, concrete and stone, rusty steel and iron, dark paints (red, brown, green, etc.).. Yellow and buff brick and stone, firebrick, fire day....................................................................................... 5 White or light-cream brick, tile, paint or paper, plaster, whitewash......................................................... 6 7 Bright aluminum paint; gilt or bronze paint.... Dull brass, copper, or aluminum; galvanized steel; polished iron......................................................... 9 Polished brass, copper, monel metal......................... Highly polished aluminum, tin plate, nickel, chromium.......................................................................... -- 0.97 to 0.99 0.90 to 0.98 0.85 to 0.95 0.85 to 0.95 0.85 to 0.95 0.90 to 0.95 0.40 to 0.60 0.20 to 0.30 0.02 to 0.05 0.02 to 0.04 0.97 to 0.99 0.90 to 0.98 0.75 to 0.90 0.70 to 0.85 0.60 to 0.75 0.30 to 0.50 0.05 to 0.15 0.05 to 0.10 0.97 to 0.99 0.85 to 0.98 0.65 to 0.80 0.50 to 0.70 0.30 to 0.50 Transparent* 0.30 to 0.50 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. fourth powers of the absolute surface temperatures (Ti4 -- TV*)- The pro-* portionality factor (<tFaFv) 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): Q ~ 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 5* 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. ** = the emissivity factor which is also dimensionless and ^ 1. This factor ac counts for the absorption and emission characteristics of the surfaces for the