Convection heat transfer coefficients for a heated horizontal surface facing upward may be determined by a gage whose specific features depend on whether the temperature of the surroundings is known. For configuration A, a copper disk, which is electrically heated from below, is encased in an insulating material such that all of the heat is transferred by convection and radiation from the top surface. If the surface emissivity and the temperatures of the air and surrounding are known, the convection coefficient may be determined from measurement of the electrical power and the surface temperature of the disk. Configuration B is used in situations for which the temperature of the surroundings is not known. A thin, insulating strip separates semicircular disks with independent electrical heaters and different emissivities. If the emissivities and temperature of the air are known, the convection coefficient may be determined from measurement of the electrical power supplied to each of the disks in order to maintain them at a common temperature.

(a) In an application of configuration A to a disk of diameter D = 160 mm and emissivity ε = 0.8, values of P e1ec = 10.8 Wand T = 67°C are measured for T∞ = Tsur = 27°C. What is the corresponding value of the average convection coefficient? How does it compare with predictions based on a standard correlation?

(b) Now consider an application of configuration B for which T∞ = 17°C and Tsur is unknown. With D = 160 mm, ε1 = 0.8 and ε2 = 0.1, values of Pe1ec,1 = 9.70 Wand P e1ec,2 = 5.67 Ware measured when T1 = T2 = 77°C. Determine the corresponding values of the convection coefficient and the temperature of the surroundings. How does the convection coefficient compare with predictions by an appropriate correlation?

Transcribed Image Text:

Quiescent air Quiescent air Tut Tu sur T, e Peec elec (b) (a)