(a) Briefly discuss the penetration theory.

(b) Calculate the time taken for the distant face of a brick wall, of thermal diffusivity, D=0.0043 cm/s and thickness, l = 0.45 m, to rise from 295 to 375 K, if the whole wall is initially at a constant temperature of 295 K and the near surface is suddenly raised to 900 K and maintained at this temperature. Assume that all the flow of heat is perpendicular to the faces of the wall and that the distant face is perfectly insulated

Data : The temperature at any distance x from the near surface at time t for unsteady state heat conduction may be given by:

0 (-1)Ne'erfcn++erfe2n [2(n+1)-x|\ erye 2Dt*erc2D,t N=0

The radiant energy emitted by dAj and intercepted by dAz per unit time may be expressed as

dQ12 cos61cosC2 dA, dAz TT Stefan-Boltzmann constant, o = 0.1712x 10-5Btu/(hr)(ft")(R)*