Standards for firewalls may be based on their thermal response to a prescribed radiant heat flux. consider a 0.25-m-thick concrete wall (rho= 2300 kg/m3, c= 880 j/kg·k, k= 1.4 w/m·k), which is at an initial temperature of ti= 25°c and irradiated at one surface by lamps that provide a uniform heat flux of qs′′= 0.85·104 w/m2. the absorptivity of the surface to the irradiation is αs= 1.0. if building code requirements dictate that the temperatures of the irradiated and back surfaces must not exceed 325°c and 25°c, respectively, after 30 min of heating, will the requirements b

Air is to be cooled in the evaporator section of a refrigerator by passing it over a bank of 0.8-cm-outer-diameter and 0.4-m-long tubes inside which the refrigerant is evaporating at -20°c. air approaches the tube bank in the normal direction at 0°c and 1 atm with a mean velocity of 4 m/s. the tubes are arranged in-line with longitudinal and transverse pitches of sl- st 1.5 cm. there are 30 rows in the flow direction with 15 tubes in each row. determine (a) the refrigeration capacity of this system and (b) pressure drop across the tube bank. evaluate the air properties at an assumed mean temperature of -5°c and 1 atm. is this a good assumption?

Afluid flows with a velocity field given by v=(x/t)i.. determine the local and convective accelerations when x=3 and t=1.

Apipe with an outside diameter of 15 cm is exposed to an ambient air and surrounding temperature of -20°c. the pipe has an outer surface temperature of 65°c and an emissivity of 0.85. if the rate of heat loss from the pipe surface is 0.95 kw per meter of length, the external convective heat transfer coefficient (h) is: (a) 12.5 w/m"k (b) 18.6 w/mk (c) 23.7 w/mk (d) 27.9 w/mk (e) 33.5 w/mk