Heat Transfer
Log mean temperature difference (LMTD) cannot be used, if

there exists an unsteady state
heat transfer co-efficient over the entire heat exchanger is not constant
the heat capacity is not constant and there is a phase change.
none of the listed here

ANSWER DOWNLOAD EXAMIANS APP

Heat Transfer
A long iron rod initially at a temperature of 20°C has one end dipped in boiling water (100°C) at time, t = 0. The curved surface of the rod is insulated so that heat conduction is one dimensional in the axial direction. The temperature at a distance 100 mm from the dipped end becomes 40°C at time, t = 200 s. The same temperature is achieved at a distance of 200 mm from the dipped end at time

t = 400 s
t = 356 s
t = 800 s
t = 283 s

ANSWER DOWNLOAD EXAMIANS APP