Question

The average heat transfer coefficient on a thin hot vertical plate suspended in still air can be determined from observations of the change in plate temperature with time as it cools. Assume the plate temperature to be uniform at any instant of time and radiation heat exchange with the surroundings negligible. The ambient temperature is ${25}^{\circ }C,$ the plate has a total surface area of $0.1m^2$ and a mass of $4kg.$ The specific heat of the plate material is $2.5kJ/kgK.$The convective heat transfer coefficient in $W/m^{2}K,$ at the instant when the plate temperature is ${225}^{\circ }C$ and the change in plate temperature with time $dT/dt=-0.02K/S,$ is

• A.
  $200$
• B.
  
  $20$
  
• C.
  $15$
• D.
  $10$

Answer 1

The correct option is D

$10$
Given data:
$A=0.1m^2;m=4kg{c}_P=2.5kJ/kgK$
Net rate of heat transfer
= Rate of change of internal energy
$hA(T-T_f)=-m{c}_P\frac{dT}{dt}h\times 0.1(225-25)$
$=-4\times 2.5\times 1000\times (-0.02)$

$h=\frac{4\times 2.5\times 1000\times 0.02}{0.1\times 200}$
$=10W/m^{2}K$