HEAT TRANSFER QUIZ 2019

Q. 1 A 20 cm diameter cylindrical solid pellet of a nuclear fuel with density 6000 kg / m³ and conductivity of 300 W/(m K) generates heat by nuclear fission at a spatially uniform rate of 10⁴ W/ kg . The heat from the fuel pellet is transferred to the surrounding coolant by convection such that the pellet wall temperature remains constant at 300 ⁰C. Neglecting the axial and azimuthal dependence, the maximum temperature (in ⁰C) in the pellet at steady state is.................(rounded off to the nearest integer).

Q. 2.Consider the two countercurrent heat exchanger designs for heating a cold stream from t_in to t_out as shown in figure. The hot process stream is available at T_in. The inlet stream conditions and overall heat transfer coefficients are identical in both the designs. The heat transfer area in Design I and Design II are respectively A^I_HX and A^II_HX

If heat losses are neglected, and if both the designs are feasible, which of the following statements holds ture:

A^I_HX > A^II_HX , T^I_out < T^II_out
A^I_HX = A^II_HX , T^I_out = T^II_out
A^I_HX < A^II_HX , T^I_out > T^II_out
A^I_HX < A^II_HX , T^I_out = T^II_out

Q. 3 The correct expression for the Colburn j-factor for mass transfer that relates Sherwood number (Sh), Reynolds number (Re) and Schmidt number (Sc) is

Q. 4 Prandtl number signifies the ratio of

Q. 5 Stream A with specific heat capacity C_PA = 2000 J / kg K is cooled from 90 ⁰C to 45 ⁰C in a concentric double pipe counter current heat exchanger having a heat transfer area of 8 m². The cold stream B of specific heat capacity C_PB = 1000 J / kgK enters the exchanger at a flow rate 1 kg/s and 40 ⁰C. The overall heat transfer coefficient U = 250 W/( m² K) . Assume that the mean driving force is based on the arithmetic mean temperature difference, that is where T_i,in and T_i,out refer to the temperature of the i^th stream (i = A,B )at the inlet and exit; respectively. The mass flow rate of stream A (in kg/s),is..................(rounded off to two decimal places).

Q. 6 Pool boiling equipment operating above ambient temperature is usually designed to operate

far above the critical heat flux
near the critical heat flux
far above the Leidenfrost point
near the Leidenfrost point

Q. 7 A solid sphere of radius 1cm and initial temperature of 25 ⁰C is exposed to a gas stream at 100 ⁰C. For the solid sphere, the density is 10⁴ kg / m³ and the specific heat capacity is 500J / (kg K) . The density of the gas is 0.6 kg / m³ and its specific heat capacity is 10³ J / KgK . The solid sphere is approximated as a lumped system (Biot number <<1) and all specific heats are constant. If the heat transfer coefficient between the solid and gas is 50 W/ m² K , the time (in seconds) needed for the sphere to reach 95 ⁰C is ...................(rounded off to the nearest integer)