The document contains 7 questions related to applied thermodynamics and heat engines: 1. Elaborate on the concept of thermal contact resistance. 2. Determine the rate of heat loss and temperature drops for steam flowing in an insulated pipe. 3. Define total and spectral blackbody emissive powers and how they are related.

1. Applied Thermodynamics and Heat Engines
S.Y. B.Tech. (Production)
Assignment - II
1. Elaborate the concept of thermal contact resistance.
2. Steam at T∞1= 320 °C flows in a cast iron pipe (k = 80 W/m·°C) whose inner and outer
diameters are D1 = 5 cm and D2 = 5.5 cm, respectively. The pipe is covered with 3-cm-
thick glass wool insulation with k = 0.05 W/m·°C. Heat is lost to the surroundings at T2 =
5°C by natural convection and radiation, with a combined heat transfer coefficient of h2 =
18 W/m2·°C. Taking the heat transfer coefficient inside the pipe to be h1 = 60 W/m2·°C,
determine the rate of heat loss from the steam per unit length of the pipe. Also determine
the temperature drops across the pipe shell and the insulation.
(ANS : Q = 121 W, ΔTpipe = 0.02 °C, ΔTinsulation = 284 °C)
3. Define the total and spectral blackbody emissive powers. How are they related to each
other?
4. Water enters a 2.5-cm-internal-diameter thin copper tube of a heat exchanger at 15 °C at
a rate of 0.3 kg/s, and is heated by steam condensing outside at 120 °C. If the average
heat transfer coefficient is 800 W/m2.ºC, determine the length of the tube required in
order to heat the water to 115 °C.
(ANS : 61 mtr)
5. Under what conditions is the effectiveness–NTU method definitely preferred over the
LMTD method in heat exchanger analysis?
6. A 6-m-long section of an 8-cm-diameter horizontal hot water pipe passes through a large
room whose temperature is 20 °C. If the outer surface temperature of the pipe is 70 °C,
determine the rate of heat loss from the pipe by natural convection.
The properties of air at the film temperature of Tf = (Ts + T∞)/2 =(70 + 20)/2 = 45 °C and
1 atm are :
k = 0.02699 W/m.°C, Pr = 0.7241
ν = 1.749 X 10-5 m2/sec, β = 1/Tf = 1/318 K.
Use the following formula for Nu.

 0.387 Ra D
1/ 6


Nu  0.6  8 / 27 
 
(ANS : 443 W)

 1  (0.559 / Pr) 9 / 16 

7. What does the Grashof number physically represent? How does the Grashof number
differ from the Reynolds number?