Thermal-Fluids II

Assignment 04
Problem 1
During air cooling of potatoes, the heat transfer coefficient for combined convection, radiation, and evaporation is
determined experimentally to be as shown:
Air Velocity, m/s
Heat Transfer Coefficient W/m2 · 0C
Consider a 10-cm-diameter potato initially at 200C with a thermal conductivity of 0.49 W/m · 0C. Potatoes are
cooled by refrigerated air at 5 0C at a velocity of 1 m/s. Determine the initial rate of heat transfer from a potato, and
the initial value of the temperature gradient in the potato at the surface.
Problem 2
Engine oil at 80 0C flows over a 6-m-long flat plate whose temperature is 30 0C with a velocity of 3m/s. Determine
the rate of heat transfer over the entire plate per unit width.
Problem 03
Combustion gases passing through a 3-cm-internal diameter circular tube are used to vaporize waste water at
atmospheric pressure. Hot gases enter the tube at 115 kPa and 2500C at a mean velocity of 5m/s, and leave at
1500C. If the average heat transfer coefficient is 120 W/m2. 0C and the inner surface temperature of the tube is
1100C, determine
(a) the tube length and
(b) the rate of evaporation of water.
Problem 04
The evaporator section of a heat pump is installed in a large tank of water, which is used as an energy source
during the winter. As energy is extracted from the water, it begins to freeze, creating an ice/water bath at 0°C,
which may be used for air conditioning during the summer. Consider summer cooling conditions for which air is
passed through an array of copper tubes, each of inside diameter D = 50 mm, submerged in the bath.
 = 0.01 kg/s, what tube length
(a) If air enters each tube at a mean temperature of T m,i = 24°C and a flow rate of m
L is needed to provide an exit temperature of T m,o= 14°C?
(b) With 10 tubes passing through a tank of total volume of 10 m3, which initially contains 80% ice by volume,
how long would it take to completely melt the ice? The density and heat of fusion of ice are 920 kg/m3 and 3.34
105 J/kg, respectively.
Problem 05
Cooling water flows through the 25.4-mm-diameter thin walled tubes of a steam condenser at 1m/s, and a surface
temperature of 350 K is maintained by the condensing steam. The water inlet temperature is 290 K, and the tubes
are 5 m long. What is the water outlet temperature? Evaluate water properties at an assumed average mean
temperature, Tm =300 K.