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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 0.66 1.00 1.36 1.73 Heat Transfer Coefficient W/m2 · 0C 14.0 19.1 20.2 24.4 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.