PRACTICAL 12: simultaneous heat and mass transfer
1a) Consider a shallow body of water. Is it possible for the water to freeze during a cold and dry night even then the ambient air and surrounding surface temperatures might never drop to OC? Explain.
1b) During evaporation of water to air, under what conditions will latent heat be equal to convection heat transfer from the air?
Copyright By PowCoder代写 加微信 powcoder
2) Jugs made of porous clay were commonly used to cool water in the past. A small amount of water leaks out keeping the outer surface of the jug constantly wet. Hot and relatively dry air causes this water to evaporate, and this latent heat causes cooling of the water inseide. If the environment conditions are 1 atm, 32C and 40% relative humidity, determine the temperature of the jug in the water under steady state conditions. (Ans: Ts = 278 K)
3)On a hot summer’s day, one way to increase heat transfer from the head is to wet it. This is more effective in windy conditions. If the head is a 30-cm sphere at 30C with an emissivity of 0.95, determine the total rate of heat loss from the head at ambient air conditions of 1 atm, 25C, 30% relative humidity, and 25 km/h winds, if the head is (a) dry and (b) wet. The surrounding temperature is 25C (Ans: a) qloss = 36.3 W, b) qloss = 366.3 W)
4) A water-soaked 10 cm x 10cm square sponge experiences parallel dry air flow over its surface. The average convective heat transfer coefficient of dry air at 20C flowing over the sponge is 30 W/m2K, and the sponge surface temperature is 30C. If the sponge is placed under an array of radiant lamps, determine (a) the rate of evaporation of water from the sponge, and (b) the net radiation heat transfer rate. (Ans: a) mevap = 8.54 x 10-6 kg/s, b) qrad = 23.75 W)
程序代写 CS代考 加微信: powcoder QQ: 1823890830 Email: powcoder@163.com