MEHB323 – Heat TransferChapter 1 1. A glass window of width 1m and height 2m is 5 mm thick and has a thermal conductivity of kg = 1.4 W/m·K. (a) If the inner and outer surface temperatures of the glass are 15°C and -20°C, respectively, on a cold winter day, what is the rate of heat loss through the glass? (b) To reduce heat loss through windows, it is customary to use a double pane construction in which adjoining panes are separated by an air space. If the spacing is 10 mm and the glass surfaces in contact with the air have temperatures of 10°C and 15°C, what is the rate of heat loss from a 1 m × 2 m window? The thermal conductivity of air is ka = 0.024 W/m·K. (a) q = 19.6 kW (b) q = 120 W 2. A 60 mm × 55 mm × 30 mm cell phone charger has a surface temperature of Ts = 40°C when plugged into an electrical wall outlet but not in use. The surface of the charger is of emissivity = 0.92 and is subjected to an air flow which provides a convection heat transfer, h = 4.5 W/m2.K. The room air and wall temperatures are 22°C and 20°C, respectively. Determine the daily cost of leaving the charger plugged in when not in use if the electricity cost is $0.18/kWh. Daily cost = $0.00869 3. A spherical, stainless steel (AISI 302) canister is used to store reacting chemicals that provide for a uniform heat flux qi” to its inner surface. The canister is suddenly submerged in a liquid bath of temperature T<Ti, where Ti is the initial temperature of the canister wall. 535 J/kg·K (a) Assuming negligible temperature gradients in the canister wall and a constant heat flux qi”, develop an equation that governs the variation of the wall temperature with time during the transient process. What is the initial rate of change of the wall temperature if qi” = 105 W/m2? (b) What is the steady-state temperature of the wall? (a) dT/dt = -0.084 K/s (b) T = 438.89 K 1 (a) Determine the steady-state rate of heat transfer through this double pane window.026 W/m.K.15 W/m·K and its centre temperature is observed to be 50ºC. The convection coefficient is 10 W/m2·K and the air and surrounding temperature is 27ºC.K and ho = 25 W/m2.25/kWh.9. The orange is exposed to open air and surrounding hence the steady-state heat transfer from the outer surface of the orange is by convection and radiation. Calculate the cost of heating if the resistance heater is to be operated 12 hours a day for 1 year (365 days).7C 5.5 kW/m 3 of heat during its ripening.78 W/m. resistance heater is used where the energy price is RM0. The corresponding convection coefficient at the inner and outer surfaces of the window to be hi = 10 W/m2. (a) q = 114. assumed to be a sphere of 8 cm diameter generating 22.2 m high and 2 m wide double pane window consisting of two layers of 3 mm thick glass (kglass = 0. Thermal conductivity of the orange is 0. Determine the outer surface temperature of the orange. Consider an orange. Consider a 1. The room temperature is maintained at 24ºC while the temperature at the outdoor is -5ºC.24 W (b) Annual cost = $125. (b) In order to maintain the room temperature.K). The surface emissivity of the orange can be assumed to be 0.10 2 .MEHB323 – Heat Transfer Chapter 1 4. Ts 45.K) separated by a 12 mm wide stagnant air space (kair = 0. (a) Ts 33. Consider a person with a skin/fat layer thickness L = 3 mm and thermal conductivity of 0. what is the skin surface temperature and rate of heat loss to the environment? (b) When the person is in water (T∞ = 24C.8 m2.K.49 C q 271. The average skin area of a person is 1.K). h = 2 W/m2.60 C q 1333. Humans are able to control their heat generation rate and heat loss rate to maintain a nearly constant core temperature of Tc = 37C under a wide range of environment. (a) When the person is in air (T∞ = 24C.K). Consider a layer of skin and fat.MEHB323 – Heat Transfer Chapter 1 6. what is the skin surface temperature and rate of heat loss to the water.96 W (b) Ts 27. h = 200 W/m2.25 W 3 . with its outer surface is exposed to environment and its inner surface is at a temperature Ti = 35C.3 W/m.