Heal transfer coefficient radiation

A person s head can be approximated as a 25-cm diameter sphere at 35°C with an eniissivity of 0,95. Heat is lost from the head to Ihe. surrounding air at 25°C by convection with a heal transfer coefficient of 11 W/m C, and by radiation to the surrounding surfaces at lOX. Disregarding the neck, determine the total rate of heat loss from the head. 

The roof of a house consists of a 25-cin-lhick concrete slab (k 1.9 W/m C) that is 8-m wide and 10 m long. The emissivity of tlie outer surface of the roof is 0.8, and the convection heal transfer coefficient on (hat surface is estimated to be 18 W/m °C. On a cleat winter nighi, the ambient air is reported to be at I0 C, while the night sky temperature for radiation heat transfer is 170 K. If the inner surface temperature of tberoofisTi = 16"C, determine (he outer surface temperature of the roof and the rate of heat loss through the roof when steady operating conditions are reached. 

Consider a 3-m-diameter spherical tank that is initially filled with liquid nitrogen at 1 atm and I96°C. The tank is exposed to ambient air at I5°(. with a combined convection and radiation heal transfer coefficient of 35 W/m °C. The temperature of the thin-shellcd spherical tank is observed to be almost the same as the temperature of the nitrogen inside. Determine the rate of evaporation of the liquid nitrogen in the tank as a result of the he.ii transfer from the ambient air if the tank is (<7) not insulated, h) insulated with 5-cm-thick fiberglass insulation k = 0.035 W/m C), and (c) insulated with 2-cm-lhick superinsulation which has an effective thermal conductivity of 0.00005 W/m C. 

A row of 10 parallel pipes that are 5 m long and have tin outer diameter of 6 cm are used to transport steam at 145°C through the concrete floor (k = 0.75 W/ui °C) of a 10-m X 5-m room that is maintained at 20°C. The combined convection and radiation heal transfer coefficient at the floor is 12 W/m °C. If the surface temperature of the concrete floor is not to exceed 35°C, determine how deep the steaip pipes should be buried below the surface of the concrete floor. 

Consider a plane wall of thickness 2L, a long cylinder of radius r , and a sphere of radius r, initially at a nnifonn temperature T,-, as shown in Fig. 4—11. At time t = 0, each geometry is placed in a large medium that is at a constant temperature T and kepi in that medium for t > 0. Heat transfer lakes place between these bodies and their environments by convection with a uniform and constant heal transfer coefficient A. Note that all three ca.ses possess geometric and thermal symmetry the plane wall is symmetric about its center plane (,v = 0), the cylinder is symmetric about its centerline (r = 0), and the sphere is symmetric about its center point (r = 0). We neglect radiation heat transfer between these bodies and their surrounding surfaces, or incorporate the radiation effect into the convection heat transfer coefficient A. 

C by the heating system and the solar radiation incident on it during the day. During the night, the outer surface of the wall is exposed to cold air at -3°C with an average heal transfer coefficient of 20 V//m °C, determine the wall temperatures at distances 15. 30, and 40 cm from the outer surface for a period of 2 h. 

Conduct the following experiment at home to determine the combined convection and radiation heal transfer coefficient at the surface of an apple exposed to the room air. You will need two thermometers and a clock. 

The absorber surface of a solar collector is made of aluminum coaled with black chrome a, = 0.87 and e = 0.09). Solar radiation is incident on the surface at a rate of 600 W/m. The air and the effective sky temperatures are 25°C and 15"C, respectively, and the convection heal transfer coefficient is 10 W/m C, For an absorber surface temperature of 70°C, determine the net rate of solar energy delivered by the absorber plate to the water circulating behind it. 

I conv convection heal transfer coefficient, as given in Table 13-5 hnd = radiation heal transfer coefficient, 4.7 W/m - °C for typical indoor conditions the eniissivity is assumed to be 0.95, which is typical dcioiNiij = outer surface area of a clothed person IciMking = average temperature of exposed skin and clothing ambient air temperature... 

The inner and outer surfaces of a 25-cm-thick wall in summer are at 27"C and 44 C, respectively. The outer surface of the wall exchanges heal by radiation with surrounding surfaces at 40°C, and convection svith ambient air also at 40 C with a convection heat transfer coefficient of 8 W/m - °C. Solar radiation is incident on the surface at a rate of 150 W/m If both the emissivity and the solar absorptivity of the outer surface are 0.8, determine the effective thermal conductivity of the wall. 

Consider a flat-plate solar collector placed on the roof of a hoifse. The temperatures at the inner and outer surfaces of the glass pover are measured to be 28°C and 25°C, respectively. I he glass bover has a surface area of 2,5 mF, a thickness of 0.6 cm, and a themial conductivity ofO.7 W/ni °C, Heat is lost from the outer surface of the cover by convection and radiation with a Convection heat transfer coefficient of lOW/m Ctuid anaiflbienljtemperature of 15°C, Determine the fraction of heal lo.st from the glass cover by radiation. 

A room is heated by a 1.2 kW electric re.sistance heater whose wires have a diameter of 4 mm and a total length of 3.4 m. The air in the room is at 23°C and the interior surfaces of tlie room are at 17°C. The convection heal liansfer coefficient on the surface of the wires is 8 W/m °C. If the rates of heal transfer from the wires to the room by convection and by radiation are equal, the surface temperature of the wire is (a) 3534X (b) 1778X (c) 1772X... 

A cylindrical resistor elefneot on a circuit board dissipates 0.15 W of power in an environment at 4ff C. The resistor is i. 2 cn long, and has a diameter of 0.3 cm. Assuming heal to be transferred uniformly front all surfaces, determine (a) the amount of heat this resistor dissipates during a 24-h period (b) the heat flux on the surface of the resistor, in W/m and (if) the surface temperature of the resistor for a combined convection and radiation heat transfer coefficient of 9 W/m C. 

The roof of a house consists of a 15-cm-thick concrete slab (k - 2 W/m - C) that is 15 m wide and 20 m long. The convection beat transfer coefficients on the inner and outer surfaces of the roof are 5 and 12 W/m °C. respectively. On a clear winter night, the ambient air is repotted to be at lO C, while the night sky temperature is 100 K. The house and the interior surfaces of the wall are maintained at a constant temperature of 20°C. The emissivity of bodi surfaces of the concrete roof is 0.9. Considering both radiation and convection heat transfers, determine the rate of heal transfer tlirough the roof, and the inner surface temperature of the roof. 

An 8-m-iDtemal-diameter spherical lank made of 1.5-cm-thick stainless steel (Ir = 15 W/m °C) is used to store iced water at 0°C. The tank is located in a room whose temperature is 25°C. The walls of the room are also at 25 C. The outer surface of the tank is black (emissivity e = 1), and heat transfer between the outer surface of the tank and the surroundings is by natural convection and radiation. The convection heat transfer coefficients at the inner and the outer sui faces of the tank arc 80 W/m. C and 10 W/m - C, respectively. Detecmine (a) the rale of heal transfer lo the iced water in (he tank and (b) the amount of ice at 0°C that melts during a 24-h period. Therheai of fusion of water at atmospheric pressure is = 333.7 kJ/kg. 

Heal transfer through a window is also affected by the convection and radiation heat transfer coefficients between the glass surfaces and sunound-ings. The effects of convection and radiation on the inner and outer surfaces of glazings are usually combined into the combined convection and radiation heat transfer coefficients /i,- and h , respectively. Under still air conditions, the combined heat transfer coefficient at the inner surface of a vertical window can be determined from... 

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