Heal transfer coefficient convection

Fur heal transfer purposes, a standing man can be modeled as a 30 Cm diameier, 170 cm-loag vertical cylinder with both the top and botiom surfaces insulared and with the side surface at an average temperature of 34"C. For a convection heat transfer coefficient of 15 W/m "C, deiemiine the rale of heat loss from ihis man by convection in still air al 20°C. What would your answer be if the convection heal transfer coefficient is Increased to 50 W/m °C as a result of winds What is the wind-chill factor in this case Ansuers 336 W, 11 0 W, 3 .7 C... 

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. 

Discussion This transistor can be used at higher power levels by attaching it to a heat sink (which lowers the thermal resistance by increasing the heat transfer surface area, as discussed in the next example) or by using a fan (vrhich lowers the thermal resistance by increasing the convection heal transfer coefficient). 

Reconsider Prob. 3-61, Using EES (or other) software, investigate the effects of the thickness of the wall and the convection heal transfer coefficient on the outer surface of the rate of heal loss from the kiln. Let the thickness vary from 10 cm to 30 cm and the convection heat transfer coefficient frotn 5 W/m °C to 50 W/m "C. Plot the rate of heat transfer as functions of wall thickness and the convection heat transfer coefficient, and discuss the results. 

Discussion The convection heal transfer coefficient should be kept below this value to satisfy the constraints on the temperature of the steak during refrigeration. We can also meet the constraints by using a lower heat transfer coefficient, but doing so would extend the refrigeration time unnecessarily. 

Tlie convection heal transfer coefficient, in general, varies along the flow (or X-) direction. The average or mean convection heat transfer coefficient for a surface in such cases is determined by properly averaging the local convection heat tran.sfcr coefficients over the entire surface. 

A 2-m X 3-m flat plate is suspended in a room, and is subjected to air flow parallel to its surfaces along its 3-m-long side. The free stream temperature and velocity of air are 20°C and 7 m/s. The total drag force acting on the plate is measured to be 0.86 N. Determine the average convection heal transfer coefficient for the plate (Fig. 5-36). 

Hot au at 60 C leaving the furnace of a house enters a 12-m-long section of a sheet metal duct of rectangular cross section 20 cm X 20 cm at an average velocity of 4 m/s. The thermal resistance of the duct is negligible, and the outer surface of the duct, whose einissivity is 0.3, is exposed to the cold air at 10°C in the basement, with a convection heal transfer coefficient of 10 W/m °C. Taking the walls of the basement to be at 10°C also, determine (a) the temperature at which the hot air will leave the basement and (ft) the rale of heat loss from llie hot air in the duct to ihe basement. 

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 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. 

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... 

Consider a spherical container of inner radius r, outer radius rj, and thermal conductivity k. Express the boundary condition on the inner surface of the container for steady one-dimensional conduction for the following cases (a) specified temperature of 50°C, (b) specified heat flux of 30 W/m toward the center, (c) convection to a medium at 7. with a heal transfer coefficient of/i. 

C Consider steady heal transfer through the wall of a room in winter. The convection heat transfer coefficient at the outer surface of the wall is three times that of the inner surface as a result of the winds. On which surface of the v/all do you think the temperature will be closer to the surrounding air temperature Explain. 

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. 

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. 

CoWsider a plane wall of thickness 2L initially at a uniform temperature of T , as shown in Fig. 4—1 In. At lime t = 0, the wall is immersed in a fluid at temperature 7 and is subjected to convection heal transfer from both sides with a convection coefficient of h. The height and the widlh of the wall are large relative to its thickness, and thus heat conduction in the wall can be approximated to be one-dimensional. Also, there is thermal symmetry about the inidplane passing through.x = 0, and thus the temperature distribution must be symmetrical about tlie midplane. Therefore, the value of temperature at any -.T value in - A "S. t 0 at any time t must be equal to the value at f-.r in 0 X Z, at the same time. This means we can formulate and solve the heat conduction problem in the positive half domain O x L, and then apply the solution to the other half. 

In (his chapter, we considered the variation of temperature with time as well as position in one- or multidimensional sysKhls, We first considered the lumped systems in which tlte lempcr. itiire varies with time but remains uniform throughout the system at any time. The temperature of a lumped body of arbitrary shape of mass in, volume L/, surface area density p, and specific heat ioitially at a uniform temperature T, that is exposed to convection at time r 0 in a medium at temperature Tr. with a heal transfer coefficient h is expressed as... 

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 unsteady forward-difference heal conduction for a coiistaiil area. A, pin fin willi perimeter, p, exposed to air wliose temperature is Tq witli a convection heat transfer coefficient of It is... 

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