Heal transfer coefficient combined

Tfe A 50-m-long section of a steam pipe whose outer Wy diameter is 10 cm passes through an open space at IS C. The average temperature of the outer surface of the pipe is measured to be 150°C. If (he combined heal transfer coefficient on the outer surface of the pipe is 20 W/m C, determine (a) the rate of heat loss from the steam pipe (b) the... 

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. 

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. 

Graphical represenlalion of the average heal transfer coefficient for a flat plate with combined laminar and turbulent flow. 

A plot of the nondimensionalized heal transfer coefficient for combined natural and forced convection on a vertical plate is given in Fig. 9 32 for different fluids. We note from this figure that natural convection is negligible when Gr/Re <0.1, forced convection is negligible when Gr/Re > 10, and neither is negligible when 0.1 < Gr/Re < 10. Therefore, both natural and forced convection must be considered in heal itaiisfer calculations when the Gr and Re are of the same order of magnitude (one is within a factor of 10 times die other). Note that forced convection is small relative to natural convection only in the rare case of extremely low forced flow velocities. 

Consider a long resistance wire of radius r, = 0.3 cm and thermal conductivity = 18 W/m C in which heat is generated uniformly at a constant rate of = 1.5 W/cm as a result of resistance healing. The wire is embedded in a 0.4-cm-thick layer of plastic whose thermal conductivity is = 1.8 W/m °C. The outer surface of Ihe plastic covet loses heat by convection to the ambient air at T = 25 C with an average combined heat transfer coefficient of A = 14 W/m °C. 

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. 

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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