Heal transfer coefficient

Figure 3. Effect of sphere diameter on overall heal transfer coefficient. Top curve, G = 2.73 kg/rn2 s bottom curve, G = 1.40 kg/m s.

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... 

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. 

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. 

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). 

W/m °C) of cross section 18 cm X 18 cm, or identical size bricks with nine square air holes (k = 0,026 W//n °C) that are 23 cm long and have a cross section of 4 cm X 4 cm. There is a l-cm-thick plaster layer (k = 0.17 W/m °C) between two adjacent bricks on all four sides and on belli sides of the wall. The house is maintained at 21°C and the ambient temperature outside is 0°C. Taking the heal transfer coefficients at the inner and outer surfaces of the waU to be 9 and 23 W/m °C, respectively, determine the rale of heat transfer through the wall constructed of (a) solid bricks and (6) bricks with air holes. 

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. 

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. 

Consider a stainless slccl spoon (k = 15.1 V/m C) partially immersed in boiling water at 95°C in a kitchen at 25°C. The handle of the spoon has a cross section of 0.2 cm X 1.3 cm, and extends 18 cm in the air from the free surface of the water. If the heal transfer coefficient at the exposed surfaces of the spoon handle is 17 W/m - C, deieimine the temperature difference across the exposed surface of the spoon handle. Stale your assumptions. Ansr/er 69.8X... 

A 1.2-m-diameIer and 6-m-long cylindrical propane tank is initially filled with liquid propane whose density is 581 kg/m. The tank is exposed to the ambient air at 30°C, with a heal transfer coefficient of 25 W/m °C. Now a crack develops at the top of the tank and the pressure inside drops to 1 atm while the temperature drops to —42 C, which is the boiling temperature of propane at 1 atm. The heat of vaporisation of propane at 1 atm is 425 kJ/kg. The propane is slowly vaporised as a result of the heat transfer from the ambient air into the tank, and the propane vapor escapes the tank at -42 C through the crack. Assuming the propane lank to be at about the same... 

Repeat Prob. 3-159 for the case of a heal transfer coefficient of 10 W/m "C on the outer surface as a result of putting a fence around the pipe that blocks the wind. 

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. 

A total of 10 rectangular aluminum fins k = 203 W/ni K) arc placed on the outside flat surface of an electronic device. Each fin is 100 mm wide, 20 mm high and 4 mm thick. The fins are located parallel to each other at a cenier-to-cenler distance of 8 mm. The temperature at the outside surface of the electronic device is 60°C. The air is at 20°C, and the heal transfer coefficient is 100 W/in K. Determine (a) the rate of heat loss from Uie electronic device to the surrounding air and (b) the fin effectiveness. 

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. 

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. 

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... 

During a picnic on a hot summer day, all the cold drinks disappeared quickly, and the only available drinks were those at (he ambient temperature of 30°C. In an effort to cool a 350 mL drink in a can, which is 13 cm Jiigli and has a diameter of 6.5 cm. a person grabs the can and starts shaking it in the iced water of-(he chest at 0°C. The temperature of the drink can be assumed to be uniform at all limes, and the heal transfer coefficient between tlie iced water and the aluminum can is 170 W/m °C. l/sing the properties of Water for the drink, estimate how long it will take for the canned drink to cool to 4°C. 

Layers of l5-cm-lhick meat slabs (k = 0.45 W/m °C and or = 1.3 X 10 mVs) initially at a uniform temperajure of lO C are cooled by refrigerated air at -5 C to a tempeiatiire of 2°C at their center in 12 h. Esliinate the average heal transfer coefficient during this cooling process. 

---