The inner and outer surfaces are at 870°C and 40°C respectively. The respective coefficient of the thermal conduciveness of the layer are 1.0, 0.12, and 0.75 W/mK and thickness are 22 cm,7.5 cm, and 11 cm. Assume close bonding of the layers at their interfaces. Find the the total thermal resistace of a furnace wall that is made up of three layers, one of fire brick, one of insulating brick and one of red brick

The inner and outer surfaces are at 870°C and 40°C respectively. The respective coefficient of the thermal conduciveness of the layer are 1.0, 0.12, and 0.75 W/mK and thickness are 22 cm,7.5 cm, and 11 cm. Assume close bonding of the layers at their interfaces. Find the the total thermal resistace of a furnace wall that is made up of three layers, one of fire brick, one of insulating brick and one of red brick

Principles of Heat Transfer (Activate Learning with these NEW titles from Engineering!)

8th Edition

ISBN:9781305387102

Author:Kreith, Frank; Manglik, Raj M.

Publisher:Kreith, Frank; Manglik, Raj M.

Chapter1: Basic Modes Of Heat Transfer

Section: Chapter Questions

Problem 1.45P: An electronic device that internally generates 600 mW of heat has a maximum permissible operating...

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1.10 A heat flux meter at the outer (cold) wall of a concrete building indicates that the heat loss through a wall of 10-cm thickness is . If a thermocouple at the inner surface of the wall indicates a temperature of 22°C while another at the outer surface shows 6°C, calculate the thermal conductivity of the concrete and compare your result with the value in Appendix 2, Table 11.
1.4 To measure thermal conductivity, two similar 1-cm-thick specimens are placed in the apparatus shown in the accompanying sketch. Electric current is supplied to the guard heater, and a wattmeter shows that the power dissipation is 10 W. Thermocouples attached to the warmer and to the cooler surfaces show temperatures of 322 and 300 K, respectively. Calculate the thermal conductivity of the material at the mean temperature in W/m K. Problem 1.4
1.37 Mild steel nails were driven through a solid wood wall consisting of two layers, each 2.5-cm thick, for reinforcement. If the total cross-sectional area of the nails is 0.5% of the wall area, determine the unit thermal conductance of the composite wall and the percent of the total heat flow that passes through the nails when the temperature difference across the wall is 25°C. Neglect contact resistance between the wood layers.
2.38 The addition of aluminum fins has been suggested to increase the rate of heat dissipation from one side of an electronic device 1 m wide and 1 m tall. The fins are to be rectangular in cross section, 2.5 cm long and 0.25 cm thick, as shown in the figure. There are to be 100 fins per meter. The convection heat transfer coefficient, both for the wall and the fins, is estimated to be K. With this information determine the percent increase in the rate of heat transfer of the finned wall compared to the bare wall.
An electronic device that internally generates 600 mW of heat has a maximum permissible operating temperature of 70C. It is to be cooled in 25C air by attaching aluminum fins with a total surface area of 12cm2. The convection heat transfer coefficient between the fins and the air is 20W/m2K. Estimate the operating temperature when the fins are attached in such a way that (a) there is a contact resistance of approximately 50 K/W between the surface of the device and the fin array and (b) there is no contact resistance (in this case, the construction of the device is more expensive). Comment on the design options.
2.30 An electrical heater capable of generating 10,000 W is to be designed. The heating element is to be a stainless steel wire having an electrical resistivity of ohm-centimeter. The operating temperature of the stainless steel is to be no more than 1260°C. The heat transfer coefficient at the outer surface is expected to be no less than in a medium whose maximum temperature is 93°C. A transformer capable of delivering current at 9 and 12 V is available. Determine a suitable size for the wire, the current required, and discuss what effect a reduction in the heat transfer coefficient would have. (Hint: Demonstrate first that the temperature drop between the center and the surface of the wire is independent of the wire diameter, and determine its value.)
The front window of an automobile has a thickness of 4 mm and is defrosted by the passage of a stream of hot air through the inner surface. The thermal conductivity of glass is around 0.78 W/(m oC). Assume the ambient air temperature is -10°C and the external convection coefficient is 65W/(m2 K). If the hot air blowing to defog the glass internally is at 40 ºC and the internal convection coefficient corresponds to 30 W/(m2 K), the internal surface temperatures will be _____________. However, if the car accelerates and maintains high speed, the external convection coefficient should _____________ [increase or decrease?] and if the condition is maintained long enough the temperature of the inner surface of the glass should _____________ [increase or decrease?].
A hot steam pipe having an inside surface temperature of 250°C has an inside diameter of 8 cm and a wall thickness of 5.5 mm. It is covered with a 9-cm layer of insulation having k =0.5 W/m-°C, followed by a 4-cm layer of insulation having k =0.25W/m-°C. The outside temperature of the insulation is 20°C. Calculate the heat lost per meter of length. Assume k =47 W/m-°C for the pipe.
The section of a vertical wall is made up of fiberglass insulation slabs separated by wooden studs. The thermal conductivity of fiberglass and wood are 0.04 W/m∙K and 0.18 W/m∙K, respectively. The thickness of the wall is 180 mm. The temperatures of the inner and outer surfaces of the wall section are 23.5 °C and 4.7 °C, respectively. The ratio of the insulation area to the total wall area is 0.8. Calculate the total heat flow rate (in W/m2) through the wall per unit area.
Calculate the rate of heat loss through the vertical walls of a boiler furnace of size 4 m by 3 m by 3 m high. The walls are constructed from an inner fire brick wall of 20 cm thick with thermal conductivity 0.4 W/mK, a layer of ceramic blanket insulation with thermal conductivity of 0.2 W/mK and 10 cm thick, and a steel protective layer with thermal conductivity of 55 W/mK and 3 mm thick. The inside temperature of the fire brick layer was measured at 600°C and the temperature of the outside of the insulation was 60°C. Also find the interface temperature of layers.
A brick wall, plastered on one face, has a thermal conductance of 0.36 m2 K/W, an inside surface resistance of 0.4 m2K/W, and an outside surface resistance of 0.075 m2K/W. Calculate the total heat resistance.
A 1.0-mm-diameter wire is maintained at a temperature of 400°C and exposed to aconvection environment at 40·C with h=120 W/㎡2.C. Calculate the thermal conductivitythat will just cause an insulation thickness of 0.2 mm to produce a “critical radius."How much of this insulation must be added to reduce the heat transfer by 75 percent from thatwhich would be experienced by the bare wire?