3-124 A DC motor delivers mechanical power to a rotating stainless steel shaft (k = 15.1 W/m-K) with a length of 25 cm and a diameter of 25 mm. In a surrounding with ambient air temperature of 20°C and convection heat transfer coefficient of 25 W/m2-K, the surface area of the motor housing that is exposed to the ambient air is 0.075 m2?. The motor uses 300 W of electri- cal power and delivers 55% of it as mechanical power to rotate the stainless steel shaft. If the tip of the stainless steel shaft has a temperature of 22°C, determine the surface temperature of the motor housing. Assume the base temperature of the shaft is equal to the surface temperature of the motor housing.

3-124 A DC motor delivers mechanical power to a rotating stainless steel shaft (k = 15.1 W/m-K) with a length of 25 cm and a diameter of 25 mm. In a surrounding with ambient air temperature of 20°C and convection heat transfer coefficient of 25 W/m2-K, the surface area of the motor housing that is exposed to the ambient air is 0.075 m2?. The motor uses 300 W of electri- cal power and delivers 55% of it as mechanical power to rotate the stainless steel shaft. If the tip of the stainless steel shaft has a temperature of 22°C, determine the surface temperature of the motor housing. Assume the base temperature of the shaft is equal to the surface temperature of the motor housing.

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.

Chapter2: Steady Heat Conduction

Section: Chapter Questions

Problem 2.30P: 2.30 An electrical heater capable of generating 10,000 W is to be designed. The heating element is...

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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.)
3.16 A large, 2.54-cm.-thick copper plate is placed between two air streams. The heat transfer coefficient on one side is and on the other side is . If the temperature of both streams is suddenly changed from 38°C to 93°C, determine how long it takes for the copper plate to reach a temperature of 82°C.
3.17 A 1.4-kg aluminum household iron has a 500-W heating element. The surface area is . The ambient temperature is 21°C, and the surface heat transfer coefficient is . How long after the iron is plugged in does its temperature reach 104°C?
Two cylindrical metal rods-one copper, the other lead-are connected in parallel with a temperature of 22.0 ∘C at one end and 113 ∘C at the other end. Both rods are 0.640 mm in length, and the lead rod is 2.76 cm in diameter. If the combined rate of heat flow through the two rods is 33.1 J/s , what is the diameter of the copper rod?
the composite wall of a refrigerator has a thermal conductivity of 0.05 W/m-K and a wall thickness of 50 mm. In a room of 25 °C, the refrigerated cold space inside the refrigerator is maintained at 4 °C. Assume the inner and outer convection heat transfer coefficients are 5 W/m² K and 10 W/m².K, respectively. Neglect radiation heat transfer. Determine the rate of heat leaking into the refrigerator per unit surface area.
A refrigerator door of area 0.6 sq.m consists of 25 mm of insulation on top of a thin metal sheet. The film heat transfer coefficients inside and outside the refrigerator are 10.0 and 15.0 W m-2K -1 , respectively, and the thermal conductivity of the insulation is 0.25 W m-1K -1 . If the working temperature of the refrigerator is 0oC and ambient temperature is 20oC, determine the heat flow through the refrigerator door and the temperature of the inside surface of the door.
A mother takes the milk to be given to her baby from the refrigerator and pours it into a cylindrical bottle for heating. The height of the milk in the bottle is 10 cm. Then he places the bottle into a large container with hot water at a temperature of 62 ° C. The heat transfer coefficient between hot water and bottle is 130 W / m2.oC. The process of heating the milk from 4 0C to 40 0C is required to be completed within 6 minutes. What should be the diameter of the bottle used? The properties of milk can be equal to the properties of water at the same temperature.
the plane Wall seen in the figure is L=20 cm and the heat conduction coefficient is 57 W/mK. Inside the wall there is a uniform distributed volumetric heat generation of 120 kW/m3. One surface of the wall is a fluid alloy with a temperature of 15 Co, the otherits surface is subjected to heat flux of Q0 = 50 kW/m2. Since the heat transfer coefficient between the wall and the fluid is 400 W / m2 K, accepting the temperature change within the wall as one-dimensional and continuous,a) draw the temperature change curve by finding the correlation that gives the temperature distribution inside the Wall b) find the maximum temperature inside the Wall? E) find in x=L?
The wall of a furnace consists of 200 mm thick refractory brick (k = 3 Btu / hr-ft2-0F), 100 mm thick bricks (k = 0.050 Btu / hr-ft2-0F) and 6 mm thick steel. The temperature on the side of the refractory brick layer of the furnace is 1150 C, and the temperature on the outer steel surface is 30C. There is a heat loss of 300 W / square meter at the wall of the furnace. It is difficult to prevent the thin air additions between the brick layers and the brick layer and the steel wall. How many mm of brick layers is the total thickness of these air layers equivalent?
Convective heat transfer coefficient is 15 W/(m*K), surface area of the fin is 2 m^2, the temperature at the base is 70 C and the ambient temperature is 20 C. Determine the fin efficiency if the actual heat transfer rate from the fin is 500 W?
A 6-m-long 2-kW electrical resistance wire is made of 0.2-cm-diameter stainless steel(? = 15.1 W/mK). The resistance wire operates in an environment at 30°C with a heattransfer coefficient of 140 W/m2K at the outer surface. Determine the maximumtemperature in the wire;(a) by using the applicable relations.(b) by setting up the proper differential equation and solving it.
One side of a copper (401 W/m-oC) block 5 cm thick is maintained at 175 oC, the other side is covered with fiberglass (k = 0.026 W/m-0C) 2.5 cm thick. The outside of the block is maintained at 38 oC, and the total heat flow is 44 kW. Find the overall heat transfer coefficient. (four decimal places for final answer)