Understanding Heat Transfer in Engineering Applications
This content covers various aspects of heat transfer in engineering, including calculations for thermal resistances, heat transfer between different surfaces and mediums, and practical applications such as heat transfer in pipes, plates, and spheres. It also includes review questions involving heat transfer scenarios with detailed property calculations for air, water, and other materials. The content provides a comprehensive overview of heat transfer concepts and calculations relevant to engineering studies.
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Heat Transfer PCE 340 2 nd Semester 2021 Prof . Yehia Hassan Magdy
b. Cylinder and Sphere The thermal resistance for cylinder or spherical shape as shown in Figure 1.17 can be calculated as the following equations. T = Q R total Figure 1.17 thermal resistances for cylinder or sphere T = Q R total = + + R R R R 1 , , 2 , total conve conduction cyl conve
r ln 2 r 1 1 1 = + + R total 2 2 2 Lk 2 2 r Lh r Lh 1 2 i o ( ) r T T = i o Q ln 2 2 r 1 1 1 + + 2 2 Lk 2 2 r Lh r Lh 1 2 i o = ( ) Q UA T T i o 1 1 = = R total A U A U o o i i
( k ) 1 A ln r r A 1 h 1 h = + + 2 1 i i U 2 L A i 0 i a o ( k ) 1 A A ln r r 1 h 1 h = + + 0 0 2 1 U 2 A L o 0 i i a For Sphere: For sphere R R + = + R conve R 1 , , 2 , total conduction Sph conve r 1 r r 1 1 = + + 2 1 R total 2 2 4 r k 4 4 r h r h 2 1 2 i o
( ) T T = i o Q r 1 r r 1 1 + + 2 1 2 2 4 r k 4 4 r h r h 2 1 2 i o = ( ) Q UA T T i o
Review Question 1. 1)A horizontal large plate with 2m is maintained at 85 C and exposed to atmospheric air at 15 C. Calculate the free-heat transfer if plat is 2m high and the air properties are: k = 0.03 W/m. C v= 12.3 x10-6 m /s Pr = 1.2 2. Air at 100 C is heated as it flows through a pipe with a diameter of 3cm at velocity of 15m/s. Calculate the heat transfer if the wall temperature is 120 C and the pipe length is 1m. The properties of air are: = 0.8741kg/m Pr = 2.377 = 2.233x10-5kg/m.s
2. Air at 100C is heated as it flows through a pipe with a diameter of 3cm at velocity of 15m/s. Calculate the heat transfer if the wall temperature is = 0.8741kg/m Pr = 2.377 = 2.233x10-5kg/m.s k = 3.221x10-5 W/m. C. 3. A stainless steel pipe is maintained at vertical position with 5cm diameter and 130 C. It exposed to atmospheric air at 25 C. Calculate the heat transfer if the pipe is . / 10 003 . 3 = s m = 5 k kW m K 5 2 . 2 = 056 10 / v Pr . 0 697
4. A square cupper plate with 10cm width is exposed to water. The plate surface temperature is 75 C and its thermal conductivity is 3.77 W/m. C. if the water temperature is 20 C and the convection coefficient from one side is 25 W/m . C and 85 W/m . C from the opposite side. Calculate the heat transfer if the plate thickness is 2 cm. 5. Air at 100 C is cooling as it flows through a pipe with a diameter of 3cm at velocity of 15m/s. Calculate the heat transfer if the wall temperature is 120 C and the pipe length is 1m. The properties of air are: = 0.8741kg/m Pr = 2.377 = 2.233x10-5kg/m.s k = 3.221x10-5 W/m. C. 6. A 2.0 cm diameter vertical heater is maintained at a surface temperature of 38 C and submerged in water at 27 C. Calculate the free-convection heat loss if the pipe long is 2m. The water properties are: k = 0.623 W/m. C v= 0.7x10-6 m /s Pr = 5.12
