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sm5_80
Course: MAE Heat Trans, Spring 2010School: Seoul National
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5.80 PROBLEM KNOWN: Initial temperature of copper and glass plates. Initial temperature and properties of finger. FIND: Whether copper or glass feels cooler to touch. SCHEMATIC: ASSUMPTIONS: (1) The finger and the plate behave as semi-infinite solids, (2) Constant properties, (3) Negligible contact resistance. PROPERTIES: Skin (given): = 1000 kg/m , c = 4180 J/kgK, k = 0.625 W/mK; Table A-1 (T = 300K), Copper:...
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5.80 PROBLEM KNOWN: Initial temperature of copper and glass plates. Initial temperature and properties of finger. FIND: Whether copper or glass feels cooler to touch. SCHEMATIC:
ASSUMPTIONS: (1) The finger and the plate behave as semi-infinite solids, (2) Constant properties, (3) Negligible contact resistance. PROPERTIES: Skin (given): = 1000 kg/m , c = 4180 J/kgK, k = 0.625 W/mK; Table A-1 (T = 300K), Copper: = 8933 kg/m , c = 385 J/kgK, k = 401 W/mK; Table A-3 (T = 300K), Glass: = 2500 kg/m , c = 750 J/kgK, k = 1.4 W/mK. ANALYSIS: Which material feels cooler depends upon the contact temperature Ts given by Equation 5.63. For the three materials of interest,
3 3 3
( k c )1/ 2 = ( 0.625 1000 4180 )1/ 2 = 1, 616 J/m2 K s1/2 skin 1/2 ( k c )cu = ( 401 8933 385)1/ 2 = 37,137 J/m2 K s1/2 ( k c )1/ 2 = (1.4 2500 750 )1/ 2 = 1, 620 J/m2 K s1/2 . glass
Since ( k c )cu >> ( k c )glass , the copper will feel much cooler to touch. the From Equation 5.63,
1/2 1/2
( k c )1/2 TA,i + ( k c )1/2 TB,i A B Ts = 1/2 1/2 ( k c )A + ( k c )B
Ts( cu ) = 1, 616 ( 310 ) + 37,137 ( 300 ) 1, 616 + 37,137 1, 616 + 1, 620 = 300.4 K = 305.0 K.
< <
Ts( glass ) =
1, 616 ( 310 ) + 1, 620 ( 300 )
COMMENTS: The extent to which a materials temperature is affected by a change in its 1/2 thermal environment is inversely proportional to (kc) . Large k implies an ability to spread the effect by conduction; large c implies a large capacity for thermal energy storage.
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Seoul National - MAE - Heat Trans
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Seoul National - MAE - Heat Trans
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Seoul National - MAE - Heat Trans
PROBLEM 6.6 KNOWN: Expression for the local heat transfer coefficient of a circular, hot gas jet at T directed normal to a circular plate at Ts of radius ro. FIND: Heat transfer rate to the plate by convection. SCHEMATIC:ASSUMPTIONS: (1) Steady-state con
Seoul National - MAE - Heat Trans
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Seoul National - MAE - Heat Trans
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Seoul National - MAE - Heat Trans
PROBLEM 6.24 KNOWN: Laminar boundary layer flow of air at 20C and 1 atm having t = 1.13 . FIND: Ratio / t when fluid is ethylene glycol for same conditions. SCHEMATIC:ASSUMPTIONS: (1) Laminar flow. PROPERTIES: Table A-4, Air (293K, 1 atm): Pr = 0.709; Ta
Seoul National - MAE - Heat Trans
PROBLEM 6.29KNOWN: Form of Nusselt number for flow of air or a dielectric liquid over components of a circuit card. FIND: Ratios of time constants associated with intermittent heating and cooling. Fluid that provides faster thermal response. PROPERTIES:
Seoul National - MAE - Heat Trans
PROBLEM 6.35 KNOWN: Air flow conditions and drag force associated with a heater of prescribed surface temperature and area. FIND: Required heater power. SCHEMATIC:ASSUMPTIONS: (1) Steady-state conditions, (2) Reynolds analogy is applicable, (3) Bottom su
Seoul National - MAE - Heat Trans
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Seoul National - MAE - Heat Trans
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Seoul National - MAE - Heat Trans
PROBLEM 6.50KNOWN: Convection heat transfer correlation for flow over a contoured surface. FIND: (a) Evaporation rate from a water film on the surface, (b) Steady-state film temperature. SCHEMATIC:ASSUMPTIONS: (1) Steady-state conditions, (b) Constant p
Seoul National - MAE - Heat Trans
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