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### sm5_80

Course: MAE Heat Trans, Spring 2010
School: Seoul National
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Word Count: 264

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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. Excerpts from this work may be reproduced by instructors for distribution on a not-for-profit basis for testing or instructional purposes only to students enrolled in courses for which the textbook has been adopted. Any other reproduction or translation of this work beyond that permitted by Sections 107 or 108 of the 1976 United States Copyright Act without the permission of the copyright owner is unlawful.
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Seoul National - MAE - Heat Trans
PROBLEM 5.99KNOWN: A 0.12 m thick wall, with thermal diffusivity 1.5 10-6 m2/s, initially at a uniform temperature of 85C, has one face suddenly lowered to 20C while the other face is perfectly insulated. FIND: (a) Using the explicit finite-difference me
Seoul National - MAE - Heat Trans
PROBLEM 6.3 KNOWN: Boundary layer temperature distribution. FIND: Surface heat flux. SCHEMATIC:PROPERTIES: Table A-4, Air (Ts = 300K): k = 0.0263 W/mK. ANALYSIS: Applying Fourier's law at y = 0, the heat flux is q = -k s u y u = - k ( T - Ts ) Pr exp - P
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
PROBLEM 6.20 KNOWN: Experimental measurements of the heat transfer coefficient for a square bar in cross flow. FIND: (a) h for the condition when L = 1m and V = 15m/s, (b) h for the condition when L = 1m and V = 30m/s, (c) Effect of defining a side as the
Seoul National - MAE - Heat Trans
PROBLEM 6.23 KNOWN: Variation of hx with x for flow over a flat plate. FIND: Ratio of average Nusselt number for the entire plate to the local Nusselt number at x = L. SCHEMATIC:ANALYSIS: The expressions for the local and average Nusselt numbers areNu L
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
PROBLEM 6.36 KNOWN: Heat transfer correlation associated with parallel flow over a rough flat plate. Velocity and temperature of air flow over the plate. FIND: Surface shear stress l m from the leading edge. SCHEMATIC:ASSUMPTIONS: (1) Modified Reynolds a
Seoul National - MAE - Heat Trans
PROBLEM 6.44 KNOWN: Species concentration profile, CA(y), in a boundary layer at a particular location for flow over a surface. FIND: Expression for the mass transfer coefficient, hm, in terms of the profile constants, CA, and DAB. Expression for the mola
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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