Thermodynamics HW Solutions 540

Thermodynamics HW Solutions 540 - Chapter 7 External Forced...

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Chapter 7 External Forced Convection 7-11C The friction coefficient represents the resistance to fluid flow over a flat plate. It is proportional to the drag force acting on the plate. The drag coefficient for a flat surface is equivalent to the mean friction coefficient. 7-12C The friction and the heat transfer coefficients change with position in laminar flow over a flat plate. 7-13C The average friction and heat transfer coefficients in flow over a flat plate are determined by integrating the local friction and heat transfer coefficients over the entire plate, and then dividing them by the length of the plate. 7-14 Hot engine oil flows over a flat plate. The total drag force and the rate of heat transfer per unit width of the plate are to be determined. Assumptions 1 Steady operating conditions exist. 2 The critical Reynolds number is Re cr = 5 × 10 5 . 3 Radiation effects are negligible. Properties The properties of engine oil at the film temperature of ( T s + T )/2 = (80+30)/2 =55 ° C = 328 K are (Table A-13) 1505 Pr C W/m. 141 . 0 /s m 10 123 kg/m 867 2 6 3 = ° = × = υ = ρ k Analysis Noting that
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