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Natural Convection
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HW # 5 /Tutorial # 5WRF Chapter 19; WWWR Chapter 20
ID Chapters 7-9
• Tutorial # 5• WRF#19.3;
WRF#19.8; WWWR #20.13,; WRF#19.23, WWWR#20.42.
• To be discussed on Feb. 16, 2016.
• By either volunteer or class list.
• Correction: #20.42: Table: Unit for Cp should be corrected from J/kg-K to kJ/kg-K.
Convective Heat Transfer Correlations
Natural Convection
Horizontal Plates
McAdam’s Correlations: Hot surface facing up or cold surface surfacing down
H
C
HC
Horizontal Cylinders
g
In each of the correlations to follow the temperature of the hotter of the two large surfaces is designed in T1, and the cooler surface is at temperature T2.
Fluid properties are evaluated at the film temperature, Tf = (T1 + T2) / 2.
Convective heat flux is expressed as
qA
= h(T1 - T2)
Example 1
H
H
Initial Guess T = 385K, Resulting T=381.1K Second Iteration
Forced Convection For Internal Flow
Turbulent Flow
Example 2
Hydraulic fluid (MIL-M-5606), in fully developed flow, flows through a 2.5 cm diameter copper tube, with a mean velocity of 0.05 m/s. The oil enters at 295K. Steam condenses on the outside tube surface with an effective heat transfer coefficient of 11400 W/m2-K. Find the rate of heat transfer to the oil.
Ro
RiRc
Rc << Ro, Ri295K
Example 2Ro
Ri
=(295+305)/2
Rc
Rc << Ro, Ri295K
305K?
=373-(373-300)*[98.1/(98.1+11400)]=372.3K
Example 3
Forced Convection For External Flow
Flow Parallel To Plane Surface
Cylinders In Crossflow
Tube Banks In Crossflow
SL : Center-to-center distance between tubes along the direction of flowST: Center-to-center distance between tubes normal to the direction of flow
Adapted from ID Page 437