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Unformatted text preview: it HOWELL/ka #5122 DL/tﬁ MWMLL 53501 laminar flow over a ﬂat plate, the local heat
' ansfer coefficient hx is known to vary as if“,
here :c is the distance from the leading edge (x =
,0) of the plate. What is the ratio of the average coef ficient between the leading edge and some location
I x on the plate to. the local coefficient at x? In flow over a surface, velocity and temperature
profiles are of the forms u(y) = Ay + By“ — Cy3 and TO) = D + Ey + F312  Gy3 ,
where the coefficients A through G are constants.
Obtain expressions for the friction coefficient Cf  and the convection coefﬁcient h in terms of Lt”, To;
and'appropriate proﬁle coefﬁcients and ﬂuid proper;
ties. ' A fan that can provide air speeds up to 50 m/S i5 be used in a lowspeed wind tunnel with atm0§a pheric air at 25°C. If one Wishes to use the Wig5 tunnel to study flatplate boundaiy layer behavigr
up to Reynolds numbers of Rex = 105, what is the
minimum plate length that should be used? At what distance from the leading edge would transi tion occur if the Critical Reynolds number were
"Raw = 5 X 10°? Assuming a transition Reynolds number of 5 X 105,
determine the distance from the leading edge of a
@ , 'ﬂat plate at which transition will occur for each of
the following ﬂuids when it, = 1 m/s: atmospheric A air, water, engine oil, and mercury. In each case the
ﬂuid temperature is 27°C. Experimental results for heat transfer over a flat
plate with an extremely rough surface were found to
be correlated by an expression of the form N “x = 0.0412339 Prl/a Where Mix is the local value of the Nusselt number
at a position x measured from the leading edge of
the plate. Obtain an expression for the ratio of the average heat transfer coefficient/3x to the local coef
ficient hx. ‘ ExPerimentai measurements of the convection heat transfer coefficient for a square bar in cross flow
yielded the following values: 131 = 50 W/m2  K when VI = 20 m/s 122 = 40 W/m1  K when V2 = 15 m/s l“ o.L5=m "l Assume__ that the functional form of the Nusselt num
ber 15 Nu = C Re'" Pr", Where C, m, and n are con
stants.  (a) What will be the convection heat transfer coefﬁ cient for a similar bar with L = 1 In when V =
15 m/s? (b) What Will be the convection heat transfer coeffi cient for a similar bar with L =, 1 111 when V =
30 m/s? (c) Would your results be the same if the side of the I bar, rather than its diagonal, were used as the
I characteristic length? A thin, ﬂat plate that is 0.2 m by 0.2 m on a side is
oriented parallel to an atmospheric airstream having
a velocity of 40 m/s. The air is at a temperature of
Tm = 20°C. While the plate is maintained at ’1} =
120°C. The air flows over the top and bottom sur
faces of the plate, and measurement of the drag
force reveals a value of 0.075 N. What is the rate of
heat transfer fromboth sides of the plate to the air? ...
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 Spring '08
 Okamoto
 Heat Transfer

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