Consider a vertical 20-cm-tall flat plate at 120°C suspended in a fluid at 100°C. If the fluid is being forced past the plate from above, estimate the fluid velocity for which natural convection becomes negligible (less than 10%) in: (a) mercury (b) air (c) water.
GIVEN
? A vertical flat plate suspended in a fluid
? Plate temperature (Ts) = 120°C
? Fluid temperature (T?) = 100°C
? Fluid is being forced past the plate from above
? Plate height (H) = 20 cm = 0.2 m
FIND
? The fluid velocity (U?) for which natural convection has a less than 10% effect in
(a) mercury (b) air (c) water
ASSUMPTIONS
? Steady state
SKETCH
PROPERTIES AND CONSTANTS
From Appendix 2, Tables 26, 28 and 13
From Equation (8.55), for laminar forced convection over a flat plate, the effect of buoyancy will be
less than 10% if
Solving for the fluid velocity
The Reynolds numbers for these fluid velocities are
These Reynolds numbers are all within the laminar regime (mercury is approaching the transition to turbulence). Therefore, the use of Equation (8.55) was valid.
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