Working in groups of two, our students design and perform experiments on forced convection phenom- ena using the general arrangement shown schemati- cally. The air box consists of two muffin fans, a plenum chamber, and flow straighteners discharging a nearly uniform airstream over the flat test-plate.The objectives of one experiment were to measure the heat transfer coefficient and to compare the results with standard convection correlations. The velocity of the airstream was measured using a thermistor- based anemometer, and thermocouples were used to determine the temperatures of the airstream and the test-plate.
With the airstream from the box fully stabilized at Too = 20°C, an aluminum plate was preheated in a convection oven and quickly mounted in the test plate holder. The subsequent temperature history of the plate was determined from thermocouple measurements, and histories obtained for airstream velocities of 3 and 9 m/s were fitted by the following polynomial:
The plate is square, 133 mm to a side, with a thickness of 3.2 mm, and is made from a highly polished aluminum alloy (p = 2770 kg/m3, c = 875 J/kg · K, k = 177 W/m · K).
(a) Determine the heat transfer coefficients for the two cases, assuming the plate behaves as a spacewise isothermal object.
(b) Evaluate the coefficients C and m for a correlation of the form NuL = C Rem Pr1/3 Compare this result with a standard flat-plate corre- lation. Comment on the goodness of the comparison and explain any differences.