Working in groups of two, our students design and perform experiments on forced convection phenomena using the general arrangement shown schematically. The air box consists of two muffin fans, a plenum chamber, and flow strengtheners 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 T_∞ = 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 temperature T and time t have units of °C and s, respectively, and values of the coefficients appropriate for the time interval of the experiments are tabulated as follows: 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 space wise isothermal object.

(b) Evaluate the coefficients C and m for a correlation of the form Nu_L = C Re^m Pr^1/3 Compare this result with a standard flat-plate correlation. Comment on the goodness of the comparison and explain any differences.

Transcribed Image Text:

Air box Slot nozzle Air stream Test-plate Insulation pad T) = a + br + cf + d? + er Velocity (m/s) Elapsed Time (s) a ("C) b(°C/S) c("Cs) d ("C/s') e ("Cs) 300 160 56.87 57.00 -0.2641 -0.1472 3 x 10 9 x 10 -2 x 10* IX 10 -4 x 107 2 x 10 10