The apparatus described in Problem 7.40 is used by our students to experimentally determine convection heat and mass transfer coefficients, to confirm the heat-mass analogy, and to compare measured results with predictions based on standard correlations. The velocity, V, of the airstream is measured using a thermistor-based anemometer, and its relative humidity is determined from measurements of the wet and dry bulb temperatures, T_wb and T_db, respectively. Thermocouples are attached to the test-plate, which is covered with a sheet of wet paper in the mass transfer experiments.

(a) Convection heat transfer coefficient. Using the data provided in Problem 7.40, determine the heat transfer coefficients for the two velocities assuming the plate to behave as a space wise isothermal object. Evaluate the coefficients C and m for a correlation of the form Nu _L = C Re'" pr^1/3. Compare this result with a standard flat-plate correlation. Comment on the goodness of the comparison and provide reasons for any differences.

(b) Convection mass transfer coefficient. A sheet of water-saturated paper, 133 mm to a side, was used as the test surface and its mass measured at two different times, m(t) and m(t + ∆.t). Thermo couples ere used to monitor the paper temperature as a function of time, from which the average temperature, T_s, was determined. The wet and dry bulb temperatures were Twb = 13°C and Tdb = 27°C and data recorded for two airstream velocities are as follows:

Determine the convection mass transfer coefficients for the two flow conditions. Evaluate the coefficients C and m for a correlation of the form Sh_L = C Re^m Sc^1/3.

(c) Using the heat-mass analogy compares the experimental results with each other and against standard correlations. Comment on the goodness of the comparison and provide reasons for any differences.

Transcribed Image Text:

Water Mass Loss Observations m(t + Ar) (g) m(t) (g) At (m/s) T(C) (s) 3 15.3 55.62 54.45 475 240 16.0 55.60 54.50