A device has been developed for measuring heat-transfer rates at many points underneath individual bubbles during boiling, in order to determine the heat-transfer coefficient as a function of position and time. The device is a planar array of small heaters that also serve as heat-flux sensors.

Ninety-Six Thin-Film Electrical-Resistance Heaters in a square array are kept at constant electrical resistance and thus constant temperature. The instantaneous power supplied to each heater is a measure of the instantaneous local heat-transfer coefficient.

Heretofore, the majority of experimental data on heat transfer in boiling have been obtained by use of devices as large as or larger than bubbles; consequently, the data have not been spatially resolved to dimensions smaller than those of bubbles, and little has been learned about local heat-transfer rates from wall surfaces under and around the bubbles as the bubbles grow and depart from the walls. The present device enables measurement of the local heat transfer from a wall surface during the growth and departure of a bubble, with very high temporal and spatial resolution. Thus, the present device is expected to contribute to better understanding of boiling heat-transfer mechanisms by indicating when and where in the bubble-departure cycle large amounts of heat are transferred from the wall. The information provided by this device could be used to validate or improve analytical and numerical models used in computational simulations of boiling. Other uses for temporally and spatially resolved heat-transfer data obtained by use of this device and similar devices include identification of time-varying structures in near-wall regions of turbulent boundary layers or impinging jets, study of heat-transfer coefficients associated with spray cooling processes, and determining shear stresses in turbulent flows.

The device (see figure) includes a quartz substrate that supports a square array of 96 electrical-resistance heaters. Each heater occupies an area of 250 by 250 µm. The resistive heating element in each heater is a platinum strip 5 µm wide, 0.4 µm thick, and 6 mm in total length; the lateral gap between adjacent parallel portions of the strip is 5µm. Each strip has an electrical resistance of ≈ 1 kΩ with a temperature coefficient of resistance of 2 × 10 –3 °C 1. Heater power is supplied through aluminum lines routed between the heaters to the edge of the array.

Each heater strip is electrically connected to one of the resistances in a Wheatstone bridge. The resistance (and thus the temperature) of the bridge is kept at a constant desired value by use of a feedback control circuit similar to circuits used in hot-wire anemometry: The circuit constantly adjusts the power supplied to the heater to keep the Wheatstone bridge balanced. Thus, the instantaneous power supplied to the heater is a measure of the local heat-transfer coefficient in that it equals the rate at which heat must be supplied to keep the affected portion of the surface at the desired temperature (e.g., the temperature of the wall in contact with the boiling liquid).

Because the heater strip is so thin, the frequency response of the heater greatly exceeds typical frequencies associated with boiling, making it possible to measure heat-transfer coefficients with high temporal resolution. The spatial resolution is, of course, determined by the pitch of the array.

This work was done by Jungho Kim formerly of the University of Denver and Richard Quine of the University of Denver for Glenn Research Center . For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Physical Sciences category.

Inquiries concerning rights for the commercial use of this invention should be addressed to

NASA Glenn Research Center
Commercial Technology Office
Attn: Steve Fedor
Mail Stop 4 —8
21000 Brookpark Road
Ohio 44135.

Refer to LEW-16825.