Computations of boiling flows

Critical heat flux

Boiling nucleation on a very small film heater subjected to extremely rapid heating

We describe a method for measuring the average surface temperature of a small square thin metallic film deposited on a silicon substrate and immersed in subcooled water during a voltage pulse of short duration. The thin film studied is a material used in the current generation of commercial ‘desk–jet’printers and comprises a mixture of tantalum and aluminium 65 µm wide and 0.2 µm thick. The experiment uses a bridge circuit with a dynamic amplifier design to measure the evolution of electrical resistance, coupled with a separate calibration of the thin film resistor element with temperature to determine average surface temperature. Voltage pulses of 5 µs typical duration are applied to the thin films. An ‘inflection’ point in the resulting evolution of heater surface temperature identifies bubble nucleation. The calibration of the heater resistance with temperature showed a hysteresis effect that required a burn–in process to stabilize the electrical resistance. With the calibration curve obtained, resistance was converted to temperature and the results analysed. For low power input the average surface temperature exhibited an oscillatory behaviour which indicated a cyclic growth/collapse process often found in nucleate boiling. At higher powers, the oscillatory behaviour disappeared and gave way to an exponential variation of temperature with time similar to a lumped capacitance behaviour of a thermal system. An inflection point in the evolution of surface temperature was found that signified bubble nucleation. The largest heating rate and highest nucleation temperature measured was 0.25 x 109 °C s–1 and 556 K, respectively. This temperature is in good agreement with homogeneous nucleation theory as applied to a surface. The contact angles needed for measured nucleation temperatures to agree with predictions are within the range that is typical for water on metallic surfaces.

Okuyama Kunito

Papers

Transient boiling heat transfer characteristics of R113 at large stepwise power generation