Study on Heat Transfer Mechanism of Isolated Bubble Nucleate Boiling With MEMS Sensors

This paper describes an experimental investigation of heat transfer mechanism beneath isolated bubble during nucleate boiling with MEMS sensors having high temporal and spatial resolution in temperature measurement. The MEMS sensor fabricated for the boiling research includes eight thin film thermocouples and an electrolysis trigger on the topside of 20 × 20 mm² silicon substrate and thin film heater on the backside. The electrolysis trigger initiates bubble growth by supplying hydrogen gasses as bubble nuclei with the electrolysis of the water by two electrodes. In the experiment, temperature fluctuation beneath an isolated bubble during saturated nucleate boiling of water was measured with the sensor. The measurement data presented strong evaporation and dry-out of the microlayer in the bubble growth phase and rewetting of the dry-out area in the bubble departure phase. Moreover, heat transfer induced by the boiling bubble was evaluated by computing local heat flux through a transient heat conduction simulation in the sensor substrate using the measured data as boundary condition. The heat transfer analysis shows that the local heat flux in the microlayer evaporation area has high value of the order of MW/m², and the maximum value of about 2 MW/m² is indicated near the center in an early phase of the bubble growth. On the other hand, the heat flux is very low of around zero at the dry-out area, where microlayer had disappeared completely, and slight increase was observed at the rewetting area. Total heat transferred from the surface reached to about half of latent heat in the bubble until the bubble departure. Finally, initial thickness of the microlayer under the bubble was estimated by integrating the derived local heat flux. As the result, it was distributed in a few μm within the measurement area.