Flow Instability (FI) for subcooled flow boiling through a narrow rectangular channel under transversely uniform and non-uniform heat flux

Transversely non-uniform heating enforces high local heat fluxes at a few areas on the heated surface and low local heat fluxes at others. Differences in the heat flux distribution may generate differences in the boiling behavior when compared with that under uniform heating. Thus, the present experiments investigate the influence of transverse heat flux distribution on the flow instability through a narrow rectangular channel (2.35 mm × 54.0 mm × 566 mm). A wide range of experimental operation conditions, such as inlet temperatures (35-65 °C), thermal power (500-6250 W), and mass flow rates (0.03-0.13 kg/s), are applied to the upward flow channel. The working fluid corresponds to demineralized water under atmospheric pressure. The channel is heated from a side, and the other side is a transparent polycarbonate window from which the bubble behavior is visualized by using a high-speed camera. The heating surface is composed of aluminum with a width of 50 mm and a length of 300 mm. The experiments are performed by using two different procedures to achieve the flow instability, namely (1) constant mass flow rate with power increases and (2) constant power with mass flow rate decreases. The results show that the flow instability occurs at similar thermal power and similar mass flow rates for both uniform and non-uniform heating conditions. However, the pressure drop and wall temperature curves exhibit differences in the trends between the two heating conditions, especially after the fluctuation in the inlet pressure. In the uniform case, bubbles are generated uniformly on the whole transverse direction of the heated surface. In the non-uniform case, more bubbles are generated at the higher local heat flux, which disturbs the velocity profile in the transverse direction. The differences in bubble generation in the transverse direction leads to differences in the flow instability in a narrow rectangular channel.