SUBCOOLED POOL BOILING ON A HEATED WIRE WITH MICROBUBBLE EMISSION

摘要

To clarify the fundamental features of subcooled boiling on a small heated wire, the experiment of pool boiling of water was conducted by employing test wires with various materials and diameters for a wide range of subcooling. At low subcoolings, thermal plume takes place from the heated wire. In contrast, at high subcoolings, tiny micro bubbles appear around the wire and the so-called microbubble emission boiling (MEB) takes place. The size of the microbubble decreases with increasing subcooling as well as heat flux, ranging from several hundreds to ten micron meters. From high-speed video observation, it is found that the microbubbles generate from the upper surface of a primary bubble fluctuating due to rapid evaporation and condensation. The boiling is noisy and the power spectrum of the sound of ebullition has two peaks at around 1kHz and 3 kHz. The critical heat flux (CHF) increases with subcooling at low subcoolings but tend to a constant value at high subcoolings. At critical condition, the transition from nucleate to film boiling takes place at low subcoolings whereas burnout takes place at high subcoolings. The attained maximum heat flux reaches over 10 MW/m~(2), which is approximately ten times of the value of saturated condition. The burnout heat flux depends on the heater material as well as heater size.