Experimental Investigation on Flow Instability in a Single Vertical Microtube

The experiments are conducted to study the flow boiling instability in a single microtube with 0.889 mm hydraulic diameter in vertical upward and downward flow directions (VU and VD). The subcooled dielectric liquid FC-72 is driven at mass fluxes varying from 700 to 1400 kg/m²·s, and the heat flux uniformly applied on the microtube surface is up to 9.6 W/cm². The onsets of flow oscillations (OFIs) in both flow directions are observed. Their oscillation types and characteristics are presented as well. The effects of mass flux and heat flux on flow instability in vertical flow directions are discussed. The results show that as the mass flux increases, the OFI occurrence is postponed, and the compounded oscillation types (Ledinegg, pressure drop and density wave oscillations) turn to pressure drop type dominant. At low mass fluxes, the OFI appears earlier in VD than in VU due to the buoyancy force impeded the bubble discharging. As the mass flux increases, the OFI appearance in VD is close to the ones in VU and its flow oscillations tend to be re-stabilized. After OFIs appeared at a given mass flux, with more heat flux added, the density wave oscillation type in VU becomes more active. However, at a constant mass flux, as the heat flux increases, the flow instability in VD becomes “stable” which may be due to the rapid flow pattern change, and this kind of “stable” is not expected because the local dryout may accompany. Hence, the microtube with vertical upward flow direction (VU) performs better from flow boiling instability point of view.