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 supplied 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 onset of flow instabilities (OFIs) in both flow directions is observed without inlet orifice. Their oscillation types and characteristics are also studied. The results show that as the mass flux increases, the compounded oscillation types (Ledinegg, pressure drop and density wave oscillations) turn to pressure drop type dominated. The buoyancy force influences the OFIs occurrence in different flow directions. After OFIs appear, with more heat flux applied, the density wave oscillation type in VU becomes more active; however, the flow instability in VD trends to be “stable” due to the rapid flow pattern change but this kind of “stable” is not expected because local dryout may accompany. Two sizes of inlet orifices are used to test their abilities for stabilizing the flow oscillations. These inlet orifices form 50% and 20% area ratios with the main microtube, respectively. The 50% inlet orifice delays OFIs occurrence as mass flux increased, and it shows better performance in VU. The 20% inlet orifice shows good ability to eliminate flow instability.
ASJC Scopus subject areas
- Fluid Flow and Transfer Processes