Experimental Investigation of Flow Boiling Instability in a Single Vertical Microtube:Effects of Hydraulic Diameter and Flow Orientation

Flow boiling in a microchannel heat sink is considered as a suitable and an efficient method to dissipate high heat flux from a small surface. Especially, this technique can achieve uniform axial temperature distribution and low noise with a little coolant and low pumping power consumption. However, the main drawback of this attractive technique is flow instability which is induced by the flow phase change. Flow instability can constrain the advantages of flow boiling heat transfer, or even damages systems. In this thesis, the fundamental investigations on the flow instability in a single vertical microtube are conducted. The objectives are to understand the flow oscillations types and features in vertical flow directions, the effects of geometric factors (hydraulic diameter of microtube and flow orientation) and operating conditions (mass flux and heat flux) on flow instability behaviors, and to investigate the inlet orifice for controlling flow instability in vertical flow directions. Three different sizes of stainless steel microtubes with 0.305, 0.533 and 0.889 mm hydraulic diameters are tested. The working fluid FC-72 maintains around 24 °C at the inlet of microtube. The mass flux varies from 700 to 1600 kg/m2•s, and the heat flux is applied on the tube surface uniformly up to 9.6 W/cm2. For the flow instability controlling study, two sizes of inlet orifices (50% and 20% area ratio) are investigated, respectively. The experimental results show that in a large hydraulic diameter, the onset of flow instability with obvious and sustained oscillation features is usually observed, and it can be delayed by large mass fluxes. In a small hydraulic diameter, the transient point is most detected and occurs earlier than in large size microtubes at a given mass flux, and the mass flux effect on its occurrence can be ignored. The buoyancy force impacts the flow instability appearance and characteristics. The irreversible flow blockage is observed in the smallest tube in downward flow direction and not sensitive to the mass flux. With more heat flux applied on the largest tube, the flow oscillations change to intensive in upward flow direction, but tend to be re-stabilized in downward flow direction. The 50% inlet orifice shows better performance at large mass fluxes or in upward flow direction. The 20% inlet orifice has a good ability to eliminate flow instability in the current investigation, but it induces higher pressure drop than 50% inlet orifice