Effect of upstream compressibility on pulsating gas-liquid flow in microchannels

Abstract

Upstream compressibility has been shown to have a significant effect on flow instabilities, heat transfer and flow patterns of flow boiling in microchannels. However, this factor has almost always been overlooked in previous research and only a few investigations have addressed well-controlled upstream conditions. This thesis aims to establish a system with well-defined upstream boundary conditions and study the effect on gas-liquid flow pressure drop in microchannels. Compared with the complex flow boiling mechanism, Taylor flow which has a much simplified and well-defined flow regime was applied in this work. A pulse generator was devised to introduce highly repeatable and reliable pulsations. Pulsating Taylor flow experiments were firstly carried out in a hydrophilic glass tube and a hydrophobic PFA tube without upstream compliance. While the effect of pulsation on average pressure drop in the hydrophilic test section was found to be negligible, significant pressure drop reductions were observed in the hydrophobic test section. With the introduction of upstream compressibility, a change in average pressure drop was observed in the hydrophilic test section and the trends of pressure drop variation were found to be strongly dependent on the pulsation frequency. A mathematical model was developed to understand the pulsating flow behaviour in the presence of upstream compressibility. Visualisation studies showed that the imposed pulsation also caused the bubble and slug lengths to vary simultaneously. The calculation of the average pressure drop using the equation proposed by Kreutzer et al. (2005) revealed that the pressure drop variation was caused by the bubble and slug lengths variations. A preliminary study of pulsating annular flow was also conducted.