Research Projects

The flow of fluid inside closed channels can be seen in the plethora of applications for the transport and heat exchange process. For the heat exchange process, the reduction in hydraulic diameter (miniaturization) results in the increase in the surface area to volume ratio of a channel and heat transfer coefficient (HTC). Thus, the miniature channels find applications where there is a need to extract high heat from a small area like in the cooling of electronics. However, there will be an increase in the pressure drop across the channel due to the confinement effect. The confinement of the generated bubble inside the mini/microchannel during flow boiling heat transfer instigates flow reversal, flow instabilities and bubble clogging. Due to rapid bubble growth in confined channels at high heat flux, the upstream movement of vapour or the backflow of vapour occurs which induces fluctuations in pressure, temperature, and mass flux, along with a significant reduction in the HTC. Moreover, these fluctuations induce mechanical vibrations and instigate premature critical heat flux (CHF) followed by device failure. Such early CHF is typically attributed to the accumulation of vapour, which moves upstream towards the inlet plenum and blocks the continuous supply of fresh fluid into the channels. Many design modifications and techniques have been devised and studied by different researchers to reduce instability. The high system pressure reduces the rapid bubble growth, which results in the reduction in fluctuations to low amplitude and high-frequency oscillations. The reduction in fluctuations/backflow can be achieved by (a) surface modifications inside the microchannel such as fabrication of nanowires, nanostructure, micropillars, and artificial nucleation sites with pressure drop element; (b) channel modification such as expanding channels, diverging channel, microchannel with inlet restrictor, microchannel with a re-entrant cavity, microchannel with micro pin fin; (c) heat sink design modification such as open channel, the orientation of ports or vapour venting manifold; (d) open form of flexible dampener. These above-mentioned modifications ensure easy removal of vapour in the downstream direction to the outlet port, thereby facilitating the rewetting and minimizing the backflow of vapour. However, these designs are complex and involve a complicated manufacturing process. Surface modification, like micro and nanostructured surfaces, requires sophisticated fabrication procedures with sustainability issues in the long term. The flexible dampener attached to the outlet port of the heat sink is pressure-sensitive, as it can operate at a specific pressure at the outlet port. To tackle the above issue of unstable flow boiling and to enhance the heat transfer flow boiling in mini and microchannel, some modifications are proposed which include a closed form of the flexible dampener and various combinations of passive flexible elements inside a channel.