Flow boiling in microchannels offers a promising and attractive solution for thermal management of electrical devices and power systems. However, a significant challenge for the implementation of microscale phase change heat spreader is associated with micro/nano flow instabilities due to insufficient micro/nano bubble removal, leading to local liquid dry-out which severely limits the heat removal efficiency. Therefore, a major issue in a flow boiling microchannel is to increase the nucleation site density, reduce the bubble diameter of detachment and increase the bubble departure frequency; these bubbles obstruct the heat transfer layer and the flow channel if they are not removed efficiently, leading to dry-out effect and malfunction of electronic devices. Developments in microfluidic...[ Read more ]

Flow boiling in microchannels offers a promising and attractive solution for thermal management of electrical devices and power systems. However, a significant challenge for the implementation of microscale phase change heat spreader is associated with micro/nano flow instabilities due to insufficient micro/nano bubble removal, leading to local liquid dry-out which severely limits the heat removal efficiency. Therefore, a major issue in a flow boiling microchannel is to increase the nucleation site density, reduce the bubble diameter of detachment and increase the bubble departure frequency; these bubbles obstruct the heat transfer layer and the flow channel if they are not removed efficiently, leading to dry-out effect and malfunction of electronic devices. Developments in microfluidic heat pipes will require sophisticated methods for handling microbubbles and fluids in the low Reynolds number regime that is imposed by the dimensions of the microchannels in these devices. In the micrometric scale, the relative importance of surface tension and viscous forces become predominant compared to inertia and buoyancy forces. Most of the previous research has focused on microchannels with homogeneous wettability smooth surfaces. However, these kinds of microchannels show certain disadvantages. On the one hand, a hydrophilic surface induces less bubble nucleation in the early stage, which increases the surface temperature, on the other hand, a hydrophobic surface decreases the frequency that bubbles detach, which deteriorates the critical heat flux. Wettability-patterned surfaces offer a promising solution to solve this dilemma. Nevertheless, for microchannels with hydrophobic islands on the hydrophilic surfaces, the questions about how the wettability-patterned dots in microchannels can affect subcooled flow boiling heat transfer, bubble dynamics and flow patterns remain unsolved.

The objective of this thesis work is to study flow boiling heat transfer, bubble dynamics and flow patterns in microchannels with wettability-patterned surfaces.

First, the effects of wettability patterns on bubble dynamics and heat transfer were explored and compared with that in a homogenous hydrophilic microchannel. It is found that the heat transfer coefficient in the wettability-patterned microchannel is about 22% higher than that in the homogenous hydrophilic microchannel. The mechanism behind the heat transfer enhancement is that the nucleation occurs easily on the hydrophobic dots, and the bubbles on the patterned surface show higher mobility and longer triple contact line. In addition, the mass flux effect on flow boiling in the wettability-patterned microchannel was investigated and it is found that the HTC improves as the mass flux increases.

After that, Microchannels composed of a hydrophilic heater with hydrophobic dots were applied to characterize the effect of pitch distance of adjacent dots on flow boiling heat transfer and pressure drop. It is found that bubbles coalesce more easily, and flow patterns change faster in the microchannel with smaller dot pitch distance. Heat transfer coefficient, critical heat flux, pressure drop, and flow instabilities are found to significantly rely on the pitch distance of dots. Based on pressure drop along rectangular microchannels containing a slug flow, the critical slug flow lengths of microchannels with various wettability-patterned surface were calculated theoretically. The results give a useful insight to explain how mass flux and wettability patterns affect flow boiling heat transfer.

Finally, based on a force-balance model, bubble detached diameters were predicted in hydrophilic, hydrophobic and wettability-patterned microchannels, respectively. This provides a useful solution to optimize the wettability pattern design and then improve flow boiling heat transfer in a microchannel.