Experimental facility design and a diffuse interface numerical model for studying evaporating bubbles in mini-channels

Abstract

The development of compact and efficient heat exchangers employing phase change as heat transfer mechanism has gained a considerable amount of attention during the last decades. One of the main reasons for the increase of popularity of these devices is due to the possibility of high heat removal in smaller volumes when using mini- and micro-channels. However, the measurements for heat transfer in these channels differ from the predicted results from models developed for conventional channels and as a consequence different models developed for mini- and micro- channels should be employed. This is particulary the case for boiling heat transfer in micro-channels. There is a considerable amount of work presented in the literature related with prediction models for boiling heat transfer in mini- and micro-channels. Nevertheless none of these models are conclusive and in particular there is still no agreement within the mechanisms of heat transfer in such channels. In order to gain a better understanding of the mechanisms of boiling heat transfer in mini- and micro-channels, investigation should be focused on the evaporation process considering the nucleation and growth of bubbles. In this context, a numerical model for simulating bubbles growing during evaporation in a mini-channel was developed during this dissertation. At the same time, an experimental facility to study similar phenomena was designed and constructed. The numerical model for simulating bubbles growing during evaporation in mini-channel was developed using a least-squares finite element framework in combination with a diffuse interface approach for modeling two phase flow phenomena. Phase change phenomena were included into the governing equations using the diffuse interface approach. Two models of evaporation were studied. The first considered the evaporation of a droplet and the second the evaporation of a bubble. For the latter, the simulated cases included the evaporation of an isolated bubble and a bubble flowing in a mini-channel with constant temperature condition at the walls and constant heat flux at the walls. During the second part of this dissertation, the design and construction of an experimental facility is presented. The facility developed allows the direct visualization of the fluid inside the channel including also the visualization in the heated part. At the moment of the writing of this thesis, channel sizes in the range between 300μm and 800μm can be inserted in the facility. The channels are made of glass and present deposition of a transparent and conductive coating made of Indium-Tin-Oxide (ITO) for heating by Joule effect. The facility was validated through the measurement of heat transfer and pressure drop for single phase liquid flow using a channel of 500μm inner diameter and refrigerant R134a as working fluid. The experimental facility provided measurements of bubble growth rate during boiling for different flow and heating conditions.