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.