Session: 7.9 - Multiphase flows in Environmental and Industrial applications

Paper Number: 156356

156356 - Experimental Investigation of Gas-Bubble Formation due to Supersaturation in a Plate Heat Exchanger 

Abstract: 

To mitigate global warming, in parallel with attempts to establish cleaner energy sources, actions must be taken to improve the efficiency of existing energy-consuming systems. Heating and cooling systems account for approximately 15% of global carbon emissions and 50% of global energy consumption. As such, an in-depth investigation of their performance is an essential element of efforts to advance the Green Transition and achieve a more sustainable society. One of the key uncertainties affecting the performance of these systems is the presence of air. Air can enter a heating system through component leaks or as dissolved gas in feed water, with the latter potentially emerging as free gas due to local supersaturation.
Given the increasing use of plate heat exchangers in heating systems due to their high power-to-volume ratio, this research will experimentally investigate the impact of bubble formation due to water supersaturation in a chevron plate heat exchanger.
The experimental setup is comprised of two separate temperature-controlled water loops, pressure and flow rate controls, a degassing unit, a nitrogen-filled saturation tank, and a plate heat exchanger. The plate heat exchanger has a channel per pass for a total of two passes and visual access to the last cold channel. Cold streams are studied at eight Reynolds numbers ranging from 520 to 2050 (corresponding to a volumetric flow rate of 0.05 to 0.2 m³/h) at a constant temperature inlet of 30^o C. The hot stream is maintained at 0.3 m³/h and 60^o C throughout all experiments. The nitrogen saturation level on the cold side is tracked with a total gas pressure sensor and varied between 10% and 100%.
High-resolution images of the last channel show the emergence of bubbles at any Reynolds number when the liquid is highly saturated while no visible bubbles can be observed at low saturation levels. In compliance with Henry’s law, pressure reduction and temperature increase occurring in the heat exchanger do not bring the fluid to the supersaturated state if at the inlet the saturation level is lower than a certain value. Surface coverage and bubble diameter are studied through imaging techniques under different system conditions. At higher Reynolds numbers, the surface coverage ratio and bubble diameter reduce, while low Reynolds numbers allow the bubbles to coalesce and grow in size.
Finally, the thermal performance of the plate heat exchanger is evaluated by comparing the convective heat transfer coefficient of the cold channels and the overall heat transfer coefficient at high and low saturation levels along the entire Reynolds number range.

Presenting Author: Roberto Bricalli Aarhus University

Presenting Author Biography: