Experimental investigation of flow boiling instability and associated pressure fluctuations in a mini-channel heat sink

Two-phase flow instability, inherently accompanied by hydraulic and thermal oscillations, is a complex phenomenon that must be overcome to design an efficient flow boiling heat transfer system. The potential factors influencing two-phase flow instability are so diverse and complex that many efforts have been devoted to identifying and classifying flow instabilities. Density wave oscillation and pressure drop oscillation in a single channel represent classical flow instabilities, and their mechanisms are relatively well established. However, parallel channel instability observed in multi-channel heat sinks is challenging to analyze due to flow interaction between neighboring channels through the inlet/outlet plenum. Even the number of channels, the size and shape of the plenum, and the inlet/outlet arrangement influence the fluid flow in multi-channel heat sinks, complicating the analysis of previous experimental data from the literature. Therefore, to address the complexity of multi-channel flow boiling and enhance the understanding of its behavior under specific conditions, additional experimental studies are essential. This study conducts flow boiling experiments using FC-72 with a mini-channel heat sink consisting of 22 parallel channels, each with dimensions of 1.6 mm in width and 4.8 mm in height, operating under two distinct pressure conditions. Flow instability occurring within the present channel flow boiling is investigated by visualizing the flow in the channel upstream and inlet plenum, as well as through spectral analysis of the pressure fluctuation. The fast Fourier transform was used to characterize the oscillation characteristics of transient pressure measured at various locations in the test loop. In addition, visualization data and fast Fourier transform results identified flow oscillation caused by vapor backflow in the channel. As a result, the present flow boiling data are classified as stable or unstable based on the frequency of pressure oscillation, and a new stability map for mini-channel flow boiling is proposed.