Two-dimensional heat transfer coefficients with simultaneous flow visualisations during two-phase flow boiling in a PDMS microchannel

•Local heat transfer measurements at high spatial resolution and frequency were achieved.•Mapping of local temperature simultaneously with high speed CCD is performed for flow boiling.•The use of PDMS allowed for the first time access to solid–liquid interface during the process.•Integrated pressure sensors allowed correlation of heat transfer and pressure data. Infrared (IR) thermography was combined with simultaneous high speed flow visualisation and pressure measurements from integrated pressure sensors inside the microchannel, in order to produce two-dimensional (2D) high spatial and temporal resolution two-phase heat transfer coefficient (HTC) maps across the full domain of a polydimethylsiloxane (PDMS) high aspect ratio (a = 22) microchannel (Dh = 192 μm). High spatial and temporal resolution two-dimensional wall temperature measurements and pressure data were obtained for a range of mass fluxes (G = 7.37–298 kg m−2 s−1) and heat fluxes (q = 13.64–179.2 kW m−2) using FC-72 as the working liquid. The 3D plots of HTC provided fine details of local variations during bubble nucleation, confinement, elongated bubble, slug flow and annular flow regime. The optical images from the channel top revealed the local flow regimes and were correlated with simultaneous thermal images obtained from the channel base. The 3D plots of the 2D two-phase heat transfer coefficient with time across the microchannel domain were correlated with vapour-liquid dynamics and liquid film thinning (from the contrast of the optical images) which caused suspected dryout. The correlation between the synchronised at the same frame rate high-resolution thermal and optical images will assist in a better understanding of the heat transfer mechanisms during two-phase flow boiling in microchannels. This work intends to give a better insight into heat transfer coefficient spatial variation during flow instabilities with two-dimensional heat transfer coefficient plots as a function of time during a cycle of liquid–vapour alternations.