High-speed imaging of flow in microchannel array water evaporators

The flow of water and steam in an electrically powered micro heat exchanger consisting of an array of 68 wavy microchannels in parallel, each 200-μm wide, 100-μm deep and approximately 40-mm long, was investigated by high-speed videography. Semi-quantitative, time- and space-resolved information about the void fraction was extracted by pixel intensity calculations. Fluctuations at sub-audio frequencies (pulsation) were always visible in the microchannel array. Pulsation at the outlet reached a minimum with increasing heating power, but below the power required to achieve complete evaporation. This minimum was accompanied by a shift in the lateral distribution of channels filled with more steam or water. The differences between the test device and typical devices for applications in thermal and process engineering are also identified.