Cooling systems incorporating convective boiling in mini- and microchannels achieve very high thermal performance. Although many investigations related to the subject have already been conducted, the basic phenomena of the heat transfer mechanisms are not yet fully understood. The development of empirical correlations based only on flow pattern maps does not lead to a deeper knowledge of the mechanisms. In this study a comprehensive measurement technique that was successfully adapted in pool boiling experiments [8,9] was used for the investigation of forced convective boiling of FC-72 in a single rectangular minichannel. This technique allows the measurement of the local temperature with very high spatial and temporal resolution. High speed video recording was used to observe the flow inside the minichannel. The inlet Reynolds number was kept constant for the first measurements to Re = 200 corresponding to a hydraulic diameter of the minichannel of 800 μm. The Bond number for the proposed setup is about Bo ≈ 1.2. Several flow pattern regimes such as bubbly flow, slug flow and partially dryout were observed for heat fluxes up to 25 kW / m2. From an energy balance at each pixel element of the thermographic recordings the local transient heat flux could be calculated and compared to the flow pattern video recordings. The results of the first experiments already give an indication about the heat transfer mechanisms at different flow regimes.

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Cooling systems incorporating flow boiling in mini- and microchannels achieve very high thermal performance. In this study a comprehensive measurement technique is used for the investigation of flow boiling in a single rectangular minichannel. This technique allows the measurement of local surface heat flux and temperature with a very high spatial and temporal resolution in combination with a synchronized flow regime observation [1]. This way, an insight into the basic phenomena is possible and will contribute to the fundamental understanding of the process. The experimental results show different phenomena such as an alternating area of high heat transfer between bubble front and end for elongated bubbles and the coalescence of bubbles in the flow. These observed phenomenon are comparable to experimental and numerical results of pool boiling.

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