Investigation of flow boiling phenomena in small-scale complex geometries

This study concerns measurement and prediction of single-phase and flow boiling heat transfer coefficients and pressure drops in mini multi-channel geometries with and without interconnecting passages, including plate channel; parallel channel; in-line pin-fin and in-line off-set pin-fin surfaces. Experiments were performed with refrigerant R113 and deionised water at atmospheric pressure. Single-phase and flow boiling heat transfer coefficients and pressure drops were obtained over a range of effective heat fluxes and mass fluxes. For the plate and parallel channel surfaces, the results obtained have been compared with several published macro- and micro-channels correlations. For the in-line and in-line off-set pin fin surfaces, as the geometries have some similarities with tube bundles, the results obtained have been predicted using the standard correlations for tube bundles. The results also have been compared with several existing correlations developed based on macro-scale tube bundles and micro-pin-fin surfaces data. The saturated flow boiling heat transfer coefficients for the parallel channel and pin-fin surfaces were similar to within the experimental uncertainty, and considerably higher than the plate channel values, all dependent on heat flux and reasonably independent of mass flux and vapour mass fraction. This indicated that the dominant heat-transfer mechanism in the saturated boiling flow regime was nucleate boiling for all surfaces. The parallel channel, in-line and off-set pin-fin surfaces improved heat transfer by increasing the surface area and the heat transfer coefficient in comparison with the plate channel surface. The two-phase pressure drops in the parallel channel and pin-fin surfaces were considerably larger than that for the plate channel surface. Thus, the reduction in wall temperature is achieved by a significant pressure drop penalty. For the pin-fin surfaces, at low vapour qualities the heat transfer coefficients were in reasonable agreement with the conventional scale tube bundles correlations however as the vapour quality increases, the correlations were not able to predict the heat transfer coefficient as unlike the conventionally-sized tube bundles, the convective enhancement does not happen in the mini-pin-fin surfaces tested. The nucleate pool boiling correlation of Cooper (1984) provided a good agreement with the data for all surfaces tests with R113 and deionised water. The measured two-phase pressure drops for both pin-fin surfaces tests with R113 and deionised water were in a good agreement with the predicted values obtained from standard correlations for tube bundles, indicating pressure drop methods maybe transferable