Two-phase flow regimes and heat transfer in small tubes and channels

The estimation of flow boiling heat transfer requires a knowledge of the prevailing flow regimes. The flow regimes present in small channels have been observed to differ from those in large channels. Some progress has been made in determining heat transfer correlations for each regime but identification of the regime for particular flow conditions is still problematic. In this paper, experimental studies of flow regimes of gas-liquid flow in small tubes and channels are reviewed, in terms of size and geometry of flow passages, flow range, flow map presentation, and heat transfer behaviour. An experimental study of air-water flow in a small diameter (2.1mm) tube has been carried out. The flow patterns observed are compared against the predicted flow regimes from the previous studies. Annular slug flow has not been observed within the flow range examined to date. It is concluded that existing flow maps can predict flow regimes, however the flow regime boundaries observed are dependent on flow geometry and experimental conditions. A general flow map for narrow channels need to be established as a tool to aid designers of compact heat exchanger.