Study of flow pattern transition criterion and heat transfer characteristics in vertical rectangular narrow channel for steam heating

Abstract

This study clarifies the heat transfer characteristics of flow boiling in a narrow vertical rectangular channel at each stage and optimizes the performance of a heat exchanger. The flow pattern and local heat transfer characteristics in a narrow vertical rectangular channel of steam heating with dimensions of 1400 × 250 × 2.75 mm (l × w × s) were experimentally investigated using deionized water as the working medium. Based on the visualization data, the mechanical model and empirical formulas in previous literature, a preliminary derivation of the flow pattern transition criterion for steam heating in a vertical rectangular narrow channel was obtained. The effects of the inlet temperature, mass flux, and heating power on the local heat transfer characteristics were analyzed in conjunction with the flow patterns. Based on the parametric analysis of the experimental data, a new flow boiling heat transfer correlation formula was obtained and applied to the experimental conditions. The results showed that an increase in the inlet temperature had a positive effect on the nucleate boiling section, whereas the effect on the local heat transfer coefficient was minimal after entering the churn flow phases. The increase in mass flux decreased the local heat transfer coefficient in the nuclear state boiling stage and enhanced the local heat transfer coefficient in the evaporation zone of the liquid film flow. The peak of the local heat transfer coefficient tended to shift to the left as the heating power increased. The proposed heat transfer correlation equation could well predict the local heat transfer coefficient of flow boiling in a vertical rectangular narrow channel heated by steam under these experimental conditions.