Experimental study on the relationship between the flow pattern and heat transfer characteristics of flow boiling in a vertical upward rectangular narrow channel

Abstract

This study investigated the evolution of flow patterns and heat transfer characteristics of flow boiling within vertical upward rectangular narrow channels. The channel, visualized and heated to simulate compact heat exchangers and the fuel channels of small modular reactors (SMRs), has geometric dimensions of 720 mm × 240 mm × 2.75 mm (l × w × s) and employs deionized water as the working fluid. Experimental conditions include inlet temperature in the range of 50 - 90 °C, mass flux between 4.04–24.24 kg/(m²·s), and heat flux between 3.32–64.12 kW/m². Images acquired via high-speed camera are presented to elucidate the flow regime physics. Different types of flow pattern maps were drawn and compared with other researchers’ data under adiabatic and steam heating conditions. A new type of dimensionless correlative flow pattern transition criterion for vertical rectangular narrow channels was obtained and showed good accuracy in predicting the critical vapor quality of flow pattern transition. The results indicated that thermo-hydraulic parameters influence the positions and length of flow patterns, and the overall trend of the local heat transfer coefficient (HTC) remains consistent during flow pattern transition. Peak HTC values occurred during the later processes of restricted bubbly flow and annular flow. Critical heat flux (CHF) exhibited a positive correlation with mass flux but showed minimal sensitivity to inlet temperature. Specially, due to the intermittent wetting behavior of the tidal flow, heat transfer deterioration demonstrated a delayed response, and the heat flux of optimal average HTC was higher than the CHF.