Narrow rectangular channels are widely used in major thermal flow fields because of the compact structure and large heat transfer area.

This study aims to improve the prediction method of critical heat flux in the narrow rectangular channel and establishing the critical heat flux (CHF) mechanism model for the enhancement of reactor safety and economy.

CHF experiments was carried out in the present study to identify the dominant mechanism in a narrow rectangular channel at different gap sizes. The visualization experiments were performed at pressures ranging from 1 MPa to 4 MPa, inlet subcooling from 60 K to 120 K, and mass flux from 350 kg·(m²·s)^-1 to 2 000 kg·(m²·s)^-1.

According to the visual experiment results, two typical bubble behaviors are investigated in the narrow rectangular channel. Based on the bubble dynamics characteristics of narrow rectangular channels, a new CHF mechanism model was proposed, and a set of constitutive relations will be provided to close the developed model.

A comprehensive assessment of new model has been conducted and analyzed by using the experimental data for the upward flow in a vertical narrow rectangular channel and it has good accuracies of less than ±30% as relative to the experimental values.