A comprehensive analysis of the gap size influence on boiling crisis in narrow rectangular channels

•Bubble growth confined factor and bubble velocity in narrow rectangular channels are modelled.•Precise characterisation of two-phase flow DNB, Dryout, and PM type boiling crisis in narrow rectangular channels.•A modified size correction factor is proposed that enables the look-up table to more accurately predict CHF values in narrow rectangular channels. The critical heat flux (CHF) is an extremely vital thermal–hydraulic parameter that must be comprehended and determined to ensure the safety and economy of operation, particularly in narrow rectangular channels, where bubbles will be confined by the gap sizes, leading to the existing research in the conventional channels cannot be directly adapted in narrow rectangular channels. The visualization results are the fundamental basis to clarify hydrodynamic characteristics, thus the visual analytical method is used to investigate the bubble dynamics and two-phase flow evolution phenomenon in narrow rectangular channels with gap sizes of 1 mm, 2 mm, 3 mm, 5 mm, and explore the influence of gap size on boiling crisis. Vast quantities of bubbles will be generated near the occurrence of a boiling crisis, based on the bubble dynamic characteristics, considering the effect of gap size, a confine factor model and a bubble velocity model are established. Two-phase flow evolution characteristics are an important priority for an in-depth understanding of heat transfer and boiling crisis, as the gap size decreases from 5 mm to 1 mm, the flow pattern in a channel corresponds to bubbly flow, slug flow, churn flow, and annular flow near the occurrence of the boiling crisis. DNB (departure from nucleation boiling) type boiling crisis can be discovered in bubbly flow, slug flow, and churn flow, Dryout type boiling crisis undertakes in annular flows, PM (premature) type boiling crisis alternate with slug flow and churn flow. The new diameter correction factors K*1 in LUT (look-up table) with the consideration of gap size limitation is put forward to accurately predict the CHF in narrow rectangular channels.