Experimental study on heat transfer deterioration and critical heat flux of DNB type in helically coiled tubes

Helically coiled tubes (HCTs) are extensively utilized in numerous engineering fields due to their exceptional heat transfer and structural capabilities. The thermal safety within HCTs is crucial for the long-term stable operation of heat transfer systems. The heat transfer deterioration and critical heat flux (CHF) of departure from nucleate boiling (DNB) type in the HCTs at subcritical pressure were experimentally studied in this paper. The special circumferential wall temperature distribution was obtained and the effects of inlet cooling, pressure, mass velocity and HCT orientation on CHF were investigated. As the inlet subcooling increased, the CHF increased almost linearly. As the mass velocity increased, the turbulence degree in the HCT increases significantly, bubbles were less likely to merge and accumulate, and the CHF increased substantially. It was found that the CHF in the vertical HCT was higher than that in the horizontal HCT under the same working conditions because of the combined action of gravity and centrifugal force. The CHF v /CHF h exhibits a reduction with elevated inlet subcooling and pressure, and the influence of mass velocity on CHF v /CHF h is related to the pressure. New orientation-specific CHF correlations for vertical and horizontal HCTs are developed, achieving high prediction accuracy across pressures of 16–20 MPa and mass fluxes of 320–560 kg m −2 s −1 . These findings can provide theoretical support for the optimization design and safe operation of the HCT heat transfer systems. • The heat transfer deterioration in helically coiled tubes were experimentally and theoretically studied. • Severe inhomogeneity of circumferential wall temperature existed when HTD occurred. • Influencing factors on CHF and circumferential wall temperature were explored & analyzed. • New orientation-specific CHF correlations for vertical and horizontal HCTs are developed.