Critical heat flux in helical coils with a circumferential heat flux tilt toward the outside surface

Abstract

A study of the forced-convection boiling of R-113 in electrically heated coils with a substantial heat flux tilt toward the outside of the coils is reported. The circumferentially local subcooled critical heat flux (CHF) with a flux tilt is increased or decreased, when compared to a coil without a flux tilt at the same bulk fluid condition, depending on the relative magnitudes of the flux tilt, mass velocity and ratio of tube diameter to coil diameter ( d/ D). Generally, there are increases in the local CHF with decreasing mass velocity and d/ D and with increasing flux tilt. The quality region CHF in a coil with a flux tilt is lower than the CHF in a coil without a flux tilt. The subcooled and quality region CHF condition (when unaffected by buoyancy) occurs at the inside surface of the coil and not at the location of highest heat flux. Operational problems, in particular upstream CHF conditions, may occur if a coiled tube is operated with low to moderate subcooling near the inlet and with moderately high heat fluxes.