Experimental Study on CHF Characteristics of R134a Flow Boiling in Horizontal Helically-Coiled Tubes

An experimental study of critical heat flux (CHF) was performed to investigate the CHF characteristics of R134a flow boiling in horizontal helically-coiled tubes. The test sections made of stainless steel were directly heated by high-power DC power sources to maintain constant heat-fluxes. The experimental parameters are the outlet pressures of 0.30–1.10 MPa, mass fluxes of 60–480 kg/m²s, inlet qualities of −0.32–0.36 and heat fluxes of 6–90 kW/²m. Thirteen helically-coiled tubes were tested with the outer diameters of 6–14 mm, inner diameters of 3.8–11 mm, coil diameters of 135–370 mm, helical pitches of 40–105 mm and valid heated lengths of 0.85–7.54 m. The detection for CHF occurrence was based on the event-driven method of Agilent BenchLink Data Logger Pro with a total of thirty-two 0.2mm T-type thermocouples set in every coil to measure the wall temperatures accurately. It was found that the wall temperatures jumped abruptly once the CHF occurred. The magnitude of CHF decreases with increasing valid heated length, coil diameter and inner diameter to some extent, while the DNB (departure from nucleate boiling) CHF seems independent when length-to-diameter L/di>200. Coil-to-diameter ratio is considered more important for CHF than length-to-diameter ratio in helically coiled tubes. The helical pitch has little effect on CHF. While CHF value increases much with increasing mass flux and decreases little with increasing absolute pressure, it decreases nearly linearly with increasing inlet qualities and critical qualities in different levels. But the CHF values at x_cr<0.5 vary acutely than those at higher critical qualities. New experimental correlations for the DNB and the dry-out CHF of R134a flow boiling in horizontal helically-coiled tubes were developed.