Experimental study on critical heat flux characteristics of R134a flow boiling in horizontal helically-coiled tubes

Critical heat flux (CHF) experiments were performed to study the R134a CHF characteristics in horizontal helically-coiled tubes. The stainless steel test sections were heated uniformly, with tube inner diameters of 3.8-11 mm, coil diameters of 135-370 mm, helical pitches of 40-105 mm and heated lengths of 0.85-7.54 m. The experimental conditions are pressures of 0.30-1.10 MPa, mass fluxes of 60-480 kg m^-2 s^-1, inlet qualities of -0.32-0.36 and heat fluxes of 6.0 × 10³-9.0 × 10⁴ W m^-2. It was found that the wall temperatures jumped abruptly once the CHF occurred. The CHF values decrease with increasing heated lengths, coil diameters and inner diameters, but the DNB (departure from nucleate boiling) CHF seems independent when length-to-diameter L/d_i > 200. The coil-to-diameter ratios are more important than length-to-diameter ratios for CHF in helically-coiled tubes, while the helical pitches have little effect on CHF. The CHF value increases greatly with increasing mass flux and decreases smoothly with increasing pressure. It decreases nearly linearly with increasing inlet and critical qualities, but it varies more acutely at x_cr < 0.5 than higher critical qualities. New correlations for R134a flow boiling CHF in horizontal helically-coiled tubes were developed for applications.