DRY-OUT CHF CHARACTERISTICS OF R134a FLOW BOILING IN A HORIZONTAL HELICALLY-COILED TUBE

An experimental study was carried out to investigate the dry-out critical heat flux (CHF) characteristics of R134a flow boiling in a horizontal helically-coiled tube. The test section was heated uniformly by DC high-power sources and the geometrical parameters are the outer diameter of 10 mm, inner diameter of 8.4 mm, coil diameter of 300 mm, helical pitch of 75 mm and valid heated length of 1.89 m. The experimental conditions are the outlet pressures of 0.30-0.95 MPa, mass fluxes of 60-500 kg/m(2)s, inlet qualities of -0.36-0.35 and heat fluxes of 7-50 kW/m(2). In total of sixty-eight 0.2 mm T-type thermocouples were set along the tube to measure wall temperatures exactly. A method based on the event-driven Agilent Bench Link Data Logger Pro software was developed to determine the occurrence of CHF. It was found that the wall temperatures jumped abruptly once the CHF occurred. The CHF usually starts to form at the front and offside (270 and 90) of the sections near outlet. The CHF value increases largely with increasing mass flux and decreases slightly with increasing pressure. It decreases nearly linearly with increasing inlet qualities, while it decreases acutely with increasing critical qualities under larger mass flux conditions. An experimental correlation was developed to estimate dry-out CHF of R134a flow boiling in horizontal helically-coiled tubes under current conditions compared with the calculated results of Bowring and Shah correlations.