Flow boiling heat transfer in the quenching of a hot surface under reduced gravity conditions.

摘要

An experimental set-up, which combined a new state of the art micro-sensor for instantaneous measurements of heat flux and surface temperature, was designed, constructed and used to study the effects of gravity, as well as inlet liquid flow rate and subcooling on rewetting of a hot horizontal surface. The experiments were conducted by injecting liquid R113 and PF5060 into an initially dry, heated channel, which was 40 mm wide, 5 mm high and 200 mm long, on the ground and in reduced gravity aboard the parabolic aircraft, KC-135 and DC-9 of the NASA. The measurements showed large instantaneous fluctuations in heat flux and surface temperature following the onset of rewetting, even after the maximum heat flux was passed, where the heat transfer mode changed from transition boiling to nucleate boiling. Heat flux and surface temperature data showed synchronized responses indicating sufficiently fast response of the sensors and the reliability of the measurements. The boiling curves covering film, transition and nucleate boiling regimes were obtained during quenching and analyzed. The heat transfer characteristics in each boiling mode, as well as rewetting temperature, quench velocity, liquid-solid contact frequency in transition boiling and maximum heat flux were examined in detail for different gravity levels, inlet liquid flow rate and subcooling. The quench velocity and rewetting temperature were found to decrease for R113 but only showed very slight decreases for PF5060 in reduced gravity. A peak in the liquid-solid contact frequency curve was found at wall superheats of 107 ∼ 118°C for R113 and 65 ∼ 83°C for PF5060 in both gravity conditions. The maximum heat flux for both fluids decreased in reduced gravity except for R113 at high flow rate.