Investigation of flow boiling heat transfer characteristics of CO2 in horizontal mini-tube

In this paper, the characteristics of heat transfer for CO2 flow boiling in horizontal micro-tube were studied. Experimental conditions: saturated temperature: -40-0 degrees C, heat flux: 5-35 W m(-2), mass flow rate: 200-1500 kg m(-2) s(-1), inner diameter: 1.5 mm. Experimental results show that: the increase of heat flux has significant effect on enhancing nucleate boiling heat transfer, while it's can also speed up the dryout process and decrease the starting vapor quality of dryout; the effect of mass flow rate on heat transfer coefficient is slight but as mass flow rate increasing, the starting vapor quality of dryout decreases, and the heat transfer coefficient increases after the dryout happens; the effect of saturation temperature on the physical properties of CO2 is the main cause of the different heat transfer characteristics in different experimental conditions, with the increasing of saturation temperature, the starting vapor quality of dryout has a decline tendency and the heat transfer coefficient has a steeper fall after dryout happens. Compared with the prediction model, the Cheng's model has a higher prediction accuracy, it is presented with 77.1% of prediction accuracy ration within +/- 30% error band and 20.5% mean absolute deviation before dryout phenomenon, while it has only 22.9% and 57.8% after dryout phenomenon.