Experimental investigation on flow patterns and pressure drop of r134a flow boiling in a horizontal helically coiled pipe

Experiments were performed to investigate the characteristics of flow boiling patterns and pressure drops of R134a in a horizontal helically coiled pipe. The test section is made of a 7.6 mm inner diameter and 10 mm outer diameter stainless steel tube, the structural parameters of which are eight coils, 300 mm coil diameter, and 30 mm pitch, which was heated directly by low-voltage and high-current direct current power supplies. The experiments were carried out under the conditions of saturation temperatures ranging from 5° to 20° refrigerant mass fluxes varying from 50 to 500 kgm ^-2s^-1, heat fluxes varying from 5 to 20 kWm^-2, and vapor qualities ranging from 0.01 to 0.9. The flow patterns were obtained through visualization experiments, and it was found that the flow patterns in the rising and declining sections of the horizontal helically coiled test pipe were somehow different under the same conditions, especially in the case of the observation of two new transition flow patterns that occurred before the formation of the annular flow; i.e., wave annular flow and super slug flow. Therefore, two different flow pattern figures have been proposed for the rising and declining sections according to different situations. The flow frictional pressure drops were experimentally determined and a new correlation has been developed to predict the flow frictional factors through the regression analysis of the experimental data.