Investigation of flow boiling characteristics in spiral mini channels

Curved channels tend to generate higher heat transfers due to their structure the generation of secondary flows and their ability to mix. This is a result of centrifugal and centripetal forces present within the channel, causing a constantly changing force distribution and resulting in flow mixing and increased heat transfer. Boiling flows are known to have a higher heat transfer coefficient and create a uniform temperature distribution, which is an essential advantage for heating and cooling applications. An experimental investigation of heat transfer, pressure drop, and boiling instabilities was conducted in a forced boiling environment in spiral mini-channels with a curved channel structure. The working fluid was used R134a. However, for a specific parameter, experiments conducted on R134a were replicated on R1234yf refrigerant, and a comparison was drawn. The operational parameters included determining two different channel lengths and three different channel inlet numbers of the spiral mini-channel. During experimental studies, three different mass fluxes (G = 100, 200 and 300 kg/m2s), heat flux ranging from 0-72 kW/m2, and the quality of vapor reaching up to x = 0.99 were tested. Three channel angles were varied from 0° (horizontal), 45° (angled) and 90° (vertical) to investigate the effect of inertial forces that may occur in the test system. All experimental parameters were tested at working pressures of 3, 4, and 5 bar. In the experimental study, boiling flow regimes for temperature, pressure and mass flux values were visualised with a high-speed camera. The experiments showed that the increase in mass flux resulted in an increase in heat transfer coefficient (HTC) of up to 150%. Based on the findings of the straight channel experiment, itvii was discovered that the centripetal and centrifugal forces on the spiral mini-channel (SMC) have a considerable impact on HTC and flow regime. Moreover, in the two-phase flow regime, the pressure drop in the SMC increased significantly, exceeding the rate of increase in heat transfer. This caused a change in the cooling coefficient of performance, which was inversely proportional to the mass flux. Despite the fact that the pressure, mass and temperature oscillations obtained for the SMC are generally similar to those reported in the literature for straight channel boiling flow instabilities, there are quantitatively large differences. In particular, it is observed that the pressure oscillations increase by a factor of 6 for the single inlet SMC compared to the other parameters. The HTC obtained for R134a refrigerant consistently higher than the values obtained for R1234yf in terms of pressure drop and pressure amplitude values.