Experimental investigation on hysteresis effect of refrigerant flowing through a coiled adiabatic capillary tube

This paper experimentally investigates the mass flow rate hysteresis of R22 in a coiled adiabatic capillary tube. The measured mass flow rate data in a coiled adiabatic capillary are found to be more dispersive than those in a straight one. The scatter bands of measured data are significantly dependent on how the state points are achieved. For increasing inlet subcooling paths, the measured data tend to become consistent even though the starting point states are different. However, for decreasing subcooling paths, the data points are much more dispersive due to their different starting point states. The measured temperature distribution along the tube length confirms that the significant scattering in mass flow rate is caused by variation of the flash point location in the tube. Further, the refrigerant flashes earlier as the coiled diameter of the capillary decreases, thus leading to lower mass flow rates. It is suggested that for a coiled capillary tube, the secondary flow effect caused by the centrifugal force increases the uncertainty of the flash point location.