Experiments of the Pressure Drop of Propane Considering Hysteresis and Metastability

Abstract

There is a broad database on two-phase pressure drop of refrigerants through capillaries in literature, since capillaries are a commonly used throttle device in heat pumps. However, the following essential aspects are insufficiently included: extensive data on natural refrigerants, precise capillary specifications, and the consideration of the hysteresis effect on vaporization delay and the metastable region of the flow, respectively. This paper contributes to a more comprehensive database of the two-phase pressure drop of propane with respect to the hysteresis curves that occur in the capillary. The present study investigates a copper capillary tube with 1 m length and 1.1 mm inner diameter. The manufacturer's specification of the capillary is refined to enable a more predictive numerical analysis of the pressure drop. Experimental data comprises a mass flux of 4300 kg/m²s, a constant inlet pressure of 15 bar and subcooling temperatures ranging from 5 °C to 9 °C. The presence and magnitude of the hysteresis becomes evident in the measurements: starting from a higher inlet temperature and increasing the subcooling temperature results in a higher pressure drop (~5 %) than starting from a lower inlet temperature and decreasing the subcooling temperature. Additionally, experimental results show that a metastable region must be present that leads to a delay in vaporization and thereby affects the measured pressure drop.