An improved model for flashing flow in short tubes [heat pumps

Abstract

Short tube restrictors are commonly used as expansion devices in refrigeration and heat pumping systems. Flashing flow through short tubes is choked, i.e. independent of downstream conditions. Flow rate is typically predicted by empirically correcting the flow rate of compressed liquid from upstream pressure to saturation pressure at upstream temperature. The empirical correction factors depend on pressure and temperature, on short tube geometry and on the refrigerant used. This work extends and improves a model of short tube flow based on the physics of the observed flow phenomena. Short tube flow is believed to consist of a core of superheated liquid surrounded by an annulus of vapor. Evaporation is driven by heat transfer from the core to the interface and the flow is choked by the evaporated vapor. Flow rate is modeled by calculating the heat transfer rate, the evaporation rate and the choking effect of the vapor. The model attempts to improve on previous work by improving the accuracy with which thermodynamic properties are approximated, by improving the heat transfer model and by including the effects of frictional heating of the liquid. In comparison with experimental data, it is found that the improved thermodynamic modeling increases accuracy, but the change to the heat transfer model reduces accuracy. For the data examined, the effects of the frictional heating are small. The heat transfer model is based on an existing analytic solution with a mixing-length turbulence model. A appears that this model must be further improved, perhaps through empirical modification.