Comparison analysis of density wave oscillation type two-phase flow instability of superheated and saturated boiling tubes

Abstract

Most of previous conclusions about two-phase flow instability come from saturated two-phase flow boiling tubes (SatTFB). Superheated two-phase flow boiling tubes (SupTFB) can generate superheated steam, which will make the thermodynamic cycle efficiency higher and the steam generator or system simpler. However, there is no systematic mathematical analysis about the difference between the two-phase flow instabilities of SatTFB and SupTFB. Algebraic criteria composed of dimensionless numbers are deduced to easily determine their flow instability boundary. Different from six dimensionless numbers used in SatTFB, SupTFB have one more dimensionless number. It is superheated number or two-phase number. The verified model is applied to investigate the parameters’ effects and the transient characteristics of density wave oscillation (DWO) type two-phase flow instability. The effects of Froude number and friction number for SupTFB are the same as that for SatTFB. When system pressure is low, only the case with high inlet resistance coefficients can be stable in SupTFB. Thus for SupTFB at low system pressures, they become stable when increasing the inverse of Froude number (or inclination angle) or decreasing friction number, which do not have the nonmonotonic phenomenon as the SatTFB. For the added superheated steam region, increasing its friction number makes SupTFB unstable. The reasonable application range of the figure expressed by subcooling number and phase change number is the same for both SatTFB and SupTFB when analyzing parameters’ effects. Different from increasing continually for SatTFB, the ratio between oscillation period and transit time for the SupTFB first increases with subcooling numbers and then keeps unchanged at high subcooling numbers.