A study of system-induced instabilities in forced-convection flows with subcooled boiling

Abstract

A combined analytical and experimental program was carried out to investigate the problem of hydrodynamic stability of forcedconvection flows with boiling. The study was restricted to the flow of water in small channels (<".250 I.D.), high length-to-diameter ratios (25-200), moderate temperature and pressure (Ti ~ 70[degree] F; pex< 60 psia), and primarily directed toward subcooled, local boiling (hex< hsat.liq.). Two types of instability were recognized: The first, a nonrecurring excursive instability, and the second, an oscillatory instability in which the operating point varies in a sustained, repetitive way around the original condition. The excursive behavior was predicted on the basis of a comparison of the slopes of the pressure drop vs. flow rate curve of the heated section and the external system. The criterion for stability was simply that the slope of the external system characteristic be more strongly negative than that of the heated section. This was verified experimentally. This excursion was found to be the limiting condition on the heat flux in a parallel channel system. The oscillatory instability was investigated analytically through the use of a linearized, lumped parameter model in which steady-state measurements of the pressure drop were assumed to be valid in characterizing the transient characteristics of the heated section. For cases in which the energy storage mechanism was a compressible volume either upstream of, or within, the heated section, a critical slope of the pressure drop vs. flow rate curve in the heated section could be computed, as well as an associated frequency. The analytical results were verified on a single-tube apparatus with controlled compressibility, and the onset of the instability agreed well with the theoretical predictions. The criterion was applied to the data of a number of other investigations and gave excellent correlation in all cases where the restriction of subcoole