(A) study on the critical heat flux for low quality flow boiling in vertical tube and tube diameter effect

Abstract

A new theoretical model of critical heat flux (CHF) is developed for the flow boiling condition from the bubble-detached to the low quality range. In a heated tube, after a significant void generation starts, a bubble layer develops and is closely packed near the wall. It is assumed that a thin liquid layer called superheated liquid layer exists under the moving bubble layer. The initial liquid layer thickness is presumed to be determined by the hydrodynamic instability. The bubble layer may act as a barrier of cooling liquid supply to the wall. So, the superheated liquid layer is depleted due to the evaporation along the heated length. The CHF condition is postulated to occur when the liquid cannot contact the heated surface any more due to the complete depletion of the superheated liquid layer at a certain location. The model is validated for the bubble-detached to the low quality range, through comparisons against the measured data for the vertical upflow of water in uniformly heated round tubes. The data set of 2313 test runs covers the following ranges of parameters: 34＜P＜180 bar, 210＜G＜7500 kg/㎡s, 0.0036＜D＜0.0375 m, 0.08 ＜L＜ 6.0 m. The model shows a very promising agreement with experimental data by simply applying well-known constitutive relationships without any tuning coefficients: a mean error of 2% and a r.m.s. error of 9.4 %. In general, the parametric trends are also sufficiently good without apparent systematic error over the examined ranges. The proposed model is also examined with the refrigerants of R-11, R-12 and R-113 without changing the proposed model. The model showed no particular loss of the prediction capability under no scalingfactor. The tube diameter effect, one of major parameters of CHF but remaining unsatisfactorily understood, is examined both experimentally and theoretically. The overall effect of tube diameter on CHF is investigated with the available experimental data. For the range of lack of available data, the CHF experim...