Vibration effects on critical heat flux in a vertical round tube

Abstract

An experimental study has been performed to find out the relationship between CHF and vibration systematically. First, CHF experiment without vibration in vertical round tubes was conducted at low and medium pressure condition where the data are scarce. 251 CHF data in total have been collected to investigate the effect of independent parameters and to give information about the assessment of prediction method. It was revealed that the parametric CHF trends agreed well with those of previous studies. Within ranges of the given data, 1995 CHF table showed the best prediction performance giving -3.54% for average error and 18.1% for RMS error, and the backpropagation neural network (BPN) also gave reasonable prediction accuracy. Second, to find out the dynamic response of a tube at CHF condition, measurement of tube vibration in boiling process has been carried out under vertical upward flow at atmospheric condition. Vibration characteristics of the heated tube are significantly changed by heat transfer mode and flow patterns, therefore it is reasonable to say that the vibration measurement in heating condition can be an alternative method in the detection of heat transfer mode. In the case of liquid film dryout (LFD) type CHF condition, vibration could be regarded as negligible, however, these results cannot be extended to departure from nucleate boiling (DNB) type CHF mechanism Third, vibration controlled CHF tests are also accomplished to reveal the relationship between CHF and mechanical vibration. The CHF generally increases with vibration intensity that is represented by vibrational Reynolds number $(Re_v)$ and the CHF enhancement is more dependent on the amplitude than on the frequency. It seems that the CHF enhancement comes from reinforced fluid turbulence induced by vibration. Based on the heat transfer enhancement correlation, an empirical correlation for the prediction of CHF enhancement by tube vibration was suggested. The correlation predicts the CH...