Condensation heat transfer coefficient during steam intermittent flows in a horizontal minichannel

Abstract

Contents of the paper refer to experimental investigations and subsequent prediction of the heat transfer coefficient for condensation of steam flowing in the 2.13 mm i.d. horizontal smooth circular minichannel. Experimental data obtained cover comprehensive span of two-phase flow conditions such that total mass flux is in the range of 178 kg/(m2·s) ≤ G ≤ 885 kg/(m2·s), steam quality varies within 0.003 ≤ x ≤ 0.118 and saturation temperature is ranging between 120.2 °C ≤ Ts ≤ 151.8 °C. The flow patterns were observed visually and images of interest were recorded and subsequently identified based on a map from the literature. There are three basic two-phase flow patterns studied such as slug, slug-annular and churn occurring in the tested minichannel during intermittent flow. Measurements of the local peripherally averaged condensation heat transfer coefficient for each of the patterns considered were carried out using of a heat flux sensor.Based entirely on empirical results achieved the correlation equations specific to the patterns studied have been developed in terms of pertinent dimensionless numbers predicting values of the condensation heat transfer coefficient measured at deviation of ±15%. The correlations show that the condensation process studied combines contributions by gravity induced falling-film condensation with superimposed influence of the liquid phase axial flow, steam quality and interactions by capillarity and surface tension forces.Validity of the correlations proposed is presented in terms of variability ranges of the dimensionless numbers employed specific for each of the pattern studied such as slug, slug-annular and churn. The data predicted on the correlations developed are then compared against those computed on well recognized contributed relationships referenced to steam in flow condensation found in the literature and discrepancies between the predicted and contributed results are displayed and discussed. The comparison reveals some cases for which the results predicted on the correlations proposed can differ substantially from those computed on existing contributions particularly when referenced to intermittent flows at slug and churn patterns.