Experimental investigation of the effects of helical wire coils on critical heat flux, boiling heat transfer and two-phase flow patterns in round tubes

Experimental studies of critical heat flux (CHF), boiling heat transfer and two-phase flow patterns were performed in round tubes in the presence of internal helical wire coils. The data were obtained using the University of Cincinnati's boiling loop and refrigerant 113 as the working fluid.;The experimental CHF tests showed that the CHF in ribbed tubes is significantly above that in smooth tubes over most of the range of interest. The CHF enhancement does not begin until liquid flow rates exceed those at which bubbly-core flow (onset of swirl) is observed. An empirical correlation of the ratio of CHF in helically ribbed tubes to that in straight tubes was developed. Extrapolation of this correlation was consistent with the limited CHF data for tight-pitch coils in the literature. Both the literature data, and the empirical CHF correlation, show that the power required to achieve very high CHF values with tight-pitch wire coils is substantially lower than that needed using smooth tubes or tubes containing twisted tapes.;The flow patterns observed in horizontal flow were essentially similar to those previously observed with air-water. Modification of the flow pattern transition correlations which had been developed for air-water flow were able to predict nearly all of the refrigerant flow pattern transitions in horizontal lines. However, the flow pattern observations in vertical lines showed the presence of a new flow pattern, designated bubbly-core flow. Here the was swirling liquid flow around the tube perimeter and a central (non-swirling) bubbly region. In contrast to earlier tests of boiling heat transfer in tubes with twisted tapes, boiling heat transfer coefficients were not increased over those seen in smooth tubes. A slight modification of the boiling suppression correlation of Gungor and Winterton (58) provided a conservative prediction of the observed heat transfer coefficients.