Post-CHF, horizontal, swirl flow heat transfer, pressure drop, and drop size distribution

Heat transfer enhancement techniques can be utilized in both single- and two-phase flow systems. A two-phase flow region that can potentially benefit considerably from such devices is the post-CHF (critical heat flux) or mist flow region. This region is characterized by a continuous vapor flow entrained with liquid drops. Since the vapor is a poor conductor, the heat transfer coefficient is quite low compared to annular flow or nucleate boiling regions.;The method of enhancing heat transfer using twisted tapes is attractive because twisted tapes can be inserted and removed from tubes of existing systems and heat exchangers. The twisted tapes are of interest to nuclear steam generator applications where periodic tube inspections are necessary.;It is the objective of this study to investigate the axial and swirl flow post-CHF region in a horizontal configuration and to develop correlations that can predict the heat transfer coefficient, the total pressure drop, and the liquid drop size distribution. For this reason, A horizontal test section of 0.00474 m inside diameter and 1.62 m heated length was designed, fabricated and connected to the existing UIC flow boiling facility. In all of these experiments, the working fluid was R-113 and four tapes were used with tape twist ratios of Y = 4, 5.27, 7.63, and 9.15.;Existing correlation to predict the swirl flow, post-CHF heat transfer in vertical tubes with larger diameters was modified to account for the small diameter, horizontal configuration and the increased drop interaction at the high mass fluxes. No articles were found in the engineering literature that measured the post-CHF liquid drops size distribution. To study the entrained liquid drops in the post-CHF region, a Malvern system was utilized which is based on the principle of laser ensemble light scattering and employs Fourier optics.;It was found that the volume distribution histograms of liquid drops in the post-CHF region for both axial and swirl flows have three or four peaks while the number distribution histograms were bimodal in nature (two peaks). This suggested that the drops were mainly of two sizes (two-drop model). Also, the axial flow volume mean, surface mean, and number mean diameters were higher than those in swirl flow. Those diameters decreased with increasing mass flux, G, for axial flow or increasing (G/Y2) for swirl flow.;New correlations to predict the number mean, maximum and minimum number diameters for both axial and swirl flows were developed. Also, the percentages of drops associated with the maximum and minimum diameters were correlated. It was found that all swirl flow correlations of drop diameters and percentages were functions of a turbulence parameter (G/Y2). All axial flow drop diameters and percentage correlations were found to be a function of mass flux G.;Finally, axial flow has the highest percentage of maximum diameter drops. The swirl flow with the tightest twisted tape (Y = 4) has the highest percentage of minimum diameter drops which was found to increase as (G/Y2) increases. No effect was found of the quality at CHF on any of the previous discussed drop parameters.