Laser-Based Diagnostics of Slug Flow Boiling in a Horizontal Pipe

Abstract

We present results from an experimental investigation on flow boiling, in the slug flow regime, of refrigerant R245fa through a 12.6-mm inner diameter horizontal plain pipe using particle image velocimetry (PIV) and an interface detection method. The study is supplemented by an overview of the state-of-the-art in experimental research of two-phase dispersed pipe flows and the development of modern optical and laser-based full-field non-intrusive measurement techniques as applied to these flows. We consider different flow conditions, with heat fluxes over the range 5.3 to 7.9 kW/m 2 and mass fluxes from 300 to 460 kg/m 2 •s. Significant disturbances in the instantaneous velocity fields are revealed in both the noses and tails of slugs, with their values being two times higher behind vapour bubbles. The slug passage frequency is determined based on the results of the interface detection method. The vapour bubble velocity is found to increase linearly with the interfacial velocity of the two-phase mixture, while its gradient grows with the heat flux.Moreover, at increased heat fluxes the bubbles may move even faster than the mixture itself, which implies that they must significantly enhance local turbulence, thereby additionally intensifying heat transfer. This research demonstrates the applicability of laser-based diagnostic techniques to boiling slug flows, to which such experimental methods have not been widely applied in macro-scale geometries. In addition to the conclusions, we provide practical recommendations for possible future research in this particular field of fluid mechanics and the further development of sophisticated laser-based measurement techniques for boiling, and similar, flows.

RS–DFID Africa capacity-building initiative programme grant: harnessing unsteady phase-change heat exchange in high-performance concentrated solar power systems

“…Furthermore, following a comprehensive review of appropriate (i.e. optically matched) solid materials and liquid substances for the design of dedicated experiments for the implementation of such measurements [23], the consortium was successful in the generation of a 'first-of-a-kind' phase and velocity-field information in boiling flows in horizontal pipes [24,25]. This effort necessitated, firstly, a systematic approach to develop and validate measurement techniques based on LIF and PIV/PTV methods, and a formal identification and quantification of the errors that arise in such measurements.…”

“…The combined application of PLIF and BBLIF allowed a reduction in errors associated with the separate application of individual approaches, but also, based on these limitations, a new technique was proposed, referred to as S-PLIF, which overcame the above errors with uncertainties of 10% and 5% in the measurement of local/instantaneous and average film thickness in annular flows, respectively [27]. In a final step, Voulgaropoulos et al [28] and Moran et al [24,25] studied the pool boiling of water and the flow boiling (in the 'slug-flow' regime) of R-245fa in horizontal pipes via the combined application of the laser-based techniques developed in the project in order to obtain spatiotemporal phase distribution, velocity and temperature fields over a range of heat and mass fluxes.…”

“…combinations of solid and liquid phases, that can allow the minimization of optical distortions and resulting errors when attempting to apply these optical measurements in a range of multiphase flows. The consortium was successful in applying-for the first time-LIF-based and PIV/PTV methods to boiling flows in horizontal pipes [24,25]. We will return to these results later in §4.3, where we outline the key findings and contributions made to the field by the consortium during the remit of this project.…”

“…In a final step, in terms of the development and application of advanced measurement techniques to these flows, Moran et al [24] investigated flow boiling via the application of the laser-based techniques described in §4.2. At the investigated conditions, the flow was found to be in the 'slug' regime, whereby large bubbles dominate the flow.…”

“…It was suggested that these high-flux, high-speed slug regimes may give rise to an enhancement in the local turbulence levels in the flow compared with lower-heat-flux regimes. For the case of flow boiling, further work by Moran et al [24,25] involved the use of LIF (described above), along with PIV, to investigate the velocity within flows of R245fa simultaneously with pressure drop measurements from pressure gauges. In their experiments, the authors used R-245fa and a bespoke flow-boiling facility consisting of a flow circuit and an optical section for visualizations.…”