No abstract

No abstract

No abstract

When a long gas bubble travels in a horizontal liquid-filled channel of circular crosssection, a liquid film is formed between the bubble and the channel wall. At low Reynolds and Bond numbers, inertial and buoyancy effects are negligible, and the liquid film thickness is a function of the capillary number only. However, as the tube diameter is increased to the millimetre scale, both buoyancy and inertial forces may become significant. We present the results of a systematic analysis of the bubble shape, inclination, and liquid film thickness for a wide range of capillary, Bond, and Reynolds numbers, namely 0.024 ≤ Ca l ≤ 0.051, 0.11 ≤ Bo ≤ 3.5, and 1 ≤ Re l ≤ 750. Three-dimensional numerical simulations of the flow are performed by employing the Volume-Of-Fluid method implemented in OpenFOAM. In agreement with previous studies, we observe that buoyancy lifts the bubble above the channel axis, making the top liquid film thinner, and thickening the bottom film. As the Bond number approaches unity, the cross-sectional shape of the bubble deviates significantly from a circular shape, due to flattening of the bottom meniscus. The simulations demonstrate the existence of a cross-stream film flow that drains liquid out of the top film and drives it towards the bottom film region. This drainage flow causes inclination of the bubble, with a larger inclination angle along the bottom plane of the bubble than the top. As buoyancy becomes even more significant, draining flows become less effective and the bubble inclination reduces. A theoretical model for the liquid film thickness and bubble speed is proposed embedding dependencies on both capillary and Bond numbers, which shows good agreement with the reported numerical results. Inertial forces tend to shrink the bubble cross-section and further lift the bubble above the channel centreline, so that the bottom film thickness increases significantly with the Reynolds number, whereas the top film thickness is less sensitive to it.

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.