Interfacial instability in turbulent flow over a liquid film in a channel

International audienceWe revisit the stability of a deformable interface that separates a fully-developed turbulent gas flow from a thin layer of laminar liquid. Although this problem has received considerable attention previously, a model that requires no fitting parameters and that uses a base-state profile that has been validated against experiments is, as yet, unavailable. Furthermore, the significance of wave-induced perturbations in turbulent stresses remains unclear. To address these outstanding issues, we investigate this problem and introduce a turbulent base-state velocity that requires specification of a flow rate or a pressure drop only; no adjustable parameters are necessary. This base state is validated extensively against available experimental data as well as the results of direct numerical simulations. In addition, the effect of perturbations in the turbulent stress distributions is investigated, and demonstrated to be small for cases wherein the liquid layer is thin. The detailed modelling of the liquid layer also elicits two unstable modes, 'interfacial' and 'internal', with the former being the more dominant of the two. We show that it is possible for interfacial roughness to reduce the growth rate of the interfacial mode in relation to that of the internal one, promoting the latter, to the status of most dangerous mode. Additionally, we introduce an approximate measure to distinguish between 'slow' and 'fast' waves, the latter being the case for 'critical-layer'-induced instabilities; we demonstrate that for the parameter ranges studied, the large majority of the waves are 'slow'. Finally, comparisons of our linear stability predictions are made with experimental data in terms of critical parameters for onset of wave-formation, wave speeds and wavelengths; these yield agreement within the bounds of experimental error

Computational study of bubble, thin-film dynamics and heat transfer during flow boiling in non-circular microchannels

Flow boiling in multi-microchannel evaporators is one of the most efficient thermal management solutions for high-power-density applications. However, there is still a lack of understanding of the governing two-phase heat and mass transfer processes that occur in these devices, which has resulted in a limited availability of applicable boiling heat transfer prediction methods based on first principles, and of reliable thermal design tools. This article presents a systematic analysis of the dynamics of bubbles and the surrounding liquid film during flow boiling in three-side-heated non-circular microchannels. The study is performed using a custom version of ESI OpenFOAM v2106 with a geometric volume-of-fluid method to capture the interface dynamics, also incorporating conjugate heat transfer through the evaporator walls. The hydraulic diameter of the channel is fixed to ℎ = 0.229 mm and the range of width-to-height aspect ratios = 0.25−4 is examined. We investigate different fluids, namely water, HFE7100, R1233zd(E), R1234ze(E), and evaporator materials, namely copper, aluminium, silicon, stainless steel, with base heat fluxes in the range = 50 − 200 kW∕m2. The results show that conjugate heat transfer acts to make the temperature distributions around the perimeter of the channel cross-section more uniform, and that the topography of the lubricating film and the extension of the dry vapour patches that develop while the film is depleted both depend on the cross-sectional channel shape and influence the heat transfer performance significantly. For highly wetting conditions, channels with = 0.25 tend to allow enhanced heat transfer rates, with a spatially-averaged Nusselt number that is 50% higher than that obtained for = 1 (square channels) and 10% higher than that for = 4. This arises thanks to an extended evaporating film that covers the vertical walls which, owing to the three-side-heated configuration, contribute twice to the spatially-averaged heat transfer performance. For more hydrophobic conditions, large dry patches develop over the vertical walls for = 0.25 due to the lower evaporator temperatures, leading to reduced heat transfer, with thermal performance weakly dependent on in the range = 0.5 − 2

Self-excited hydrothermal waves in evaporating sessile drops

Pattern formation driven by the spontaneous evaporation of sessile drops of methanol, ethanol, and FC-72 using infrared thermography is observed and, in certain cases, interpreted in terms of hydrothermal waves. Both methanol and ethanol drops exhibit thermal wave trains, whose wave number depends strongly on the liquid volatililty and substrate thermal conductivity. The FC- 72 drops develop cellular structures whose size is proportional to the local thickness. Prior to this work, hydrothermal waves have been observed in the absence of evaporation in shallow liquid layers subjected to an imposed temperature gradient. In contrast, here both the temperature gradients and the drop thickness vary spatially and temporally and are a natural consequence of the evaporation process

Crude Oil Fouling: Fluid Dynamics, Reactions and Phase Change

In the present study, the fluid dynamics and phase behavior of crude-oil fouling in a closed-end heat-exchanger is studied. The deposition process associated with fouling is assumed to be due to two routes: asphaltene precipitation, and a two-step chemical reaction. The SAFT-γ Mie theory is employed to describe the phase behavior of an asphaltene-containing crude oil system, which comprises pseudo-components (C13 ~ C20+). The predicted phase equilibrium constants are used to quantify the asphaltene precipitation rate. A computational fluid dynamics framework is then used to simulate the fouling process, accounting for the multiphase flow dynamics, heat transfer, and the two deposition routes. Fouling is simulated due to the two routes individually and in concert. In the latter case, it is found that the interaction of the two routes is due to the fouling layer adhering to the heatexchanger walls, which influences heat transfer from the hot walls to the cooler oil in the bulk. The delicate interplay between heat transfer and fluid dynamics, which accompanies the flow, leads to enhancement and suppression of chemical reaction- and precipitation-driven fouling, respectively, and an overall rise in the fouling rate

Investigation of oil-water flow in horizontal pipes using simultaneous two-line planar laser-induced fluorescence and particle velocimetry

The flow of oil and water in pipes represents a challenging configuration in multiphase flows due to complex hydrodynamics which are still not fully understood. This can be observed in the large number of flow regimes encountered, which extend from smooth stratified flows to complex dispersions such as droplets of oil-in-water and water-in-oil. These flow configurations are the result of the inherent properties of the liquid phases, e.g., their densities and viscosities, interfacial tension and contact angle, as well as of flow conditions and related phenomena, such as turbulence, which have a direct effect on the interface instabilities giving rise to flow regime transitions. In this paper, experimental data are reported that were acquired at low water cuts and low mixture velocities using an aliphatic oil (Exxsol D140) and water as the test fluids in an 8.5 m long and 32 mm internal diameter horizontal pipe. A copper-vapour laser, emitting two narrow-band laser beams, and two high-speed cameras were used to obtain quantitative simultaneous information of the flow (specifically, spatiotemporally resolved fluid-phase and velocity information in both phases) based on simultaneous two-line Planar Laser-Induced Fluorescence (PLIF) and Particle Image and Tracking Velocimetry (PIV/PTV). To the best knowledge of the authors this is the first such instance of the application of this combined technique to these flows. It is found that the rms of the fluctuating velocity show peaks in high shear regions, i.e. at the pipe wall and interface.Papers presented to the 12th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Costa de Sol, Spain on 11-13 July 2016

Multi-Physics Modeling of Light-Limited Microalgae Growth in Raceway Ponds

This paper presents a multi-physics modeling methodology for the quantitative prediction of microalgae productivity in raceway ponds by combining a semi-mechanistic model of microalgae growth describing photoregulation, photoinhibition and photoacclimation, with models of imperfect mixing based on Lagrangian particle-tracking and heterogeneous light distribution. The photosynthetic processes of photoproduction, photoregulation and photoinhibition are represented by a model of chlorophyll fluorescence developed by Nikolaou et al. (2015), which is extended to encompass photoacclimation. The flow is simulated with the commercial CFD package ANSYS, whereas light attenuation is described by the Beer-Lambert law as a first approximation. Full-scale simulation results are presented on extended time horizons. Comparisons are made in terms of areal productivities under both imperfect and idealized (CSTR) mixing conditions, and for various extraction rates and water depths