Yan Delauré

Photooxygenations in a bubble column reactor

A novel column reactor was constructed and successfully applied to dye-sensitized photooxygenation reactions in aqueous alcohol solutions. The air flow pattern within the narrow glass column could be controlled via the size of the air inlet capillary. Using a 500 μm capillary, a slug flow pattern was realized which allowed for superior mass transfer and light transparency within a thin solvent layer. These features subsequently gave higher conversion rates and isolated yields.

A simultaneous PIV and heat transfer study of bubble interaction with free convection flow

Abstract Whole field velocity and point temperature and surface heat flux measurements were performed to characterise the interaction of a single rising ellipsoidal air bubble with the free convection flow from a heated flat surface immersed in water at different angles of inclination. Two thermocouples and a hot film sensor were used to characterise heat transfer from the surface, while a time-resolved digital particle image velocimetry technique was used to map the bubble induced flow in a plane parallel to the surface. Heat flux fluctuations, preceding and following the bubble passage, were shown to correlate with the variation in both local flow velocities and fluid temperatures. The largest increases in heat transfer were recorded when both flow and temperature effects combined to enhance the convective cooling simultaneously. Such conditions were shown to be most likely met when the block was inclined at 45°, thus forcing the bubble to slide closer to the heated surface and hence to the thermal boundary layer.

Convective Heat Transfer of an Air Bubble in Water With Variable Thermophysical Properties

A numerical study of convective heat transfer of an air bubble in water with variable thermophysical properties is considered. Two-dimensional simulations of multifluid flows with heat transfer include the Navier–Stokes, energy, and volume of fluid (VOF) advection equations. The solver computes the flow field and temperature by solving the systems of Navier–Stokes equations and the energy equation using the finite–volume method with the SIMPLE algorithm and tracks the position of interface between two fluids with different fluid properties by the VOF method with piecewise linear interface construction technique. Empirical correlations in terms of temperature for thermophysical properties are considered in the simulations. The convective heat transfer model is assessed with a benchmark problem of cooling of water and compared with previous literature data showing good agreement. Finally, a numerical study of the effect of the bubble diameter in the range from 2 mm to 3 mm on heat transfer is performed.

Numerical investigation of volume of fluid and level set interface capturing methods for bubble growth and detachment

The injection of an air bubble in a liquid at rest is an interface flow problem where surface tension and its modeling at solid boundaries is a key factor. It is the subject of this study. Numerical simulations have been performed to study 3D axi-symmetrical bubble growth from an orifice through a horizontal wall. The gas inflow velocity used was sufficiently small to ensure that the bubble growth is quasi-static so that surface tension and buoyancy forces are dominant. The wall was considered non-wettable to avoid spreading of the interface along the wall. The Navier-Stokes equations were solved with two different interface capturing methods based on Volume of Fluid (VOF) and Level Set (LS) as well as coupled CVOFLS. In the VOF method the bubble interface was tracked using either an algebraic solver which results in some diffusion of the interface (compressive scheme implemented in OpenFOAM), or it was determined using a geometric reconstruction scheme (Geo-Reconstruct Scheme from Fluent). The TransAT code was used for the LS model which captures the interface using signed distance function. The bubble volume and center of gravity have been investigated during the growth using the three solvers and numerical results have been assessed against experimental data. These results have shown that reconstructing the interface using the LS method gives good agreement with the experiments. In VOF (compressive scheme), the bubble detaches at earlier times resulting in a smaller detachment volume. The coupled CVOFLS-GeoReconstruct was found to be more computationally expensive than the VOF-GeoReconstruct and to present bubble oscillation during the growth.

An Assessment of Suitability of a SIMPLE VOF/PLIC-CSF Multiphase Flow Model for Rising Bubble Dynamics

AV olume of Fluid (VOF) - Youngs’ model for the solution of an incompressible immiscible two-phase flows is presented. The solver computes the flow field by solving the family of Navier Stokes equations on a fixed (Eulerian) Staggered Cartesian grid using the Finite Volume formulation of Semi-Implicit Pressure Linked Equation (SIMPLE) method and tracks the position of interface between two fluids with different fluid properties by Piecewise Linear Interface Construction (PLIC) Method. The suitability of the SIMPLE type implementation is assessed by investigating the dynamics of free rising bubbles for different fluid properties and flow parameters. The results obtained with the present numerical method for rising bubbles in viscous liquids are compared with reported numerical and experimental results.

Bouncing bubble dynamics and associated enhancement of heat transfer

Heat transfer enhancement resulting from the effects of two phase flow can play a significant role in convective cooling. To date, the interaction between a rising gas bubble and a horizontal surface has received limited attention. Available research has been focused on bubble dynamics, although the associated heat transfer has not been reported. To address this, this study investigates the effect of a single bubble bouncing against a heated horizontal surface. Local heat transfer measurements have been performed for four orifice to surface distances, with a bubble injection orifice of 1 mm in diameter. High-speed photography and infrared thermography have been utilized to investigate the path of the bubble and the associated heat transfer.

Optical considerations for time-resolved digital PIV measurement in a single bubble flow against heated boundaries

Simultaneous whole field velocity and heat transfer measurements were performed to characterize the interaction of a single rising ellipsoidal bubble with the free convection flow from a heated flat surface. This design note reports on the optimization of a purpose built high frame rate time-resolved digital particle image velocimetry system, which was used to map the bubble wake interaction with a heated solid boundary. The system used here was highly optimized; a refractive index matching method has been used to accommodate the depth of field limit of the imaging camera, and a special laser sheet optical arrangement was employed to accommodate spectral dispersion of the laser operating in multicolour mode.

Sliding bubble dynamics and the effects on surface heat transfer

An investigation into the effects of a single sliding air bubble on heat transfer from a submerged, inclined surface has been undertaken. Existing literature has shown that both vapour and gas bubbles can increase heat transfer rates from adjacent heated surfaces. However, the mechanisms involved are complex and dynamic and in some cases poorly understood. The present study utilises high speed, high resolution, infrared thermography and video photography to measure two dimensional surface heat transfer and three dimensional bubble position and shape. This provides a unique insight into the complex interactions at the heated surface. Bubbles of volume 0.05, 0.1, 0.2 and 0.4 ml were released onto a surface inclined at 30 degrees to horizontal. Results confirmed that sliding bubbles can enhance heat transfer rates up to a factor of 9 and further insight was gained about the mechanisms behind this phenomenon. The enhancement effects were observed over large areas and persisted for a long duration with the bubble exhibiting complex shape and path oscillations. It is believed that the periodic wake structure present behind the sliding bubble affects the bubble motion and is responsible for the heat transfer effects observed. The nature of this wake is proposed to be that of a chain of horseshoe vortices.

A thin film Fluid Structure Interaction model for the study of flexible structure dynamics in centrifugal pumps

This paper describes a fluid-structure interaction (FSI) model for the study of flexible cloth-like structures or the so-called rags in flows through centrifugal pumps. The structural model and its coupling to the flow solver are based on a Lagrangian formulation combining structural deformation and motion modeling coupled to a sharp interface immersed boundary model (IBM). The solution has been implemented in the open-source library OpenFOAM relying in particular on its PIMPLE segregated Navier–Stokes pressure–velocity coupling and its detached eddy simulation (DES) turbulence model. The FSI solver is assessed in terms of its capability to generate consistent deformations and transport of the immersed flexible structures. Two benchmark cases are covered and both involve experimental validation with three-dimensional (3D) structural deformations of the rag captured using a digital image correlation (DIC) technique. Simulations of a rag transported in a centrifugal pump confirm the suitability of the model to inform on the dynamic behavior of immersed structures under practical engineering conditions.

Coupling of a thermo-mechanical model for NTE to a fluid solver

A one dimensional thermomechanical model for a Negative Thermal Expansion material (NTE) wire was developed. The model uses a first order approximation, based on a finite difference method, to account for thermal expansion of a wire using experimentally obtained characterization data to define the material properties. The model was coupled to a three dimensional fluid solver in order to model the reaction of the wire under fluctuating fluid temperatures. The wire model was coupled to a piston and a dynamic mesh layering technique was used to allow for motion of the piston in reaction to thermal expansion. A series of mesh refinements were studied in order to assess the impact of the refinement on the piston motion. The overall displacement achieved using the model is representative of the expansion expected from material.