Mohammed SAID's thesis defense

Microfluidic devices have attracted a great deal of interest in the field of thermal management due to their precise control of fluid flow, which enhances heat transfer. Applications for microfluidic devices include electronic cooling, microscale heat exchangers and microreactors.

While many studies have focused on single-phase flow in microchannels, research into two-phase flow, particularly Taylor flow, remains limited. Taylor flow is a periodic dispersion of droplets or plugs in a continuous carrier phase. According to the literature, this type of flow offers better heat transfer performance than single-phase flow. It is therefore imperative to gain knowledge of this type of flow and understand its behavior in rectangular and square microchannels.

This thesis proposes a numerical and experimental study of the hydrodynamics and heat transfer of a Taylor flow in a rectangular microchannel.

The first part is devoted to 3D numerical simulation in ANSYS Fluent, with interface modeling using the Volume of Fluid (VOF) method. The impact of key parameters such as flow ratio, viscosity ratio and capillary number was studied. It was observed that the thickness of the liquid film increases as the flow ratio decreases, strongly influencing droplet velocity. Capillary number, based on the velocity of the two phases, proved to be the most decisive parameter for droplet and plug shape.

After the hydrodynamic analysis, the thermal performance of the Taylor flow was investigated numerically by first assuming constant thermophysical properties, then incorporating a temperature-dependent viscosity via a user-defined function (UDF). The results show an improvement in heat transfer of up to 440% compared with single-phase flow. The introduction of temperature-viscosity dependence resulted in a further 20.8% improvement in thermal efficiency.

The second part of the thesis presents an experimental study aimed at validating the numerical results and further analyzing Taylor flow in T-shaped microchannels. Various fluid combinations were tested, varying the flow ratio, and the capillary number. Experimental results show that the mechanism of droplet formation and the competition of forces at the junction strongly influence the hydrodynamics of Taylor flow. The velocity of the two phases and the viscosity ratio have a noticeable effect on the length, shape, velocity and frequency of droplet and plug generation.

In addition, the effect of flow ratio on droplet generation frequency depends on the phase (continuous or dispersed) employed to vary the flow ratio. We also found that droplet/plug shape can depend on the type of junction used to generate the droplets, with the radii of curvature of the front and back decreasing with increasing flow ratio or Ca number, in agreement with our numerical results.

The effect of the flow ratio on droplet generation frequency depends on the type of junction used to generate the droplets.

Reporters

• Abdel EL ABED, MCF-HDR, ENS Paris Saclay
• Sefiane KHELLIL, University Professor, University of Edinburgh

Examiners

• Rachid BENNACER, Professeur des Universités, ENS Paris Saclay
• Hakim NACEUR, University Professor, UPHF -Valenciennes-

Invited

• Nora NAIT BOUDA, University Professor, USTHB -Alger-

Thesis director

• Souad HARMAND, University Professor, UPHF -Valenciennes-