Name of Project Leader: Professor Dhahri Hacen  

Names of the involved researchers:  

• MHIMID Abdallah
• SLIMI Khalifa
• JEMNI Abdelmadjid
• ZILI Leila
• SGHAIER Nour
• GUELLOUZ Mohamed Sadok 

Names of PhD students:

• GUESMI Hela
• NEFZI Yassine
• GUEDRI Firas
• RACHDI Zakia
• BOUTABBA Amel Hayeti
• HMILA Mohamed Wassim
• AYED Oussama

1. Summary and objectives:

This project focuses on quantifying mass and heat transfers in porous media within various systems. The main goals are:

• Understanding Transport Phenomena: Explore how flow, heat, and mass transfer occur in porous and granular materials.
• Model Development: Develop and solve models to simulate transfer mechanisms using the Lattice Boltzmann Method (LBM).
• Optimization of Energy Storage: Enhance energy storage systems in porous media at both macro and micro scales by incorporating nanofluids.
• Characterization of Materials: Assess the properties of these materials at the micro scale, which is crucial for effective design and sizing of storage mediums.
• Solar Air Conditioning Systems: Contribute to environmental protection by exploring solar air conditioning systems with variable refrigerant flow and desiccation methods.
• Hybrid Systems Investigation: Investigate a hybrid solar/geothermal system for generating electricity, domestic hot water, and heating, particularly in the Tunisian climate.
• Carbon Footprint Assessment: Evaluate the carbon footprint of the energy sector in Tunisia to identify sustainable solutions.

2. Research Programme and Methodology:

• Establish equations governing physical phenomena for each studied system.
• Develop LBM-based calculation codes for simulating heat and mass transfers in porous media.
• Improve existing calculation codes for better efficiency and usability.
• Evaluate the characteristics of porous materials for energy storage at micro and macro scales.
• Assess the performance and feasibility of hybrid solar/geothermal systems in Tunisia.
• Analyse the carbon footprint in Tunisia's energy sector to inform environmental strategies.

3. Research results:

Porous media are of significant interest in various industrial applications, aiming to maximize heat transfer and serve as thermal storage supports. Indeed, designing and sizing a storage medium presents a promising solution for renewable energy systems.  An environmentally conscious contribution involves investigating solar air conditioning systems using variable refrigerant flow or desiccant-based methods. The study of a hybrid solar/geothermal system for electricity generation, domestic hot water production, and space heating constitutes a key motivation for the team. Figures I-1-1 and I-1-2 illustrate the proposed renewable energy systems targeted by the project for the 2023–2034 period. The results are currently being finalized.

Figure I-1-1. Solar air conditioning system with a dual-porosity heat exchanger.

Figure I-1-2. Hybrid system for a net-zero energy building.

Name of Project Leader: Professor JEMNI Abdelmajid 

Names of the involved researchers:          

• LAATAOUI Zied
• BRAHIM Taoufik
• CHAABANE Raoudha             

Names of PhD students:

• GHEDIRA Aroua
• MAHJOUB Hammouda (co-supervised thesis)

1. Objectives:

The primary goals of this research include

• Operational Study: Investigate the various operational aspects of different heat pipe types.
• Model Development: Create detailed models for thermosiphons and capillary pumping heat pipes.
• Experimental Procedures: Establish protocols to characterise the impact of operational parameters on heat pipes.
• Heat and Mass Transfer Understanding: Develop specific models and conduct experiments to enhance the understanding of heat and mass transfer phenomena in heat pipes, leading to optimisation of their performance.
• Renewable Energy Application: Explore the utilisation of heat pipes for renewable energy applications, particularly solar energy.
• Validation of Numerical Results: Implement experimental methodologies to validate numerical simulations.

2. Research Programme and Methodology:

To achieve the objectives, the following methodologies will be employed:

• Generalised Tool Development: Create a generalised design tool for heat pipes, focussing on thermosiphon and grooved types, incorporating the operational limits for better design considerations.
• Experimental Setup Optimisation: Optimise the vacuum filling process of heat pipes with appropriate heat transfer fluids suitable for specified temperature ranges.
• Simulation Techniques: Use OpenFOAM to simulate operational characteristics of heat pipes.
• Two-Phase Flow Modelling: integrate models to simulate the dynamics of heat pipes utilising porous capillary structures.
• Gravity-Assisted Heat Pipes: Investigate the benefits and challenges of gravity-assisted heat pipes, focussing on their simplicity and cost-effectiveness.
• Efficiency Optimisation: Optimise operational parameters to enhance the efficiency of heat pipes.

​​​​​​ 3- Coooperation

This research programme is part of the PHC-Utique cooperation between LESTE and the Thermal Team at Institute P’ (ENSMA-Poitiers, France). Additionally, a co-supervised thesis is currently underway.

4. Research results

A non-equilibrium two-phase flow model was developed using the OpenFOAM platform, specifically to simulate the operation of thermosyphon-type heat pipes (Fig. I-2-1). This model has helped resolve several numerical challenges, particularly those related to unexplained pressure jumps. Although full validation is still ongoing, an initial validation was conducted by comparing the results with existing experimental data.
As part of the project, a preliminary design of the heat pipes was carried out, and a prototype is currently under development to ensure better alignment between theoretical and experimental aspects.

Figure I-2-1. Example of results: fluid velocity contours (a) at the evaporator and (b) at the condenser.

This work also led to the development of a simplified model for a porous wick heat pipe (Fig. I-2-2). The model is based on the Volume of Fluid (VOF) approach and the Lee liquid-vapor phase-change model, and has been validated on simple test cases, such as flow through a cylindrical porous medium and condensation on a horizontal surface.

Figure I-2-2.

The model was then extended to three-dimensional configurations for both transient and steady-state simulations, allowing tracking of the liquid and vapor phase distributions within the porous wick. Model validation was also carried out using a simple example of transpiration through a porous medium. A parametric study is still underway to validate the technical choices and identify key parameters for optimizing the operating conditions of heat pipes.

In summary, the results demonstrate significant progress both theoretically and practically, with well-validated numerical models and promising simulations. This work paves the way for a better understanding of heat pipe operation and their optimization in real-world applications, particularly in the field of solar energy.

Figure I-2-3. 3D numerical simulations

Name of Project Leader: Professor BEN CHIEKH Maher             

Names of the involved researchers:        

• JEMNI Abdelmajid

• GUELLOUZ Mohamed Sadok

• BOUGHAMOURA Ayda          

Names of PhD students:

• GUIZANI Mouna (co-supervised thesis)

• YAHMADI Wiem (co-supervised thesis)

1. Objectives:

The streaming generated by ultrasound in a liquid can significantly enhance mass transfer at the wall. This process can be implemented using focused ultrasound. In this context, the objective of this project is to stimulate wall transfer through focused acoustic streaming in a conduit.

The project's objectives are:

• To analyse the effect of focused ultrasound on a flow, particularly in terms of mixing and shear at the wall,

• To understand the evolution of the flow structure as a function of the flow rate,

• To evaluate the theoretical potential action of ultrasound in terms of the effectiveness of local drug treatment in a simplified configuration of the iliac artery.

2. Research Program and Methodology:

The general structure of the flow subjected to the action of focused ultrasound will also be the subject of this research. Indeed, the formation of flow structures (large-scale vortex structures such as Dean vortices, recirculation or stagnation zones), as well as the small-scale mixing of the fluid (turbulence, particularly near the wall), are important parameters concerning wall transfer.

In the configuration of a steady or pulsed flow, experiments and numerical simulations of flow, along with wall remodelling models, will be implemented to evaluate the effect of a focused ultrasonic force on the flow:

• Experimental measurements of velocity fields in a straight tube configuration and stenosis subjected to an ultrasonic field will be conducted.

• The results obtained will allow for the validation and adjustment of ultrasonic parameters in numerical simulations.

• The results from the numerical simulations will help estimate wall transfers and feed into a wall remodelling model to assess the evolution of the stenosis.

3. Cooperation

This research is part of the PHC-Utique cooperation between LESTE and the Labtau U1032 laboratory at INSERM in Lyon. Additionally, two co-supervised theses are in progress

4. Research results

This study investigates the effects of High-Intensity Focused Ultrasound (HIFU) in a closed tube, simulating the confined environments encountered in medical and industrial applications. In a first stage, a Newtonian fluid was examined to explore the influence of HIFU in a context aimed at stimulating fundamental biological mechanisms. Specifically, this phase focuses on the generation of mixing within the fluid and the induction of wall shear stress (WSS)—both critical elements for therapeutic and biological applications.

Numerical simulations were used to evaluate the variations in wall shear rates induced by acoustic streaming. A parametric study was carried out to support the interpretation of experimental results regarding the impact of streaming generated by focused ultrasound (HIFU), depending on the position of the focal point. A detailed force analysis was conducted using momentum balance equations (see Figure I-3-1).

Figure I-3-1. Schematic of the forces acting within a closed tube subjected to a focused ultrasonic wave (HIFU) applied along the central axis.

HIFU was found to effectively induce mixing within the fluid (Fig. I-3-2), regardless of the ultrasound focal position—except when it is applied too close to the wall. In this case, mixing efficiency decreases, but conversely, WSS increases significantly. This could be particularly relevant for applications requiring localized stimulation near the wall.

Figure I-3-2. Axial velocity contours and corresponding streamlines for an acoustic Grashof number of 542. HIFU applied at (a,c) the center of the tube and (b,d) near the wall.

In a second phase, the Bird–Carreau model, representing blood, showed reduced mixing efficiency at low acoustic Grashof numbers. Achieving mixing levels comparable to those of the Newtonian fluid requires much higher Grac values. However, the non-Newtonian behavior has no significant effect on WSS values, which remain similar to those observed in Newtonian fluids under all configurations.

These results highlight the potential of HIFU to simultaneously generate fluid mixing and wall shear stress, two phenomena capable of preventing fluid stagnation and enhancing molecular transport to the wall in localized drug delivery treatments. Furthermore, the generation of localized shear may be of particular interest for tissue regeneration processes.

Name of Project Leader: Dr GUELLOUZ Mohamed Sadok         

Names of the involved researchers:        

• ORFI Jamel
• BEN CHIEKH Maher
• LOUSSIF Nizar
• KHABBOUCHI Imed
• BEN KHEDHER Nidhal           

Names of PhD students:

• BEDOUI Maroua

1. Summary:

The present project focuses on studying the intensification of thermal and mass exchanges in conduits and micro-conduits. This investigation will consider various types of heat transfer fluids, including simple and hybrid nanofluids, as well as mixtures like saline solutions. The conduits examined may feature a variety of configurations, such as circular, rectangular, trapezoidal, or concentric tubes. These conduits can be either impermeable or permeable, incorporating mechanisms for blowing or suction.

The intensification of transfers can be achieved through several methods, including:

• Insertions of obstacles and fins
• Application of uniform or variable electric and/or magnetic fields

A particular focus will be on conduits designed to promote vortex formation. This research builds on previous laboratory projects related to flows in complex channels.

The findings from this study will primarily aim to enhance exchanges in microsystems and improve separation processes, particularly in the desalination of saline waters.

2. Methodology:

These studies will primarily be conducted through theoretical approaches in both steady and dynamic regimes, considering correlations that express the properties of reliable and precise heat transfer fluids. Some experimental investigations for validation and calibration purposes are also planned. In general, the research programme is structured around the following phases:

• Development and validation of computational codes.
• Study of flows and transfers in micro-conduits.
• Investigation and quantification of transfer intensification through various approaches, such as the insertion of obstacles and the application of electric and/or magnetic fields.
• Application to desalination processes for saline waters.

3. Cooperation

This project could benefit from collaboration with the following partners:

• Institut Universitaire de Technologie de Saint-Malo, Université de Rennes, France
• King Saud University, Riyadh, Saudi Arabia
• University of Birmingham, England
• Université de Sherbrooke, Canada.

4. Research results

A numerical study of laminar flow in channels formed by one or two rectangular sub-channels with a slit in the middle or on one of the lateral sides was conducted.

First, the mathematical model describing the physical phenomenon—derived from the Navier-Stokes equations—was developed, along with tools for solving the base flow and for analyzing its linear stability.

A sample of results is shown below in the form of iso-contours and velocity profiles for a rectangular channel with a slit. Similarly, some results from the stability analysis are presented, which identified the conditions for vortex formation and characterized them.

Figure I-4-1. Velocity iso-contours for a rectangular channel with a slit for (a) δ/L = 0.1, (b) δ/L = 0.3, (c) δ/L = 0.7, and (d) δ/L = 0.9.

Figure I-4-2. Dimensionless velocity profiles at mid-height of the slit (z = δ/2).

In a second stage, numerical simulations of axial flow in a rectangular channel containing two parallel fins and a microchannel with a slit were performed using the unsteady Reynolds-averaged Navier–Stokes (URANS) equations coupled with a Reynolds Stress Model (RSM). The formation of quasi-periodic vortex structures is clearly observed in the temporal variation of the velocity components (Figure I-4-5) and in their contours (Figure I-4-6).

Figure I-4-3. Variation of the critical Reynolds number for a rectangular channel with a slit (●) and for two rectangular channels connected by a slit (▲).

Figure I-4-4. Variation of the critical Strouhal number with slit height.

Figure I-4-5. Temporal variation of the longitudinal and transverse velocities at different positions between the two fins.

Figure I-4-6. Contours of transverse velocity between the two fins.