Najla El Gharbi

An improved near-wall treatment for turbulent channel flows

The success of predictions of wall-bounded turbulent flows requires an accurate description of the flow in the near-wall region. This article presents a comparative study between different near-wall treatments and presents an improved method. The study is applied to fully developed plane channel flow (i.e. the flow between two infinitely large plates). Simulations were performed using Fluent. Near-wall treatments available in Fluent were tested: standard wall functions, non-equilibrium wall function and enhanced wall treatment. A user defined function (UDF), based on an analytical profile for the turbulent kinetic energy (Absi, R., 2008. Analytical solutions for the modeled k-equation. ASME Journal of Applied Mechanics, 75 (4), 044501), is developed and implemented. Predicted turbulent kinetic energy profiles are presented and validated by DNS data.

Numerical Simulations of Heat Transfer in Plane Channel Flow

Reynolds-averaged Navier–Stokes “RANS” turbulence models (such as k-ε models) are still widely used for engineering applications because of their relatively simplicity and robustness. In fully developed plane channel flow (i.e. the flow between two infinitely large plates), even if available models and near-wall treatments provide adequate mean flow velocities, they fail to predict suitable turbulent kinetic energy “TKE” profiles near walls. TKE is involved in determination of eddy viscosity/diffusivity and could therefore provide inaccurate concentrations and temperatures. In order to improve TKE a User Defined Function “UDF” based on an analytical profile for TKE was developed and implemented in FLUENT. Mean streamwise velocity and TKE profiles were compared to DNS data for friction Reynolds number Reτ = 150. Simulation results for TKE show accurate profiles. Simulation results for horizontal heated channel flows obtained with FLUENT are presented. Numerical results are validated by direct numerical simulation “DNS” data for Reτ = 150.

Near-Wall Models for Improved Heat Transfer Predictions in Channel Flow Applications

This work aims to improve the accuracy of the predicted temperature profile in the near-wall region of turbulent boundary layers. It is well known that practical understanding of turbulent boundary layers is central to heat transfer enhancement in many engineering applications. In one case, an eddy viscosity formulation based on a near-wall turbulent kinetic energy k+ function and the van Driest mixing length equation are used. In the second case, the turbulent Prandtl number is not assumed constant but rather is modeled by incorporating improved turbulent Prandtl number relations into existing models. The test case is the fully developed plane channel flow, which is considered to be the simplest and most idealized boundary-layer flow. These new formulations show improved agreement against the temperature profiles of direct numerical simulations of turbulent channel flow versus simulated cases using a commercial computational fluid dynamics package.

A Porous Medium Materiel Used In Solar Energy Conversion

Intense research on development of ozone and global warming protection heat pump machine technology is observed, because of the international environment protection. In arid areas, electricity network is not often available or it is at high price while the sun is shining and the cooling demand is important during almost the year. Absorption refrigeration has been intensively investigated over the world, during the last twenty years. This report is concerned by the solar energy conversion and use in an absorption cooling process where a specific porous medium is used to improve the efficiency. This process is the best appropriate solution to refrigeration and air conditioning problems in the desert areas and other remote countries where electricity power network is not available. The simulation of absorbing/desorbing chemical reaction in a solar cooling system, working according to absorption principle has given some results allowing to confirm the efficiency improve, the economical and technical feasibility and to avoid difficulties.

Exhaust Thermal Energy Use and Optimization in Remote Areas

In these recent thirty years, several investigations have been encouraged and done about the environment improve and the enhancement of the energy efficiency. In some arid areas, an important agrarian activity is developed because the fecundity of the soil and the presence of an important underground reservoir of water. The fruits production is important and must be stored for few months before its delivery to consumers. In order to protect the production some cold rooms have been constructed for more than 30 years. They need to be renewed. The extracted water is hot and must be cooled for its use in irrigation. This work is concerned by these two topics; renewing the cold rooms as recommended in Kyoto protocol and use of heat recovered from that released by the hot water. Fur this purpose an absorption refrigeration process is adopted using a not prohibited refrigerant and waste heat recovered as energy source. A reduced cold room is designed for theoretical simulation and experimental tests. Calculations and results are presented and discussed.