Papers by Author: Rachid Bennacer

Abstract: We present the results of an experimental investigation of the evaporation of a liquid meniscus in a high aspect ratio micro-channel. The study investigates evaporation rates of a stationary liquid meniscus in a high aspect ratio microchannel, the wall of which is electrically heated using transparent resistive coating. Four different liquids are used as working fluids. We report on the dependence of the measured overall evaporation rate on the applied power. The results indicate, and consistently, that the evaporation rate increases with the applied power then peaks before declining. In order to gain insight into these results, we used thermographic infra red imaging to map the temperature field on the external wall of the microchannel. The measurements show that there is a good correlation between the maximum in the evaporative rate and the onset of instabilities of the interface. These instabilities, to our mind, are induced by an increasing temperature gradient along the microchannel wall around the three phase contact line region. These instabilities are revealed by a high speed camera used to record the behaviour of the interface during evaporation.

Abstract: The present study deals with a compression-absorption machine. The proposed hybrid cooling system uses water-ammonia as a working fluid and operates at three pressure levels. The absorber is at an intermediate pressure (Pint) taken between the evaporator pressure (PEV) and the condenser pressure (PCD), unlike the single stage machine which works between two pressure levels. The proposed new system is studied and compared to the conventional machine. In order to evaluate the performance of the invoked machine, a procedure based on the MAPLE software is set up to compute accurately the thermodynamic properties of the working fluid. The analyses of the numerical results highlight that the performance of the novel proposed configuration is better than that relative to the conventional system. The study reveals the great impact of the intermediate pressure on the performance improvement and on reducing the generator temperature allowing the system to work at low enthalpy. In fact, for an evaporator temperature and a condenser temperature fixed respectively at -10°C and 40°C, the proposed hybrid refrigeration cycle operates at a generator temperature TGE = 75°C and the installation’s COP is about 0.56. While for the same conditions, the single stage machine COP cannot exceed 0.51 with a generator temperature of about 135°C. Consequently, our enhanced novel configuration presents the opportunity to operate at low enthalpy sources.

Abstract: This paper presents a numerical study of mixed convection heat and mass transfer in horizontal rectangular channels partially filled with porous medium. The main contribution of this research is to characterize how the porous block will create a heterogeneity that will induce a change on the Poiseuille-Rayleigh-Benard (PRB) fluid circulation dynamics. For a broad range of dimensionless parameters, which control the mixed convection, we show that the effect of the insertion of the porous block changes the thermal and solutal boundary layers; we find that the exchanges are intensified near the sidewalls in the porous region compared to upstream and downstream of the porous medium; and inversely in the core region. We describe, also, the onset of the longitudinal rolls at both upstream and downstream of the porous region. And finally, we compared the heat transfer, for different positioning of the porous medium with the purely fluid mixed convection.

Abstract: The present study deals with a solid adsorption refrigerator analysis using activated carbon/methanol pair. It is a contribution to technology development of solar cooling systems. The main objective consists to analyse the heat and mass transfer in an annular porous adsorber that is the most important component of the system. The porous medium is contained in the annular space and the adsorber is heated by solar energy. A general model equation is used for modelling the transient heat and mass transfer. Effects of the key parameters on the adsorbed quantity, the coefficient of performance, and thus on the system performance are analysed and discussed.

Abstract: A 3D numerical analysis is carried out to investigate heat and mass transfer in a partly porous cavity of high aspect ratio. The goal is to determine the best physical and geometrical parameters that allow optimal heat and mass transfer rate in such domain used in a solar adsorption cooling system. The computational domain consists of a tall cavity heated on the left vertical wall and cooled on the opposing wall. The SIMPLE algorithm is used to handle the velocity pressure coupling. Simulation results allow determining the optimal configuration of the used porous substrate and plain fluid position in the cavity in order to optimize the performance of such solar adsorption cooling installation.

Abstract: In this work the influence of thermal effects on the performance of a finite porous journal bearing has been investigated using a thermo-hydrodynamic analysis. The Reynolds equation of thin viscous films is modified taking into account the oil leakage into the porous matrix, by applying Darcy’s law to determine the fluid flow in the porous media. The governing equations were solved numerically using the finite difference approach. Obtained result show a reduction in the performance of journal bearings when the thermal effects are accounted for and, this reduction is greater when the load capacity is significant.

Abstract: Salinity currently has the highest priority of all environmental issues nowadays. The only means of preventing the development of soil salinity was combining leaching and artificial drainage. Recently a new method of capturing and removing salt from the soil surface called a dry drainage method [1-4] has been studied (fig.1) to help arid soils prevention against such threat. This work is a numerical simulation of the simultaneous water, solute and heat transfer in an unsaturated porous media during the simultaneous evaporation and drainage of saline dry land soils. A detailed mathematical model was formulated to describe the non-isothermal transport of water in the unsaturated porous media [5]. The model consists of the coupled conservative equations of mass for liquid phase, gas phase, water vapour, species and the energy. The boundary conditions at the soil–atmosphere interface include dynamical mass flux and energy flux. Comparison of numerical simulations with analytical results demonstrated that the present model is able to describe water and energy transport dynamics.

Abstract: Nucleate boiling is an efficient mechanism of heat transfer. The rate of bubble growth and the subsequent bubble motion has a tremendous influence on heat transfer. The study of bubble dynamics is a coupled problem. The rate of evaporation controls the interface speed. One approach to study bubble dynamics is to decouple the problem from energy conservation equation and use an input value of rate of evaporation. The objective is to observe how irregular evaporation rate controls bubble dynamics and the shape of bubble and to study the local over-pressure. The level set method is used to track the liquid-vapor interface. The model consists of the Navier-Stokes equations which govern the momentum and mass balances and the level set equation which governs the interface motion due to phase change. The dynamics of a single bubble under different rates of evaporation and varying levels of gravity have been studied. The results of the numerical simulation show that this model adequately describes bubble dynamics in nucleate boiling, including conditions of microgravity.

Abstract: The shape and surface texture of a liquid droplet were studied in two-dimension when a droplet impinges on a solid substrate under isothermal condition. The lattice Boltzmann moment model was applied to simulate the fluid dynamics considering the adhesive interaction between fluid particles and surfaces. The results show the influence of wetting on the process and the drop shape. For the hydrophobic surface, the process after impinging may be divided into two stages: the spreading process driven by inertial forces and the subsequent oscillation (recoiling process) driven by surface tension forces. While for the hydrophilic surface, the droplet will only deposits on the surface and there is no the recoiling stage. In addition, the effects of the impinging speed on the shape and texture of the droplet were studied. The spreading speed and the maximum diameter of the spreading droplet increase with the rise of the impinging speed.

Abstract: The contribution of this work is to characterize the travelling wave’s appearance and to generalize the behavior of Poiseuille-Rayleigh-Benard (PRB) systems for a broad range of dimensionless parameters, which control the double diffusive mixed convection. The numerical results consist to analyzing the flow regimes of the steady longitudinal thermoconvectives rolls for the case of purely thermal mixed convection and for both thermal and mass diffusion. The passage from an opposed volume forces to cooperating one at fixed Rayleigh (Ra), Reynolds (Re) and Lewis (Le) numbers, affects considerably the birth and the development of the longitudinal rolls R//. The distribution of the heat and mass transfer, presented by the average Nusselt and Sherwood numbers, is also examined.