Slimane Gabsi

Low-Pressure Superheated Steam Drying of a Porous Media

The present study aimed at developing a theoretical model to simulate the drying process of a porous particle in low-pressure superheated steam drying (LPSSD). Spherical and porous particles of ceramic and coal are used as the model material in this work. Experimental data for ceramic and coal particles reported in the literature were used for the model validation. The effect of particle thermophysical properties and operating variables is tested. Results showed that a decrease in pressure and an increase in the fluid temperature during drying lead to an elevation of mass flux during the constant rate period (CRP) and the falling rate period (FRP) of the drying process. Moreover, this increase in mass flux is more significant for particles with the highest effective porosity. Thus, we conclude that decreasing pressure and increasing the drying fluid temperature reduce the mass flow.

Numerical Prediction of Flow Patterns in Bubble Pumps

In the present study, the ammonia-water mixing flow in a bubble pump is numerically simulated. The flow patterns of a two-phase flow in a bubble pump were studied under different conditions of heat flux and tube diameter. A one-dimensional two-fluid model was developed under constant heat flux. This model was used to predict the variations in void fraction and liquid and vapor velocities throughout the tube. Then, the void fraction profile and the curve of liquid velocity versus vapor velocity were used to predict the flow patterns along the tube length. It was found that at heat fluxes below 15 kW m ―2 , bubbly, slug, and churn flows are the dominating regimes, and the length of these flow regimes depends on the tube diameter. For heat fluxes higher than 15 kW m ―2 , the bubble pump operates under the churn and annular regimes, and the bubble pump performance is improved when the tube diameter increases.

Low-Pressure Superheated Steam Drying—Vacuum Drying of a Porous Media and the Inversion Temperature

This article deals with comparative study of low-pressure superheated steam drying (LPSSD) and vacuum drying using coal particles as model material. The focus is made on the inversion temperature, which means that the drying rate in the two drying processes becomes equal. This temperature is of great importance because it allows the choice of drying agent for given operating conditions. The effect of external and internal parameters on the drying kinetics and the inversion temperature were investigated. Several studies have shown discrepancies of inversion temperature values basing on the constant rate period (CRP) and the falling rate period (FRP). This work points out gaps in these temperature values using the two calculation methods.

Simulation of an autonomous solar vacuum membrane distillation for seawater desalination

AbstractSeveral papers deal with membrane distillation of seawater and mainly focus on the choice of membranes and comparison of this technique with other desalination techniques, but there are very few studies that quantitatively estimate the potential of this technology coupled with solar energy. The objective of this work is to develop a model describing the operation of a desalination membrane powered by solar energy. This model determines the performance of the unit over time and for any day of the year. This model is established from the balance equations of mass and heat on the different units (membrane, exchanger, condenser, and field of solar collectors). The model is used to evaluate the evolution of the distillate flow rate and temperature changes for different flows. The model also allows to estimate the productivity of the unit during the year. The simulation of the operation of the unit shows that the daily production of distilled water is between 63 and 188 kg/m2 for the days of 21 December...

Study of the performances of different configurations of seawater desalination with a solar membrane distillation

AbstractThe object of this study is the comparison between two configurations of hollow fiber module for seawater vacuum membrane distillation. The first one is a module membrane in series with a solar compound parabolic collector (CPC). The second configuration is a hollow fiber module integrated in a cylindrical absorber of CPC. A model describing the operation of a hollow fiber module, with and without recycling, will be developed. This model determines the instantaneous variation of the temperature of each element of the installation with the distillate flow variation. A comparison of each module production is carried out. A mathematical model describing the performances of different configurations for membrane hollow fibers shows that: (1) The permeate flow for the integrated configuration is always higher than that of not integrated. It can be multiplied by two and even more. (2) The recycling of concentrate makes it possible to improve the production. This is due to the high level temperature at th...

Performance Simulation of Two-Bed Adsorption Refrigeration Chiller with Mass Recovery

The technology of adsorption chiller is an efficient way of heat conversion. It can significantly reduce environmental pollution and improve energy efficiency. This paper deals with numerical study of refrigeration systems with silica gel/water pairs with mass recovery. The model is validated with experimental results from the ENERBAT platform. Numerical results are in good agreement with those of experiment. A process of mass recovery is added to study its the effect on the performances of the system. The effect of hot water temperature, cooling water temperature, chilled water temperature and cycle time, on the coefficient of performance (COP), the specific cooling capacity (SCP), the cycled mass, the evaporator outlet temperature and efficiency system are investigated in order to extrapolate the results in the Tunisian climate and to determine their optimum values able to maximize the performance of the system under analysis. The simulation calculation indicates a COP value of 0.7 with a driving source temperature of 85°C in combination with coolant inlet and chilled water inlet temperature of 40°C and 15°C, respectively. The most optimum adsorptiondesorption cycle time is approximately 1240s based on the performance from COP and SCP, achieving a SCP of 400 W/kg.

MODELING OF BOILING TWO-PHASE FLOW IN THE BUBBLE PUMP OF DIFFUSION-ABSORPTION REFRIGERATION CYCLES

ABSTRACT This work presents a numerical model developed to simulate steady-state refrigerant flow along a tube of a bubble pump at operating conditions of interest for diffusion-absorption refrigeration systems. The flow is considered one-dimensional, and non-metastable flow is neglected. The two-fluid model was employed to simulate the two-phase flow, considering the hydrodynamic nonequilibrium between the liquid and the vapor phases. Comparisons were made for void fraction versus vapor quality using the homogeneous model, the Rouhani and Axelsson drift model, and the Zivi annular model. The results show a great similarity among the values achieved in the present work and those calculated using the Rouhani and Axelsson and Zivi correlations. On the basis of the numerical simulations, a detailed description of the flow characteristics was obtained. The influence of the heat source on the flow characteristics was numerically examined.

Numerical analysis of heat, air, and moisture transfers in a wooden building material

The present paper aims to predict the hygrothermal behavior of massive wood panel considered as bio-based building material. In this context, we developed a macroscopic model coupled no linear heat, air, and moisture transfers that incorporates simultaneously the effect of thermal diffusion and infiltration phenomenon on the building material. The model inputs parameters were evaluated experimentally according to the recognized standards of material’s characterization. Therefore, numerous series of hygrothermal calculation were carried out on the 1-D and 2-D configuration in order to assess the dimensionless effect on such wooden material. Two types of boundary conditions were considered and examined. The first are at the material scale of wood drying process. The second type of conditions is at the wall scale, where the conditions of the building ambiance are considered. Moreover, the model sensitivity to the driving potentials coupling and to the parameters variability was considered and examined. It has been found that the coupling in the model had a remarkable impact on both kinetics of temperature and moisture content.

Numerical Investigations of Coupling a Vacuum Membrane DesalinationSystem with a Salt Gradient Solar Pond

This work proposes new configurations for the desalination of salt water using systems based on coupling of membrane distillation with solar energy. This study is a comparison between two coupling configurations of the vacuum membrane distillation (VMD) hollow fiber module with salinity gradient solar pond (SGSP). The first configuration is a module membrane in series with SGSP and the second one is a hollow fiber module immersed in the SGSP. Two models describing the heat and mass transfer in the hollow fiber module and in the SGSP will be developed. The coupling of the two models allows the determination of the instantaneous variation of temperature and salinity in the SGSP and the permeate flow variation. A comparison of each module production was carried out. The mathematical model shows that the immersed module production presents more than one and a half times that of the separated module, their production reached 75 kg.day-1 per m² of the membrane in the third year. Thus, immersing the module in the solar pond improves the performance of the hollow fiber module.

Correlation for Lewis number for evaluation of mass flow rate for simple/hybrid solar still

In isolated and arid areas, especially in the almost Maghreb regions, the abundant solar radiation intensity along the year and the available brackish water resources are the two favorable conditions for using solar desalination technology to produce fresh water. In this paper, a theoretical correlation named the “Lewis number” correlation is developed and used for the calculation of a simple and a hybrid solar distiller mass flow rate. Uses of single glass cover for periodically oriented still or double glass cover for fixed still are studied. Calculated results are compared with those obtained experimentally. It was found that the theoretical mass flow rate is in good agreement with that obtained experimentally.