Belkacem Zeghmati

A new Navier-Stokes and Darcy's law combined model for fluid flow in crossflow filtration tubular membranes

Numerical simulations were performed for laminar fluid flow in a porous tube with variable wall suction, a model of a crossflow filtration tubular membrane. The variable wall suction is described by Darcys law, which relates the pressure gradient within a flow stream to the flow rates through the permeable wall. The feed stream in the tube, which flows mainly tangentially to the porous wall, is modelled by the Navier-Stokes equations. A method of coupling the Navier-Stokes and the Darcy equations in a solution scheme was considered to develop a fluid dynamic model of crossflow filtration. The governing equations were solved numerically using a finite difference scheme. The solution depends on both the Reynolds axial and filtration number. Some assumptions adopted in simplified models are discussed.

Theoretical and experimental study of natural convection on a horizontal plate

The authors report a numerical and experimental study of the natural convection flow above an upward facing horizontal heated plate placed in a semi-infinite medium. The equations, which govern the phenomenon, are discretized by using the control volume approach. The linkage between the momentum and continuity equations is realized with the SIMPLE algorithm. The results show the existence of two regions situated near the sides of the plate, which break away in the middle of the heated surface and form a thermal plume. The local heat transfer coefficient is higher near the sides of the plate and decreases at the central heated region. The validity of the above numerical results is experimentally checked with measurements of the temperature in the air surrounding the plate, which leads to experimental Nusselt numbers.

Performance analysis of a new sustainable evaporative cooling pad made from coconut coir

This paper reports a performance analysis for a new sustainable engineering application to beneficially reuse an abundant agricultural waste, coconut coir (Cocos nucifera), in evaporative cooling pads. Two small coconut coir pads of different configurations were fabricated and tested using a laboratory‐scale experimental arrangement. The air supply velocity was controlled and varied between 1.88 and 2.79 m s−1. Heat and mass transfer coefficients, evaporative cooling efficiency and pressure drop across the two types of coconut coir pad were analysed and compared with those of a commercial rigid media paper pad. Results show that the cooling efficiency of the manufactured coconut coir evaporative cooling pad was fairly good (about 50%) and close to that of the commercial paper pad (about 47%). The average pressure drop across the two coconut coir pads was 1.5 and 5.1 Pa respectively. Correlations for heat and mass transfer coefficients expressed using Nusselt and Sherwood numbers are also reported. In addition, the cooling potential of the coconut coir pads was analysed using the average climatic conditions of the central region in Thailand throughout the year. The analysis showed that the air temperature leaving the coconut coir pad varied from 23 to 28°C. Commercial development appears feasible given the coconut coir pads good performance, lower cost and its availability throughout the country.

Forced convection laminar film condensation of downward flowing vapor on a single horizontal elliptic cylinder or a bank of elliptical tubes

A numerical study is performed on the laminar film condensation of pure saturated vapor flowing in the direction of gravity on a single horizontal elliptic cylinder or a bank of elliptical tubes. Temperature, velocity distribution, and heat transfer coefficient of the fully developed flow are carried out with a fully implicit finite difference scheme. The equality of shear stress at the liquid-vapor interface is used as the coupling condition between the two phases. The inertia and convection term are retained in the analysis. Outside of the vapor boundary layer, the vapor phase velocity is obtained from potential flow. The method of source density distribution on the body surface is used for determination of the external vapor velocity in elliptical tube banks. The effect of inundation produced by condensate on upper ellipses is taken into account by assuming that the vapor velocity field is not affected by the condensate flow from one elliptic cylinder to another. Based on the obtained solutions of flow field, the effect of surface tension, the interaction because of the ellipse spacing, and the inundation on the heat transfer coefficient and the boundary layer separation point have been evaluated. The results of this analysis are discussed especially in function of eccentricity e (effect of the surface tension). The heat transfer in interellipse space is analyzed and compared with the theoretical and experimental results of other authors. Good agreement is shown.A numerical study is performed on the laminar film condensation of pure saturated vapor flowing in the direction of gravity on a single horizontal elliptic cylinder or a bank of elliptical tubes. Temperature, velocity distribution, and heat transfer coefficient of the fully developed flow are carried out with a fully implicit finite difference scheme. The equality of shear stress at the liquid-vapor interface is used as the coupling condition between the two phases. The inertia and convection term are retained in the analysis. Outside of the vapor boundary layer, the vapor phase velocity is obtained from potential flow. The method of source density distribution on the body surface is used for determination of the external vapor velocity in elliptical tube banks. The effect of inundation produced by condensate on upper ellipses is taken into account by assuming that the vapor velocity field is not affected by the condensate flow from one elliptic cylinder to another. Based on the obtained solutions of flow...

Study of transient laminar free convection over an inclined wet flat plate

Abstract A new analysis of the transient natural convection between an inclined wet flat plate and ambient air is presented. The problem is treated by considering two separate regions—i.e. the boundary layer and the capillary-porous plate—for which a specific differential system of equations is developed. The two systems are linked with the wall heat and mass balances from which the local and average Nusselt and Sherwood numbers are deduced. For some particular cases, quantitative comparisons with previous works reported in the literature agree with each other. Moreover, the agreement between theoretical results and experimental data is satisfactory in the boundary-layer region.

Design optimization of a new hot heat sink with a rectangular fin array for thermoelectric dehumidifiers

The objective of this research was aimed at conducting an experimental investigation to study the heat sink performance of a new rectangular fins array. Various operating conditions were considered, namely the distance between the fan and the fins, varied from 0 mm to 40 mm, heat supplied to the sink and the fan voltage. It was concluded that a fan installed at 30 mm height from the fins is recommended as the hot side temperature was the lowest. Next a pre-experimentation of small scale prototype of thermoelectric Dehumidifier (TED) was designed and constructed. It was composed of two thermoelectric (TE) cooling modules, MT2-1, 6-127, (two in serial) mounted between the rectangular fin heat exchangers with dimension 140 × 240 × 35 mm. The hot side was cooled by a ventilation fan whereas the air flow on the cold side was free convection. The effect of position of fan was investigation experimentally. Preliminary tests confirmed the good performance of the hot heat sink design for the intended thermoelectric application.

Numerical Study of Boundary-Layer Transition in Flowing Film Evaporation on Horizontal Elliptical Cylinder

A numerical study of the onset of longitudinal transition between turbulent and laminar regimes during the evaporation of a water film is presented. These water film streams along a horizontal elliptical tube under the simultaneous effects of gravity, pressure gradients, caused by the vapor flow and curvature, and viscous forces. At the interface of water vapor, the shear stress is supposed to be negligible. Outside the boundary layer, the vapor phase velocity is obtained from potential flow. In the analysis Von Karman’s turbulence model is used and the inertia and convection terms are retained. Transfers equations are discretised by using the implicit Keller method. The effects of an initial liquid flow rate per unit of length, Froude number, temperature difference between the wall and the liquid–vapor interface and ellipticity on the transition position have been evaluated. The transition criterion has been given in term of the critical film Reynolds number (ReΓ)C.

NUMERICAL STUDY OF LAMINAR MIXED CONVECTION IN A VERTICAL SATURATED POROUS ENCLOSURE: THE COMBINED EFFECT OF DOUBLE DIFFUSION AND EVAPORATION

Steady laminar mixed convection in a fluid-saturated, vertical, and homogeneous porous enclosure has been studied numerically based on a two-dimensional mathematical model. The buoyancy forces that induce the fluid motion are due to the temperature and concentration gradients on the vertical walls. The fluid movement is described by Darcys law, and transfers are given by the equations conservation of heat and mass transfer in the liquid phase. Results are presented in the form of stream lines, isotherms, concentration lines, and the evaporation velocity as a function of dimensionless physics parameters that govern the phenomena (R ct , R r/c , Pr, Sc,...). This treatment takes into consideration the effects of evaporation on the upper side wall of the system.

Prediction of onset of boundary layer transition in film condensation on a horizontal elliptical cylinder

This article is devoted to a numerical study of prediction of the onset of the boundary layer transition in film condensation of pure saturated vapor on a horizontal elliptical tube, under the simultaneous effects of gravity, shear stress, and the imposed pressure gradients, caused by the vapor flow and curvature, on the condensate film. The inertia and convection terms are retained in the analysis. The hypothesis of Shekriladze and Gomelauri is used at the liquid-vapor interface. Outside the boundary layer, the vapor phase velocity is obtained from potential flow. Temperature, velocity distribution, and dimensionless apparent turbulent stress of the fully developed flow are carried out using the implicit Keller method. The effects of pressure gradients characterized by Froude number, ellipticity, and Bond number on the transition position have been evaluated. The transition criterion has been given in terms of the critical film Reynolds number ( {\bf Re}_{\bGamma} ) C .

Numerical Simulation of Chaotic Natural Convection in a Differentiated Closed Square Cavity

This article reports a numerical study on the roads to chaos for a transient laminar natural convection in a square enclosure with horizontal adiabatic walls. The other walls, composed of two regions of the same size, were assumed isothermal. Transfer equations were solved using the vorticity-stream function formulation. The effects of Rayleigh number on heat transfer were also analyzed, and the roads to chaos borrowed by the convective flow. As the Rayleigh number increased, the flow structure started from four-cells to oscillatory three-cells. The first Hopf bifurcation was observed. Quasi-periodic flows appeared before the chaos onset.