M. Khamis Mansour

Numerical simulation of flashing process in MSF flash chamber

abstract Multistage flash (MSF) technology is widely used in saline water desalination particularly in Middle East and North Africa and Gulf Cooperation Council countries. Enhancement in the thermal performance of this technology in different plant sections and processes including the brine flashing process is still promising. This study addresses the optimal position of jumping plate (weir) location and its number inside the MSF chamber. The optimization exercise has been carried out in terms of maximum flashing vapor production and lower pressure drop using commercial computational fluid dynamics software ANSYS CFX 12.1. The Eulerian–Eulerian (free surface flow) two-phase model was adopted while a k–ϵ model was used a turbulent flow model. The theoretical model was verified by comparing the predicated results with those obtained from the reference case study. The maximum deviation between both results was found to be within 8.3%. Prediction of velocity vectors, phase volume fraction, and temperature pro...

Buoyancy-induced flow and heat transfer in a porous annulus between concentric horizontal circular and square cylinders

ABSTRACT This paper reports on natural convection heat transfer in a porous annulus between concentric horizontal circular and square cylinders. The heated inner circular cylinder is maintained at the uniform hot temperature Th, whereas the cooled outer square duct is held at the uniform cold temperature Tc. A pressure-based collocated finite-volume method is used to numerically investigate the effects on the total heat transfer of Rayleigh number (Ra), Prandtl number (Pr), Darcy number (Da), porosity (ϵ), and annulus aspect ratio (R/L). Results demonstrate that at low Ra values, conduction is the dominant heat transfer mode. Convection contribution to total heat transfer becomes more important beyond a critical Ra value, which decreases with an increase in Da and/or ϵ. Furthermore, an increase in the enclosure aspect ratio (R/L) leads to an increase in total heat transfer. A similar behavior is obtained with Prandtl number, where predictions indicate higher heat transfer rates at higher Pr values with its effect increasing as Ra increases. Streamlines and isotherms reveal flow separation for some of the reported cases. Limited computations are also performed for natural convection in a porous annulus between two horizontal concentric circular cylinders having the same inner and outer perimeters as the investigated enclosure. Comparison of the predicted average Nusselt number estimates with similar ones obtained in the original enclosure reveals a large percentage difference in values, demonstrating the strong influence of geometry on natural convection in enclosures.

Novel approach of thermal modeling of partially dry–wet chilled water cooling coil under unit and nonunit Lewis number conditions

ABSTRACT The main objective of this work is to present a new modeling approach of thermal performance and design of partially dry–wet cooling coils working under unit or nonunit Lewis number conditions. The innovative model is presented as a new simplified and practical correlation that interrelates the cooling coil effectiveness (ε) with its number of transfer unit, and vice versa. The simplified model was constructed on a basis of solving the heat and mass transfer equation “enthalpy potential method” simultaneously coupled with the thermodynamics equations. The validity of the new correlations was tested through predictions of its thermal performance. The output results of those correlations show satisfactory agreement with those obtained from the referenced data with deviation of less than 10%. The main feature of this novel correlation is its simplicity and easiness in calculation by knowing input Lewis number and some other key parameters. Also, the main benefit of this new model is to provide helpful guidelines for optimization of fully wet or partially dry–wet cooling coils’ performance and developing suitable control strategies to achieve higher thermal behavior of the cooling coil during its operation.

Effect of natural convection on conjugate heat transfer characteristics in liquid minichannel during phase change material melting

This article presents numerical and experimental simulation of three-dimensional conjugate heat transfer problem in mini-scaled thermal storage system. The conjugate problem includes melting process of phase change material in the presence of natural convection during laminar flow of heat transfer fluid through circular minichannel. The paraffin wax is used as a phase change material while the water is used as a heat transfer fluid. The main objective of this study is to investigate the effect of the phase change material natural convection during the melting process on the heat transfer fluid thermal characteristics as well as the impact of the natural convection on the melting performance itself. The thermal characteristics are represented by local Nusselt number (Nu) and local surface temperature. The melting performance is evaluated by fusion time and liquid fraction profile. Two inlet temperatures and velocities of the heat transfer fluid are adopted to highlight the effect of the natural convection. Combination of the inlet temperatures and velocities of the heat transfer fluid forms four cases: case_1 (at T f , in  = 353 °K, V f , in  = 1 m/s), case_2 (at T f , in  = 453 °K, V f , in  = 1 m/s), case_3 (at T f , in  = 353 °K, V f , in  = 0.1 m/s), and case_4 (at T f , in  = 453 °K, V f , in  = 0.1 m/s). Experimental test rig was constructed to verify the computational results and good agreement between both results was achieved. The study shows that the heat transfer fluid encounters an erratic thermal behavior during the phase change material melting process. For example, the local surface temperature experiences dramatic increase and decrease at certain sections of the channel length. The magnitude of this temperature inconsistency interrelates closely to the strength of natural convection impact, and this can expose the minichannel (which has short length) to severe wall thermal stress. The local Nu experiences improvement in some section of the channel and at the same time it suffers from drastic deterioration in its value particularly at the channel end at which the convection current accommodates. The case with the lowest inlet velocity and the highest inlet temperature has the smallest fusion time at expense of the largest heat transfer fluid bulk temperature gradient before reaching the fusion time. The study is considered as a benchmark and helpful guidelines in the design of small-scaled thermal storage systems of phase change material.

Buoyancy-induced flow, heat, and mass transfer in a porous annulus

ABSTRACT This paper reports on double-diffusive natural convection heat transfer in a porous annulus between concentric horizontal circular and square cylinders. A pressure-based segregated finite volume method is used to solve the problem numerically. The diffusion fluxes are discretized using the MIND fully implicit scheme. Furthermore, a modified pressure correction equation is derived that implicitly accounts for the nonorthogonal diffusion terms, which are usually neglected in the standard SIMPLE algorithm. Results indicate that convection effects increase with an increase in Rayleigh number, Darcy number, porosity, and enclosure aspect ratio. Further, at low Darcy values, porosity has no effect on the flow, temperature, and concentration fields.

Thermal investigation of the conjugate heat transfer problem in multi-row circular minichannels

ABSTRACT A numerical three-dimensional flow and conjugate heat transfer in circular minichannel-based multi-row heat sink is presented in this article. Effects of geometrical parameters including channel dimensions, channel arrangements (inline or staggered), and the number of channel rows with a single-pass flow on the thermal performance of the heat sink are presented. The determination of the bottom surface temperature, average heat transfer coefficient, thermal resistance as well as the pressure drop was reported. The number of rows and the diameter of the circular channel for a constant Reynolds number were found to have a remarkable cooling effect on the heat sink. It was found out that in the case of using four channel rows with the channel diameter of 1 mm, the cooling capacity is 88.5 W/cm2 compared to 28 W/cm2 for a single row 1 mm diameter.

The Effect of Axial Wall Conduction on Heat Transfer Parameters for a Parallel-Plate Channel Having a Step Change in Boundary Conditions

This article investigates the effect of an axial wall condition on the heat transfer characteristics for laminar flow through a thick-walled channel. A partition separates the channel of interest into two regions (upstream and downstream region), in which two different coolant fluids bind the channel in those regions. A numerical model was developed using a finite-difference technique to solve the conjugate heat transfer problem. The theoretical model was verified by comparing the predicated results with those obtained from the available analytical and experimental data, and good agreement was achieved. The wall thickness ratio, Biot number, and coolant fluids’ temperature difference step change were selected as the key parameters. The study shows that the significant change in those parameters could lead to an enhancement of 24% for the average Nusselt number at certain conditions.

Comparative study for different demister locations in multistage flash (MSF) flash chamber (FC)

ABSTRACTMultistage flash (MSF) is a widely used technology in large capacity salted water desalination plants. The enhancement in the thermal performance of this technology is still prospective and promising. In this research, vapor flow through the flash chamber (FC) was studied. Flow development in 2D simulation model of a real FC was investigated. Trajectory of liquid droplets was calculated using Lagrange approach. The continuity and Navier–Stokes equations for the continuous phase “vapor” were solved simultaneously with the particle equation using two equations k–ϵ turbulence model. The computational model was verified by comparing the predicated results (vapor pressure drop through the FC demister and moist separation efficiency) with those obtained from the published experimental data. The comparison showed a good agreement between both results with maximum deviation of less than −19.16%, however, most of the disagreement between both results is fewer than 10%. Four different demister locations wer...