Jamel Orfi

Evaporation of water by free or mixed convection into humid air and superheated steam

Abstract This paper concerns a numerical analysis of the evaporation of water in pure air, humid air and superheated steam in an externally insulated channel. Results were obtained for mixed and free convection driven by combined thermal and mass buoyancy forces. For natural convection case, the analysis is restricted to situation in which combined buoyancy forces act in the downward direction. The results show that below a certain temperature of the free stream, water evaporation rate decreases as the humidity of air increases and above it this relation reverses. This temperature “inversion point temperature” was treated in previous experimental and numerical studies in the case of forced convection. In this work, particular attention is paid to study the effect of the ambient conditions on evaporation rate of water and the inversion temperature phenomenon in the condition of free and mixed convection.

Evaporation of water by free convection in a vertical channel including effects of wall radiative properties

Abstract The present study consists of a numerical investigation of coupled heat and mass transfers by natural convection during water evaporation in a vertical channel heated symmetrically by a uniform flux density by taking into account radiative heat transfer between the plates. The governing equations are solved numerically by a finite difference method. The spatial profiles of velocity, temperature and moisture are presented. The effect of ambient conditions, channel width and walls radiation are also analysed in this study.

Developing laminar mixed convection with heat and mass transfer in horizontal and vertical tubes

Abstract The effects of the solutal and thermal Grashof numbers on the flow, temperature and concentration fields in tubes with uniform heat flux and concentration at the fluid-solid interface have been investigated numerically using a three-dimensional axially parabolic model. Results show a complex development of the flow field which is strongly influenced by the values of the two Grashof numbers and by the tube inclination. For vertical tubes the flow field is also influenced by the relative direction of the flow and the buoyancy forces. In general, very close to the tube inlet forced convection boundary layer development dominates. Further downstream, the effects of solutal buoyancy predominate while those of thermal origin determine the flow field far downstream and, in particular, the fully developed conditions. The axial evolution of the wall shear stress τ z , the Nusselt number Nu z and the Sherwood number Sh z in both horizontal and vertical tubes are presented for different combinations of the two Grashof numbers. For horizontal tubes and vertical tubes with upward flow these three variables are greater than the corresponding ones for forced convection. The opposite is true for downward flow in vertical tubes.

Bifurcation in steady laminar mixed convection flow in uniformly heated inclined tubes

The fully developed laminar mixed convection flow in inclined tubes subject to axially and circumferentially uniform heat flux has been studied numerically for a Boussinesq fluid. Dual solutions characterized by a two‐ and a four‐vortex secondary flow structure in a cross‐section normal to the tube’s longitudinal axis have been found for different combinations of the Grashof number Gr and of the tube inclination α for all Prandtl numbers between 0.7 and 7. In the two‐parameter space defined by Gr and α dual solutions occur: at a given α, if the Grashof number exceeds a critical value Grl (for horizontal tubes Grl is approximately 5.5 × 105, 1.7 × 105 and 1.7 × 104 respectively for Pr = 0.7, 7 and 70); at a given Gr, if the tube inclination is below a critical value αc (for Gr = 106 this critical angle is approximately 62.5° and 83.5° respectively for Pr = 0.7 and 7). Numerical experiments carried out for developing flows indicate that the two‐vortex solution is the only stable flow structure.

Développement simultané hydrodynamique et thermique d'un écoulement laminaire dans un tube incliné en régime de convection mixte

Abstract In this work, we study numerically the heat transfer and fluid flow inside a circular inclined tube. The thermal boundary condition is that of a constant and uniform (axially and circumferentially) heat flux on the tube wall. A finite volume method is used to solve, in dimensionless form, the parabolic equations of mixed convection. The results, obtained for water with different combinations of the Grashof number and the tube inclination, show that the average heat transfer is improved and the wall shear stress is increased compared to those of pure forced flow.

Desalination With a Solar-Assisted Heat Pump: An Economic Optimization

The solar-assisted heat pump (SAHP) desalination, based on the Rankin cycle, operates in low temperature and utilizes both solar and ambient energy. An experimental SAHP desalination system has been constructed at the National University of Singapore, Singapore. The system consisted of two main sections: an SAHP and a water distillation section. Experiments were carried out under the different meteorological condition of Singapore and results showed that the system had a performance ratio close to 1.3. The heat pump has a coefficient of performance of about 8, with solar collector efficiencies of 80% and 60% for evaporator and liquid collectors, respectively. Economic analysis showed that at a production rate of 900 L/day and an evaporator collector area of around 70 m2 will have a payback period of about 3.5 years.

Numerical study of flow, temperature, and salinity distributions of a brine discharge problem

In this study, the problem of brine discharge into sea is studied numerically. A 3D model for the heat and brine dispersion in the vicinity of discharge and intake ports is developed using Fluent package. The flow is taken turbulent and the fluid properties are considered variable with salinity and temperature. The results are expressed in terms of streamlines, isotherms, and salinity contours as well as velocity, temperature, and salinity profiles. These results give the excess temperature and salinity relative to the nominal values of the free stream. Effect of discharge mass flow rate on patterns of temperature and salinity contours is presented and analyzed. It is observed in particular that for the different simulations undertaken, the excess temperature and salinity can be important and the intake is always affected by the discharge conditions.

Evaporation of water/ammonia binary liquid film by mixed convection inside isothermal vertical plates

Abstract An ammonia/water mixture can be used as an efficient working fluid in several industrial applications such as in heat pumps, heat transformers and separation processes. This paper presents a numerical analysis of coupled heat and mass transfer by mixed convection during water/ammonia binary liquid film evaporation inside vertical channels. The two channel walls are isothermal, but maintained at different temperatures. One wall maintained at (T w) is covered with an extremely thin liquid film, while the other wall which is maintained at (T p) is dry. The conducted simulations enable the analysis and the description of the evaporation process and the heat and mass transfers for a wide range of parameters including the inlet variables of each liquid and the inlet properties of the gas. The results focused on the effect of the inlet amount of water vapour and that of ammonia vapour on the evaporating rates of these two components as well as on the distribution of the concentration profiles.

Performance analysis of a gas turbine unit combined with MED-TVC and RO desalination systems

AbstractThe present work deals with a theoretical study on the performance of a combined power and desalination plant. The power is a simple gas turbine unit (GT) while the desalination is composed of thermal and membrane systems. The thermal desalination consists of a multiple effect distillation (MED) with thermal vapor compression (TVC), and the membrane plant is a reverse osmosis (RO) with an energy recovery device (pressure exchanger system). The modeling is based on appropriate evaluation of the sea water properties including that of flow exergy and the solution of mass, energy, and exergy balances for the main components of the power and desalination plant. The simulations include typical data on actual GT units. The effect of the main controlling parameters of the GT plant mainly the pressure ratio and the turbine inlet temperature on the total production rates of fresh water is presented and discussed. An exergy analysis showing the destruction rates of the overall system as well as its main comp...

Thermodynamic analysis of a cogeneration gas turbine and desalination plant

Abstract This work deals with a thermodynamic analysis of the performance of a cogeneration gas turbine and thermal desalination plant. A heat recovery steam generator links the two units together. The analysis is based on the application of mass, energy and exergy balances on the whole cogeneration plant as well as on each of its components. The analysis is also based on an accurate evaluation of the seawater properties, which leads to correct calculations of the flow exergy and exergy efficiency of any component of the desalination plant. The main results of the study concern the influence of the pressure ratio of the compressor and the turbine inlet temperature on the exergy efficiency of the gas turbine and the whole combined system. It was found in particular that as the performance ratio of the desalination plant increases the specific exergy destruction rate decreases.