Mechanical Engineering

The objectives of energy systems diagnosis are: i) identifying components responsible for highest losses increments in comparison with design conditions, ii) quantifying energy-efficiency recovery when design conditions are restored in a particular system component. In this paper the energy diagnosis of an existing steam power plant is faced on the basis of three different formulae expressing global losses. A simulation model of a real 320 MW steam power plant has been implemented with a commercial modular energy systems simulation software (Aspen+). The functional decay of different components of the plant has been simulated using the model. The results show that the components actually responsible for the additional power losses can be identified and the effect of restoring design conditions can be quantified in most cases with sufficient approximation by using the General Formula for the Efficiency or the Lost work Impact Formula (developed in the ambit of thermoeconomics).

This paper presents and discusses the results of a complete thermoeconomic analysis of an integrated power plant for co-production of electricity and hydrogen via pyrolysis and gasification processes fed by various coals and mixture of coal and biomass, applied to an existing large steam power plant (ENEL Brindisi power plant – 660 MWe). Two different technologies for the syngas production section are considered: pyrolysis process and direct pressurized gasification. Moreover, the proximity of a hydrogen production and purification plants to an existing steam power plant favors the inter-exchange of energy streams, mainly in the form of hot water and steam, which reduces the costs of auxiliary equipment. The high quality of the hydrogen would guarantee its usability for distributed generation and for public transport. The results were obtained using WTEMP thermoeconomic software, developed by the Thermochemical Power Group of the University of Genoa, and this project has been carried out within the framework of the FISR National project “Integrated systems for hydrogen production and utilization in distributed power generation”.

In this paper, we simulate steady inviscid flows over a cylinder using potential and stream functions, including entropy and vorticity corrections for incompressible, subsonic, transonic and supersonic flows. The present hierarchical formulation is equivalent to Euler equations satisfying conservation of mass, momentum and energy. Standard numerical schemes and iterative algorithms are used for the mixed type potential (or stream function) equation and for the convection equations of entropy and vorticity augmented with the proper artificial dissipation necessary for stability of computations. Typical results for a cylinder in uniform and shear flows are presented and the merits of the present approach are briefly discussed.

In this paper, the hierarchical formulation for steady viscous transonic flow simulations of Refs. [Hafez M, Wahba E. Numerical Simulations of Transonic Aerodynamic Flows. AIAA paper 03-3564, 2003] and [Hafez M, Wahba E. Viscous/Inviscid Interaction Procedures for Compressible Aerodynamic Flow Simulations (in press)] is reviewed and a simplified version for the calculation of the vortical velocity components is presented. The results are compared to available solutions of standard Navier-Stokes equations for laminar flows.

In a companion paper, the authors used a hierarchical formulation based on a Helmholtz velocity decomposition to simulate transonic ows over airfoils. The potential ow formulation is augmented with entropy and vorticity corrections and the numerical results are compared to standard Euler and Navier-Stokes calculations. For many aerodynamic applications, the corrections are limited to relatively small regions; the ow in the near and far ÿelds is irrotational and isentropic. The entropy and vorticity corrections are governed by convection=di usion equations while the non-homogeneous potential equation is of mixed type; elliptic in the subsonic domain and hyperbolic in the supersonic one. The forcing function represents the necessary correction for mass conservation. Upwind schemes are used for the convection terms of the scalar equations of the corrections and for the potential equation in the supersonic region. The formulation can be viewed as an implementation of a viscous=inviscid interaction procedure which is equivalent to Navier-Stokes equations in the inner ÿeld.

In this paper, steady two-dimensional potential ows over airfoils are calculated with and without entropy and vorticity corrections, due to the presence of shock waves and=or laminar boundary layers. The results are in agreement with the corresponding solutions of Euler and Navier-Stokes equations. The present approach is brie y described and some extensions are discussed.

Steady two-dimensional laminar incompressible flows over airfoils are simulated using a Helmholtz velocity decomposition where the velocity vector is split into a gradient of a potential and a correction representing the rotational components. For most aerodynamic flows, the rotational components of the velocity vanish outside the boundary layer and the wake region. Therefore, the near and far velocity fields can be represented by a potential function and the pressure can be obtained there from BernoulliÕs law. In the viscous flow region, the momentum equations are integrated to calculate the rotational velocity components. Conservation of mass leads to a PoissonÕs equation for the potential function, where the right hand side, the forcing function, is given in terms of the divergence of the rotational velocity component. The latter represents a distribution of sources in the viscous layer and results in a displacement effect on the outside potential field. In this formulation, the pressure in the momentum equations does not play an essential role since the conservation of mass is imposed through the augmented potential equation. After the velocity field is obtained, the correct pressure distribution in the viscous flow region is calculated by integrating the normal momentum equation. Numerical results are presented to confirm the validity and the merits of the present formulation. Solving a PoissonÕs equation for the potential function, rather than the pressure as in standard methods for solving Navier-Stokes equations, is the main advantage since the calculations of the rotational velocity components are restricted to the viscous flow region only. Standard numerical methods are applicable to the present formulation including multigrid acceleration techniques for the augmented potential equation.

The subject matter of this research is that of improving and enhancing the results of the mathematical models of the classical turbulent flows with increasing Reynolds numbers over the surfaces of complex configurations to improve its applicability in diverse realistic disciplines. As the sinusoidal solid surface with the wavy boundary in the mainstream direction develops periodic pressure gradient in the fluid flow, successive acceleration and deceleration associated with multiple fluid flow separations and reattachments, leads to enrich the analysis and the consequent results. Also, as this issue represents the focal point of many researchers over the previous three decades and consequently the numerical and experimental results available in the literature are enough for conducting its investigation. Therefore, turbulent flow over a sinusoidal solid surface is investigated using two versions of the standard k-e turbulence model. In this regard, the present investigation is performed within the framework of the 2D modeling to simplify the involved rigorous mathematical processing and to introduce a reliable physical interpretation of the numerical results, which validated against the available results of the Direct Numerical Simulations (DNSs) and experimental works at moderate Reynolds numbers with the recirculation zones captured well. Also, the influences of alternating pressure gradients induced by the fluctuating surface curvatures, the sequential

This chapter introduces a comprehensive overview about the principles, challenges and applications of adsorption refrigeration systems (ARSs), as a promising sustainable solution for many of cooling and heating applications. In addition to the features and the basics of ARSs, the following topics have been covered such as characteristics of working pairs, trends in improving the heat and mass transfer of the adsorber; advanced adsorption cycles and performance and operational data of some adsorption refrigeration applications. In some details, the operating range and the performance of ARSs are greatly affected by the employed working adsorbent/refrigerant pairs. Therefore, the study, development and optimum selection of adsorbent/refrigerant pairs, particularly the composite adsorbents, can lead to improving the performance and reliability of ARSs. Regarding the enhancement of heat and mass transfer in the adsorbent bed, two methods are commonly used: one is the development of adsorbents through different coating technologies or new materials such as metal-organic frameworks, and the second is the optimization of the adsorber geometrical parameters and cycle modes. Finally, a brief on some adsorption chillers applications have started to find their share in markets and driven by solar or waste heats.

The subject matter of this research is that of improving and enhancing the results of the mathematical models of the classical turbulent flows with increasing Reynolds numbers over the surfaces of complex configurations to improve its applicability in diverse realistic disciplines. As the sinusoidal solid surface with the wavy boundary in the mainstream direction develops periodic pressure gradient in the fluid flow, successive acceleration and deceleration associated with multiple fluid flow separations and reattachments, leads to enrich the analysis and the consequent results. Also, as this issue represents the focal point of many researchers over the previous three decades and consequently the numerical and experimental results available in the literature are enough for conducting its investigation. Therefore, turbulent flow over a sinusoidal solid surface is investigated using two versions of the standard k-e turbulence model. In this regard, the present investigation is performed within the framework of the 2D modeling to simplify the involved rigorous mathematical processing and to introduce a reliable physical interpretation of the numerical results, which validated against the available results of the Direct Numerical Simulations (DNSs) and experimental works at moderate Reynolds numbers with the recirculation zones captured well. Also, the influences of alternating pressure gradients induced by the fluctuating surface curvatures, the sequential

Direct numerical simulations of low-Reynolds number turbulent channel flow are carried out where thin rectangular riblets are distributed uniformly at only one of the channel walls. A second-order accurate finite volume code is modified in order to reconstruct the adjacent cells to the riblets at Cartesian coordinates. A quadratic interpolation scheme is used to estimate the fluxes at the reformed cells. The riblets height/spacing ratio is fixed at 0.5 which doubles the surface area at the ribbed wall and the thickness/spacing ratio is set to 0.02. The riblet spacing ranges from 13 to 41 viscous units. Comparison with experiments shows excellent agreement in detecting both reduction and the increase of drag at the studied range of spacings. The maximum drag reduction obtained is about 11% which occurs at a spacing of 18 viscous units. The increase of drag is addressed when the spacing is larger than 30 wall units. Wide spacing permits the near-wall eddies to reside inside the riblet-to-riblet span which in turn makes them interacting with larger wetted area and increases the friction drag.

The present study numerically investigates the characteristics of three-dimensional turbulent flow in a wavy channel. For the purpose of a careful observation of the effect of the wave amplitude on the turbulent flow, numerical simulations are performed at a various range of the wave amplitude to wavelength ratio (0.01pa/lp0.05), where the wavelength is fixed with the same value of the mean channel height (H). The immersed boundary method is used to handle the wavy surface in a rectangular grid system, using the finite volume method. The Reynolds number (Re ¼ U b H/n) based on the bulk velocity (U b) is fixed at 6760. The present computational results for a wavy surface are well compared with those of references. When a/l ¼ 0.02, the small recirculating flow occurs near the trough at the instant, but the mean reverse flow is not observed. In the mean flow field, the reverse flow appears from a/l ¼ 0.03 among the wave amplitude considered in this study. The domain of the mean reverse flow defined by the locations of separation and reattachment depends strongly on the wave amplitude. The pressure drag coefficient augments with increasing the wave amplitude. The friction drag coefficient shows the increase and decrease behavior according to the wave amplitude. The quantitative information about the flow variables such as the distribution of pressure and shear stress on the wavy surface is highlighted.

The present study numerically investigates the aerodynamic characteristics of two-dimensional wings in the vicinity of the ground by solving two-dimensional steady incompressible Navier-Stokes equations with the turbulence closure model of the realizable k ε − model. Numerical simulations are performed at a wide range of the normalized ground clearance by the chord length (0.1 / 1.25 h C ≤ ≤) for the angles of attack (0 1 0 α°≤ ≤°) in the prestall regime at a Reynolds number (Re) of 6 2 10 × based on free stream velocity U ∞ and the chord length. As the physical model of this study, a cambered airfoil of NACA 4406 has been selected by a performance test for various airfoils. The maximum lift-to-drag ratio is achieved at 4 α =°and / 0.1 h C =. Under the conditions of 4 α =°and / 0.1 h C = , the effect of the Reynolds number on the aerodynamic characteristics of NACA 4406 is investigated in the range of 2×10 5 ≤ Re ≤ 2×10 9. As Re increases, l C and d C augments and decreases, respectively, and the lift-todrag ratio increases linearly.

The effect of Coriolis force on turbulent channel flow has been sought in a more general manner by taking into account the alignment between the rotation axis and the direction of mean pressure gradient and the rotation rate as well. Three different, but orthogonal rotation vectors coincident with the Cartesian coordinates have been imposed on a plane channel, in which homogeneity is presumed in the planes parallel to the wall. A series of DNS has been performed for each case starting from the non-rotating plane channel, while increasing the rotation number and keeping the Reynolds number based on the friction velocity at 150. Detailed statistics are obtained including mean quantities, turbulent intensities, vorticities, and higher-order moments. The budgets of transport equations of the quantities relevant to turbulence modeling are prepared for the three orthogonal cases in order to help assessing turbulence models. An attempt to investigate the near-wall structures has been tried.

The present study provides an energy saving design of a desalination plant that uses the waste heat of the exhaust of a diesel generator. The proposed model consists of a multistage flash (MSF) desalination unit in which the sea water is heated by the flue gases instead of steam. A mathematical model is derived to design the entire plant including the waste heat recovery heat exchanger and the MSF plant. The model is flexibly designed in order to deal with wide range of diesel generators rates and any number of stages of MSF. Database of diesel generator sets are imported including temperature of exhaust, gas flow rates and the back pressures for each set. Two test cases are carried out with two different salinity levels and diesel generator rates, results show that the amount of distilled water in kg/s can be correlated to the rate of diesel generator according to the second order polynomial: in case of brackish water and in case of seawater. The survey of power generation in Lebanon reveals that the waste heat from diesel generator can be utilized to get around 100 million of distillate water usable for drinking and for domestic use. This quantity can provide fresh water for more than 300,000 person/year. The performance of proposed plant is assessed during summer and winter conditions and the model can deal with any salinity ranges.

A three-dimensional buoyancy-driven turbulent boundary layer is generated in fully developed turbulent channel flow.Setting the buoyancy forces perpendicular to the mean flow direction and using high Grashof number result in strong crossflow which is expected to have a similar influence to that of the previously studied pressure- and shear-driven threedimensionalturbulence boundary layers (3DTBLs). The present buoyancy-driven configuration has the advantage of being inequilibrium and three-dimensional from inception. Few issues are presented and discussed in order to give betterunderstanding of the underlying physics associated with buoyancy-induced 3DTBL flows. The characteristics of the resulting3DTBL are examined in comparisons with the available data in the literature. Reduction of the structure parameter A1(defined as the ratio between the total Reynolds shear stress to twice the turbulent kinetic energy TKE) is noticed in the edgeof the inner layer which is attributed to the r...

The present paper investigates the aerodynamic characteristics of an arbitrary airfoil and its impact on the performance of a Ground Effect Vehicle GEV. The approaches that have been previously suggested and conducted for studying the WIG problem are demonstrated. Few published papers are accompanied by reasonable background information to enable an independent researcher to directly incorporate the available mathematical model into his calculations. A numerical technique for investigating the aerodynamic characteristics of an arbitrary airfoil in the ground effect zone is adapted. Wide ranges of the dominant parameters, i.e., ground clearance, angle of attack and Reynolds number are considered. The calculations are performed within the framework of a 2-D formulation to provide a simple mathematical model with a meaningful physical interpretation of the numerical results. Prominence is given to the investigation of the numerical results and there physical interpretation rather than ...

A simple prototype air conditioning unit driven entirely by solar energy is proposed aiming at replacing the conventional vapor compression air-conditioning systems which are reasonable for the global warming. The proposed model is supposed to be used in conditioning the governmental offices during the working hours in the weekdays when both the sunshine and the need for air-conditioning reach their maximum levels at the same instance. Solar adsorption refrigeration devices have no moving parts consequently they are noiseless, non-corrosive, cheap to maintain, long lasting in addition to being environmentally friendly with zero ozone depletion as well as zero global warming potentials. For these reasons, the research activities are of increasing interest in this aspect in order to provide optimum solutions for the crucial points that impede making these systems capable to meet the criteria for commercialization. INTRODUCTION The world is facing energy crisis due to the expected shor...

Multi Stage Flash (MSF) technology is widely used in salted water desalination. The enhancement in the thermal performance of this technology is still prospective and promising. The main purpose of the present study is to highlight the key parameters which contribute significantly to the improvement in MSF Flash Chamber (FC) efficiency, as the critical component of MSF unit. In this work, vapor flow through demister is studied. Flow development in 2D simulation model of a real flash chamber has been investigated using ANSYS FLUENT 12.1, commercial Computational Fluid Dynamics (CFD) software. The two equations k-ε turbulence model is used. Prediction of velocity vectors, pressure contours, phase volume fraction, temperature profile and demister pressure drop are presented. The significance of the results and the factors affecting the MSF chamber performance are analyzed. Verification of the results will be presented in this work that simulates a typical experimental set-up (under con...