Multi-dimensional scavenging analysis of a free-piston linear alternator based on numerical simulation

A free-piston linear alternator (FPLA) is being developed by the Beijing Institute of Technology to improve the thermal efficiency relative to conventional crank-driven engines. A two-stroke scavenging process recharges the engine and is crucial to realizing the continuous operation of a free-piston engine. In order to study the FPLA scavenging process, the scavenging system was configured using computational fluid dynamics. As the piston dynamics of the FPLA are different to conventional crank-driven two-stroke engines, a time-based numerical simulation program was built using Matlab to define the piston’s motion profiles. A wide range of design and operating options were investigated including effective stroke length, valve overlapping distance, operating frequency and charging pressure to find out their effects on the scavenging performance. The results indicate that a combination of high effective stroke length to bore ratio and long valve overlapping distance with a low supercharging pressure has the potential to achieve high scavenging and trapping efficiencies with low short-circuiting losses.     

Numerical simulation of a mini PEMFC stack

Fuel cell modeling and simulation has aroused much attention recently because it can probe transport and reaction mechanism. In this paper, a computational fuel cell dynamics (CFCD) method was applied to simulate a proton exchange membrane fuel cell (PEMFC) stack for the first time. The air cooling mini fuel cell stack consisted of six cells, in which the active area was 8 cm² (2 cm × 4 cm). With reasonable simplification, the computational elements were effectively reduced and allowed a simulation which could be conducted on a personal computer without large-scale parallel computation. The results indicated that the temperature gradient inside the fuel cell stack was determined by the flow rate of the cooling air. If the air flow rate is too low, the stack could not be effectively cooled and the temperature will rise to a range that might cause unstable stack operation.     

Numerical Simulation of a Negative Impulsive Wave

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Numerical simulation of a steam pipeline network

The present paper describes a computer program that can simulate the behaviour of a network of steam pipelines, such as those used in Larderello geothermal field, fed by superheated steam. Also given are some applications of this program to a network in operation.     

Operational simulation of wind power plants for electrolytic hydrogen production connected to a distributed electricity generation grid

Two procedures are analyzed to control the flow of hydrogen produced by an electrolyzer in a plant connected to a distributed electricity grid. The general idea of both procedures is to approximate the consumption power of the electrolyzer to the tracked hourly mean useful power of a wind generation system. The first technique uses a perceptron to predict hourly wind-speed values as the basis for the power consumption of the electrolyzer. The second approximates the hourly consumption of the electrolyzer to the useful power of the wind generation system over the previous hour. Calculations have shown that the control procedure, using either one of these two techniques, leads to substantial improvements in the main parameters of the plant, compared to an installation in which electrolyzer consumption is constant. In particular, the number of batteries in the accumulation system may be reduced. Moreover, considering the possibility that the hydrogen production plant might supply electricity to the external electricity grid, various objectives for operational optimization of the installation are analyzed. A function that defines the joint exploitation of the wind energy by the electrolyzer and the external electricity grid is introduced and then, by using that function, an optimal operating regime for the plant is determined.     

Numerical simulation of laser irradiation to a randomly packed bimodal powder bed

Bimodal packing arrangements can be used to overcome the balling phenomena in selective laser sintering (SLS). However, little attention has been paid to the effects of bimodal packing structures on radiative transfer in the SLS powder beds. In this study, a sequential addition packing algorithm is firstly employed to generate 3-D random packing of opaque, diffusively or specularly reflecting spherical particles with the same or different sizes. Then, a Monte Carlo based ray tracing algorithm is formulated, for the first time, to simulate the radiative transfer in the bimodal random packing structures composed of particles with different sizes and emissivities. The credibility of the computer code is verified with published experimental data. A comparison is also made between the calculating results and those obtained by two-flux model. By using the present algorithm, the radiative heat fluxes at both levels of particle and entire bed as well as the transmitted laser energy are statistically evaluated. The influences of bimodal size distribution and particle surface emissivity on the radiative transfer process are examined. Such information is expected to be helpful for optimizing the SLS process.     

Numerical simulation of a supercritical CO2 geothermosiphon

The thermo-hydraulic performance of a CO₂ geothermosiphon has been numerically investigated using the commercially available software CFX. A simple Engineered (or Enhanced) Geothermal System, EGS, consisting of an injection and a production well as well as a reservoir is numerically simulated. Both water and carbon dioxide have been examined as the working fluid. While the former fluid has been very popular for its availability, the latter offers advantages such as favorable thermodynamic properties as well as the inherent possibility of geosequestration. However, detailed analysis of such CO₂ geothermosiphon systems is not available in the open literature. Higher heat extraction rate from the reservoir at lower pressure drops for a CO₂ geothermosiphon, compared to water-based systems, can be achieved and general criteria for that are presented.     

Numerical simulation of a liquid propellant rocket motor

IntroduedonModelling and numerical simulation are presentedin nearly every aspect of aerospace engineeringnowadays thanks to availability of powerful andrelatively inexpensive computing tools. This workpresents a numerical simulation of the flow field in aliquid Propellant rocket engine chamber and exit nozZleusing techniques to allow the results to be taken asstaring points for designing those PrOPulsive systems.iiis was dolle using a Finite VolUme method sthe different flow regimes which …     

Numerical simulation of a water spray—Radiation attenuation related to spray dynamics

Radiation attenuation by a water spray is predicted on the basis of a detailed simulation. First of all, a two-way coupling treatment of the spray dynamics is achieved through an Eulerian–Lagrangian modeling. Droplet distribution combined with water vapor and carbon dioxide volume fractions are then used to compute the radiative properties of the medium. A simulation of radiation propagation is performed, aimed at the computation of the spectral transmittance through the spray. A Monte Carlo technique is used to describe radiation absorption and scattering phenomena for a real droplet polydispersion and an equivalent monodispersion. Numerical results of spray attenuation are compared to experimental data obtained on a laboratory spray with low flow rate. Satisfying accuracy can be obtained for the numerical prediction if a realistic size distribution is used for the pulverization. The mean Sauter diameter and the volumetric fraction of droplets are found to vary with the position in the spray. Tentative predictions with a monodispersion therefore fail in predicting the attenuation ability of the spray at various vertical positions below the injection point.     

Numerical simulation of a hydrogen fuelled gas turbine combustor

The interest for hydrogen-fuelled combustors is recently growing thanks to the development of gas turbines fed by high content hydrogen syngas. The diffusion flame combustion is a well-known and consolidated technology in the field of industrial gas turbine applications. However, few CFD analyses on commercial medium size heavy duty gas turbine fuelled with pure hydrogen are available in the literature. This paper presents a CFD simulation of the air-hydrogen reacting flow inside a diffusion flame combustor of a single shaft gas turbine. The 3D geometrical model extends from the compressor discharge to the gas turbine inlet (both liner and air plenum are included). A coarse grid and a very simplified reaction scheme are adopted to evaluate the capability of a rather basic model to predict the temperature field inside the combustor. The interest is focused on the liner wall temperatures and the turbine inlet temperature profile since they could affect the reliability of components designed for natural gas operation. Data of a full-scale experimental test are employed to validate the numerical results. The calculated thermal field is useful to explain the non-uniform distribution of the temperature measured at the turbine inlet.     

Numerical simulation of boiling enhancement on a microstructured surface

Significant efforts have been made to augment nucleate boiling by surface modification with micro-machined structures, but a general predictive approach for heat transfer enhancement has not yet been developed. In this work, complete numerical simulations are performed for boiling enhancement on a microstructured surface by employing the sharp-interface level-set method, which is modified to handle the contact angle and the evaporative heat flux from the liquid microlayer on an immersed solid surface. The effects of cavity diameter and surface modification such as concentric grooves and multi-step cavities on bubble growth and boiling heat transfer are investigated.     

Numerical simulation of a submerged cylindrical wave energy converter

In this study, a numerical model based on the complete solution of the Navier–Stokes equations is proposed to predict the behavior of the submerged circular cylinder wave energy converter (WEC) subjected to highly nonlinear incident waves. The solution is obtained using a control volume approach in conjunction with the fast-fictitious-domain-method for treating the solid objects. To validate the model, the numerical results are compared with the available analytical and experimental data in various scenarios where good agreements are observed. First, the free vibrations of a solid object in different non-dimensional damping ratios and the free decay of a heaving circular cylinder on the free surface of a still water are simulated. Next, the wave energy absorption efficiency of a circular cylinder WEC calculated from the model is compared with that of the available experiments in similar conditions. The results show that tuning the converter based on the linear theory is not satisfactory when subjected to steep incident waves while the numerical wave tank (NWT) developed in the current study can be effectively employed in order to tune the converter in such conditions. The current NWT is able to predict the wave-body interactions as long as the turbulence phenomena are not important which covers a wide range of Reynolds and Keulegan-Carpenter numbers.     

Numerical simulation of a practical chemical vapor deposition reactor

ABSTRACT

Numerical simulation was set up to study a rotating vertical impinging chemical vapor deposition (CVD) system for the fabrication of thin films. It has been becoming very costly for manufacturers to produce high potent thin films due to the excessive waste of materials and high energy costs. A numerical study can be used to model and optimize the CVD reactor to yield favorable operating conditions. A simple geometry consisting of a rotating susceptor and flow guide is considered. The study shows visually how the temperature changes as the carrier gases respond to the thermal transport, and the effects of rotation and buoyancy. Commercially available software is used, with modifications, and the results obtained are discussed in detail.     

Numerical simulation of flow characteristics in a subway station

Transient simulation on the process of subway trains pulling in and out of a station was carried out to investigate the air flow characteristics in a subway station in various situations by utilizing computational fluid dynamics (CFD) methods. The influence of the piston effect on the flow field in the station and its role in natural ventilation are discussed. The results indicate that the piston effect has significant influence on the flow field in the station and it plays an important role in natural ventilation. The ventilation shafts established at both ends of the station can effectively enhance natural ventilation while reducing the velocity of airflow in the station. © 2009 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20239     

Numerical simulation of cellulose pyrolysis

A mathematical model of transport phenomena and chemical processes of the thermal degradation of cellulose is presented. The kinetic model developed by Bradbury et al. (J. Appl. Polym. Sci. 23, 3271, 1979) for primary pyrolysis is extended to include secondary reactions of volatiles:

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From the physical point of view, the model describes convective, conductive and radiative heat transfer, mass convection and diffusion and velocity and pressure variations interior to the porous solid (Darcy law). Furthermore, porosity, mass diffusivity, permeability and thermal conductivity vary with the composition of the reacting medium. Time and space evolution of the main variables, and reaction product distribution, are simulated by varying the reactor temperature and the reactor heating rate.     

Numerical simulation of thermocapillary nonwetting

The nonwetting phenomenon that occurs when a liquid drop is pressed against a solid wall held at a sufficiently lower temperature, is analyzed numerically. An interstitial gas film, induced by thermocapillary convection, separates the drop from the wall, forming a self-lubricating system. The temperature differences and wall distances were probed to evaluate their nonwetting effect. The results indicate that increasing the temperature difference or decreasing the wall distance can enhance the wetting suppression, whether with silicone-oil or water. The thermocapillary nonwetting phenomenon using 5 cSt silicone-oil droplet is more apparent than that obtained with water when the wall distance is small enough, because the capillary number of silicone oil is much larger than that of water. Alternately, when a cold liquid drop is moved towards a hot wall, the thermocapillary flow encourages the occurrence of wetting.     

Numerical Simulation of Air-cooling Tower

A mathematic model for packed air-cooling tower thermodynamic calculation is set up in this paper on the basis of fundamental heat and mass transfer equations. Based on the Double Film theory, direct equation-solving method is used to simulate air-cooling tower, and variation of parameters is taken to analyze the data and results of the program.     

Numerical simulation of the devolatilization of a moving coal particle

The devolatilization of an isolated coal particle moving relative to the surrounding gas is numerically simulated using a competing reaction model of the pyrolysis and assuming that the released volatiles burn in an infinitely thin diffusion flame around the particle or not at all. The temperature of the particle is assumed to be uniform and the effects of the heat of pyrolysis, the intraparticle mass transfer resistance, and the variation of the particle radius are neglected. The effects of the size and velocity of the particle and of the temperature and oxygen mass fraction of the gas on the particle and flame temperature histories, the devolatilization time and the yield of light and heavy volatiles are investigated. The motion of the particle may have an important effect on the shape and position of the flame of volatiles, but it has only a mild effect on the devolatilization process for the particle sizes typical of pulverized coal combustion. This effect increases for large particles or in the absence of radiation. The relative motion enhances the heat transfer between the particle and the gas, causing the devolatilization time to decrease at high gas temperatures and to increase at low gas temperatures. The numerical results are compared with a blowing-corrected Nusselt number correlation often used in heat transfer models of the process.     

Numerical simulation of melting of ice around a horizontal cylinder

The problem of outward melting of ice around a horizontal isothermal cylinder is considered. A numerical model in which natural convection induced in the molten water encompassing density inversion is taken into consideration has been developed. Via finite-difference solution of the melting model, numerical simulation of melting of ice has been performed for the cylinder surface temperature T_i = 4, 6, 8, 9 and 10°C with cylinder radius of 25.4 mm. The results of the present simulation were found qualitatively valid when compared with the existing experimental data.     

Numerical simulation of bubbly two-phase flow in a narrow channel

An advanced numerical simulation method on fluid dynamics - lattice-Boltzmann (LB) method is employed to simulate the movement of Taylor bubbles in a narrow channel, and to investigate the flow regimes of two-phase flow in narrow channels under adiabatic conditions. The calculated average thickness of the fluid film between the Taylor bubble and the channel wall agree well with the classical analytical correlation developed by Bretherton. The numerical simulation of the behavior of the flow regime transition in a narrow channel shows that the body force has significant effect on the movement of bubbles with different sizes. Smaller body force always leads to the later coalescence of the bubbles, and decreases the flow regime transition time. The calculations show that the surface tension of the fluid has little effect on the flow regime transition behavior within the assumed range of the surface tension. The bubbly flow with different bubble sizes will gradually change into the slug flow regime. However, the bubbly flow regime with the same bubble size may be maintained if no perturbations on the bubble movement occur. The slug flow regime will not change if no phase change occurs at the two-phase interface.