Numerical investigation on mixing process of a sonic fuel jet into a supersonic crossflow

The mixing process of a fuel jet in a supersonic crossflow is one of the significant issues for the design of the scramjet combustor. In this paper, the orthogonal analysis was employed to investigate the influences of the parameters of the supersonic mainstream and the fuel jet on the mixing process. Eight variables were considered and 27 cases were performed by the three-dimensional Reynolds-averaged Navier-Stokes (RANS) coupled with the shear stress transport (SST) turbulence model. The results show that the jet patterns can be divided into three categories by calculating the velocity ratio, named attachment pattern, transition pattern, and separation pattern, respectively. The extreme difference analysis indicates that the total pressure and Mach number of the mainstream, the total pressure of the fuel jet, and the diameter of the jet hole have a remarkable impact on the penetration depth and total pressure recovery. Additionally, a new dimensionless number named BS was proposed. And the penetration depth and total pressure recovery can be fitted to different functions of the BS. The fitted curves show that the larger penetration depth and smaller total pressure loss are generated as the BS increases. Finally, another new dimensionless number named LJ was proposed. And a positive correlation between the LJ and mixing efficiency has been elaborated based on analyzing the influence mechanism of the streamwise vortexes and the shockwaves on the mixing process. These correlations can provide help for primary optimization of supersonic combustor.     

Fundamentals of coal combustion during injection into a blast furnace

The efficiency of coal combustion is important for the blast furnace process. Incomplete combustion of coal does not reduce coke consumption as can be expected and decreases burden permeability which results in improper gas flow and temperature distribution. Consequently, this reduces the throughput of the blast furnace.

This paper describes combustion conditions and mechanisms of coal combustion in the blast furnace, and discusses factors affecting coal combustion such as injector location, coal type, injection rate, maceral composition, and air blast parameters. Also, mathematical models of coal and coal/coke combustion in the blast furnace are considered.     

Numerical investigation of coke oven gas (COG) injection into an ironmaking blast furnace (BF)

Blast furnace (BF) ironmaking is the predominating process for producing hot metal (HM). It consumes huge quantities of carbonaceous fuel materials and leads to massive CO₂ emissions. The injection of coke oven gas (COG) into the BF is considered a promising solution. It recovers the COG that is a kind of off-gas in the steelwork, and reuses the COG as an H₂-intensive fuel in the BF to partially replace the use of carbonaceous fuel materials. However, thus far, the technology is not widely adopted, mainly due to the lack of understanding regarding the effects of key operational parameters of COG injection on BF performance. In addition, the coupling effect of COG injection and BF operation particularly the control at furnace top is not clear, leading to the low utilization efficiency. In this work, a continuum-based BF process model is developed and validated to consider the injection of COG into a commercial scale BF through the tuyere. The model is validated by comparing the calculated key performance indicators with those measured in production. The impact of COG injection rate is studied and its coupling effects with top burden distribution have also been clarified. The simulation results show that an increased COG injection rate leads to improved BF performance, in terms of increased productivity and decreased consumption of carbonaceous fuel materials. However, the utilization efficiency of COG and the replacement ratio of carbonaceous fuel materials by COG is decreased. An optimum top burden distribution can be identified, which can improve the utilization efficiency of injected COG and achieve a relatively high replacement ratio. The findings from this work should be useful to guide production of BF with H₂-intensive fuel injection, which helps to save the use of carbonaceous fuel materials and reduce CO₂ emission.     

Numerical study on supersonic combustion with cavity-based fuel injection

The present study describes the numerical investigations concerning the combustion enhancement when a cavity is used for the hydrogen fuel injection through a transverse slot nozzle into a supersonic hot air stream. The cavity is of interest because recirculation flow in cavity would provide a stable flame holding while enhancing the rate of mixing or combustion. Several inclined cavities with various aft wall angle, offset ratio and length are evaluated for reactive flow characteristics. The cavity effect is discussed from a viewpoint of total pressure loss and combustion efficiency. The combustor with cavity is found to enhance mixing and combustion while increasing the pressure loss, compared with the case without cavity. But it is noted that there exists an appropriate length of cavity regarding the combustion efficiency and total pressure loss.     

Numerical simulation on the behavior of a liquid jet into an air flow

A numerical simulation has been performed to clarify the effects of turbulence in a liquid on the deformation of the liquid jet surface into an air flow. The turbulences in the liquid jet were simulated by the Rankin vortices, and the liquid jet surface was tracked numerically by the volume of fluid method. By numerical simulations, the onset of the protrusions on the liquid jet surface is caused by the vortices in the liquid, and the surrounding air flow plays an important role in the amplification of the protrusions. The amplification rate of the trough displacement is proportional to the air‐to‐liquid velocity ratio. At large imposed vortex intensities, the trough displacement increases with the vortex intensity. On the other hand, at small imposed vortex intensities, the amplification of the trough displacement is also affected by factors other than vortex intensity. © 2001 Scripta Technica, Heat Trans Asian Res, 30(6): 473–484, 2001     

Numerical investigation of a transient free jet resembling a laser-produced vapor jet

In the present study, the transiently developing free jet emanating from a laser-impacted surface is considered. The jet velocity profiles are varied with time in connection with the vapor jet velocity profiles emanating from the laser-produced cavity. Consequently, jet expansion from the laser cavity situation is modelled in the simulations. The jet exiting profiles measured previously are employed in the present simulations. Since the thermophysical properties of the laser-produced vapor are unknown, air properties are used for the jet in the simulations. A numerical method employing control volume approach is introduced to discretize governing equations of flow and energy. It is found that in the early period, jet behavior is similar to slowly flowing jets as reported in the literature. The self-similar transient jet behavior occurs as the time progresses; in which case jet exit velocity profiles become similar.     

Numerical simulation on liquid jet behavior issued into still air

Numerical simulations have been conducted to clarify the effects of turbulence, in the onset of protrusions on liquid jet surfaces. The turbulences in the liquid jet were simulated by the Rankin vortices. The liquid jet surface was tracked numerically by the VOF method. From numerical simulations, the protrusions on the liquid jet surface are induced by the vortices in the liquid, whose rotational direction decelerates the jet surface. Despite the distance between vortices, the displacement of the liquid jet surface from the initial surface location increases linearly, in time, at almost the same growth rate. In the initial region, the growth rate of the displacement increases as the major semiaxis‐to‐minor semiaxis ratio of the ellipsoidal vortex increases. The initial growth rate of displacement is almost proportional to the vortex intensity. © 2003 Wiley Periodicals, Inc. Heat Trans Asian Res, 32(2): 141–152, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10078     

Numerical computation of a circulating fluidized bed combustor

A mathematical model to describe a circulating fluidized-bed combustor is presented. A modified two-phase model which was used in the bubbling fluidized-bed combustor is considered to simulate the dense zone of the bottom section. For the upper section of the bed the momentum and energy-balance equation are used to predict the temperature and velocity profiles for the gas and the particles. The model performs mass balances for the chemical gas species (O₂, H₂O, CO, CO₂ and SO₂) with consideration being given to the last for retention by limestone particles. The model is applied to typical conditions of a circulating atmospheric fluidized-bed boiler and the simulation results show the expected trends. © 1998 John Wiley & Sons, Ltd.     

Numerical simulation of biodiesel fuel combustion and emission characteristics in a direct injection diesel engine

The effect of the physical and chemical properties of biodiesel fuels on the combustion process and pollutants formation in Direct Injection (DI) engine are investigated numerically by using multi-dimensional Computational Fluid Dynamics (CFD) simulation. In the current study, methyl butanoate (MB) and n-heptane are used as the surrogates for the biodiesel fuel and the conventional diesel fuel. Detailed kinetic chemical mechanisms for MB and n-heptane are implemented to simulate the combustion process. It is shown that the differences in the chemical properties between the biodiesel fuel and the diesel fuel affect the whole combustion process more significantly than the differences in the physical properties. While the variations of both the chemical and the physical properties between the biodiesel and diesel fuel influence the soot formation at the equivalent level, the variations in the chemical properties play a crucial role in the NO_x emissions formation.     

Numerical simulation of biodiesel fuel combustion and emission characteristics in a direct injection diesel engine

The effect of the physical and chemical properties of biodiesel fuels on the combustion process and pollutants formation in Direct Injection (DI) engine are investigated numerically by using multi-dimensional Computational Fluid Dynamics (CFD) simulation. In the current study, methyl butanoate (MB) and n-heptane are used as the surrogates for the biodiesel fuel and the conventional diesel fuel. Detailed kinetic chemical mechanisms for MB and n-heptane are implemented to simulate the combustion process. It is shown that the differences in the chemical properties between the biodiesel fuel and the diesel fuel affect the whole combustion process more significantly than the differences in the physical properties. While the variations of both the chemical and the physical properties between the biodiesel and diesel fuel influence the soot formation at the equivalent level, the variations in the chemical properties play a crucial role in the NO _x emissions formation.     

Numerical simulation of fuel reactor for a methane-fueled chemical looping combustion using bubbling fluidized bed with internal particle circulation

Chemical-looping combustion (CLC) is recognized as a promising technique to efficiently and economically capture emitted carbon dioxide in common combustion processes. In this study, the bubbling fluidized bed (BFB) fuel reactor performance of the CLC system was examined through numerical simulation. The reduction reaction performance obtained from conventional BFB fuel reactor and BFB fuel reactor incorporated with internal particle circulation denoted as internal circulation bubbling fluidized bed reactor (ICBFB), were compared under the same fuel flow rate and operating conditions. By using CH₄ as fuel and ilmenite as the oxygen carrier, it was found the reduction reaction can be enhanced by using the ICBFB fuel reactor due to particle circulation. The particle circulation increased the mixing and contact time between fuel and oxygen carrier that produced reduction reaction enhancement. Moreover, the simulation results indicated that higher reduction reaction performance can be achieved by higher reduction reaction temperature and initial oxygen carrier volume fraction.     

直喷式柴油机两段喷射计算模拟

以KIVA-3为计算平台，计算模拟4气门直喷式柴油机不同预喷油提前角的两段喷射燃烧历程。其结论是：150CA BTDC以后的预喷提前角，其预喷与主喷柴油都以扩散燃烧为主；预喷柴油燃烧与缸内流动的共同作用，使缸内温度、质量分数分布更加复杂。氧质量分数分布差异是不同预喷油提前角对主喷燃烧放热率影响的主要因素。     

One-dimensional turbulence model simulations of autoignition of hydrogen/carbon monoxide fuel mixtures in a turbulent jet

The autoignition of hydrogen/carbon monoxide in a turbulent jet with preheated co-flow air is studied using the one-dimensional turbulence (ODT) model. The simulations are performed at atmospheric pressure based on varying the jet Reynolds number and the oxidizer preheat temperature for two compositions corresponding to varying the ratios of H₂ and CO in the fuel stream. Moreover, simulations for homogeneous autoignition are implemented for similar mixture conditions for comparison with the turbulent jet results. The results identify the key effects of differential diffusion and turbulence on the onset and eventual progress of autoignition in the turbulent jets. The differential diffusion of hydrogen fuels results in a reduction of the ignition delay relative to similar conditions of homogeneous autoignition. Turbulence may play an important role in delaying ignition at high-turbulence conditions, a process countered by the differential diffusion of hydrogen relative to carbon monoxide; however, when ignition is established, turbulence enhances the overall rates of combustion of the non-premixed flame downstream of the ignition point.     

Numerical simulation of transient 3-D turbulent heated jet into crossflow in a thick-wall T-junction pipe

The present work is to investigate the transient three-dimensional heated turbulent jet into crossflow in a thickwall T-junction pipe using CFD package.Two cases with the jet-to-crossflow velocity ratio of 0.05 and 0.5 are computed,with a finite-volume method utilizing k-ε turbulent model.Comparison of the steady-state computations with measured data shows good qualitative agreement.Transient process of injection is simulated to examine the thermal shock on the T-junction component.Temporal temperature of the component is acquired by thermal coupling with the fluid.Via analysis of the flow and thermal characteristics,factors causing the thermal shock are studied.Optimal flow rates are discussed to reduce the thermal shock.     

多功能流态化热处理炉的研制

介绍多功能流态化炉的结构组成，关键部件的作用及设计要点。     

Time-averaged characteristics of a reacting fuel jet in vitiated cross-flow

This paper describes an experimental study of reacting jets in a high-temperature (1775 K) vitiated crossflow at 6 atm. We present an extensive data set based on high speed chemiluminescence imaging and exhaust gas sampling showing the characteristics of the time-averaged trajectory, width of the flame, flame standoff (or ignition) location, and NO_x emissions over a momentum flux ratio range of 0.75 < J < 240. Key observations are: (1) Depending upon ignition times, reaction can initiate uniformly around the jet, initiate on the leeward side of the jet and spread around to the windward side farther downstream, or initiate further downstream. (2) The time-averaged trajectory generally follows nonreacting trajectories, but penetrates further in the far-field than for what would be expected of a nonreacting jet. (3) The width of heat release zone increases monotonically with downstream location, J, and flame flapping amplitude, but seems to be dominated by the size of the counter-rotating vortex pair. (4) The measured ignition locations were of the same order of magnitude as values based on calculated ignition time scales and mean jet exit velocities, but with some additional variability. (5) The incremental NO_x emissions were controlled primarily by the global temperature rise associated with burning the jet fuel (for the fixed crossflow conditions studied here), and the NO_x emissions increased roughly linearly with the temperature rise.     

室内燃气火灾的数值模拟研究

通过采用FLUENT软件进行数值模拟计算的方法,将实际房间进行简化和抽象,建立模型用丙烷代替液化石油气作为燃料,来研究室内燃气火灾火焰蔓延过程。运用通用有限速率燃烧模型和大涡模拟模型来模拟750mm×500mm×50mm与750mm×500mm×250mm的箱体,研究了反应热、化学反应速率、火焰面分布以及箱体内各组分质量分数的分布,发现化学反应速率与反应热的变化趋势是一致的,大箱体的火焰面发展较小箱体快并且燃烧充分。     

Characteristics of a fluidized bed electrode for a direct carbon fuel cell anode

The characteristics of a fluidized bed electrode applied as a direct carbon fuel cell anode, which has an inner diameter of 35 mm and height of 520 mm and employed bamboo-based activated carbon (BB-AC) as a feedstock, are vigorously studied under various experimental conditions. The optimal performance of the fluidized bed electrode direct carbon fuel cell (FEBDCFC) anode with the BB-AC as a fuel is obtained under the following conditions with a limiting current density of 95.9 mA cm⁻²: reaction temperature, 923 K; N₂ flow rate, 385 ml min⁻¹; O₂/CO₂ flow rate, 10/20 ml min⁻¹; nickel particle content, 30 g; and a cylindrically curved nickel plate as a current collector. Under the same optimal conditions, the limiting current density of the FEBDCFC anode with oak wood-based activated carbon and activated carbon fiber as the fuel is determined to be 94.5 and 88.4 mA cm⁻², which is lower than that determined for BB-AC as the fuel. Comparatively, the limiting current density for graphite, which is utilized as the carbon fuel for this fuel cell system, could not be unequivocally determined because no plateau of the limiting current density against the overpotential is observed.     

Numerical study of a nuclear fuel element dissipating fission heat into its surrounding fluid medium

The objective of the present work is twofold – the first to establish the criterion for the boundary layer solution to be accurate enough in the study of conjugate heat transfer problem associated with a rectangular nuclear fuel element washed by upward moving coolant and the second to predict the critical thermal performance characteristics of the fuel element with uniform volumetric energy generation. Accordingly, employing stream function–vorticity formulation, equations governing the steady, two-dimensional flow and thermal fields in the coolant are solved simultaneously with the steady, two-dimensional heat conduction equation for the fuel element using second-order accurate finite difference schemes. Keeping the Prandtl number constant at 0.005 for liquid sodium as coolant, numerical results are presented for wide range of aspect ratio, conduction–convection parameter, energy generation parameter and Reynolds number. It is found that for all value of aspect ratio greater than 15, numerical prediction using the boundary layer approximation based model is quite accurate enough. It is also concluded that other parameters being kept constant, the increase in the maximum fuel element temperature due to increase in aspect ratio beyond 15 is negligible. Further, it is found that a relatively higher value of conduction–convection parameter reduces the coolant pumping power requirement to a large extent.     

Numerical investigation of transient responses of a PEM fuel cell using a two-phase non-isothermal mixed-domain model

In this paper, a transient two-phase non-isothermal PEM fuel cell model has been developed based on the previously established two-phase mixed-domain approach. This model is capable of solving two-phase flow and heat transfer processes simultaneously and has been applied herein for two-dimensional time-accurate simulations to fully examine the effects of liquid water transport and heat transfer phenomena on the transient responses of a PEM fuel cell undergoing a step change of cell voltage, with and without condensation/evaporation interfaces. The present numerical results show that under isothermal two-phase conditions, the presence of liquid water in the porous materials increases the current density over-shoot and under-shoot, while under the non-isothermal two-phase conditions, the heat transfer process significantly increases the transient response time. The present studies also indicate that proper consideration of the liquid droplet coverage at the GDL/GC interface results in the increased liquid saturation values inside the porous materials and consequently the drastically increased over-shoot and under-shoot of the current density. In fact, the transient characteristics of the interfacial liquid droplet coverage could exert influences on not only the magnitude but also the time of the transient response process.