Numerical modeling of combustion processes and pollutant formations in direct-injection diesel engines

The Representative Interactive Flamelet (RIF) concept has been applied to numerically simulate the combustion processes and pollutant formation in the direct injection diesel engine. Due to the ability for interactively describing the transient behaviors of local flame structures with CFD solver, the RIF concept has the capabilities to predict the auto-ignition and subsequent flame propagation in the diesel engine combustion chamber as well as to effectively account for the detailed mechanisms of soot formation, NO_X formation including thermal NO path, prompt and nitrous NO_X formation, and reburning process. Special emphasis is given to the turbulent combustion model which properly accounts for vaporization effects on the mixture fraction fluctuations and the pdf model. The results of numerical modeling using the RIF concept are compared with experimental data and with numerical results of the commonly applied procedure which the low-temperature and high-temperature oxidation processes are represented by the Shell ignition model and the eddy dissipation model, respectively. Numerical results indicate that the RIF approach including the vaporization effect on turbulent spray combustion process successfully predicts the ignition delay time and location as well as the pollutant formation.     

An experimental study on spray and combustion characteristics based on fuel temperature of direct injection bio-ethanol-gasoline blending fuel

This study investigated the spray and combustion characteristics of a direct injection spark ignition type system based on the changes in the temperature of the blended fuel (with bio-ethanol and gasoline). The test was performed in a chamber with a constant volume. The diameter and width of the chamber were 86 mm and 39 mm, respectively. The bio-ethanol test fuel was blended at volume ratios of 0 %, 10 %, 20 % and 100 %. The temperature of the fuel was set as −7, 25 and 35 °C. The fuel injection pressure and ambient pressure were set as 4.5 and 0.5 MPa, respectively. The shape and characteristics of the spray were investigated through a spray experiment. The increase in the fuel temperature changed its density and viscosity; this in turn increased spray penetration and spray area and increased the bio-ethanol blending ratio. The combustion visualization and experimental analysis indicated that the decrease in the fuel temperature and the increase in the bio-ethanol blending ratio led to the high viscosity and low heating value. This resulted in an increase in the ignition delay and a decrease in the rate of heat release. It is necessary to adjust the spray strategy and ignition timing to adopt bio-ethanol blended fuel as an alternative fuel.

     

Effects of turbulent mixing on spray ignition in spray system

When a column of droplets freely falling from an ultrasonic atomizer was ignited behind a reflected shock, no ignition occurred at a temperature below 1100 K, even if the pressure was as high as IMPa. Although, a higher temperature condition ensured ignition, no luminous flame was observable by high-speed photography, and even if a luminous flame lump appeared at an extremely high temperature, it disappeared without spreading over the entire column of droplets in this case. It is known however that, if a fuel is injected into a diesel cylinder or an electric furnace, ignition occurs even at a temperature as low as 650 K with a luminous flame spreading over the entire spray. These differences could be caused by the effects of turbulent mixing between fuel droplets and hot air, in fact, turbulence-generating rods were placed on the upstream side of the spray column. Experimental results indicates that the ignition limit was lowered to 840 K, and the ignition delay period was decreased by increasing the intensity of turbulence. Furthermore, the light emission of the flame was intensified, and normal spray combustion was maintained in the low-temperature atmosphere after the shock tube ceased its operation.     

A study on soot formation in premixed constant-volume propane combustion

The effects of pressure, temperature and equivalence ratio on soot formation in premixed propane-oxygen-inert gas combustion have been investigated over wide ranges of pressure (0.1 to 6 MPa), temperature (1200 to 2100 K) and equivalence ratio (1.5 to 2.7) in a specially designed disk-type constant-volume combustion chamber. To observe the soot formation under high pressure, premixtures are simultaneously ignited by eight spark plugs located on the circumference of chamber at 45 degree intervals. The eight converging flames compress the end gases to a high pressure. The soot volume fraction in the chamber center during the final stage of combustion at the highest pressure is measured by the in situ laser extinction technique and the burnt gas temperature during the same period by the two-color pyrometry method. The pressure and temperature during soot formation are varied by changing the initial charge pressure and by changing the volume fraction of inert gas in the premixture, respectively. It is found that the soot yield is dependent on the pressure, temperature and equivalence ratio, and the soot yield increases under the following conditions; (1) decreasing temperature and increasing equivalence ratio at constant pressure, (2) increasing pressure and decreasing temperature at constant equivalence ratio, (3) increasing equivalence ratio at constant temperature and pressure.     

Characteristics of auto-ignition and micro-explosion behavior of a single droplet of water-in-fuel

The characteristics of auto-ignition and micro-explosion behaviors of a single fuel droplet have been investigated experimentally with varying droplet sizes, ambient temperature, and water content. The fuel used for this experiment was pure n-decane, which was emulsified with several water content varied from 10% to 30% to compare the effects of water content in the emulsified fuel. Imaging with a high-speed digital camera was adopted to measure the ignition delay and flame life-time, as well as to observe micro-explosion behavior. The increase of droplet size and furnace temperature causes a decrease of the ignition delay time. The flame life-time is augmented as the droplet size increases, however it doesn’t seem to be affected by the ambient temperature relatively. As the water content increases, the ignition delay increases and the micro-explosion behavior is strengthened. The start timings of micro-explosion and fuel puffing are compared for different droplet sizes and the amount of water content.     

Analytical study on soot formation in a diesel engine

The soot formation inside a diesel engine was studied by the analytical model. The soot is formed from gaseous carbon atoms by the phase change under the saturation condition. This soot model is implemented into the KIVA-3V code. From the results of the model, it is found that the fuel rich core of spray inside a flame is the main source of soot. The effect of injection timing is investigated by the soot model. When the injection timing is advanced, the formation of soot is suppressed because of high saturation pressure. The soot formation is increased when the injection timing is retarded mainly due to the decreased soot oxidation at low cylinder temperature.     

The 2D measurement of soot diameter and number density in a diesel engine using laser induced methods

It is necessary to diagnose accurately the characteristics of soot formation and oxidation in a diesel engine. Whereas past measurement techniques for soot concentration give limited information for soot, laser-based two-dimensional imaging diagnostics have a potential to provide temporally and spatially superior resolved measurements of the soot distribution. The technique using laser sheet beam has been applied to an optically accessible diesel engine for the quantitative measurement of soot. The results provided the information for reduction of soot from the diesel engine. Both LIS (Laser Induced Scattering) and LII (Laser Induced Incandescence) techniques were used simultaneously in this study. The images of LIS and LII showed the quantitative distribution of the soot concentration in the diesel engine. In this study, several results were obtained by the simultaneous measurements of LIS and LII technique. The diameter and number density of soot in combustion chamber of the test engine were obtained from ATDC 20 degree to 110 degree. The soot diameter increased about 37% between ATDC 20 degree and 110 degree. The number density of soot, however, decreased significantly between ATDC 40 degree and 70 degree.     

Investigation of soot formation in a D.I. diesel engine by using laser induced scattering and laser induced incandescence

Soot has a great effect on the formation of PM (Paniculate Matter) in D.I. (Direct Injection) Diesel engines. Soot in diesel flame is formed by incomplete combustion when the fuel atomization and mixture formation were poor. Therefore, the understanding of soot formation in a D.I. diesel engine is mandatory to reduce PM in exhaust gas. To investigate soot formation in diesel combustion, various measurements have been performed with laser diagnostics. In this study, the relative soot diameter and the relative number density in a D.I. engine was measured by using LIS (Laser Induced Scattering) and LII (Laser Induced Incandescence) methods simultaneously which are planar imaging techniques. And a visualization D.I. diesel engine was used to introduce a laser beam into the combustion chamber and investigate the diffusion flame characteristics. To find the optimal condition that reduces soot formation in diesel combustion, various injection timing and the swirl flow in the cylinder using the SCV (Swirl Control Valve) were applied. From this experiment, the effects of injection timing and swirl on soot formation were established. Effective reduction of soot formation is possible through the control of these two factors.     

Group vaporization of liquid fuel sprays

In the simplest models of spray combustion it is assumed that the droplets burn individually according to the “d²-law.” Both theory and experiment show that this model only will be valid if the spray is extremely dilute, otherwise the droplets burn collectively in what is known as the group combustion mode. Most of work on spherical droplet cloud is limited to monodisperse sprays; however it is important to determine how the particle size distribution will affect group combustion, particularly the sheath combustion mode. In order to gain preliminary indication of the effect of the droplet size distribution of the vaporization process, the behavior of spays with a bimodal droplet size distribution is examined. The present study shows that for a cloud which is initially in saturated equilibrium the droplet size distribution changes the profiles of temperature and fuel vapor mass fraction within the vaporization wave at the edge of the cloud; however, the size distribution does not affect the overall evaporation characteristics such as the vaporization rate and the cloud lifetime in the sheath combustion limit considered here. It is shown that as before the cloud radius decreases according to a “d²-law” with a modified vaporization constant.     

一种研究柴油甲醇双燃料的定容燃烧弹试验装置

设计并研制了一种定容燃烧弹试验装置,用于对柴油在甲醇/空气预混均质混合气中燃烧特性的基础研究。介绍了该试验装置的各子系统的原理、结构及特点。定容燃烧弹试验结果表明:甲醇抑制了柴油的着火燃烧,随着甲醇/空气混合气浓度的增大,燃烧火焰变暗,碳烟生成受到抑制。与空气热氛围相比,甲醇/空气混合气氛围延长了柴油的滞燃期,加长了火焰的浮起长度。火焰稳定后,甲醇氛围中火焰的浮起长度随时间的变化比在纯空气氛围中大。     

Characteristics of a liquefied butane spray

The characteristics of a butane spray from pintle-type injector were studied by droplet velocity and diameter measurements and high speed photography. The accumulator type injector operated off a common rail fuel supply system operated at 13 MPa, and was controlled by a high-speed solenoid valve. Injection was carried out in a chamber at ambient temperature and at the pressure above (0.37 MPa) and below (0.15 MPa) the fuel vapor pressure. Two component phase/Doppler particle analyzer and traverser were used to obtain the droplet diameter and the velocity at numerous locations in the spray. The entire injection event was analyzed as a time-average and also subdivided into three temporal intervals. A, B, and C. The high-speed photographs showed a narrower cone angle during the quasi-steady spray period at the 0.37 MPa chamber pressure compared to the 0.15 MPa case.     

Swirl effect on the spray characteristics of a twin-fluid jet

In the liquid fuel combustion chamber, employed fuel must be atomized before being injected into the combustion zone. Therefore, the complete fuel atomization is the most important condition for the combustion efficiency. The atomization quality strongly affects the combustion performance, exhaust pollutant emissions, and flame stability. Therefore, the whole process of spray atomization is of fundamental significance. During past decades many experimental and theoretical studies in this field have been carried out and some improved results have been obtained. Two-phase atomizers, having a variety of advantages such as spray uniformity, appreciable atomization, and smaller SMD with an increase of ambient gas, are considered to be applied in various industrial processes. The purpose of present study is to investigate the mean velocity, turbulence shear stress, turbulence intensity, mean drop size distribution, and droplet data rate in a two-phase swirling jet using PDPA systems.     

An experimental study on the evaporation of freely falling droplet under high temperature and high pressure gas stream

An experimental apparatuses and measuring system have been made to obtain characteristics connected with evaporation, ignition delay, combustion of a freely falling liquid fuel droplet in high temperature and high pressure gas stream. In this study some systematic experiments were performed to test the utility of the system. The newly devised apparatus was ensured reliability and utility from the tentative experimental results.     

Numerical simulation of a pool fire

Thermo-fluid mechanical characteristics as well as flame structure of a pool fire were numerically simulated in this study. The small scale pool fire was treated as a gaseous jet flame with low initial velocity as an approximation. The combustion model postulated infinitely fast chemical reaction and the soot formation model included soot nucleation, surface growth, coagulation, thermophoresis, and oxidation. Valuable data were obtained in the temperature distribution, velocity profiles, soot volume fraction and major species concentrations. Especially, the periodic formation of the large scale structures were successfully simulated. Its frequency agreed well with experimental results. Research Institute of Industrial Science and Technology Heat and Fluid Flow Research Team     

Ignition and combustion characteristics of the gas turbine slinger combustor

This paper describes the ignition and combustion characteristics of a gas turbine slinger combustor with rotating fuel injection system. An ignition test was performed under various airflow, temperature and pressure conditions with fuel nozzle rotational speed. From the test, there are two major factors influencing the ignition limits: the rotational speed of the fuel nozzle, and the mass flow parameter. Better ignition capability could be attained through increasing the rotational speed and air mass flow. From the spray visualization and drop size measurement, it was verified that there is a strong correlation between ignition performance and drop size distribution. Also, we performed a combustion test to determine the effects of rotational speed by measuring gas temperature and emission. The combustion efficiency was smoothly enhanced from 99% to 99.6% with increasing rotational speed. The measured pattern factor was 15% and profile factor was 3%.     

低散热压燃点火天然气发动机的设计与试验

为实现单一燃料天然气的压燃着火，针对天然气自燃温度高的特点，设计了具有低散热结构的分隔式燃烧宣，同时设置了进气管空气加热装置和分隔室内陶瓷电热塞加热装置。实验研究了分隔宣通道尺寸、进气管空气加热温度、燃烧室陶瓷隔热涂层对天然气压燃着火过程的相关影响。     

An analysis on structure of impinging and free diesel spray with exciplex fluorescence method in high temperature and pressure field

Because an injected spray development process consists of impinging and free spray in the diesel engine, it is needed to analyze the impinging spray and free spray, simultaneously, in order to study the diesel spray behavior. To dominate combustion characteristics in diesel engine is interaction between injected fuel and ambient gas, that is, process of mixture formation. Also it is very important to analyze liquid and vapor phases of injected fuel on the investigation of mixing process, respectively and simultaneously. Therefore, in this study, the behavior characteristics of the liquid phase and the vapor phase of diesel spray was studied by using exciplex fluorescence method in high temperature and injection pressure field. Finally, it can be confirmed that the distribution of vapor concentration is more uniform in the case of the high injection than in that of the low injection pressure.     

A study on the shaped combustion system using diesel spray impinged on a wall

In modern small compact combustion chambers of diesel engines with high speed injection spray, the wall impaction of spray is encountered. A number of combustion systems have been developed which deliberately employ wall impaction as a means of breaking up the fuel spray and/or directing it in a desired direction. As a link of this, in a further type of geometry, which has been presented by Park et al. cut-off pips were provided for a number of sprays directed not only straight down into combustion chamber centre but also across and down into the bowl in the normal radial direction. The sizes of the pips and their positions were tested for axisymmetrical shape, and their results were analyzed. The further type was simulated in here by using 3D non-orthogonal spray engine code. The simulation results show that the further type is much better than general conventional shapes, i.e. well mixing of fuel vapour and spray high qualified droplet atomization, etc. Thus the improvement of the engine performance is expected by the shaped engine.     

Effect of water induction on the performance and exhaust emissions in a diesel engine (II)

This study was to investigate the effects of water induction through the air intake system on the characteristics of combustion and exhaust emissions in an IDI diesel engine. The fuel injection timing was also controlled to investigate a method for the simultaneous reduction of smoke and NOx when water was injected into the combustion chamber. The formation of NOx was significantly suppressed by decreasing the gas peak temperature during the initial combustion process because the water played a role as a heat sink during evaporating in the combustion chamber, while the smoke was slightly increased with increased water amount. Also, NOx emission was significantly decreased with increase in water amount. A simultaneous reduction in smoke and NOx emissions was obtained when water was injected into the combustion chamber by retarding more 2°CA of the fuel injection timing than without water injection.     

煤粉颗粒燃烧过程可视化研究

基于McKenna型燃烧器,研制了双光路成像系统,对煤粉颗粒燃烧过程中的背光投影轮廓图像和自发光光强分布信息进行同步采集,观察了不同高度位置煤粒的运动、着火和燃烧过程。针对烟煤和无烟煤颗粒燃烧实验中出现的挥发组分着火燃烧、碳黑形成、焦炭着火燃烧等现象进行了分析,发现两种煤种分别以均相着火和非均相着火为主。同时对煤颗粒燃...