Two-stage Combustion Research Articles

Flame stability and emissions characteristics of liquid ammonia spray co-fired with methane in a single stage swirl combustor

Direct combustion of liquid ammonia spray can reduce the cost and start-up time required for gas turbines fuelled with otherwise gaseous ammonia. This article reports the first successful study on liquid ammonia spray combustion in a novel swirl combustor with preheated air at 500 K. Particle Image Velocimetry (PIV) was employed to measure the non-reacting air flow field in the combustor while the liquid ammonia spray was visualized by Mie scattering techniques. Emissions from the flame were analysed using a Fourier Transform Infrared spectrometer. The present combustor encouraged a large central reverse flow, counter flowing with the spray. Hence, the preheated air enhanced the dispersion and evaporation of the liquid droplets, and thus promoted shorter spray heights. Stable combustion of liquid ammonia spray alone was achieved, while co-combustion with methane enhanced the combustion of the spray and reduced the flame height. For 70% ammonia fraction in the fuel by lower heating value, the flame was stable for global equivalence ratios, ϕ, between 0.66 and 1.37. The stability range narrowed as ammonia fraction and swirler mean inlet velocity increased. Emission analysis from the co-combustion shows that an optimum low emission ϕ exists at 1.06, thus rich-lean combustion can be employed in two-stage combustors to further control emissions from the spray flame when the primary zone ϕ is slightly rich. Numerical analysis indicates that the large latent heat of vaporization of liquid ammonia may contribute to a shift of the low emission ϕ towards leaner values.

Hypervelocity penetration of concrete targets with long-rod steel projectiles: experimental and theoretical analysis

Hypervelocity penetration experiments were performed on semi-infinite concrete targets with 7 mm-diameter 40CrNiMo steel long-rod projectiles at impact velocities ranging from 2117 m/s to 3086 m/s by using a two-stage combustion light-gas gun. After the experiments, the crater dimensions and penetration depth were carefully measured, also the residual projectiles were recovered. The depth of penetration was found to be greater than the hydrodynamic limit penetration depth. Numerical simulation was conducted to analyze the penetration process, and combined with the experimental results, it was obtained that the head of the long-rod projectile eroded and deformed into a hemisphere during hypervelocity penetration. According to the experimental and numerical simulation results, the penetration model of long-rod projectiles penetrating concrete targets at hypervelocity was constructed based on the Alekseevskii-Tate model. The penetration model was compared with the experimental depths of penetration, showing that the model had good applicability.

Two-stage autoignition and combustion mode evolution in boundary layer flows above a cold flat plate

Boundary layers are omnipresent in fundamental kinetic experimental facilities and practical combustion engines, which can cause ambiguity and misleading results in kinetic target acquisition and even abnormal engine combustion. In this paper, using n-heptane as a representative large hydrocarbon fuel exhibiting pronounced low-temperature chemistry (LTC), two-dimensional numerical simulation is conducted to resolve the transient autoignition phenomena affected by a boundary layer. We focus on the ignition characteristics and the subsequent combustion mode evolution of a hot combustible mixture flowing over a colder flat plate in an isobaric environment. For cases with autoignition occurring within the boundary layer, similarity is observed in the first-stage ignition as manifested by a constant temperature at all locations. The first-stage ignition is found to be rarely affected by heat and radical loss within the boundary layer. While for the main ignition event, an obvious dependence of ignition process on boundary layer thickness is identified, where the thermal-chemical process exhibits similarity at locations with similar boundary layer thickness, and the main ignition tends to first occur within the boundary layer at the domain end and generates a C-shape reaction front. It is found that sequential spontaneous autoignition is the dominant subsequent combustion mode at high-pressure conditions. At low to intermediate pressures, auto-ignition assisted flame propagation is nevertheless the dominant mode for combustion evolution. This research identifies novel features of autoignition and the subsequent combustion mode evolution affected by a cold, fully developed boundary layer, and provides useful guidance to the interpretation of abnormal combustion and combustion mode evolution in boundary layer flows.

Investigation of Multistage Combustion Inside a Heavy-Duty Natural-Gas Spark-Ignition Engine Using Three-Dimensional Computational Fluid Dynamics Simulations and the Wiebe-Function Combustion Model

Abstract The conversion of existing heavy-duty diesel engines to lean natural-gas (NG) spark ignition can be achieved by replacing the diesel injector with a spark plug and fumigating the NG into the intake manifold. While the original fast-burn diesel chamber will offset the lower NG flame speed, it will result in a two-stage combustion process (a stage inside and another outside the bowl). However, experimental data at more advanced spark timing, equivalence ratio of 0.8, and mean piston speed of 6.5 m/s suggested an additional combustion stage (i.e., three combustion stages). A three-dimensional (3D) computational fluid dynamics (CFD) simulation and a zero-dimensional triple Wiebe-function model were used to better understand the phenomena. While 78% fuel burned inside the bowl, burning rate reduced significantly when the flame approached the squish entrance and the bowl bottom. Moreover, the triple Wiebe-function indicated that the burn inside the squish was also divided into two separate combustion stages, due to the particularities of in-cylinder flow before and after top dead center. The first stage was fast and took place inside the compression stroke. The second took place in the expansion stroke and produced a short-lived increase in the burning rate, probably due to the increasing squish height during the expansion stroke and the increased combustion-induced turbulence, hence the third heat-release peak. Overall, these findings support the need for further investigations of combustion characteristics in such converted engines, to benefit their efficiency and emissions.

SCIENTIFIC AND ENGINEERING PRINCIPLES OF EFFICIENT FUEL USE AND ENVIRONMENTALLY FRIENDLY GAS COMBUSTIONIN STOVE PLATES. PART 2. STANDARD BASIS AND METHODOLOGY OF EVALUATION THE POWER EFFICIENCY AND ECOLOGICAL CHARACTERISTICS OF DOMESTIC GAS DEVICES

Gas stoves belong to the number of the most wide — spread domestic devices. Research activity concerning these appliances has been stopped in Ukraine some decades ago despite an increase of the natural gas using in municipal economy.
 Operation data, power efficiency characteristics and pollution indicators related to on air state by gas combustion in the living accommodation are regulated by the normative documents: national and international standards, regional technical (engineering) conditions of Ukraine, EU countries, Russia, USA, China and other states.
 Practically any gas stove is equipped with an atmospheric ejection burner. The principal characteristics of the gas burners, operation peculiarities peculiarities for the atmospheric burners are systematized and analyzed in the presented paper. The following qualitative and quantitative indicators of the atmospheric burners have been considered in this paper: the fuel types to be used (natural gas, liquefied gas), working gas pressure ahead of stove, nominal heat capacity, limit range of operation adjustment, noise by burner operation.
 Gas burning under the atmospheric burners operation makes the two-stage process. The physical background of the stable combustion have been considered along with the specific reasons and generalized criteria of the unstable combustions modes: flash-back, blow-off, appearance of the «yellow tips» in the flame.
 The atmospheric burners possess both the power and environmental advantages in comparison with the burners of total preliminary mixing of fuel gas and combustion air. Application of primary air excess lpr < 1.0, provides higher efficiency of the burners and need for lower gas pressure ahead of the domestic stoves while the two-stage combustion makes one of the principle techniques of environmentally benign combustion technologies. It has been shown that energetic experiences in ejection burners are proportional to the cube of the air access coefficient, in case of the atmospheric burners — are raised two the third power: ea ~ l3a,pr.
 An original methodology experimental researchers for the atmospheric burners of various design and of different companies — manufacturers has been proposed. The methods of the tests performing fully meet to Ukrainian norms and international standards. The computerized firing rig stand with a diagnostic facility has been created providing both power and environmental research of the atmospheric burners with definition of the boundaries of stable operation and breach the combustion stability. An example of the water heating thermogram within the test vessel has been presented providing an opportunity to evaluate the peculiarities of head transfer process by time and space for the system «atmospheric burner – the vessel to be heated and liquid to be boiled». Bibl. 31, Fig. 7, Tab. 5.

Effects of OH concentration and temperature on NO emission characteristics of turbulent non-premixed CH4/NH3/air flames in a two-stage gas turbine like combustor at high pressure

This study examined the effects of OH concentration and temperature on the NO emission characteristics of turbulent, non-premixed methane (CH4)/ammonia (NH3)/air swirl flames in two-stage combustors at high pressure. Emission data were obtained using large-eddy simulations with a finite-rate chemistry method from model flames based on the energy fraction of NH3 (ENH3) in CH4/NH3 mixtures. Although NO emissions at the combustor exit were found to be significantly higher than those generated by CH4/air and NH3/air flames under both lean and stoichiometric primary zone conditions, these emissions could be lowered to approximately 300 ppm by employing far-rich equivalence ratios (ϕ) of 1.3 to 1.4 in the primary zone. This effect was possibly due to the lower OH concentrations under far-rich conditions. An analysis of local flame characteristics using a newly developed mixture fraction equation for CH4/NH3/air flames indicated that the local temperature and NO and OH concentration distributions with local ϕ were qualitatively similar to those in NH3/air flames. That is, the maximum local NO and OH concentrations appeared at local ϕ of 0.9, although the maximum temperature was observed at local ϕ of 1.0. Both the temperature and OH concentration were found to gradually decrease with the partial replacement of CH4 with NH3. Consequently, NO emissions from CH4/NH3 flames were maximized at ENH3 in the range of 20% to 30%, after which the emissions decreased. Above 2100 K, the NO emissions from CH4/NH3 flames increased exponentially with temperature, which was not observed in NH3/air flames because of the lower flame temperatures in the latter. But, the maximum NO concentration in CH4/NH3 flames was occurred at a temperature slightly below the maximum temperature, just as in NH3/air flames. The apparent exponential increase in NO emissions from CH4/NH3 flames is attributed to a similar trend in the OH concentration at high temperatures.

Evaluating performance and emissions of a CI engine fueled with n-octanol/diesel and n-butanol/diesel blends under different injection strategies

Considering climate change and energy crisis, the high-carbon alcohol has become a popular substitute fuel for diesel engines. In this study, diesel (D100), n-octanol/diesel (D50O50), and n-butanol/diesel (D50B50) blends were tested in a single-cylinder CI engine to evaluate the effects of n-octanol and n-butanol on the engine combustion characteristics, performance, and emissions formation. During the testing, the engine was operated at two engine loads (30% and 50%) with two injection pressures (40 and 60 MPa) at 1200 rpm. Both single injection and split injection strategies were used. Experimental results suggest that D50O50 and D50B50could result in serious knocking combustion under certain conditions with the single injection strategy. In contrast, the split injection strategy can lower the peak pressure and peak heat release rate, and consequently mitigating the knocking tendency for D50O50 and D50B50. Besides, noticeable two-stage combustion was observed for D100 and D50O50 due to the NTC behavior of diesel and n-octanol. Generally, because of the lower fuel reactivity of n-octanol and n-butanol, the ignition delay periods of D50O50 and D50B50 were significantly extended with the single injection; however, the difference in ignition delay was not evident with split injection strategy. Moreover, D50O50 can exhibit the highest thermal efficiency with proper injection pressure, and combustion phasing, etc. NOx and black carbon can be simultaneously reduced by D50O50 with split injection at an injection pressure of 60 MPa. Nevertheless, adding n-octanol and n-butanol increased the HC emissions due to their lower fuel reactivity at all tested conditions.

Oxidation, ignition and combustion behaviors of differently prepared boron-magnesium composites

Effect of magnesium on ignition and combustion characteristics of boron was explored for three materials containing ca. 23 wt% of magnesium. Boron was sintered with elemental Mg, leading to the formation of MgB2. This material showed the most homogeneous distribution of Mg. Boron was also mechanically milled with elemental Mg, as well as with commercial MgB2, respectively. The presence of Mg delayed the oxidation of boron in all materials. The delay was nearly the same for both milled powders and it was somewhat greater for the sintered material. The observed oxidation rate of all materials, however, was greater than for pure boron. All materials could be ignited by a heated wire at nearly the same temperature, although boron could not. Particles of all materials, including elemental boron, could be ignited by a CO2 laser with the lowest power required for the sintered material. The addition of Mg had a weak effect on particle burn times. Boron milled with elemental Mg showed distinct two-stage combustion patterns interpreted as delayed, and separate, Mg combustion. A small increase in the maximum pressure and rate of pressure rise is observed for the Mg-bearing powders in constant volume explosion experiments.

Optimization of Two-Stage Combustion System Fueled by Lean-Burn Compressed Natural Gas Mixtures for Light-Duty Vehicle Engines

Optimization of Two-Stage Combustion System Fueled by Lean-Burn Compressed Natural Gas Mixtures for Light-Duty Vehicle Engines

Thermodynamic cycles variability of TJI gas engine with different mixture preparation systems

Gas engines are a viable source of propulsion due to the ecological indicators of gas fuels and the large amount of the needed natural resources. Combustion of lean homogeneous gas mixtures allows achieving higher thermal efficiency values, which is a key factor in current engine development trends. Using the spark-jet ignition system (also called as Turbulent Jet Ignition or Two-stage combustion) significantly improves the efficiency and stability of the combustion process, especially in the part-load operation on lean or very lean mixtures. This paper presents the impact of using two different fuel injection methods: Port Fuel Injection or Mixer on the operation stability of a gas engine designed for LDVs. Comparative studies of two different mixture preparation systems were carried out on a single-cylinder AVL 5804 test engine. By re-cording the cylinder pressure for a significant number of engine cycles, it became possible to determine the repeatability of engine operation and to correlate the results with the mixture formation system and the air-fuel ratio. In the performed research the beneficial effect of the mixer system application on the engine operation stability in the part-load conditions was found.

FGM을 이용한 에탄올 연료의 MILD 연소특성에 대한 수치해석 연구

MILD combustion has been considered as one of the promising combustion technology for high thermal efficiency and reducing nitrogen oxides and carbon dioxide emissions. Recently, many studies have revealed the potential of MILD combustion in various power systems but most studies have been focused on gas-phase fuel MILD combustion. Therefore further study on MILD combustion using liquid fuel is needed. In this work, we will focus on the numerical simulation of the effects of diluted gas velocity on the formation of liquid fuel MILD combustion using FGM using ethanol fuel as a carbon-neutral fuel. A two-stage combustion structure similar to ‘Deft jet in hot co-flow’ configuration was used to study the formation of liquid fuel MILD combustion for the validation purpose. The diluted burnt gas velocity from the first premixed combustion chamber can be controlled by an open ratio of the nozzle between two combustion chambers. The results show that the local high-temperature region was decreased and the flame temperature was uniformly distributed due to the intense mixing by the higher momentum of ejected diluted burnt gas from the venturi type nozzle considering atomization and vaporization of the injected liquid fuel.

Hypervelocity Impact Cratering on Semi-Infinite Concrete Targets of Projectiles with Different Length to Diameter Ratios

Impact cratering experiments were performed on semi-infinite concrete targets with 7 mm-diameter 40CrNiMo steel long-rod projectiles at impact velocities ranging from 2117 m/s to 3086 m/s by using a two-stage combustion light-gas gun. After the impact experiments, the crater diameter and depth as well as the crater volume were carefully measured. The concrete fragments were collected from the target chamber and the fragment mass was measured. The size of the crater (including the volume, diameter, and depth) and the fragment mass increased with increasing impact velocities, while the fragment distributions at different impact velocities were almost the same. Scaling laws for the crater volume impacted by the rod-shaped projectile were discussed and an empirical formula of crater volume was determined by the experimental data from the literature. Through the verification of the present experimental results, the predictive ability of the empirical formula proved to be reliable. Scaling laws for the size distribution of concrete fragments were also discussed. The normalized fragment mass distribution was proportional to the impact velocity raised to the power 1.5.

Cyclone-Bed Furnaces: Experimental Studies and Thermal Design

Furnace processes occurring in laboratory setups, pilot commercial facilities, and small-capacity boilers of various sized equipped with cyclone-bed furnaces containing fixed and fluidized beds burning solid biofuels (wood pellets, straw pellets, sunflower husk pellets, peat pellets, wood waste, wood chips, milled peat, and crushed peat bricks) are investigated. The distribution features of temperature and gas concentrations (CO, CO2, H2, O2, CH4) in the combustion and secondary combustion chambers have been established in the course of experiments. With this information available, it becomes possible to determine the zones in which fuel actively reacts with the oxidant under the conditions of two-stage vortex combustion. Dependences of the carbon monoxide (CO) and nitrogen oxide (NOх) concentrations in the flue gases on the excess air ratio, bottom blast share (primary air), furnace power, and the combustion chamber’s outlet hole diameter are obtained. The furnace processes in cyclone-bed furnaces are numerically simulated using the standard k–e turbulence model and the Magnussen combustion model, and satisfactory agreement between the calculation and experimental results is shown. A semiempirical method for designing cyclone-bed furnaces has been developed, according to which their main geometrical and operating parameters are determined in two stages: a preliminary thermal design analysis is first carried out, after which the furnace process is numerically simulated. Based on extensive generalization of experimental data on the burning of solid biofuels in cyclone-bed furnaces having different diameters, the parameter М = 0.52, which appears in the well-known Gurvich formula for the thermal design of furnace chambers, has been determined. This parameter takes into account the temperature field structure and the heat transfer pattern over the furnace height, thus making it possible to use the standard method in carrying out thermal design computations of furnaces.

Characterizing Two-Stage Combustion Process in a Natural Gas Spark Ignition Engine Based on Multi-Wiebe Function Model

Abstract The Wiebe function is a simple and cost-effective analytical approach to approximate the burn rates in internal combustion (IC) engines. Previous studies indicated that a double-Wiebe function model can better describe the two-stage combustion process inside diesel engines retrofitted to natural gas (NG) spark ignition (SI) compared with a single-Wiebe function. Specifically, the two Wiebe functions are associated with the bowl burn and the squish burn. However, the long tail in the energy release at the end of combustion produces some differences between experiment and model, which can be attributed to the complexity of the late oxidation process inside the post-flame zone and the incomplete combustion of the unburned mixture flowing out from engine crevices. To improve the matching between the model and experimental data, this paper investigated the effect of adding a third Wiebe function just to describe the long tail in the energy release at the end of combustion. The results indicated that such a methodology greatly improved the fitting accuracy in terms of phasing and magnitude of the heat release rate in each combustion stage.

Experimental investigation on ignition and combustion characteristics of n-butane/air mixtures by glow plug in miniature chamber

The aim of this work is to understand the ignition and combustion behaviors of n-butane/air mixtures under glow plug ignition in the start-up condition of miniature IC engines. We utilized a centimeter-scale constant-volume chamber setup to investigate the effects of glow plug heating power, equivalence ratio, and initial pressure on the characteristics of ignition and flame propagation of n-butane/air mixtures. Results indicated that the ignition time and ignition temperature decrease as the heating power increases and the flame propagation time is weakly sensitive to that. Increasing the initial pressure can reduce the ignition time, decrease the ignition temperature and slow flame propagation; except for the flammable limit, the ignition time and ignition temperature increase as the equivalence ratio increases, in addition, the flame propagation is fastest at φ = 1.2. Furthermore, an obvious two-stage combustion phenomenon was observed under fuel-rich flame propagation conditions. Overall, the order of influencing factors on the ignition characteristics shows heating power > initial pressure > equivalence ratio, and that on the flame propagation characteristics shows equivalence ratio > initial pressure > heating power in present experimental conditions.

Investigation of the effects of split-injection on particle emissions from a Dieseline CI engine

Premixed compression ignition (PCI) combustion techniques utilising low cetane-number (<40) fuels are associated with low engine-out oxides of nitrogen (NOx) and soot emissions. However, fuel-air over-mixing and high combustion noise at low and high engine loads, respectively, have been the challenges with PCI. Fuel split-injection can effectively govern the fuel-air mixing process and subsequently control combustion and emissions characteristics; it is of interest to characterise particle emissions under these PCI conditions. In this study, the effects of double-injection of low-cetane G75-Dieseline (75% gasoline in diesel based on volume) on controlling charge premixing and consequently size distribution of particle emissions were investigated in a production light-duty 4-cylinder CI engine. First injection-quantity ratio was varied from 10% to 50% while using different injection timings at engine loads from 1.4 to 12 bar BMEP and speed of 1800 RPM. Brake specific emissions results were compared to diesel baselines to quantify particles concentration reduction while maintaining the same range of NOx and brake thermal efficiency (BTE). G75 combustion emitted lower number and mass of particles (in all size classes) by up to 99.9% and generally lower NOx by up to 65% compared to diesel. Negligible particle number and mass emissions (as low as 1.91 × 1012 parts/kWh and 138 µg/kWh respectively) were measured for G75 at low-medium loads (3–6 bar BMEP). In the absence of two-stage combustion, the timing of second-injection controlled the combustion-phasing effectively. Lower first injection-quantity ratios (e.g. 10%) are recommended for overall high BTE and low gaseous and particle emissions.

Experimental Analysis of a Micro Gas Turbine Combustor Optimized For Flexible Operation With Various Gaseous Fuel Compositions

Abstract With the push to curb dangerous atmospheric pollutant production, energy generation technologies that reduce green-house gas emissions, while still providing adequate electrical supply, are of high importance. With major energy infrastructure already in place, developing enhanced pollutant-reducing combustor systems for micro gas turbines (MGTs), that can utilize low calorific fuels from renewable resources, is a major goal. The current work focuses on the experimental testing of an optimized two-stage combustor designed to operate with various fuel types, including natural gas and syngas produced via biomass gasification. Atmospheric experimental tests were performed and the results indicate larger flame lift-off heights and slightly higher CO gas emissions levels, while displaying lower NOx gas emissions levels for all thermal loads and air-to-fuel equivalence ratios tested, compared to that of the previous combustor designs. Additionally, steady-state computational fluid dynamics (CFD) simulations were conducted and the results are in general good agreement with the experimental data. Overall, the results indicate high fuel flexibility of the combustor, as well as the ability to comply with the NOx emissions limits for a larger range of operating points, compared to that of the previously tested combustors.

Multiple Combustion Stages Inside a Heavy-Duty Diesel Engine Retrofitted to Natural-Gas Spark-Ignition Operation

Abstract Converting existing diesel engines to natural-gas (NG) spark-ignition (SI) operation can reduce the dependence on oil imports and increase energy security. NG-dedicated conversion can be achieved by the addition of a gas injector in the intake manifold and of a spark plug in place of the diesel injector. Previous studies indicated that lean-burn NG inside the traditional diesel chamber (i.e., a bowl-in-piston geometry) is a two-stage combustion (i.e., a fast burn inside the bowl followed by a slower burn inside the squish). However, a triple-peak apparent heat release rate (AHRR) was seen at specific operating conditions (e.g., advanced spark timing (ST) at medium load and engine speed), suggesting that one of the two combustion stages may separate again. Specifically, the burn inside the squish region divided in two events before and after top dead center (TDC). This was due to the different flow motion inside the squish during the compression stroke compared to the one in the expansion stroke, which affected the combustion environments. The result was the apparition of two close peaks in pressure trace, which suggest larger gradients in pressure and temperature than at a more delayed ST. In addition, the phasing and magnitude of three peaks of the heat release changed cycle-to-cycle. As an advanced ST is the usual strategy used in lean-burn SI combustion, understanding phenomena such as the one presented here can be important for reducing engine-out emissions and increase engine efficiency.

NOx reduction by 13A/hydrogen mixed combustion using two-stage combustion

NOx reduction by 13A/hydrogen mixed combustion using two-stage combustion

Numerical analysis on two-stage combustion in a combustion furnace using ammonia/methane as fuel

Numerical analysis on two-stage combustion in a combustion furnace using ammonia/methane as fuel