Study on ignition and combustion of gas jet and liquid spray fuels

If ignition delay is too long, the accumulated fuel available for simultaneous explosion is large and thus it is causes rapid combustion with high pressure and temperature, contributing

Study on Ignition and Combustion of Gas-Jet and Liquid-Spray Fuels

September 2009

Nguyen Ngoc Dung

CHAPTER 2 METHODOLOGY FOR THE RESEARCH OF IGNITION AND COMBUSTION BY CONSTANT-VOLUME VESSEL UNDER DIESEL-ENGINE CONDITIONS 18

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CHAPTER 3 SPONTANEOUS IGNITION AND COMBUSTION OF GASEOUS

CHAPTER 4 FUNDAMENTAL STUDY ON IGNITION AND COMBUSTION OF

4.3.2 Effects of t emperature at an ambient pressure of 4 MPa 61 4.3.3 Effects of temperature at an ambient pressure of 2 MPa 66

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5.4.1 Spray penetration and flame development of GTL fuels at base condition 78

in which total energy demand in the non-Organization for Economic Cooperation and Development (non-OECD) countries increases by 73 percent, compared with an increase of

15 percent in the OECD countries In terms of global consumption, liquids are expected to remain the most important energy source World consumption of liquids and other petroleum is expected to grow from 85 million barrels per day in 2006 to 91 million barrels per day in 2015 and 107 million barrels per day in 2030 [3]

Fossil fuels possess many useful properties such as high energy density, safety and ease of use compared to others energy sources These advantages have made fossil fuel popular during last century In 2006, burning fossil fuels satisfied about 86 percent of world energy consumption Unfortunately, fossil fuels are non-renewable, and the limited resources are currently being depleted Shafiee and Topal [1] have computed and derived depletion times

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for fossil fuels reserves, and report that crude oil, coal and natural gas are likely going to

be depleted in 35, 107 and 37 years, respectively

The burning of fossil fuels produces harmful pollutants, which are destroying the environment and harming people‘s life The most dangerous product is the emission of carbon dioxide (CO2) Carbon dioxide is one of the greenhouse gases that enhances radioactive forcing and contributes to global warming, causing the average surface temperature of the Earth to rise World carbon dioxide emissions are predicted to rise from

29 billion metric tons in 2006 to 33.1 billion metric ton in 2015 and 40.4 billion metric tons

in 2030-an increase of 39 percent over this projection period, according to the IEA [3] In addition, about 700 million tons of carbon monoxide, 150 million tons of nitrogen oxides,

200 million tons of solid particles, and 200 million tons of sulphur dioxides are released annually in the atmosphere [4] The majority of these substances are produced by the transport sector

In the last three decades, the world has been confronted with energy crises due to the decrease in fossil resources, with the increase in environment constraints, and with the increasing prices of oil Lowering of world CO2 emissions to reduce the risk of climate change requires a major restructuring of the energy system This situation brings as a consequence the search of alternatives and renewable fuels, which have to be not only sustainable, but also techno-economically competitive Gaseous fuels of hydrogen, natural gas and bio-fuels like ethanol, vegetable oil, biomass, biogas, synthetic fuels, biodiesel, etc are starting to be of high interest to the developed countries [4]

Liquid oils from petroleum are the main energy source for internal combustion engines Recently, the reduction of nitrogen oxides (NOx) and particulate matter (PM) in the exhaust

of diesel engines to meet the upcoming automobile emission regulations have become the main directives for engine manufactures For this purpose, engine makers have introduced various techniques such as new modes of combustion [5,6], the high-pressure common-rail injection system with electronic controlled [7-9], the exhaust gas recirculation system (EGR) [10,11] and the aftertreatment system [12,13] for modern diesel engines because

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they satisfy the increasingly more stringent emission regulations Additionally, in order to provide engine manufacturers with phenomenological models to guide their designs, many researchers have started to develop methods to investigate diesel combustion mechanisms with a focus on the processes that lead to the production of soot and nitrogen oxides As a result, a wide range of engine modifications have being developed and implemented to achieve higher engine fuel efficiency and larger emissions reductions

In direct-injection (DI) diesel engines, combustion sequence starts from the injection of fuel toward the end of the compression stroke in a small volume of high-pressure, high-temperature gases at which point the injected fuel autoignites and combusts in the expansion stroke The combustion process is usually composed of a premixed combustion phase, followed by a diffusion-combustion phase During premixed combustion, the levels

of mixing are high and the local air-fuel ratio of the mixture range from above 14.5:1, the stoichiometric value, to beyond the limit of flammability in the outer boundary of the spray [14,15]

The efficiency of the combustion process is strongly dependent upon, firstly, the charge-air, its temperature, pressure, motion and contents, secondly, the combustible fuel, its type, injection, atomization, evaporation, and thirdly, their states and interaction, leading to the auto-ignition and combustion of the charge [14], [16] Equivalence ratio () is used to present a measure of the relative amount of fuel and air The fuel-air mixture is defined rich

if  >1 and contrarily it is called lean if  < 1 The fuel-air mixing process during combustion produces soot particles in the highly rich regions of each fuel spray, and a high temperature during premixed-combustion contributes to the formation of NOx emissions

For liquid diesel fuels, the injection spray consists of a cold, liquid phase core surrounded

by a mixture that contains fuel droplets and vaporized fuel In order to burn, injected fuel must be in a vapor state The injected fuel is immediately atomized, vaporized and mixed with hot air at the time just after injection A high injection pressure and small nozzle-hole size normally provide smaller fuel droplets and faster fuel evaporation In high-temperature

Both physical delay and chemical delay take place during ignition delay period Physical delay is associated with atomization, vaporization, and mixing, while chemical delay or the induction period process depends on factors including temperature, pressure, fuel properties and oxygen mole fraction These processes are affected by engine design, operating variables and fuel characteristics If ignition delay is too short or there is no ignition delay, the injected fuel will burn at the injector and there will not be enough oxygen around the injector resulting in incomplete combustion with high HC and soot emissions If ignition delay is too long, the accumulated fuel available for simultaneous explosion is large and thus it is causes rapid combustion with high pressure and temperature, contributing to an increase NOx emissions [18] and combustion noise

The ignition characteristics of a fuel are very important in determining diesel engine operating characteristics such as fuel conversion efficiency, smoothness of operation, misfire, smoke emission, noise and ease of starting because they affect ignition delay Cetane number is often used to rate the ignition quality of a given fuel relative to a reference fuel Cetane number depends on the chemical composition of a fuel Fuels containing higher n-paraffinic hydrocarbons often have a higher the cetane number, while fuels containing high iso-paraffins are known to usually have lower cetane numbers The

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low cetane number fuels have a long ignition delay which causes a very rapid burning rate once combustion occurs with a high rate of pressure rise, a high peak pressure and a characteristic sharp knocking sound For high cetane number fuels, ignition delay is often short and the ignition occurs during injection Heat release-rate is controlled by injection rate and fuel-air mixing, and results in smoother combustion and smoother engine operation [14,15,18]

The autoignition of air/fuel mixtures can be modeled by a detailed kinetic mechanism as a sequence of elementary chemical reactions In the case of diesel fuel, however, the number

of intermediate species and possible reactions is very large and becomes prohibitive in terms of computing resources required Halstead et al [19] have published a simplified model that regroups all possible species under a limited number of generic species and reactions

Recently, the constant-volume combustion vessel has become popular facility for diesel engine research The data obtained in this facility is useful for model development and validation because of the well-defined boundary conditions and the wide range of conditions employed Naber et al [20] performed experiments on effects of gas density and vaporization on penetration and dispersion of diesel sprays Siebers [21] performed a study of liquid-phase penetration length in diesel sprays The visualization of diesel spray penetration, cool-flame, ignition, high-temperature combustion, and soot formation using high-speed imaging are presented by Pickett et al [22] The work described in this thesis presents the study of ignition delay and combustion characteristics for several alternative fuels by using the constant-volume combustion vessel Prior to the main discussion, the following part of this chapter presents a background for the alternative fuels in this study

1.2 Alternative fuels

This part focuses on analyzing potential, prospects and combustion characteristics of several alternative diesel fuels The fuels described in this part include hydrogen (H2), natural gas (NG), biodiesel fuel (BDF) and gas-to-liquid (GTL) fuel

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1.2.1 Hydrogen

Hydrogen offers tremendous advantages as a clean energy carrier In the coming decades, hydrogen and fuel cells will assume a greater role in meeting the world‘s energy consumption needs Hydrogen, like electricity, is an energy carrier, and can be produced directly from all primary energy sources, enabling energy feedstock diversity for the transportation sector These alternative energy sources include wind, solar power, and biomass (plant material), which are all renewable fuel resources Hydrogen derived from renewable energy sources has the potential to provide an inexhaustible supply of energy to fuel our cars, homes and industry without generating pollution

Recently, electricity produced from nuclear fission, or fusion, has also been mentioned with increasing frequency as a possible source of H2 production through electrolysis of water or thermo chemical cycles A major benefit of increased H2 usage for power generation and transportation is that all of these sources minimize our dependence on non-renewable fossil fuels and diversify our energy supply for utilization in end-use energy sectors Alternatively, H2 can be produced through coal gasification, or by ―steam reforming‖ of NG, both of which are non-renewable fossil fuels but are abundantly available throughout the world Combining the latter technologies with carbon capture and storage would provide a significant increase in sources of clean burning H2 while at the same time eliminating greenhouse gas emissions

The literature on hydrogen fueled internal combustion engines is surprisingly extensive and papers have been published continuously from the 1930‘s up to the present [23] One of the most important features of hydrogen engine operation is that it is associated with less undesirable exhaust emissions than compared to engines operating on other fuels Hydrogen has very high flame propagation rates within the engine cylinder in comparison

to other fuels [24] This permits stable lean mixture operation and control in fueled engines The operation on lean mixtures, in combination with the fast heat-release rates near top dead center associated with the very rapid burning of hydrogen–air mixtures results in high-output efficiency values The wide flammability limits of hydrogen in air

hydrogen-7

allow ignition and combustion of very lean mixtures, lowering peak cylinder temperatures and NOx emissions, at a higher thermal efficiency The fast burning characteristics of hydrogen permit much more satisfactory high-speed engine operation This should allow an increase in power output with a reduced penalty for lean mixture operation In addition, the extremely low boiling temperature of hydrogen leads to fewer problems encountered with cold weather operation [25]

However, hydrogen-fuelled engines suffer from reduced power output, due mainly to the low energy density of hydrogen on a volume basis and resorting to lean mixture operation The mass of the intake air is reduced for any engine size because of the relatively high stoichiometric hydrogen to air ratio There are serious potential operational problems associated with the uncontrolled pre-ignition and backfiring into the intake manifold of hydrogen engines [26,27] Hydrogen engines are prone to produce excessively high cylinder pressure and to the onset of knock The high burning rates of hydrogen produce high pressures and temperatures during combustion in engines when operating near stoichiometric mixtures This may lead to high exhaust emissions of oxides of nitrogen [28] Hydrogen engine operation may be associated with increased noise and vibrations due mainly to the high rates of pressure rise resulting from fast burning In addition, hydrogen requires a very low ignition energy, which leads to uncontrolled preignition problems [29,30] To improve hydrogen engines, an efficient combustion system with direct-injection and spontaneous-ignition operation may be preferable, in which combustion may be fully controlled by injection [31-33]

1.2.2 Natural gas

Natural gas is a vital component of the world's energy supply In 2005, natural gas supplied 20.9 percent of total world energy consumption [34] Natural gas is composed primarily of methane It is one of the cleanest, safest, and most useful energy sources Worldwide, total natural gas consumption is projected to increase by an average of 1.6 percent per year, from

104 trillion cubic feet in 2006 to 153 trillion cubic feet in 2030 In addition, because NG produces less CO2 when it is burned than does either coal or petroleum, governments

a higher power output for the same engine displacement by increasing the boost pressure level Accordingly, NG engines using high compression ratio, lean burn mixture or high exhaust gas recirculation would be expected to outperform gasoline engines in torque and power, and would allow a significant reduction in pollutant emissions and an improvement

in thermal efficiency [37] In addition, NG engines produce lower PM than diesel engines

do, because natural gas does not contain aromatic compounds such as benzene and it contains less dissolved impurities (e.g., sulphur compounds) than petroleum fuels do The relatively low flame speed and low combustion temperature of NG engines help to reduce engine NOx emissions when operating with a high compression ratio, or when the engine is supercharged

Conventional NG engines are limited in power and thermal efficiency output relative to diesel engine due to knock under high load conditions and they need to operate at stoichiometric fuel/air ratio conditions together with three-way catalytic converters to get the advantages of emissions reduction In order to get higher thermal efficiency for natural gas engines and to overcome the problems of abnormal combustion, direct-injection and spontaneous-ignition operation have been introduced and applied [38,39] Recently, glow-plug assisted direct-injection NG engines have been attempted to attain a higher thermal efficiency [40] However, the control of mixing is still the main obstacle for this technology application, especially at low load operation conditions

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1.2.3 Biodiesel fuel

Biodiesel, an alternative diesel fuel, is made from renewable resources such as vegetable oils and animal fats Biodiesel fuel is biodegradable and nontoxic It has low emissions profiles and is thus environmentally beneficial Prior to biodiesel, biofuels were developed

by Rudolph Diesel, the inventor of compression ignition (CI) engine around 1900 Peanut oil was used in the early period as a fuel to run the diesel engine At that time, crude oil was available in plenty and it was convenient to refine it to run the diesel engines Continued and increasing use of petroleum will intensify local air pollution and magnify the global warming problems caused by CO2 The oil crisis of the 1970s was a rude awakening for most countries that depended on imported oil The price increases and fuel shortages of the 1970s and early 1980s spurred interest in the development of alternative fuels around the world In particular cases, such as the emission of pollutants in the closed environments of underground mines, biodiesel fuel has the potential to reduce the level of pollutants and the level of potential or probable carcinogens [41,42]

Biodiesel is composed of long-chain fatty acids with an alcohol attached It is produced through a chemical reaction termed transesterification Transesterification, also called alcoholysis, is the reaction of a fat or oil with an alcohol to form esters and glycerol A catalyst is usually used to improve the reaction rate and yield Potassium hydroxide (KOH)

or sodium hydroxide (NaOH) is commonly used as a catalyst Because the reaction is reversible, excess alcohol is used to shift the equilibrium to the products side Methanol and ethanol are used most frequently, especially methanol because of its low cost and its physical and chemical advantages (polar and shortest chain alcohol) [43,44] The biodiesel fuel is called a methyl ester if methanol is used in the reaction and it is called an ethyl ester

if ethanol is used Currently, typical raw materials of biodiesel are rapeseed oil, canola oil, soybean oil, sunflower oil, palm oil and jatropha curcas oil Beef and sheep tallow and poultry oil from animal sources and waste cooking oil are also common sources of raw materials [41], [45]

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Properties of biodiesel are somewhat similar to conventional diesel fuel The main differences are acid content, density, viscosity, cetane number and heat of combustion Biodiesel fuel often exhibits higher density, higher viscosity, lower cetane number and lower heating value than diesel fuel Biodiesel fuel usually enjoys a carbon-neutral classification When it replaces diesel fuel, it reduces the global warming gas emissions of carbon dioxide Biodiesel fuel helps reduce tailpipe particulate matter (PM), hydrocarbon (HC), and carbon monoxide (CO) emissions from most modern four-stroke CI engines These benefits are due to the fact that neat biodiesel fuel contains about 11% oxygen by weight The presence of oxygen in fuel allows the fuel to burn more completely, so fewer unburned fuel emissions result This same phenomenon reduces toxic air pollutants, because the toxic air pollutants are associated with unburned or partially burned HC and

PM emissions The drawbacks of biodiesel are lower energy density than conventional diesel fuel and increased NOx emissions in many engine types [46,47]

1.2.4 Gas-to-liquid fuel

As previously discussed, NG is a clean, versatile fuel with abundant reserves around the world, and it is projected to increase in consumption, as it is a relatively clean primary energy for the near future According to the International Energy Outlook-2005 forecast, natural gas is going to be the highest growing primary energy source in the near future The consumption of natural gas is projected to increase by nearly 70% between 2002 and 2025, with the most robust growth in demand expected among the emerging developing countries [48] However, the drawbacks of NG are that there are relatively few users of the gas reserves with limited markets due to difficulties and extra expenses occur when transporting it from the remote locations where it is produced The challenge is to find more ways of bringing natural gas to markets, and to turn it into a diversified basket of products that customers need

New technology is being developed and applied to convert natural gas to liquids using to-liquids (GTL) technology [49,50] GTL technologies are well proven and have been in development for nearly a century They were developed in 1922 by two German scientists,

Gas-to-Liquids fuel is a pollution-free alternative fuels, made from natural gas, and can be used directly in conventional diesel engines It is a colorless, odorless liquid, with the easy handling features of diesel fuel, and can be used as a stand-alone fuel, or as blended with diesel [52,53] GTL fuel delivers a superior environmental performance with significantly lower local emissions than ultra-low sulfur diesel [54,55] It is also cost-effective, and can

be used in existing infrastructures and vehicles without modification In addition, GTL fuel

is not only added value, but capable of producing products that could be sold or blended into refinery stock as superior products with less pollutants Although the original form is a gas, gas-to-liquids processes produce diesel fuels with an energy density comparable to conventional diesel, but with a higher cetane number permitting a superior engine performance [56] Low sulfur content leads to significant reductions in particulate matter generated during combustion, and the low aromatic content reduces the toxicity of the particulate matter These properties are favorable considering worldwide trend towards the reduction of sulfur and aromatics in fuel [57-59]

1.3 Objectives and approaches

Many alternative and renewable fuels have been introduced and applied for diesel engine with the purpose being to get high fuel conversion efficiency, to reduce the emissions and partially replace conventional diesel fuel Before adapting a new fuel to a diesel engine, a thorough understanding of the effects of its properties on the diesel combustion processes is

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very important Therefore, the main objective of the work described in this thesis was to provide fundamental data of ignition delay and combustion characteristics for various gas-jet and liquid-spray fuels under simulated diesel combustion conditions, in terms of:

 Jets and sprays penetration and dispersion with time

 Vapor phase penetration and mixture formation with time

 Flame propagation with time

 Ignition delay and combustion characteristics under various injection and ambient conditions

The work described in this thesis was conducted at the Combustion and Power Engineering Laboratory, Kyoto University by a using constant-volume combustion vessel This research facility has been specifically designed to study ignition and combustion characteristics for various alternative and renewable fuels under simulated direct-injection (DI) diesel combustion conditions The vessel allowed full optical access for the visual investigation of spray (jet) penetration, mixture formation and flame propagation by using a high-speed CMOS digital camera The tested fuels included (a) gaseous fuels of hydrogen, natural gas and methane; and (b) liquid fuels of biodiesel fuels derived from waste-cooking oil, synthetic fuels produced from natural gas, and including conventional diesel fuel as a reference fuel The main steps of this research were to:

 Study ignition delay and combustion characteristics for hydrogen, natural gas and methane jets under direct-injection diesel combustion conditions Jet penetration, mixture formation and flame propagation for these gaseous fuels were visually studied by using shadowgraph images

 Study effects of ambient temperature and ambient pressure on ignition and combustion for different biodiesel fuels from waste cooking oil The penetration of biodiesel sprays was also visually investigated and compared to gas-oil

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 Fundamentally study spray penetration, flame development, and ignition and combustion characteristics for several GTL fuels The results obtained for synthetic fuels were analyzed and compared to those of gas-oil

1.4 The structure of this thesis

This thesis comprises 6 chapters, including this chapter as the introduction

Chapter 2 provides a description of the experimental apparatus and the procedure for conducting the experiments Methods used to determine ignition delay and to analyze combustion processes are described in this chapter

Chapter 3 shows work undertaken to study the ignition and the combustion for various gaseous fuel jets In this chapter, I concentrate on discussing the effect of various injection and ambient parameters on the ignition and combustion characteristics for hydrogen and natural gas jets

Chapter 4 presents the ignition and combustion characteristics of biodiesel fuel from waste cooking oil In this chapter, I focus on studying the effects of ambient temperature and ambient pressure on ignition and combustion for different biodiesel fuels

Chapter 5 studies combustion characteristics of GTL fuels Three kinds of GTL fuels with different properties have been tested in this research within a large variation of injection and ambient conditions Characteristics of sprays, of the ignition and combustion of GTL fuels are compared to standard gas-oil

Chapter 6 concludes this thesis by summarizing the obtained results through this research and the prospects of possible future directions

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In this research, ignition delay and combustion characteristics for various gas-jet and liquid-spray fuels were investigated by using a constant-volume combustion vessel A wide range of ambient (charge-gas) environments can be simulated at the time of fuel injection

by using this facility, allowing the effect of each variable to be assessed The combustion vessel provides full optical assess and can be used to generate temperature from 600 to

1300 K; pressure from 2 to 6 MPa and oxygen mole fraction from 0% to 21% Depending

on fuel types, fuel is injected into the vessel by different injectors The gaseous fuel is injected by using gas injector, which can injected the gas at pressure above ambient up to

15 MPa with nozzle sizes from 0.4 to 1.2 mm On the other hand, liquid fuel is injected by using common-rail fuel injectors with injection pressure from 40 to 120 MPa and nozzle-hole diameters from 0.14 to 0.25 mm

2.2 Experimental setup and procedure

2.3.1 Constant-volume combustion vessel

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The experimental apparatus was made of two main parts: a constant-volume combustion vessel and fuel injector The constant-volume vessel was built similar to previously studies

[5,6] Figure 2-1 shows a schematic cross section of the combustion chamber The

combustion chamber is circular cylinder, 80 mm in diameter, 30 mm in depth, approximately 150 cm3 in volume and fitted with quartz windows in both sizes for allowing full optical assessment

The chamber has six important parts, necessary to simulate diesel combustion conditions The first one was a manual intake valve used to charge and adjust the initial combustible premixed gas in the evacuated chamber The second one was an exhaust valve for removing burned-gas The port from exhaust valve was connected to vacuum pump to evacuate the combustion chamber The third one was a conventional spark ignition system for igniting premixed gas in the combustion chamber The fourth one was a stirrer (or mixing fan) used

to maintain a uniform temperature in the chamber before fuel injection; stirrer speed was then kept constant The fifth one was a pressure transducer, piezoelectric absolute-pressure transducer (Kistler 6052A), used to record combustion pressure and corresponded to calculate ignition delay and heat-release rate The last part was a single-shot injector mounted at the top of the chamber The new vessel allowed working with both gas and liquid injector The gas injector was a high-pressure solenoid-type gas injector with electronically controlled and was developed by Toyota Motor Corporation This special

Figure 2-1 Cross-section of constant-volume combustion vessel

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injector could inject gaseous fuels to a maximum of 15 MPa Liquid fuel was supplied using common-rail system The common-rail injection system, a modified version of Denso‘s common-rail injection system, was used to drive and control the injected fuel This injection system could inject the fuel at pressure of 40 to 120 MPa

2.3.2 Optical diagnostics

Shadowgraph technique was applied to visually study liquid sprays (gas jets) penetration and flames development Generally, shadowgraph is used to visualize the changes in density within a system that is transparent For vaporizing diesel sprays, this technique shows the boundary between vaporized fuel and background ambient gases because (a) refractive index differences exist between the fuel and ambient gases and (b) density gradients are created in the ambient gases as the vaporized fuel spray cools the ambient [7] These changes are shown by the ray displacement resulting from deflection To make the differences in density visible, the light passing through the system must be of uniform intensity The deviated light beam falls on a perpendicular surface or is directed into a camera The intensity of the light on the screen will change as it travels through the media

of varying density [8] Figure 2-2 shows the schematics arrangement of shadowgraph system The setup is essentially following the Z-type schlieren setup described in [9] The

Lamp

Gas injector

Combustion chamber

Concave mirror (FL =1910mm)

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light source, which is a 1 kW Xenon lamp (Katou light LS-1000) with a condensing lens and a small pinhole of 0.3 mm in diameter, provides a sharp and intense illumination throughout the whole system The pinpoint source of light is then reflected by a flat folding mirror and is captured via a concave mirror (focus length 1910 mm) This concave mirror reflects the parallel light rays passed through the combustion vessel to the opposite concave mirror Once the parallel ray hits this second mirror, it again reflects to flat folding mirrors and re-focused into a pinpoint and entering high-speed CMOS camera (Phantom v7.1 or Fastcam SA1) The capture rate and shutter speed of the camera are varied and optimized depending on the burning speed of the mixture and the brightness of the flame The image that the camera receives with the Shadowgraph system in place is very sensitive to density variation and allows us to see the changes in density of the mixture as the combustion event takes place To reduce vapor condensation on the combustion windows and to get high-quality shadowgraph photos, these windows were heated to 2300C before every experiment

In this study, photographs was captured at a speed of 10,000 fps and an exposure duration

of 30 µs (gaseous jets, biodiesel fuels) and 30000 fps and 14 µs (GTL fuels)

2.3.3 Experimental procedure

Diesel combustion condition in constant-volume combustion vessel was simulated by step combustion The first step was to generate high temperature and high-pressure environment by spark ignition and burning premixed combustible gas The second step was injected fuel into the simulated environment, and the fuel injection then autoignited and burned Figure 2-3 shows a pressure history of combustion chamber for a typical diesel combustion simulation To start the experiment, premixed gas (combustible-gas mixture) prepared in mixing tank was introduced into the combustion chamber through the intake valve In order to maintain uniform temperature in the vessel, stirrer was set to run 20 seconds before the spark-ignited At the time zero second in Figure 2-3, this lean mixture was ignited by the spark plug, generating a high temperature, a high-pressure environment like diesel-engine conditions The premixed burn ended at 0.03 s, reaching a pressure of 6.7 MPa and then the hot gases started to cool down owing to heat transfer When the desired

two-temperature and pressure were obtained, at t = 0.24 s, a signal was transmitted to actual the

22

injector The second pressure rises in Figure 2-3 was caused by the autoignition and combustion of the injected fuel The ambient gas temperature was determined as a thermodynamically average temperature, from knowing the pressure at the time of fuel injection and the initial mass and composition of gas within the vessel The ambient gas at injection in this example had average temperature of 750 K and pressure of 4 MPa

Premixed gas generated high-pressure and high-temperature consisted of ethylene (C2H4), hydrogen (H2), oxygen (O2) and nitrogen (N2) As shown in Figure 2-3, the ambient conditions simulated oxygen mole fraction 21% The following reaction gave the reactant and product compositions of the premixed combustible gas mixture used to generate the high-pressure, high-temperature, 21% oxygen environment:

3.05C2H4 + 0.50H2 + 29.9O2 + 62.42N2 → 21.00O2 + 69.33N2 + 6.11CO2 + 3.56H2O (1)

The reaction was normalized to produce 100 moles of product and was also assumed that complete-combustion products were generated In addition, the engine exhaust gas recirculation (EGR) environment could be simulated by changing the reactant O2 and N2concentrations Table 2-1 shows the mole fraction and molecular weight of the reactants and products for a given oxygen mole fraction applying in this research The products listed

Figure 2-3 Pressure history of the simulated diesel combustion process

23

in Table 2-1 determined by experiments and slightly different with the calculation As shown in this table, the resulting ambient gases had CO2 1.23% and H20 9.87% higher than air However, the influence of these compositions on ignition was reported to be negligible [2], [10]

2.3 Determination of ignition delay

Both liquid and gaseous fuels were fundamentally studied ignition and combustion Two definitions of ignition delay will be referred to the results: ignition delay definition for gaseous fuel and ignition delay definition for liquid fuels

2.3.1 Ignition delay for gaseous fuel jets

The definition of ignition delay these gaseous fuels have been described detail in previous work by Naber et al [2] and Ishiyama et al [5] In those works, ignition delay was defined

as the time from start of injection until the net of pressure-rise exceeds a set value

Figure 2-4 shows combustion vessel pressure history for n-heptanes, hydrogen and natural

gas The effective pressure-rise (p f - p a) was defined as the difference between the pressure

with combustion (p f ) and pressure decaying by heat loss to the combustion wall (p a) Thus,

p f - p a indicated the pressure-rise due to heat release and absorption excluding the effect of

heat loss As shown in Figure 2-4, after the injection, p f - p a of n-heptanes decreased due to fuel evaporation and then rose at the time fuel injection autoignited and burned On the contrary, for hydrogen and natural gas, vessel pressure increased just the time fuel injection because those gases were compressed at high pressure Continue with previous work [5], the ignition delay for hydrogen and natural gas jets was determined by the time from start

Table 2-1 Percentages of ambient gas at the time of fuel injection and reactants prior to spark ignition

Products (mol%) (Composition at injection start)

Reactants composition (Composition before spark ignition)

21 64.19 1.23 9.87 27.75 2.15 5.53 29.9 62.41 27.01

15 70.24 1.23 9.87 27.52 2.15 5.53 24.02 68.29 26.79

10 75.25 1.23 9.87 27.33 2.15 5.53 19.14 73.17 26.58

24

of injection until rate of net pressure-rise d(p f - p a )/dt exceeds the employing threshold level

of 50 MPa/s

2.3.2 Ignition delay for liquid fuel sprays

Figure 2-5 shows the definition of ignition delay for liquid fuels (standard gas-oil) Ignition delay or pressure-recovery delay was defined as the time from start of injection until the

pressure in the vessel rises just above p a , (p f - p a >0) In this figure, the dash line p a

displayed pressure history generated by burning combustible premixed gas without

injection fuel, and the solid line p f indicated the actual chamber pressure with injection fuel During delay period, fuel was continuously injected and vaporized, and both physical and chemical processes took place at the same time This definition was similar to pressure delay definition used in engine tests

Figure 2-5 Definition of ignition delay for liquid fuelsFigure 2-4 Pressure histories for different fuels

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2.4 Combustion analysis

Combustion characteristics of the tested fuels were analyzed and compared using

heat-release rate dq/dt calculated via the obtained pressure history Essentially, the heat-heat-release rate is the heat value generated per unit of time by combustion The value of dq/dt depends

on ignition delay, the properties of the fuel, and injection and combustion conditions The

method for calculating dq/dt for a constant-volume combustion vessel in present study is

The total enthalpy, including sensible enthalpy and enthalpy of formation, is expressed as follows:

T

i

i T fi i

T

T fi i

dT T C n dn

d C dn

h

d C h

n d

dH

0 0

0 0

The third part to the right of equation (2) is sensible enthalpy change due to temperature

The second part is sensible enthalpy change caused by alteration of dni and chemical

reaction The first part is enthalpy of formation, which is changed by chemical reaction and

is known as heat quantity release by combustion The first part is replaced by dQ B If

enthalpy of formation is negative, dQ B is positives The gases are regarded as ideal gases

By substituted equation (2) into equation (1) and differentiating over time, the following equation are derived and used to calculate heat-release rate

0   is the total enthalpy derived from Prothero‘s equation

Fuel is assumed to burn completely The mole rate of burned fuel dn i /dt is calculated by dividing dQB/dt by low heating value Once dni/dt has been calculated, the total mole

number can then be calculated, and temperature in the chamber can be determined by using state equation for ideal gases and we can then be calculated the rate of heat release

As the calculation of inlet enthalpy by fuel injection, the enthalpy of the species j in the fuel

can be calculated like above description The variation of mole number is derived from the following equation

where mF is the total mass of fuel, MF is the mean molar weight of fuel, tij is the injection

period, and rFj is the molar fraction of the species j in fuel

The half-experimental equation of engine could not calculate heat loss in the volume combustion vessel In this research, heat loss rate was calculated as:

where nt is the total mole number of operating gas in the chamber, C v is the average specific

heat at constant-volume, k is the heat transfer rate from combustion chamber to wall Tw is

the wall surface average temperature of chamber:

By using state equation, rate of pressure are derived from (5) and (6)

 

V

RT n p C n

k B p

p B

dt

w v t

Assuming that nt, Cv, k, and Tw are as constant (in the case that injection and combustion

have not done yet, this assumption is realized approximately), when the pressure p1 and p2

t1 and t2 are defined as the time before injection After the injection starts and combustion

happens, dQc/dt is estimated by extrapolating equation (9) In reality, temperature and the gas flow is changed due to the combustion, and thus it causes changing k Therefore, the

heat transfer coefficient is unpredictable This method is equivalent to correct a heat loss rate in the case that injection has not done yet In this research, heat-release rate was the quantity per 1kg of mixture including fuel mass in case of gaseous fuel and the quantity of entire chamber in case of liquid fuel

2.5 Experimental conditions

The research conducted with various alternative and renewable fuels These included gaseous and liquid fuel Experiments were conducted in constant-volume diesel combustion under wide range of injection and ambient conditions Table 2-2 shows the typical experimental conditions in this research The underlined numbers in the table corresponded

to the base conditions For gaseous fuels, the source gas was compress in a large tank, connected to gas-injector, and the gas was injected at injection pressure from 6 to 10 MPa For liquid fuels, the fuels were supplied using common-rail system and injected into the vessel at pressure of 60 to 100 MPa The similar ambient and injection conditions were applied to study the ignition and the combustion for gaseous and liquid fuels The ambient

temperature T i was controlled in the range of 600 to 1,300 K to study effects of T i on ignition delay and combustion characteristics This temperature range covers temperature from cold start to warm-up conditions for current compression-ignition engines Ambient

pressure p i was reduced to 2 MPa to simulate pre-mixed charge compression ignition

28

(PCCI) combustion, future diesel engine Furthermore, shadowgraph images were taken at several conditions to visually study sprays (jets) penetration length, flames development and mixture formation for the tested fuels

2.6 Conclusion

All experiments were made by using constant-volume combustion vessel Principle and specification of the vessel have been described in this chapter In addition, methods determined ignition delay and analyzed combustion processes were also presented Based

on the description, experiments were conducted to study ignition delay and combustion characteristics for H2, NG, CH4, biodiesel from waste cooking oil and the synthetics GTL fuel Experimental results and detail of discussion are presented in the next chapters

References

[1] D.L Siebers, Ignition Delay Characteristics of Alternative Diesel Fuels: Implications

on Cetane Number, SAE Paper No 852102 (1985) 673-685

[2] J.D Naber, D.L Siebers, J.A Caton, C.K Westbrook, S.S Di Julio, Natural Gas Autoignition Under Diesel Conditions: Experiments and Chemical Kinetic Modeling, SAE Paper No 942034 (1994)

[3] H.S Shen, J Vanderover, M.A Oehlschlaeger, A shock tube study of the auto-ignition

of toluene/air mixtures at high pressures, Proceedings of the Combustion Institute 32 (2009) 165-172

Table 2-2 Typical experimental conditions

Test fuels H2, NG, CH4 Gas-oil, Biodiesel, Synthetic

Nozzle-hole diameter, d N

Injection pressure, p j (MPa) 6, 8, 10 60, 80, 100

Ambient pressure, p i (MPa) 2, 3, 4, 5 2, 4

Oxygen mole fraction, r O2 (%) 21, 15, 10 21, 15, 10

Ambient temperature, T i (K) 700–1300 700–1200

Direct-[7] J.D Naber, D.L Siebers, Effects of Gas Density and Vaporization on Penetration and Dispersion of Diesel Sprays, SAE Paper No 960034 (1996)

[8] F Parsinejad, J Keck, H Metghalchi, On the location of flame edge in Shadowgraph pictures of spherical flames: a theoretical and experimental study, Experiments in Fluids 43 (2007) 887-894

[9] G.S Settles, Schlieren and shadowgraph techniques, Springer, 2001

[10] J.D Naber, D.L Siebers, Hydrogen combustion under diesel engine conditions, International Journal of Hydrogen Energy 23 (1998) 363-371

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CHAPTER 3

Spontaneous Ignition and Combustion of

Gaseous Fuel Jets under Diesel Combustion Conditions

3.1 Introduction

Hydrogen and natural gas are considered to be some of the most promising fuel replacement of conventional petroleum Hydrogen can be produced entirely from water and can meet ever-stringent regulations concerning the future emission of greenhouse and exhaust gas In addition, natural gas is a vital component of the world‘s energy supply as well as one of the cleanest burning alternative fuels and is produced abundantly from natural resources all over the world

Researchers have been interested in applying hydrogen as a fuel for internal combustion engines over many decades [1–8] Table 3-1 shows some combustion properties that have much influence on the potential behavior of H2, NG and CH4 as a fuel in general and for engine applications in particular The corresponding values for commercial diesel are also shown for comparison Hydrogen has unique properties as a fuel and is more advantageous than other alternative fuels These advantages include minimum emissions of hydrocarbons,

Table 3-1 Combustion properties of H2, NG, CH4 and diesel fuel

Fuel H2 NG (13A) CH4 DieselDensity (g/cm3) 0.071 - 0.415 0.8-0.84 Auto ignition temperature (K) 844 813 905 530 Minimum ignition energy (MJ) 0.018 - 0.2 - Flammability limits (vol%) 4.0-75 4.0-14.0 5.0-15.0 0.7-5 Flames velocity (cm/s) 291 39 37 30 Lower heating value (MJ/kg) 120.95 43.93 50.17 46.93 Stoichiometric air/fuel ratio (kg/kg) 34.32 16.74 17.16 14.5

31

carbon dioxide, smoke [9], and greatly improved cold-start capability Hydrogen fuel jets exhibit desirable characteristics for spark-ignition engines This gaseous fuel has broad flammability limits, high flame propagation, high burning velocity, low ignition energy, and high autoignition temperature over a wide range of temperatures and pressures With this broad flammability range, hydrogen engines can easily combust and produce energy over a wide range of fuel–air mixtures; thus, hydrogen engines could operate with a very lean equivalence ratio Lean-mixture operation in combination with fast combustion near top dead center (TDC) results in a short combustion period, high thermal efficiency output, and low NOx levels [10] The low ignition energy could reduce cyclic variation for hydrogen engines even for very-lean-mixture operation This leads to reduction in emissions, improvement in thermal efficiency, and smoother engine operation [11] Furthermore, the fast-burning characteristics satisfy the requirements of hydrogen engines regarding high-speed engine operation This may permit increased direct power output for lean-mixture operation [12,13] These unique features have attracted many researchers to create high-efficiency hydrogen engines [14,15]

However, there are many reports concerning the performance problems of hydrogen engines Engines fueled with hydrogen are subject to reduced power output owing to the low heating value of hydrogen in basic volume and lean-mixture operation [11] The low ignition energy causes problems such as premature ignition and flashback as a result of its autoignition by hot gases and hot spots in the combustion chamber The smaller quenching distance may increase the tendency to backfire since the flame from a hydrogen–air mixture more readily passes a nearly closed intake valve 10,16-18] In addition, the high burning rate of hydrogen produces high pressure and high temperature during combustion, which leads to knock, high exhaust gas emissions of NOx, and increases in engine vibration and noise Many methods have been proposed to overcome these problems These include decreasing the temperature of the ignition sources, optimizing injection and spark timing, using lean-burn techniques, reducing crevice volume, and eliminating abnormal discharges However, these techniques have not been successful in controlling backfire and engine performance Recently, internal mixing of hydrogen achieved by high-pressure injection

Direct-injection technology has been developed and applied to both hydrogen and natural gas engines in an effort to reduce the effects of abnormal combustion This method is very effective in increasing combustion efficiency and engine power and in obtaining cleaner emissions in an onboard engine Previous studies have successfully applied direct-injection

of hydrogen and natural gas to both spark-ignition and compression-ignition engines [15,27] However, the ignition and combustion of transient and high-pressure gaseous jets are not well understood at present [28,29] To increase thermal efficiency and to benefit from potential reductions in exhaust-gas emissions, the utilization of direct-injection combustion by the optimum control of ignition and mixing is needed

Continuing with our previous research on the spark-ignition and combustion characteristics

of high-pressure hydrogen injection and intermittent natural-gas jets [21], the main objective of the present research was to obtain fundamental data regarding the ignition delay and combustion characteristics of hydrogen, natural gas, and methane jets under

33

direct-injection compression-ignition conditions In particular, we studied effects of ambient temperature, nozzle-hole diameter, injection pressure, ambient pressure and oxygen mole fraction on the ignition and the combustion of H2, NG and CH4

3.2 Experimental

Three gaseous fuels, hydrogen, natural gas (CH4 88%, C2H6 6%, C3H8 4%, C4H10 2%), and methane, were studied The detail test conditions for this study are described in Table 3-2 The underlined numbers in the table correspond to the base conditions Fuel quantity was selected so as to obtain an equivalence ratio  = 0.33 at the base condition of T i = 1,000 K for each fuel, and the injection duration was fixed irrespective of ambient temperature The effects of injector parameters on ignition and combustion were studied by varying the

nozzle-hole diameter d N = 0.56, 0.8, 1.1 mm and injection pressure p j = 6, 8, 10 MPa; the

effects of combustion environments were investigated by varying ambient pressure p i = 3, 4,

5 MPa and reducing oxygen mole fractions from 21 to 10% For each test parameter,

ambient temperature (T i) was varied in the range of 700–1300 K so as to observe its effects

on ignition and combustion

3.3 Results and discussion

The results and discussions are presented in six parts In first part, I concentrate on discussing penetration of hydrogen and natural-gas jets at several conditions In the second

part, we focus on analyzing the effect of ambient temperature T i on ignition and combustion for hydrogen, natural gas, and methane jets Ignition delay is plotted in Arrhenius forms,

Table 3-2 Experimental conditions

Fuels H2, NG, CH4

Nozzle-hole diameter, d N (mm) 0.56, 0.8, 1.1

Injection pressure, p j (MPa) 6, 8, 10

Ambient pressure, p i (MPa) 2, 3, 4, 5

Oxygen mole fraction, r O2 (%) 21, 15, 10

Ambient temperature, T i (K) 700–1300

34

reflecting the importance of chemical kinetics, and Arrhenius lines are then defined as basic ignition lines The next two parts discuss the effects of nozzle-hole diameter and injection pressure on ignition and combustion The fifth part concerns the effects of ambient pressure Finally, we discuss the effects of oxygen mole fraction

3.3.1 Penetration of hydrogen and natural gas jets

In diesel engine, spray penetration and the mixing process are strongly influenced to the ignition and the combustion Prior to the main study of ignition delay and combustion characteristics of gaseous fuels, the study of jet penetration is important The penetration of hydrogen and natural-gas jets were investigated at several injection and ambient conditions Penetration length of the jets was determined as a maximum axial distance from nozzle-tip

to tip of the jet by using Phantom software

Figure 3-1 shows the penetration length of non-reacted gaseous jets at room temperature

298 K The graph exposes the penetration length for hydrogen and natural-gas jets, x t,

Figure 3-1 Jet penetration length of H2 and NG with different injection pressures

Chamber pressure and temperature are 1.2 MPa and 298 K, respectively

35

versus the time from injection start (t) The gaseous jets were injected to a pressurize combustion vessel, which is charged by nitrogen at p i = 1.2 MPa The observation result indicates that penetration length of natural-gas jets are somewhat longer than those of H2

jets The longer x t of natural-gas jets can be explained by the higher density, which contributes to higher momentum fuel jets In addition, for every gaseous fuel, the longer of

x t corresponds to a larger of d N and a higher of p j

Moreover, penetration of hydrogen and natural-gas jets were studied at high-temperature and high-pressure conditions For this purpose, combustion vessel was charged with combustible premixed gas The initial charge pressure and its compositions were selected to

generate high T i and p i with oxygen mole fraction approximately 0% Figure 3-2 show the

penetration of hydrogen and natural-gas jets at r O2 = 0% with (a) different T i = 900, 1000,

1200 K; (b) different d N = 0.56, 0.8, 1.1 mm; (c) different p j = 6, 8, 10 MPa; and (d)

different p i = 2, 3, 4, 6 MPa As shown in this figure, at p i = 4 MPa, the different of T i do not affect penetration length of both gaseous fuels The various penetration lengths of

hydrogen jets are much more than those of natural-gas jets under the variation of d N and p j

Similar to the result in Figure 3-1, longer x t is observed for natural-gas jets, larger of d N and

higher of p j jets More specifically, the variations setting of d N and p j do not significantly

affect x t values of natural-gas jets in comparison between the results at high-pressure and high temperature conditions with the results at room temperature (Figure 3-1) Furthermore,

in Figure 3-2, at constant injection pressure 8 MPa, the change of p i exhibits great effects

on x t of hydrogen and natural-gas jets The longer of x t values corresponds to a lower of p i

setting

3.3.2 Effects of temperature

In diesel-combustion conditions, studies of various liquid sprays found that ambient temperature had a large effect on ignition delay This dependence is associated with mixture formation and chemical kinetics during the autoignition period of fuels In this part,

we focus on discussing the effects of ambient temperature on the ignition delay and combustion characteristics for hydrogen, natural gas, and methane jets at the base condition

36

Figure 3-2 Reactive jet penetration length of H2 and NG jets with different T i , d N , p j

and p i Chamber pressure and temperature are generated by burning premixed gases