ANALYSIS ON COMBUSTION OF A PRIMARY REFERENCE FUEL

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ANALYSIS ON COMBUSTION OF A

PRIMARY REFERENCE FUEL

S.PERIYASAMY

Assistant Professor, Mechanical Engineering,

Government College of Technology, Anna University Coimbatore, Tamilnadu, India. speriyyasamy@gmail.com

Dr.T.ALWARSAMY

Professor and liaison officer,

Director of Technical Education, Chennai-600025. alwar_samy@yahoo.co.in

V.RAJASEKAR*

PG Scholar, Thermal Engineering

Government College of Technology, Anna University Coimbatore, Tamilnadu, India. rajasekar.vel@gmail.com

Abstract

This paper presents a numerical combustion study of primary reference fuel methane (CH4) in a variable volume

reactor. The GRI 3.0 mechanism constructed using 53 species and 325 reactions have been used for the analysis purpose. The software used for the analysis is COMSOL. The combustion characteristics such as pressure rise, formation of species, and ignition delay characteristics were studied in Homogeneous Charge compression ignition engine.

Keywords: Chemical kinetic model, combustion, homogeneous charge compression ignition engine, primary

reference fuel.

1. Introduction

Combustion is the sequence of exothermic chemical reactions between a fuel and an oxidant accompanied by the production of heat and conversion of chemical species. Light and flame may be produced as the result of combustion. Emissions and fuel economy are the two major challenges that are faced by automotive industries. To achieve improvements in these areas the combustion process has to be studied thoroughly. Lot of researches is going on in the combustion area both numerically and experimentally. As foretold, analysis can be done by both experimental methods and numerical methods. Numerical study is more advantageous as it provides a great range of operating conditions that cannot be easily in experimental conditions. The limiting parameter in numerical study is the computational capacity. As computational capacity improves, numerical study is getting more attractive. The combination of detailed fluid mechanics model and a detailed chemical kinetics model would be the perfect analysis tool for studying engine combustion numerically. A direct numerical simulation of this kind would require greater computational capacity, time and cost. So, to reduce the cost and time, simplifications are thus made in either fluid mechanics or the chemical kinetic part of models. The simplified models show great validation with the experimental results. Now a day more attention is paid to detailed chemical kinetic models than the fluid mechanics models as the combustion chemistry is becoming critical for improving engine performance. To make the analysis much simpler Homogeneous Charge Compression Ignition Engine (HCCI) is mainly used for the numerical analysis purpose.

2. Kinetic Modelling

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reactions offer the best accuracy and reliability. Moreover, the knowledge of a specific elementary reaction can be re-used for completely different operating conditions and in different species mixtures. Knowledge acquired about reactions at this level becomes a re-usable asset. This is more advantageous than the approximate methods those have its parameters determined strictly by fitting to experimental measurements and have very limited applicability. Using basic expressions will provide more accuracy and extensibility than using global reaction expressions. The detailed mechanism can be used over a wide range of pressure, temperature, and gas composition when intermediate species and reactions associated with the minor species are included. The neglect of minor species in operational models makes it difficult for them to accurately describe the production of unwanted byproducts or pollutants like NOx in combustion systems. The minor species can have large effect on the overall reaction rates. So, inclusion of the minor species in the chemical reaction mechanism becomes important to study the combustion properly.

1.1 Primary reference fuel

Large number of hydrocarbons, organic matter, aromatics and other compounds forms the practical fuels such diesel and gasoline. And also the composition of the automotive fuels varies depending on the fuel’s source and production history. So, it is not practical to include all the components of the practical fuels in the modeling of combustion process. So, the combustion characteristics of automotive fuels are often represented using blends of hydrocarbons known as primary reference fuel. Basic hydrocarbon fuels methane, ethane, propane and such things are known as primary reference fuel.

3. Homogeneous Charge Compression Ignition Engine

Homogeneous charge compression ignition engines are being considered as an alternative to traditional spark ignition and compression ignition engines. As the name implies, a homogeneous fuel/oxidant mixture is auto-ignited by compression with simultaneous combustion occurring throughout the cylinder volume. Combustion temperatures under lean burn operation are relatively low, resulting in low levels of NOx emission. Furthermore, the fuel’s homogeneous nature as well as the combustion process itself leads to low levels of particulate matter being produced. Although HCCI combustion shows much promise, the method also suffers from a number of recurring problems one of the most important is being ignition timing. The following model examines the HCCI of methane, investigating ignition trends as a function of initial temperature. This model solves the mass and energy balances describing the detailed combustion of methane in a variable volume system.

3.1 Model definition

The chemical kinetic model used incorporates a detailed reaction mechanism of 53 species and 325 reactions.

3.2 Variable volume reactor

This model represents the combustion cylinder with a perfectly mixed batch system of variable volume, a reactor type that is predefined in the reaction engineering lab.

The volume change as a function of time is described by the slider- crank equation:

2[R+1-cosα-((R^2-(sin α)^2))^1/2]

1)) -((CR + 1 =

V/Vc

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Where, V is the cylinder volume (m3), Vc gives the clearance volume (m3), CR equals the compression ratio,

and r denotes the ratio of the connecting rod to the crank arm (Lc/La). Further, α is the crank angle (rad), which

is also a function of time.

α=2ΠN/60t (2) Where N is the engine speed in rpm, and t is the time (s).

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Table 1. Engine Specifications

Engine specification

Variable

name Value Unit

Bore D 85 mm Stroke S 110 mm Connecting rod Lc 235 mm

Crank arm La 55 mm

Engine speed N 1500 rpm Compression

ratio

CR 18:1 - The clearance volume can be calculatedusing,

(CR 1)

Vs = Vc

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The volume swept by the piston during a cycle can be calculated using

4 S D Π = Vs 2 (4)

3.3 Mass and energy balances

The mass balances describing a perfectly mixed reactor with variable volume are summarized by

dt =VRi

) Vci ( d

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Where Ci represents the species concentration ((mol/m3) and Ri denotes the species rate expression (mol/m3.s)

For an ideal gas mixture, the reactor energy balance is

dt dp V + Q + Q = dt dT c c

V _ext _r

i i , p i r (6)

Where Cp,i is the species molar heat capacity (J/ mol.K), T is the temperature (K) and p gives the pressure (Pa).

In this equation, Q is the heat due to chemical reaction (J/s)

j j jr H Vr = Q (7)

Where, Hj is the enthalpy of reaction (J/mol/K), and rj equals the reaction rate (mol/m3.s). Qext denotes heat

added to the system (J/s). The model being described assumes adiabatic conditions, that is Qext = 0.

3.4 Initial conditions

In this model methane is combusted under lean conditions that is supplying more than the stochiometric amount of oxidizer. The stochiometric requirement of the oxidizer to combust methane is found from the overall reaction:

CH4 + 2 (O2+3.76 N2)  CO2 +2H2O +7.52 N2

Assuming that the composition of air is 21% oxygen and 79% nitrogen, the stochiometric air-fuel ratio is

fuel air stochio fuel air m * 1 m * 2 * 76 . 4 = m m = stochio ) F / A ( (8)

The equivalence ratio relates the actual air-fuel ratio to the stochiometric requirements.

(A/F)

) F / A ( = φ stochio (9) This model assumes the equivalence ratio to 0.5

The molar fraction of the reacting mixture is calculated from

φ 1 =

(4)

And subsequently the initial concentration is

init

init fuel fuel

RgT P * x = C

(11) The initial pressure and initial temperature are variable model parameters.

4 Results and Discussion

The combustion of a primary reference fuel has been analyzed considering the above mentioned initial conditions and the following results were achieved. The results achieved were

1) Pressure rise for various inlet temperatures. 2) Pressure rise for various speeds.

3) Molar fractions of species CO, CO2, NO, NO2, N2O for various inlet temperatures.

These graphs clearly show the variations that occur during the combustion process. Fig 4.1 shows the pressure rise curves for various engine speeds varying from 1000-1500 rpm. As the speed of the engine increases induction delay is shortened.

Fig. 1. Effect of engine speed on pressure rise

Fig. 2. Effect of inlet air temperature on pressure rise

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Fig. 4. Molar fractions of species at inlet air temperature of 300 K.

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Fig. 2 and fig. 3 shows the pressure rise curves for various inlet temperatures. The induction delay decreases with increasing initial temperature. The induction delay time can be evaluated from the pressure gradient. For instance, the induction delay is 0.0193 s when Tinit = 500 K When the inlet air temperature is increased peak

pressure achieved varies in a notable manner. Maximum pressure is achieved for the inlet temperature 425 K. This is because methane starts ignites above 400 K. Further increment leads to improper combustion and thus reduces the peak pressure. Fig . 4 - 9, shows the molor concentration of species CO, CO2, NO, N2O, and NO2 at

various inlet temperatures varying from 300-550 K. The molar fraction graphs clearly represent the different molecular formation temperatures and the amount of formations. This would greatly help in the NOx formation studies and thus by leading to finding effective solutions to control it.

5 Conclusion

The analysis showed that methane does not ignite at an initial temperature of 400 K. Furthermore, the induction delay decreases with increasing initial temperature. The induction delay time can be evaluated from the pressure gradient. An increase in pressure means an increase in the species concentrations in the fuel-air mixture, resulting in the expected advance ignition times. The results just discussed show that the inlet temperature of the fuel air mixture is a potential tuning parameter for ignition. However, relatively high inlet temperatures are often required for proper timing. This adversely affects the engine performance because the trapped mass as well as the volumetric efficiency decreases. NOx does not form until the inlet temperature reaches 450 K. The increased inlet temperature increases the combustion temperature and thus by leads to NOx formation.

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