combustion bomb

7/29/2019 combustion bomb

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Energy Sources, 23:345 361, 2001

Copyright 2001 Taylor & Francis

0090-8312 / 01 $12.00 1 .00

Laminar Burning Velocity of Some

Coal Derived Fuels

MOHSEN RADWAN

MOSTAFA ISMAIL

MOHAMED YOUNES SELIM

HOSAM SALEH

Faculty of Engineering

Helwan University

Mattaria, Cairo, Egypt

HINDAWI SALEM

Faculty of Engineering

Cairo University, Egypt

The laminar burning velocity of seven different coal-derived liquid fuels have been mea-

sured in a constant-volume combustion vessel using transient pressure technique. Thetest conditions included the type of fuel, equivalence ratio, initial mixture temperature,

and pressure. The results showed that coal-derived liquid fuels generally exhibit lower

laminar burning velocity than iso-octane fuel. The maximum laminar burning velocityoccurred at nearly stoichiometric mixture, but burning velocity decreases as the mixture

becomes more lean or more rich. Over the range of the studied test conditions, a corre-

lation was developed to fit the laminar burning velocity data with the main governingparameters.

Keywords coal-derived fuels, combustion bomb, laminar burning velocity, pressure-time history

Introduction

The world energy resources and the rate of their consumption, when related to the estimated

population growth, indicate very clearly the necessity of searching for alternative fuels. The

attention devoted to coal is related to its large natural resources as well as to its competitive

stable prices. However, one of the main drawbacks of coal is that it produces ash upon com-

bustion in addition to its difficulty of handling. Thus it is rendered unsuitable for use inengines and some practical power units. As a consequence, efforts have been made to produce

345

Received 16 February 2000; accepted 21May 2000.Address correspondence to Dr. Mohamed Younes Selim, Mechanical Engineering Department,

Faculty of Engineering, United Arab Emirates University, P O Box 17555 - Al-Ain, United Arab Emi-

rates. E-mail: [email protected]


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liquid coal-derived fuels, e.g., Sefer et al. (1985). Coal-derived fuels are important to be con-

sidered because they are expected to have long term contributions. Coal-derived fuels have

been tested in engines from the performance and exhaust emission point of view, e.g., Davies

& Freese (1981) and Needham & Doyle (1985). However, one of the main combustion char-

acteristics of these fuels, the burning velocity, is still lacking.

Knowledge of the burning velocity enables the determination of several factors affectingthe combustion chamber design, such as rate of pressure rise, peak cylinder pressure, exhaust

gas temperature, and exhaust emissions. It has also been suggested that the ignition delay

time, engine knock, and wall quench layer thickness (Blizard & Keck, 1974; Lewis & Von

Elbe, 1987) are functions of the laminar burning velocity. The experimental data of the lami-

nar burning velocity and heat release rate-temperature profile are not only of practical impor-

tance for design and analysis of practical power units, but are also of fundamental importance

for developing and assessing theoretical models of laminar flame propagation (Aung et al.,

1997). Hence the main objective of the present work is to provide accurate measurements of

laminar burning velocity of seven different coal-derived liquid fuels. The laminar burningvelocity data of this type of coal-derived fuels have not been presented before. The examined

fuels are obtained from the Synthetic Fuel Center for the United States Department of Energy,

the South West Research Institute, Texas. The characteristics of the tested fuels are given in

Table 1.

In the present work, the laminar burning velocity of spherical flames in a cylindrical com-

bustion chamber has been measured using the transient pressure measuring technique. The

reliability of the present test set-up for providing accurate measurements of the laminar burn-

ing velocity have been extensively tested. The test conditions cover some reference fuels (e.g.,

propane and iso-octane) at different conditions. The laminar burning velocity of coal-derivedfuels was determined over a wide range of operating conditions, equivalence ratio, initial pres-

sure, and initial temperature. An analytical correlation was also derived to fit the laminar burn-

ing velocity measurements at the different operating conditions.

Experimental Set-Up

There are many existing measuring techniques to find the laminar burning velocity. However,

many authors, e.g., James (1987), Andrews & Bradley (1972), Metghalchi & Keck(1982),

and Rallis & Garforth (1980), have shown that the closed combustion vessel techniques thatemploy either high-speed Schlieren photography or transient pressure measurement tech-

niques are both versatile and accurate. In the present work, the bomb method together with the

transient pressure technique was adopted to determine the laminar burning velocity.

The present test rig permits the control of the initial conditions of equivalence ratio, pres-

sure, and temperature. Figure 1 illustrates a schematic of the test set-up, which comprises a

combustion section, an air supply system, a fuel admission system, an ignition system, and

instrumentation.

The combustion section is an electrically heated double-walled steel cylindrical chamber

with 250 mm inner diameter and 315 mm height. Both inner and outer cylinders have 6 mm

thickness. Four equispaced high-speed bladed fans are used to insure complete mixing of the

fresh incoming mixture. A piezo electric pressure transducer is mounted centrally to the

bomb, to be close to the spherical flame, to measure the pressure-time, see Figure 1. The bomb

section is also furnished by a circumferential vacuum and pressure gauge to measure the ini-

tial pressure of the mixture. An iron-constantan thermocouple was used to measure the initial

temperature of the mixture.

346 M. Radwan et al.


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Combustion Characteristics of Coal Derived Fuels 347

Table 1

Characteristics of Coal-Derived Liquid Fuels

SRC-II SRC-II SRC-II EDS 50/50

Coal Coal med. low middle middle blend D-2

Fuel case 5A case 12 CN CN dist. distillate and EDS

Composition, vol. %

Kerosene petroleum 17.3 19.3 0 0 0

Diesel petroleum 66.7 45.7 65.0 50.0 0

Coal SRC-II 16.0 13.0 35.0 50.0 100.0

HC kerosene 0 2.0 0 0 0

Properties:

Gravity, API 31.1 32.8 26.8 23.3 12.3 21.4 27.9

Specific gravity 0.8702 0.8612 0.8939 0.9141 0.9840 0.9254 0.8877

Distillation, D-86, F

IBP/5 % recovered 378/432 326/416 346/409 329/387 368/400 412/424 382/419

10/20 453/472 434/455 424/446 420/438 412/428 431/440 430/440

30/40 487/499 468/480 464/476 455/470 / 449/461 454/469

50/60 512/528 491/504 489/502 485/500 473/ 473/489 485/503

70/80 546/576 521/547 522/543 517/538 / 509/534 524/551

90/95 626/667 603/652 577/610 570/600 553/577 573/612 586/600

EP 690 683 638 626 613 649 613

Recovery, % 98.5 98.5 98.5 99.0 99.0

Residue 1.5 1.5 1.2 1.0 1.0

Loss 0.0 0.0 0.3 0.0 0.0

Viscosity, Cstat 40c 3.08 2.61 2.83 2.95 3.68 2.53 2.48

Flash point, F 176 166 176 176 176 199 168

Pour point, F 21 10 0 -6 54 54 18

Hydrocarbon type, vol.

Olefins 1.4 1.3 49.5 57.8 91.2 6.5 0.0

Saturates 63.7 64.4 1.8 1.4 0.7 18.3 36.9

Aromatics 34.9 34.3 48.7 40.7 8.1 75.2 63.1

Elemental analysis, wt%

Carbon 86.47 86.6 86.03 86.02 86.15 88.5 87.31

Hydrogen 12.38 12.55 11.47 10.86 8.64 10.9 12.04

Oxygen 0.032

Nitrogen 0.15 Est 0.12 Est 0.39 Est 0.44 Est 0.82 0.028

Sulphur 0.1 0.08 0.33 0.43 0.26 0.01 0.16

H/C atom ratio 1.71 1.73 1.59 1.50 1.19

Heat of combustion

Gross, MJ/kg 45.32 45.56 44.08 42.87 41.84 43.701 44.441

Net, MJ/kg 42.73 42.89 41.64 40.56 40.01 41.388 41.886

Cetane number 42 41.1 31.4 25.4 16.2 23.5 34.8


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An automotive coil ignition system was employed to generate a spark between two steel

electrodes located at the center of the bomb section. The electrodes extend from the left side to

the right side of the chamber, one is fixed and the other is positioned with an accuracy of 0.01

mm using a micrometer. Spark gaps were adjusted by trial so that minimum ignition energies

were obtained in order to minimize the effects of spark disturbances. The flame is started at the

center of the cylindrical bomb so that it propagates radially to obtain a spherical flame.

During experiments, the test section is evacuated to a complete vacuum and then charged

with the required amount of the test fuel and the hot air to reach the required initial pressureof the test condition. The mixture was prepared as follows: the charged air was heated in a

lagged air tank by a gas burner prior to mixing with the test fuel, Figure 1. The required

amount of test fuel was injected first at the throat of the venturi using an accurate syringe. An

electric heater, to prevent condensation of fuel on its inner surface, was used to externally heat

the venturi. After injection, the hot air was charged to the cylinder. The air fuel ratio was

adjusted by measuring the partial pressure for both the injected fuel and the hot air. The mix-

ture passes through a controlled electric heater to adjust the desired temperature of the mix-

ture. The bomb and all piping connections are externally heated so that the temperature inside

the bomb and piping is always higher than the evaporation temperature for the fuels, whichmay be seen in Table 1. It should be noted here that the fuel is always admitted to the bomb

under a vacuum, which will ensure that the fuel is always in vapor form. The gases were

mixed using fans within the chamber and then allowed to become quiescent prior to ignition.

After combustion was complete, the chamber was vented and purged with air to remove con-

densed water vapor prior to evacuating and refilling the chamber for the next test.

Instrumentation

The laminar burning velocity for all modes of spherical flame is defined as the velocity, rela-

tive to and normal to the flame front, with which unburned gas moves into the front and is

transformed to product mixtures under laminar flow conditions (Lewis & Von Elbe, 1987).

The present model has been designed and programmed to calculate the laminar burning veloc-

ity according to the pressure-time history and based on the thermodynamic analysis by Lewis

& Von Elbe (1987) and Manton et al. (1953). The model is briefly presented in the next sec-

tion. The transient pressure measurement technique has been adopted in the present work to

determine the burning velocities. The measuring circuit, Figure 1, used to record the pressure-


Figure 1. Schematic diagram of combustion bomb.


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time history, consists mainly of a piezoelectric pressure transducer, charge amplifier, storage

oscilloscope connected to a plotter, and a personal computer. The computer facilitates data

storage and processing, through the use of an A/D converter. The pressure data are sampled

over a period of 130 ms by a personal computer using PCL-818 High Performance Analog to

Digital converter to digitize the signal with a sampling frequency of 0.1 ms/point. Figure 2

shows a typical pressure-time history obtained by the present circuit.

Burning Velocity Thermodynamic Model

The model used to calculate the laminar burning velocity, SL, is the model suggested by Lewis

and Von Elbe (1987) and used by many authors, e.g., Metghalchi & Keck(1982). The main

end equation of the model is the following one, which calculates the laminar burning veloc-

ity from the pressure-time data:

(1)

where

(2)

(3)

(4)

(5)Tu Ti 5 p p icu2 1 cu

rb 5 R 1 2p i

pTu

Ti

pe 2 p

pe2 p i

1 3

dri dt5 R 3 pe 2 p i p 2 p i pe 2 p i2 2 3 dp dt

ri 5 R p 2 p i pe 2 p i1 3

SL5 dri dt ri rb2 p i p

1 cu


Figure 2. Pressure-time history for isooctane combustion.


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where

SL = laminar burning velocity

pe = pressure of the burned gases at thermodynamic equilibrium

pi = initial pressure before combustion

ri = radius of cylinder at any instant before combustion

rb = radius of cylinder at any instant after combustionTi = initial mixture temperature before combustion

p = pressure at time tat which the burning velocity is calculated (in order of 1.1 Pi for this

model)

R = combustion bomb radius

g = specific heat ratio

t= time

Tu = unburned gas temperature

In this model, the indices i (for initial) and e (for end) refer to the gas before ignitionand after combustion is complete. Indices u (for unburned gases) and b (for burned gases)

refer to the progress of the wave from the center to the wall. Before ignition, the combustible

gas mixture is at the pressure Pi and the temperature Ti. After ignition, the burned gas occu-

pies a core surrounded by unburned gas that has been compressed isentropically to the tem-

perature Tu. The pressure throughout the vessel isp.

The main input to the burning velocity model is (p/pi) and it is chosen not to exceed 1.1

to avoid the pressure rise effect (constant volume combustion). Within this range of pressure

ratio, the combustion may be assumed to be at constant pressure. During this period, the flame

radius is calculated and plotted with time and the flame circle volume divided by the bombvolume was found to be below 7%. The pressure gradient at the required value of(p/pi) is cal-

culated from the pressure-time record. The burning velocity is plotted with time during com-

bustion and found to increase with time as the pressure increases during combustion. Then the

maximum value of burning velocity is estimated directly from the results atp = 1.1pi accord-

ing to the Lewis and Von Elbe model.

The present model has been tested using the results of the laminar burning velocity of

a regular type of gaseous fuel, propane, at an initial pressure of 1 bar and an initial temper-

ature of 300 K, which was measured by different measuring techniques, e.g., Palm-Leis &

Strehlow (1959), Yamaoka & Tsuji (1984), Gray et al. (1952), Anderson & Fein (1949), andFaeth et al. (1993)(by photographing the flame), and found to fit well within the experi-

mental error with a maximum burning velocity of 42 cm/s at a relative fuel air ratio slightly

< 1 (see Figure 3).

Other preliminary experiments were carried out with a regular type of liquid fuel such as

isooctane. Figure 4 shows the influence of the equivalence ratio of isooctane air mixture on

the laminar burning velocity at an initial temperature of 350 K. The influence of the mixture

temperature on the laminar burning velocity at an equivalence ratio of 1 and initial pressure

of 1 bar has also been carried out. A comparison of the burning velocity data obtained to the

available isooctane data from the literature given in Keck(1981) and Gulder (1984) were

found to agree with the present result.

Test Conditions

The laminar burning velocity for the coal-derived fuels is presented and discussed in the next

sections. A wide range of different operating parameters affecting the laminar burning veloc-

ity has been studied. These operating parameters include:



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1. The equivalence ratio (f); it is varied from 0.8 to 1.3 with a step of 0.05.

2. The initial mixture temperature (Ti); it is varied from 300 to 400oC with a step of 25oC.

(The minimum initial temperature is chosen to ensure that the admitted fuel is in fully

vaporized form at the atmospheric pressure.)


Figure 3. Laminar burning velocity of propane.

Figure 4. Effect of equivalence ratio on laminar burning velocity for isooctane.


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3. The initial mixture pressure (Pi); it is varied from 0.5 to 1 bar with a step of 0.1 bar.

4. The fuel composition.

Preliminary experiments were carried out to ensure that the present combustion bomb with

its aggregates and instrumentation are functioning properly and precisely. Ten identical exper-

iments were carried out using isooctane at the same equivalence ratio, initial pressure, and ini-

tial temperature. The ten experiments were recorded to evaluate the laminar burning velocity

and the standard deviation was found to be 0.028. The maximum error in measuring the

amount of fuel by the syringe is 0.01 cc while the minimum amount of test fuel is 1 cc,

which results in a maximum relative error of 1%. Error in measuring the initial pressure is 0.02

bar with minimum pressure of 0.5 bar and results in a relative error of 4%. Initial temperature

measured with an error of 3oC with a minimum value of 300oC results in 1% relative error.

Equivalence ratio is measured with a relative error of 6%, as calculated based on fuel volume,

pressure, and temperature. Laminar burning velocity is estimated from pressure gradient and

the model equations shown above. This includes an error in pressure transducer reading, time

measurement error of the A/D converter, error in calculating pressure gradient, errors in equiv-

alence ratio, initial temperature, and initial and instantaneous pressures. The summation of

errors gives an error of < 8% in the estimation of the laminar burning velocity.

The isooctane resultsas a typical liquid fuelare presented for comparison with the pre-

sent coal-derived liquid fuels. These data have been used to develop an analytical correlation to

characterize the laminar burning velocity of each type of the used fuel at different operating con-

ditions. The isooctane results and their analytical correlation showed good agreement with other

investigators, which gives confidence in the present results and their correlations.

Results and Discussion

The results of laminar burning velocity of the tested fuels are presented and analyzed in the

next sections. The results are compared with the data of isooctane fuel as a regular type of

liquid fuel.

Effect of Equivalence Ratio

The effects of equivalence ratio on the laminar burning velocity of the different types of coal-derived liquid fuels are shown in Figures 5 a, b, and c at an initial pressure and temperature

of 1 bar and 573 K, respectively. On each graph, the laminar burning velocity of isooctane was

superimposed for comparison. The figures show that the relation between the equivalence

ratio and the laminar burning velocity tends to take the bell shape. The burning velocity starts

with low values near a lower flammability limit in the lean mixture side and then increases as

equivalence ratio increases until it reaches a maximum value at nearly the stoichiometric mix-

ture. On the rich side, it begins to decrease again. The maximum burning velocity occurs at

an equivalence ratio of 1, which is similar to the conventional hydrocarbon fuels. The lower

values of the laminar burning velocity in the lean side could be explained due to the excess air,

which dilutes the reactant mixture ahead of the flame front. On the other hand, laminar burn-

ing velocity decreases in the rich side due to incomplete combustion, which leads to a

decrease in the flame temperature that tends to slow down the reaction rate.

Comparison of data with those of isooctane shows that the laminar burning velocity of

isooctane is always greater than that of any of the coal-derived fuels. This appears to be due

to no aromatic content of isooctane, which is one of the iso-paraffin groups, which have a high

pyrolysis and cracking rate. Most fuels having higher contents of aromatics are expected to



9/17


Figure

5a

Figure5b

Figure5c

Figure

5.

Effectofequiva

lenceratioonlaminarburning

velocityforcoal-derivedfuel.


10/17

have low pyrolysis and cracking rates that leads to a slow down of the combustion process as

has been shown by Bradley et al. (1991). The aromatic contents and the other characteristics

of the coal-derived liquid fuel are given in Table 1.

It has been shown by Blizard & Keck(1974) and others that the ignition delay period is

a function of the laminar burning velocity. Higher burning velocity SL leads to smaller igni-

tion delay such that

(6)

whereLe is the characteristic radius entrained by the flame front and Su is the laminar veloc-

ity for fuel air mixture. Radwan et al. (1991) have measured the ignition delay for the same

type of coal-derived liquid fuels used here. It was found that the ignition delay runs in step

with the Cetane number such that a higher Cetane number leads to a shorter delay. The coal-

derived fuel found with lowest burning velocity is SRC-II middle distillate and it has the low-

est Cetane number, and Coal case 5A was found to have the highest burning velocity andhighest Cetane number. So the higher the Cetane number the lower the ignition delay and the

higher the burning velocity. Generally, it is found that the burning velocity is higher for the

fuels, which have a lower ignition delay.

It is also found that the laminar burning velocity is running in step with the heat of com-

bustion of used fuels such that the highest burning velocity is found to be for Coal case 5A,

which has a high heat of combustion. The SRC-II middle distillate has the lowest heat of com-

bustion and also the lowest burning velocity, and all other fuels have in-between heat of com-

bustion and in-between burning velocity. Also, all fuels used have lower heat of combustion

compared to isooctane.

Effect of Initial Mixture Temperature

The effect of initial temperature (temperature of unburned mixture before ignition) on the

laminar burning velocity of isooctane and different types of coal-derived liquid fuels is pre-

sented in Figures 6 a, b, and c at an equivalence ratio and initial pressure of 1 and 1 bar,

respectively. Generally, the figures show that increasing the initial mixture temperature leads

to an increase in the laminar burning velocity. This can be explained in the manner of the

Arrhenius equation

(7)

whereRR m Kand K=Z.Tz eE/RT

whereRR is the reaction rate, Kis the specific reaction rate, Tis the absolute temperature,R

is the universal gas constant,Eis activation energy, andZand x are empirical constants. The

equation showed that the higher the temperature, the higher the reaction and cracking rate, so

these lead to a speed up of the combustion process.

Effect of Initial Mixture Pressure

The effect of initial mixture pressure (pressure of unburned mixture before ignition) on the

laminar burning velocity of isooctane and the different types of coal-derived liquid fuels is

depicted in Figures 7 a, b, and c. Measurements with varied initial mixture pressure were

performed at a constant equivalence ratio and temperature of 1 and 573 K, respectively. The

SL ~ RR

r5 Le Su



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Figure6a

Figure6b

Figure6c

Figure6.

Effectofinitialtemperatureonlaminarburningvelocityforcoal-derivedfuel.


12/17


Figure7a

Figure7b

Figure7c

Figure7.

Effectofinitialpr

essureonlaminarburningvelo

cityforcoal-derivedfuel.


13/17

figures show that as the initial pressure increases, the laminar burning velocity tends to

decrease. Generally, the burning velocity of the hydrocarbon-air mixtures can be expressed

as

(8)

where Pi is the initial mixture pressure and n is small in magnitude ( 0.5) and is negative

(Goldenberg & Pelevin, 1958).

It has been shown recently that in most hydrocarbon air flames, the diffusion of hydro-

gen atoms into the preheat zone is the primary triggering mechanism for the chain-branching

reactions which support flame propagation. It is well known that the hydrogen atom under-

goes two primary competing reactions during any hydrocarbon oxidation process. These are

the chain-branching reaction

(9)

and the recombination reaction

(10)

Furthermore, the chain-branching reaction has a first-order pressure sensitivity (because

it is a second-order reaction) and has a large activation energy, which means that its rate

increases very rapidly with the increased temperature. On the other hand, the recombination

reaction has a second-order pressure sensitivity (because it is a third-order reaction) and hasessentially no temperature sensitivity at all. In the preheat zone of the flame, hydrogen atoms

diffuse towards the cold gas and at the same time react. In the higher temperature regions of

the flame, the chain branching reaction dominates and recombination is slow. However, as one

travels toward the incoming gas, the temperature drops and at some point the two reactions

become competitive. In the colder regions, the recombination reaction dominates and destroys

hydrogen atoms. If the pressure level at which the flame is burning increases, the rate of the

recombination reaction increases relative to the chain branching reactions and therefore tends

to lower the burning velocity. In other words, we should expect that for low burning velocity

flames, the competition between recombination and chain branching reactions in the preheatzone is important.

Analytical Correlation

The analytical correlation is necessary, especially in CFD simulations, and therefore this sec-

tion will express the present data in a form of an analytical correlation. The correlation of lam-

inar burning velocities SL will be expressed as a function of (f, Ti, Pi). Figures 5, 6, and 7

propose the correlation to be in the following form:

S (11)

where the positive sign applies for the region of lean fuel-air mixtures, while the negative sign

applies for the region of rich fuel-air mixtures.

(12)SL 5 A2 T Toc

L 5 A1 u u o b

H 1 O2 1 M H O2 1 M

H1 O2 OH1 O

SL ~ Pin



14/17

(13)

whereA1,A2,A3, b, c, and dare constants and they mainly depend on the type of fuel. This

relation form agreed with most of previous investigators (Goldenberg & Pelevin, 1958).

From the previous equations the present analytical formula can be produced describing

the overall dependence of the laminar burning velocity on the temperature, pressure, andequivalence ratio. This can be expressed in the form

(14)

where subscript o refers to the reference conditions for f, T, P of 1, 573 K and 1 bar, respec-

tively, and SLo refers to the value of the burning velocity at this reference state.A, b, m, and n

are constants that depend on the fuel characteristics. To calculate the values of these constants,

the experimental data were fed into a regression computer program prepared for this purpose.

It utilizes a multivariable linear regression technique.

It may be noted that all the calculations have been performed under the following range

of validity: T= 573 K to 673 K, P = 0.5 to 1.0 bar. The resulting values of different constants

in the above correlation for various operating parameters for isooctane are listed in Table 2.

Values of the burning velocity calculated from the obtained correlation were compared with

the experimental data as presented in Figure 8a for isooctane. From Table 2, it may be seen

that the experimental data are scattered around the ideal line with a maximum deviation (dif-

ference between measured value and correlated value) of 28% for isooctane when the range

0.8 f 1.3 is used. However, when the correlation is divided into two ranges, a maximumdeviation of 4.5% is observed in the lean mixture range f 1 and 7.5% in the rich mixture

range f 1. So in the present correlation the equivalence ratio range 0.8 f 1.3 is divided

into two subranges, a lean mixture range where f 1 and a rich mixture range where f 1,

see Table 2 and Figure 8a. The figure indicated that the above correlation predicts the data

with higher accuracy for the range f 1 and f 1 individually. The present pressure and tem-

perature exponents for isooctane are in close agreement with those obtained by Gulder (1984),

where both were obtained for combustion in constant volume bomb and related to the pres-

sure range of combustion (p =1.1pi).

Since the burning velocity data for the used coal-derived liquid fuels has the same trendof the burning velocity profile for isooctane fuel, so the present correlation is used to charac-

terize the coal-derived fuels data. The calculated constants for various operating conditions for

each fuel are listed in Table 3 for the lean mixture side (f 1) and Table 4 for the rich mix-

ture side (f 1).

SL 5 SLoAu

u o

b T

To

m P

Po

2 n

SL 5 A3 P Po2 d


Table 2Values of Constants for the Iso-octane Laminar Burning Velocity Correlation

f A b m N Maximum deviation

0.8 to 1.3 0.8523 0.2535 3.0720 0.6233 28%

1 0.9990 1.5049 1.6483 0.2310 4.5%

1 1.0309 1.6502 1.6483 0.2310 7.5%


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Figure 8. Correlated versus measured laminar burning velocity for isooctane and coal-derived fuel.

Table 3

Values of Constants for the Laminar Burning Velocity Correlation

f 1

Type of fuel A b m n

Coal case 5A 1.0109 1.9743 2.9101 0.4457

Coal case 12A 1.0075 1.9697 2.9660 0.4644

SRC-II med CN 0.9993 1.9322 2.9600 0.5331

SRC-II low CN 0.9993 1.9455 2.8523 0.5508

SRC middle dist. 0.9964 2.0326 2.9546 0.4253

EDS 0.9912 1.96266 3.2748 0.5607EDS +50 % D-2 1.0046 2.1896 3.4273 0.6607

Table 4

Values of Constants for the Laminar Burning Velocity Correlation

f 1

Type of fuel A b m n

Coal case 5A 1.0456 2.0347 2.6650 0.3857Coal case 12A 1.0449 2.2076 2.7011 0.3996

SRC-II med CN 1.0300 2.0134 2.7427 0.4799

SRC-II low CN 1.0301 2.0134 2.7427 0.4799

SRC middle dist. 0.9921 2.0134 2.7427 0.4799

EDS 1.0328 1.9298 2.9681 0.4856

EDS +50 % D-2 1.0575 0.2513 3.0445 0.5669

Figure 8a Figure 8b


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Values ofSLo for isooctane and coal-derived fuels are tabulated in Table 5. The obtained

correlation was compared with the experimental data as presented in Figure 8b for coal-

derived liquid fuels. It may be seen that the experimental data are scattered around the ideal

line with a maximum deviation of 4%. The figure indicated that the obtained correlation is

able to well predict the laminar burning velocity data.

ConclusionsFrom the present study conducted on coal-derived diesel engine liquid fuels, and by compar-

ison with the isooctane fuel results, the following conclusions may be drawn:

1. The combustion bomb measuring technique used in the present work proved to be

suitable for laminar burning velocity measurements.

2. The maximum value for the laminar burning velocity occurs at the stoichiometric

mixture. Richer or leaner mixtures result in a lower laminar burning velocity.

3. Coal-derived fuels, in general, exhibited a lower burning velocity in comparison with

isooctane.4. The lower laminar burning velocity of coal-derived fuel may primarily be attributed

to its chemical characteristics.

5. Higher initial temperature leads to higher laminar burning velocity.

6. Higher initial pressure leads to lower laminar burning velocity. Thus any design or

operational factor that affects the initial pressure and/or temperature will affect the

laminar burning velocity.

7. Coal-derived fuels with lower heat of combustion have lower laminar burning veloc-

ity and vice versa.

8. An analytical correlation was developed to predict the laminar burning velocity of

coal-derived fuel, which can be expressed in the form

whereA, b, m, and n are constants that mainly depend on the type of fuel.

SL 5 SLoAu

u o

b T

To

m P

Po

2 n


Table 5

Values of Laminar Burning Velocity at Reference Conditions

Fuel SLo (cm/s) Fuel SLo (cm/s)

Isooctane 123 SRC-II middle dist. low 77.3

cetane blendCoal case 5A 94 SRC-II middle dist. 64.6

Coal case 12A 89 EDS coal 85.5

SRC-II middle 82 50/50 blend EDS and D-2 77

dist. Med

cetane blend


17/17

References

Anderson, J. W. and R. S. J. Fein. 1949. Chem. Phys. 17:12681273 (cited in Faeth et al., 1993).

Andrews, G. E. and D. Bradley. 1972. Determination of burning velocities: A critical review.

Combustion and Flame 18:133135.

Aung, K. T., M. I. Hassan, and G. M. Faeth. 1997. Flame stretch interactions of laminar pre-

mixed hydrogen/air flames at normal temperature and pressure. Combustion and Flame

109: 124.

Blizard, N. C., and J. C. Keck. 1974. Experimental and Theoretical Investigation of Turbulent

Burning Model for Internal Combustion Engines. SAE Paper 740191, Detroit, MI.

Bradley, D., S. Habik, and S. El-Sherif. 1991. A generalization of laminar burning velocity

and volumetric heat release rates. Combustion and Flame 87:336346.

Davies, G. O., and R. G. Freese. 1981. The Preparation and Performance of Coal-Derived

Fuel. In Proceedings of the Fourteenth International Congress on Combustion Engines,

CIMAC, Helsinki.

Faeth, G. M., M. A. Ismail, and L . K.,Tseng. 1993. Laminar burning velocities and markstein

numbers of hydrogen / air flames. Combustion and Flame 95:410426.

Goldenberg, S. A., and V. S. Pelevin. 1958.Influence of Pressure on Rate of Flame Propaga-

tion in Turbulent Flow, 7th Symposium (Int.) on Combustion, Oxford University, UK.

Gray, K. L., J. W. Linnett, and C. E. Mellish. 1952. Trans. Farad. Soc. 48:115 (cited in Faeth

et al., 1993).

Gulder, O. L. 1984. Laminar burning velocities of ethanol-isooctane blends. Combustion and

Flame 56:261268.

James, E. H. 1987. Laminar Burning Velocity of Iso-octane-Air MixturesA Literature

Review, SAE paper 870170.

Keck, J. C. 1981.Laminar Combustion at High Temperatures and Pressures. Final report for

U.S. Army Research Office, Contract No. DAAG29-78-C0010.

Lewis, B., and G. Von Elbe. 1951. Combustion Flames and Explosion of Gases. Academic

Press, New York.

Lewis, B., and G. Von Elbe. 1987. Combustion, Flames, and Explosions of Gases. Third Edi-

tion, Academic Press, New York.

Manton, J., G. Von Elbe, and B. Lewis. 1953.Burning Velocity Measurements in a Spherical

Vessel with Central Ignition, Fourth (Int.) Symp. on Combustion, pp. 358363, Williams

and Wilkins, Baltimore, MD.Metghalchi, M., and J. C. Keck. 1982. Burning velocity of mixtures of air with methanol, iso-

octane, and indolene at high temperature and pressure. Combustion and Flame 48:191210.

Needham, J. R., and D. M. Doyle. 1985. The Combustion and Ignition Quality of Alternative

Fuels in Light Duty Diesels-SAE 852101, 1985 SAE Int. Fuels and Lubricants Meeting

and Exp., Tulsa, OK, October 2124.

Palm-Leis, A., and R. A. Strehlow. 1959. Combustion and Flame 82:361129 (cited in Faeth

et al., 1993).

Radwan, M. S., A. Y. Abdalla, and K. E. Mahrous. 1991. A shock tube investigation of the

thermal ignition delay of some coal-derived diesel engine liquid fuels. EngineeringResearch Journal April, Vol. 2, Egypt.

Rallis C. J., and A. M. Garforth. 1980. The determination of laminar burning velocity. Prog.

Energy Combustion Science and Technology 6:303329.

Sefer, N. R., L. Erwin, and J. A. Russel. 1985. Synthetic Fuel Center Construction and Alter-

native Test Fuels Production, Final Report, U.S. Department of Energy, September.

Yamaoka, I., and H. Tsuji. 1984. Determination of Burning Velocity Using Counter Flow

Flames. 20th Symposium (int.) on Combustion, The Combustion Institute, Pittsburgh, PA.


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