Burning velocities of ethanol-isooctane blends

C O M B U S T I O N A N D F L A M E 56: 261-268 (1984)

261

Burning Velocities of Ethanol-Isooctane Blends* OMER L. GtILDER National Research Council, Division of Mechanical Engineering, Ottawa, Ontario K IA OR6, Canada

Laminar burning velocities of isooctane, ethanol, and isooctane-ethanolblends in air have been determinedover a practical range of mixture strength at various initial mixture temperatures using a constant volume spherical bomb. Measurementswere made duringthe constantpressurecombustionperiodand a detaileddensitycorrectionschemewas employed tbr calculationof burningvelocitiesfrom the measuredflamegrowth rates. A strong promotionof isooctane combustion by ethanol has been observed. Maximumburningrates were found to occur at an equivalenceratio of approximately 1.08, independentlyof unburnedmixturetemperatureand fuel type. Mixturestrength, unburnedmixture temperature, and fuel type dependenceof burningvelocityis representedby empiricalfunctionsover the rangeof ~ = 0.75-1.4, Tu = 350-61DK,and up to 20% ethanol(by liquidvolume)in isooctane-ethanolblends.

INTRODUCTION Laminar burning velocity is one of the most essential parameters for analysis and performance predictions of various combustion engines. The majority of turbulent combustion models require a knowledge of laminar burning velocity of the fuelair mixture as a function of mixture strength, temperature, and pressure. Also reliable experimental data are needed in order to test and calibrate thermokinetic combustion models which have been quite successful for combustion predictions of simple hydrocarbon fuels [1-3]. Fundamental experimental data on combustion characteristics of alcohol fuels are very limited [46] ; in particular, only a few studies have been reported for high pressure and temperature conditions [7, 8]. Although the majority of alcohol fuel applications, at present, are in the form of hydrocarbon blends, no data, to the author's knowledge, have been reported on the burning rates of hydrocarbon-ethanol blends in archival literature. Ex* Based in part on an oral presentation at the Spring Technical Meeting of the Canadian Section of the Combustion Institute, Kingston, Ontario, May 9-10, 1983. Copyright ~) 1984by The CombustionInstitute Publishedby ElsevierSciencePublishingCo., Inc. 52 VanderbiltAvenue, New York, NY 10017

perimental data are available only for isooctanemethanol blends [9]. There exists a number of burning velocity prediction techniques for multicomponent gaseous fuel mixtures [10-13]. It has been shown recently that no existing technique for multicomponent fuel gas is reliable [14]. It should be noted that available techniques are based on thermal considerations and that they do not account for any chemical kinetic interactions. In the work reported here the measured laminar burning velocities of isooctane-ethanol-air flames and their dependence on mixture composition and unburned mixture temperature are presented. Two blends of isooctane-ethanol, namely, isooctane containing 10% and 20% ethanol by liquid volume, were used as fuel mixtures.

EXPERIMENT AND DATA EVALUATION A spherical high pressure vessel with an internal diameter of 0.325 m was used for the measurements. Ionization probes were used to measure the spatial velocity of the flame during the prepressure period of combustion in the vessel. The description

O010-2180/84/503.00

262

OMER L. GI2LDER

of the experimental setup has been reported in a previous paper [8]. The burning velocity can be calculated from the measured flame growth rate by employing the following relation,

Su =

ct%__, -pu dt

z = ~b/~.d

I dr b ' Tad N dt

Tu --

(2)

Tad ~--~-~

(3)

and Pad is the burned gas density calculated using the adiabatic flame temperature, Tad. N is the ratio of the number of moles of products to that of reactants. Procedures were suggested for the estimation of I for constant volume bomb measurements [8, 9, 15, 16]. The analysis of Andrews and Bradley [ 15 ] yielded a density correction factor of nearly 1.2 for a stoichiometric methane-air flame propagating spherically at a flame radius of 25 mm with P = 1 atm and T u = 300K, using a flame front thickness value of 1.1 mm. Rallis and Garforth [17] reported that the correction factor is approximately 1.05 for the same conditions. A much simpler scheme [8] yielded a correction factor of 1.13 + 0.05 for stoichiometric methanol-air flame at atmospheric conditions. In this work a physically more realistic scheme has been employed for density correction. For a steady one-dimensional planar flame, the energy equation for the preheating zone can be expressed as follows,

(1)

--Pb

which is valid only for the prepressure period of combustion, drb/dt, ~ , and ,oh are the measured flame growth rate, unburned mixture density, and mean burned gas density, respectively. The main difficulty in employing Eq. (1) is the evaluation of the mean burned gas density. This problem arises due to presence of the preheat zone, which has a considerable effect at low pressures and temperatures. The radial temperature distribution in a spherical kernel during the constant pressure combustion period, in which the increase in temperature resulting from the compression of the burned gases can be neglected, is shown in Fig. 1. If the burned gas within radius r b is assumed to be at the adiabatic flame temperature, then the corresponding value of Pb is less than the actual one and Eq. (1) underestimates the burning velocity. Assuming ideal gas behavior, Eq. (1)can be put into the following form,

Su -

where I is the density correction factor defined as

d IXd~xI dx _

_

- -

d(CpT)+H~b, Su Pu dx _

_

_

(4)

where X is the thermal conductivity, T is the temperature, Cp is the specific heat, H is the enthalpy of combustion, and w is the reaction rate. Integrating this expression, assuming that the reaction rate is negligible and the transport properties are constant, gives the temperature profile as [18]

unburned fuel air mixture

Tu--

,r FLAME

RADIUS

Fig. 1. Radial temperature distribution in a spherical kernel during constant pressure combustion period.

T = Tu +A exp I S u ~ C P x1 .

(5)

In order to take the variation of thermal conductivity and specific heat with temperature into consideration, the preheat zone was divided into a number of elemental layers and in each layer apappropriate mean values of X and Cp can be used.

ETHANOL-ISOOCTANE BURNING VELOCITIES Then the sum of these elemental layers gives the thickness of the preheat zone 6 as follows:

q-' ~ X In V T i - - T E ] " 8 = ~'~ i=1 Sup.Cp L T / + , - ru

(6)

263 or estimated by the methods recommended by Reid et al. [22].

RESULTS AND DISCUSSION Density Correction

In this expression T 1 = T u 4- 5 ° C , Tq = Tb and q is the number of layers with appropriate mean temperatures within the preheat zone. The peak of the ionization signal is expected to correspond to the location of the luminous zone within the flame front. Some of the ionization signals observed during this work did not present a distinct peak, but it was observed that the signals start with a sharp deviation from the baseline in all cases. Then, it was assumed that the point where a pulse starts deviating from the baseline corresponds to a position near to the upstream boundary of the preheat zone, i.e., to T 1, and A T was arbitrarily taken as 5°C (Fig. 1). Since the reaction zone has been neglected and with Tq = T b , 6 actually represents the total flame front thickness. Knowing the temperature profile, the mean burned gas density is given by

=--3 Pb

rb 3

r / r b Oh(r)r2 dr.

(7)

.Io

Using Eqs. (2), (3), (6), and (7), the burning velocity can be evaluated from the measured flame growth rate by a simple iterative scheme. The burning velocities presented in this work were evaluated from the measurements made at a flame diameter of 0.05 m. This diameter is well within the volume corresponding to the prepressure combustion period, and cutoff pressure corresponding to this diameter is less than 1% of the initial mixture pressure. Measurements were limited to a range of mixture strength covering equivalence ratios from 0.75 to 1.4. Unburned mixture temperature was varied from 350 to 600K at 100 kPa pressure. Adiabatic flame temperatures were computed by employing an equilibrium computer program which considers 14 species in the combustion products. Thermodynamic and transport properties were either taken from standard tables [ 19-21 ]

In the procedure of density correction only the effect of the preheat layer was taken into consideration. The heat exchange between the mixture and the ignition electrodes and ionization probes was assumed to be negligible. Adiabatic combustion temperatures and mole ratios, necessary for data evaluation, for ethanol and isooctane at Tu = 350K and P = I00 kPa are shown in Fig. 2. Adiabatic temperatures of isooctane-ethanol blends are tabulated in Table 1. Computed values of preheat layer thickness and correction factors were plotted in Fig. 3 as a function of mixture strength. Figure 4 shows the variation of computed density correction factors with unburned mixture temperature. The measured flame front thicknesses of common fuel hydrocarbons [15, 23-25] and those computed by both detailed thermokinetic [3, 26] and simplified thermal models [27] are of the order of 1 mm at room temperature and atmospheric pressure. Our results obtained at 350K are comparable to the 1.2

i

Tu:350 K 23

P=100 kPa

----~\'ad "f--

~

1.16

o o 2.2

-/

"/

N,S '\,"#

~"

o

//, m

1.12

-1.08

I-

,,,

/"

//

z 2£ < LL 1.9

4t.~ ~ ~/ 0.7

. ~

o -

ISOOCTANE -ETHANOL

J , h , ~ 0.8 0.9 1-0 1.1 1.2 EQUIVALENCE RATIO

1.04 ,

1.3

A

1.4

Fig. 2. Adiabatic c o m b u s t i o n temperatures and mole ratios of isooctane and ethanol.

OMER L. GULDER

264 TABLE 1 Adiabatic Flame Temperatures of Isooctane-Ethanol Blends: T u = 350K, P = 100 kPa

C8H18

10% C2H5OH, 90% C8H18

20% C2HsOH, 80% C8H18

1922 2083 2216 2299 2303 2244 2168 2091

1921 2081 2214 2296 2300 2241 2166 2089

1920 2080 2212 2294 2297 2238 2163 2087

0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4

ISOOCTANE

1.1o

"~

--

.

-

~

ETHANOL

80% ISOOCTANE / 20% ETHANOL

.

___1.05 P=100 kPo o

,~ =1.0

1.00

~

4;0

,

UNBURNED MIXTURE

measured and c o m p u t e d values reported in the literature. Burning Velocities

The majority of experimental studies dealing with the determination of the laminar burning velocity have been devoted to methane-air mixtures (for critical reviews see [15] and [17]), and, as a re]

i

-

E .\ ,E 1.0

-----. . . .

\

LU Z

,

-

~c 0.8 (.3

\

i

i

l

ISOOCTANE

ETHANOL 80% ISOOCTANE / 20% ETHANOL

j ,

500

j

6;0

TEMPERATURE- K

Fig. 4. Variation of density correction factor with unburned mixture temperature.

suit, there exists a considerable amount of data on burning characteristics of methane-air mixtures. To validate the method and the apparatus used in the present work, a set of measurements were taken with methane-air mixtures at 300K and 100 kPa, to be compared to recently reported burning velocity data. Measured velocities are shown in Fig. 5 along with other investigators' data. The agreement is very good and this confirms the validity of the method used. Variation of the laminar burning velocities of

\

0.5!

METHANE-AIR P=I arm

~ 0.6 <

2 0.4

EO.Z

Tu=350K P=IO0 o

i

kPa o _J

\ 1.16

0.3 z

z

z

_Q1,12

en

..... o

LU rr O~ 1.0 8 (J

0.2 01.7

018

019 110 111 112 EQUIVALENCE RATIO

1=3

•

[3/.] 293 K [35] 293 K [36l 293 K Present Work 300 K

1'.4

Fig. 3. Predicted flame thickness and density correction factors for ethanol, isooctane, and 20% ethanol-80% isooctane blend.

0'8

' ¢o ' 1'.2 EQUIVALENCE RATIO

'

14

Fig. 5. Comparison of burning Velocity data of methaneair mixtures.

ETHANOL-ISOOCTANE BURNING VELOCITIES ethanol, isooctane, and isooctane-ethanol blends with mixture strength at 350K unburned mixture temperature and i00 kPa pressure is shown in Fig. 6. Maximum burning rates occur at equivalence ratios between 1.05 and 1.1, independently of the fuel. Burning velocity data of isooctaneethanol blends indicate a promotion of isooctane combustion by the addition of ethanol. For mixtures leaner than 0.95, isooctane-ethanol blends exhibit faster burning rates than those of neat isooctane and ethanol. During the oxidation of ethanol the major intermediate species are acetaldehyde and formaldehyde, the concentration of the former being 3-6 times more than that of the latter [28]. In an experimental study investigating the influence of aldehydes on hydrocarbon combustion processes, Salooja [29] showed that, excepting the formaldehyde, the aldehydes generally promote the oxidation process at various preflame stages and lower the ignition temperature. Of the large number of aldehydes studied, acetaldehyde was found to be the most reactive [29]. The observed promotion of isooctane combustion by ethanol, thus, could be attributed to the effect of acetaldehyde formed as an intermediate during the oxidation of ethanol and which acts as degenerate chain-branching ini

0.60

0.5(3 i

(:3 -J UJ

> 0.4C

P =100 kPa

z

• 150 O C T A N E

Z ¢Y

zx ETHANOl.

0.3C ~ / / 0.7 0.8

t3 9 0 */* ISO0 CTANF./10*/* E T HA N 0 l, o 8 0% IS0 0CTANE/20%ETHANOL 019

1'.0

111

1:2

1:3

11/.

EOUIVALENCE RATIO Fig. 6. Burning velocities of isooctane, ethanol, and isooctane--ethanol blends as a function of mixture strength.

265 TABLE 2 Values of the Constants in Eq. (8) at 7"=0 = 350K Fuel CgHsOH C8H18 CsHla--C2HsOH

F

B (m/s)

1 0.609 1 0.5924 1+0.07V o.a5 0.5924

c=

#

0.25 6.34 -0.326 4.48 -0.326 4.48

termediate in isooctane combustion. A previous study carried out with isooctane-methanol blends showed that isooctane combustion is inhibited by methanol [9]. Measured burning velocities can be represented as a function of equivalence ratio by a fitted curve. The following empirical expression estimates the burning rates for the range 0.75 ~< ¢ ~< 1.4 at Tuo = 350K and Po = i00 kPa with small deviations: Suo($) = FBc~e' exp{--/~[¢ -- 1.0751 z).

(8)

The values of F, B, t~, and/3 are tabulated in Table 2, for ethanol, isooctane, and isooctane-ethanol blends. In Table 2, V is the liquid volume fraction of ethanol in the blend for the range 0 ~< V ~ 0.2. Prediction of burning velocities from Eq. (8) yields a maximum difference of 5% from the experimentally determined values. Variation of the maximum burning velocities, which were obtained at an equivalence ratio of approximately 1.08, of ethanol, isooctane, and isooctane-ethanol blends with the initial mixture temperature is shown in Fig. 7. Measured burning velocities of stoichiometric mixtures were normalized with respect to the burning velocity of isooctane and results are shown in Fig. 8, which gives a clear picture of the effect of ethanol on isooctane's burning rate at different unburned mixture temperatures. At 350K unburned mixture temperature and 100 kPa pressure, the laminar burning velocities of the two isooctane-ethanol blends are almost the same at all equivalence ratios considered (Fig. 6). However, at increased unburned mixture temperatures the 80% isooctane-20% ethanol blend exhibited faster burning rates than the 90% isooctane20% ethanol blend (Figs. 7 and 8). Figure 8 also

266

OMER L. GULDER Mole

1.6

1.4

fraction

1.5

o ETHANOL o" [] ISOOCTANE / v 90% ISOOCTANE/ / A 10% ETHANOL / /v. A 80*/* ISOOCTANE/ ,o' //n 20*,/, E T H A N O L / j / ~ ~ ~ / ~ "

of i s o o c t a n e ,

1.0

:~ 2400

.-'2 "6 2 3 7 5 Isooctane-

2350

"~1.2 i

Et h ~ n o t - Air

- - o - - MeQsured 1.35 -------Thermal mixing theory

uJ

Suj/

E

(D z0.8

>, 1.25

o /

m

kPo

8

0.4 , 300

c~ 1.15~-

0

J L J 400 500 500 UNBURNED MIXTURE TEMPERATURE-K

Mole

7. Maximum burning velocities as a function of unburned mixture temperature.

indicates that percentage increase in burning velocity of the isooctane-ethanol blends caused by ethanol addition increases with the increase in unburned mixture temperature. Figure 9 shows the variation of the combustion temperatures and the burning rates with the mole fractions of each fuel in the unburned mixture of 550K. Also shown in Fig. 9 are the predicted burning velocities of isooctane-ethanol mixtures employing a thermal mixing rule [30]. Recently Margolis and Matkowsky [31] and -T

i

- - - ISOOCTANE o ETHANOL >- 1.15 v 90% ISOOCTANE/ ~10% ETHANOL •" 80% ISOOCTANE/ .J uJ 20*,/* ETHANOL > 1.10 I''110 ~ O

rr

~

~

I

f

o_

/ o

/

P=I00 kPa @=1.0

l

L

|

2

4

6

fraction

of

ethanol

%

-

ation with the mole fractions of each fuel in the unburned mixture. The dot-dash line shows the burning velocity of the mixture assuming a linear dependence on mole fractions of the two fuels.

Margolis [32] have formulated the steady flame propagation through a premixed combustible mixture, in which the mixture consists of two distinct fuels, and solved through the method of matched asymptotic expansion. The solution obtained for large activation energies gives propagation velocity as a function of Lewis numbers, activation energies of each fuel, modified Damk6hler numbers, and heat release fractions. Since these parameters do not account for any chemical interactions between the intermediates of each fuel during oxidation, it is unlikely that observed behavior can be explained by their solution. In the range 350-600K, unburned mixture temperature dependence of burning velocity is represented by the following power law:

v/v

o ~ e

Tu~'550 K

-'~ ~ ~"

F i g . 9. Burning velocity and combustion temperature vari-

Fig.

g 1.05

~I

E

j-/ -JJ~

~> 1.20

0.6

LU N

%

0.5

~

~b= 1.0 P=IO0

0 kPa 1.00 . . . . . . . . . . . . . . . . . . . . . . . . . . . oz L I I 300 400 500 600 U N B U R N E D MIXTURE TEMPERATURE-K F i g . 8 . Normalized burning velocities of stoichiometric mixtures at different unburned mixture temperatures.

Su(T, ¢) = Suo(¢)[ Tu/To]rn.

(9)

Values of the temperature exponent m are tabulated in Table 3. Maximum difference between experimentally determined values and those calculated from Eq. (9) is less than 5%. Comparison of the experimental burning veloc-

ETHANOL-ISOOCTANE BURNING VELOCITIES TABLE 3

Values of the Temperature Exponent m, Eq. (9) Fuel C2HsOH C8H18

C8H18-CzH50H

Exponent m 1.75 1.56 1.56 + 0.23 V0.46

ities obtained for isooctane with the available data from various sources has been reported elsewhere [8, 9]. It was found that the velocities of isooctane reported by [4], [5], and [37] agree with present results. The data of [7], [38] and [39] contain considerably lower values than present velocities. The only experimental value reported in the literature for the burning velocity of ethanol, to the author's knowledge, is 0.7 m/s measured at Tu = 100°C and ~ = 1.09 [4]. This value agrees well with the velocity predicted from Eqs. (8) and (9), which is 0.695 m/s. As far as the burning velocities of isooctane-ethanol flames are concerned, there exist no data in the literature to which present results can be directly compared. The maximum uncertainty in the measured burning velocities was estimated as -+0.02 m/s. The uncertainty in equivalence ratio is +-0.02. CONCLUSIONS Laminar burning velocities of isooctane, ethanol, and two blends of isooctane-ethanol have been determined over a broad range of equivalence ratio and initial mixture temperature using the constant volume bomb technique and employing a detailed density correction scheme. Experimental results show a strong promotion of isooctane combustion by ethanol, especially at elevated unburned mixture temperatures. This promotion mechanism is most likely to be caused by the chemical kinetic effect of acetaldehyde formed as an intermediate during the oxidation of ethanol. The experimental results obtained for methaneair mixtures suggest that the density correction scheme and the constant volume bomb technique, in which the measurements were confined to the

267 prepressure combustion period, yield reliable burning velocity data. NOMENCLATURE A B Cp H 1 m N

constantin Eq. (5) constant in Eq. (8) specific heat at constant pressure enthalpy of combustion density correction factor temperature exponent ratio of number of mols of products to that of reactants P pressure r distance along radial axis r b flame radius Su laminar burning velocity T temperature t time V liquid volume percentage of ethanol in isooctane-ethanol blend reaction rate Greek Letters

a ~3 6 p

constant in Eq. (8) constant in Eq. (8) preheat zone thickness thermal conductivity density fuel-air equivalence ratio

Subscripts ad adiabatic b burned i 1, 2, "",q q number of computational segments in preheat layer 0 reference condition u unburned An overbar denotes mean value.

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268

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21.

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