30120130406014

International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 6, November - December (2013) © IAEME

110

THE MEASUREMENT OF LAMINAR BURNING VELOCITIES AND

MARKSTEIN NUMBERS FOR HYDROGEN ENRICHED NATURAL GAS

Miqdam Tariq Chaichan

Mechanical Eng. Dep.-University of Technology-Baghdad-Iraq ABSTRACT

Laminar burning velocities and Markstein lengths were obtained at various ratios of hydrogen (volume fraction from 0 to 100%) enriched to natural gas, and equivalence ratios (Ø from 0.2 to 1.7). The effect of stretch rate on flame was also analyzed. It was found that the laminar burning velocity is increased nonlinearly with the increase of hydrogen fraction for lean equivalence ratios. For stoichiometric equivalence ratio there is a linear relationship between flame radius and time, while for rich mixtures there is approximate linear relationship.

The Markstein length decreases and flame instability increases with the increase of hydrogen enrichment in mixture. For a fixed hydrogen fraction, the Markstein number shows an increase and flame stability increases with the increase of equivalence ratios. The laminar flame speed results were compared with other researches and it was found a good agreement.

Keywords: Laminar Burning Velocity; Markstein Length; Stretch; Constant-Volume Bomb.

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INTERNATIONAL JOURNAL OF MECHANICAL ENGINEERING AND TECHNOLOGY (IJMET)

ISSN 0976 – 6340 (Print) ISSN 0976 – 6359 (Online) Volume 4, Issue 6, November - December (2013), pp. 110-121

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IJMET

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111

INTRODUCTION

The laminar burning velocity is an important intrinsic property of combustible fuels and of air and burned gas mixtures. It is the velocity at which the flame propagates into a quiescent premixed unburned mixture ahead of the flame. In the case of reciprocating internal combustion engines, an additional feature is the presence of residual or burned gas from the earlier cycle, and this causes a reduction in the laminar burning velocity (Rahim, 2006 & Balusamy, 2009).

Knowledge of the burning velocity enables the determination of several factors affecting the 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 the wall quench layer thickness are functions of the laminar burning velocity (Fairweather, 2009). The experimental data of the laminar burning velocity and heat release rate-temperature profile are not only of practical importance for design and analysis of practical power units, but are also of fundamental importance for developing and assessing theoretical models of laminar flame propagation (Kelley, 2007 & Singh, 2007).

With recent dramatically increased crude oil prices, the inevitable decline in petrol resources and the increasing concern on environmental protection, investigation on alternative fuels has attracted more and more attention. Natural gas (NG) is regarded as one of the most promising clean fuels and has already been used in passenger cars, power generation, domestic usage etc. Due to its unique tetrahedral molecular structure, the main constituent of natural gas, methane, has narrow operational limits and is relatively difficult to be ignited (Powell, 2009 & Locka, 2008).

Consequently the utilization of natural gas can increase the severity of cyclic variations under fuel-lean conditions, leading to low thermal efficiency and high unburned hydrocarbon emissions (Kishore, 2007). One of the effective methods to improve its lean operation is to add fuels with faster burning velocity. Hydrogen seems to be the best candidate, which is difficult to be used directly by transport engines due to safety, storage and economic reasons. Experiments showed that engines fueled with hydrogen-enriched natural gas can operate at leaner conditions with reduced cyclic variations (Saxena1, 2009 & Miao, 2008).

So far, there is lack of information on the fundamental combustion characteristics of hydrogen-enriched natural gas (Lafay, 2008). The combustion of natural gas (or methane) and hydrogen has been studied intensively investigated the combustion of hydrogen–methane mixtures and found that adding hydrogen can effectively increase the burning velocity (Burluka1, 2007 & Coppens, 2007). Hydrogen could be an important future energy carrier, offering CO2 free emissions at the point of combustion. Laminar burning speeds for the most common stoichiometric hydrocarbon-air mixtures are on the order of only half a meter per second. The laminar burning speed of stoichiometric hydrogen-air equals approximately 3.5 m/s (Lamoureux, 2003 & Ilbas, 2006).

Recently (Hermanns, 2007) used same setup for determining effect of hydrogen addition to the laminar burning velocities of methane-air mixtures. (Konnov et. al, 2007) had used a replica of this setup and worked extensively on the determination of adiabatic burning velocities of methane (CH4), ethane (C2H6) and methane-hydrogen mixtures with different dilution ratios of artificial air having carbon dioxide (CO2), nitrogen (N2) or Argon (Ar).

This poses a challenge to combustion scientists and engineers in designing the combustion systems which runs efficiently on a wide range of operating conditions. In order to understand the impact of the variability in fuel compositions on the combustion performance and emissions, understanding of the changes in combustion properties of NG in presence of hydrogen is required (Tanoue, 2003 & Miao, 2009).

This study aims at obtaining fundamental combustion information of hydrogen-enriched natural gas, which can be used for design and optimization of hydrogen-enriched natural gas fueled


112

combustion systems. Resultant laminar burning velocities are compared with experimentally derived data, and differences discussed below. EXPERIMENTAL SETUP

The experiment was conducted as shown in Fig. 1; a detailed description of the experimental setup is in (Saleh, 2006). The flame is propagated inside the flame chamber passing over many thermocouple junctions located or inserted inside the chamber (Fig. 2). The thermocouple can be used as a sensor probe. The computer control system is used to read the flame speed inside the cylinder. Six thermocouples were fixed and used inside the cylinder, and it were distributed regularly as shown in Figure 1. They are connected to computer in order to collect, process and display the data. These collected data are getting at a very short time period. An interface circuit is built between the sensors and the computer. The thermocouple signals are reading through the parallel port.

To prepare the mixture (fuel-air), a gas mixer has been designed and constructed for gaseous fuels compounds that have a low partial pressure like (hydrogen, methane, propane, LPG & butane). The main purpose of preparing the fuel-air mixture in the mixing unit rather than in the cylinder is to increase the total pressure of the mixture and consequently increase the partial pressure of fuel to increase the accuracy.

Fig. 1: Block diagram of experimental apparatus


113

Fig. 2: The Internal Structure of Combustion Chamber

The mixer (Fig. 3) is made of (iron-steel); it has a cylindrical shape without any skirt to

improve the efficiency of mixing operation. Mixer dimensions are (435mm) length, (270mm) diameter and (5 mm) thickness. It undergoes a pressure of more than (60 bar) and also withstands high temperatures. The mixing unit has six holes of (12.7mm) in diameter. Two holes are used to fix the pressure gauge and vacuum gauge, the third is for admitting the dry air to the mixing unit from the compressor through the filter dryer, the fourth hole is for admitting the NG fuel from the fuel cylinder through pressure gauge regulator, the fifth hole is for admitting gaseous hydrogen from cylinder through pressure regulator, and the last hole admits the homogeneous mixture to the combustion chamber.

Fig. 3: mixture preparing unit diagram


114

A cover is added on one side of the mixer in order to connect the fan to a power source of (12 volt) DC. Through a glass sealed electrical connections, which are sealed completely to prevent any leakage of gas to improve the mixing operation and obtain a homogenous mixture. All welding in the cylinder is done using Argon welding, and tested by increasing the internal pressure of the cylinder to avoid any leakage.

The mixture preparation process has an important role in measuring the burning velocity. The process performed is based on partial pressure of mixture components and according to Gibbs-Dalton Law, to obtain an accurate equivalence ratio, because the ratio has effect on flame speed. The preparation of the mixture is done inside a mixing box, which was designed for this purpose. The partial pressure for hydrogen and NG are low so this method is used to obtain an increasing partial pressure for the used hydrogen- NG-air mixture.

The combustible mixture was prepared by adding natural gas, hydrogen and air according to their calculated partial pressures. Natural gas was admitted first, followed by hydrogen, then finally air.

The partial pressures were determined by initial pressure, hydrogen fraction (HVF) (the volume fraction of the hydrogen in the fuel of hydrogen-enriched natural gas), equivalence ratio Ø (the ratio of the actual fuel/air ratio to the stoichiometric ratio). In this study, hydrogen, with purity of 99.99% was used. The natural gas constitution is listed in Table 1. Based on Table 1, considering the formula of natural gas as CαHβOγ, it can be calculated that α is 1.1838, β is 4.3676, and γ is 0.059305. The combustible mixture in the bomb can be expressed as (1−x) CαHβOγ +xH2 + L (O2 + 3.762N2), and the equivalence ratio of the natural gas–hydrogen–air mixture is defined as:

� � ��α � β

4 � γ � 12 �1 � x � 1

2 /L

Flame temperatures were recorded in the experiment by means of thermocouples, where the

initial pressure and temperature kept the same value for mixtures with different hydrogen fractions. The initial conditions were strictly controlled in the experiments to realize the same initial pressure and temperature. For avoiding the influence of wall temperature on mixture temperature, an enough interval between two experiments is set, providing enough time for wall to cool down and keeping the same initial temperature.

Table 1: Compositions of natural gas

Items CH4 C2H6 C3H8 C4H10 C5H12

Volumetric fractions 84.32 13.27 2.15 0.23 0.03

Laminar burning velocity and Markstein numbers

To calculate the affecting variables (Haung, 2006) calculation method was used, applying the reading from the experimental rig. For a spherically expending flame, the stretched flame velocity, Sn, reflecting the flame propagation speed, is derived from the flame radius versus time data as

S� � �� 1

Where ru is the radius of the flame and t is the time. Sn can be directly obtained from the

flame measurements.


115

Flame stretch rate, α, representing the expanding rate of flame front area, in a quiescent mixture is defined as

α � ��

� �� 2

where A is the area of flame. For a spherically outwardly expanding flame front, the flame

stretch rate can be simplified as

α � ��

��

�� S� �3

In respect to the early stage of flame expansion, there exists a linear relationship between the

flame speeds and the flame stretch rates; that is,

Sl −Sn = Lbα, (4)

Where Sl is the unstretched flame speed, and Lb is the Markstein number (Markstein length) of burned gases. From eqs. (1) and (3), the stretched flame speed, Sn, and flame stretch rate, α, can be calculated.

The unstretched flame speed, Sl, can be obtained as the intercept value at α = 0, in the plot of Sn against α, and the burned gas Markstein number Lb is the slope of Sn–α curve. Markstein number can reflect the stability of flame. Positive values of Lb indicate that the flame speed decreases with the increase of flame stretch rate. In this case, if any kinds of protuberances appear at the flame front (stretch increasing), the flame speed in the flame protruding position will be suppressed, and this makes the flame stability. In contrast to this, a negative value of Lb means that the flame speed increases with the increase of flame stretch rate. In this case, if any kinds of protuberances appear at the flame front, the flame speed in the flame protruding position will be increased, and this increases the instability of the flame. When the observation is limited to the initial part of the flame expansion, where the pressure does not vary significantly yet, then a simple relationship links the spatial flame velocity Sl to unstretched laminar burning velocity ul, given as

ul = ρbSl/ρu (5)

where ρb and ρu are the densities for burned gases and unburned gases. The equation

u� � S �S� ρ�ρ�

�6

is used to determine the stretched laminar burning velocity un, and the stretched mass burning

velocity unl, proposed by (Bradley et al., 1998), is calculated from

u�� ρ�ρ��ρ�

�u� � S� �7

in which S is a rectified function and it depends upon the flame radius and the density ratio,

and accounts for the effect of the flame thickness on the mean density of the burned gases. The expression for S in the present study used the formula given by (Bradley et al., 1998),


116

S � 1 � 1.2 !δ"�� #ρ�

ρ�$�.�% � 0.15 !δ"

�� #ρ�ρ�

$�.�%�

�8

Here δl is laminar flame thickness, given by δl = ν/ul, in which ν is the kinetic viscosity of the

unburned mixture. RESULTS and DISCUSSION

The effect of hydrogen enriched NG on the flame propagation and Markstein length (i.e.

flame stability) will be analyzed at lean, stoichiometric and rich conditions. The flames of premixed natural gas–hydrogen–air mixtures speeds were measured and the flame radiuses were obtained. Figures 4, 5& 6 show the measured flame radius versus time after ignition for natural gas with hydrogen volume fractions (HVF) of 25, 50 and 75% vol. at various equivalence ratios (representing lean (Ø=0.5), stoichiometric (Ø=1.0) and rich (Ø=1.3) fuels respectively).

After ignition, the flame expands spherically, whose radius increases at different speeds depending on hydrogen fraction (i.e. fuel constitution), equivalence ratio (mixture lean or rich). With the increase of hydrogen fraction from 0% to 100% vol., the flame expands much more rapidly at all tested conditions (as shown by the fig. 4). Equivalence ratio has greater effect on flame propagation speed of the fuel with low hydrogen fraction than that of the fuel with high hydrogen fraction. In (Huang, 2006) and (Miao, 2008) those effects had been discussed in detail.

Different behavior of flame radius with time is demonstrated at different equivalence ratios, and this is reflected from the gradient of ru–t curves. In the case of lean mixture combustion (Ø= 0.5) at fig. 4, the flame radius increases with time but the increasing rate (gradient of curve) decreases with flame expansion for natural gas and for mixtures with low hydrogen fractions, while there exists a linear correlation between flame radius and time for mixtures with high hydrogen fractions.

In the case of rich mixture combustion (Ø= 1.3), as fig. 6 shows, the flame radius shows a slowly increasing rate at early stages of flame propagation and a quickly increasing rate at late stages of flame propagation for natural gas and for mixtures with low hydrogen fractions, and there also exists a linear correlation between flame radius and time for mixtures with high hydrogen fractions. In contrast to these, combustion at stoichiometric mixture demonstrates a linear relationship between flame radius and time for natural gas–air flame, hydrogen–air flame, and natural gas– hydrogen–air flame, as fig. 5 represents.

The gradient of the ru–t curve reflects the stretching effectiveness of flame. For an unstable flame, the gradient of the ru–t curve will decrease with the flame expansion, while for a stable flame; the gradient of the ru–t curve will increase with the flame expansion.

Fig. 7 illustrates the increments of the unstretched laminar burning velocity against hydrogen fraction for natural gas–hydrogen–air mixture combustion for wide range of equivalence ratio. The increments of the unstretched laminar burning velocity increase exponentially with the increase of hydrogen fraction in fuel blends. For fixed hydrogen fraction, little variation in increments is observed among various equivalence ratios. The arbitrary unstretched laminar burning velocities at different hydrogen fractions and equivalence ratios can be calculated from Eqs. (4).

As shown in Fig. 7. It was also found that the hydrogen-enriched natural gas with high hydrogen fraction can sustain relatively wider equivalence ratios than the fuel with low hydrogen fraction does. It was observed that the fuel with high hydrogen fraction has stronger capability to maintain its flame propagation speed under ultra lean conditions than the fuel with low hydrogen fraction.

For the fuel with 25% vol. hydrogen fraction, the flames propagate at relatively slow speeds after the ignition, followed by a faster process. For the fuel with 75% vol. hydrogen fraction, there exists a rather linear correlation between the flame radius and the time, According to the theory of


117

Markstein as (Gu, 2000) mentioned the gradient of the flame radius–time curve reflects the stretching effectiveness of the flame. For a stable flame, the gradient of the flame radius–time curve will increase with the increase of the radius. The experimental results showed that for the fuels with low hydrogen fraction the flame instability increased.

The stretched flame speed is the derivative of flame radius with time, which reflects the flame moving speed relative to the combustion wall. Fig. 8 illustrates the stretched flame speeds of stoichiometric mixtures versus flame radius. It is clearly shown that the more the hydrogen gas is added, the faster the flame propagates. For the fuel with (HVF=25%), the stretched flame speed increases with the increase of flame radius in most cases. However, in the case of the fuel with 75% vol. hydrogen the speed decreases slightly with the increase of flame radius. Thus, we can conclude that a stable flame is presented for natural gas–hydrogen flames under rich conditions, while the flame stability will decrease with the decrease of equivalence ratio (variation from rich mixture to lean mixture).

To obtain unstretched flame speed, the stretched flame speed versus the flame stretch rate was plotted (see Fig. 9). The unstretched flame speed Sl can be obtained by extending the measured data to the zero flame stretch rate, while the gradient of the Sn-α curve determines the value of Markstein length (Fig. 10). At all given equivalence ratios, the flame speed increased with the increase of hydrogen fraction.

The Markstein length also depends on the hydrogen fraction of the fuel. For the fuel with low hydrogen fraction (25% vol. in this study), the Markstein length tends to increase from negative value to positive one, indicating that the flame tends to become more stable. On the other hand, for the fuel with high hydrogen fraction (75% vol. in this study), the Markstein length tends to be slightly decreased with the increase of equivalence ratio.

Measurements showed that the addition of hydrogen can decrease the Markstein number of the laminar premixed hydrogen-NG flames as the flame tends to be unstable due to the effect of fast diffusing component (hydrogen), as shown in Fig. 9. Further study needs to be conducted to obtain a better understanding to the effect of the hydrogen-enriched NG/air on flames stability.

A comparison of unstretched laminar burning velocities versus equivalence ratios for methane–air flames (fig. 11), while the comparison of unstretched laminar burning velocities of hydrogen–air flames is illustrated in Fig. 12. The results show that the present work gives data consistent with those of others both for various methane, hydrogen-fraction flames and pure hydrogen flames. For methane–air flames,( as fig 11 shows) except for the data from Haung, which give values lower than those of others, the present work gives values consistent with other experimental results. For hydrogen–air flames (as fig.12 represents), except for the data from Aung, which give lower values compared to those of others, the present work gives values consistent with other experimental results. This proves the data correctness obtained from this study, as well as it provides data for wider range of equivalence ratio, compared with other works.

For methane– hydrogen–air flames at HVF=50%, comparable data are presented (fig. 13) between the present work and those of others, except for Tanoue whose values were approximately higher at rich equivalence ratios more than Ø=1.1. The figure also distinguishes this study wide range of equivalence ratios, and the increment of this range with increasing HVF.


118

Fig. 4: time versus flame radius for Fig. 5: time versus flame radius for lean equivalence ratio (Ø=0.5) stoichiometric equivalence ratio (Ø=1.0)

Fig. 6: time versus flame radius Fig. 7: unstretched flame speed for wide range for rich equivalence ratio (Ø=1.3) of equivalence ratios with variable hydrogen volume fractions

Fig. 8: Stretched flame speed versus flame Fig. 9: Stretched flame speed versus radius for variable hydrogen volume fraction the flame stretch rate at Ø=1

0

5

10

15

20

25

30

0 20 40 60

Fla

me

rad

ius

(mm

)

Time (ms)

Ø= 0.5, Pu= 1 bar, Tu= 300 K

HVF=0%

HVF=25%

HVF=50%

HVF=75%

HVF=100%

0

5

10

15

20

25

30

0 5 10

Fla

me

rad

ius

(mm

)

Time (ms)

Ø=1.0, Pu=1 bar, Tu=300K

HVF=0%

HVF=25%

HVF=50%

HVF=75%

HVF=100%

0

5

10

15

20

25

30

0 10 20 30 40

Fla

me

rad

ius

(mm

)

Time (ms)

Ø=1.3, Pu=1 bar, Tu=300K

HVF=0%HVF=25%HVF=50%HVF=75%HVF=100%

0

0.5

1

1.5

2

2.5

3

3.5

0 0.5 1 1.5 2

Un

stre

ched

fla

me

spee

d S

l (m

/s)

Equivalence ratio

Pu=1 bar, Tu= 300K

HVF=0%HVF=25%HVF=50%HVF=75%HVF=100%

0

0.5

1

1.5

2

2.5

3

3.5

4 14 24

Str

etch

fla

me

spee

d S

n (

m/s

)

Flame radius (mm)

Ø=1.0, Pu=1 bar, Tu=300 K

HVF=0%

HVF=25%HVF=50%

0

1

2

3

4

5

6

7

8

0 500 1000 1500

Sn

(m

/s)

α (s-1)

Ø=1.0, Pu=1 bar, Tu=300

HVF=0%

HVF=25%

HVF=50%

HVF=75%

HVF=100%


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Fig. 10: Markstein length versus Fig. 11: unstretched laminar burning velocity equivalence ratio for variable HVF’s versus equivalence ratio for NG

Fig. 12: unstretched laminar burning Fig. 13: unstretched laminar burning velocity velocity versus equivalence ratio for hydrogen versus equivalence ratio for hydrogen VF=50% CONCLUSIONS

Laminar flame characteristics of NG–hydrogen–air flames were studied in a constant volume

bomb at normal temperature and pressure, using thermocouples method. Laminar burning velocities and Markstein lengths were obtained at various ratios of hydrogen to NG (volume fraction from 0 to 100%) and equivalence ratios (Ø from 0.2 to 1.7). The influence of stretch rate on flame was also analyzed. The results are summarized as follows:

(1) For lean mixture combustion, flame radius increases with time, but the rate of increase decreases with flame expansion for NG and for mixtures with low hydrogen fractions, while at high hydrogen fractions, there exists a linear correlation between flame radius and time. For rich mixture combustion, there is also exists a linear correlation between flame radius and time for mixtures with high hydrogen fractions. Combustion at stoichiometric mixture demonstrates the linear relationship between flame radius and time for NG–air, hydrogen–air, and NG–hydrogen–air flames.

(2) Hydrogen–air flame gives a very high value of the stretched flame speed compared to those of natural gas–air flame and natural gas–hydrogen– air flames, even for high hydrogen fraction.

-1

0

1

2

3

4

5

0 0.5 1 1.5 2

Ma

rkst

ein

len

gth

Lb

(m

m)

Equivalence ratio

Pu=1 bar, Tu= 300 K

HVF=0%

HVF=25%

HVF=50%

HVF=75%

HVF=100%

0

5

10

15

20

25

30

35

40

45

0.4 0.9 1.4

Un

stre

tch

ed la

min

ar

bu

rnin

g

vel

oci

ty U

l

(cm

/s)

Equivalence ratio

0% Hydrogen, Pu=1 bar, Tu= 300K

Present workHaungBurlukaLiawKishore

0

50

100

150

200

250

300

350

0 0.5 1 1.5

Un

stre

tch

ed l

am

ina

r b

urn

ing

vel

oci

ty U

l (

cm/s

)

Equivalence ratio

100% Hydrogen, Pu= 1 bar, Tu= 300K

Present work

Haung

Dahoe

Lamoureux

Aung 0

10

20

30

40

50

60

70

80

90

100

0.2 0.7 1.2 1.7

Un

stre

tch

ed l

am

inar

bu

rnin

g

vel

oci

ty U

l (

cm/s

)

Equivalence ratio

50% Hydrogen, Pu=1 bar, Tu= 300K

Present workHaungGuBurlukaTanoue


120

(3) Enrichment of 25 & 50% hydrogen to NG-air has very little effect on burning velocities for lean and rich mixtures and remarkable effect on stoichiometric mixtures.

(4) Enrichment the mixture with hydrogen expands the equivalence ratio range and makes it wider.

(5) Unstretched laminar burning velocities are increased with the increase of hydrogen fraction. Markstein lengths are decreased with the increase of hydrogen fraction, indicating that the flame instabilities are increased with the increase of hydrogen fraction.

(6) For a fixed hydrogen fraction, the Markstein length and flame stability increase with the increase of equivalence ratios.

(7) Very good agreement is obtained between values of laminar burning speed and Markstein length values determined with the present methodology and values found in the literature.

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