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Insert Gas Dilution and Temperature Effects on Laminar Burning Velocity of DME+Air Mixtures Akram Mohammad, and Khalid Al Juhany Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b00739 • Publication Date (Web): 23 Apr 2018 Downloaded from http://pubs.acs.org on April 24, 2018
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Energy & Fuels
Insert Gas Dilution and Temperature Effects on Laminar Burning Velocity of DME+Air Mixtures Akram Mohammad*, Khalid A. Juhany Department of Aeronautical Engineering, King Abdulaziz University, Jeddah, Saudi Arabia
*Corresponding Author: Akram Mohammad Department of Aeronautical Engineering, Faculty of Engineering, King Abdulaziz University, Jeddah, Saudi Arabia, 21589 Fax: +966-12-2641686 Email:
[email protected],
[email protected] Abstract: The influence of blending the mixture of dimethyl ether and air using inert gases and initial temperature on laminar burning velocity (LBV) was investigated experimentally at ambient pressure. In the present work, premixed planar (flat/one dimensional) flames of laminar nature at various temperatures were achieved using an increasing aspect-ratio channel by placing an electric heater externally beneath the channel made of quartz material. These tests were performed for a wide range of mixture compositions with equivalence ratio varying (0.8 ≤ Φ ≤ 1.3) and temperature ranging from 330-570 K. Comparison between the obtained burning velocities with the numerical prediction and detailed flame structure using Zhao mechanism for dimethyl ether+air mixtures were also made which were validated against the experiments very well. The results show the burning velocity to decrease monotonously with the increase in percentage dilution for both inert gas dilution while the peak burning velocity obtained is for somewhat rich mixture for all the dilution cases. In addition to this, the trend of power-law temperature exponent α (from curve fitting) with the diluted mixture equivalence ratio (Φ) is also reported. The addition of CO2 has more deterring influence mutually on LBV as well as on α compared to N2 addition.
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Keywords: laminar burning velocity; dimethyl ether; inert gas dilution; temperature exponent; CO2 utilization; etc
1. Introduction:
Environmental pollution is increasing at an alarming rate over the past few decades due to the excessive amount of exhaust emission, particularly CO and NOx emissions which intensified the search for high-efficient, low-emission alternative fuels for the automobile industry in recent years. An array of studies has been dedicated to understand and explore the new mechanisms for the better utilization CO2 1,2 from the combustion of hydrocarbon fuels. The exhaust gas recirculation (EGR) or mixture dilution techniques have been widely used to further reduce the NOx emissions and the combustion temperature
3–9
. The other forms of
techniques for the ultra-low emission also includes the ‘flameless’ or “moderate or intense low-oxygen dilution” (MILD) or HiCOT combustion in which the lean reactant mixture is preheated to its auto ignition temperature which substantially lowers the oxygen concentration, also the adiabatic temperature of the non-visible flame front will be less than the actual mixture temperature, this can also be achieved by recirculating (dilution) the product gases into fresh unburned mixture which increases the mixture temperature before combustion and also reduces the oxygen concentration 10–16. This sort of study is essential in designing the combustion devices which operates at premixed lean conditions as MILD combustion helps in temperature uniformity and efficiency along with reduced hazardous emissions.
Dimethyl ether (as a clean fuel) has received great attention in recent times and has been termed as a promising non-conventional fuel 3,17–26. The LBV of the dimethyl ether and air mixture has been studied experimentally by various researchers 17–19,21,22,27–30. The LBV of non-diluted dimethyl ether and air mixtures is presented in Fig. 1. Although most of them used the spherical vessel method
17,19,22,27–29
, Zhao et al.
18
incorporated the jet-plate
stagnation flame technique with the linear extrapolation method, while Wang et al.
21
incorporated the counterflow method with nonlinear extrapolation to report the LBV of dimethyl ether and air mixtures. The reported LBVs of the pure dimethyl ether and air blends have a large scattering at ambient conditions with its LBV maximum at slightly rich mixture 2 ACS Paragon Plus Environment
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as shown in Fig. 1. The LLNL mechanism
31
and Aramco mech 1.3
32
overestimate the
burning velocities for near stoichiometric and rich mixtures compared to Zhao et al. 34
Wang et al.
33
and
mechanisms and recent experimental data.
55 DME + Air T = 300 K 50 u Pu = 1.0 atm 45
Daly 2001 Zhao 2004 Qin 2005 Huang 2007 Chen 2009 Wang 2009
40 35 30
SL, cm/s
25
0
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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20 de Vries 2011 Chen 2012 Song 2013 Wu 2014 Yu 2014 Varghese 2017
15 10 5 0
Wang mech Zhao mech Aramco mech 1.3 LLNL mech
0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9
φ
Figure 1. The LBV of non-diluted dimethyl ether + air mixtures at ambient temperature and pressure in the literature. Lines: Computational results:
31–34
. Symbols: experiments.
17–
19,21,27,30,35–39
.
Recently Varghese et al. 30 measured the LBV of pure dimethyl ether and air mixtures at various high initial temperatures utilizing the diverging channel method. They extrapolated the high-temperature data to ambient conditions using popular power-law correlation. Their magnitudes were in good with the available literature at ambient pressure and temperature conditions. The presence of inert gases in DME used in engines with the exhaust gas recirculation (EGR) practice lowered the combustion instabilities and diminish NOx emissions
4,20,37
. In the meantime, the pollution and safety norms have stimulated
investigations on combustion characteristics of inert gas diluted fuel+air mixtures. Collectively, these reasons elucidate the rising interest in LBVs of dimethyl ether and air at different initial mixture temperatures and inert gas dilutions. The worsening effects of inert gas dilution on LBVs were reported for CH4+air 40 and C3H8+air 41 mixtures. However, very 3 ACS Paragon Plus Environment
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few studies are available on the effect of CO2/N2-dilution on dimethyl ether and air mixture in the literature18,20,37,42. However, the effect of these dilutions at various high initial temperatures for an array of mixture compositions is not available in the literature. The objective of the present effort is to determine the impact of CO2 and N2 gas dilutions separately on the LBV of dimethyl ether and air mixtures for a range of mixture compositions and initial mixture temperatures. The diverging channel method with increasing aspect-ratio introduced by Akram et al.
42–44
and later confirmed by Varghese et al.
30
is utilized in the
present investigations. 2. Experimental Methodology:
In this present work, an externally heated mesoscale rectangular diverging channel with a diverging angle of 10° and inlet dimensions of (25mm × 2mm) was chosen. This high aspect ratio channel is made up of a quartz material and was chosen due to high specific heat capacity and optical transparency for the flame visualization. A schematic diagram of the complete experimental setup is presented in Fig. 2. MFC
Gas lines
MFC
Pressure regulators
Mixer
Diverging channel
MFC
Manual valve
Electric heater
Signal lines
Pressure gauge
Diluent
Air
Control module
DME
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Figure 2: Schematic of the experimental setup. Red lines: mixture flow, blue lines: signal flow.
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External preheating was provided to the channel, and hence to the mixture, by coilplate type electric heater, was placed below the channel. The overlap distance and elevation to the channel was maintained at 20 mm, a constant power was supplied to the heater through a power controller. This external heating steadies the flat flames at various initial mixture temperatures. The temperature of the gas (bottom wall inside surface the channel) was measured with external heating conditions and without feeding fuel in both transverse and axial directions to check the uniformity using a K-type thermocouple and the measured temperature has uncertainty under ± 5 K of the real magnitudes. The CO2 or N2 diluted dimethyl ether and air mixture were then supplied to the combustor section via suitable mass flow electric controllers. This diluted unburned blend gets ignited near the exit of the channel where the temperature reaches its auto-ignition temperature. The flame front moves against the flow and stands still wherever the local flow velocity equals the laminar burning consumption rate or velocity at that specific initial temperature. The one-dimensional stabilized flat flame front position was captured using a digital camera. The wall temperature measurement at the stabilized flame location was carried out using a K-type thermocouple. Complete particulars of the investigation system and data acquisition are available in ref. 43,41,44. The detailed flame dynamics inside the high aspect ratio diverging channels have been studied by Akram et al.
45
. The thermal-wall coupling between wall temperature and
flow field affect the shape of a flame. The formation of planar flame inside the channel is governed by the uniformity of temperature and velocity profile which confirms the existence of low hydrodynamic strain or near zero stretch 45,46. These planar flames were later used for extracting the LBV of particular fuel-air mixtures. The stabilized planar flames at different locations for different mixture velocities were used for extracting the LBVs of the combustible mixtures at different conditions. The position of the stationary planar flame inside the diverging channel is captured using a digital camera and the exact local flame area ( ) was calculated. It is assumed here that the unburned gases attains the wall temperature ( ) of the channel. The burning velocity is obtained from mass conservation relation of combustible mixture entering the stabilized flame front as follows. = ⁄ ⁄,
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(1)
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The key parameters which influence the burning velocity are boundary layer effects, thermal feedback from flame front to the reactants through walls, the difference between wall and the reactant temperature, and thermal management between the reaction zone and thermally thick channel walls. These key parameters were included in the present study to compute the overall uncertainty in the measurement. Suitable mass flow controllers (MFCs) with flow rates of the blends ~70-80% of the full-scale were chosen to reduce the uncertainty in equivalence ratios. The overall effect of these constraints extends the uncertainty to lower than ±5% in the present work. Detailed analyses of uncertainty estimation have been described elsewhere 43,41
3. Computational Method: The LBV computations are executed for steady adiabatic freely propagating flames. The PREMIX solver resolving these flames
47
was incorporated. The thermodynamic and
transport properties of the mixtures studied in present computations were evaluated with available respective databases
48,49
. The transport data is calculated considering the diffusion
of multi-component and temperature gradient based diffusion. The upstream difference scheme is chosen in calculations. This is helpful for refining and adapting the grid. The grid independence was assured with GRAD and CURV values of 0.01 and 0.05 respectively. Recently, Varghese et al. 2017 30 reported that two DME+air reaction mechanisms by Wang mech
21
and Zhao mech
33
predict similar results even at high temperatures. Due to this the
detailed DME+air kinetic mechanism proposed by Zhao et al. 33 is used in present work. To investigate the influence of CO2 and N2 a pseudo-CO2 analysis is also carried out along with sensitivity analysis. These analyses have been presented in the following sections.
4. Result and Discussion:
The present work emphasizes to understand the influence of inert gas dilutions on LBVs of dimethyl ether and air mixture blends. The LBVs of these mixtures were obtained for a wide range of mixture compositions (Ф = 0.7 - 1.4) and different elevated mixture temperatures (330 - 570 K). The inert dilution is conveyed in the percentage of the volume supplemented gas to the volume of combined DME and inert gas. For instance, a 30 % N2 6 ACS Paragon Plus Environment
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addition means that the total fuel feed line consists of 30 % N2 and 70 % dimethyl ether. The percentage dilution ( )is calculated as the ratio of the volume flow rate of diluent inert gas added in the fuel stream to the total volume flow rate of the fuel stream including diluents 41,50
. The is then subsequently given by the expression: Percentage dilution ( )=
Volume of the inert gas diluent
(2)
Volume of (fuel + inert gas diluent)
The experimentations were made at ambient pressure and different initial temperatures. The variation of LBV at any temperature was plotted alongside the ratio of mixture initial temperature and the initial reference temperature , by applying the power-law correlation as follows. !
= × ⁄,
(3)
The derived LBV at ambient pressure and temperature and the temperature exponent " can then be utilise to obtain the LBV at any unburned mixture temperature within the range over which the experiments are carried out.
4.1 Laminar burning velocity at ambient conditions:
50 45
Tu = 300 K Pu = 1.0 atm
40 35
SL, cm/s
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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30 25 20 PURE DME + Air 90% DME + 10% CO2+Air 80% DME + 20% CO2+Air 70% DME + 30% CO2+Air
15 10 0.6
0.7
0.8
0.9
1.0
1.1
1.2
1.3
Φ
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1.4
1.5
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Figure 3: Variations of LBV of CO2 diluted dimethyl ether + air blends at ambient conditions. Symbols-present experiments (power-law extrapolated data), lines-predictions of Zhao et al. mechanism 33. The LBVs of non-diluted and CO2 added dimethyl ether + air blends are shown in Fig. 3. The symbols indicate the present experiments while lines represent a computational prediction of Zhao et al. 2008 mechanism. The experimental data for pure dimethyl ether and the air is taken from Varghese et al. 2017
30
. The maximum LBV is witnessed for a marginally rich
mixture of Φ = 1.1 decreases on both sides of the stoichiometry for all CO2 diluted mixtures. The numerical predictions and experimental results differ slightly low dilution cases. However, this difference increases with dilution rates.
50 45
Tu = 300 K Pu = 1.0 atm
40 35
SL, cm/s
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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30 25 20 PURE DME + Air 90% DME + 10% N2+Air 80% DME + 20% N2+Air 70% DME + 30% N2+Air
15 10 0.6
0.7
0.8
0.9
1.0
1.1
1.2
1.3
1.4
1.5
Φ
Figure 4: Variations of the LBV of N2 diluted dimethyl ether + air blends at ambient conditions. Symbols-present experiments (power-law extrapolated data), lines-predictions of Zhao et al. mechanism 33.
Figure 4 shows the results of the LBV variation with fuel and equivalence ratio for different inert gases dilution cases of N2. The magnitude of the LBV of the diluted N2 mixture decreases with the increase in dilution ratio. The occurrence of maximum LBV has witnessed again at Φ = 1.1 for N2 dilution as well as different dilution levels of 10, 20 and 30 8 ACS Paragon Plus Environment
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%. The effect of CO2 dilution was evident compared to the dilution of the N2 case. The numerical predictions are slightly higher throughout the mixture compositions compared with the power-law extrapolated magnitudes. The predictions of
Zhao et al. mechanism
33
matches well and are in rationally good covenant with the obtained magnitudes, except at higher percentage inert dilutions. The slight over predictions of the LBV for higher dilution cases indicate the need for modification of mechanism for higher dilution ratio conditions.
4.2 Temperature exponent of various blends: Temperature exponent (α) is a resilient function of mixture composition which is obtained by using the power-law correlation. The trend of α in the present experiment is observed to be minimum for marginally rich mixtures for all the dilution cases, where the LBV is observed to be maximum. The magnitudes of the α were perceived to escalate with the dilution rates for both the inert gasses. The trend of α for CO2 dilution case is shown in Fig. 5 and N2 dilution is shown in Fig. 6.
2.2
2.1
2.0
1.9 α
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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1.8
1.7 PURE DME + Air 90% DME + 10% CO2+Air 80% DME + 20% CO2+Air 70% DME + 30% CO2+Air
1.6
1.5 0.6
0.7
0.8
0.9
1.0
1.1
1.2
1.3
1.4
1.5
Φ
Figure 5: Variations of temperature exponent (⍺) of CO2 diluted dimethyl ether + air blends. Symbols-present experiments (Tu = 330-570 K), lines-predictions of Zhao et al. mechanism 33
.
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A higher value of alpha for the lean and rich mixtures suggested the higher LBV sensitivity to the unburned mixture temperatures. This behavior resembles higher changes in adiabatic flame temperatures of these lean and rich dimethyl ether and air mixtures compared to near stoichiometric mixtures for a specific increase in unburned gas temperature. This could be due to dissociation of stable species at higher temperatures. 2.2
2.1
2.0
1.9 α
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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1.8
1.7 PURE DME + Air 90% DME + 10% N2+Air 80% DME + 20% N2+Air 70% DME + 30% N2+Air
1.6
1.5 0.6
0.7
0.8
0.9
1.0
1.1
1.2
1.3
1.4
1.5
Φ
Figure 6: Variations of temperature exponent (α) of N2 diluted dimethyl ether + air blends at ambient conditions. Symbols-present experiments (Tu = 330-570 K), lines-predictions of Zhao et al. mechanism 33.
The magnitudes of the α were found to be slightly higher for all the CO2 dilution cases compared with the non-diluted and N2 inert gas diluted dimethyl ether and air blends. This could due to the higher specific heat capacity of CO2 as compared to N2 and thus absorbs more the released heat which results in a larger drop in temperature of the flame front compared to N2 dilution case 20.
4.3 Laminar burning velocities at elevated temperatures
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Many practical devices work at high initial temperatures compared to ambient temperature. Figure 7 shows the LBV of dimethyl ether + dilution + air blends at ambient pressure and different mixture temperatures of 300 K, 373 K, 473 K, and 573 K for various dilutions of CO2. LBV increases with initial unburned gas temperatures for all the reported conditions. A reasonably good match can be observed seen between different measurement data and simulations at all unburned gas temperatures except at 573 K for 20 % and 30 % dilution mixtures.
140 PURE DME+Air
140 90% DME + 10% CO2 + Air
120
120
Tu = 300 K Tu = 373 K Tu = 473 K Tu = 573 K
80
80 SL, cm/s
100
SL, cm/s
100
60
60
40
40
20
20
0 0.6 0.7 0.8 0.9 1.0 140 80% DME + 20% CO2 + Air
1.1
1.2
1.3
1.4
1.5
0 0.6 0.7 0.8 0.9 1.0 140 70% DME + 30% CO2 + Air 120
100
100
80
80 SL, cm/s
120
SL, cm/s
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60
40
20
20
0.7
0.8
0.9
1.0
1.1
1.2
1.3
1.4
1.5
1.2
1.3
1.4
1.1
1.2
1.3
1.4
60
40
0 0.6
1.1
0 0.6
0.7
0.8
Φ
0.9
1.0 Φ
Figure 7: The LBV of CO2 diluted blends of dimethyl ether and air at ambient and various initial temperatures. Symbols-present experiments (power-law interpolated data), lines-predictions of Zhao et al. mechanism 33.
Similarly, Fig. 8 shows the LBV of dimethyl ether and air mixtures at ambient pressure and different mixture temperatures of 300 K, 373 K, 473 K, and 573 K for various dilutions of N2. The LBV rises with rise in unburned gas temperature for various reported conditions. A 11 ACS Paragon Plus Environment
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good match can be seen between different measurement data and simulations at all unburned gas temperatures except at higher temperatures for 30 % dilution mixtures.
140
PURE DME+Air
120
140 90% DME + 10% N2 + Air
Tu = 300 K Tu = 373 K Tu = 473 K Tu = 573 K
120
80
80 SL, cm/s
100
SL, cm/s
100
60
60
40
40
20
20
0 0.6 0.7 0.8 0.9 1.0 140 80% DME + 20% N2 + Air
1.1
1.2
1.3
1.4
1.5
0 0.6 0.7 0.8 0.9 1.0 140 70% DME + 30% N2 + Air 120
100
100
80
80 SL, cm/s
120
SL, cm/s
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60
40
20
20
0.7
0.8
0.9
1.0
Φ
1.1
1.2
1.3
1.4
1.5
1.2
1.3
1.4
1.1
1.2
1.3
1.4
60
40
0 0.6
1.1
0 0.6
0.7
0.8
0.9
1.0 Φ
Figure 8: The LBV of N2 diluted dimethyl ether + air blends at ambient and various unburned gas temperatures. Symbols-present experiments (power-law interpolated data), lines-predictions of Zhao et al. mechanism 33.
4.4 Heat release rates:
The effect of various parameters for example temperature, inert dilution and the mixture equivalence ratio on LBV was studied in detail using the premix code mechanism
51
and Zhao et al.
33
. The overall discrepancy of total heat release rate is plotted against the non-
dimensional temperature defined as (τ = (T-Tu)/(Tad-Tu), where T is instantaneous 12 ACS Paragon Plus Environment
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temperature, Tu is initial mixture temperature and adiabatic flame temperature Tad ) for pure and diluted cases as shown in Fig. 9. The net heat release rate is relatively higher for pure dimethyl ether and air mixtures at higher unburned gas temperatures and decreases with an increment in dilution at same unburned gas temperature. The inert gas dilution reduces the heat release rate, which acts as a heat absorbent. Due to this, the ultimate heat release rates shift slightly towards higher non-dimensional temperature and necessitates additional heat to produce radicals which result in the delay in chain initiation.
14
PURE DME Tu = 300 K PURE DME Tu = 500 K 70% DME + 30% CO2 Tu = 500 K 70% DME + 30% N2 Tu = 500 K
12
3
10 Heat release rate, MW/m
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8
6
4
2
0 0.0
0.1
0.2
0.3 0.4 0.5 0.6 0.7 Non-dimensional temperature τ
0.8
0.9
1.0
Figure 9: The heat release rates of diluted dimethyl ether + air blends at two different temperatures. Lines-calculations of Zhao et al. mechanism 33.
4.5 Thermal and chemical effect of CO2 dilution on LBV:
The LBV of dimethyl ether and air mixture decreases with the increase in percentage dilution of inert gas. The presence of CO2 has better extinguishing effect on the variation of LBV as compared to the presence of N2. Both thermal, as well as chemical effects, may lead to this drop in reactivity and exothermicity. Generally, the addition of CO2 reduces the 13 ACS Paragon Plus Environment
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concentration of the active species, particularly H and OH (light and highly reactive) radicals, and increases the specific heat capacity of the burned gases, thus reducing the temperature of the flame. To further evaluate the effect of chemical and thermal influences of CO2 presence on LBV, a numerical study was carried out using (i) normal CO2 participating in the reaction as chemically inert gas and (ii) fake-CO2 or FCO2 or pseudo CO2 not participating in the reaction with same thermophysical properties as that of normal CO2. The LBVs calculated with the one-dimensional code and detailed reaction mechanism of Zhao et al. 33 were used as a reference here. The difference in LBV for N2 diluted dimethyl ether and air mixture and the diluted FCO2 and CO2 mixture gives the thermal and chemical effect respectively. The difference in chemical and thermal effects for the diluted cases is shown in Fig. 10. 110
70% DME + 30% N2 Tu = 500 K 70% DME + 30% CO2 Tu = 500 K 70% DME + 30% F-CO2 Tu = 500 K
100
Thermal effect Chemical effect
90 SL, cm/s
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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80
70
60 0.6
0.7
0.8
0.9
1.0
1.1
1.2
1.3
1.4
1.5
Φ
Figure 10: LBV of N2, F-CO2, and CO2 diluted dimethyl ether + air blend at 500 K. Linespredictions of Zhao et al. mechanism 33.
Taking N2 dilution as a reference, the relative involvement of chemical and thermal effects at two different percentage dilutions (10% and 30%) at 300 K and 30% at 500 K is plotted for a wide range of mixture compositions as shown in Fig. 11. The change in LBV due to thermal effect is estimated with the subsequent expression of Galmiche et al. 52. 14 ACS Paragon Plus Environment
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$% ('( ))$% (*+,( )
(5)
$% ('( ))$% (+,( )
The chemical effect of the diluted mixture is obtained using the same correlation in the form (1− [ (./ ) − (012/ )⁄ (./ ) − (12/ )]). Due to the high heat capacity of the diluted mixtures, the adiabatic temperature of the inert gas diluted blend decrease as an intensification in the percentage dilution, hence, the thermal effect becomes more dominant with an increase in dilution. The increase in unburned gas temperature mitigates the thermal effect of CO2 for a fixed percentage dilution. Whereas, the increase in dilution of CO2 exaggerates the thermal effect of CO2 for a fixed initial temperature. The dilution diminishes the number of molecules which lacks the energy to deplete the barrier of activation energy. Whereas an increase in temperature enhances the number of molecules that have enough activated complex with sufficient energy to overpower the activation energy barricade. Therefore, the chemical effect increases while thermal effect decreases. This influence is significant for lean mixture compared to rich mixture due to the presence of enough O2. 100 90
DME+Air Pu = 1.0 atm
10% CO2, 300 K 30% CO2, 300 K 30% CO2, 500 K
80 70 Percentage thermal effect
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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Chemical effect
60 50 40 30 20 Thermal effect 10 0 0.6
0.7
0.8
0.9
1.0 Φ
1.1
1.2
1.3
1.4
Figure 11: Percentage thermal effect (pattern bar) and chemical effect (empty bar) on the LBV of the 10 % and 30 % CO2 diluted dimethyl ether + air blends at different temperatures compared to N2 dilutions. 15 ACS Paragon Plus Environment
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A sensitivity analysis is performed using one-dimensional code (CHEMKIN) to identify the dominant reactions that show dependence on the prediction of LBV. The sensitivity coefficients for critical reactions in the case of pure DME at 300 K, 500 K, 30% N2 and 30% CO2 at 500 K are shown in Fig. 12. The figure 12 shows that the most contributing reactions for DME decrease when the temperature is increased from 300 K to 500 K. But, the comparison of the reaction rate coefficients in 30% N2 and 30% CO2 diluted DME at 500K show that the sensitivity of major contributing reactions is more for CO2 dilution than that for N2 dilution. This indicates that the change in burning velocity of DME is more affected due to CO2 dilution.
Figure 12: Sensitivity coefficients for critical reactions that affect burning velocity predictions
5. Conclusion:
The LBV of dimethyl ether (DME) + inert gas dilution + air blends at various high temperatures are measured and analysed over a range of mixture compositions using the 16 ACS Paragon Plus Environment
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Energy & Fuels
diverging channel setup. Detailed reaction mechanism predicts well the LBV of pure dimethyl ether and air mixtures and lower dilution cases at high temperatures. However, for high percentage dilutions, somewhat over-estimates the LBV and under-estimates the α. The presence of both inert gases devalues and lowers the magnitudes of LBV of dimethyl ether + air blends substantially.
CO2 presence having a superior quenching effect on LBV of
dimethyl ether + air mixtures due to a combination of both chemical and thermal effects in comparison with the presence of N2 where the thermal effect is prevalent. A higher presence of these inert gases enhances the thermal influence whereas mitigates the chemical effects. Inert gas dilution to the dimethyl ether and air mixture as in EGR needs to be investigated prudently.
Acknowledgment: This present work was supported by Deanship of Scientific Research (DSR), King Abdulaziz University, Jeddah, under grant No. (D-052-135-1438). The authors, therefore, gratefully acknowledge the DSR technical and financial support.
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