Autoignition of Dimethyl Ether and Air in an Optical Flow-Reactor

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Autoignition of Dimethyl Ether and Air in an Optical Flow-Reactor Alessandro Schönborn,*,† Parisa Sayad,† Alexander A. Konnov,‡ and Jens Klingmann† †

Division of Thermal Power Engineering, Lund University, Box 118, SE-221 00, Lund, Sweden Division of Combustion Physics, Lund University, Box 118, SE-221 00, Lund, Sweden



S Supporting Information *

ABSTRACT: Autoignition of dimethyl ether in air was studied in a turbulent flow-reactor with optical access, at conditions relevant to micro gas turbine combustors. The ignition process was visualized using OH*-chemiluminescence imaging, showing the formation of multiple autoignition kernels along the central axis of the reactor. Ignition delays in the range of τ = 112−310 ms were measured at temperatures of T = 739−902 K, pressures of P = 0.2−0.4 MPa, equivalence ratios of ϕ = 0.225−0.675, and initial flow velocities of Ui = 8−46 m/s. The effect of adding nitrogen to the reactants as a diluent was investigated for mole fractions of additional nitrogen ranging from 0 < XN2 < 0.1. The experimental ignition delays were compared with homogeneous gas-phase chemical kinetic modeling. Comparison between the modeling and experiments showed significant discrepancies, but agreement was improved when heat transfer in the reactor was taken into account in the modeling.

1. INTRODUCTION

Autoignition delays for second stage ignition of DME have not been reported at conditions of low pressure (1000 K). Wada et al.11 carried out measurements of first stage ignition at atmospheric pressure and temperatures of 555−585 K. © 2014 American Chemical Society

2. MATERIAL AND METHODS 2.1. Experimental Apparatus. The experiments were carried out in a pressurized and optically accessible flow-reactor of circular crosssection. An overview of the reactor can be seen in Figure 1. The experimental apparatus consisted of an air supply system delivering air to the flow-reactor at variable temperature and pressure, a fuel system supplying pressurized DME at room temperature, a pipe of circular cross-section representing the test-section of the flowreactor, and an exhaust system, in which the gas mixture was cooled by water injection and subsequently expanded to atmospheric pressure by means of an exhaust throttling valve. The reactor test-section was heavily insulated with glass-wool and aluminum foil, but contrary to a previously published investigation,12 no electric heating of the testsection was used to reduce the heat loss of the reactants to the walls. This resulted in a significant heat loss along the reactor, as shall be discussed in detail in section 3.5. The overall length of the test-section was 4.45 m. A centrally located axial fuel jet represented the first point of fuel injection and was defined as the start of the reactor test-section. The point of water injection was defined as the end of the test-section. DME was added to the air using one centrally located axial fuel jet ahead of a Venturi tube and four radially inward pointing jets (⌀ = 1 mm) located in the Received: December 16, 2013 Revised: April 30, 2014 Published: April 30, 2014 4130

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converging section of the Venturi tube. The Venturi tube constricted the diameter of the tube from 23.7 mm to 11.85 mm. A more detailed description of the test apparatus, a schematic of the injector, and a typical flow velocity profile through the reactor can be found in an earlier paper.12 The straight part of the circular test-section consisted of a pipe having an internal diameter of 23.7 mm. The optically accessible section of the flow-reactor was located at the end of this test-section and featured a transparent quartz tube of 135 mm length and wallthickness of 2.5 mm. The quartz tube was located within a pressurized chamber that had three quartz windows of 100 mm length, 30 mm height, and 20 mm thickness, through which the inner quartz tube could be observed. Apart from the quartz tube and windows, the material of the test-section was austenitic alloyed steel (AISI 253 MA). The test-section was instrumented with Chromel−Alumel (K-type) thermocouples for temperature measurements. A diaphragm pressure sensor (Tecsis E113) was used to measure static pressure within the tube at a distance of 69 mm upstream of the quartz tube. The inlet temperatures of DME and air into the reactor were measured using Ktype thermocouples. The mass flow rate of air was measured using a Coriolis mass flow meter (Micromotion), and the mass flow rate of DME was measured using a thermal mass flow meter (Bronkhorst). The air used was compressed using a screw compressor (Ceccato CSB 25-13), cooled, and filtered to keep impurities below 0.1 ppm. The DME was supplied from bottles at a mass purity of ≥99.9%. A photoelectric cell (Hamamatsu UVtron R9454) was used to sense the emission of ultraviolet (UV) light at wavelengths of 185−260 nm, through the top window of the optically accessible section of the flowreactor. A high-speed CMOS camera (Vision Research Phantom V 7.1) was used in conjunction with an image intensifier (Hamamatsu C4598), a band-pass filter (Acton Research 310.5 ± 5.75 nm), and a phosphate glass lens (UV-Nikkor 105 mm, f/4.5) to photograph OH*chemiluminescence of the flame around 306 nm. OH*-chemiluminescence images were recorded at a resolution of 512 × 512 pixels, at a frame rate of 4000 frames per second, each frame having an exposure time of 100 μs. 2.2. Experimental Method. The experimentally determined ignition delay time τ of the mixture at a specific reaction temperature, pressure, and equivalence ratio was defined as the critical plug-flow residence time in the reactor necessary to initiate autoignition. The temperature used to correlate the data was the adiabatic mixing temperature (Tad.m.) of DME and air. This mixing temperature was calculated using the inlet temperature and mass flow rates of fuel and air respectively, assuming adiabatic conditions during the mixing process. The experimental procedure for determining the critical residence time was as follows: the mass flow rate of the reactants in the test section was gradually decreased in small steps, as the mixture pressure, equivalence ratio, and temperature were kept constant. The limiting residence time, at which UV light was detected by the photoelectric cell, was recorded as the critical autoignition delay. When second-stage ignition was detected by UV-light, the fuel flow was terminated about one second later, and the temperatures in the reactor where stabilized again, in order for residual heat in the walls flow variations from the foregoing experiment not to affect the ensuing experiments. The residence time of the reactants was calculated by assuming plug-flow conditions. This is similar to the procedure reported in the earlier autoignition studies carried out in flow-reactors (Freeman and Lefebvre;13 Lefebvre et al.;14 and Beerer and McDonell15). 2.3. Image Processing. The OH*-chemiluminescence was recorded in 12-bit grayscale images with a resolution of 512 × 512 pixels. The images were corrected for background noise, calibrated spatially to the dimensions of the reactor tube, and false colored for improved clarity. 2.4. Modeling. Chemical kinetic modeling of autoignition in the reactor was carried out using homogeneous gas-phase reactions at constant pressure using the plug-flow reactor (PFR) model in Chemkin. Some of the calculations assumed simple adiabatic conditions, while in the more accurate calculations, heat transfer was included in the modeling. The peak fraction of OH radicals was used

no N2 75% 100−700 555−585 0.835 flow reactor Wada et al.11

Li et al.

8

0.1

Ar 93−95% 0.06−3 1100−1500 0.5−2 shock tube Hu et al.10

2

Ar 92% 0.01−1.33 1200−1600 0.12−0.53 1 shock tube Hu et al.9

N2 71−86% 0.16−2.91 697−1239 2.2−2.3 0.5−1.5 shock tube

Ar 95% 0.08−3 1134−2105 0.1−1 1 shock tube Tang et al.7

Cook et al.6

Dagaut et al.

yes

yes no

yes no

yes yes

yes no

yes no

pressure & laser absorption pressure & photomultiplier pressure & photodetector pressure & photodetector pressure & photodetector thermocouples Ar 93−97.5% 80−3000 1175−1900 0.16−0.67

N2 27−32% 0.1−100 615−735 1−2 0.43−1.5

rapid compression machine shock tube Mittal et al.5

Ar 0.006−27 1200−1600 0.35 0.5−2 shock tube

1 Pfahl et al.3

4

shock tube

0.5−2

yes yes

yes no

yes

yes

not specified not specified steel, chrome stainl ess steel not specified stainl ess steel not specified not specified stainless steel

87

Article

pressure & photomultiplier pressure & photomultiplier pressure N2 72% 0.07−10 650−1250 1.3 and 4.0

P [Mpa] ϕ technique source

Table 1. Autoignition Delay Studies of Dimethyl Ether

T [K]

τ [ms]

diluent [mol/mol]

detection

first stage ignition

second stage ignition

reactor surface

reactor cross-section [mm]

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Figure 1. Flow reactor schematic. (1) Air inlet from compressor, (2) safety burst disc, (3) electric air heater, (4) thermocouples, (5) fuel injector, (6) ignition delay section, (7) pressure transducer, (8) cooling air for outer quartz windows, (9) outer quartz windows, (10) inner quartz tube, (11) water injector, (12) air dilution, (13) throttling valve, (14) exhaust duct. to determine the point of ignition in the simulations. Details of the modeling are indicated in the respective results section. All calculations were performed using the gas-phase reaction mechanism of Zhao et al.16 This mechanism had demonstrated good performance in reproducing product formation for flow reactor studies of Alzueta et al.,17 Fischer at al,.18 and Curran et al.,19 and the well-stirred reactor study of Dagaut et al.4 It has also been shown to perform well in predicting the ignition delays measured in shock tubes by Dagaut et al.,4 Cook et al.,6 Pfahl et al.,3 Tang et al.7 Li et al.,8 and Hu et al.9,10 2.5. Uncertainty Analysis. The uncertainty in reaction temperature and pressure, equivalence ratio, and the ignition delay, occurred due to measurement uncertainty, heat transfer from the reactants to the reactor walls, and deviation from plug-flow conditions. The mixing temperature Tad.m. of the reactants was determined using an energy balance of the mixing flows and was estimated to have a maximum relative uncertainty of 6%, and a maximum absolute uncertainty of 49 K, based on temperature and mass flow measurements. The reaction pressure bore an uncertainty of 5%, based on pressure measurements and limitations in controlling the pressure in the reactor. The equivalence ratio was accurate to 2%, based on mass flow stability. Heat transfer coefficients were estimated to have a total uncertainty of 6%. This was the result of an error of 3.8% resulting from changes in Nusselt number and Prandtl number as a result of changing the medium from a fuel air mixture to a pure air mixture for heat transfer measurements, as well as an error of 4.6% incurred as a result of heat conduction from the thermocouples during temperature measurements. The ignition delay was estimated to have a maximum uncertainty of 63% by assuming plug-flow conditions through the reactor. This uncertainty in ignition delay was calculated by adding the variance in measurement (10%) used to calculate the plug-flow residence time, to the variance in autoignition delay resulting from using a finite mixing time (15.3 ms) of the reactants during fuel injection. The variance in plug-flow residence time was calculated on the basis of temperature, pressure, mass flow, and spatial dimension measurements. The variance in autoignition delay resulting from using a finite mixing time was estimated using a transient gas-phase chemical kinetic model. This model assumed that during a finite mixing time, fuel mixed with air causing the equivalence ratio to progress from infinity to that of the final mixture of reactants, and air mixed with fuel causing the equivalence ratio to progress from zero to that of the final mixture. This was done in order to account for radial mixing and diffusion of the fuel and air close to the point of fuel injection. A detailed description of this model has been reported in an earlier paper.12

Table 2 gives an overview of the calculated error incurred as a result of fuel and air mixing at the beginning of the flow reactor and the total

Table 2. Calculated Uncertainty in Ignition Delay τ Due to Fuel and Air Mixing within the Reactor Tm [K]

P [Mpa]

ϕ

error due to fuel and air mixing [%]

total error [%]

855 882 902 796 823 840 739 756 783 823 853 864 760 763 787

0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.2 0.2 0.2 0.4 0.4 0.4

0.225 0.225 0.225 0.45 0.45 0.45 0.675 0.675 0.675 0.45 0.45 0.45 0.45 0.45 0.45

0.37 −5.49 −11.49 10.64 12.82 10.67 62.07 42.92 27.31 5.44 −4.18 −15.9 39.75 36.88 25.53

9.87 11.29 15.14 14.51 16.17 14.53 62.85 44.04 29.04 11.26 10.71 18.71 40.95 38.18 27.37

error. The error due to fuel air mixing was markedly higher in the present experiments with DME (62%), than in experiments conducted with propane (1.7%), and strongly contributed to the discrepancy between experiment and modeling. The propane study was in fact a later study, and a further reasons for the improved accuracy of the experiments conducted with propane12 was that electric heating of the test-section provided for a more uniform temperature distribution, and reduced uncertainty in temperature of the reactants.

3. RESULTS Autoignition delays of dimethyl ether in air were measured at temperatures of Tad.m. = 739−902 K, pressures of P = 0.2−0.4 MPa, equivalence ratios of ϕ = 0.225−0.675, and initial flow velocities of Ui = 8−46 m/s. The Reynolds numbers of the flow in the reactor lay between ReD = 11000 for low pressure and low temperature conditions and ReD = 28000 for high pressure and high temperature conditions. Autoignition was visualized 4132

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Figure 2. OH* chemiluminescence at 306 nm showing the development of consecutive autoigntion kernels at the same spatial location. Images are to be read line by line from left to right. Image interval 250 μs, exposure time 100 μs. Reactant conditions at Tad.m. = 814 K, P = 0.3 MPa, ϕ = 0.45, τ = 162 ms, Ui = 27.43 m/s.

using OH*-chemiluminescence, and ignition delays were measured in the range of τ = 112−310 ms. The effect of adding nitrogen as a diluent to the reactants was investigated for mole fractions of additional nitrogen ranging from 0 < XN2 < 0.1. The experimental results were compared with homogeneous gas-phase chemical kinetic modeling, which in some cases included modeling of the heat transfer from the reactants to the reactor walls. Images of OH*-chemiluminescence, the influence of equivalence ratio, pressure, nitrogen dilution, and heat transfer are presented in the following sections. 3.1. Visualization of Autoignition. High-speed OH*chemiluminescence images were recorded in order to gain insight into the spatial and temporal distribution of the flames during the autoignition process. The formation of autoignition kernels was a stochastic process, with different kernel patterns developing during each event. A sample of autoignition kernel formation is shown in Figure 2.

Ignition commenced in the form of an autoignition kernel appearing in the second frame of Figure 2. This autoignition kernel grew in size and intensity, while moving with the reactant flow. During the later frames a second, third, and fourth autoignition kernel appeared upstream of the initial kernel. The images show that autoignition started in the central regions of the circular pipe, suggesting that heat-transfer to the walls may have played a significant role in affecting ignition during the experiments. Further sequences of images at this condition are given in the Supporting Information of this article. 3.2. Effect of Equivalence Ratio. A series of experiments were carried out to investigate the influence of equivalence ratio on ignition delays of DME air mixtures. The equivalence ratio was varied between values of ϕ = 0.225, ϕ = 0.45, and ϕ = 0.675, while the pressure was kept constant at P = 0.3 MPa. Autoignition delays were modeled using adiabatic homogeneous gas-phase chemical kinetic modeling at constant 4133

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pressure, showing first stage and second stage ignition delays. The experimental results only measured second stage ignition emitting UV light. The results are shown in Figure 3.

into account in the chemical kinetic modeling presented in Figure 3, ignition delays were longer for the experiments than for the chemical kinetic modeling. The heat transfer was modeled in section 3.5, where it is discussed in further detail. The modeled ignition data shown in Figure 3 indicate that the first stage ignition delay increases for increasing temperatures of 1000/T < 1.3. This can be explained by s direct (brute-force) sensitivity analysis for first stage ignition at 800 K and 0.3 MPa. The sensitivity analysis showed that first stage ignition delay was most strongly delayed by the β-scission of CH2OCH2O2H to OH and two CH2O molecules. The increasing rate of this reaction at higher temperatures reduces the competing decomposition of CH2OCH2O2H via the reactions O2CH2OCH2O2H = HO2CH2OCHO + OH and HO2CH2OCHO = OCH2OCHO + OH. As the β-scission pathway becomes more important, only one OH radical is produced instead of two, increasing the first stage ignition delay. This observation has also been reported by Mittal et al.5 for higher reaction pressures. 3.3. Effect of Pressure. A second series of experiments was conducted to investigate the effect that pressure had on the autoignition delay times. Autoignition delay measurements were carried out at P = 0.2 MPa, P = 0.3 MPa, and P = 0.4 MPa, while the equivalence ratio was kept constant at ϕ = 0.45. Adiabatic homogeneous gas-phase chemical kinetic modeling of the first stage and second stage ignition delays was conducted to serve as a comparison to the experimental data. In the homogeneous gas-phase chemical kinetic modeling, pressure had a consistently accelerating effect on autoignition. The results are presented in Figure 4.

Figure 3. Influence of equivalence ratio on autoignition delays of dimethyl ether at P = 0.3 MPa. Symbols: Experiments (blue open circles) ϕ = 0.225, (red open triangles) ϕ = 0.45, (green open squares) ϕ = 0.675. Lines: Modeling (blue dash-dotted line) ϕ = 0.225, (red line) ϕ = 0.45, (green dotted line) ϕ = 0.675.

Figure 3 illustrates that for the experimental data, an increase in equivalence ratio resulted in a decrease in ignition delay at the same temperature. The difference was greater between low equivalence ratios of ϕ = 0.225 and ϕ = 0.45 than it was at higher equivalence ratios of ϕ = 0.45 and ϕ = 0.675. The activation energy represented by the slope of the experimental data may be calculated as 49 kJ/mol for ϕ = 0.225 and 94 kJ/ mol for ϕ = 0.45, and 36 kJ/mol for ϕ = 0.675, when the data is approximated by a straight line fit using the least-squares method. Given the limited number of data points, the activation energy results should merely be used as indication. The activation energy may be affected by experimental error in reaction temperature, pressure, equivalence ratio, and the ignition delay, owing to deviation from plug-flow conditions such as radial velocity and temperature gradients, a finite mixing time of the fuel and air streams, heat transfer from the reactants to the reactor walls and catalytic wall reactions. At the highest equivalence ratio (ϕ = 0.675), autoignition was frequently accompanied by shattering of the quartz tube in the optical flow reactor, which is why the data set was not extended further at this condition. Figure 3 shows that the homogeneous chemical kinetic calculations did not agree with the experimental data. The experimental ignition delays were consistently longer than the delays calculated a negative slope of activation energy representative of negative temperature coefficient behavior is visible for higher equivalence ratios, and the first stage ignition delay was shortest around values of 1000/T of 1.3. To either side of this value, the cool flame chemistry was inhibited. Since the test-section of the reactor was insulated, but not heated, the reactants experienced significant heat transfer as they flowed through the reactor. This influenced the results of the autoignition delay measurement by lowering the temperature of the reactants with respect to adiabatic reaction conditions, and increasing ignition delays. Since heat transfer was not taken

Figure 4. Influence of pressure on autoignition delays of dimethyl ether at ϕ = 0.45. Symbols: Experiments (blue open circles) P = 0.2 MPa, (red open triangles) P = 0.3 MPa, (green open squares) P = 0.4 MPa. Lines: Modeling (blue dash-dotted line) P = 0.2 MPa, (red line) P = 0.3 MPa, (green dash line) P = 0.4 MPa.

The experimental data in Figure 4 also show a reduction in ignition delays with increasing pressure. The activation energy represented by the slope of the experimental data may be calculated as 63 kJ/mol for P = 0.2 MPa and 94 kJ/mol for P = 0.3 MPa, and 75 kJ/mol for P = 0.4 MPa, when the data are approximated by a straight line fit using the least-squares method. 4134

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experimentally measured ignition delays as well as adiabatic homogeneous gas-phase chemical kinetic modeling of second stage ignition are shown in Figure 6.

A comparison between the experimental data and the chemical kinetic modeling showed that the experimental ignition delays were longer than the delays calculated by the chemical kinetic modeling for the higher pressures of P = 0.3 MPa and P = 0.4 MPa. For the lowest pressure of P = 0.2 MPa, the experimental ignition delays were shorter than predicted by the chemical kinetic modeling. As discussed in section 3.2, the experimental ignition delays would be expected to display longer ignition delays than the modeling, due to the heat losses of the reactants, which were not taken into account in the modeling. The overprediction of the ignition delays by the chemical kinetic modeling at P = 0.2 MPa may thus be due to other sources of error. It is possible that catalytic wall reactions played an increasing role in accelerating ignition at low pressures, since the ratio of reactor surface area to amount of reactant was greatly increased at low pressures. It is also possible that inaccuracies in the chemical kinetic mechanism may account for a small part of the discrepancy observed between experiment and model. The data and modeling reported by Hu et al.9 at P = 0.12 MPa suggested that a slight overprediction of ignition delays by the model of Zhao et al.16 existed at low temperatures and pressures. A direct (brute-force) sensitivity analysis for constant volume combustion is shown in Figure 5, to illustrate the varying influence reactions on ignition delay for different pressures.

Figure 6. Effect of additional N2 diluent mole fractions of 0 < XN2 < 0.1 on autoignition times of DME in air at Tad.m. = 820 K, P = 0.3 MPa, and ϕ = 0.45. (open blue circles) Experiment, (blue line) modeling.

Figure 6 shows that the ignition delay increased as the mole fraction of additional N2 was increased from 0 to 0.1. This would be expected as the nitrogen molecules moderated reactions by diluting the fuel and oxidant. The same trend was visible in the chemical kinetic modeling, but the experimentally determined ignition delays were consistently longer than the modeled values. As previously mentioned in section 3.2, the reactants experienced heat losses to the reactor walls, which were not taken into account in the homogeneous gas-phase chemical kinetic modeling shown in Figure 6. This will be discussed further in Section 3.5. 3.5. Heat-Transfer Modeling. The temperature of the reactor walls was not controlled during the experiments. Instead, the walls were heavily insulated in the nonoptical parts of the reactor and only slightly insulated in the optical part of the reactor. This meant that a significant amount of heat transfer took place from the reactants to the surroundings. Comparison of the experimental results with adiabatic homogeneous gas-phase chemical kinetic modeling (Figures 3−6) showed that the experimentally determined ignition delays were generally longer than calculated by the chemical kinetic model. In order to quantify the effect of these heat losses on the autoignition delays, the amount of heat transfer for the different experimental conditions was measured and incorporated into the modeling. First, the experimental heat losses were quantified for different temperatures, pressures, and mass flow rates of the reactants. This was accomplished by installing thermocouples along the centerline of the reactor and measuring temperature profiles of the reactants in the absence of heat release at different pressures and mass flow rates. A sample plot of the temperature profiles in the reactor at P = 0.3 MPa is shown in Figure 7. In order to simplify the measurements and to allow safe operation without any heat release from reactions, the DME and air mixtures were replaced with pure air in these measurements. It was thus assumed that the lean DME and air mixtures had the same heat transfer properties, as a flow of pure air of the same mass flow. The reactor was divided into six sections, in order to account for the varying heat transfer properties along the reactor length. A heat transfer coefficient

Figure 5. Sensitivity of reactions on second-stage ignition delay at [gray bars] 0.2 MPa, [white bars] 0.3 MPa, [black bars] 0.4 MPa.

The analysis shows that as the pressure is reduced from 0.4 to 0.2 MPa the most sensitive reaction for second stage ignition delay is shifted from the CH3OCH2 ↔ CH2O + CH3 to the βscission reaction producing two formaldehyde molecules and a hydroxyl radical. As mentioned in section 3.2, the increasing rate of this reaction at lower pressures reduces the competing decomposition of CH 2 OCH 2 O 2 H via the reactions O 2 CH 2 OCH 2 O 2 H = H O 2 CH 2 O C H O + O H a n d HO2CH2OCHO = OCH2OCHO + OH and reduces chainbranching by producing only one OH radical instead of two. 3.4. Diluent Effect. In a third series of experiments, the influence of nitrogen dilution on the autoignition delays was studied. In these experiments additional N2 was added to the DME and air mixture in mole fractions of 0 < XN2 < 0.1. The 4135

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Figure 7. Temperature profiles for air measured along the reactor for different initial temperatures and mass flow rates at P = 0.3 MPa. Measurements: (blue dash-dotted line) ṁ = 24.5 g/s, (red line) ṁ = 18.5 g/s, (green dotted line) ṁ = 14.0 g/s.

Figure 8. Reactor modeling with heat transfer. Measurements: (blue dash-dotted line) ṁ = 18.5 g/s of air at Tgi = 855 K, Modeling: (red line) ṁ = 18.5 g/s of air at Tgi = 855 K, (green dotted line) ṁ = 18.5 g/s of DME mixed with air at ϕ = 0.45 and Tgi = 823 K. Temperature Tgi was reduced from 855 to 823 K due to hot air mixing with colder DME.

(h) was calculated as a function of mass flow (ṁ ) of the reactants for each of the reactor sections, according to eq 1. This was done for each pressure condition used in the experiments and may be written as h = mċ P(Tgi − Tgo)/(A(Tamb − Tgm))

reported for the turbulent flow reactor study of Freeman and Lefebvre.13 The modeling was performed for three representative conditions at each of the equivalence ratios used during the experiments. Figure 9 shows the experimental results alongside the modeling results which incorporated heat transfer modeling, as well as with the adiabatic gas-phase chemical kinetic modeling.

(1)

where cp is the specific heat capacity of the gases at constant pressure, Tgi is the inlet temperature of the gases, Tgo is the outlet temperature of the gases, A is the wall area of the reactor section, Tamb is the ambient temperature surrounding the reactor insulation, and Tgm is the mean temperature of the gases in the reactor section. Equation 1 allowed the calculation of a heat transfer coefficient at various mass flow rates and pressures. These values were interpolated between three different mass flow rates at one pressure, to yield linear equations for heat transfer coefficients, at all operating conditions. The net heat transfer to the walls Q̇ could thus be calculated for every reactor section, depending on the pressure, mass flow rate, and local mean temperature Tgm by using eq 2. Q̇ = hA(Tamb − Tgm)

(2)

The value of the ambient temperature was taken as Tamb = 293 K for all measurements. An example of the temperature profiles measured for air at P = 0.3 MPa, mass flow rate ṁ = 18.5 g/s of air at Tgi = 855 K, together with a modeling example of the nonreacting flow of air and an example of a reacting mixture of DME and air, is shown in Figure 8. This method was used to investigate the effect that heat transfer had on the experimentally determined ignition delay times. The heat transfer coefficient of the reactor was measured as 12 W/m2 K for the steel section, and 98 W/m2 K for the short 135 mm optically accessible quartz section. In the steel section of the reactor, this coefficient was higher than the value reported by Wada et al.11 for a laminar flow reactor. When comparing the temperature drop of the current study to other turbulent flow reactors, the typical temperature drop of 19 K/m observed for the steel section of the present study is only slightly higher than the temperature drop of around 16 K/m

Figure 9. Reactor modeling with heat transfer. Influence of equivalence ratio on autoignition delays of dimethyl ether at P = 0.3 MPa. Experiments: (blue open circles) ϕ = 0.225, (red open triangles) ϕ = 0.45, (green open squares) ϕ = 0.675. Modeling without heat transfer (blue dash-dotted line) ϕ = 0.225, (red line) ϕ = 0.45, (green dashed line) ϕ = 0.675. Modeling including heat transfer: (blue solid circles) ϕ = 0.225, (red solid triangles) ϕ = 0.45, (green solid squares) ϕ = 0.675. Dotted lines: error bars. 4136

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for the overprediction of ignition delays by the chemical kinetic model at low pressures are not entirely clear. It is possible that, as a result of the reduced amount of reactants per surface area of the reactor, catalytic wall reactions may have been of increasing importance at lower pressures and could have shortened ignition delays in the experiments. Accurate modeling of a heterogeneous system involving gas phase reactions and surface reactions requires the development of a chemical kinetic surface reaction model for the DME reactions on stainless steel. Wada et al.11 used the simplified assumption that heterogeneous reactions at the wall of the stainless steel reactor consumed radicals infinitely fast but did not cause any reaction of DME in their modeling approach. As discussed in section 3.3, it is further possible that the chemical kinetic mechanism may have minor inaccuracies at low pressures. The disagreement observed at P = 0.4 MPa may largely be explained by error incurred by initial mixing of the reactants in the reactor. The error analysis showed that at these conditions, the maximum error due to fuel and air mixing in the reactor was as high as 40%. In addition to this, further nonidealities in the experiments, which were not modeled in the chemical kinetic calculations, such as axial diffusion, turbulence, and recirculation zones, could have further affected the agreement between model and experiments. Figure 11 shows the influence of heat transfer in the modeling of the experiments with additional nitrogen dilution.

Figure 9 shows that better agreement between experiments and modeling was achieved at the lower equivalence ratios (ϕ = 0.225 and ϕ = 0.45) when modeling heat transfer. Significant differences between the modeling and experiments remained for ϕ = 0.675, particularly at lower temperatures. It is possible that this discrepancy may be due to the error incurred due to the mixing process at the beginning of the reactor. Table 2 in section 2.5 showed that the uncertainty derived from fuel and air mixing at the beginning of the reactor influenced the ignition delays much more strongly at higher equivalence ratios and low temperatures. At ϕ = 0.225 and ϕ = 0.45, the maximum error was estimated at 12% and 13%, respectively, while at ϕ = 0.675, the maximum error increased to 62% of the overall ignition delay time. Modeling of the heat transfer in the experiments was also performed for different pressures. The modeling was carried out for three representative conditions at each of the pressure levels used during the experiments. Figure 10 shows the

Figure 10. Reactor modeling with heat transfer. Influence of pressure on autoignition delays of dimethyl ether at ϕ = 0.45. Symbols: Experiments (blue open circles) P = 0.2 MPa, (red open triangles) P = 0.3 MPa, (green open squares) P = 0.4 MPa. Lines: Modeling (blue dash-dotted line) P = 0.2 MPa, (solid red line) P = 0.3 MPa, (green dotted line) P = 0.4 MPa. Modeling including heat transfer: (solid red triangles) P = 0.3 MPa, (green solid squares) P = 0.4 MPa. Dotted lines: error bars.

Figure 11. Effect of additional N2 diluent mole fractions of 0 < XN2 < 0.1 on autoignition times of DME in air at Tm = 820 K, P = 0.3 MPa, and ϕ = 0.45. (blue open circles) Experiment, (blue line) modeling without heat transfer, (blue solid circles) modeling with heat transfer.

Agreement between modeling and experiment is improved when heat transfer is taken into account, but discrepancies remain at higher dilution ratios. The slope of the modelled ignition delays with additional nitrogen mole fraction did not change when heat transfer was taken into account. Both slopes deviate slightly from the experiments at high N2 mole fractions.

experimental measurements for second stage ignition alongside the modeling results for second stage ignition which included heat transfer modeling, as well as with the adiabatic modeling results for first stage and second stage ignition. Figure 10 shows that improved agreement between experiments and modeling was achieved when including heat transfer into the modeling at P = 0.3 MPa, but that significant differences remained for P = 0.4 MPa and P = 0.2 MPa. For the experiments carried out at P = 0.2 MPa, the modeling did not predict any ignition in the range of mass flow rates for which heat transfer coefficients could be acquired experimentally, because the necessary residence time required flow rates that were significantly below those for which heat transfer coefficients were measured experimentally. Thus, no reasonable modeling results including heat transfer could be obtained at P = 0.2 MPa. As previously discussed in section 3.3, the reasons

4. SUMMARY AND CONCLUSIONS Autoignition of dimethyl ether and air mixtures were studied at different pressures in a turbulent flow reactor with optical access. The experiments were performed at conditions relevant to micro gas turbines. Mixing temperatures were in the range of Tad.m. = 739−902 K, reactant pressures were in the range P = 0.2−0.4 MPa, and the equivalence ratios of the reactants were in the range ϕ = 0.225−0.675. The autoignition process was visualized using OH*-chemiluminescence imaging at a rate of 4 kHz. The images showed that autoignition occurred via the 4137

dx.doi.org/10.1021/ef402476r | Energy Fuels 2014, 28, 4130−4138

Energy & Fuels

Article

development of multiple flame kernels forming around the central axis of the reactor. The ignition delays were measured by assuming plug-flow conditions through the reactor and were compared with homogeneous gas-phase chemical kinetic modeling. The results showed significant discrepancies between experiment and modeling when adiabatic conditions were assumed in the modeling. Heat transfer from the reactants to the reactor was measured for different reactant mass flow rates and temperatures. When this heat transfer was incorporated into the chemical kinetic modeling, better agreement between experiment and model was achieved, but discrepancies persisted particularly at high equivalence ratios and at low and high reaction pressures. The discrepancies observed at higher equivalence ratios may be explained by the lower heat losses incurred by the air and DME mixture compared with air only, as well as by larger uncertainties incurred due to fuel and air mixing. The discrepancy observed at low reaction pressure could be owed to an increasing importance of catalytic wall reactions and to a small extent to a slight overprediction of the chemical kinetic model of Zhao et al.16 at low pressures. Nonidealities such as deviations from ideal plug-flow conditions, wall reactions, and detailed fuel and air mixing were not modeled in the chemical kinetic simulations and could account for further discrepancies between modeling and experiment.



(11) Wada, T.; Sudholt, A.; Pitsch, H.; Peters, N. Combust. Theor. Modell. 2013, 17, 906−934. (12) Schönborn, A.; Sayad, P.; Konnov, A. A.; Klingmann, J. Combust. Flame 2013, 160, 1033−1043. (13) Freeman, G.; Lefebvre, A. Combust. Flame 1984, 58, 153−162. (14) Lefebvre, A. H., Freeman, W. G., Cowell, L. H., Spontaneous Ignition Delay Characteristics of Hydrocarbon Fuel/Air Mixtures, Report No. CR-175064; National Aeronautics and Space Administration: Washington, DC, 1986. (15) Beerer, D. J.; McDonell, V. G. Proc. Combust. Inst. 2011, 33, 301−307. (16) Zhao, Z.; Chaos, M.; Kazakov, A.; Dryer, F. L. Int. J. Chem. Kinet. 2008, 40, 1−18. (17) Alzueta, M. U.; Muro, J.; Bilbao, R.; Glarborg, P. Isr. J. Chem. 1999, 39, 73−86. (18) Fischer, S. L.; Dryer, F. L.; Curran, H. J. Int. J. Chem. Kinet. 2000, 32, 713−740. (19) Curran, H. J.; Fischer, S. L.; Dryer, F. L. Int. J. Chem. Kinet. 2000, 32, 741−759.

ASSOCIATED CONTENT

S Supporting Information *

Figures of chemiluminescence patterns. This material is available free of charge via the Internet at http://pubs.acs.org.



AUTHOR INFORMATION

Corresponding Author

*Phone: + 46 46 222 4771. Fax: + 46 46 222 47 17. E-mail: [email protected]. Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS This work was financially supported by the Swedish Research Council (Vetenskapsrådet) under contract A0088001.



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dx.doi.org/10.1021/ef402476r | Energy Fuels 2014, 28, 4130−4138