Marius PopescuPage 11AE 6766 Spring 2014|Project #1

Project #1AE/ME 6766Dr. Seitzman

A Study of Equilibrium Combustion Temperatures

1/16/2014Marius Popescu

Table of ContentsI. Abstract.1II. Introduction1III. Methods..1IV. Discussion..11V. Conclusion.12

Terms and Symbols Used = Fuel Equivalence RatioK =Kelvinatm = Pressure in atmospheres (101.325 kPa)Tf = Flame Temperature in KAbstractGasEq was used to determine equilibrium values for various conditions to determine what factors contribute to changes in efficiency or emissions. Higher pressures resulted in higher combustion temperatures, and higher reactant temperatures consequently also rose combustion temperature. Both propane and methane peak temperatures were in the slightly rich range, at s of around 1.05. Constant pressure flame temperatures were lower than constant volume temperatures. Higher pressures result in both higher combustion temperatures and lower pollutant emissions. At pressures and temperatures typical in gas turbine combustion, sensitivity analysis suggests that linear increases in pressure resulted in less significant increase in combustion temperatures compared to the decrease in emissions of most pollutants.IntroductionThis report outlines a preliminary study of flame temperatures of various fuels and conditions using a chemical equilibrium solver, in this case primarily GasEq. This first part of the study was the difference between methane and propane gas adiabatic constant pressure flame temperatures. Then further analysis is done between propanes constant pressure temperatures and constant volume temperatures while varying fuel equivalence ratios. Lastly, pressures and temperatures were varied as well for propane in constant pressure adiabatic flames and this time the molar fractions of the products are analyzed. Although the goal of this study is primarily to gain a general insight of trends in typical combustion processes, it also provides the first clues to designing more efficient and less polluting combustors. It is important to note, however, that this solver uses formulas based on equilibrium and therefore neglect other effects and tell us nothing about the rates of the reaction, and therefore may be off by many magnitudes on some quantities. MethodsFor this report GasEq is used to determine most of the properties of the flames. GasEq uses Lagrangian multipliers to solve for the lowest Gibbs free energy of the products, curve fitted enthalpies to get initial and final enthalpies to match, and atomic conservation equations to solve for the flame temperatures. For the calculations done in this report, GasEq cannot determine anything about the rate of the combustion, and molar fractions cannot be fully trusted, but flame temperatures are going to be relatively close to actual, since it is mostly given by the energy balance for which it is solving for.ResultsPart 1: Analysis between Methane and Propane Flame TemperaturesFigures 1 and 2 show varying from 0.4 to 2.5 while keeping the reactants at 1atm and 298K for methane and propane respectively.

Figure 1. Methane adiabatic flame temperature at constant pressure, reactants @ 298K and 1atm. Not shown: Peak @ =1.035 and Tf = 2233.3K.

Figure 2. Propane adiabatic flame temperature at constant pressure, reactants @ 298K & 1atm. Not shown: Peak @ =1.046 and Tf = 2277.8K.Figure 3 shows the two graphs overlapped with in a logarithmic scale. This help visualize the regions of fuel rich and fuel lean better.

Figure 3. Propane and Methane adiabatic temperatures. Horizontal axis in log2 scale.Part 2: Difference between Adiabatic Constant Volume and Constant Pressure Flame TemperaturesFigure 4 shows the same conditions for propane except for constant volume flame temperatures and Figure 5 shows the two graphs overlapped with on a logarithmic scale.

Figure 4. Adiabatic flame temperatures at constant volume, Propane. Not shown: Peak @ =1.093, Tf = 2654.3 K, final pressure = 9.553 atm.

Figure 5. Adiabatic Flame Temperatures with varying logarithmically. Part 3: Analysis of Propane Varying Initial Conditions Figures 6, 7 and 8 show the major and minor species mole fractions of propane at constant pressure of 1 atm and final Temperature while varying Temperatures from 250 K to 850K, for a stoichiometry/fuel equivalence ratio of 0.7, 1, and 1.4. Minor species were considered to be anything with a mole fraction never reaching above 0.1. Species below 1ppb were not considered, except for Ammonia, and N2O is ignored as it never reached more than NO2 unless it is less than 1ppb .

Figure 6a. Major Species for = 0.7. Reactant temperature varied from 250K to 850K, pressure @ 1atm.

Figure 6b. Minor Species for = 0.7. Reactant temperature varied from 250K to 850K, pressure @ 1atm. Mole fractions in log10 scale.

Figure 6c. Product Temperature for = 0.7

Figure 7a. Major Species for = 1. Reactant temperature varied from 250K to 850K, pressure @ 1atm.

Figure 7b. Minor Species for = 1. Reactant temperature varied from 250K to 850K, pressure @ 1atm. Mole fractions in log10 scale.

Figure 7c. Product Temperature for = 1

Figure 8a. Major Species for = 1.4. Reactant temperature varied from 250K to 850K, pressure @ 1atm.

Figure 8b. Minor Species for = 1.4. Reactant temperature varied from 250K to 850K, pressure @ 1atm. Mole fractions in log10 scale.

Figure 8c. Product Temperature for = 1.4Figures 9, 10 and 11 show the major and minor species mole fractions of propane at constant temperature of 800K and pressure varying from 0.1 to 40 atm varying exponentionally, for a stoichiometry/fuel equivalence ratio of 0.7, 1, and 1.4 respectively.

Figure 9a. Major Species for = 0.7. Pressure varied from 0.1 to 40, initial Temp @ 800K.

Figure 9b. Minor Species for = 0.7. Pressure varied from 0.1 to 40, initial Temp @ 800K.

Figure 9c. Product Temperature for = 0.7

Figure 10a. Major Species for = 1. Pressure varied from 0.1 to 40, initial Temp @ 800K.

Figure 10b. Minor Species for = 1. Pressure varied from 0.1 to 40, initial Temp @ 800K.

Figure 10c. Product Temperature for = 1

Figure 11a. Major Species for = 1.4. Pressure varied from 0.1 to 40, initial Temp @ 800K.

Figure 11b. Major Species for = 1.4. Pressure varied from 0.1 to 40, initial Temp @ 800K.

Figure 11c. Product Temperature for = 1.4Part 4: Jet-A ComparisonUsing NASAs CEA, some properties of Jet-A at compressor exit (assumed to be 800K and 30 atm and =1) were found and compared to propane. TfNOCOHOH

Jet-A1.0252492K0.00170.01920.00260.002

Propane1.062635.5K0.003010.025770.0006430.00375

DiscussionFrom the first figures, Figure 1 and 2, we can see that difference in temperatures are only about 4% between the two hydrocarbons. This is probably accounted by the fact that the products are similar and that the ratio of carbon dioxide to water in the products is slightly higher for larger alkanes since the ratio of carbon to hydrogen atoms slightly increase. Since carbon dioxide has a higher enthalpy of formation this can lead to higher temperatures. The large difference in temperatures between Figure 2 and 4 for the difference between adiabatic constant pressure and constant volume flame temperatures, is likely because the pressure of the system is allowed to increase. Higher pressures tend to result in higher temperatures because of less dissociation and since no heat or work is done out of the system this work goes back into raising internal energy.Looking at varying initial conditions of propane on mole fractions of products and flame temperatures, a few things can be inferred. If reducing NOx emissions is desired, burning rich works better and raising pressures reduces NOx when burning rich but does the opposite effect when burning lean, in general raising temperatures has a relatively strong effect on raising NOx emissions. Reducing CO emissions is strongly correlated with burning leaner, once again raising pressures reduces these emissions, but this is regardless of burning rich or lean. Hydrogen and Hydrogen products rise with richer burns and higher temperatures, although more strongly correlated to higher temperatures than other products.ConclusionFor most hydrocarbons it seems that burning slightly fuel rich produces the highest adiabatic flame temperatures. Burning fuel rich also seems to produce generally less pollutants and generally higher temperatures. If it were possible to burn rich at first and then the rest of the fuel in a lean environment, this could result in burning of more of the fuel yet produce less pollutants overall since remaining CO will burn off.Sources1. GasEqs Calculations Document2. Determination of Pollutant Emissions Characteristics of General Electrics CF6-6 and CF6-50 Model Engines by T.F. Lyon, W.J.Dodds, D.W. Bahr, March 1980, ADA088927.