Thesis AIAApaper

8/13/2019 Thesis AIAApaper

1/10

Oxygen Methane Combustion Simulation Towards

Pollution Free Solution For Industry and

Transportation

Arni Steingrimsson and Reda Mankbadi

Embry-Riddle Aeronautical University, Daytona Beach, Florida, 32114, USA

Computational fluid dynamic combustion simulation of pure methane, CH4, and pure

oxygen, O2 was laid out to investigate the effects of different turbulence models, combustion

models, advection schemes, average time stepping, time accurate time stepping and mesh

sizing. All the results were compared with empirical data for validation and seem to

agree quite well. The results show that steady state, lower fidelity turbulence model is

able to show comparable result to high fidelity turbulence model with time accurate time

stepping. Pulsating injection combustion of methane and oxygen was simulated as well

and the results compared with non-pulsating combustion. Results show that there is a

performance penalty in the combustor, mainly due to pressure drop in the inlet, while C

efficiency is comparable to non-pulsating combustion.

Nomenclature

m Mass Flow RatemCH4 Methane Mass Flow Rate AmplitudemO2 Oxygen Mass Flow Rate Amplitude

S

dA Integrated Surface Area

Densityf Frequencystep Heaviside Step Functiont Simulation TimeU Fluid Velocity

I. Introduction

With more evidence being released for global warming and growing acceptance of the severity and theimpact it will have on our lives it is crucial that big steps are taken to reduce emissions that cause

global warming. Electricity, heating and transportation account for almost 60% of carbon dioxide (CO2)gas emissions in the us. The biggest pollutants are road based transportation with 21.6% while residencebuildings account for 15.3% and 12% for the commercial buildings, the values account for both heating and

electric usage.The purpose of this study is to design a pollution free system that can be used in transportation and

energy production. The system consist mainly of combusting pure methane and pure oxygen. Water isinjected into combustion chamber and super heated steam from combustor enters steam turbine whichextracts energy out of the exhaust. The steam turbine spins an electric generator which charges a supercapacitor that stores the energy. High level view or the core components of the system is shown in figure 2.

Aerospace Engineer, Siemens Energy, and AIAA Member.Distinguish Professor of Aerospace Engineering, Embry-Riddle Aeronautical University, Daytona Beach FL 32114, Florida

Center for Advanced Aero-Propulsion Technology (FCAAP) and AIAA Fellow.

1 of10

American Institute of Aeronautics and Astronautics


2/10

A more detailed system diagram is shown in figure 3 which shows how a system could be used in ships andboats. What is different about this system compared with e.g. diesel generators in some ships today is thatthere is no greenhouse emission. Once the steam turbine has extracted energy out of the steam the exhaust(H2Oand C O2) is cooled through condenser and the two chemical compounds are separated. The separatedcompounds, H2O and C O2, are stored and the water is reused. One of the difficult tasks in this study is todesign the combustion chamber. The main task of this study is to investigate the combustion chamber and itscombustion behavior. In this study the use of computational fluid dynamics was used entirely to investigatethe combustion behavior. An empirical data from Salgues et. al.2 was used to validate simulation model and

simulation setups. There are limited empirical and simulation data available for combustion of pure methaneand pure oxygen and thus CFD simulation was fitted to match Salgues et. al.2 empirical experiment. Inaddition to match empirical data and validate the CFD models a pulsating combustion was studied. Notto be confused with pulse combustion like the known Schmidt, Helmholtz and Rijke type combustor, thepulsating simulation injects fuel and oxidizer in pulses into the combustion chamber. No available data ison this subject thus simulation used the same geometry and models for the pulsating combustion as for theCFD validation case. The ultimate goal is to design very efficient power system that is pollution free.

Figure 2: Simple process flow diagram of the corecomponent

Figure 3: Example of ship drive train processdesign

II. Model

A. The Validation

Salgues et. al. performed combustion experiments using pure methane and pure oxygen. Their main resultsare listed in table 4. Other experiments were performed using imaging technology, such as shadowgraph.The geometry used for this study is shown is shown in figure4 and figure5 shows the cross sectional sketchof the combustor that Salgues et. al. used.

2 of10



3/10

Figure 1: US greenhouse gas emission flow chart1

Figure 4: Geometry created us-ing Siemens NX showing com-bustion chamber dimension

The geometry is a rocket type combustor with two inlets, cylindershaped at the center axis is the oxygen inlet and an annulus inlet for themethane. The combustion chamber is a straight cylinder and its outletconsist of convergence-divergence sections. Two meshes were created forthe geometry, one course another fine. The fine mesh contained boundarylayer but is comparible to Boundiers et. al.3 fine LES mesh, wherethey performed mesh sensitivity study in complex combustor geometry.Typical LES mesh sizes with wall model were used for the fine mesh,see table 2 for typical values. Boudiers et. al. mesh sensitivity studyhas shown that in complex geometry the mesh size plays a mayer role incapturing all physics and producing accurate results, comparison of themeshes are shown in table 1. Saluges et. al. measured the massflow ratefor the methane and the oxygen very close to the inlet and is shown in the table 4. The boundary conditions

for the combustion model are shown in table 3. 8 different combustion simulations were performed. Eachsimulation had a unique CFD setup.

Figure 5: Original schematic ofthe validation combustor2

The CFD simulation setups are shown in table 5and6. Simulationswere both time accurate as well as time averaged and two turbulencemodels were tested one for each time model. For the time average simu-lationsk turbulence model with Eddy dissipation and thermal energyheat transfer model was used. Both 1st order and second order advec-tion scheme was used and also for the turbulence numerics. For the timeaccurate simulation two turbulence models were used, URANSkandWALE-LES. Finite rate chemistry and eddy dissipation combustion mod-els were used. Total energy and thermal energy heat transfer were alsotested. 3 advection schemes were tested, high resolution, specified blend

factor and central difference. 3 different transient schemes were simulated as well, high resolution, 1st and

2nd order backward euler. Course and fine mesh was then tested for both the time accurate simulations andthe time average simulation.

B. Pulsating Simulations

(a) Consine wave pulse

(b) Square wave pulse

The other task was to study the affect of pulsat-ing combustion. Like mentioned before this simula-tion test was different than the known pulse combus-tion, Schmidt, Helmholtz and Rijke type. The re-

3 of10



4/10

Coarse mesh Fine mesh My RANS mesh My LES mesh

Total num. of points 230,118 7,661,005 153,761 799,983

Total num. of cells 1,242,086 43,949,682 1,514,128 8,787,466

Max. cell volume 3.12671108 4.05748109 4.79152109 5.819991010

Min. cell volume 1.817951011 1.18281012 6.46551013 3.043781015

Table 1: Comparison of mesh sizes3

DNS Wall-Resolved LES LES with Wall Model

Streamwise x+ 10-15 50-150 100-600

Spanwise z+ 5 10-40 100-300

Wall-normal (y+) 1 1 30-150

Number of points in 0 < y+


5/10

Boundary Parameter Value

Inlet Oxygen Mass flow (kg/s) 0.1 177

Inlet Oxygen Temperature (K) 122

Inlet Methane Mass flow (kg/s) 0.03923

Inlet Methane Temperature (K) 300

Outlet Pressure (Pa) 1 01 32 5

Ou tl et Tem pera ture (K) 300

Wall Smooth no slip

Table 3: Model boundary condi-tions

Type of injector Type of measurement Values

Sh ea r C oa xi al LOX m ass fl ow (kg/s) 0.118

Sh ea r C oa xi al C H4 m ass fl ow (kg/s) 0.039

Swirl Coaxial #1 Chamber Pressure (MP a) 4.23

Swirl Coaxial #2 Chamber Pressure (MP a) 4.03

Shear Coaxial Chamber Pressure (MP a) 3.82

Swirl Coaxial #1 Cexp/C

theo. (%) 95

Swirl Coaxial #1 Cexp/C

theo. (%) 91

Shear Coaxial Cexp/C

theo. (%) 85

Table 4: Validation parametersobtained from Salgues et. al.2

Identification Turbulence Model Combustion Model Heat Transfer

Sim V1 k model Eddy Dissipation Thermal Energy



Sim V4 k URANS model Eddy Dissipation Thermal Energy

Sim V5 WALE-LES model Finite Rate Chemistry Total Energy




Table 5: Simulation identification and solver set-tings 1

Identification Advection Scheme Turbulence Numerics / Mesh

Transient Scheme

Sim V1 Specified Blend Factor 1st Order Course mesh

Sim V2 High Resolution High Resolution Course mesh

Sim V3 High Resolution High Resolution Fine mesh

Sim V4 High Resolution High Resolution Course mesh

Sim V5 Specified Blend Factor 1st Order Backward Euler Course mesh

Sim V6 Central Difference 2nd Order Backward Euler Course mesh

Sim V7 Specified Blend Factor 1st Order Backward Euler Fine mesh

Sim V8 Specified Blend Factor 2nd Order Backward Euler Fine mesh

Table 6: Simulation identification and solver set-tings 2

III. Results

There were two different experiments studied, validation simulation and pulsating simulation.

A. Validation

8 different simulations were performed for validation. Each simulation had different setting and they can beseen in table5 and in table6. Oxygen combustion is very volatile and almost immediately after the fuel andthe oxygen enters the combustion chamber the combustion occurs. The maximum temperature occurs alongthe chamber axis immediately after the combustion. The comparison of the average chamber temperatureis shown in figure6. The Sim V3, Sim V7 and Sim V8 all use the fine mesh and show the highest averagechamber temperature. The average chamber pressure results are shown in figure7. The empirical resultsare shown with black horizontal line and all the simulation results show higher pressure. Advection schemeseems to be the biggest factor for the pressure results as can be seen from the results, see Sim V1, Sim V5,Sim V7 and Sim V8. Combustion chamber performance measures are listed in figure 8 and in figure9 withspecific impulse and thrust specific fuel consumption. Combination of advection scheme and mesh size seemsto contribute to the results and the use of second order scheme with courser mesh results in higher specificimpulse. Another performance and efficiency measure for combustion is the characteristic exhaust velocityefficiency and its results are shown in figure 10. Again the advection scheme and mesh size are the biggestfactors for the results. The turbulence structure inside the combustion chamber is shown in figure11andthe eddy viscosity were computed by the WALE-LES turbulence model.

S im V1 S im V2 S im V3 S im V4 S im V5 S im V6 S im V7 S im V8

3,500

4,000

Theoretical Temperature

3,

630

3,

640

4,

041

3,

650

3,

686

3,

682

4,

174

4,

153

ChamberTemperature(K)

Figure 6: Comparison of combustion chambertemperature

S im V1 S im V2 S im V3 S im V4 S im V5 S im V6 S im V7 S im V83.8

3.9

4

4.1

4.2106

Theoretical

Experiment

4.

03

106

4.

09

106

4.

09

106

4.

07

106

4.

02

106

4.

05

106

3.

93

106

3.

92

106

ChamberPressure

(Pa)

Figure 7: Comparison of combustion chamberpressure

5 of10



6/10

Sim V1 Sim V2 Sim V3 Sim V4 Sim V5 Sim V6 Sim V7 Sim V8300

305

310

315

320

311

.82

318

.96

311

.54

316

.6

310

.86 3

14

.54

312

.15

312

.01

Specificimpulse,

Isp(s)

Figure 8: Comparison of combustion chamberspecific impulse

Sim V1 Sim V2 Sim V3 Sim V4 Sim V5 Sim V6 Sim V7 Sim V81.1

1.12

1.14

1.16

1.18

1.2

1.

18

1.

15

1.

18

1.

16

1.

18

1.

17

1.

18

1.

18

TSFC

Figure 9: Comparison of combustion chamberthrust specific fuel consumption

S im V 1 S im V 2 S im V 3 S im V 4 S im V 5 S im V 6 S im V 7 S im V 8

85

90

95

100

Experiment

89

.67 9

1.

53

89

.4

90

.47

89

.23

90

.27

87

.28

87

.21

C Exp

/C Theo

(%)

Figure 10: Comparison of characteristic exhaust ve-locity C*

Figure 11: Turbulence structure from theinside of the combustion chamber (Q-criterion) as computed by WALE-LES

B. Pulsating Simulation

The pulsating results for the pressure inside the combustion chamber are shown in figure 12. From theresults the inlet the pressure osculates back and forth from lower to higher pressure and as we increase thefrequency the amplitude of the wave gets smaller. After combustion the pressure levels off at about 2 MP a.During combustion the oxygen is fully combusted about 1/4 way through the combustion chamber as can

be seen in figure 13. The methane mass fraction results are shown in figure14where there is about 5 or6 % of the methane unburnt, this was expected as the mixture was slightly rich. The turbulence structureand eddy viscosity is larger for the lower frequency than the higher as can be seen in figure 15. Figure16shows a contour plot of the eddy viscosity and the turbulence structure (q-criterion) as computed by theturbulence model. The plot is only showing the inlet and the small section of the combustion chamber.The temperature is only affected by the pulsation only at the inlet and before the combustion. After thecombustion the temperature evens out and all the simulations result at around 4500K. Finally table8 listsaverage chamber pressure, average chamber temperature, characteristic exhaust velocity efficiency and thespecific impulse of all the pulsating simulations and the validation which had the same simulation settings.The results show that the pulsation does not affect the temperature much and the characteristic exhaustvelocity. The pressure drop on the other hand tremendous and the average chamber pressure is half of thenon pulsating cases. The specific impulse is much lower for the pulsating cases than for the non-pulsatingcase.

IV. Conclusion

Validation of fluid dynamics and combustion model was compared with Salgues et al.2 empirical results.Not all dimensions were available to design the combustor to be identical to the empirical data and thussome educated guesses had to be included in this study. Because this fluid dynamic study and the empiricaltest were performed completely independently of each other the results for the empirical data were limited.The results show that all the simulations slightly over predict the theoretical chamber temperature. Thiscan be expected when simulating combustion simulations using equation of state as ideal instead of real

6 of10



7/10

Figure 12: Combustion chamber pressure comparison of the pulsating simulations (results given in Pascals)

Parameter Avg Chamber Temperature Avg Chamber Pressure %C ISP

Non Pulsating 3686.26K 4015281.75P a 89.23% 310.86s

Pulse 10, 000Hz 3699.21K 2090637.63P a 92.06% 284.08s

Step 10, 000Hz 3715.25K 2104722.5P a 90.12% 283.77s

Pulse 50, 000Hz 3738.83K 2079055.25P a 89.81% 286.88s

Step 50, 000Hz 3695.56K 2064676.5P a 89.95% 284.89s

Pulse 100, 000Hz 3691.10K 2000926.63P a 89.15% 283.37s

Step 100, 000Hz 3680.79K 1994451.88P a 89.07% 283.53s

Table 8: Comparison of pulsating combustion results and non-pulsating results

gas. The results for the chamber pressure showed that some simulations were able to predict pressureunder the theoretical chamber pressure although all simulations were slightly over-predicting the empiricalchamber pressure. It is known that usage of ideal gas for equation of state will cause simulations to overpredict temperature and pressure.5 Another factor is that the geometric details were not know, such as theconverging diverging exit nozzle and the inlet pipe length etc.

The mesh size seemed to influence the chamber temperature results significantly, it did not have the sametrend on the chamber pressure. The mesh size did not have much or any change on the RANS simulationchamber pressure. For the LES simulations the mesh size reduced the chamber pressure and the results wereunder the theoretical chamber pressure. Using the specific blend factor advection scheme seemed to influencethe results more than changing the transient model from 1st to 2nd order as can be seen from results. TheSim V7 and Sim V8 which compare 1st order transient model with 2nd order show very little difference as

can be seen in all the results. Sim V1 and Sim V5 seem to agree very closely with each other even thoughthey have very different solver settings. Sim V1 is time averaged and uses lower fidelity turbulence modelwhile Sim V5 is time accurate and uses LES turbulence model. They both run on course mesh and thiscould be the reason for them agreeing so closely with each other. The mesh is not small enough for theLES model to capture the smaller eddies and thus computing similar results as the k model. If we thencompare Sim V3 and Sim V8 the results show bigger difference and especially in turbulence structure. Thiscomparison should not be taken fervently as turbulence structure is usually not computed for steady statesolutions, only time dependent simulations.

Pulsating simulations with injection fuel and oxygen alternating either as cosine wave or step function atspecific frequency was performed and compared with validated simulations above. The pulsating simulations

7 of10



8/10

Figure 13: Oxygen mass fraction, ratio of oxygen mass to the mass of the total mixture.

were performed using the same simulation setup as Sim V4 and the results show that there is a drop inefficiency by pulsating the injection. This is mainly due to the pressure drop in the combustion chamber.Even though characteristic exhaust velocity efficiency is very close to the non-pulsating simulations theresults show that TSFC and Isp are lower than non-pulsating simulations. It could be feasible, at least withsimulation, to increase the injection pressure and see the results of changing the amplitude. This causesanother problem though as the injection pressure is already close to the supercritical state of the methaneand oxygen.

In this study bothkturbulence model and LES turbulence model have been validated within acceptablelimit. Results have shown that course mesh and k turbulence model will be able to predict acceptableresults and much faster than LES with fine mesh. The main reason for this could be that the geometry isrelatively simple and it is well known that LES is able to predict very complex geometries while lower fidelityturbulence models cannot.6 Thek model gives similar results to the LES but the flow field is averaged

and does not show the correct flow field when compared with combustion images such as shawdowgraph orOH-PLIF images.

References

1Baumert, K. A., Herzog, T., and Pershing, J., Navigating the Numbers Greenhouse Gas Data and International ClimatePolicy, Tech. rep., World Resources Institude, 2005.

2Salgues, D., Mouis, G., Lee, S.-Y., Kalitan, D. M., Pal, S., and Santoro, R., Shear and Swirl Coaxial Injector Studies ofLOX/GCH4 Rocket Combustion Using Non-Intrusive Laser Diagnostics, American Institude of Aeronautics and Astronautics ,, No. 1, January 2006, pp. 114.

3Boudier, G., Gicquel, L. Y. M., and Poinsot, T. J., Effects of mesh resolution on Large Eddy Simulation of reacting owsin complex geometry combustors, Combustion and Flame, Vol. 155, No. 1-2, October 2008, pp. 196214.

4Jiang, X. and Lai, C.-H., Numerical Techniques for Direct and Large-Eddy Simulations , Taylor and Francis Group, LLC,2009.

5

Cutrone, L., Battista, F., Ranuzzi, G., Bonifacio, S., and Steelant, J., A CFD Method for Simulation of Mixing andCombustion in High-Pressure LOx/Methane Rocket Engines, American Institute of Aeronautics and Astronautics, , N o.AIAA 2008-949, January 2008.

6Jiang, X. and Lai, C.-H.,Numerical Techniques for Direct and Large-Eddy Simulations , CRC Press Taylor Francis Group,2009.

8 of10



9/10

Figure 14: Methane mass fraction, ratio of methane mass to the mass of the total mixture.

Figure 15: Comparison of eddy viscosity for all the pulsating simulations

9 of10



10/10

(a) 10, 000Hz cosine wave (b) 50, 000Hz cosine wave (c) 100, 000Hz cosine wave

Figure 16: Contour plot of the eddy viscosity of the pulsating simulation using cosine wave. The structureis the turbulence structure (q-criterion) as computed by the turbulence model

Figure 17: Combustion chamber temperature results for all the pulsating simulations

10 of10


Thesis AIAApaper

Documents

Transcript of Thesis AIAApaper