Thermochemistry study of internal combustion engine

Available online at www.sciencedirect.com

Energy Procedia 18 (2012) 1086 – 1095

Conference Title

Thermochemistry study of internal combustion engine Fethi BOURAS*, Azeddine SOUDANI and Mohamed SI AMEUR a

University of HL-Batna, 05000-BATNA- Algeria.

Abstract

The majority of aerospace engines such as turboreactor and rocket engines work due to the process of combustion, which is also responsible for the formation of harmful and pollutants chemical species in the environment. The progress in engine technology asks constructors to respect the environmental regulations; this is required to develop ecological and cleaner engines. Our investigation was based on a configuration of cylindrical combustion chamber similar to the turboreactor, using the injection of methane and air with different inlet velocity, in order to calculate the temperature field and estimate the mass fraction of carbon monoxide (CO) at different region of the burner, using the solution of aerothermochemistry equations. The validation is based on experimental data of C.P. David (Sanford-USA. 2001). ____________________________

* Corresponding author. Tel.: +213 33 86 89 75; fax: +213 33 86 89 75. E-mail address: [email protected]

by Elsevier Ltd. Selection and/or peer reviewpeer-review under responsibility The TerraGreen © 2012 2010Published Published by Elsevier Ltd. Selection and/or under of responsibility of Society. [name organizer] Open access under CC BY-NC-ND license. Keywords Combustion; Turbulence; Large eddy simulation (LES); Probability density function (PDF).

Nomenclature Roman Symbols C n p P(…) r t

progress variable Boundary-normal coordinate Pressure Probability density function Radial coordinate Time

1876-6102 © 2012 Published by Elsevier Ltd. Selection and/or peer review under responsibility of The TerraGreen Society. Open access under CC BY-NC-ND license. doi:10.1016/j.egypro.2012.05.123

Fethi Bouras et al. / Energy Procedia 18 (2012) 1086 – 1095

T

Temperature

u

Velocity component

x yi Z

Spatial coordinate Mass fraction of species i Mixture fraction

Greek Symbols ij

Kronecker delta Thermal conductivity Molecular viscosity; Distribution mean Kinematics viscosity Mass density Viscous stress tensor

Abbreviation LES PDF WALE

Large eddy simulation Probability Density Function Wall Adapting Local Eddy Viscosity.

Introduction Advancement of science has helped to develop many domain of life, which helped to create the conditions of human comfort. This shows clearly that the machine replaced human in agriculture and industry. The main result of this advancement is an abundant production with less effort in less time. As well as the means of transportation helped to transport for individuals and goods in a less time in terms of comfort and away from danger. With all these advantages for the humanity, can that assure a clean environment? The pollution has begun since centuries; the development of amalgamation process into industrial operation in 1551 stimulated the massive production of silver in the New World but left behind an unprecedented quantity of mercury pollution. The annual loss of mercury in the silver mines of Spanish America averaged 612 tonnes/year between 1580 and 1900. Approximately 90% of the mercury consumed to the environment due to the production of precious metals in the Americas totaled 257400 tonnes. Approximately 60-65% of the mercury lost is believed to have been released to the atmosphere, suggesting that gold and silver mines were a dominant source of atmospheric mercury pollution. Because of its high volatility, any deposited mercury can readily be re-emitted to the atmosphere [1]. The pollution doesn't only touch the environment where humans live, but can arrive to the sea and ocean. J.G.B. Derraik discussed the pollution of the marine environment and also some of the ways to mitigate the problem. The deleterious effects of plastic and metal debris on the marine environment were reviewed by bringing together most of the literature published so far on the topic. A

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large number of marine species is known to be harmed and/or killed by plastic and metal debris [2, 3], which could jeopardize their survival, especially since many are already endangered by other forms of anthropogenic activities. Marine animals are mostly affected through entanglement in and ingestion of plastic litter. Other less known threats include the use of plastic and metal debris by ‘‘invader’’ species and the absorption of polychlorinated biphenyls from ingested plastics. Less conspicuous forms, such as plastic pellets and ‘‘scrubbers’’ are also hazardous. S.E Atkinson and D.H Lewis (1974) have investigated in the linear programming models purport to minimize the costs of emission control to achieve ambient air quality standards. Many of the simulations incorporate the simplifying assumption that improvements in ambient air quality are proportional to reductions in regional emissions [4]. This approach minimizes the cost of mass emission reduction, but not the cost to achieve a prescribed ambient air quality. The costs of this emissions least-cost strategy are compared to an ambient least-cost strategy which does achieve prescribed ambient air quality at minimum cost. The cost saving achieved by this strategy relative to the emissions least-cost strategy is as much as 50%. In addition, both are compared to strategy typical of those currently used by the states, which found to be as much as ten times as expensive as the ambient least-cost strategy. Progress in engine technology provides powerful engines but with emission of harmful species (CO, N2O and NOx). In the 80s M. Kavanaugh interested on the emissions of CO, N2O and NOx from combustion and were estimated using a common set of demographic, economic and regulatory assumptions [4,5]. The estimates represent a sum of emissions from six large geographical regions in which energy usage was predicted using an economic forecasting model. Analysis was performed for 1960, 1975, 2000 and 2025. Future global CO emissions from combustion are likely to decrease because of regulation in the developed nations and fuel-switching in the developing nations. Future global N2O and NOx combustion emissions are likely to increase unless there is new regulation [5-7]. A. Abu-Jrai et al. (2007) dealt with the reduction of

NOx

and particulate

matter emissions while maintaining efficient combustion performance is one of the main drivers for internal combustion engine research. Modern diesel and premixed charge compression ignition engines have improved engine fuel economy and significantly reduced NOx and emissions achieved by advances in both combustion and exhaust after treatment technologies [8]. To date, it has been shown that vehicle emissions can be further improved by several catalytic systems including fuel reformers (i.e. partial oxidation, autothermal, and exhaust gas reforming) and after treatment systems, such as the selective catalytic reduction of NOx under oxygen-rich conditions. Among the most promising on-board reforming technologies is the exhaust-gas reforming, which allows the fuel/air feed to the engine to be enriched with reformate containing

H2

and CO. This method is a combination of reforming and exhaust-gas

recirculation [4-8]. In 2002, the European research project called MOLECULES (MOdelling of Low Emissions Combustors Using Large Eddy Simulations) was launched. The main objective of this project is the control of combustion air and fuel which has a vital capacity in the development of new


propulsion systems and the sustainability of its business [9, 10]. One of the challenges facing today's aviation industry to support the development of air transport is to reduce the impact of pollutant species on the environment and this, both in terms of noise pollution in areas airport than to the release of chemical species polluting first and foremost found nitrogen oxides (NOx) and gases involved in global warming, such as carbon oxides CO and CO2. Emission reduction of pollutant species from 50 to 25 % and forcing motorists to offer ever more innovative solutions, since the only reduction in specific fuel consumption due to the continuous improvement of current systems is insufficient to achieve the objectives. Among the new technologies that are already in operation include the combustion chamber with two heads (Double Annular Combustor DAC) (A320 CFM56-5B and CFM56-7B B737) proposed since 1995 by CFMInternational (CFMI) and that can lead reductions in NOx emissions up to 40% in some configurations. One of the great difficulties encountered in the development of these systems is to achieve the desired reductions in pollution regardless of the current engine speed and thus to achieve precise control at all engine speeds. This involves in particular by improving injection systems (e.g. Twin-Annular Pre-Swirl by CFMI) to optimize the spray and the vaporization of kerosene and its mixture with air, to burn as much as possible in Lean Premixed Prevaporized (LPP), which is one of the concepts used for the development of future combustion chambers called LPP [9, 10]. C. D. Pierce (2002) focused his work in the development of a large eddy simulation based prediction methodology for turbulent reacting flows with principal application to gas turbine combustors. It is in the gas turbine industry where accurate, high fidelity prediction methods for turbulent combustion are desperately needed for the design of next generation, low emissions combustors. Current practice in the industry relies heavily on method calculations, backed up by expensive physical testing. The trend in the industry and in engineering in general, is towards shorter design cycles through increased reliance on numerical prediction. Mathematic models for turbulent combustion do not appear capable of meeting the needs of industry in this regard, and the industry is beginning to look towards more sophisticated prediction tools that can take advantage of new computational capabilities. In this fact, the advance of computer technology that is the driving force behind increasing expectations for computational fluid dynamics and the main motivation for this study [11,12]. Combustion is a very complex phenomenon and used widely in our lives and in the industry, but it is the source of the pollutant species. The difficult to measure the characteristic parameters in all regions of the burner and given the limitation in the experimental measurement, we choose the simulation using new coupling models LES/PDF to estimate the fields of temperature and mass fraction of carbon monoxide (CO). Following of the work of F. Bouras et al.[12] we continue the thermochemistry part in this paper, using a configuration similar to a combustion chamber of a gas turbine, inject CH4/Air in cylindrical burner, the results will be validated by experimental results [11].

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I. Experimental configuration and computational domain

The experimental reference case used for the validation of the results of the present simulation is depicted in Fig.1. The coaxial jet combustor configuration was the focus of numerous experimental investigations because of both its relatively simple geometry and it’s representatively to the gas turbine burner. The combustion is a cylindrical of ray R4=0.06115 and L=1m in length. The burner is supplied by two coaxial jets. The central one having the internal ray R1=0.03157 m and external R2=0.03175 m, injects the methane with velocity and temperature respectively V1=0.985 m/s and T1= 300 K. The annular one having internal ray R3= 0.04685m, injects the air, with higher velocity V2=20.63 m/s and preheated at temperature T2=750K. The combustion chamber is pressurized to 3.8 atm and has constant temperature wall of Twall=500K [11,12].

Fig 1. Schematic of the coaxial jet combustor experiment.

II. Governing equation Our study, concerns the behaviour of a non-premixed turbulent methane flame in three dimensions using numerical simulation. The equations governing gaseous combustion are summarized below. The filtered governing equations in LES for compressible flow can be written in Cartesian coordinates for a mixture of ideal gases as [10-12]: Continuity:

Momentum:

Mixture fraction

t u~i t

xi

t

( u~i u~ j )

~ Z

xi

xi xi

~ ( ui Z )

( u~i )

[ (u i u j

xi

( D

0

(1)

u~i u~ j )]

xi

~ Z)

p xi

ij

xj

(2)

(3)

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c~ t

Progress variable

xi

( u~i c~ )

Where:

xi

c

xi

.( D

xi

c~ )

c

(4)

i=1, 2, 3 and j=1, 2, 3

~ RmT

p

Thermodynamic state

(5)

The majority of the subgrid models are based on the assumption of Boussinesq which present

~ t ij to the tensor velocity of deformation Sij by the intermediary

the tensor of the unsolved constraints

of a turbulent viscosity. The small scales influence the large scales via the subgrid-scale stress [10-12]:

2

ij

Where

t

~ S ij

1 k ll 3

(6)

ij

k ll is the subgrid kinetic energy. The filtered strain rate tensor is defined by [10-12]: u~ j 1~ 1 u~i ~ S ij u ll ij 3 2 xj xi

(7)

We selected the WALE (Wall-Adapting Local Eddy-Viscosity) eddy viscosity model from Nicoud and Ducros [10,12] to represent the eddy viscosity term in Eq8. The main idea of this model is to recover the proper behavior of the eddy viscosity near the wall in case of wall-bounded flows, while preserving interesting properties such as the capacity to provide no eddy-viscosity in case of vanishing turbulence (property required for the transition from laminar to turbulent states). The major interest of this model first relies on the fact that it needs no information about the direction and distance from the wall (avoiding the use of any damping function) thus being really suitable for unstructured grids, where evaluating a distance to the wall is precarious. The residual stress tensor of the WALE eddy viscosity model can be found as [10,12]:

( s d s d )3 / 2 (Cw ) 2 ~ ~ 5 /ij2 ij d d 5 / 4 ( sij sij ) ( sij sij )

t

1 ~2 ( g ij 2

s ijd

And

g~ij

Where

g~ 2ji ) u~i xj

Cw: WALE model constant (Cw =0.49). The

1 ~2 g kk 3

ij

(8)

(9)

(10) is the spatial filter width, which is generally

related to the grid size of the resolved field. When Favre filtering is used for the scalar variables, it is more appropriate to evaluate filtered quantities using the joint Favre PDF of the subgrid scalar fluctuations. Favre-filtered quantities would be evaluated using [11,12],

~ y

1 Z 0

~ y ( Z ) P ( Z )dZ

(11)

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In this work, the probability density function (PDF) methodology is employed as a subgrid scale (SGS) closure in LES of turbulent non-premixed Methane/Air flames. The joint probability density function of the SGS scalars is determined via the solution of its modelled transport equation. III. Results and discussion Simulation is performed under the same experimental conditions, using the coupling models LES/ PDF. The parameters that are used to validate the simulation are the temperature and mass fraction of carbon monoxide, were compared with experimental data, considering R R3. III.1. Temperature Distribution of the temperature field is shown in Fig.2. Comparison of predicted temperature profiles with experimental data is shown in Fig. 3. Temperature is a quantity that is derived from the mixture fraction and progress variable by assuming isothermal walls and neglecting thermal radiation. Where these assumptions are valid, the temperature can be expected to behave very similarly to product mass fraction, but where the assumptions breakdown, an overprediction of temperature is expected. Therefore, if product mass fraction predictions are in good agreement with experimental data, discrepancies between predicted and measured temperature profiles must be due to the breakdown of these assumptions or to experimental error, which owing to differences in measurement technique between species concentrations and temperature can be significant. One of the investigators involved with the experiment has stated that the temperature data have been measured using a rather large, invasive, and dynamically unresponsive thermocouple probe, are in fact subject to considerable experimental uncertainty, especially in regions with large temperature fluctuations [11]. It could also be the case in which thermal radiation is nonnegligible in some regions of the flow, particularly in fuelrich, slow moving regions where soot formation is likely and residence times are long enough for radiative effects to accumulate. It is also important to note that the axial measurement stations used for temperature are different from those used for species concentrations. Since the experiment had isothermal, water-cooled walls at roughly 500K, thermal boundary layers would be expected to develop, affecting the temperature close to the wall (Fig. 3). The annular air stream (at 750K) tends to create an insulating sheath between the hot combustion products and the wall, although it appears that the flame in Fig. 2 does occasionally brush up against the wall. These factors should account for the good agreement of the selected model.

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Fig.2 Distribution of the temperature in the burner.

2400

2400

x/R=4,52

x/R=5,20 1800

T(k)

T(K)

1800

LES/PDF EXP

1200

600

0,0

LES/PDF EXP

1200

600 0,2

0,4

0,6

0,8

1,0

1,2

0,0

0,2

r/R

0,4

0,6

0,8

1,0

1,2

r/R

Fig. 3 Radial profiles of temperature.

III.2.

Carbon Monoxide

The improvement is to consider tow new variable (the mixture fraction and the progress variable) without need to use Arrhenius equation, in order to estimate production or consumption of each species considered in the reaction. Fig. 4 presents the CO mass fraction distribution in the burner, and shows that CO is more found in the center of the burner at the fuel-rich region (the injector of CH4). In addition Fig .5 shows the results of the simulation compared to the experimentally predict values of CO mass fraction. The results obtained of the simulation clearly show good agreement with experiment data in tow stations (Fig.5). Carbon monoxide is a significant species in the fuel-rich interior region of the flame, because dissipation rates are low in this region, which have low temperatures and high concentrations of CO in fuel-rich mixtures (Fig.5). This part is mapped by the present simulation, so that such high CO chemical states can be accessed relatively, regarding previous study using the same configuration, for more detail see [11].

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Fig.4 Distribution of the carbon monoxide in the burner.

0,5

0,5

x/R=3,16

0,4

LES/PDF EXP

0,2

0,2

0,1

0,1

0,0 0,0

0,2

0,4

0,6

LES/PDF EXP

0,3

yCO

yCO

0,3

x/R=3,84

0,4

0,8

1,0

1,2

0,0 0,0

0,2

0,4

0,6

0,8

1,0

1,2

r/R

r/R

Fig. 5 Radial profiles of CO mass fraction.

IV. Conclusion The relationship between industrial pollution and economic development created the challenges facing the industry today on supporting the development of motor transport is to reduce the impact of harmful gases and products on the environment and this, both in terms of noise pollution in airport zones to that of rejection of chemical species polluting first and foremost found nitrogen oxides (NOx) and gases involved in global warming, such as carbon oxides CO and CO2. Emission reductions of pollutant species from 25 to 50% and the agenda and forcing motorists to offer ever more innovative solutions, since the only reduction in specific fuel consumption due to the continuous improvement of current systems is insufficient to achieve the objectives. In this paper, we have used new models in order to estimate the distribution of the temperature field and Carbone monoxide (CO) in combustion chamber, using the filtered governing equations for gaseous combustion, basing on the experimental data for the validation. Finally, the following conclusion may be drawn from our investigation:


LES/PDF is a modeling technique that offers much potential for studies where the resolutions of flow details are important like detection of the morphology of flow (flame holder, recalculating region), The predicted of coupled models LES/PDF result shows a good agreement with the experimental data, and also gave results of somewhat quantitatively and qualitatively satisfactory and they may give some information for different physical parameters (velocity, pressure...) and other chemical species (CO2, OH, H2O, C(s)…) for the mean axial velocity, the position of peak velocity and turbulent intensity. The advantage of the utilization of PDF is to evaluate the mean of scalar parameters and to integrate the mixture fraction in order to predict different species mass fraction without using Arrhenius’s law. Results obtained in this work allow us to exploit them directly in other domains: Exergy analysis, environment. References [1] Nriagu, J.: Mercury pollution from the past mining ogf gold and silver in the Americas. The science of the total environment. 149, 167-181 (1994). [2] Derraik, J.G.B.: The pollution of the marine environment by plastic debris: a review. Marine Pollution Bulletin. 44, 842–852(2002). [3] Rubio, B., Nombela M. A. and Vilas F.: Geochemistry of Major and Trace Elements in Sediments of the Ria de Vigo (NW Spain): an Assessment of Metal Pollution. Marine Pollution Bulletin. 40, 968-980(2000). [4] Atkinson, S.E. and Lewis, D.H.: A Cost-Effectiveness Analysis of Alternative Air Quality Control Strategies. Journal of environmental economics and management. 1, 237- 250 (1974). [5] Kavanaugh , M.: Estimates of future co, N2O and NOx emissions from energy combustion. Atmospheric environment . 21, 463-468 (1987). [6] Wang, L., Haworth, D.C., Turns, S.R. and Modest, M.F.: Interactions among soot, thermal radiation, and NOx emissions in oxygen-enriched turbulent nonpremixed flames: a computational fluid dynamics modeling study. Combustion and Flame. 141, 170–179(2005). [7] Guo, H. and Smallwood, G. J.: The interaction between soot and NO formation in a laminar axisymmetric coflow ethylene/air diffusion flame. Combustion and Flame. 149, 225–233(2007). [8] Abu-Jrai ,A and Tsolakis., A.: The influence of H2 and CO on diesel engine combustion characteristics, exhaust gas emissions, and after treatment selective catalytic NOx reduction. International Journal of Hydrogen Energy.32, 3565-3571(2007). [9] Nguyen, P.D., Bruel, P. and Reichstadt, S.: An Experimental Database for Benchmarking Simulations of Turbulent Premixed Reacting Flows: Lean Extinction Limits and Velocity Field Measurements in a Dump Combustor. Flow, Turbulence and Combustion. 82,155–183(2009). [10] Bouras, F and Soudani, A .: Impact of the equivalence ratio and the mass flow rate on turbulent lean premixed prevaporized combustion. Energy Procedia. 6, 251–260(2011). [11] Pierce, C. D. and Moin, P.: Progress-variable approach for large-eddy simulation of non-premixed turbulent combustion. Journal of Fluid Mechanics. 504, 73-97 (2004). [12] Bouras, F., Soudani,A. and Si Ameur, M.: Beta-pdf approach for large-eddy simulation of nonpremixed turbulent combustion. International Review of Mechanical Engineering. 4, 358-363(2010).

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