Introduction to Complex Combustion

1. Turbulent mixing of fuel and air
2. Heat and mass transfer (multiple species)
3. Chemical kinetics associated with different phases
4. Phase change (from liquid to vapor) of fuel
5. Solid fuel (coal) particle interaction with air or combustible gas
6. Formation of char
7. Devolatilization of solid fuel (coal)
8.  Pyrolysis of  fuel
9. Formation of soot and emissions.
10. Formation of ash

• Rocket engine combustion where multi-phase combustion plays significant role due to fuel injection in the combustion chamber

                                         

Domestic cooking or heating

• • In a domestic kerosene  stove, fuel evaporates by absorbing heat and mixed with air
  • The flame is formed where evaporated fuel and air meet at stoichimetric level

• • Principle of Liquid Fuel Combustion: phase of liquid fuel is important

Automotive Power Generation

• • In, the diesel engine fuel is injected into the cylinder. Here, the phase change of diesel from liquid to gas is achieved with the help of injector
  • Atomization of droplets is modeled as multi-phase  CFD modeling
  •  Schematic diesel fuel combustion

• Dual Fuel Combustion Engine
  •  In dual fuel combustion engine, diesel fuel is injected. Hence, understanding of multi-phase combustion is essential

•  Dual fuel injection system for power generation

 Solid Fuel Combustion

• Wood and coal are used for domestic and power plant generation
• Understanding  Solid combustion is essential as it is the most complex combustion phenomenon due to much more complex physical and chemical processes
• The following example shows devolatilization of coal. It results in the formation of volatile matter (hydrocarbon in a gaseous state) and char (hydrocarbon in solid-state)

  CFD Modeling of Coal Fired Boiler

• Computational fluid dynamics (CFD) is used for simulating combustion devices connected with fluid flow, heat transfer, and chemical reactions. 
• The method uses well-known complex mathematical equations (Conservation of Mass, Momentum, and Energy) which are not enabled to solve in even simple cases without powerful computers. Basic CFD modeling is available on this website which covers basic governing equations and numerical schemes in ANSYS FLUENT and Open FOAM
• With the increasing of computers power, CFD methods found application in mechanical and thermal power engineering.
•  CFD is able to obtain more accurate and cost-effective information
  • Velocity and temperature of flue gases
  • Mixing of fuel and air
  • Flame pattern

•  PC coal-fired furnace and selection typical CFD sub-models are required to model physical and chemical  processes

• The onset of devolatilization is generally set at a particle temperature  and is  modeled using a single-rate kinetic model
• Governing equation for rate of volatilization is as follows

• The char oxidation is considered using the diffusion-kinetics limited model
• The gas-phase volatile and CO reactions are modeled using the two-step global mechanism
• In this model, the pseudo volatile molecule is assumed to be released from the solid phase during devolatilization
• The volatile reacts to form carbon monoxide which together with the carbon-monoxide formed during char oxidation, reacts with oxygen to form carbon dioxide 
•  The gas-phase reaction rates is calculated using the eddy-dissipation finite rate model
• The reaction rates is given as below. Where A and B are model empirical constants.

• The heat transfer to the furnace walls, platen superheater, and final superheater for the case study boiler is mainly due to radiation. 
• The radiation transport is generally solved using the Discrete Ordinates Method (DO).
• The gas mixture absorption coefficient was calculated using the domain-based weighted sum of gray gases model (WSGGM) which accounts for the radiation from the tri-atomic gas species CO2 and H O2
•  The radiation properties of the particles are set to constant values for the emissivity and scattering factor 

•   Boiler model Swirl burners

•  Boiler with tangential burners

1. OpenFOAM_Steady-State Lagrangian Coal Combustion Solver
2. OpenFOAM_Report_Combustion_Modeling
3. Modeling Turbulent Non-Premixed Combustion in Industrial Furnaces

 Type of  Liquid Fuel Combustion 

a) Pool Fire 

• A pool fire is a type of diffusion flame where a layer of volatile liquid fuel is evaporating and burning. 
• The fuel layer can be either on a horizontal solid substrate or floating on a higher-density liquid, usually water.
•  Pool fires are an important scenario in fire safety science, as large amounts of liquid fuels are stored and transported by different industries.

• Spray combustion is a commonly used method for  burning of  liquid fuels which are relatively less volatile 
• This method is primarily adopted to burn heavy fuel oils
• Major spray regions are shown below: 1) Primary breakup, 2) Secondary breakup

• Diffusion flame due to spray mixing of fuel and air: the following regions are formed
  • Air entertainment zone
  • Droplet collision and coalescence
  • Ignition zone
  • Diffusion flame
  • Formation of soot and NOx emissions

Fundamentals of Liquid Sprays

 Spray Regimes

• Diesel engine sprays are generally  of the full-cone type
•   Classification of spray breakup: 1) Primary break up, 2) secondary break up

a) Primary Break Up

• The primary breakup mechanism is related to the breakup of the intact liquid core 
• It can be divided into four regimes:  the Rayleigh regime, the first and second wind-induced regimes and the atomization regime.
• For quantitative classification of the primary regimes, the Ohnesorge number Oh is introduced:

• The Ohnesorge diagram represents  four primary breakup regimes, and region for diesel injection application indicated

b) Secondary Breakup 

• The secondary breakup mechanism deals  the breakup of droplets due to aerodynamic forces that are induced by the relative velocity between the droplets and the surrounding gas
•  On the gas-liquid interface, instable growth of waves occur, while in the same time surface tension counteracts the disintegration process
• Effect of weber number on break up process

ANSYS FLUENT For Spray Combustion

 VOF – DPM spray model

• The VOF -DPM spray model is a new hybrid multi-phase model  in FLUENT that will  simulate spray processes with the finest details
• In this model, two primary models are well established
  • The Volume of Fluid (VOF) model tracks the liquid-gas interface
  • The Discrete Phase Model (DPM) is a separate solver to  tracks discrete particles suspended in an Eulerian (continuous) phase
• This hybrid model changes from a full VOF to a DPM solution is shown in the picture below.

2) Pressure-Swirl Atomizer Model

• It is also referred to by the gas-turbine community as a simplex atomizer.
• This type of atomizer accelerates the liquid flow through nozzles called as swirl ports into a central swirl chamber
•  The swirling liquid pushes against the walls of the swirl chamber and develops a hollow air core. 
• It then emerges from the orifice as a thinning sheet, which is unstable, breaking up into ligaments and droplets.
•  The pressure-swirl atomizer is very widely used for liquid-fuel combustion in gas turbines, oil furnaces, and direct-injection spark-ignited automobile engines.
•  The transition from internal injector flow to fully-developed spray can be divided into three steps: film formation, sheet breakup, and atomization. 
• A schematic  of how this process is presented as below

• In ANSYS FLUENT, the pressure-swirl atomizer model  is called as the Linearized Instability Sheet Atomization (LISA) model of Schmidt et al. [ 308]. 
• The LISA model is divided into two stages: i) film formation, ii) sheet breakup and atomization
• For more detail refer: FLUENT_Pressure-Swirl Atomizer Model_Theory

3) The Air-Blast/Air-Assist Atomizer Model:

• To accelerate the process of  breakup of liquid sheets from an atomizer, an additional air stream is often directed through the atomizer
• The liquid is formed into a sheet by a nozzle, and air is then directed against the sheet to promote atomization. 
• This method is called air-assisted atomization or air-blast atomization process which depends on the amount of air and its velocity. 
• By adding the external air flow stream past the sheet results in smaller droplets without the air. However,  the exact mechanism for this enhanced performance is not well understood, it is considered that the additional air can  accelerate the sheet instability
• The air may  help disperse the droplets, preventing collisions between them.
•  Air-assisted atomization is used in many of the similar  applications: pressure-swirl atomization, where especially fine atomization is required.
• The air-blast atomizer model does not contain the sheet formation equations
• The air-blast atomizer model assumes that the sheet breakup is due to short waves 
• Click here for more detail:ANSYS_The Air-Blast_Air-Assist Atomizer Model_Theory

a) Fire FOAM

b) SprayFOAM

• The FireFOAM solver provide  a  simulation of  Lagrangian sprays Example:  sprinkler sprays for fire suspension. 
• Application of SprayFOAM:  turbulent dispersion, liquid injection, liquid atomization, droplet breakup or evaporation, droplet-wall interaction and surface film.

Author links open overlay panel

•    Diesel engine cycle for power generation

1. Fluent Diesel Engine Simulatio_PDF
2.   U. V. Kongr , V. K. Sunnapwar CFD Modeling and Experimental Validation of Combustion in Direct Ignition Engine Fueled with Diesel, Int. J. Applied Eng. Research, Vol.1, 3(2010)
3.  Mahaer et al., CFD Modeling of Spray Formation in Diesel Engines, Athens J. of Technology and Engineering (2017)

• DieselFOAM is the solver for DI engine combustion modeling 
• The dieselFoam solver uses the dieselSpray library to simulate the combustion of diesel spray
•  A  sprayFoam solver has now been introduced that can simulate flow and combustion in any spray using the lagrangianSpray library and other lagrangian libraries. 
• The aachenBomb tutorial, formerly an example case for the dieselFoam solver, has now been set up for sprayFoam to allow comparison between the old and new libraries.
• Meaning of sub-models 

1. 1. atomizationModel:  How atomization is treated
   2. breakupModel: If secondary break up is used
   3. injectorModel: Which injector model to use
   4. collisionMode:l Particle – particle interaction
   5. evaporationModel Which evaporation model to use
   6. heatTransferModel: Particle heat transfer model
   7. dispersionModel: If turbulent dispersion is used or not
   8. dragModel Particle drag model
   9. wallModel:  What happens to particles hitting the walls

Jet Engine Combustion Modeling

• The combustion chamber is a critical part of the jet engine in any aircraft
• In jet engine, air enters the front intake and is compressed. Then the air is forced into combustion chambers where fuel is sprayed into it, and the mixture of air and fuel is ignited
• These gases exert equal force in all directions, providing forward thrust as they escape to the rear.
• The thrust from one or more engines pushes a plane forward, forcing air past its scientifically shaped wings to create an upward force called lift that powers it into the sky.
• Combustion Chamber in a typical aircraft

• In the combustion chamber, fuel is mixed with air to produce the bang, which is responsible for the expansion that forces the air into the turbine.
•  Inside the typical commercial jet engine, the fuel burns in the combustion chamber at up to 2000 degrees Celsius.
•   Combustion Chamber 

• Early gas turbine engines used a single chamber known as a can-type combustor.
•  Today three main configurations of combustion chambers are used in aircraft: can, annular, and cannular (also referred to as can-annular tubo-annular). 
• Afterburners are another type of combustor.

• CFD modeling of jet engine combustion is similar to spray combustion modeling, only additional swirling effects are applied to fuel jet for faster and strong turbulent mixing with air
• Velocity and Temperature contours of the jet engine combustion chamber. CFD results are presented from CSIR -NAL projects

• Combustion Model: The phenomenon of afterburner  modeled as  turbulent diffusion (non-premixed) combustion using the PDF approach (mean mixture fraction and its variance)
• Turbulence Model: the flow through  the afterburner has a swirling component, hence RNG k-e turbulence model is used
•  CFD results of afterburner combustion for thrust vectoring nozzle

(a) Velocity contours

Summary

• Physical understanding of turbulent multiphase combustion is complex due to turbulent flow, phase change or interaction of particles, unclear chemical kinetics due to pyrolysis of solid fuels, combined convective and radiative heat transfer from flue gases containing water vapor and carbon dia-oxide, etc.
• Numerical modeling of turbulent multiphase combustion is possible using commercial and open-source CFD solvers. The combustion models need to be well validated against power plant data for the heat absorption rate of boiler components to optimize the performance of burners.

1.  Kenneth K. Kuo, Ragini Acharya,  Fundamentals of Turbulent and Multiphase Combustion, Wiley Pub., 2012
2. Alan Williams, The Combustion of Liquid Fuel Sprays , Butterworth-Heinemann Ltd (1 March 1990)
3. Kenneth K. Kuo, Recent Advances in Spray Combustion: Spray Atomization and Drop Burning Phenomena, AIAA (1996)
4. Lakshmi, P. A., Aghav, Yogesh V., Modeling Diesel Combustion, Springer (2010)

Research Article:

1. A. Kourmatzisa,∗ , P.X. Phama , A.R. Masr , Characterization of atomization and combustion in moderately dense turbulent spray flames 1, Elsevier
2. Refer : International Journal of Spray and Combustion Dynamics
3. Masayuki Taniguchi,Fundamental Experiments of Coal Ignition for Engineering Design of Coal Power Plants,Zhao F. Tian, Peter J. Witt, Mark P. Schwarz, and William Yang, Numerical_Modelling_of_Pulverised_Coal_Combustion, Springer (2016)
4. Masayuki Taniguchi,Fundamental Experiments of Coal Ignition for Engineering Design of Coal Power Plants, Zhao F. Tian, Peter J. Witt, Mark P. Schwarz, and William Yang, Numerical_Modelling_of_Pulverised_Coal_Combustion, Springer (2016)

For more detail : NPTEL_Video_Spray Theory and Applications by Prof. Mahesh Panchagnula