CFD Modeling of Turbulent Multiphase Combustion
(Introduction to Complex Combustion Models in ANSYS FLUENT and OpenFOAM)
Dr. Sharad N. Pachpute (PhD, IIT Delhi)
Modeling the Combustion with Complex Physics and Chemistry
1. Introduction to Complex Combustion
- Turbulent mixing of fuel and air
- Heat and mass transfer (multiple species)
- Chemical kinetics associated with different phases
- Phase change (from liquid to vapor) of fuel
- Solid fuel (coal) particle interaction with air or combustible gas
- Formation of char
- Devolatilization of solid fuel (coal)
- Pyrolysis of fuel
- Formation of soot and emissions.
- Formation of ash
Application of Liquid Fuel Combustion
- Thrust Generator for aviation
- Multiphase combustion is useful in jet engines, where, fuel injected into the combustion chamber
- Turbojet or turbofan engines are examples of multi-phase combustion
- In space application, fuel and oxidizer are stored in liquid phase with high pressure
- Rocket engine combustion where multi-phase combustion plays significant role due to fuel injection in the combustion chamber
- Liquid jet Combustion for Rocket V2 Engine is shown below
Domestic cooking or heating
- In 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 duel 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 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 gaseous state) and char (hydrocarbon in solid state)
2. Modeling of Coal Fired Boiler
2.1 Processes for Pulverized Coal Fired Furnace
- With advances in computing power and CFD modelling techniques, computational
- Computational Fluid dynamics (CFD) modeling of coal fired furnace is feasible for scientists and engineers
- Combustion in furnaces is a complex topics of physics and chemistry. Hence, CFD modeling of coal fired boiler has been a topic research for many years
Major physical and chemical processes that occur during the burning of pulverized coal particles in a PC coal fired furnace.
- Fine coal particles which are pulverized in mills. After that they are blown into the furnace through burners
- Once the particles enter the furnace, they are heated by hot furnace gasesand radiation from the flame, they start to dry when their temperature reaches about100–110 C
- When the particles are heated further up to a certain critical temperature (depending on the coal type and size), devolatilisation starts and volatiles are released from the particles
- The products of devolatilisation comprises non-condensable volatiles (light gases), condensable volatiles (tars) . The remaining solid particles generally contains char and minerls
- The volatiles (hydrocarbon) react with oxygen from the combustion air and other oxidants in the combustion chamber ( furnace)
- Finally char particles react with gases in the furnace, leaving mineral matter and probably a small fraction of unburnt char in the solid particles.
- These particles (ash) and the furnace gas flow through convective heat transfer sections such as superheaters, reheaters and economisers, exchanging heat with the working fluid (water/steam) in the convective devices.
- Most exhaust gases comprise CO2, N2, O2, H2O, and small amounts of NOx, SOx, CO and particulate matters (PM).
- After leaving the convective passes exhaust gases normally go through various air pollution control equipment (NOx and particle control units) before being discharged through the stack.
- A general layout of Boiler is shown as below for pulverized coal fired sub-critical boiler
- CFD is used for simulating processes and 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 enable to solve in even simple cases without powerful computers.
- With 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
Selection of CFD models.
- As per physics and chemical kinematics of boiler, the various CFD models are selected
- Following are key parameters for selection of models
- Type of fuel and fuel compostion
- Type of burners ( fuel staged or air low NOx staged)
- Configuration of furnace or boiler
- Type of combustion: premixed, non-premixed, partially premixed
CFD models for Coal combustion
- There are a variety of models for different physics of flow and heat transfer. It is important to select well trusted validated CFD models.
- Note that none of CFD model can give correct results unless they are well validated against experimental or power plant data
2.3 Modeling of Coal Combustion
Apart from the selection of turbulence model and enthalpy transport , we have to selection the coal combustion as following ways:
- Fuel (coal) Composition:
- Define fuel composition data based on proximate and ultimate analysis
- Select the Multi-phase model for coal and air combustion:
- The DPM coal combustion simulation is commonly used for thee non-premixed combustion.
- Select the devolatile and char model in the DPM model:
- No gas phase fuel inlets will be included and the sole source of fuel will come from the coal devolatilization and char burnout.
- Set Coal (particle) Injection Properties:
- Particle size
- Particle Distribution
- particle flow rate (kg/s) or velocity (m/s)
- moisture content
- temperature of particle
- Select the Mixture Material:
- Select Species Transport (Non-premixed combustion) Model:
- Select the number of volumetric and solid species,
- Select Volumetric and Particle Surface reactions
- Turbulence -Chemistry Interaction
- Coal composition and reaction type
- Set Combustion reactions:
- Set the volumetric and particle surface reactions with reactant or product species
- Select the stoichimetric coefficients
- reaction constants as per the chemical kinetics
- Select Turbulence-Chemistry Interaction Model
- Finite rate rate
- Finite rate /EDC
- Eddy Dissipation Concept (EDC)
- Define the Inlet Properties of Air:
- Select the mass flow rate
- temperature of air
- mass /mole fraction oxygen in the air
- Select the Pollution models:
- Define types of models for Soot formation.
- Select NOx or SOx model:
- Define the pathway Thermal, Prompt or Fuel
- Define fuel stream or raeburn species, equivalent ratio
- Single variable: Temperature or mixture fraction
- Click here: a pollution modeling lecture on CFD modeling of NOx, SOx and soot
2.4 Devolatilization Model:
- The onset of devolatilization is generally to set at a particle temperature and is modeled using a single-rate kinetic model
- Governing equation for rate of voliatilization is as follows
2.5 Char Oxidation Model:
- 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, 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.
- A video Tutorial on CFD modeling coal combustion and NOx pollution modeling is presented below. This is a 2D axis symmetric multiphase coal problem. There are 6 reactions are involved
- 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
Modeling of Slagging /fouling of tubes;
2.7 CFD Modeling for Different Type of Burners in Coal Fired Boiler
- Using CFD analysis , various results can be presented
2.8 Modeling of Coal Combustion using ANSYS FLUENT:
- FLUENT Solver: CFD MODELING OF PULVERIZED COAL COMBUSTION IN AN INDUSTRIAL BURNER
- FLUENT Solver: Simulation of Combustion and Thermal-flow Inside a Petroleum Coke Rotary Calcining Kiln
- FLUENT Paper:R. Laubscher, P. Rousseau, CFD study of pulverized coal-fired boiler evaporator and radiant superheaters at varying loads, Applied Thermal Engineering 160(2019) 114057
2.9 Modeling of Multiphase Combustion using OpenFOAM:
- A new steady lagrangian solver which uses ‘coal’ parcels has been implemented: simpleCoalParcelFoam.
- The solver is very similar to the existing transient coalChemistryFoam solver.
- The solver employs a single cloud of coal particles, which can undergo evaporation of any liquid/vapour content and devolatilisation to the carrier phase, and surface reactions.
- The carrier phase includes support for turbulence, heat transfer and combustion modelling.
- The solver is based on the rhoThermoCombustion combustion class and therefore the thermo type must be set to heRhoThermo.
3. Modeling of Liquid Fuel or Spray Combustion
3.1 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
3.2 Fundamentals of Liquid Sprays
- The method of injecting liquid fuel through small holes is called the process of spray formation
- The flow physics of spray formation proves to be extremely complex. However, the analysis of liquid spray formation is carried out either numerically or experimentally
- CFD models are used to get more details of spray combustion with the help of correct multi-phase models
- Details of full cone example are shown below
- The spray regions are explained in next sections
3.3 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
- Based on the relative size of droplet and nozzle, the primary breakup regimes are defined
- Three major regions : 1) Rayleigh regime, 2) Wind-induced regime, 3) Atomization regime
- 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
3.4 ANSYS FLUENT For Spray Combustion:
1) 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 a 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
3.5 OpenFOAM for Modeling of Liquid Fuel Combustion
a) Fire FOAM:
- Transient solver for fires and turbulent diffusion flames with reacting particle clouds, surface film and pyrolysis modelling
- It considers the evolution of particles, a liquid film on the surface of solid boundaries and the influence of pyrolysis within the framework of a compressible solver able to consider combustion and radiation.
- FireFOAM is used for modelling problems relevant to thermo- and fluid-dynamics and multiphase flow.
- it is able to run simulations using both Large Eddy Simulation (LES) and Reynolds-Averaged Navies-Stokes (RANS) turbulence models
- Fore more detail click here: Wiki_FireFoam_Equation_Solution
- 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.
- C. A. Sedano,Prediction of a Small-Scale Pool Fire with FireFoam, International Journal of Chemical Engineering ,Hindawi (2017)
- T Myres, A. Trouve, A. Marshall, Predicting sprinkler spray dispersion in FireFOAM, Fire Safety Journal (2008) 93-102
4. Modeling of Diesel Combustion
4.1 Introduction to Diesel Engine combustion:
- In the diesel engine, the air and fuel mixture is ignited by compressing air in the combustion chamber to the point that the air becomes very hot,
- Just after the compression, the air fuel mixture is injected in at very high pressure.
- The heat from the compressed air ignites the air-fuel mixture
- The Self Ignition Temperature of Diesel is 210°C
- Diesel engine cycle for power generation
Operating range of diesel engine cycle
4.2 Selection of Models for Diesel Combustion :
- Select Turbulence Model:
- The RNG k ε model can derived used a thorough statistical technique. This model includes the effect of swirl , which is essential for ICE combustion for strong mixing of fuel and air
- Other models can be selected as per scope of modeling
- Specify the motion of Piston: The motion of the piston is specified as a function of the engine’s crank angle, crank radius, connecting rod length and engine speed. The piston location is calculated using the UDFs
- Spray Model:
- FLUENT offers two spray breakup models example, the TAB and the wave model.
- The TAB model is based on the analogy between an oscillating and distorting droplet and a spring mass system.
- The distorting droplet effect is considered
- Select Droplet collision model
- Droplet collision model consider tracking of droplets; for estimating the number of droplet collisions and their outcomes in a computationally efficient manner
- Wall-film model:
- Spraywall interaction is an important part of the mixture formation in diesel engines
- In a DI engine, fuel is injected directly into the combustion chamber, where the spray can impinge upon the piston.
- The modeling of the wallfilm inside a DI engine is compounded by the occurrence of carbon deposits on the surfaces of the combustion chamber.
- This carbon deposit soak up the liquid layer.
- It is understood that the carbon deposits adsorb the fuel later in the cycle. The wallfilm model ANSYS FLUENT allows a single constituent liquid drop to impinge upon a boundary surface and form a thin film.
- Interactions during impact with a boundary and the criteria by which the regimes are detached are based on the impact energy and the boiling temperature of the liquid
- Combustion Models:
- Select non-premixed combustion models
- The combustion model is combined with species transport and finite rate chemistry with simplified chemistry reactions to simulate the overall combustion process in a diesel engine.
- This approach is based on the solution of transport equations for species mass fractions.
- The reaction rates that emerge as source terms in the species transport equations are computed from well known Arrhenius rate expressions.
- Emission Models
4.3 Modeling of DI Engine using ANSYS FLUENT
- Fluent Diesel Engine Simulatio_PDF
- 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)
- Mahaer et al., CFD Modeling of Spray Formation in Diesel Engines, Athens J. of Technology and Engineering (2017)
4.4 Modeling of DI Engine using OpenFOAM
- DieselFOAM is the solver for DI engine combustion modeling
- The dieselFoam solver uses thedieselSpray library to simulate 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 and 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
- atomizationModel: How atomization is treated
- breakupModel: If secondary break up is used
- injectorModel: Which injector model to use
- collisionMode:l Particle – particle interaction
- evaporationModel Which evaporation model to use
- heatTransferModel: Particle heat transfer model
- dispersionModel: If turbulent dispersion is used or not
- dragModel Particle drag model
- wallModel: What happens to particles hitting the walls
5. Jet Engine Combustion
5.1 Combustion in Aircraft Jet Engine
- The combustion chamber is a critical part of 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
- Location of Jet Engine
- For Airbus A380, the jet engine is placed just below of wings to generate thrust
- Rolls -Royce Trent 900 engine includes fans, compressor, combustion chamber and gas turbine
- Schematic of Jet Engine
- General Electric (GE) jet engine has a fan made for composite materials with aerodynamic design
- Cut way of Rolls Royce Combustion chamber is shown below
5.2 Basic Principle of Jet Engine Combustion
- 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
- Principle of Jet Engine Combustion and Combustion Zones
Classification Combustion Chamber in Jet Engine
- Early gas turbine engines used a single chamber known as a can type combustor.
- Today three main configurations of combustion chambers are used in aircrafts: can, annular and cannular (also referred to as can-annular tubo-annular).
- Afterburners are another type of combustor.
5.3 CFD Modeling of Jet Engine Combustion
- 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.
6. Afterburner Jet Engine Combustion
- After burners are widely used in modern fighter jets to produce additional thrust by burning some fuel in the exhaust nozzles just after the turbines
- After burners provides additional power during quick take off
6.1 Working principle of after burner:
- The following figures shows how after burner combustion is carried out in the nozzle by injecting fuels
- Cooling channels are provided to avoid overheating of exhaust nozzles and to maintain structural strength
- The exhaust nozzle can be 2D or 3D thrust vectoring in order make the fighter jet more manurebale
- Principle of After burner combustion in a jet engine
- After burner with thrust vectoring nozzle
- The exhaust gas nozzle angle can be shifted as per maneuverability of fighter jet
6.2 CFD Modeling of after burner
- Combustion Model:The phenomenon of after burner modeled as turbulent diffusion (non-premixed) combustion using the PDF approach (mean mixture fraction and its variance)
- Turbulence Model: the flow through the after burner has a swirling component, hence RNG k-e turbulence model is used
- CFD results of after burner combustion for thrust vectoring nozzle
(a) Velocity contours
Reference for CFD of after burner:
- D. Sathish, CFD Anaylsis of Improving Thrust in Afterburner By Configuration Changes, Int. Global J. for Research, ISSN No 2277 – 8160 (2014,)
- S. Unaune , V.Ganeshan, Analysis of reacting flows in an aero-engine afterburner using CFD , Indian J. Eng.of Mat.Sc.(2003)
- X.Z.Chen, I. Langella, N. Swaminathan, The Role of CFD in Modern Jet Engine Combustor Design, Int. Tech. Open access,(2019)
- CFD Analysis of Flow in After Burner – wseas
- G.S Rao, A. Saija,Computational Analysis of Reacting Flows in Afterburner, vol.41, Heat Trans. Engg., vol.41,(2020)
- Kenneth K. Kuo, Ragini Acharya, Fundamentals of Turbulent and Multiphase Combustion, Wiley Pub., 2012
- Alan WIlliams, The Combustion of Liquid Fuel Sprays ,Butterworth-Heinemann Ltd (1 March 1990)
- Kenneth K. Kuo,Recent Advances in Spray Combustion: Spray Atomization and Drop Burning Phenomena, AIAA (1996)
- Lakshminarayanan, P. A., Aghav, Yoghesh V., Modeling Diesel Combustion, Springer (2010)
- A. Kourmatzisa,∗ , P.X. Phama , A.R. Masr ,Characterization of atomization and combustion in moderately dense turbulent spray flames 1, Elseiver
- Refer :International Journal of Spray and Combustion Dynamics
- 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)
- 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)
Fore more detail :NPTEL_Video_Spray Theory and Applications by Prof. Mahesh Panchagnula