CFD Modelings of Oil Fired Burner

How to model an Oil-fired burner in CFD simulation?

 

Introduction to Oil Burner

  • An oil burner is used in many domestic and industrial application like heating oils, diesel fuel or other similar fuels.
  • The diesel with the grade of ultra-low 2  is the common fuel in the United States
  • An oil burner is commonly used in oil furnace, water heater, or boiler
  • It provides the ignition of heating oil biodiesel fuel used to heat either air or water via a heat exchanger
  •  The oil fuel is atomized into a fine spray by forcing it under pressure through a nozzle or fuel injector. Droplet evaporates and mixes with the surrounding air
  • The resulting flame formation depends on a specific flow rate, angle of spray and pattern (variations of a cone shape)
  • The mixture is ignited by an electric spark or pilot in the oil-fired burner

Working Principle of Oil-Fired Burner

Fuel Oil Preparation

  • Filtering the oil and pumping the oil and heating are the major functions for the preparation of oil for firing
  • Filtration of oil can be carried out in one or more stages to remove any dirt, dust, and sediments
  • This can lead to long trouble-free service of the pump, fuel valves, and atomizer or injector work properly
  • Pumping the oil to overcome pressure drop in the long oil supply lines and delivery

Fuel Oil Atomization

    • Atomization is the process of spraying the oil to form fine droplets which result in mist. It helps for better mixing of evaporated fuel droplets with the combustion air
    • While oil passes through the spray nozzles of the oil gun, the pressure energy of the steam converts into velocity energy, which breaks up the oil stream into fine particles.
    • Poor atomization of oil fuel could result in bigger spray particles.  It may take a longer burning time, resulting in poor mixing of droplets and air due to a low rate of heat liberation. It results in incomplete combustion. Apart from fuel pressure, the viscosity of the oil plays an important role in atomization.
  • For satisfactory atomization, the viscosity of oil should be between 15 to 20 centistokes

Oil Recirculation

  • it  is carried out using a fuel pump

Design Parameter for Oil Burner

  • Combustion parameters
    • Fuel firing rate
    • Composition of oil: Pyrolysis Oil or Kerosene
    • Oil temperature
    • Air mixing mechanism
  • Fuel pump
  • Fuel Filtration
  • Firing operation: single or two-stage
  • Oil Pressure
  • Number of nozzles in the burner
  • Type of oil atomization: High-Pressure fuel atomization using the nozzle
  • Compatible with any type of combustion chamber.
  • Adjudgment of fuel: Manual or automatic
  • Type of combustion chamber
  • The shape of the combustion chamber
  • Oil igniter

CFD Modeling of Oil Burner

  • A numerical simulation of the oil-fired burner is carried out to find out the flame pattern. The oil consist of heavy hydro-carbon which reacts with oxygen and produces carbo-di-oxide and water vapor

  • Single-step combustion reaction with Arrhenius rate constants is considered for combustion
  • Fuel oil is sprayed with droplets of size 40 microns
  • Inlet  Conditions of Air and fuel for combustion
    • Heat release per burner: 4300 KW
    • Excess Air ratio: 25%
    • Oil supply rate to burner: 380 kg/hr
    • Heating value (LHV): 40.26 MJ/kg
    • Air to fuel ratio: 17.7
    • Inlet Air temperature: 295 K
    • Inlet Fuel temperature: 353 K

Geometry of Oil Burner

  • The burner geometry was created in ANSYS space claim.
  • The burner is at the Centre of the heater

Meshing of Computational Domain

  • The computational domain is divided into a finite number of volumes. Fine mesh used in a burner with hybrid mesh elements in the burner
  • Hexahedral elements used in the cylindrical heater

Numerical Models

Simulation of oil burner is carried out using turbulent multiphase combustion. This is given on this webpage. CFD users need to understand the basics of combustion, turbulent combustion modeling, and multiphase flow modeling. All variables must be selected correctly in CFD solvers like ANSYS FLUENT or Open FOAM

The details of solver setting is given as follows:

  • The steady-state pressure-based solver used
  • Turbulence Model: k-ε realizable turbulence model
  • Multiphase model:
    • Discrete phase model
    • Evaporating oil droplet
  • Turbulent chemistry model: Finite rate chemistry
  • Single-step chemical reaction
  • Temperature-dependent properties (viscosity and thermal conductivity) used for numerical simulation
    • Evaporation temperature: 560 K
    • Pressure and temperature-dependent curve
  • Type of Oil: C19H30

CFD Results

  • Contours of fuel obtained from numerical simulations are presented for velocity 

  • Concentration of oil

  • Concentration of Oxygen

  • The temperature of flue gases

  • The temperature in the burner section

  • Burner Flames is presented on the stoichiometric mole fraction of CO2

Concluding Remark

  • CFD simulation of oil-fired burner carried out using turbulent multiphase combustion model
  • Turbulent multiphase combustion is used for simulations.
  • A longer flame is observed due to a longer mixing zone formed above the burner

Reference

 

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