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

Introduction to Oil Burner

An oil burner is liquid fuel-fired burner used in many domestic and industrial applications 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
It is commonly used in many oil-fired furnaces, some water heaters, or ignition of coal-fired burners in boilers.
In the fired heater, both gaseous and oil fuels are used for heating process fluids.
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

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

- 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
- As fuel oil passes through the spray nozzles with 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

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

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: 393 K

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

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 the solver setting are 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
- Size droplet: 50 micron (uniform)
Turbulent chemistry model: Eddy Dissipation
Chemical Kinetics: Single-step chemical reaction
Temperature-dependent properties (viscosity and thermal conductivity) used for numerical simulation
- Density of heavy oil: 980 kg/m3
- Evaporation temperature: 126 °C
- Boiling Temperature: 310° C
- Kinematic viscosity of oil: 15 mm2/s (cp)
- Saturation Pressure and temperature-dependent curve
Type of Oil: C19H30

Contours of fuel obtained from numerical simulations are presented for velocity
High velocity of oil jet is observed in the combustion chamber

The concentration of fuel oil:
- A high concentration of oil is near the nozzle
- As fuel jet atomizes and fuel droplet evaporates and mixes with the surrounding air
- When the vaporized oil and air mix with stoichiometric proportions, then combustion reactions occur and the amount of heat is released as exothermic reactions.

Concentration of Oxygen
- A high concentration of oxygen is around the fuel jets. It is in red color (high mole fraction of air)

The temperature of flue gases
- The high temperature of combustion is observed away from the burner.
- The flame is surrounded by unmixed cold air

Burner Flames is presented on the stoichiometric mole fraction of CO2
The flame anchors from the oil nozzle and it gets elongated inside the combustion chamber

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