Turbulent combustion in a burner

### TITLE : Combustion and NOX formation in a burner

BY : Dr S V Zhubrin, CHAM Ltd

DATE : December, 2001

FOR : Demonstration case for V3.4

### INTRODUCTION

The application of an Eddy-Dissipation Model of Magnussen and Hjertager blended with Arrhenius-type reaction rates is presented aimed at the demonstration of the method for the simulation of the two-step turbulent combustion in some typical burner design.

Special attention is given to NOX formation by incorporating the simple realistic model for the rate of the oxidation of atmospheric nitrogen present in the combustion air. It is known as the steady-state simplification of Zeldovich mechanism with partial-equilibrium assumptions.

### PROBLEM DESCRIPTION

The present example consists of an plane-symmteric burner in which methane burns in a stream of heated air. The fuel and oxidant are introduced through separate streams into the burner. The fuel enters through a duct on the symmetry plane of the burner, and the air is introduced through an annual inlet into a chamber surrounding the fuel duct. Therein, the air is divided in two streams: one of them, primary air, is injected straight into the fuel stream, with the remaining, secondary, air being supplied through the orifice into the downstream recirculation region as depicted here.

The fuel is ignited on entry and steady combustion is in progress producing the high temperature combustion products.

The task is to calculate the temperatures and composition of combustion gases along with all related flow properties.

### Conservation equations

The independent variables of the problem are the two components of cartesian coordinate system, namely X and Y.

The main dependent (solved for) variables are:

• Pressure, P1
• Two components of velocity, U1 and V1,
• Turbulence energy and its dissipation rate, KE and EP,
• Gas specific enthalpy, H1,
• Mass fractions of gas mixture, namely, fuel methane (YCH4), oxygen (YO2), carbon monoxide (YCO), water vapour (YH2O), carbon dioxide (YCO2), nitrogen (YN2) and nitric oxides, collectively reffered to as (YNOX).

Although, the transport equations are usually solved for all of these variables, the nitric oxides are supposed to be presented only in trace quantities non-contributing to the state of the gas mixture and mass conservation.

### Turbulence model

The standard K-epsilon model, KEMODL, is used to calculate the distribution of turbulence energy and its dissipation rate from which the turbulence viscosity is deduced.

### Combustion model

Combustion is treated as a two-step irreversible chemical reaction of methane oxidation as follows:

Step 1: CH4 + 1.5 ( O2 + 3.76N2) = CO + 2H2O + 5.64N2

Step 2: CO + 0.5( O2 + 3.76N2 ) = CO2 + 1.88N2

The reaction rates of combustion are obtained as the limiting blend of a Arrhenius kinetics and eddy-dissipation rates:

RcomCH4 = - min ( RkCH4, ReCH4 ) and

RcomCO = - min ( RkCO, ReCO ),

where Rk and Re, in kg/m3s, are the kinetic and eddy-dissipation rates:

ReCH4 = 4 RHO1 EP/KE min( YCH4, YO2/3)
ReCO = 4 RHO1 EP/KE min( YCO, YO2/0.57)
RkCH4 = 1.15 109RHO12e-24444/T YCH4-0.3YO21.3
RkCO = 5.42 109RHO12e-15152/T YO20.25YH2O0.5YCO

The remaining rates are defined through associated stoichiometric coefficients:

Step 1:
R1O2 = 3 RcomCH4
R1CO = -1.75 RcomCH4
R1H2O = -2.25 RcomCH4

Step 2:
R2O2 = 0.57 RcomCO
R2CO2 = -1.57 RcomCO

The net rates of species generation, i.e. the source terms, are the balances of formation and combustion as appropriate:

SCH4 = RcomCH4
SCO = RcomCO + R1CO
SO2 = R1O2 + R2O2
SCO2 = R2CO2
SH2O = R1H2O

### NOX formation model

The NOX formation rate, in kg/m3/s, is given by:

SNO = 2 RHO1 K1 YN2 [O] MNO/ MN2

where MNO = 30, is NO molecular mass,
MN2 =28, is N2 molecular mass, and,
K1 = 1.8 108e-38370/T,
stands for the reaction-rate constant for the forward reaction:

N2 + O --> NO +H

The equilibrium O-atom concentration, [O], can be obtained from the expression:

[O] = 3.97 105T-0.5(RHO1 YO2/MO2) 0.5e-31090/T

wherein MO2 =32, is the molecular mass of oxygen.

### Properties and auxiliary relations

The gas density is computed from the ideal-gas law.

The specific enthalpies are related to gas temperatures and specific heat:

H1 = CPT

The specific heat, CP, is assumed to be equal for all gas components and is a function of temperature as follows:

CP = 1059 + 0.25( T - 300 )

### Reaction heats

The constant reaction heats, i.e. the heats of combustions for fuel methane, H°CH4, and carbon monoxide, H°CO, are released with the rates of their consumptions. In terms of the transport equation for H1, the source term for specific enthalpy, in J/s, due to combustion of CH4 and CO is:

SH1 = RcomCH4CH4 + RcomCOCO

### Boundary conditions

Uniform velocities, air and fuel temperatures are assumed at the burner inlets, and the pressure is assumed constant at the outlet plane.

The pure methane, YCH4=1, enters its inlet and dry air, YN2=0.768, YO2=0.232, enters the oxidant inlet.

At the walls, standard "wall-functions" are used for the gas velocities, and the condition of zero flux is assumed for enthalpy and mass fractions.

### THE RESULTS

The plots show the distribution of temperatures, velocities and other related fields within the burner.

Pictures are as follows :

### THE IMPLEMENTATION

All model settings have been made by PIL commands and PLANT settings of PHOENICS 3.4