BY : CHAM Development Team: S Zhubrin
DATE : 2000
FOR : v3.3 Validation case
The purpose of a secondary combustion chamber in an incineration unit is to prevent the release of certain chemicals emitted by the incinerator from entering the atmosphere. One method of doing so is to raise the gases to such a temperature in the presence of oxygen, as will destroy the chemicals by pyrolysis and/or oxidation/combustion.
The thermal design task is to calculate the field distribution of gas mixture composition and temperature.
The combustion chamber chosen for the present computations retains all major features of real-life industrial equipment: rectangular shape of 920x2100x2400 mm; inlet of primary gas mixture at the bottom; high temperature burner product mixture input at the chamber wall; injection of secondary air through ports in the other; and secondary combustion product discharge at the top.
Chamber walls are supposed to be well insulated. The low conductivity plate is located on the supports inside a chamber with the aim to provide the flow recirculations resulting in the increase of residence time for carbon monoxide of primary gas mixture to be burned out.
Through the inlet at the floor the primary mixture of five gases enters a chamber vertically upward at mass flow rate 1340 kg/h. The mixture temperature is 850C and it has a composition of carbon monoxide, carbon dioxide, nytrogen and water vapour. The mass fractions of each species, namely, mCO, mCO2, mN2 and mH2O are 11.0, 2.0, 60.0 and 27 percent correspondingly.
Dry secondary air (mN2/mO2 is 0.768/0.232 ) of room temperature is injected horisontally in the chamber through three equally spaced circular wall openings at the total flow rate 1204 kg/h.
The hot burner products also enters the chamber horisontally through the opening made in the other wall. They are free of carbon monoxide and bring in carbon dioxide, oxygen, nytrogen and water vapour with total mass flow rates 53.0, 34.0, 295.0 and 23.0 kg per hour at 1462 C.
The independent variables of the problem are the three components of cartesian coordinate system.
The main dependent (solved for) variables are:
The model employed postulates a physically controlled, one-step instant reaction, with fuel, CO, and oxygen, O2, unable to coexist at the same location:
Fuel + S.Oxygen + Diluent -> (1+S).Product + Diluent or
2CO + O2 + Diluent -> 2CO2 + Diluent,
where the stoichiometric ratio, S, of oxygen to carbon monoxide is 0.571, i.e. 4 kg of oxygen are required to complete combustion of 7 kg of carbon monoxide since the molecular masses of CO and and O2 are respectively 28 and 32.
Products are supposed to be a pure carbon dioxide. The mixture of an CO2, N2 and H2O is regarded as a single substance, i.e. simple total diluent, entering no chemical reaction at all.
The mixture fraction, f, is represented as
f = (S.mCO - mO2 + mO2a)/(s.mCOp + mO2a)
from which the stoichiometric value, fst, for mO2=0 can easily be deduced.
In above, the mass fraction of CO in the primary mixture, mCOp, and mass fraction of O2 in the secondary air, mO2a, are known from their inlet compositions.
The local mass fractions of carbon monoxide, mCO, combustion product carbon dioxide, mCO2pr, and oxygen, mO2, are then derived from calculated mixture fraction values as:
if f is less or equal fst, then
mCO=0.0 and mO2 = mO2a.(fst-f)/fst,
if f is greater than fst, then
mO2=0.0 and mCO = mO2a.(f-fst)/(S.fst) and also
for any f, mCO2pr=1.-mCO-mO2-mDil.
The mass fractions of other components of local gas composition are evaluated as follows:
mH2O = mDil - mCO2d - mN2
mCO2 = mCO2d + mCO2pr
The gas density is computed from the local pressures, gas temperatures and local mixture molecular masses.
The specific enthalpies are related to gas temperatures as follows:
H = mixture specific heat.Tgas + heat of combustion.mCO
At all inlets, values are given of all dependent variables together with the prescribed flow rates.
Fixed exit pressure. As the fluid is assumed incompressible, this pressure is set equal to zero and the computed pressures are relative to this pressure.
The smooth-wall 'wall functions' are used to provide the non-slip conditions for momentum equations.
It is assumed that there is no heat exchange to the wall, ie. an adiabatic boundary conditions are employed.
The plots show the flow distribution, mixture composition as represented by the model, gas temperature and velocity within the combustion chamber and at the outlet.
Pictures are as follows :
|Water vapour, %||14.4||12.8|
|Carbon dioxide, %||12.7||10.3|
All model settings have been made in VR-Editor of PHOENICS 3.3.1. PLANT menu has been used to introduce the combustion model features.
The relevant Q1 file with PHOTON USE commands supplied can be inspected by clicking here.