Combustion 2008
Background
In recent years, because attention has been concentrated on
non-reacting flows, it must be admitted that the development of
combustion modelling in PHOENICS has stagnated. Specifically:
- The Simple Chemically Reacting system has continued to be used,
but still with the restriction to a single fuel-supply stream.
- Temperatures have continued to be calculated by way of enthalpy,
which has conflicted with the direct solution for TEM1 that is
preferred for problems involving conjugate heat transfer (i.e.
interaction with heat conduction in neighbouring solids).
- The Extended SCRS has been little used because of its limitations
and complexity.
Recently however attention to the problem are has been renewed, with
the results that:
multiple fuel-supply streams can be handled;
solution for enthalpy can be abandoned; and
direct solution for temperature (i.e. TEM1 ) can be
recommended for all circumstances.
These developments involve:
- replacing the MIXF (i.e. mixture fraction ) variable by
MIX1, MIX2, MIX3 etcetera which represent the mass fractions of
material introduced by the first, second, third etcetera
fuel-supplying streams;
- introducing the concept of the adiabatic-flow temperature,
T1AD,
which is the temperature which would prevail if heat transfer to the
solids surrounding the flowing gases were absent;
- recognising that the solved-for TEM1 variable, which has no
chemical-reaction-related sources, represents the deviation of the
actual temperature T1 from the adiabatic temperature, thus:
T1 = T1AD + TEM1; and
- recognising that the enthalpy continues to have value as an
auxiliary variable, but only as stored, not solved-for.
The extreme scenarios distinguished
In order to simplify the simulation process where possible, it is
useful to distinguish the following extreme scenarios:
- Adiabatic; mixed-is-burned; one fuel-bearing stream
- Adiabatic; mixed-is-burned; several fuel-bearing streams
- Adiabatic; mixed-is-burned; several different-fuel-bearing streams
- Non-adiabatic; mixed-is-burned; one fuel-bearing stream
- Non-adiabatic; mixed-is-burned; several fuel-bearing streams
- Non-adiabatic; mixed-is-burned; several different-fuel-bearing streams
- Adiabatic; finite reaction rate; one fuel-bearing stream
- Adiabatic; finite reaction rate; several fuel-bearing stream
- Adiabatic; finite reaction rate; several different-fuel-bearing stream
These scenarios will be discussed individually, one-by-one.
a. Adiabatic; mixed-is-burned; one fuel-bearing stream
The solved-for variable which is relevant to combustion is MIXF, the
mass fraction of material emanating from the fuel-bearing in-flow
stream, whatever its state of chemical aggregation.
Variables which it may be useful to store but not solve include:
- the enthalpy H1,
- the temperature T1,
- the unburned fuel mass fraction FU,
- the unburned oxygen mass fraction O2,
- the product-of-combustion mass fraction PROD
- If the fuel mass fraction of that stream is fuin,
Relevant material constants are:
- The stoichiometric ratio (= bass of oxygen needed to burn unit
mass of fuel), fst.
b. Adiabatic; mixed-is-burned; several fuel-bearing streams
c, Adiabatic; mixed-is-burned; several different-fuel-bearing streams
d, Non-adiabatic; mixed-is-burned; one fuel-bearing stream
e. Non-adiabatic; mixed-is-burned; several fuel-bearing streams
f. Non-adiabatic; mixed-is-burned; several different-fuel-bearing streams
g. Adiabatic; finite reaction rate; one fuel-bearing stream
h, Adiabatic; finite reaction rate; several fuel-bearing stream
i. Adiabatic; finite reaction rate; several different-fuel-bearing stream