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ALAM is calculated from the inlet flows and is defined as the quotient of
Thus ALAM=1 means that precisely the amount of air is supplied that allows everything to be entirely burned. ALAM=2 then means that twice the amount of air is supplied as would be required for a complete combustion.
ALAMT is calculated from the composition of the exhaust gas according to
ALAMT = O2_air / (O2_air - O2_exhaust gas),
where O2_air is the mole fraction of the oxygen in the air (=0.2094615993) and O2_exhaust gas is the mole fraction of the oxygen in the exhaust gas.
ALAMT=1 thus means that no oxygen is left in the exhaust gas. ALAMT=2 means that half the concentration of the oxygen from the air is left in the exhaust gas.
In the case of the complete combustion with pure air of substances where 1 mole O2 is required for the combustion of 1 mole fuel, both values coincide. This is shown in the sample model, top left:
For the combustion of 100 kg/s pure C, 1151.307 kg/s air are required. If twice the amount of air is supplied (2302.614 kg/s), i.e. ALAM=2, the O2 fraction in the exhaust gas is 10.473 mole percent, which is half the fraction in the air. Thus ALAMT=2.0 as well.
For the combustion of pure H (2nd row left), this is not the case. Here significantly more, namely 3430 kg/s air, is required for 100 kg/s fuel. If twice the amount is supplied here (ALAM=2), this will result in 9.48 mole percent in the exhaust gas, which corresponds to an ALAMT of 1.827.
Differences between ALAM and ALAMT also occur in the case of humid air. As the water from the air turns up in the exhaust gas, the O2 fraction decreases (see 3rd row left).
However, no differences will occur if part of the air is supplied via the fuel line (see 4th row left). At ALAM=1, 1051.307 kg/s are still supplied via the air line as 100 kg/s air are already in the fuel line. For ALAM=2, the total air quantity is doubled (to 2302.614 kg/s), thus with the 100 kg/s from the fuel line, 2202.614 kg/s are calculated for the air line. The condition after the combustion is the same as in the case above, i.e. ALAMT=2.
On the right-hand side, further special cases are shown that lead to discrepancies between ALAM and ALAMT.
If O is already present in the fuel, less air is required (1st row).
If this lower air quantity is doubled (ALAM=2), less O2 will therefore be present in the exhaust gas than in the case of the combustion of pure C (ALAMT=1.888).
Water (H2OB) in the fuel (2nd row) shifts the O2 fractions as well.
As ALAM refers to a complete combustion as a rule, for an incomplete combustion there will be more O2 in the exhaust gas, which even applies to the case ALAM=1 (see rows 3 and 4).