Week 4.2 - Combustion Efficiency Calculation after Preheating
Aim : comparing the effect of pre-heated oxidizer on AFT
comparing the effect of pre-heating oxidizer on combustion efficiency
Given: Fuel - CH4 , constant pressure heat exchange system -Recuperator, LHV=50MJ/Kg
Exhaust temperature = 1700 K
Theory:
1.Comparing the effect of pre-heated oxidizer on AFT:
AFT increases linearly with an increase in air pre-heating from 298.15 K to 600 K. This is because pre-heating the air enhances the system enthalpy even before combustion has occurred. This finds extensive application in for example in gas turbines, where the pre-heated air from the compressor enhances the Adiabatic flame temperature.
CODE:
https://drive.google.com/file/d/1BWLEpzeMuNI-ng9vlubBLdzHTyhtMche/view?usp=sharing
As learned previously, the AFT is highest at the Stochiometric equivalence ratio followed by rich combustion and then lean combustion.
2.Comparing the effect of pre-heating oxidizer on combustion efficiency:
Recuperator is used to extract excess heat from the flue gases and transfer that into the inlet of the system as pre-heated air that enhances the overall system efficiency as well but more importantly combustion efficiency. It supports the accelerated evaporation of the fuel for complete or efficient combustion.
1. Combustion efficiency = heat utilized per unit kg of fuel/heat released per unit kg of the fuel
Fuel savings = 1 - (fuel used without pre_heat)/fuel used with pre_heat)
Code:
https://drive.google.com/file/d/18kq51_BuagkZ49kngGaF8cij_uu1D8ce/view?usp=sharing
Combustion Efficiency % linearly increases with increase in Pre-heating temperature [K].
However, Fuel saving has a bit of a non-linear relationship as it increases with an increase in Pre-heating temperature. For example, for the given conditions, a pre-heat temperature of 450K will result in ~10% fuel saving.
3. It is right to assume a fixed exhaust temperature value as the downstream component as well as the heat extraction from the flue gas depend on it for the optimal functioning of the combustor as well as the design, durability, and functioning of the recuperator, especially in this example. Also from a safety factor, only a minute variation in exhaust temperature should be allowed. Large variations or fluctuations indicate the malfunctioning combustor.