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Bjorn’s Corner: New engine development. Part 17. Combustion.
By Bjorn Fehrm
July 26, 2024, ©. Leeham News: We do an article series about engine development. The aim is to understand why engine development now has longer timelines than airframe development and carries larger risks of product maturity problems.
To understand why engine development has become a challenging task, we need to understand engine fundamentals and the technologies used for these fundamentals.
We have covered the compression in the compressor (Figure 1) and now go on to combustion in the combustor.
Figure 1. The gas turbine cycle and its parts. Source: Rolls-Royce: The Jet Engine.
Combustion, a challenging task
In the combustor, the high-pressure air from the compressor burns the injected fuel to produce a high-volume combustion gas that drives the turbines. The gas ultimately exits in the nozzle and produces jet thrust.
The combustion process is made complicated by several factors:
- The combustor is required to convert the chemical energy of the fuel into thermal energy in a small volume. To achieve this, the flow in the combustor must be highly turbulent. It’s, therefore, almost impossible to model the flow, even with today’s supercomputers. Combustor design requires extensive knowledge and a lot of cut and try.
- The combustor flow differs greatly between engine start, ground idle, maximum power, cruise power, and flight idle. Yet the sensitive flame (which doesn’t like fast airflows) must burn steadily or relight if it flames out in all these conditions.
- Since 1970, NOx and Soot emissions have been under the spotlight, and the ICAO Committee on Aviation Environmental Protection (CAEP) has issued reduced NOx limits four times (CAEP/2, CAEP/4, CAEP/6, and CAEP/8). We are now at about 40% of the values of CAEP/2, Figure 2. Soot or Non-Volatile Particulate Matter (nvPM) values do not yet have a limit as their environmental impact is not yet fully clear.
Figure 2. NOx limits versus the CAEP levels and the NOx emissions for engines of different generations and OPRs. Source: EASA.
The above makes combustion chamber design particularly hard as:
- Soot is created when the combustor has a rich air-fuel ratio (below 10) for the safe ignition and continuous burn of the flame (Figure 3).
- NOx is created when excess air, in a later stage, is directed at the Soot to burn it off, as NOx is created from the air’s Nitrogen and Oxygen when combustion temperatures are over 1400C° for a certain time.
Figure 3. The formation and consumption of Soot (i.e., smoke) and NOx in a modern combustor. Source: Cumpsty, Jet Propulsion.
The requirement for very stable and efficient combustion, close to stochiometric level ( = where all air Oxygen is combined with the fuel’s Carbon to CO and CO2) at all engine power levels/altitude/speeds and ever lower emissions while delivering ever higher levels of heated gas in the smallest possible volumes (to keep engine mass and size low) has made combustor design a real “Rocket Science.”
We will look at the history of combustor design and where it is now in the next Corner.
Often you divide the combustor into different zones. First the diffuser from the compressor exit and the dome that splits the airflow into the cooling air outside the burner liner and the airflow into the primary zone for mixing with fuel for hot and quick burn, then you want to quench the flame as the fuel is consumed and lower its temperature below NOx generation for finally mix with more cooling air to get desired temp profile into the turbine nozzle. The chemical reactions during combustion are complex and fast to finally ideally only produce H2O and CO2. The air coming into combustion can contain sand and salt, that recrystallize in different areas downstream.