How is thrust produced in a turbofan engine?

In a turbofan engine, a significant portion of the air bypasses the core (compressor, combustion chamber, turbine) and is accelerated by the fan, producing additional thrust. This bypass air provides a quieter and more fuel-efficient operation compared to pure jet (turbojet) engines. The ratio of the air that bypasses the engine core relative to the air passing through the core is known as the bypass ratio, and engines with a high bypass ratio are typically more efficient and quieter. This process, combining both the jet thrust from the core and the thrust from the bypassed air, propels the aircraft forward.

There are few key stages for turbofan engine to produce thrust. Most basically these are following: Air Intake: The engine does it by using a fan module, which draws air. Compression: The air is then compressed by compressor blades, increasing the pressure/temperature of the air. Combustion: The compressed air is mixed with fuel in the combustion chamber, which ignites by ingiter plugs. As a result, a high-pressure, high temperature gas expands rapidly. Turbine/Exhaust: The expanding gases flow through high and low pressure turbines causing them to rotate. Each turbine module is linked to compressor and fan LPT - Fan, HPT - Compressor. Fan creates up to 75% of engine thrust through bypass duct. The rest 25% is made by exhausted gasses.

The air intake must operate efficiently over the entire flight envelope of the aircraft. At very low aircraft speeds, or when just sitting on the runway, free stream air is pulled into the engine by the compressor. At high speeds, a good air intake will allow the aircraft to maneuver to high angles of attack and sideslip without disrupting flow to the compressor. Because the air intake is so important to overall aircraft operation, it is usually designed and tested by the airframe company, not the engine manufacturer. The amount of disruption of the flow is characterized by a numerical inlet distortion index.

Fun fact!! There is a misconception that most of the thrust is produced by the core engine known with different names like, (Primary flow, Hot stream). In which the flow of air passes through the compressors, burned in the combustion chamber and drives the turbines then exits through the nozzle. This flow thought by a lot is responsible for the thrust, but in fact!! in modern Turbofan engines around 80% and in newer high-bypass turbofan engines like the CFM LEAP 90% of the thrust is produced by the Bypass flow (Secondary flow, cold stream) which is the air that bypasses the core and is directly expelled through the nozzle contributing to the overall thrust of the engine.

In recent years the aviation industry is moving toward fuel efficiency. Therefore, companies are developing high-bypass turbofan engines. Did you know that the new CFM LEAP engines installed on A320 and A321 produces around the same thrust force in comparison to the older engine CFM56-5B but with approximately 30% more fuel efficiency.

It’s true but it’s not the only one to contribute for thrust. The exhaust nozzle also accelerates the flow and converts the high pressure emanating from combustion chamber. The thrust equation describes the requirements of mass flow, as well, which is obtained from the by pass ratio.

Based on the forgoing, I would gather that the thrust equation for these engines contains two distinct terms, one associated with the thrust generated by the core flow and the other with the thrust generated by the bypass flow that are then added together, as well as a thrust coefficient term or terms probably obtained from engine tests. Is that correct?

Some details about the Compressor is that The compressor blades decrease in size as you progress through the stages, and this design is intentional for several reasons but most importantly is the Compression Ratio. The primary purpose of the compressor is to increase the pressure of the incoming air. Each compressor stage contributes to this pressure increase. As the air moves through each stage, its pressure rises, and decreasing the size of the blades allows for a higher compression ratio.

A combustor usually has an outer casing and an inner liner. The liner is often perforated to enhance mixing of the fuel and air. There are three main types of combustors, and all three designs are found in modern gas turbines: The annular combustor with the liner sitting inside the outer casing. Many modern burners have an annular design. The older can or tubular design. Each can has both a liner and a casing, and the cans are arranged around the central shaft. A compromise design is a can-annular design, in which the casing is annular and the liner is can-shaped. The advantage to the can-annular design is that the individual cans are more easily designed, tested, and serviced.

The compressor, combustion chamber and the high pressure turbine (HPT) are know as "the hot section". Typically, the HPT and compressor are mechanically connected via a hollow shaft. The hot combustion gasses spin the HPT which spins the compressor which in turn adds air air pressure to the combustion chamber which further spins the HPT. This cycle results in the high rotational energy of a gas turbine. Since turbines have been optimized for years, on of the few ways to gain thermal efficiency is to increase the heat of the engine. To achieve the highest heat possible, HPT blades are hollow and iternally air cooled, with ceramic coatings which allow metals to withstand temps above their melting point.

Not to forget about a major design element in modern day Turbines which is the cooling holes integrated into the turbine blades.. These cooling holes contribute to a film cooling effect. The cooling air passing through the holes creates a protective film along the surface of the turbine blade, insulating it from the hot gases generated in the combustion process and in doing so increases the efficiency, durability, and reliability of turbofan engines in demanding operational conditions.

This is slightly misleading. Aft of the combustion chamber there is the high pressure turbine but it is mechanically linked to the compressor and is really part of the "hot section": compressor, combustion chamber and HPT all working as a unit. Aft of the hot section is the low pressure turbine (LPT). This is typically a multi stage section (multiple bladed disks) which is driven by the exhaust of the HPT. The LPT drives the fan via a shaft which passes through the hot section's hollow shaft. The exhaust gasses of a high bypass turbo fan commonly seen on commercial aircraft actually provides only a small fraction of the thrust compared to the large fan which displaces a high volume of air.

Turbofan engines often employ a co-annular nozzle. The core flow exits the center nozzle while the fan flow exits the annular nozzle. Mixing of the two flows provides some thrust enhancement and these nozzles also tend to be quieter than convergent nozzles.

I would rather say the turbine swallows the energy to drive the compressor and other accessories compromising thrust production capacity. Whereas the nozzle effectively produces the kinetic energy required to propel the aircraft from the exhaust and by pass air.

The issue with the thrust equation is that it does not explain how thrust is transferred to the engine, but rather how the energy is transferred to the working fluid. To understand thrust in a Gas Turbine Engine, one needs to integrate all the pressures over all the surfaces of the engine exposed to the working fluid to see it generate a propelling force, opposite to the momentum added to the working fluid. The air acceleration is but the flipside of the engine thrust coin, NOT the actual coin side we are in fact interested in.

When the engine is running, air is taken in by the intake fan at the front. A large part of this air is directed around the engine core through a bypass duct and is pushed out of the back, creating a forward thrust - this is the bypass air. The rest of the air enters the engine core where it gets compressed, mixed with fuel, and ignited in the combustion chamber. This ignition process produces a high-energy exhaust gas which is expelled out the rear of the engine, and powers the engine's turbines which in turn drive the fan and the compressors. In modern high-bypass-ratio engines, a majority of the thrust comes from the bypass air. This process is more fuel-efficient and quieter than earlier jet engine designs.

The equation defines two factors requirement for effective momentum generation i.e., mass and velocity. This does not describe from where we should get these in the engine. The designer has to analyse and experiment for effective combination of these two factors and optimise.

One other thing that has a major contribution to the turbofan engine construction, is the spooling. Turbofan engine can be of either 2 spools or 3 spools. In a twin-spool engine, there are two concentric shafts, each with its own set of compressors and turbines. One spool is connected to the LP compressor and turbine, while the other is linked to the HP compressor and turbine. This design allows for better control of airflow and improves engine efficiency. On the other hand, a triple-spool turbofan engine has three concentric shafts, each associated with a specific set of compressors and turbines. The additional spool allows for even finer control of the engine components, optimizing performance and fuel efficiency further.

During my career as an aerospace engineer, I was tasked with designing a new turbofan engine for a commercial airliner. I used my understanding of thrust production to optimise the engine's performance for both fuel efficiency & thrust output. This resulted in a more efficient & powerful engine that helped to reduce the airline's operating costs More recently, I was involved in a project to upgrade the engines of a fleet of military aircraft. I used my knowledge of turbofan engine technology to identify & implement a number of improvements that significantly enhanced the aircraft's performance.These improvements included a new high-bypass-ratio fan that increased fuel efficiency and a more advanced combustion system that reduced emissions.

But how do we transfer this thrust force to the Aircraft structure and propel the Aircraft forward!? This can be simplified by first explaining that the Engine is connected to the Wings through the Pylon. And the Engine is supported to the Pylon by two Mounts, Forward and AFT Mount. The Forward Mount is designed to support axial loads and transfer the thrust force to the pylon, which, in turn, transfers it to the aircraft structure. This mount is crucial for managing the thrust load and ensuring its effective transmission to propel the aircraft forward. The AFT Mount plays a dual role by supporting both axial and rotational loads. This mount further contributes to the stability and structural integrity of the engine attachment.

The geared turbofan has been a very popular choice for moden turbofan designs. Typically, a turbofan has two main shafts: the hollow hot section shaft (compressor & high pressure turbine) and a longer shaft connecting the LPT (low pressur turbine) at the rear to the large fan it drives at the front. This meant that the LPT and fan had to have the same RPM. The addition of a gear means that the LPT and main fan can rotate at different speeds which allows engineers to tune the turbine and fan speeds independantly to obtain optimal fan and turbine efficiency. This does come at a cost of higher mechanical and maintenance complexity but the gains are worth it.

Air Intake and Compression - Air is drawn into the engine and compressed by the fan and compressor stages. Picture this as a deep breath before a powerful exhale, where the air is readied for combustion. Combustion and Exhaust - The compressed air mixes with fuel and ignites, creating high-pressure exhaust gases. These gases pass through the turbine, driving the fan, and are expelled out the back, producing thrust. It's akin to releasing a tightly coiled spring, unleashing a forceful push that propels the aircraft forward.