High bypass turbojets are quieter than pure turbojets primarily due to the difference in their design and the way they produce thrust.
In a pure turbojet engine, the majority of the thrust is generated by the hot exhaust gases expelled from the combustion chamber. These gases are produced by burning fuel with compressed air. The high-velocity exhaust creates significant noise as it exits the engine at high speeds, resulting in a loud jet noise commonly associated with jet engines.
On the other hand, high bypass turbofan engines, which are used in commercial airliners, have an additional component called a fan. The fan is located at the front of the engine and is driven by the turbine in the same way as the compressor. The fan draws in a large amount of air and accelerates it, creating a separate stream of air that bypasses the combustion chamber. This bypass air provides a significant portion of the engine’s thrust, while only a smaller portion is generated by the exhaust gases produced by burning fuel.
The bypass air produces much less noise compared to the high-velocity exhaust gases of a pure turbojet. The larger volume of air in the bypass stream helps to reduce the overall noise level. The fan also acts as a noise-reducing device by shielding the core engine noise and reducing the jet noise produced at the exhaust.
Additionally, the design of high bypass turbofan engines allows for slower exhaust velocities, which further contributes to noise reduction. The slower-moving exhaust gases create less turbulence and noise as they mix with the surrounding air.
Overall, the combination of the larger bypass air stream, the shielding effect of the fan, and the slower exhaust velocities in high bypass turbofans significantly reduces noise compared to pure turbojets. This makes high bypass turbofan engines more suitable for commercial aviation, where noise reduction is an important consideration for minimizing environmental impact and meeting noise regulations at airports.
Here are some additional details about why high bypass turbofan engines are quieter than pure turbojets:
Bypass Ratio:
The bypass ratio is a measure of how much air bypasses the core of the engine compared to the amount of air that passes through the combustion chamber. In high bypass turbofan engines, the bypass ratio is typically much higher than in pure turbojets. A higher bypass ratio means that a more significant portion of the air is directed around the combustion chamber rather than through it. This bypassed air contributes to a quieter operation because it generates less noise compared to the high-velocity exhaust gases in the core. The bypass ratio in modern commercial turbofan engines can be as high as 8:1 or even higher.
Noise Reduction Techniques:
High bypass turbofan engines employ various noise reduction techniques to further mitigate noise. These techniques include the use of acoustic liners inside the engine nacelle, which help absorb and dissipate noise. The nacelle itself is carefully designed to minimize noise radiation. Additionally, some engines use chevron-shaped nozzle designs that reduce the noise generated by the exhaust gases by altering the flow patterns.
Lower Exhaust Velocity:
The exhaust gases in high bypass turbofan engines exit the engine at lower velocities compared to pure turbojets. The slower-moving exhaust stream reduces the intensity of the noise associated with the engine’s operation. The lower exhaust velocity also leads to reduced turbulence and mixing with the surrounding air, resulting in less noise generation.
Overall Engine Design:
High bypass turbofan engines are designed with noise reduction as a key consideration. The engine components, such as the fan, compressor, and turbine, are optimized to reduce noise emissions. The integration of these components and the overall engine architecture aim to minimize noise generation and maximize efficiency.
Applications and Regulations:
High bypass turbofan engines are commonly used in commercial aviation, where noise reduction is essential for meeting stringent noise regulations at airports. These engines are designed to comply with noise standards set by organizations like the International Civil Aviation Organization (ICAO) and local aviation authorities. The quieter operation of high bypass turbofan engines allows airports to manage noise pollution and minimize the impact on nearby communities.
These factors combined contribute to the quieter operation of high bypass turbofan engines compared to pure turbojets, making them the engine of choice for many commercial aircraft due to their improved environmental performance and reduced noise footprint.
By Dan
