Figure 1 – Noise Sources, and their Relative Level and Direction in a Typical Turbofan Engine
Noise emitted by civil aircrafts can be classified into external noise, caused by the flow around the aircraft and the jet exhaust, and internal engine noise radiating outside. In conventional turbofans, a large fan stage, powered by a highly efficient compressor/turbine core, pushes a relatively slow and cold flow into a secondary surrounding bypass duct. This allows the exhaust jet velocity to be reduced for the same thrust, which dramatically reduces the noise created by the exhaust jet. The noise from the fan does increase because of the additional loading, but is more amenable to design techniques, whereas jet noise is difficult to reduce without altering the exhaust speed. This has led to a greater emphasis on the study of blade-related noise. It is now a dominant source of flyover noise in the critical takeoff and approach flight phases, when the aircraft has a relatively slow speed and is flying close to the ground, Figure 1. Crucial for airplane certification, addressing the fan noise issue is the ultimate goal of this proposed work.
A typical fan noise consists of a broadband spectrum, on top of which exists a succession of high-pitched discrete frequency components and their harmonics. Minimization of this tonal content has higher impact than broadband noise reduction. Firstly, the noise reduction regulations are written in terms of effective perceived noise level, which is based on the measurement of highest clean tonal peaks. Secondly, the tonal content of the fan is greater than the broadband noise, and travels greater distances unattenuated. Passive technologies alone are insufficient to satisfy the increasing regulatory demands. Therefore, noise reduction via active sound cancellation is becoming a trending mechanism, the basic concept of which relies on local annihilation of an unwanted pressure field through the creation of an out-of-phase sound wave at the same amplitude and frequency. This typically involves an array of distributed vibro-mechanical loudspeakers. However, the geometric limitations prevent their implementation in a distributed manner. Therefore, the common noise cancellation is localized to the observer, rather than holistic elimination. In contrast, if the noise source and the transducer are co-planar and in close proximity, then the pressure fluctuations in the original signal can be annihilated completely for all observers, resulting in spatially global noise cancellation, Figure 2.
Figure 2 – Local versus Global Noise Cancellation
With advancements in manufacturing technologies of electrodynamic and piezo-actuators, there has been significant efforts in order to cancel tonal fan noise, such as EU-funded RANNTAC or RESOUND programs, which led to integration efforts in SILENCE(R) and OPENAIR, as well as US-funded Thunder technology developed by NASA. These pioneering works have solved majority of the control related issues associated with application of distributed active noise cancellation systems in turbomachines. Considering that rotor wake/stator interaction is the dominant fan noise source, this approach may hold the key towards eliminating tonal turbomachinery noise. However, the major issue that prevents large scale implementation of active noise cancellation is associated with the speaker technology itself. We aim to resolve it in Objective 4 of this proposal.
All conventional speaker technologies produce sound via vibro-acoustics. Thus, sound is produced by moving components which require a free volume on both sides. This forced the earlier investigations to cut open windows, and place actuators inside stators. Common to all vibro-acoustic sources is this inherent need to design for unrestricted openings that allow diaphragm movement. Although this is sufficient for feasibility demonstration of active noise cancellation, such an approach cannot be implemented in flying platforms. In general, due to the moving parts involved in vibro-mechanical devices, not only their miniaturization presents significant challenges, but also the design is inherently fragile, requires clearance gaps around diaphragm boundaries and not suitable for aero-engine environment. In contrast to the vibro-acoustic transducers, the concept of a truly static sound emitter would address all these potential issues, Figure 3. The enabling disruptive technology is a surface-deposited motionless thermo-acoustic transducer, comprising of a periodically Joule heated electrically conductive thin layer, deposited on the noise source itself. We will develop, build and test these devices in the scope of Objective 3.
The structure of such heat flux transducers (also known as thermophones) is simple and can be formed by conventional deposition techniques. Since the device is thin and static, it can be placed on any surface in the vicinity of which the noise field is being generated. Such emitters, not requiring any macroscopic mechanical motion for acoustic generation, theoretically create distortion-free sounds without any resonances that are characteristic to mechanical structures of vibrating elements. Furthermore, these elements neither take up significant space within the system in which they are installed nor increase the weight. They can also be constructed to withstand high temperatures. The above characteristics all point to their suitability for application in turbomachinery, such as fan outlet guide vanes.
Although sound production via Joule heating has been studied since 19th century, only in 21st century, the thermophones regained the interest of the scientific community. Therefore, there is still no consensus in the literature as to the correct approach to model thermophone sound production, prohibiting design optimization for high efficiency thermo-acoustic speakers. We will formulate this model in the scope of Objective 2.
The first attempt to model the phenomenon analytically was through classic parabolic Fourier heat transfer for the case of a thin strip, neglecting the transient response. An energy balance relating the rate of heat production to the rate of heat storage, as well as the rate of heat transfer to the surrounding medium was constructed. Although this approach was sufficient to account for the physics at the time, inconsistency of the model with more recent experimental results has been observed. There have been attempts to compensate these differences by pragmatic addition of loss terms. However, the findings indicate contradicting trends in acoustic response with respect to frequency. In another work, the thermophone acoustics have been modelled by a closed thermodynamic system of a pressure wave emanating from the transducer surface, while modelling the energy balance according to the ratio of thermal capacitances of the thermophone, the fluid and the substrate. Although these attempts were successful in predicting the acoustic propagation, the existing theoretical frameworks show limited success at modelling the experimental observations, with typical deviations of 40-50% from the empirical trend. The principal issue is associated with modelling the movement of thermal energy through systems with large temperature gradients, small spatial dimensions, and short temporal intervals. Although Fourier heat conduction model is highly successful in calculating the evolution of temperature distributions in less exotic applications, it is an empirical law unsuitable under these conditions. Clearly, there is a fundamental gap in heat transfer modeling across nano to macro scales, and bridging this gap is our Objective 1.