Honeycomb Liner Research Articles

Investigations into rubbing wear behavior of honeycomb land against labyrinth fin with periodic-cell model

The periodic-cell model was proposed to simulate the successive contacts between the labyrinth fin and multiple honeycomb cells. With the experimental data, the finite-element-analysis (FEA) method with the periodic-cell model was validated. The effects of incursion parameters (i.e. incursion depth, incursion rate and sliding velocity) on the contact force, frictional temperature, material loss, and worn geometry of the honeycomb seal during the incursion process were studied. With the predicted worn geometry, the sealing performance degradation in the honeycomb seal was analyzed. The results showed that the proposed periodic-cell model has an excellent accuracy in predicting the wear behavior of honeycomb seal in rubbing events. The contact force between the honeycomb liner and labyrinth fin is pronounced especially at low sliding velocity and high incursion rate conditions, which increases the possibility of wear damage in the rotor part. At low sliding velocity and low incursion rate conditions, the frictional heat transferring to rotor part is increased, which increases the thermal stress near the contact region of the rotor part. As the clearance gap of honeycomb seal increases from 0.6 mm to 0.9 mm in the rubbing event, the leakage rate is increased by about 12%, and the carry-over effects downstream of the worn cells are increased.

High temperature and high strain rate properties of brazed honeycomb liner material Haynes 214

Abradable honeycomb sealing systems are essential for gas turbine efficiency. Experimental data on the rubbing performance of Haynes 214 metal sheets brazed with the nickel-chromium-silicon filler metal BNi-5 is presented. The brazing layer in the double foiled honeycomb sections turned out to have a major impact on rubbing behavior. Damage on the rotor was analyzed with a large chamber scanning electron microscope and energy-dispersive X-ray spectroscopy. The results demonstrate a significant influence of the brazing process on the mechanical properties, especially after exposure to 1000 °C. A hard aluminum nitride phase formed in the brazed metal sheets can cause severe damage to the rotor.

A comparative analysis of 3D woven honeycomb-based aircrew helmet liners against Nomex and Aluminum alternatives

This study investigates the energy dissipation efficiency of an aircrew helmet liner developed using 3D woven honeycomb structural composites compared to different commercially available honeycomb liner materials such as Nomex, and Aluminium. In contrast to liners with discrete density differences, the use of a honeycomb-based liner reduces the concern about delamination, back face deformation and fracture propagation. The research involves comparing several parameters related to crashworthiness, such as specific energy absorption (SEA), crush force efficiency (CFE), and margin of safety, which are crucial for head protection and ensuring helmet injury tolerance. These parameters play a vital role in assessing the ability of different configurations of the honeycomb liner during impacts. Flatwise compression and dynamic impact tests were conducted to evaluate the helmet liner’s performance while maintaining consistency in the helmet shell component. Finite element analysis (FEA) and 3D x-ray Tomography techniques were utilized to analyse the back face deformation (BFD) at high-velocity impact and the internal damage resulting from impacts on the helmet liners, respectively. The results revealed that the 3D woven honeycomb liner configuration performs optimally in terms of energy absorption by demonstrating sufficient and balanced competency across these three critical factors. Additionally, the simulation result revealed that the 3D woven honeycomb liner exhibits wave propagation. This phenomenon enhances its energy absorption capacity and reduces back-face deformation attributed to its crushing behaviour. This research offers valuable insights for improving the performance of aircrew helmet liners, with a particular focus on utilizing 3D woven honeycomb liners featuring 3D woven solid structure to maintain exceptional structural integrity.

Optimisation of an elastomeric pre-buckled honeycomb helmet liner for advanced impact mitigation

Advances in computational modelling now offer an efficient route to developing novel helmet liners that could exceed contemporary materials’ performance. Furthermore, the rise of accessible additive manufacturing presents a viable route to achieving otherwise unobtainable material structures. This study leverages an established finite element-based approach to the optimisation of cellular structures for the loading conditions of a typical helmet impact. A novel elastomeric pre-buckled honeycomb structure is adopted and optimised, the performance of which is baselined relative to vinyl nitrile foam under direct and oblique loading conditions. Results demonstrate that a simplified optimisation strategy is scalable to represent the behaviour of a full helmet. Under oblique impact conditions, the optimised pre-buckled honeycomb liner exceeds the contemporary material performance when considering computed kinematic metrics head and rotational injury criterion, by up to 49.9% and 56.6%. Furthermore, when considering tissue-based severity metrics via finite element simulations of a human brain model, maximum principal strain and cumulative strain density measures are reduced by 14.9% and 66.7% when comparing the new material, to baseline.

Design and Analysis of Bicycle Helmet Made of PETg and ABS with Honeycomb Structure Against External Impacts using Abaqus Software

The bicycle helmet has a significant role in reducing and preventing impact because of reducing the deceleration of the skull, spreading the area over which the forces of the impact reach them and preventing direct contact between the skull and the impacting object. Honeycomb structure, due to its elastic properties, extends the energy absorption time of the whole structure and also increases the ability of the whole structure to absorb energy. Therefore, it can be used in the liner designing of a helmet to reduce velocity, energy, and acceleration in impacts. In this paper, intending to identify the minimum stress transmitted to the helmet during an impact, we used Rhino software to model a helmet with honeycomb liner and outer shell and then analyzed it in Abaqus software. Due to the fact that the size of various parts of the head is different in people, so for more comfort and safety, the use of customized-helmet is emphasized. To design and make a customized-helmet, the materials used in designing the helmet are ABS and PETg filaments, which can be used in 3D printing. These two materials have been analyzed with four compositions for the liner and the shell of the helmet. The results show that the best combination of the helmet with Minimum stress transmission and appropriate plastic strain due to impact is the helmet case with honeycomb liner of PETg and a shell made of ABS.

On the influence of lateral cells and local perforation of resonance liner samples on the accuracy of determining their acoustic characteristics in experimental studies

When determining the acoustic characteristics of a single-layer sample of a honeycomb liner by experiments in interferometers with normal wave incidence, a significant effect of incomplete lateral cells of the sample on acoustic characteristics was revealed. The experiments were carried out on two interferometers with channel diameters of 30 and 50 mm in the frequency range 500-3500 Hz at sound pressure levels of 120-155 dB. Based on the results of the experiments, an assessment was made of the effect on the acoustic characteristics of the percentage of perforation, which is the same both for one honeycomb cell and for the entire SAS sample. Based on the results of the research, recommendations were made to improve the accuracy of obtaining the acoustic characteristics of samples of honeycomb liner when testing them in interferometers of normal wave incidence.

The Effect of Brazing on Microstructure of Honeycomb Liner Material Hastelloy X

Labyrinth seals with abradable metallic honeycomb structures are common in gas turbines to ensure a tight sealing between rotor and stator. These seals have a big impact on the turbine efficiency. The honeycombs made from nickel-based superalloys are brazed onto backing segments in order to form liners. In this work, the microstructure of Hastelloy X honeycombs after brazing with a nickel-chromium-silicon braze filler alloy is investigated. It was found that the braze filler can wet the honeycomb foils over their entire height up to the contact surface with the rotor. After solidification, the braze filler alloy forms γ and silicides (NiSi and Niy(Mo, Cr)xSi). The intermetallic silicides give high hardness to the honeycomb liners. Between the braze filler alloy and the honeycomb foils, interdiffusion zones are observed. For the rub-in behavior of the honeycomb liners, not only the foil material but also the comportment of the composite made up of the foil material, the braze filler material and the interdiffusion zones has to be considered.

Continued Experimental Study on the Friction Contact between a Labyrinth Seal Fin and a Honeycomb Stator: Slanted Position

Labyrinth seals are a state-of-the-art sealing technology to prevent and control leakage flows at rotor–stator interfaces in turbomachinery. Higher pressure ratios and the economical use of cooling air require small clearances, which lead to potential rubbing events. The use of honeycomb liners allows for minimal leakage by tolerating rub events to a certain extent. A previous study within an EU project investigated the complex contact conditions of honeycomb liners, with the idealized contact of a seal fin and a single parallel metal foil representing the honeycomb double foil section. In the present work, the results for the slanted foil position are shown and compared to the previous results. The variation of rub velocity, incursion speed, incursion rate, and seal geometry in a test rig allows for the identification of the influence on contact forces, temperatures, and wear. For the slanted position, significantly lower friction temperatures are observed, leading to a higher ratio of abrasive wear. Overall, the rub test results demonstrate strong interactions between the contact forces, friction temperatures, and wear.

Temperature distribution of a simplified rotor due to a uniform heat source

In gas turbines, high combustion efficiency as well as operational safety are required. Thus, labyrinth seal systems with honeycomb liners are commonly used. In the case of rubbing events in the seal system, the components can be damaged due to cyclic thermal and mechanical loads. Temperature differences occurring at labyrinth seal fins during rubbing events can be determined by considering a single heat source acting periodically on the surface of a rotating cylinder. Existing literature analysing the temperature distribution on rotating cylindrical bodies due to a stationary heat source is reviewed. The temperature distribution on the circumference of a simplified labyrinth seal fin is calculated using an available and easy to implement analytical approach. A finite element model of the simplified labyrinth seal fin is created and the numerical results are compared to the analytical results. The temperature distributions calculated by the analytical and the numerical approaches coincide for low sliding velocities, while there are discrepancies of the calculated maximum temperatures for higher sliding velocities. The use of the analytical approach allows the conservative estimation of the maximum temperatures arising in labyrinth seal fins during rubbing events. At the same time, high calculation costs can be avoided.

Numerical investigation of a honeycomb liner grazed by laminar and turbulent boundary layers

Direct numerical simulations are used to study the interaction of a cavity-backed circular orifice with grazing laminar and turbulent boundary layers and incident sound waves. The flow conditions and geometry are representative of single degree-of-freedom acoustic liners applied in the inlet and exhaust ducts of aircraft engines and are the same as those from experiments conducted at NASA Langley. The simulations identify the fluid mechanics of how the sound field and state of the grazing boundary layer impact the in-orifice flow and suggest a simple flow analogy that enables scaling estimates. From the scaling estimates the simulations are then used to develop reduced-order models for the in-orifice flow and a time-domain impedance model is constructed. The liner is found to increase drag at all conditions studied by an amount that increases with the incident sound pressure amplitude.

Experimental Study on the Friction Contact Between a Labyrinth Seal Fin and a Honeycomb Stator

This paper presents results from an extensive experimental study on the rubbing behavior of labyrinth seal fins (SFs) and a honeycomb liner. The objective of the present work is to improve the understanding of the rub behavior of labyrinth seals by quantifying the effects and interactions of sliding speed, incursion rate, seal geometry, and SF rub position on the honeycomb liner. In order to reduce the complexity of the friction system studied, this work focuses on the contact between a single SF and a single metal foil. The metal foil is positioned in parallel to the SF to represent contact between the SF and the honeycomb double foil section. A special test rig was set up enabling the radial incursion of a metal foil into a rotating labyrinth SF at a defined incursion rate of up to 0.65 mm/s and friction velocities up to 165 m/s. Contact forces, friction temperatures, and wear were measured during or after the rub event. In total, 88 rub tests including several repetitions of each rub scenario have been conducted to obtain a solid data base. The results show that rub forces are mainly a function of the rub parameters incursion rate and friction velocity. Overall, the results demonstrate a strong interaction between contact forces, friction temperature, and wear behavior of the rub system. The presented tests confirm basic qualitative observations regarding blade rubbing provided in literature.

Impedance assessment of a dual-resonance acoustic liner

Acoustic liners are commonly used to reduce noise from commercial aircraft engines. Engine liners are placed in the nacelle inlet and aft bypass duct to attenuate the noise radiated from the engine. Traditional engine liners are constructed of a perforated facesheet over a honeycomb structure to create a quarter-wave absorber. With this design, the low frequency performance of the liner is limited by the depth of the honeycomb. However, with advances in engine design, lower frequency sound absorption is becoming more critical while liner depth must be minimized. Acoustic metamaterials can exhibit unique acoustic behavior using periodically arranged sub-wavelength resonators. Researchers have shown that acoustic metamaterials can effectively block the propagation of low-frequency acoustic waves. Therefore, acoustic metamaterial-inspired concepts are being investigated to improve the low frequency performance of engine liners. A proposed dual-resonance liner is presented here that combines the idea of a Helmholtz resonator metamaterial with a traditional quarter-wave acoustic liner. The low frequency acoustic absorption of a traditional liner can be significantly increased by adding a second, low frequency resonance to the system. The normal incidence absorption coefficient of the proposed liner is more than 10 times larger than a conventional honeycomb liner at the designed Helmholtz resonance frequency while retaining similar performance at higher frequencies.

Impedance assessment of an acoustic metamaterial-inspired acoustic liner

Acoustic liners are commonly used to reduce noise from commercial aircraft engines. Engine liners are placed in the nacelle inlet and aft bypass duct to attenuate the radiated noise from the fan, turbine, combustor, and other sources within the engine. Current engine liners are constructed of a metal perforate facesheet over a honeycomb structure. With this design, the low frequency performance of the liner is limited by the depth of the honeycomb. However, with advances in engine design, lower frequency performance is becoming more critical. Acoustic metamaterials can exhibit unique acoustic behavior using periodically arranged sub-wavelength resonators. Researchers have shown that metamaterials can effectively block the propagation of low-frequency acoustic waves. Therefore, acoustic metamaterial-inspired concepts are being investigated to improve the low frequency performance of engine liners. By combining the idea of a split-ring resonator metamaterial with a traditional quarter-wave acoustic liner, the low frequency acoustic absorption of the device can be significantly increased. The normal incident absorption coefficient of a metamaterial-inspired liner shows an increase from 0.05 to 0.8 at 600 Hz relative to a conventional honeycomb liner with the same depth while retaining the same 0.5 absorption coefficient at 1550 Hz.

Angular Impact Mitigation system for bicycle helmets to reduce head acceleration and risk of traumatic brain injury

Angular acceleration of the head is a known cause of traumatic brain injury (TBI), but contemporary bicycle helmets lack dedicated mechanisms to mitigate angular acceleration. A novel Angular Impact Mitigation (AIM) system for bicycle helmets has been developed that employs an elastically suspended aluminum honeycomb liner to absorb linear acceleration in normal impacts as well as angular acceleration in oblique impacts. This study tested bicycle helmets with and without AIM technology to comparatively assess impact mitigation. Normal impact tests were performed to measure linear head acceleration. Oblique impact tests were performed to measure angular head acceleration and neck loading. Furthermore, acceleration histories of oblique impacts were analyzed in a computational head model to predict the resulting risk of TBI in the form of concussion and diffuse axonal injury (DAI). Compared to standard helmets, AIM helmets resulted in a 14% reduction in peak linear acceleration (p<0.001), a 34% reduction in peak angular acceleration (p<0.001), and a 22–32% reduction in neck loading (p<0.001). Computational results predicted that AIM helmets reduced the risk of concussion and DAI by 27% and 44%, respectively. In conclusion, these results demonstrated that AIM technology could effectively improve impact mitigation compared to a contemporary expanded polystyrene-based bicycle helmet, and may enhance prevention of bicycle-related TBI. Further research is required.

Effectiveness of combined aircraft engine noise suppressors

We consider the design features of fan noise suppressors in application to air intakes and the bypass duct of a turbofan engine. A combined liner is developed that has increased acoustic efficiency in comparison to conventional honeycomb liner. We demonstrate the important role of the area of the sound-absorbing liner between fan Rotor and Stator ensuring significant noise reduction.

First wall design considerations of JT-60 and related experiments

JT-60 will have a molybdenum first wall with a temperature of ~400–500°C, axisymmetric metal and magnetic limiters. Molybdenum has advantageous properties, i.e. low sputtering yield and high melting point. Furthermore, at temperatures of about 450–500°C, the hydrogen recycling ratio will become unity mainly by thermal desorption. Surface carbon and oxygen can be reduced by chemical reaction with oxygen and hydrogen gases, and surface damage such as blistering and grain ejection can be avoided. An axisymmetric limiter is useful to reduce the heat load per unit area, and a magnetic limiter will prevent the recycling of metallic impurities. An in-situ coating of molybdenum to attain a good clean surface, and a honeycomb liner to reduce the flux of sputtered wall atoms are now under development.