Air Film Research Articles

Effect of wall-seeping gas film under different working media on stability of conical hypersonic boundary layer

Wall-seeping gas film (WSGF) is a promising method of controlling hypersonic boundary layer transition and reducing friction drag and heat transfer. Experiments are conducted in a Mach 6 hypersonic quiet wind tunnel by using nano-tracer planar laser scattering (NPLS) and high-frequency fluctuating pressure measuring technique. This work investigates the effects of wall-seeping helium, air, and carbon dioxide gas films under identical volume flow rate condition on conical boundary layer thickness, disturbance wave structure, wavelength, frequency, amplitude, and nonlinear interaction. The experimental results reveal that the WSGF significantly thickens the hypersonic boundary layer, with the thickest position appearing at the downstream boundary of the seeping zone. The boundary layer thickness is thinnest for helium gas film but thickest for carbon dioxide gas film. Generally, air gas film and carbon dioxide gas film induce the regular, rope-like, and interlaced second-mode waves to appear in advance in the boundary layer. However, under a higher volume flow rate for carbon dioxide gas film, the disturbance wave structure resembles interface fluctuations, with a characteristic wavelength of approximately 18 mm and a peak frequency as low as about 35 kHz, but no the rope-like interlaced characteristic. At this time, the influence of shear layer instability becomes significant. The disturbance waves do not exhibit second-mode wave characteristics for wall-seeping helium gas film, whose shape is irregular and undergoes deformation with time and space. Additionally, the power spectral density of wall fluctuating pressure exhibits insignificant variation with volume flow rate and flow direction, which is similar to the characteristic of power spectral density in the laminar boundary layer and has no peak frequency. The wavelength of second-mode waves is about 2－3 times the boundary layer thickness for air gas film, and increases to more than 3 times for carbon dioxide gas film. The application of carbon dioxide gas film results in smaller peak frequency and bandwidth of disturbance wave, larger characteristic wavelength and amplitude, longer propagation distance, and stronger nonlinear interaction than the application of air gas film. In the future, attention should be paid to understanding disturbance wave characteristics in the boundary layer for the helium gas film and shear layer instability under larger volume flow rates.

Research on simulation method of multiple time scales dissolution process in tube electrode pulse electrochemical machining

Tube electrode pulsed electrochemical machining (TE-PECM) has received widespread attention as an electrochemical perforation technique that is widely used for machining deep small holes/oblique small holes such as air film holes in blades. However, the TE-PECM process is characterized by complex multi-physics field coupling relationships and multiple time scales for multi-physics interactions, making the simulation of the pulsed electrolytic processing process computationally huge and difficult to perform. To accurately describe the multi-physical field coupling relationship for pulsed electrochemical processing. In this paper, a k-ε gas-liquid two-phase flow model has been introduced to model the interpolar gas-liquid two-phase flow field variation law, and on this basis, a coupled multi-physics field model for pulsed electrochemical machining processing is established to provide an accurate description of the interpolar multi-physics interaction mechanism. A quasi-steady-state iterative simplification algorithm based on heat source averaging is proposed to solve the problem that the time scale span of the dissolution process leads to a huge amount of simulation calculations. The simulation analyzes the dissolution characteristics at the micro time scale and macro time scale and compares them with the full precision method. The results show that the simulation accuracy of the two methods is the same, but the simulation calculation time of the simplified algorithm is reduced by two-thirds. The experimental platform was designed and built to carry out a series of experiments under different processing times, and the results effectively verified the correctness of the simulation method.

A computational study of air film evolution during droplet impact on a flat solid surface

A computational study of air film evolution during droplet impact on a flat solid surface

Research on the Load Characteristics of Squeezed Film Gas Bearings

In this paper, a gas microbody is proposed to address the issues related to the limited load‐carrying capacity of squeezed film gas bearings, complex mechanical structures, large overall dimensions, high manufacturing costs, and complicated installation. Initially, the gas continuity equation of squeezed film was established for modal analysis, without regard to the gas inertial force and the gas volume force which are all far less than the gas viscous force. Then the load‐carrying characteristics of squeezed film gas bearing were studied and analyzed with the fluid‐structure coupling method. The load‐carrying capacity of the squeezed film gas bearing is determined by factors such as bearing amplitude, rotor‐axial clearance, and rotor‐axial tile contact area. In conclusion, the load‐carrying capacity can be enhanced by increasing the amplitude value, reducing the clearance, and increasing the contact area. Among them, when adjusting the amplitude and clearance to improve the bearing’s load‐carrying capacity, the stability of bearing operation should be considered. When the clearance is too small, the pressure distribution of the air film will be uneven, and when the contact area is too large, the development of the air film in all directions will be asymmetrical.

Flow evolution of mixed layer on convex curvature wall under hypersonic conditions

With the continuous upgrading of hypersonic vehicles, a new requirement for designing imaging window i.e. conformal window for improving aerodynamic characteristics, is put forward, in which the supersonic cooling film and optical window are required to maintain the same curvature shape as the aircraft body. In this work, the mixed-layer flow evolution on a convex wall (CV) is investigated. A nanoparticle-based planar laser scattering technique is used to design the flow field structure of the mixed layer in <i>Ma</i> = 6 hypersonic static wind tunnel, and the location of the mixed-layer instability is studied by combing fractal dimension. The results of pressure, and impulse of compression (<i>I</i><sub>p</sub>) evolution along the flow direction are obtained by numerical simulation, showing that the total incoming pressure (<i>P</i><sub>0</sub>) has a significant effect on the flow evolution of the mixed layer: as <i>P</i><sub>0</sub> increases, the ratio of static pressure (RSP) decreases, that the position of the mixed-layer instability is delayed, and that the flow velocity of the typical vortex structure increases. The favorable gradient existing at the CV wallleads the pressure to drop along the flow direction, and the pressure is enhanced when the supersonic air film along the tangential direction of the wall is under the operating condition. However, as <i>P</i><sub>0</sub> increases, the RSP decreases, and the lifting effect of the pressure on the CV decreases. The flow field is affected by the expansion effect of the CV, and <i>I</i><sub>p</sub> decreases along the flow direction. The supersonic air film can weaken the expansion effect on the CV and thus suppressing the decrease of <i>I</i><sub>p</sub>. The change rate of <i>I</i><sub>p</sub> (Δ<i>I</i><sub>p</sub>) is significantly affected by <i>P</i><sub>0</sub>, in a range of bending impulse |<i>I</i><sub><i>Φ</i></sub>| = 0.191–3.62, Δ<i>I</i><sub>p</sub> decreases from 178.67% to 12.02% when <i>P</i><sub>0</sub> = 0.5 MPa, and Δ<i>I</i><sub>p</sub> decreases from 40.38% to 5.64% when <i>P</i><sub>0</sub> = 1.0 MPa. Δ<i>I</i><sub>p</sub> decreases as |<i>I</i><sub><i>Φ</i></sub>| increases, but the decrease becomes less as <i>P</i><sub>0</sub> increases. The results reveal the flow evolution law of hypersonic mixed layer under the influence of convex curvature, and provide a certain reference for designing the shape of hypersonic vehicle to achieve aerodynamic drag reduction and thermal protection characteristics.

Preparation and Characterization of Tung Oil Loaded Melamine Formaldehyde Microcapsules

In this study microcapsules were prepared by in-situ polymerization route with melamine formaldehyde as a shell material and tung oil as core material. Melamine formaldehyde (MF), a thermosetting polymer, is one of the most widely used monomers in microencapsulation due to its superior mechanical strength and thermal stability. Tung oil contains unsaturated double bonds that can be oxidized to form a film in air. Tung oil is fast drying and biodegradable, besides it is low cost and does not pollute the environment. Most importantly, tung oil is a versatile substance in industry. Therefore, tung oil is a good choice as core material. The chemical structure of microcapsules were characterized by Fourier Transform Infrared (FTIR) spectroscopy. The surface morphology and particle size and distribution were evaluated by Scanning Electron Microscopy (SEM). The thermal behavior of microcapsules and tung oil were studied by thermogravimetric analysis (TGA). The results showed that the spherical microcapsules (particle size of mostly 4-5 μm) were produced with a filling content of 15.64 wt.%, and a yield of 49.78 wt.%. The microcapsules exhibit a good thermal stability

Computational Study on Air Film Approach in Reentry Blackout Mitigation

Communication blackouts during the atmospheric reentry phase are a significant challenge, as flight data are lost due to interruptions caused by plasma gas generated by aerodynamic heating. This study explores a novel mitigation method using an air film, a thin insulating coolant layer on the surface. The researchers successfully reduced the reentry blackout by employing a gas injection system. Through a coupled approach using computational fluid dynamics and a frequency-dependent finite-difference time-domain method, the plasma flow properties and electromagnetic propagation were analyzed around a test model in a wind tunnel in DLR (German Aerospace Center). The numerical results indicated that the injected nitrogen gas formed an insulating air film layer on the surface. The thin layer advected backward, maintaining a low temperature without ionization, and covered the object in the wake region. The electromagnetic waves propagated and reached a distant area because the electron density was low. It means that the air film layer acted as a propagation window for the telecommunication waves. Thus, communication blackouts will be avoidable because electromagnetic waves can transmit through the air-film layer. It concluded that the air film effect, developed as a thermal protection technique, is a novel mitigation scheme for reentry blackouts.

Development and Investigation of Non-Destructive Detection Drive Mechanism for Precision Type Cylindrical Roller Dynamic Unbalance

In the process of dynamic unbalance detection for precision cylindrical rollers, challenges such as difficulty in achieving effective driving and susceptibility to surface damage during driving significantly impact the accuracy of unbalance detection. This hinders the industry’s ability to achieve the non-destructive detection of cylindrical rollers. Therefore, this paper proposes a novel driving method to enable non-destructive driving of precision cylindrical rollers. The structural principles of the driving mechanism are presented, and a mechanical model for the cylindrical roller is established to analyze the force distribution. Subsequently, a mathematical model for the air film bearing the cylindrical roller is developed to study the variation characteristics of the air film’s load-bearing capacity. The optimal air film thickness is determined, and the rationality of the mathematical model is validated through simulation analysis. Finally, an experimental platform for non-destructive driving is constructed to further verify the effectiveness of the proposed method. This research provides a prerequisite for the non-destructive detection of dynamic unbalance in precision cylindrical rollers.

Rapid Surface Reconstruction in Air-Processed Perovskite Solar Cells by Blade Coating.

Blade coating has been developed to be an essential technique for large-area fabrication of perovskite solar cells (PSCs). However, effective surface treatment of the perovskite layer, which is a critical step for improving PSC performance, remains challenges during blade coating due to the short interaction time between the modification solution and the perovskite layer, as well as the limited selection of available organic solvents. In this study, a novel modifier N,N-diphenylguanidine monohydrobromide (DPGABr) dissolved in acetonitrile (ACN) is blade coated on the MA0.7 FA0.3 PbI3 surface in air to reconstruct the perovskite surface in hundreds of milliseconds. This work finds that the solvent ACN rapidly dissolves organic iodide of the perovskite layer and leads to a PbI2 -rich surface, providing reactive sites for DPGABr to form a thin DPGABr/PbI2 complex layer. This surface reconstruction can effectively passivate defects and induce n-type doping on the perovskite surface to facilitate electron transfer. The resultant devices show a 15% improvement in average power conversion efficiency. More importantly, the devices with the surface reconstruction show outstanding long-term stability, with negligible performance degradation even after 1-year storage in air. This study presents a convenient and effective approach for improving the performance of blade-coated PSCs prepared in air.

Wide-velocity range high-energy plasma sprayed yttria-stabilized zirconia thermal barrier coating — Part I: Splashing splat formation and micro-adhesive property

Regulation on the disked-shape splat is generally regarded as the traditional way to enhance micro-adhesive properties of the splat/substrate interface. In this paper, a conception of impact-induced splashing splat is proposed in order to improve these micro-adhesive properties. Three contrasting types of splashing splat were studied, utilizing a wide-velocity high-energy plasma spraying method. The experimental results suggested that the needle-shaped splat formed by the impingement of supersonic particles possessed a greater micro-adhesive strength on an metallic substrate, as much as twice that of its subsonic counterpart. The threshold velocity was 572 m/s for supersonic adhesion. In relation to air film formation and interlocking reconstruction, the solidification texture and micro-adhesive at the central and peripheral regions was also investigated. The formation process of splashing splat was divided into three stages: particle deformation, edge fragmentation and impingement. This results will provide guideline for tailoring the high-performance thermal barrier coatings.

Effect of drainage on antibubble collapse

Antibubbles are a special spherical structure generated in liquids surrounded by a thin air film. The collapse of antibubbles has been regarded as a feasible method for generating microbubbles. This paper demonstrates an experimental study on the antibubble collapse focusing on the effect of rupture time and location. With the help of a high-speed camera, the detailed collapse dynamics for different rupture times and locations was presented. The collapse velocity was found to vary significantly with the rupture time and location. An upgraded theoretical model was proposed to elucidate the effect of the air film thickness distribution due to drainage on the collapse velocity. This study should stimulate further investigation on the antibubble collapse, especially the size distribution of the generated microbubbles.

Manipulation of bubble collapse patterns near the wall of an adherent gas layer

This paper aims to apply experimental methods to investigate the effect of the thickness of gas layers on the wall on the collapse direction of spark-induced bubbles. In the experiment, two high-speed cameras synchronously record the time evolution of the bubbles and the corresponding parameters such as the normalized collapse position and bubble collapse time. Experiments yielded results for individual bubbles over a range of normalized distances from 0 to 4.0 for different air layer thicknesses. Based on the morphology of the bubbles, the experimental jets were visualized into six different modes, namely, forward jet (FJ), merging jet (MJ), bidirectional jet (BJ), reversing jet (RJ), forward followed by reversing jet (FRJ), and non-directional jet (NDJ). The height of the air layer on the wall is affected by the fluctuation of the bubble volume and shows the opposite trend to the change of the bubble volume. The air film reaches its maximum height when the bubble collapses, which affects the final jet pattern. In addition, as the thickness of the air layer increases, the center of the bubble gradually migrates away from the wall. The different collapse modes and the migration of the bubble centers have positive significance for reducing cavitation erosion in engineering.

Optimization of ultrasonic levitation bearing load characteristics based on rotor surface microstructure

Ultrasonic levitation bearings have lower load capacity and stiffness than other non-contact bearings, which restricts their practical use. This study proposes a method to improve the load characteristics of the bearings by adding microstructure on the rotor surface. The effects of different microstructure shapes, sizes, and quantities on the bearing load capacity and stiffness were investigated, and the mechanism of how the microstructures and their parameters affect the bearing static performance was revealed by analyzing the air film pressure field analysis of the bearing. Using the groove microstructure as an example, an optimization design was carried out to maximize the load capacity and stiffness of the ultrasonic levitation bearing, and the validity of this optimization was verified by the experiment.

Critical analysis of Filipino TV's militarism and impact on societal awareness

In the digital age, militarisation, encompassing police power, soldiers, and various military resources, has become increasingly prevalent in various forms of Philippine media. Thus, "Multimedia Militarisation" best describes how Philippine media instills ideals and manipulates mass audiences to accept the status quo. Right-wing propaganda normalises violence and warfare. In this paper, the authors argue that TV series like "Ang Probinsyano," "The General's Daughter," and "A Soldier's Heart" have played a significant role in shaping social consciousness. They have influenced how people view military personnel and what is philosophically referred to as state apparatuses, which encompass the police, military, justice system, and prison system, along with their respective leaders, as integral members of an ideal society. Furthermore, the authors consider it essential to reference historical instances, such as Hitler's Nazi Germany and Marcosian Philippines, where propaganda played a pivotal role in manipulating social consciousness through the dissemination of false information. The study identifies three main themes: the glorification of the armed branch of government, the normalisation of violence, and the introduction of revolutionary ideology to the public. Finally, this paper adopts a social-historical approach and critically analyses the current state of the country's mainstream media, highlighting its reactionary implications. It emphasises how these media outlets air films that advocate for the necessity of militarisation, all in the context of the progress of Philippine society and the evolving definition of patriotism within a populist cultural setting.

Regulating Air‐Annealing Time for Boosting the Performance of Cu2ZnSn(S, Se)4 Solar Cells

Excellent morphology and low harmful defect states of the absorber layer are essential to fabricate the high‐performance Cu2ZnSn(S, Se)4 (CZTSSe) devices. Herein, the annealing time of precursor films in air is proved to have much impact on the quality of absorber layer and the performance of devices. Appropriately extending the air‐annealing time can promote the diffusion of Na into films and O absorption on the surface of precursor films, boost the growth of crystal grain, and lessen the harmful defect density and band‐tailing states of absorber. The appropriate extension of air‐annealing time also regulates the electrical properties of the absorber. The efficiency of CZTSSe devices is enhanced from 6.92% (1 min) to 10.1% (7 min), with the decreased VOC, Def. These enhanced properties demonstrate that regulating the air‐annealing time of precursor films can be a simple and direct way for improving the performance of CZTSSe devices.

Hybrid ANFIS-PSO algorithm for estimation of the characteristics of porous vacuum preloaded air bearings and comparison performance of the intelligent algorithm with the ANN

Air bearings are commonly used in systems requiring high precision, including semiconductor manufacturing, optical and electronic devices, and aerospace systems, and the usage of vacuum preloading and porous material provides a homogeneous pressure distribution on the bearing surface. The prediction of the characteristics of these air bearings, such as air film thickness and location, is crucial for designing and optimizing the performance of the systems. They exhibit highly nonlinear behavior and are challenging to accurately predict their characteristics using conventional analytical or numerical techniques. In this study, the characteristics of vacuum preloaded porous air bearings are estimated using a hybrid ANFIS-PSO predictor soft computing method with a certain pressure (410 kPa), vacuum (50.8 kPa), and with three different diameters (50, 75 and 90 mm). The usage of different algorithms in the characteristics and selection and design criteria of air bearing has been applied. Six different cases are simulated with different input and output parameters to predict the air film thickness and location of the air bearing. The location of the air bearing has been determined with two angle parameters, α, and β. The dataset required for training and testing the algorithm was obtained through experimental study. In the training and testing process, 40 and 20 datasets were used, respectively. Before the starting analysis, a sensitivity analysis was executed to find optimum PSO parameters such as the number of particles, acceleration coefficients, number of membership functions, and inertia weight. The proposed hybrid ANFIS-PSO method was compared with classical ANFIS and ANN, and it showed better performance and a more accurate solution to estimate the characteristics of vacuum preloaded porous air bearings in both training and testing phases with the R2 of 0.9958 and MSE value of 0.102. The maximum training performance of the ANFIS-PSO is better than ANFIS and ANN with the ratio of 6.8% and 8.5%, respectively. On the other hand, the maximum testing performance of the ANFIS-PSO algorithm is higher than ANFIS and ANN, with the ratio of 22.2% and 24.1%, respectively.

Investigation of boiling heat transfer process in the barrel of a gravity heat pipe type extruder

Gravity heat pipe barrels produce gas films during the processing of thermoplastic materials, which affect the heat transfer, so there is an urgent need to study the nucleate boiling process and mechanism at different locations in the barrel. The results showed that the local heat flux at the vapor-liquid-solid three-phase contact was the largest, and different process conditions had different effects on the departure diameter and departure time of boiling bubbles. The tail vortex disturbance generated by bubble departure drives the merging and growth of bubbles to departure, which in turn improves the heat transfer. The rounded bottom of the barrel reduced the air film and increased the heat transfer efficiency compared to the horizontal bottom of the barrel. This study can deepen the understanding of the mechanism of bubble boiling heat exchange in gravity heat pipe-type barrels and provide theoretical support for more relevant application scenarios.

A comparative study of purge slot exit shape on the film cooling effectiveness of a gas turbine shroud

To achieve higher efficiency and performance, a modern gas turbine withstands the extremely high thermal environment. In the turbine endwall passage, a high and non-uniform heat transfer appears due to various secondary flow. The purge flow injected through the upstream slot between the stationary and rotating pars can thermally protect the endwall surface by forming air film layer on it. However, since an excessive amount of purge flow reduce the system efficiency, it is needed to effectively inject the purge flow with a limited amount. Accordingly, in the present study, the effect of purge slot exit shape on the film cooling effectiveness (hereby FCE) of a shroud surface was discussed. As nondimensional design parameters of the purge slot, L/Ws, RW/Ws, and RL/Ws were defined, representing the geometric features: the slot width (Ws), the fillet radius at the windward side (RW) and fillet radius at the leeward side (RL) of slot exit, respectively. The measurement of the FCE was performed in a five-blade linear cascade rig using the PSP (Pressure sensitive paint) technique. The results showed that the FCE increased for wider slot width at a given blowing ratio condition. In addition, the fillet radius at the windward side (RW) has a negative effect on the FCE regardless of blowing ratio condition since the purge flow was easily disturbed by the accelerated mainstream injection. On the other hand, the Case 7 with the RW/Ws = 0.9 showed 5.1 % of the improvement of FCE at a higher blowing ratio M = 0.25 compared with the Case 2 with RW/Ws = 0, which represents the sharp edge at the windward side of the slot exit.

Turbine stage expansion model including internal air film cooling and novel method of calculating theoretical power of a cooled stage

Turbine stage expansion model including internal air film cooling and novel method of calculating theoretical power of a cooled stage

Experimental investigation on the aero-optical effects of supersonic gas films with various cooling fluids.

Despite the fact that a supersonic cooling gas film can efficiently insulate aerodynamic heating, its interaction with the mainstream generates a sophisticated flow structure which may cause significant aero-optical ramifications. This study aims to analyze the fluid structure and wavefront distortion of supersonic gas film when subjected to varying nozzle pressure ratios (NPR) by employing two distinct cooling refrigerants, namely C O 2 and air. Within the NPR range of 0 to 2, a linear relationship exists between the wavefront distortion of both C O 2 and air films, while the C O 2 film exhibits higher wavefront distortion than the air. Additionally, the influence of condensation on the discrepancies in aero-optical effects of the two refrigerants is discussed.