Gas Film Thickness Research Articles

Effects of Structure Parameters on Static Performance of Gas Foil Bearings Based on a New Fully Coupled Elastic–Aerodynamic Model

Abstract: Accurately predicting the performance of gas foil bearings (GFBs) is important. This paper presents a new fully coupled elastic–aerodynamic model for analyzing the static performance of gas foil journal bearings (GFJBs). The gas compressible lubrication model was solved in MATLAB to obtain the gas pressure. The foil structure deformation was solved in COMSOL by considering the Coulomb friction and allowing the contact surfaces to be separated from each other. Under given load and rotational speed conditions, the calculated minimum gas film thickness and attitude angle match well with the literature data, validating the accuracy of the developed model. Based on the model developed, a comprehensive and systematic analysis of the effects of the structural parameters on the static performance was performed. The results showed that as the bump height, top foil thickness, and bump foil thickness increased, the load capacity could be improved to different degrees. The bump foil thickness had the greatest effect on the load capacity. These results provide theoretical guidance for the structural design and practical applications of GFJBs.

The thermal-mechanical deformations of CO2 mixture gases dry gas seal based on two-way thermal-fluid-solid coupling model

PurposeThis study aims to investigate the influence mechanism of thermal-mechanical deformations on the CO2 mixture gases dry gas seal (DGS) flow field and compare the deformation characteristics and sealing performance between two-way and one-way thermal-fluid-solid coupling models.Design/methodology/approachThe authors established a two-way thermal-fluid-solid coupling model by using gas film thickness as the transfer parameter between the fluid and solid domain, and the model was solved using the finite difference method and finite element method. The thermal-mechanical deformations of the sealing rings, the influence of face deformation on the flow field and sealing performance were obtained.FindingsThermal-mechanical deformations cause a convergent gap between the two sealing end faces, resulting in an increase in the gas film thickness, but a decrease in the gas film temperature and sealing ring temperature. The axial relative deformations of rotating and stationary ring end faces caused by mechanical and thermal loads in the two-way coupling model are less than those in the one-way coupling (OWC) model, and the gas film thickness and leakage rate are larger than those in the OWC model, whereas the gas film stiffness is the opposite.Originality/valueThis paper provides a theoretical support and reference for the operational stability and structural optimization design of CO2 mixture gases DGS under high-pressure and high-speed operation conditions.

On the Machinability and Formation of Gas Film Thickness in ECDM of Silica based Pyrex Glass

Electrochemical discharge machining (ECDM) stands out as a preferred method for microfluidic channel fabrication due to its ability to produce required shapes with the least thermal damage along with close tolerances and dimensional accuracy. Recently, there has been significant progress made in the domain of ECDM for parametric analysis, focusing on discharge regime characteristics, but precise control of the hydrodynamic regime remains a challenge. With this view, the present study has been targeted on analyzing the effect of various auxiliary electrodes on the formation of gas film thickness and discharge energy during the fabrication of microchannels using ECDM on silica-based Pyrex glass under varying conditions. Experiments were conducted on an ECDM setup to achieve near damage-free edges of microchannels on the surface silica-based Pyrex glass. Results show the effect of various auxiliary electrodes, applied voltage, and electrolyte concentration on gas film thickness, spark energy, rate of material removal (MRR), and width of overcut (WOC). The optimum parametric level of applied voltage, electrolyte concentration, and auxiliary electrode were 68 V, 35 wt%, and A2 auxiliary electrode, respectively and the best response values were 0.4134 mg min−1, and 40 μm, respectively, for MRR and WOC. In addition, tool wear rate, heat affected zone, and surface morphology of fabricated microchannels was examined using scanning electron microscopy, optical microscope, and energy-dispersive X-ray spectroscopy.

Lubrication Characteristics of Dry-Gas Seals with Spiral Grooves

To obtain an optimal range of structural parameters for dry-gas seals with good performance, this study employed advanced sensing technology to monitor and analyze the internal flow characteristics of dry-gas seals in real time. Additionally, the validity of the calculation program was verified through experimentation. Using steady-state performance parameters as evaluation indices, a calculation model with lubrication characteristics was developed. The results indicate that when there are 12 grooves, the gas film pressure distribution is uniform and has a high value. At pressures greater than 2 MPa, the opening force, leakage, and gas film stiffness change significantly due to enhanced dynamic pressure effects with high-pressure differences, which reduces the local contact forces and frictional forces. At a constant speed, decreasing the gas film thickness increases the pressure difference while increasing both the opening force and film stiffness; however, at higher rotational speeds where the gas flow becomes non-uniform, the stability of the gas film is affected, leading to increased frictional forces. When there are between 10 and 16 grooves with depths ranging from 5.0 to 6.0 μm, dynamic pressure effects caused by pressure gradients become apparent, resulting in good dry-gas sealing performance being achieved. This research provides a theoretical reference for optimizing the design of dry-gas seals, as well as their steady-state seal performance.

Observation of Gap Phenomena and Development Processing Technology for ECDM of Sapphire

The main purpose of this study was to develop observation techniques and processing technology for the electrochemical discharge machining (ECDM) of sapphire wafers. To measure the effect of gas-film thickness, discharge-spark conditions, and droplet sliding frequency on machining quality and efficiency in ECDM, this research utilized high-speed cameras to observe the gas film thickness and formation of the gas film during ECDM. Additionally, this study observed the machining-gap phenomena during ECDM. The formation mechanism and machining characteristics of the gas film were understood through experiments. The machining parameters included the liquid level, working voltage, rotation speed, and duty factor. This study analyzed and discussed the effect of each machining parameter on the gas-film thickness, current, electrode consumption, and droplet sliding frequency. Moreover, this study aimed to obtain optimized machining parameters to overcome the difficulty of machining sapphire. The experimental results indicated that utilizing a high-speed camera to capture the phenomena between electrodes during electrochemical discharge could effectively observe the gas-film thickness and the coverage of the gas film. A higher bubble coalescence rate enhanced the machining capability and reduced the lateral discharge. Therefore, this study could obtain better machining-hole depths through observation and analysis to improve gas-film stability and machining capability. This study demonstrated that a liquid level of 700 µm, a working voltage of 48 V, a duty factor of 50%, and a tool electrode rotational speed of 200 rpm could achieve a hole depth of 86.7 µm and a hole diameter of 129.5 µm.

High-speed tilting-pad gas journal bearing flow field analysis

Tilting-pad gas journal bearing are widely used in the field of high-speed turbine machinery, especially in the special environment of low temperature. However, the performance is limited by structure, so a clear understanding of the field is particularly important. In this paper, finite volume method is used to simulate the three-dimensional flow field of high-speed tilting-pad gas bearing, then the effects of shaft speed, eccentricity and gas film thickness on the bearing capacity are studied. The simulation results show that the two factors of shaft speed and gas film clearance play a decisive role in producing sufficient bearing capacity of tilting tile radial gas bearings compared with other factors.

A comprehensive heat transfer prediction model for tubular moving bed heat exchangers using CFD-DEM: Validation and sensitivity analysis

This study established a complete heat transfer prediction model that integrates four heat transfer sub-models, including contact heat conduction, gas film heat conduction, heat radiation, and heat convection, along with an artificial softening correction based on combined approach of computational fluid dynamics and discrete element method (CFD-DEM). The model’s precision was confirmed by experimental results with relative errors below 5 %. The discussion on the heat flux contributions of the above four heat transfer mechanisms shows that with an initial particle temperature of 200℃, tube wall temperature of 30℃, and particle descent velocity of 1.3 mm/s, gas film conduction heat flux is the primary contributor to the total tube wall heat flux at 60.5 %, followed by radiation heat flux (19.5 %), contact conduction heat flux (12.1 %), and convection heat flux (7.9 %). Besides, the presentation of physical fields related to particles and the fluid showcases the model’s capability in elucidating the heat and mass transfer characteristics of the particle–fluid-wall system within tubular MBHEs. Furthermore, a local nominal range sensitivity analysis, along with a study of factors’ influence rules, was performed to quantify the effects of the 8 key model parameters on heat transfer results. The analysis indicates that fluid thermal conductivity is the predominant factor with its nondimensionalized first-order sensitivity coefficient of 54.9 %, succeeded by particle-to-wall angle factor (15.4 %), wall emissivity (14.4 %), gas film thickness (9.6 %), wall thermal conductivity (2.3 %), wall roughness (-1.6 %), particle Young’s modulus (-1.4 %), and particle specific heat capacity (0.4 %). Finally, their impacts on the heat prediction results were presented in detail.

Response of underwater photosynthesis to light, CO2, temperature, and submergence time of Taxodium distichum, a flood-tolerant tree.

Partial or complete submergence of trees can occur in natural wetlands during times of high waters, but the submergence events have increased in severity and frequency over the past decades. Taxodium distichum is well-known for its waterlogging tolerance, but there are also numerous observations of this species becoming partially or complete submerged for longer periods of time. Consequently, the aims of the present study were to characterize underwater net photosynthesis (PN) and leaf anatomy of T. distichum with time of submergence. We completely submerged 6 months old seedling of T. distichum and diagnosed underwater (PN), hydrophobicity, gas film thickness, Chlorophyll concentration and needles anatomy at discrete time points during a 30-day submergence event. We also constructed response curves of underwater PN to CO2, light and temperature. During the 30-day submergence period, no growth or formation new leaves were observed, and therefore T. distichum shows a quiescence response to submergence. The hydrophobicity of the needles declined during the submergence event resulting in complete loss of gas films. However, the Chlorophyll concentration of the needles also declined significantly, and it was there not possible to identify the main cause of the corresponding significant decline in underwater PN. Nevertheless, even after 30 days of complete submergence, the needles still retained some capacity for underwater photosynthesis under optimal light and CO2 conditions. However, to fully understand the stunning submergence tolerance of T. distichum, we propose that future research concentrate on unravelling the finer details in needle anatomy and biochemistry as these changes occur during submergence.

Optimizing Load Capacity Predictions in Gas Foil Thrust Bearings: A Novel Full-Ramp Model

Gas film thickness significantly influences the performance prediction of Gas Foil Thrust Bearings (GFTB). However, the Classical Model (CM) for GFTBs exhibits inaccuracies in describing gas film thickness. In this paper, we explore the differences in the details of gas film thickness modeling and propose a Parallel Segmentation Model (PSM), which fixes the errors of the CM in describing the gas film thickness in the ramp section, and a Full-Ramp Model (FRM), to which a more realistic description of the gas film in the flat section is also added. Comparative analysis, utilizing a publicly available test dataset based on the open-source GFTB structure, establishes that the FRM surpasses the CM and PSM in accurately predicting load capacity. In-depth analysis shows that the location of the minimum gas film thickness for determining the load capacity is located at the innermost circle of the free end of the top foil, whereas the FRM is subjected to the same load with a larger film thickness at this location, which may be due to the unique geometry of the top foil of the FRM. Subsequently, employing the FRM, a parametric study explores load capacity in GFTB, considering variables such as ramp height, top foil thickness, bump foil stiffness, ramp section extent, and top foil area. The results demonstrate that GFTB load capacity exhibits a linear increase with the expansion of the top foil area. Moreover, the load capacity increases with augmented top foil thickness and bump foil stiffness, albeit at a decreasing rate. Additionally, an increase in ramp section extent initially enhances load capacity, reaching a maximum value before declining. Similarly, an increase in ramp height initially augments load capacity, attaining a maximum before subsequent diminution.

Numerical Analysis on the Static Performance of Gas Journal Bearing by Using Finite Element Method

In this paper, finite element method is used to calculate the static performance of gas journal bearing, in which rotation speed term is introduced into the stiffness matrix of linear triangular element to realize the performance calculation of the bearing with rotation speed. The results indicate that the average gas film thicknesses corresponding to the maximum load capacity and stiffness, and the minimum attitude angle increase with the growth of orifice diameter. Load capacity and stiffness significantly improved with the increase of rotation speed, eccentricity ratio and supply pressure when the bearing has thin average gas film thickness. Attitude angle increases with the growth of rotation speed, while the growth rate slows down or even decreases at high speed. The most effective way of reducing attitude angle is to increase supply pressure. It can be found that rotation speed affects attitude angle through changing gas pressure difference between two orifices, while other parameters have the same effect by changing gas pressure at orifice outlet.

Rotordynamics of a rotor radially and axially supported by active bump-type foil bearings and bump-type thrust foil bearings

The rotordynamics of a rotor supported by gas foil bearings (GFBs) has been studied, focusing mainly on the relation between radial loads and lateral vibration. Nevertheless, the various axial loads occurring during operation must be compensated by thrust GFBs. Their effect on lateral and axial vibrations of the rotor must be addressed. This research studied the effect of axial load on the rotordynamics of a rotor supported by active bump-type foil bearings (ABFBs). Two ABFBs and a pair of bump-type thrust foil bearings (BTFBs) supported a rigid rotor along the radial and axial directions, respectively. The 5 degree of freedom nonlinear dynamic model for the rigid rotor is shown in this paper and was validated through experimental measurements. The study considered the coupling of the time-variant nonlinear bearing forces of ABFBs and BTFBs. The rotordynamic behaviors of a rotor under various axial loads, rotor unbalances, and voltages are predicted. Moreover, the effects of various disk misalignments on the minimum gas film thickness of the BTFBs are discussed.

Static performance analysis of gas foil thrust bearings under inclined thrust disc considering the slip flow effect

As one of the important components in air compressors for fuel cell vehicles, the static performance of the gas foil thrust bearings greatly affects the operating stability of the rotor system. In actual operation, thrust disc tilt and the slip flow effect often occur, which will change the characteristics of bearings. Therefore, this paper focuses on the static performance of gas foil thrust bearings under tilting conditions with account of the slip flow effect. Firstly, the gas film thickness equation is established when the thrust disc is inclined, and the Reynolds equation is modified by the boundary slip conditions. Secondly, the load capacity and friction torque of bearings are solved by the finite difference method and successive over-relaxation method. Finally, the sensitivity of rigid and elastic bearings to tilt angles is analyzed by comparing the static characteristics under tilting conditions. The results show that the load capacity will be overestimated without considering the slip flow effect, and the tilting of the thrust disc increases both the load capacity and friction torque. Furthermore, compared with rigid bearings, elastic foil bearings have better adaptability to tilting conditions, which is conducive to the stable operation of the rotor system.

Numerical Analysis on Improving the Rectangular Texture Floating Ring Gas-film Seal Characteristics with Different Bottom Shapes

Finding the right texture pattern to effectively improve the sealing performance of the floating ring gas film seal has always been a topic of interest for engineers. Herein, to reveal the effect of the rectangular textured base shape on the sealing characteristic parameters of floating ring gas film seal, four bottom shapes of rectangle, isosceles triangle, left triangle and right triangle were proposed. The governing equations were solved by the finite difference method and the correctness of the theoretical results was verified by the test bench. The effect of operating conditions parameters on the sealing performance were also analyzed. The results reveal that the trends of the experimental results is consistent well with the theoretical results, in which the relative errors are all less than 9%. It indicates that the theoretical model is scientific and valid. As the speed and inlet pressure increase, the texture bottom shape significantly changes the distribution of pressure and temperature field. And the average gas film thickness determines whether the texture produces an effect or not. Under the same operating conditions, a right-angled triangular bottom shape can obtain a good stability and cooling effect for the sealing device. With a bottom shape of the right triangle and a bottom shape of the rectangle, the maximum difference in opening force and gas film temperature rise were calculated to be 234.91 N and 0.61 oC, respectively. The research results provide theoretical support for further study of the textured floating ring gas film seal and a reference for texture optimization.

Study of Gas Film Characteristics in Electrochemical Discharge Machining and Their Effects on Discharge Energy Distribution.

Glass is a hard and brittle insulating material that is widely used in optics, biomedicine, and microelectromechanical systems. The electrochemical discharge process, which involves an effective microfabrication technology for insulating hard and brittle materials, can be used to perform effective microstructural processing on glass. The gas film is the most important medium in this process, and its quality is an important factor in the formation of good surface microstructures. This study focuses on the gas film properties and their influence on the discharge energy distribution. In this study, a complete factorial design of experiments (DOE) was used, with three factors and three levels of voltage, duty cycle, and frequency as the influencing factors and gas film thickness as the response for the experimental study, to obtain the best combination of process parameters that would result in the best gas film quality. In addition, experiments and simulations of microhole processing on two types of glass, quartz glass and K9 optical glass, were conducted for the first time to characterize the discharge energy distribution of the gas film based on the radial overcut, depth-to-diameter ratio, and roundness error, and to analyze the gas film characteristics and their effects on the discharge energy distribution. The experimental results demonstrated the optimal combination of process parameters, at a voltage of 50 V, a frequency of 20 kHz and a duty cycle of 80%, that achieved a better gas film quality and a more uniform discharge energy distribution. A thin and stable gas film with a thickness of 189 μm was obtained with the optimal combination of parameters, which was 149 μm less than the extreme combination of parameters (60 V, 25 kHz, 60%). These studies resulted in an 81 μm reduction in radial overcut, a roundness error reduced by 14, and a 49% increase in the depth-shallow ratio for a microhole machined on quartz glass.

Drop impact on superheated surfaces: from capillary dominance to nonlinear advection dominance

Ambient air cushions the impact of drops on solid substrates, an effect usually revealed by the entrainment of a bubble, trapped as the air squeezed under the drop drains and liquid–solid contact occurs. The presence of air becomes evident for impacts on very smooth surfaces, where the gas film can be sustained, allowing drops to bounce without wetting the substrate. In such a non-wetting situation, Mandre & Brenner (J. Fluid Mech., vol. 690, 2012, p. 148) numerically and theoretically evidenced that two physical mechanisms can act to prevent contact: surface tension and nonlinear advection. However, the advection dominated regime has remained hidden in experiments as liquid–solid contact prevents rebounds being realised at sufficiently large impact velocities. By performing impacts on superheated surfaces, in the so-called dynamical Leidenfrost regime (Tran et al., Phys. Rev. Lett., vol. 108, issue 3, 2012, p. 036101), we enable drop rebound at higher impact velocities, allowing us to reveal this regime. Using high-speed total internal reflection, we measure the minimal gas film thickness under impacting drops, and provide evidence for the transition from the surface tension to the nonlinear inertia dominated regime. We rationalise our measurements through scaling relationships derived by coupling the liquid and gas dynamics, in the presence of evaporation.

Condensation characteristics of air–water vapor mixture on the surface of vertical flat plate

The condensation heat transfer of air–water mixed vapor is an important heat transfer phenomenon in production life. An experimental system for condensation heat transfer of the air–water vapor mixed vapor is built in this paper. The condensation characteristics of the mixed vapor and the double boundary layer structure characteristics were investigated. The study mainly discussed the influence of flow velocity on the condensation performance of mixed vapor and summarized the experimental correlation equations considering flow velocity. Results indicate that the condensation performance decreases significantly as the air concentration increases. The mixed vapor flow velocity was increased from 1 to 3 m/s, and the condensation heat transfer coefficient was increased by 34.9–121.0%. The enhanced effect of flow velocity was also observed at high air concentration and low subcooling. The ratio of convective and condensation heat transfer coefficients increases with the rise in air concentration, reaching a maximum of 0.12. The proportion of convective heat transfer can be reduced by increasing the mixed vapor flow velocity. The gas film thickness is substantially higher than the liquid film thickness in mixed vapor condensation, and the gas/liquid film thermal resistance ratio can be more than 30 times. And increasing the flow velocity can significantly reduce the gas film/liquid film thermal resistance ratio. A new experimental correlation equation is fitted based on the experimental results, and this equation can be used as a reference for the design of related heat exchangers.

Analysis on the dynamic characteristics of spiral groove dry gas seal based on the gas film adaptive adjustment model

PurposeThis study aims to acquire the influence mechanism of gas film adaptive adjustment (GFAA) acted on the dynamic characteristics of spiral groove dry gas seal (S-DGS) and then propose a sealing stability enhancement measure.Design/methodology/approachThe gas film dynamic stiffness and damping of S-DGS are obtained by numerically solving the transient Reynolds equation based on perturbation method and finite difference method. The dynamic coefficients in GFAA model and constant gas film thickness (CGFT) model are compared and analyzed.FindingsThere is the risk to misestimate the instability of DGS with rotational speed or medium pressure grows under the condition of CGFT assumption. Based on GFAA model, increasing balance ratio B properly is an effective measure to improve the stability of DGS. The balance ratio can stimulate the sensitivity of gas film dynamic coefficients to the variation of rotational speed. Increasing medium pressure in small balance ratio range will be conducive to reducing the risk of angular instability.Originality/valueThe influence mechanism of GFAA on S-DGS dynamic characteristics is analyzed. The interactions between rotational speed and balance ratio, medium pressure and balance ratio acted on gas film dynamic characteristics are explored based on the GFAA model.

Investigations of the Bump Foil Bearing With Thick Top Foil Based on Contact Mechanics

AbstractTraditional bump foil bearing, though widely used, is considered to have the problem of inadequate load capacity due to the effect of foil sag between adjacent bumps. In this paper, the thick top foil is applied to cope with the heavy load conditions, which has one order of magnitude larger top foil thickness than traditional top foil. Thick plate and curve shell models are developed to calculate the load-carrying performance of thick top foil bearing based on contact mechanics, and the sag ratio is introduced to describe the degree of foil sag and estimate the bearing load capacity. The impacts of top foil thickness on the stiffness of bearings in different sizes as well as on the adaptability to rotor tilt are discussed. The results indicate that the thick top foil (tt = 2 mm) bearing performs a nearly 100% enhanced load capacity at small gas film thickness whether rotor tilts or not. Thick top foil will increase the bearing stiffness of small size (d = 35 mm) and tends to have little influence on large-size bearing stiffness (d = 70 mm).

Steady-State Characteristics of Spiral Groove Floating Ring Gas-film Seal Considering Temperature-Viscosity Effect

During the operation of the floating ring gas film seal, a certain amount of heat is generated inside the seal gap, giving rise to thermal deformation of the seal rings, and further leading to operation unstable and increased leakage rate. Based on the gas lubrication theory, the control equations of gas pressure and gas film thickness of the floating ring gas film seal are obtained. And the energy and temperature-viscosity equation are also introduced. The above equations were solved by the finite difference method and their correctness was verified by experiments. The variation of opening force, leakage rate, friction force, and gas film temperature rise with rotating speed, inlet pressure, and eccentricity were analyzed. The results reveal that, for leakage rate, the difference between the modeled and tested values is only 2.94% at high speeds, taking into account the influence of the temperature-viscosity effect. The experiment substantiates that the temperature-viscosity effect model is scientifically valid. Operation parameters also have different effects on sealing performance. Compared with isothermal flow, the pressure distribution in the gas film flow field will change significantly with increasing gas temperature, which means that the temperature-viscosity effect cannot be neglected in the flow field calculation. These results provide grounds for further study of the thermoelastic effect of air film seal of floating ring and have important engineering significance.

Theoretical analysis on macro-mesoscopic gas flow performances in gas dynamic bearing with three pads

The Reynolds equation based on the continuum medium assumption fails to meet the accuracy requirements of numerical simulation for mesoscale gas flow. In this research, the gas flow performances and bearing performances of gas dynamic bearing with three pads (GDBTPs) are theoretically analyzed from macroscopic to mesoscopic perspectives. A modified lattice Boltzmann equation is exploited considering the wall effect ψ(y/λ) with gas density ratio ρ/ρref, and the dimensionless gas flow velocity is analyzed for smooth, square cavity, half-sine asperity, triangular asperity, and a combination of surface morphologies. A modified Reynolds equation considering the gas compressibility and gas rarefaction effect is developed to study the static bearing performances of GDBTPs. Results show that the relative roughness Δh and asperities geometries are key factors to affect the mesoscale gas flow characteristics. The load-carrying capacity of GDBTPs increases with the growth of length-to-diameter ratio L/D, rotational speed ω, and eccentricity ratio ɛ and decreases with the increase of gas film thickness hg.