Extension of the Friction Mastercurve to Limiting Shear Stress Models

2003 ◽  
Vol 125 (4) ◽  
pp. 739-746 ◽  
Author(s):  
B. Jacod ◽  
C. H. Venner ◽  
P. M. Lugt
Keyword(s):  
Shear Stress ◽  
Friction Coefficient ◽  
Numerical Simulations ◽  
Coefficient Of Friction ◽  
Rheological Model ◽  
Operating Conditions ◽  
Limiting Shear Stress ◽  
Wide Range ◽  
Two Parameters ◽  
Stress Models

A previous study of the behavior of friction in EHL contacts for the case of Eyring lubricant behavior resulted in a friction mastercurve. In this paper the same approach is applied to the case of limiting shear stress behavior. By means of numerical simulations the friction coefficient has been computed for a wide range of operating conditions and contact geometries. It is shown that the same two parameters that were found in the Eyring study, a characteristic shear stress, and a reduced coefficient of friction, also govern the behavior of the friction for the case of limiting shear stress models. When the calculated traction data is plotted as a function of these two parameters all results for different cases lie close to a single curve. Experimentally measured traction data is used to validate the observed behavior. Finally, the equations of the mastercurves for both types of rheological model are compared resulting in a relation between the Eyring stress τ0 and the limiting shear stress τL.

Influence of Longitudinal Roughness on Friction in EHL Contacts

2004 ◽  
Vol 126 (3) ◽  
pp. 473-481 ◽  
Author(s):  
B. Jacod ◽  
C. H. Venner ◽  
P. M. Lugt
Keyword(s):  
Numerical Simulations ◽  
Operating Conditions ◽  
Simplified Approach ◽  
Curve Fit ◽  
Longitudinal Roughness ◽  
Wide Range ◽  
Equivalent Wave ◽  
Harmonic Components ◽  
Comparison Of The Results ◽  
Relative Friction

The effect of longitudinal roughness on the friction in EHL contacts is investigated by means of numerical simulations. In the theoretical model the Eyring equation is used to describe the rheological behavior of the lubricant. First the relative friction variation caused by a single harmonic roughness component is computed as a function of the amplitude and wavelength for a wide range of operating conditions. From the results a curve fit formula is derived for the relative friction variation as a function of the out-of-contact geometry of the waviness and a newly derived parameter characterizing the response of the lubricant to pressure variations. Subsequently, the case of a superposition of two harmonic components is considered. It is shown that for the effect on friction such a combined pattern can be represented by a single equivalent wave. The amplitude and the wavelength of the equivalent wave can be determined from a nonlinear relation in terms of the amplitudes and wavelengths of the individual harmonic components. Finally the approach is applied to the prediction of the effect of a real roughness profile (many components) on the friction. From a comparison of the results with full numerical simulations it appears that the simplified approach is quite accurate.

scholarly journals Pressure losses and oscillations in a compact valve of a steam turbine

2021 ◽  
Vol 345 ◽  
pp. 00027
Author(s):  
Václav Sláma ◽  
David Šimurda ◽  
Lukáš Mrózek ◽  
Ladislav Tajč ◽  
Jindřich Hála ◽  
...  
Keyword(s):  
Numerical Simulations ◽  
Pressure Oscillation ◽  
Operational Reliability ◽  
Operating Conditions ◽  
Steam Turbines ◽  
Valve Seat ◽  
Pressure Losses ◽  
Oscillation Analysis ◽  
Seat Angle ◽  
Wide Range

Characteristics of a new compact valve design for steam turbines are analysed by measuring pressure losses and oscillations on the valve model. It is the model of an intercept valve of the intermediate-pressure turbine part. This valve is relatively smaller hence cheaper than usual control and intercept valves. Besides, four different valve seat angles were tested in order to investigate the valve seat angle influence. In order to further clarify measured phenomena, the wide range of numerical simulations were also carried out. Measurements were performed in the Aerodynamic laboratory of the Institute of Thermomechanics of the Czech Academy of Sciences in an air test rig installed in a modular aerodynamic tunnel. Numerical simulations were performed in the Doosan Skoda Power Company using a package of ANSYS software tools. Measurement results are compared with numerical and generalized in the form of valve characteristics and pressure oscillation maps. As a result of the pressure loss analysis, pressure losses in similar valve assemblies can be predicted with required accuracy for each new turbine where modern compact valves are used. As a result of the pressure oscillation analysis, operating conditions at which dangerous flow instabilities can occur were identified. Thanks to this, the areas of safe and dangerous operating conditions can be predicted so that the operational reliability of the valve can be guaranteed.

Towards Primary Breakup Simulation of a Complete Aircraft Nozzle at Realistic Aircraft Conditions

2020 ◽  
Author(s):  
Katharina Warncke ◽  
Amsini Sadiki ◽  
Max Staufer ◽  
Christian Hasse ◽  
Johannes Janicka
Keyword(s):  
Numerical Simulations ◽  
Liquid Fuel ◽  
Fuel Injection ◽  
Operating Conditions ◽  
Nozzle Geometry ◽  
Nozzle Flow ◽  
Spray Characteristics ◽  
Wide Range ◽  
Primary Breakup ◽  
Turbulent Scales

Abstract Predicting details of aircraft engine combustion by means of numerical simulations requires reliable information about spray characteristics from liquid fuel injection. However, details of liquid fuel injection are not well documented. Indeed, standard droplet distributions are usually utilized in Euler-Lagrange simulations of combustion. Typically, airblast injectors are employed to atomize the liquid fuel by feeding a thin liquid film in the shear zone between two swirled air flows. Unfortunately, droplet data for the wide range of operating conditions during a flight is not available. Focusing on numerical simulations, Direct Numerical simulations (DNS) of full nozzle designs are nowadays out of scope. Reducing numerical costs, but still considering the full nozzle flow, the embedded DNS methodology (eDNS) has been introduced within a Volume of Fluid framework (Sauer et al., Atomization and Sprays, vol. 26, pp. 187–215, 2016). Thereby, DNS domain is kept as small as possible by reducing it to the primary breakup zone. It is then embedded in a Large Eddy Simulation (LES) of the turbulent nozzle flow. This way, realistic turbulent scales of the nozzle flow are included, when simulating primary breakup. Previous studies of a generic atomizer configuration proved that turbulence in the gaseous flow has significant impact on liquid disintegration and should be included in primary breakup simulations (Warncke et al., ILASS Europe, Paris, 2019). In this contribution, an industrial airblast atomizer is numerically investigated for the first time using the eDNS approach. The complete nozzle geometry is simulated, considering all relevant features of the flow. Three steps are necessary: 1. LES of the gaseous nozzle flow until a statistically stationary flow is reached. 2. Position and refinement of the DNS domain. Due to the annular nozzle design the DNS domain is chosen as a ring. It comprises the atomizing edge, where the liquid is brought between inner and outer air flow, and the downstream primary breakup zone. 3. Start of liquid fuel injection and primary breakup simulation. Since the simulation of the two-phase DNS and the LES of the surrounding nozzle flow are conducted at the same time, turbulent scales of the gas flow are directly transferred to the DNS domain. The applicability of eDNS to full nozzle designs is demonstrated and details of primary breakup at the nozzle outlet are presented. In particular a discussion of the phenomenological breakup process and spray characteristics is provided.

scholarly journals A Study on the Performance of Combustion in a HCCI Engine Using n-Heptane by a Multi-Zone Model

2009 ◽  
Author(s):  
Hongsheng Guo ◽  
Hailin Li ◽  
W. Stuart Neill
Keyword(s):  
Numerical Simulation ◽  
Numerical Simulations ◽  
Chemical Species ◽  
Numerical Data ◽  
Operating Conditions ◽  
Zone Model ◽  
Nox Emissions ◽  
Hcci Engine ◽  
Operation Conditions ◽  
Wide Range

A study of n-heptane combustion in an HCCI engine was carried out by a multi-zone numerical simulation that covers a complete engine cycle. A reaction mechanism that includes 177 chemical species and 1638 reactions was used. The results of the numerical simulations were compared to existing experimental data for a range of air/fuel ratios, compression ratios and engine speeds. It is shown that the numerical simulation is able to reasonably capture the experimental cylinder pressure data over a wide range of operation conditions. It also provides a qualitative trend of CO emissions. The numerical simulation overpredicted the combustion at some operating conditions, such as at extremely high air/fuel ratios and higher engine speeds. Some differences were observed between the experimental and numerical data for NOX emissions. The numerical simulation predicted a monotonic decrease in NOX emissions as air/fuel ratio increased or compression ratio decreased, while an increase in NOX emissions was observed experimentally when combustion became very weak at extremely high air/fuel ratios or low compression ratios. It is suggested that further experiments and numerical simulations should be performed to explain this discrepancy.

Development of Friction Drive Transmission

2005 ◽  
Vol 127 (4) ◽  
pp. 857-864 ◽  
Author(s):  
Xiaolan Ai ◽  
Matthew Wilmer ◽  
David Lawrentz
Keyword(s):  
Friction Coefficient ◽  
Performance Optimization ◽  
Operating Conditions ◽  
Elastic Support ◽  
Outer Ring ◽  
Design Parameters ◽  
Fe Model ◽  
Frictional Contacts ◽  
Actual Operating ◽  
Wide Range

A cylindrical friction drive was developed for electric oil pump applications. It was comprised of an outer ring, a sun roller, a loading planet, two supporting planets, and a stationary carrier. The sun roller was set eccentric to the outer ring to generate a wedge gap that facilitates a torque actuated loading mechanism for the friction drive. The loading planet was properly assembled in the wedge gap and elastically supported to the carrier. By altering the stiffness ratio of the elastic support to contact, the actual operating friction coefficient of the friction drive can be changed regardless of the wedge angle to suit for performance requirement. This provided a greater freedom for design and performance optimization. Design analysis was presented and a FE model was developed to quantify design parameters. Prototypes of the friction drive were fabricated and extensive testing was conducted to evaluate its performance. Results indicated the performance of the friction drive far exceeded the design specifications in speed, torque, and power ratings. The friction drive offered a consistent smooth and quiet performance over a wide range of operating conditions. It was capable of operating at an elevated speed of up to 12 000 rpm with adequate thermal characteristics. The friction drive demonstrated a peak efficiency above 97%. Results confirmed that the stiffness of the elastic support has an important impact on performance. The elastic support stiffness, in conjunction with the contact stiffness, determines the actual operating friction coefficient at the frictional contacts.

Thermal Elastohydrodynamic Analysis Using a Generalized Non-Newtonian Formulation With Application to Bair-Winer Constitutive Equation

1994 ◽  
Vol 116 (1) ◽  
pp. 37-46 ◽  
Author(s):  
M. M. Khonsari ◽  
D. Y. Hua
Keyword(s):  
Shear Stress ◽  
Constitutive Equation ◽  
Fluid Model ◽  
Quantitative Agreement ◽  
Operating Conditions ◽  
Original Form ◽  
Shear Strain Rate ◽  
Limiting Shear Stress ◽  
Traction Coefficient ◽  
Local Shear

The governing equations together with a solution methodology are given which enables one to effectively handle an EHL line contact problem with simple non-Newtonian fluids including thermal effects. A computational algorithm is proposed that determines the equivalent viscosity as a function of shear strain rate for a specified constitutive equation. It is shown that the method effectively handles Bair-Winer’s rheological equation in its original form and without the need for an approximate perturbation analysis. Among the performance parameters presented are the local behavior of the shear stress as predicted by the Bair-Winer’s model and its comparison to that of the Ree-Eyrings constitutive equation. It is shown that these rheological equations predict a qualitatively similar trend for the traction coefficient. Nevertheless, depending on the operating conditions, the local shear stress as predicted by the Ree-Eyring equation may exceed the material limiting shear stress. A comparison study of the traction coefficient as predicted by the Bair-Winer’s fluid model and actual experimental measurements is also presented. The results are found to be in good quantitative agreement.

scholarly journals Natural Circulation Characteristics at Low-Pressure Conditions through PANDA Experiments and ATHLET Simulations

2008 ◽  
Vol 2008 ◽  
pp. 1-14 ◽  
Author(s):  
Domenico Paladino ◽  
Max Huggenberger ◽  
Frank Schäfer
Keyword(s):  
Numerical Simulations ◽  
Large Scale ◽  
Natural Circulation ◽  
Low Pressure ◽  
Operating Conditions ◽  
Quality Data ◽  
Experimental Investigations ◽  
Stability Performance ◽  
Wide Range ◽  
Circulation Characteristics

Natural circulation characteristics at low pressure/low power have been studied by performing experimental investigations and numerical simulations. The PANDA large-scale facility was used to provide valuable, high quality data on natural circulation characteristics as a function of several parameters and for a wide range of operating conditions. The new experimental data allow for testing and improving the capabilities of the thermal-hydraulic computer codes to be used for treating natural circulation loops in a range with increased attention. This paper presents a synthesis of a part of the results obtained within the EU-Project NACUSP “natural circulation and stability performance of boiling water reactors.” It does so by using the experimental results produced in PANDA and by showing some examples of numerical simulations performed with the thermal-hydraulic code ATHLET.

A Generalized Traction Curve for EHL Contacts

2000 ◽  
Vol 123 (2) ◽  
pp. 248-253 ◽  
Author(s):  
B. Jacod ◽  
C. H. Venner ◽  
P. M. Lugt
Keyword(s):  
Coefficient Of Friction ◽  
Sliding Friction ◽  
Operating Conditions ◽  
Wide Range ◽  
The Coefficient Of Friction ◽  
Friction Curve ◽  
The Subject ◽  
Friction Prediction ◽  
Elliptic Contact ◽  
Theoretical Results

In this paper the subject of friction prediction is revisited, with the aim of obtaining a general formula predicting the coefficient of friction over a wide range of operating conditions. By means of full numerical simulations of the smooth isothermal elliptic contact, and assuming an Eyring non-Newtonian behavior, the coefficient of friction is computed for a wide range of operating conditions. It is shown that with respect to sliding friction, all results can be presented on a single generalized friction curve relating a reduced coefficient of friction to a characteristic nondimensional shear stress. Finally, it is shown that some measured data presented in the literature when presented in terms of the derived parameters closely follow the derived behavior, which provides a validation of the theoretical results.

Lubricant Limiting Shear Stress Effect on EHD Film Thickness

1980 ◽  
Vol 102 (2) ◽  
pp. 213-220 ◽  
Author(s):  
B. Gecim ◽  
W. O. Winer
Keyword(s):  
Shear Stress ◽  
Film Thickness ◽  
Fluid Model ◽  
Operating Conditions ◽  
Stress Effect ◽  
Limiting Shear Stress ◽  
Viscous Fluid Model ◽  
Low Shear Stress ◽  
New Variables ◽  
Low Shear

A Grubin-like EHD inlet analysis utilizing a non-linear viscous fluid model with a limiting shear stress is reported. The shear rheological equation requires only a low shear stress viscosity and the limiting shear stress both functions of pressure. Values employed for these properties are taken from measurements on typical lubricants. Reductions of EHD film thickness are found to be up to 40 percent compared with the standard Grubin prediction for typical operating conditions. Slide-roll ratio, limiting shear stress dependence on pressure, and atmospheric pressure value of limiting shear stress are new variables required to determine film thickness with the first two being more important than the last. The EHD film thickness is reduced by increasing slide-roll ratio and/or decreasing the pressure dependence of the limiting shear stress.

A Rheological Model for Elastohydrodynamic Contacts Based on Primary Laboratory Data

1979 ◽  
Vol 101 (3) ◽  
pp. 258-264 ◽  
Author(s):  
Scott Bair ◽  
W. O. Winer
Keyword(s):  
Shear Stress ◽  
Material Properties ◽  
Rheological Model ◽  
Laboratory Data ◽  
Maxwell Model ◽  
Limiting Shear Stress ◽  
Low Shear Stress ◽  
Temperature And Pressure ◽  
Elastic Shear ◽  
Low Shear

A shear rheological model based on primary laboratory data is proposed for concentrated contact lubrication. The model is a Maxwell model modified with a limiting shear stress. Three material properties are required: Low shear stress viscosity, limiting elastic shear modulus, and the limiting shear stress the material can withstand. All three are functions of temperature and pressure. In applying the model to EHD contacts the predicted response possesses the characteristics expected from several experiments reported in the literature and, in one specific case where direct comparison could be made, good numerical agreement is shown.

Sign in / Sign up

Export Citation Format

Share Document