Frictional Power Dissipation Research Articles

Frictional power dissipation in a seismic ancient fault

The frictional power per unit area Q˙ (product of frictional traction τ and slip rate u˙ in MW m−2) dissipated during earthquakes triggers fault dynamic weakening mechanisms that control rupture nucleation, propagation and arrest. Although of great relevance in earthquake mechanics, Q˙ cannot, with rare exceptions, be determined by geophysical methods. Here we exploit theoretical, experimental and geological constraints to estimate Q˙ dissipated on a fault patch exhumed from 7-9 km depth. According to theoretical models, in polymineralic, silicate rocks the amplitude (< 1 mm) of the grain-scale roughness of the boundary between frictional melt (pseudotachylyte) and host rock decreases with increasing Q˙. The dependence of grain-scale roughness with Q˙ is due to differential melt front migration in the host rock minerals. This dependence is confirmed by friction experiments reproducing seismic slip where pseudotachylytes were produced by shearing tonalite at Q˙ ranging from 5 to 25 MW m−2. In natural pseudotachylytes across tonalites, the grain-scale roughness broadly decreases from extensional to compressional fault domains where lower and higher Q˙ are expected, respectively. Analysis of the natural dataset calibrated by experiments yields Q˙ values in the range of 4-60 MW m−2 (16 MW m−2 average value). These values, estimated in small fault patches, are at the lower end of broad estimates of Q˙ (3-300 MW m−2) obtained from frictional tractions (30-300 MPa) and fault slip rates (0.1-1 m/s) assumed as typical of upper crustal earthquakes.

Analysis of Rolling Force for Extra-Thick Plate with CA Criterion

In order to solve the nonlinear integral difficulty of the Mises yield criterion, a linear yield criterion, called the collaborative approximation (CA) yield criterion, is proposed by the collaborative control method. According to the approximation method, the mathematical expression of the CA yield criterion is derived as a linear function of the three principal stresses. The theoretical results based on the yield criterion in the form of the Lode parameter are verified with the classical test data, and a good agreement is found. Meanwhile, for the purpose of proving the effectiveness of the yield criterion, its specific plastic power is derived and applied to establish the rolling force model of an extra-thick plate. In the modeling, the internal power of plastic deformation is obtained by using the derived specific plastic power, while the shear power dissipation and the frictional power dissipation are obtained by using the methods of strain vector inner product and average velocity integration. Then, the analytical solution of the rolling force is obtained and then extended to the one accounting for the temperature rise. The maximum errors of the predicted rolling torque and rolling force without considering the temperature rise are 12.72% and 11.78%, respectively, while those considering the temperature rise decrease to 3.54% and 5.23%, respectively. Moreover, the influence of relative reduction, friction factor, surface temperature, and the temperature rise of the workpiece on the theoretical results is discussed.

Integrated Energy and Thermal Management for Electrified Powertrains

This study presents an integrated energy and thermal management system to identify the fuel-saving potential caused by cold-starting an electrified powertrain. In addition, it quantifies the benefit of adopting waste heat recovery (WHR) technologies on the ultimate fuel savings. A cold-start implies a low engine temperature, which increases the frictional power dissipation in the engine, leading to excess fuel usage. A dual-source WHR (DSWHR) system is employed to recuperate waste heat from exhaust gases. The energy harvested is stored in a battery and can be retrieved when needed. Moreover, the system recovers waste heat from an electric machine, including power electronics and a continuous variable transmission, to boost the heating performance of a heat pump for cabin heating. This results in a decrease in the load on the battery. The integrated energy and thermal management system aims at maximizing the fuel efficiency for a pre-defined drive cycle. Simulation results show that cold-start conditions affect the fuel-saving potential significantly, up to 7.1% on the New European Driving Cycle (NEDC), yet have a small impact on the optimal controller. The DSWHR system improves the fuel economy remarkably, up to 13.1% on the NEDC, from which the design of WHR technologies and dimensioning of powertrain components can be derived. As the optimal solution is obtained offline, a complete energy consumption minimization strategy framework, considering both energy and thermal aspects, is proposed to enable online implementation.

Study of different cooling structures on the thermal status of an Internal Combustion Engine

Different cooling structures for cylinder head and block and the corresponding thermal status, thermal dissipation and frictional power dissipation were investigated in this study.In a conventional engine cooling configuration, the coolant flow travels from the block to the head, leading to the coolant temperature of the head being higher than that of block. Furthermore, the over-cooling problem in the block will occur because the cooling system is designed for the cooling requirements of the cylinder head without considering the cooling load of the block.This paper developed a 3D single cylinder model to analyse the influence of coolant flow direction and split structure cooling for the head and block. The analysis indicated that the top-bottom flow cooling structure can effectively reduce the cylinder head thermal load and slightly increase the temperature of the cylinder liner. The combined split and top-bottom cooling structure is proven as the optimal solution with the advantages of lower thermal and frictional power dissipation compared with that of the conventional cooling structure.

Understanding white etching cracks in rolling element bearings: State of art and multiple driver transposition on a twin-disc machine

Among the prevalent tribological failures affecting rolling element bearings, an unconventional rolling contact fatigue mode has been identified as white etching cracks. Those correspond to three-dimensional branching crack networks partially bordered by white etching microstructure, eventually leading to premature and unpredictable failure. Recent work supports that this failure mode may be associated with various combinations of operating conditions depending on the application or test rig, but that all seem to converge towards similar tribological drivers related to surface-affected hydrogen evolution at asperity scales, which is known to embrittle the bearing steel. Nevertheless, as white etching cracks remain delicate to reproduce without artificial hydrogen charging, the underlying formation mechanisms remain unsettled. The present work aims to better understand how some of the main tribomechanical and tribochemical drivers may trigger white etching cracks and premature failures. In this study drivers such as sliding kinematics, water contamination, and electrical potential and lubricant additives are progressively transposed on a twin-disc machine that provides an enhanced control of contact parameters. Various attempts advocate that the tested drivers are not self-sufficient to reproduce the failure mode in such apparatus, but confirm that specific lubricant additives may reduce the fatigue life by promoting surface-initiated embrittled cracking similar to white etching cracks. A local criterion accounting for the local sliding frictional power dissipation and the lubrication regime is further proposed to assess the risk of white etching cracks based on the analysis of various reproduction and occurrences.

The role of frictional power dissipation (as a function of frequency) and test temperature on contact temperature and the subsequent wear behaviour in a stainless steel contact in fretting

Temperature is known to affect the fretting wear behaviour of metals; generally, a critical temperature is observed, above which there are substantial reductions in wear rate, with these being associated with the development of protective oxide beds in the fretting contact. This work has examined the gross-sliding fretting behaviour of a stainless steel as a function of bulk temperature and fretting frequency (with changes in the fretting frequency altering the frictional power dissipated in the contact amongst other things). An analytical model has been developed which has suggested that at 200Hz, an increase in the contact temperature of more than 70°C can be expected, associated with the high frictional power dissipation at this frequency (compared to that dissipated at a fretting frequency of 20Hz). With the bulk temperature at either room temperature or 275°C, the increase in contact temperature does not result in a transition across the critical temperature (and thus fretting behaviour at these temperatures is relatively insensitive to fretting frequency). However, with a bulk temperature of 150°C, the increase in temperature associated with the increased frictional power dissipation at the higher frequency results in the critical temperature being exceeded, and in significant differences in fretting behaviour.

Optimal Control of a Mechanical Hybrid Powertrain With Cold-Start Conditions

This paper investigates the impact of cold-start conditions on the fuel-saving potential and the associated optimal energy controller of a mechanical hybrid powertrain. The mechanical hybrid powertrain uses a flywheel system to add fuel-saving functionalities to a conventional powertrain, which consists of an internal combustion engine and a continuously variable transmission (CVT). The cold-start conditions refer to a low powertrain temperature, which increases the frictional power dissipation in the engine and transmission, and a stationary (or energyless) flywheel system, which must be energized to a minimum energy level before it can be effectively utilized. The heating of the powertrain and the initialization of the flywheel system can be influenced by the energy controller, which controls the power distribution between the engine, the flywheel, and the vehicle. The energy controller aims at minimizing the overall fuel consumption for a given driving cycle. The optimal energy controller is found analytically for a simplified model to gain qualitative insights in the controller and numerically using dynamic programming for a detailed model to quantify the impact on the fuel consumption. The results show that the cold-start conditions have a significant impact on the fuel-saving potential, yet a negligible impact on the optimal energy controller. The latter result implies that the temperature state can be eliminated from the state space of the energy controller, which is an important step toward the design of an effective yet simple energy controller suitable for real-time implementation.

Prediction of Wear in Reciprocating Dry Sliding via Dissipated Energy and Temperature Rise

An alternative technique aimed at facilitating the calculation of frictional power dissipation in reciprocating dry sliding is presented. The proposed technique can be employed for the prediction of wear in circumstances where the direct measurement of power dissipation is encumbered by practical limitations. Experimental tests are carried out to investigate the relationship between the system’s wear rate, power dissipation, and thermal response. A convenient technique is also proposed to estimate the average contact temperature in a reciprocating sliding contact. The predicted temperatures agree with the experimental measurements. It is also shown how the predicted temperatures can be used for the estimation of wear under reciprocating dry sliding configuration.

Far-field dynamics of tidal energy extraction in channel networks

Tidal hydrokinetic power generation involves the conversion of the kinetic power in swiftly moving tidal currents to renewable electricity. Resource assessment is critical to understand the tidal hydrokinetic potential, but is complicated by a number of factors, including far-field effects. These are changes to the tidal regime caused by the increased resistance to flow as power is extracted from a channel network. This study addresses far-field effects in four prototypical channel networks: multiply-connected flow around an island, a branching network in which the flow bifurcates but does not converge downstream, and a network with multiple constrictions in series. These networks are modelled as one-dimensional channels with hydrokinetic power extraction in high current constrictions. Changes to tides, transport, frictional power dissipation, and kinetic power density are quantified for a range of extraction options. Depending on the type of network, the tidal regime may be either locally augmented or reduced by kinetic power extraction. The changes to kinetic power density throughout the network have important implications for resource assessment, particularly for networks with multiple extraction sites. Results suggest that existing analytical methods tend to over- or under-estimate the hydrokinetic resource because they do not allow for changes to the tidal forcing as a consequence of extraction. In general, site-specific numerical modelling is required to quantitatively predict far-field extraction effects and assess the hydrokinetic resource.

A new energy optimal control scheme for a separately excited DC motor based incremental motion drive

This paper considers minimization of resistive and frictional power dissipation in a separately excited DC motor based incremental motion drive (IMD). The drive is required to displace a given, fixed load through a definite angle in specified time, with minimum energy dissipation in the motor windings and minimum frictional losses. Accordingly, an energy optimal (EO) control strategy is proposed in which the motor is first accelerated to track a specific speed profile for a pre-determined optimal time period. Thereafter, both armature and field power supplies are disconnected, and the motor decelerates and comes to a halt at the desired displacement point in the desired total displacement time. The optimal time period for the initial acceleration phase is computed so that the motor stores just enough energy to decelerate to the final position at the specified displacement time. The parameters, such as the moment of inertia and coefficient of friction, which depend on the load and other external conditions, have been obtained using system identification method. Comparison with earlier control techniques is included. The results show that the proposed EO control strategy results in significant reduction of energy losses compared to the existing ones.

Thermoelastic evolution of contact area and mound temperatures in carbon graphite electrical brushes

Using the finite element method to estimate coupled electrical, thermal and thermoelastic fields, and an exact solution to analyze elastic contact stresses, electrical and frictional heating of a thermal mound present in the interface between a carbon graphite block sliding against a fast moving conductor is simulated. Heating due to frictional power dissipation is modeled by internal heat sources uniformly distributed within a very thin layer beneath the sliding contact interface. Internal Joule heat sources are estimated from the electric field and resistivity. The thermal mound possesses the highly temperature-dependent electrical resistivity, thermal conductivity and specific heat common to carbon graphite materials. Evolution of the mound temperatures from cold to hot and the contact radii from large to small is studied. Results show that the type of thermal boundary condition prescribed on the contact surface has a marked effect on the entire evolution. To simulate the extreme reduction of contact area observed in the evolution to thermal mounding from normal contact, an insulated thermal boundary condition was needed.

Thermal Evolution of Hot Spots in Thermally Nonlinear Carbon Graphite Sliders

Frictional heating of a thermal mound (or hot spot) present on the interface between a carbon graphite block sliding against a fast moving conductor is simulated. Heating of this mound due to frictional power dissipation is modeled as a collection of internal heat sources uniformly distributed within a very shallow volume (or layer) located directly beneath the sliding contact interface. The thermal mound, assumed to be motionless on and originating from the carbon graphite block, possesses the extreme temperature dependent thermal conductivity and heat capacity common to carbon graphite materials. Evolution of thermal mound temperatures from cold to hot is studied as a function of the intensity of the internal heat source distribution and the thickness of the heat source layer. For a fast moving conducting body sliding against the graphite block, it is shown that (a) an optimal heat source layer thickness exists, whereby temperatures maximize for this thickness and (b) for a sufficiently high heat source intensity, thermal instability of the mound is possible.