High Friction Losses Research Articles

A comparative investigation on wear behaviors of physical and chemical vapor deposited bronze coatings for hydraulic piston pump

The cylinder block/valve plate interface is one of the most critical frictional interfaces of the swashplate-type axial piston pump. However, the poor lubrication interface caused rapid wear and high friction loss in an elastohydrodynamic lubrication system, decreasing the pump lifetime. Wear resistant bronze coatings were fabricated on 38CrMoAl substrate by Physical Vapor Deposition (PVD) and Chemical Vapor Deposition (CVD), respectively. Ball-on-disc wear tests were performed to comparatively investigate the wear behaviors of the coatings and bulk ductile iron samples. It can be found that the PVD-bronze coating exhibited better wear resistance than the other two samples. This enhanced wear resistance was attributed to the unique composite microstructure and desired mechanical strength, which could resist to mechanical shear and spallation, decreasing friction loss. The appropriate hardness of (1.33 ± 0.07) GPa could be beneficial for enhancing its wear resistance. The PVD-bronze coating possessed a much lower and stable coefficient of friction (about 0.1) and wear rate (about 6000 μm3·N−1·m−1) under the loading forces of about 100 N after 20 min. The wear mechanism was the abrasive wear.

Galinstan liquid metal as the heat transfer fluid in magnetic refrigeration

Room-temperature magnetic refrigeration is a promising technology in which cooling is achieved by exploiting a special property of some materials, the “magnetocaloric effect” (MCE), i.e., the ability of the material to change its temperature when it undergoes changes in the applied external magnetic field. For magnetic field variations on the order of 1–1.5 T, the MCE results in small adiabatic temperature changes of a few degrees so, in most cases, the designs and prototypes presented by various research groups employ active magnetic regeneration to increase the temperature span required for practical cooling devices. The effectiveness of the regenerator, and thus of the refrigeration device, depends heavily on the properties of the heat transfer fluid, which is usually a water-based mixture. High values of the fluid thermal conductivity greatly affect the heat transfer between the MCE and the fluid allowing faster operation and higher cooling capacity. In this study, the use of a gallium based liquid metal, i.e., Galinstan, is investigated to quantify this improvement. Galinstan was found not to be aggressive to gadolinium at room temperature and it resulted not affected by magnetic induction. The authors report the results of an extensive simulation campaign to characterize the performance of an active magnetic regenerator made of 0.5 mm thick gadolinium sheets using Galinstan as a heat transfer fluid. The results are compared to those of the same device using water; friction is also accounted for since the high density of this liquid metal leads to a high demand for pumping power. On average, a more than tenfold increase in cooling capacity was observed when using Galinstan, but the COP was severely penalized by the higher frictional losses. To avoid this effect, the cycle time and the utilization factor must be adjusted, or the geometry changed by using thicker sheets and ducts for the same Gd weight. In this way a cooling increase of 400% is achieved with the same COP.

An Efficient and Stable Embedded Multi-U-Shaped Column Rotary Transformer

Inductively Coupled Power Transfer (ICPT) technology can be used to solve the problems of high friction loss, overheating and low stability caused by conductive slip rings on rotating power supply equipment. An improved Embedded Multi-U-Shaped Column Rotary Transformer (EMUCRT) based on the ICPT and spliced core structure is proposed in this paper. It has a higher coupling coefficient and can transmit power more efficiently and stably due to the primary magnetic core is embedded inside the secondary magnetic core. Compared with the one-piece core structure, the spliced type is more flexible due to its loose magnetic core arrangement. Its primary coil can still transmit energy stably after offset by a certain distance, so the structure has good anti-misalignment ability. On the basis of analyzing its magnetic core arrangement and magnetic circuit model, the optimal magnetic core specifications and quantities of primary and secondary side are obtained by theoretical calculation and simulation. In order to verify the high efficiency, rotational stability and anti-misalignment characteristics of the structure, an experimental prototype was built. The results show that EMUCRT can transmit energy efficiently and has good rotational and offset stability.

A Study on Weight Reduction and High Performance in Separated Magnetic Bearings

Recently, high-speed motors are receiving a lot of attention in the industrial field. When driving a motor at high speed, the advantages include high power density, high efficiency, and miniaturization. However, the disadvantages of the high-speed operation of motors are mechanical and structural safety. This is because the bearings used in high-speed motors require characteristics such as precision and low friction. There are two prominent types of bearings mainly used in high-speed motors: rolling bearings and magnetic bearings. A feature of rolling bearings is that they reduce frictional resistance by contacting points or lines between the shaft and the bearing. However, the disadvantages of rolling bearings are high mechanical friction losses due to the need for contact with the lubrication system. Maintenance costs are high. For this reason, a lot of research on bearings is being conducted to reduce the frictional loss of bearings and increase their efficiency and reliability. Bearings that are advantageous for high-speed operation are magnetic bearings that do not require lubricants, have no friction loss, and have low maintenance. However, magnetic bearings have disadvantages such as high cost and difficulty in miniaturization. In this paper, a stator with a separated teeth structure was used to compensate for these disadvantages. Using this, a model with miniaturization, light weight, and high manufacturability was proposed. The model name proposed in this study is called the STMB (separated teeth magnetic bearing). There are also disadvantages of the STMB model proposed in this paper. A model that compensates for this drawback is called the HSTMB (hybrid separated teeth magnetic bearing). The HSTMB reduces the weight by removing the back yoke of the stator and has advantages of a high filling rate and high productivity in the form of a module. As a result, high productivity, light weight, and high performance are possible when the HSTMB is applied, which was proven through FEA (finite element analysis).

On the Wear Behaviour of Bush Drive Chains: Part II—Performance Screening of Pin Materials and Lubricant Effects

In this second part of the paper series, parameter investigations of the tribological system chain pin/bush contact, carried out on a specifically developed pin on bush plate model test technique, are presented. Both the pin material and the lubricant varied widely. In case of the pin materials, a Cr-N monolayer coating and a Cr-N-Fe-based multilayer coating were investigated. As for the lubricants used, two different performing engine oils from the field were tested as well as fresh oils, some of which were diluted with a soot surrogate (carbon black) and diesel fuel in different amounts. The results show, among other things, that friction and wear performance strongly depend on the combination of pin material and lubricant used. In this context, especially the Cr-N-Fe in combination with the used engine oils showed a high wear resistance and low friction losses compared to the Cr-N reference. In the case of fresh oils with soot, the friction losses were higher but comparable between the pin materials, and a slightly better wear performance of the Cr-N was observed due to an agglomeration effect of the soot surrogate. In general, it was found that especially soot-free oils show clear wear advantages independent of the pin material used. Thus, soot clearly has a wear-promoting component. The investigations of this study suggest that a leading mechanism that is based on a corrosive–abrasive effect in the tested system, but this is more related to the soot surrogate carbon black than engine soot.

Investigation of a centrifugal pump for energy loss due to clearance thickness while pumping different viscosity oils

The clearance between the stationary volute and rotating impeller of a centrifugal pump is crucial; however, it affects the internal flow and deteriorates the pump performance. This study performed a flow investigation through a numerical simulation to understand the effect of clearance and pumping fluid viscosity under the design and off-design conditions of a centrifugal pump. Numerical simulations were conducted by solving unsteady Reynolds-averaged Navier-Stokes (URANS) equations and validated using experimental results. Five three-dimensional centrifugal pump flow domains were created, by varying the clearance thickness, and simulated with three different fluid properties. Consequently, the numerical simulation results aided in understanding the internal flow pattern and estimating energy loss due to disk friction and leakage through the clearance. It was found that the disk friction and leakage losses, which cause secondary losses in the pump, were highly affected by the fluid properties and clearance thickness. The clearance thickness drastically affected both the volumetric and hydraulic efficiencies to leakage; however, beyond a particular value, it became less significant. Furthermore, the viscous oils caused a high disk friction loss; however, the leakage loss was less affected by the clearance thickness.

Novel multi-stage vortex cooling with bypass for turbine blade leading edge design considering rotational effect

This work aims to comprehensively improve vortex cooling performance by suggesting a novel design in the blade leading edges of gas turbines. Under considering the effect of blade rotation, different multi-stage configurations with bypass are proposed. A series of validated calculations are performed based on solving steady-state Reynolds Averaged Navier-Stokes equations and the SST k-ω turbulence model. Heat transfer and flow characteristics of the different vortex cooling structures are analyzed at different rotational speeds. From the results, it can be concluded that: (1) under stationary conditions, compared to the single-stage model, the multi-stage design of vortex cooling can greatly enhance the heat transfer rate, and its average Nusselt number can be 87.6% higher, but the cost is the high total pressure loss. Nevertheless, the comprehensive thermal performance in the multi-stage configuration is still 7.2% higher at least. (2) When blade rotates, the heat transfer rates in all configurations generally decrease by the effect of the Coriolis force (at least 8.8% when rotational number is 0.072). With the increase of rotational speed, the unfavorable high friction loss in multi-stage design is also decreased. For example, the total pressure loss coefficient in multi-stage vortex cooling without bypass is decreased by 68.4% when Rotational number increases to 0.072. (3) The bypass design in the multi-stage model provides an effective way to control the heat transfer enhancement and total pressure loss. At a high rotational number of 0.072, using a bypass with a thickness of 0.25 mm, although the averaged Nusselt number drops by 9.9%, the decrease of total pressure loss reaches to 117%. This phenomenon implies that a proper multi-stage vortex cooling model with bypass design can significantly decrease the friction loss, at a negligible price of heat transfer reduction.

Self-consistent solution of magnetic and friction energy losses of a magnetic nanoparticle

We present a simple simulation model for analyzing magnetic and frictional losses of magnetic nanoparticles in viscous fluids subject to alternating magnetic fields. Assuming a particle size below the single-domain limit, we use a macrospin approach and solve the Landau-Lifshitz-Gilbert equation coupled to the mechanical torque equation. Despite its simplicity the presented model exhibits surprisingly rich physics and enables a detailed analysis of the different loss processes depending on field parameters and initial arrangement of the particle and the field. Depending on those parameters regions of different steady states emerge: a region with dominating magnetic relaxation and high magnetic losses and another region region with high frictional losses at low fields or low frequencies. The energy increases continuously even across regime boundaries up to frequencies above the viscous relaxation limit. At those higher frequencies the steady state can also depend on the initial orientation of the particle in the external field. The general behavior and special cases and their specific absorption rates are compared and discussed.

Assessment of the impact of the number and the arrangement of cylinders of the KAMAZ disel engines on their fuel efficiency

Most of the natural aspirated truck diesel engines had an eight-cylinder V-twin (V8) configuration. Modern turbocharged diesel engines have a six-cylinder in-line configuration (L6), although some V8 diesel engines continue to be produced. A comparison was made of the performance of the KAMAZ-740 (V8) diesel engine with two turbochargers and KAMAZ-910 (L6) diesel engines with one turbocharger, having almost the same displacement with the same pattern of torque along the full load characteristic. The AVL BOOST software and experimental compressor and turbine maps were used. The factors influencing fuel efficiency were analyzed: indicated efficiency, friction and gas exchange losses, turbocharger efficiency. The L6 diesel engine had a uniform pressure pulsation in the exhaust manifold and at the turbine inlet. In the V8 diesel engine, the pulsations were uneven and had higher amplitude, which led to higher gas exchange losses and a decrease in turbine efficiency. The L6 diesel engine had a higher indicated efficiency but also higher friction losses. On average, the brake specific fuel consumption of the L6 diesel engine was by 5.5 % lower than that of the V8 diesel engine. Keywords V8 and P6 diesel engines; modeling in AVL BOOST; pressure pulsations in the exhaust system; turbocharger efficiency; friction and gas exchange losses

Research on Heat Dissipation Scheme of Active Magnetic Bearing Based on ANSYS

Compared to mechanical bearings, active magnetic bearings eliminate mechanical friction, resulting in lower energy losses. However, due to the magnetization characteristics and high-frequency working characteristics of the core material of the active magnetic bearing, there will be high wind friction loss, eddy current loss, hysteresis loss and copper loss in the active magnetic bearing. These losses will generate heat and increase the internal temperature of the bearing, which will not only affect the control accuracy of the active magnetic bearing and the stability of the rotor suspension, but may also cause irreversible damage to the components inside the active magnetic bearing. In this paper, different heat dissipation schemes are established to discuss the influence of the active magnetic suspension bearing on the loss and heat generation of the active magnetic bearing.

A Review on Two-Phase Volumetric Expanders and Their Applications

The importance of volumetric expanders has been increasing in the last decades because several studies confirmed that they lead to improved energy savings, limit the environmental impact, and reduce the energy intensity of industrial and domestic applications. In particular, several applications of the two-phase volumetric expanders, in which the operating fluid consists of liquid and vapor phases, were recently proposed. Nevertheless, the contributions in the scientific literature related to the overview of the state-of-the-art aspects of this technology are rare. For this reason, the present work discussed the potentialities and drawbacks of the available typologies of volumetric expanders that process a two-phase pure working fluid by analyzing a summary of leading studies in this field to go beyond previous efforts in the literature. The analysis revealed that twin-screw machines represent the best candidates, while reciprocating piston devices seemed the least well-adapted because of their reduced tolerance to droplets and high friction losses. Flash evaporation appeared to have the most significant impact on the expander because it affects both inlet and expansion phases, thus, determining the shape of the indicated cycle and the isentropic efficiency.

Mechanochemically driven formation of protective carbon films from ethanol environment

Wear and degradation of interfaces remain a significant roadblock in commonly used mechanical assemblies across various industries, resulting in loss of their efficiency and ultimately functionality. To advance the state-of-the-art in tribological applications, new materials must be developed not only to resist degradation, wear, and higher frictional losses in extreme environments (i.e. high temperatures, contact/shear forces, etc.), but also to benefit from them. By adopting hard, catalytic interfaces, a wide range of contact conditions can emerge where mechanochemical interactions lead to the formation of protective or self-healing lubricious films. Here, we demonstrate the mechanochemically driven formation of protective carbon films on Pt-Au alloys during sliding in an ethanol environment. We demonstrate the effect of temperature and contact pressure on film formation. The films formed on Pt-Au alloys exhibit a highly graphitic structure as indicated by Raman and Transmission Electron Microscopy (TEM) analyses. The observed results are further supported by molecular dynamics simulations that show the changes in the dissociation and transformation of ethanol molecules with applied pressure and temperature. The results create a new understanding of transformations in the contact and suggest a solution for addressing tribological challenges in the mechanical systems operated in low viscosity fuels.

Optimization of Magnetic Gear Patterns Based on Taguchi Method Combined with Genetic Algorithm

Magnetic gears (MGs) have gained increasing attention due to their sound performance in high torque density and low friction loss. Aiming to maximize the torque density, topology design has been a popular issue in recent years. However, studies on the optimization comparisons of a general MG topology pattern are very limited. This paper proposes a Taguchi-method-based optimization method for a general MG topology pattern, which can cover most of the common types of radially magnetized concentric-surface-mounted MGs (RMCSM-MGs). The Taguchi method is introduced to evaluate the influence of each parameter in MGs. Moreover, the parameter value range is re-examined based on the sensitivity analysis results. The genetic algorithm (GA) method is adopted to optimize the topology pattern in the study.

Analysis of multi-field coupled air friction loss and temperature field of high-speed permanent magnet machine

The high-speed rotation of the high-speed permanent magnet machine (HSPMM) causes high air friction loss. The multi-field calculation of air friction loss is usually unidirectional coupling, so the interaction between temperature and air friction loss cannot be considered. By using the improved analytical model, the air friction loss considering the effect of temperature on air density and dynamic viscosity is calculated. The influence of this method on the temperature and air velocity of the machine is compared. The accuracy of this method is verified by comparing with the improved air friction loss numerical calculation model. The results show that the improved calculation model of air friction loss is more accurate.

Exergy-Based Thermo-Hydraulic Performance of Roughened Absorber in Solar Air Heater Duct

This paper presents the thermo-hydraulic performance of small conical ribs on the absorber plate of a solar energy air heater (SAH) using exergy analysis. Application of conical protrusion ribs on the absorber is an attractive solution for enhancing the thermal performance of a SAH. However, these ribs are also responsible for high friction losses and increased fan power consumption caused by the turbulent air flow. To optimize the rib design, it is vital to consider both thermal and hydraulic performance at the same time. The SAH was assessed using an analytic method which predicts the exergy efficiency under operating parameters (e.g., Reynolds number, solar insolation and temperature increase parameter). The following geometric quantities of ribs were evaluated for optimum exergy efficiency: the relative rib height (e/D), which was in the range between 0.200 and 0.044, and the relative rib pitch (p/e), which was in the range between 6 and 12. The combination of a relative rib height of 0.044 and relative rib pitch of 10 exhibits the highest exergy efficiency of 0.0202. The optimization of the rib geometric quantities parameters was performed by considering the temperature increase parameter, aiming to achieve maximum exergy efficiency. The combination of rib parameters e/D = 0.044 and p/e = 10 are noted to yield best performance when operating at a temperature increase parameter above 0.0141 K∙m2/W.

Exergy analysis of solar heat collector with air jet impingement on dimple-shape-roughened absorber surface

The indented dimple-roughened jet impingement solar heat collector (CJISHC) performs better thermally and is more cost-effective than conventional single-pass solar heat collectors (SPSHCs) under the same operating conditions. However, due to the presence of jet impingement and dimple roughness, pumping power is required to obtain the appropriate airflow, resulting in a higher friction loss. The exergy analysis based on the second law of thermodynamics is appropriate for designing CJISHCs because the efficiency of useable energy production as well as pumping power are considered. The effects of the Reynolds number (Re), arc angle (αd), indented dimple height ratio (e/Dh), and indented dimple pitch ratio (p/Dh) with the ranges of 3000–21 000, 30°–75°, 0.016–0.027, and 0.27–0.81, respectively, between the dimples on the exergetic efficiency of a CJISHC were studied considering examined flow and roughness parameters. In addition, a cost-effectiveness analysis was performed to examine the cost-benefit ratios of the CJISHC and conventional SPSHC. The results showed that the CJISHC is more cost-effective in the m˙a range of 0.01–0.07 kg/s. Moreover, the exergy analysis revealed that at optimal dimple roughness values, viz. αd = 60°, e/Dh = 0.027, and p/Dh = 0.27, which depicts the maximum value of ηexe = 0.026 at temperature rise parameter, TRP = 0.03 Km2/W.

A comparative performance assessment of a hydrodynamic journal bearing lubricated with oil and magnetorheological fluid

This work presents the investigation results of a journal bearing lubricated with magnetorheological fluid that is activated by a local constant magnetic field to vary both the local flow resistance and pressure. The bearing performance is assessed via Finite Element Modelling (FEM) and results are corroborated by experiments. The FEM model uses the Bingham model to describe the fluid film. A dedicated test rig is used to assess the hydrodynamic behavior of the bearing with magnetorheological fluid, and the results are compared with the same geometrical bearing lubricated with oil. Thicker fluid films at low speeds, beneficial pressure distribution, and higher friction losses under all operating conditions are observed in the bearing with magnetorheological fluid compared to the oil lubrication.

A numerical investigation into the use of directionally drilled wells for the extraction of geothermal energy from abandoned oil and gas wells

Geothermal energy is an ideal renewable resource for power production since it is not limited by the intermittency issues associated with other renewables like wind and solar. Despite its ability to provide a steady supply of electricity on demand, the adoption of geothermal is not widespread, largely due to the high cost of drilling. One way to mitigate this cost is to use abandoned oil and gas wells, which are abundant (with an estimated 2.3 million in the United States) but pose a potential hazard to the environment. Studies in the literature have proposed installing heat exchangers in vertical wells to solve this problem. Vertical designs, however, would undergo high friction and temperature losses due to the use of internal piping. Such designs also do not enable the use of the L-shaped directionally drilled wells which have greater contact with the ground at high temperatures. This paper proposes an approach for extracting geothermal energy from abandoned oil and gas wells by using directionally drilled wells, thereby improving upon the existing designs by eliminating internal piping and maximizing contact with high ground temperatures. Through additional drilling, two adjacent wells can be connected to create a continuous loop from which thermal energy can be collected for use in a power cycle. The proposed geometry was studied numerically to determine the outlet temperatures and heat extraction rates from the system for the prediction of thermal energy extraction and electrical power production. The results show that a system with 4000 m deep vertical wells and a 4800 m horizontal section could produce approximately 2 MW of thermal energy and 200 kW of power using an organic Rankine cycle. Moreover, these wells are capable of providing a heat source for electricity generation for several decades.

Secondary Flows in the Return System of a Centrifugal Compressor Stage

Abstract This paper presents an analysis of the flow in the return system of a centrifugal compressor with a flow coefficient of 0.15. Based on the detailed experimental and numerical data, the areas of high losses and potentials for improving the return system geometry are revealed. Special emphasis is placed on the interaction of the flow in the return system components, including the U-bend, the vaned return channel, and the final L-bend. Strong flow redistribution occurs due to the sharp curvature of the U-bend, forming a blockage area at the hub near the vane leading edge. This causes a strong passage vortex, which is further intensified by the pressure gradient induced by the L-bend. Additionally, the flow near the shroud accelerates due to the large blockage area near the hub at the exit of the U-bend, resulting in high friction losses. To identify the main causes of loss, a method was evolved. It was validated and supplied by pneumatic measurement data. On this basis, analytical approaches were taken to quantify the total pressure losses due to friction, secondary flows, incidence, and trailing edge flow. As a result, approximately 60% of the return system losses arise due to friction. Another 30–40% are caused by secondary flows. It can be concluded that the results of the investigation contribute to the understanding of the secondary flow structures inside a centrifugal compressor return system of high mass flowrates. By combining the knowledge acquired in respect to the sources of highest losses with the experimental data, a well-founded basis for future optimization is achieved and the validation of numerical approaches is possible.

Friction and Texture Retention of Concrete Pavements

In the late 1980s and early 1990s, the Alabama Department of Transportation (ALDOT), U.S., noticed a decline in skid trailer numbers on concrete pavements shortly after grinding operations. The engineers at the time suspected that the coarse aggregate caused the decline in these numbers and the resulting conclusion led to a ban of carbonate aggregates in mainline concrete pavement in Alabama that is still in place. This detailed laboratory study re-examines the fundamental friction issues that led to this policy. A total of 48 aggregate, grinding, and grooving combinations were tested as part of this study. Three aggregate sources were examined: a siliceous source, a “hard” limestone source, and a “soft” limestone source. Two blade spacings were examined for grinding operations: 52 blades/ft and 60 blades/ft. Some ground specimens were also grooved. Finally, a set of specimens had the Next Generation Concrete Surface (NGCS) applied to them. The specimens were polished with the National Center for Asphalt Technology (NCAT) three-wheel polishing device (TWPD). The dynamic friction tester was used to evaluate friction values at various points through the polishing process. After the polishing, the macrotexture was characterized using the circular track meter. Across the board, the highest performing texture was that with no grooves and 52 blades/ft. Very generally, the loss of friction decreased with increasing siliceous content. However, some of the trends were extremely minor and, in a few cases, siliceous aggregates caused higher friction loss. There were numerous instances when blended carbonate/siliceous concrete pavement surfaces performed better than sole siliceous concrete pavement surfaces.