Computational Fluid Dynamics Predictions Research Articles

Progress in the validation of rotor aerodynamic codes using field data

Abstract. Within the framework of the fourth phase of the International Energy Agency (IEA) Wind Task 29, a large comparison exercise between measurements and aeroelastic simulations has been carried out featuring three simulation cases in axial, sheared and yawed inflow conditions. Results were obtained from more than 19 simulation tools originating from 12 institutes, ranging in fidelity from blade element momentum (BEM) to computational fluid dynamics (CFDs) and compared to state-of-the-art field measurements from the 2 MW DanAero turbine. More than 15 different variable types ranging from lifting-line variables to blade surface pressures, loads and velocities have been compared for the different conditions, resulting in over 250 comparison plots. The result is a unique insight into the current status and accuracy of rotor aerodynamic modeling. For axial flow conditions, a good agreement was found between the various code types, where a dedicated grid sensitivity study was necessary for the CFD simulations. However, compared to wind tunnel experiments on rotors featuring controlled conditions, it remains a challenge to achieve good agreement of absolute levels between simulations and measurements in the field. For sheared inflow conditions, uncertainties due to rotational and unsteady effects on airfoil data result in the CFD predictions standing out above the codes that need input of sectional airfoil data. However, it was demonstrated that using CFD-synthesized airfoil data is an effective means to bypass this shortcoming. For yawed flow conditions, it was observed that modeling of the skewed wake effect is still problematic for BEM codes where CFD and free vortex wake codes inherently model the underlying physics correctly. The next step is a comparison in turbulent inflow conditions, which is featured in IEA Wind Task 47. Doing this analysis in cooperation under the auspices of the IEA Wind Technology Collaboration Program (TCP) has led to many mutual benefits for the participants. The large size of the consortium brought ample manpower for the analysis where the learning process by combining several complementary experiences and modeling techniques gave valuable insights that could not be found when the analysis is carried out individually.

Experimental Force Coefficients for a Fully-Partitioned Pocket Damper Seal and Comparison to Other Two Seal Types

Abstract Labyrinth seals (LSs), honeycomb seals (HCSs) and hole-pattern seals, pocket damper seals (PDSs), and fully-partitioned damper seals (FPDSs) serve as balance pistons on the discharge side of barrel-type centrifugal compressors. These seals, facing large pressure differentials along with significant changes in gas density, produce sizeable lateral forces that impact compressor stability and rotordynamic performance. There is extensive archival data on the dynamic forced performance of LS and textured surface seals. However, the experimental database for a FPDS is insufficient, thus a comprehensive comparison of their dynamic force performance against that of textured surface seals is missing. The paper presents experimentally derived rotordynamic force coefficients for a FPDS along with a direct comparison to published data for a HCS and a LS, both similar in size and in operating conditions. The dynamic load tests with the FPDS include operation at a shaft speed (Ω) of 10 krpm (rotor surface speed of 60 m/s) while supplied with air at Pin = 70 bar and Tin =10 °C, and a discharge at Pout = 0.25, 0.5, and 0.65 of Pin. The preswirl circumferential velocity at the seal inlet is low, approximately 6% of rotor surface speed. The FPDS having eight pockets × eight axial blades produces a direct stiffness (K) increasing with excitation frequency (ω), though K < 0 for the lowest pressure ratio PR = (Pout/Pin) = 0.25. The cross-coupled stiffness (k) is invariant to frequency whereas the direct damping (C) steadily decreases as ω/Ω → 1, and then increases for larger frequencies. Hence, the effective damping coefficient (Ceff = C − k/ω) is approximately constant for ω < Ω, while increasing for higher frequencies. For the three PRs, the FPDS leakage is roughly up to 26% larger than the HCS leakage, while the 20-blade LS leaks in between both. The comparison of results shows the LS has insignificant forces compared to those from the HCS and FPDS, both producing a comparable Ceff and similar crossover frequencies (Ceff > 0). The HCS produces a large direct stiffness (K), three to four times that of the FPDS. Comparison of computational fluid dynamics (CFD) predictions for the FPDS against the experimental force coefficients shows significant differences, in particular an overestimation of direct stiffness as the frequency grows along with a lower direct damping for frequencies equal and above the shaft speed. Besides manufacturing considerations, a FPDS is a better alternative to a HCS whose large centering stiffness (K) may affect the compressor's critical speed, hence shortening the separation margin with respect to the operating speed.

Full-scale computational fluid dynamics study on wind condition of the long-span Queensferry Crossing Bridge

To date, the majority of numerical modelling [computational fluid dynamics (CFD)] studies on long-span bridges have been carried out on scaled physical models, and without field-data for validation. For the first time, a full-scale bridge aerodynamic CFD study was conducted in this paper. A full-scale three-dimensional CFD model of the middle span and central tower of the Queensferry Crossing, United Kingdom, was created. The aim of this work was accurately simulating the wind field around the bridge. The CFD simulations were developed in OpenFOAM with the k − ω SST turbulence model. Atmospheric boundary layer inflows were configured based on wind profiles provided by a full-scale Weather Research and Forecasting (WRF) model. CFD predictions were validated with field data which were collected from an on-site Structural Health Monitoring System. The simulated fluctuating wind field closely satisfied the characteristic of field data and demonstrated that the modelling approach had good potential to be used in practical bridge aerodynamic studies. Meanwhile, comparisons and sensitivity analyses on mesh density provided a reference modelling approach for any future works on full-scale bridge aerodynamic models. Additionally, a cylindrical-like domain was applied in bridge aerodynamics for the first time and verified as being a convenient and reliable way to be used in bridge studies that involve changes in yaw angle.

Towards an accurate CFD prediction of airflow and dispersion through face mask

Given the difficulty of experimental measurement of respiratory airflow and dispersion through a face mask, accurate numerical simulation is an important method to increase the understanding of the health effect of face masks and to develop high-performance ones. The objective of this study is to develop such an accurate modeling framework based on computational fluid dynamics (CFD) theory and method. For model validation, the flow characteristics through the face mask were tested experimentally, and the air speed and exhaled pollutant concentration in the breathing zone were measured with human subjects. The influence of gird division, time step size, and turbulence model on simulation accuracy were investigated. The result shows that the viscous resistance coefficient and inertial resistance coefficient of face masks (surgical masks) were 3.65 × 109 and 1.69 × 106, respectively. The cell size on the surface of face masks should not be larger than 1.0 mm; the height of the first layer cells near the face masks should not be larger than 0.1 mm; and the time step sizes discretizing the breathing and coughing periods should not be more than 0.01 s and 0.001 s, respectively. The results given by LES model show closer agreement with the experimental data than RANS models, with approximately 10% relative deviation for the air speed near the face mask. Overall, the SST k-ω model performs the best among the RANS models, especially for the air speed. The findings obtained form a CFD modeling framework for an accurate prediction of airflow and dispersion problems involving face masks.

Review of Wind Resistant Design for Tiled Roofs and Publication of New CFD Guidebook for Urban Wind Environment: Japan Country Report 2021

This paper introduces recent activities related to the structural and environmental engineering in Japan. Part 1 in this paper illustrates outline of recent research and standardization related to ensuring wind resistant performance of tiled roofs. We introduce the results of damage survey on tiled roofs caused by Typhoon Faxai in 2019, which was conducted by National Institute for Land and Infrastructure Management and Building Research Institute in collaboration with clay roof tile industry organization. Then, the outline of the wind load calculation method stipulated in the Building Standards Law and the amended Notification for roof tiles is illustrated. In addition, “Guideline for Standard Structural Design/Construction for Tiled Roof” was revised in July 2021 as a supplement to the law. The outline of monotonous pull-up test and the standard specifications, both of which are newly added to the revised version of the Guideline, are also introduced. These activities are aimed to reduce structural vulnerability of tiled roofs, based on the lessons learned from the damage caused by the typhoon. In Part 2, we introduce activities to promote the appropriate use of computational fluid dynamics (CFD) in wind environment prediction. A working group within the Architectural Institute of Japan compiled a new CFD guidebook and guidelines for urban wind environment prediction, mainly aiming to include the application of large-eddy simulation to pedestrian-level wind problems or dispersion problems relating to concentration and temperature in cities. Reflecting the latest domestic and international research trends and the WG’s validation works, achievements of their activities were published as “Guidebook for CFD Predictions of Urban Wind Environment” (2020, in Japanese), papers in international journals, and an extended experimental database for verification and validation of analysis codes.

Examining the universality of the hemolysis power law model from simulations of the FDA nozzle using calibrated model coefficients.

Computational fluid dynamics (CFD) is widely used to predict mechanical hemolysis in medical devices. The most popular hemolysis model is the stress-based power law model that is based on an empirical correlation between hemoglobin release from red blood cells (RBCs) and the magnitude of flow-induced stress and exposure time. Empirical coefficients are traditionally calibrated using data from experiments in simplified Couette-type blood-shearing devices with uniform-shear laminar flow and well-defined exposure times. Use of such idealized coefficients in simulations of real medical devices with complex hemodynamics is thought to be a primary reason for the historical inaccuracy of absolute hemolysis predictions using the power law model. Craven et al.(Biomech Model Mechanobiol 18:1005-1030, 2019) recently developed a CFD-based Kriging surrogate modeling approach for calibrating empirical coefficients in real devices that could potentially be used to more accurately predict absolute hemolysis. In this study, we use the FDA benchmark nozzle to investigate whether utilizing such calibrated coefficients improves the predictive accuracy of the standard Eulerian power law model. We first demonstrate the credibility of our CFD flow simulations by comparing with particle image velocimetry measurements. We then perform hemolysis simulations and compare the results with in vitro experiments. Importantly, the simulations use coefficients calibrated for the flow of a suspension of bovine RBCs through a small capillary tube, which is relatively comparable to the flow of bovine blood through the FDA nozzle. The results show that the CFD predictions of relative hemolysis in the FDA nozzle are reasonably accurate. The absolute predictions are, however, highly inaccurate with modified index of hemolysis values from CFD in error by roughly three orders of magnitude compared with the experiments, despite using calibrated model coefficients from a relatively similar geometry. We rigorously examine the reasons for the inaccuracy that include differences in the flow conditions in the hemolytic regions of each device and the lack of universality of the hemolysis power law model that is entirely empirical. Thus, while the capability to predict relative hemolysis is valuable for product development, further improvements are needed before the power law model can be relied upon to accurately predict the absolute hemolytic potential of a medical device.

Results of the Interlaboratory Computational Fluid Dynamics Study of the FDA Benchmark Blood Pump.

Computational fluid dynamics (CFD) is widely used to simulate blood-contacting medical devices. To be relied upon to inform high-risk decision making, however, model credibility should be demonstrated through validation. To provide robust data sets for validation, researchers at the FDA and collaborators developed two benchmark medical device flow models: a nozzle and a centrifugal blood pump. Experimental measurements of the flow fields and hemolysis were acquired using each model. Concurrently, separate open interlaboratory CFD studies were performed in which participants from around the world, who were blinded to the measurements, submitted CFD predictions of each benchmark model. In this study, we report the results of the interlaboratory CFD study of the FDA benchmark blood pump. We analyze the results of 24 CFD submissions using a wide range of different flow solvers, methods, and modeling parameters. To assess the accuracy of the CFD predictions, we compare the results with experimental measurements of three quantities of interest (pressure head, velocity field, and hemolysis) at different pump operating conditions. We also investigate the influence of different CFD methods and modeling choices used by the participants. Our analyses reveal that, while a number of CFD submissions accurately predicted the pump performance for individual cases, no single participant was able to accurately predict all quantities of interest across all conditions. Several participants accurately predicted the pressure head at all conditions and the velocity field in all but one or two cases. Only one of the eight participants who submitted hemolysis results accurately predicted absolute plasma free hemoglobin levels at a majority of the conditions, though most participants were successful at predicting relative hemolysis levels between conditions. Overall, this study highlights the need to validate CFD modeling of rotary blood pumps across the entire range of operating conditions and for all quantities of interest, as some operating conditions and regions (e.g., the pump diffuser) are more challenging to accurately predict than others. All quantities of interest should be validated because, as shown here, it is possible to accurately predict hemolysis despite having relatively inaccurate predictions of the flow field.

Annular Dump Diffuser and Deswirl System for Back-Pressure Control in Engine-Scale Transonic Annular Cascade

Abstract A common requirement for turbomachinery testing facilities is the ability to independently control Mach number and Reynolds number. In practice, this means independent control of the inlet total pressure and exit static pressure in a test facility. In this paper, we present a solution to this problem with particular applicability to large-scale annular test facilities. We describe the design and commissioning of a combined annular dump diffuser and deswirl system for back-pressure control in environments with high-whirl transonic flow. The particular application was an annular cascade of nozzle guide vanes from a current civil engine. The purpose was to provide independent control of Mach number and Reynolds number, by controlling the back-pressure in the intermediate annular plenum which forms the dump diffuser. The dump diffuser is necessary to facilitate optical and probe access (without interaction effects) and to reduce the risk of exit static pressure disturbances (due to particular duct design). The system has been installed and validated in the engine component aerothermal (ECAT) facility at the University of Oxford. In this implementation of the system, the high-whirl transonic flow from the nozzle guide vanes passes through a short, parallel annular duct, and is dumped into an annular plenum, before being re-accelerated into a deswirl vane ring. The deswirl ring turns the flow to the axial direction. The flow is then discharged through a variable choke plate into a silencer. Conditioning the flow to have zero whirl at the choke plate reduces the sensitivity of the choke plate effective blockage to the whirl angle. The design, deswirl vane aerodynamic performance, and overall system performance are assessed with detailed experiments and 3D unsteady computational fluid dynamics predictions. The control of high-whirl transonic flow is notoriously challenging, and the deswirl system has application to exhaust conditioning in a number of applications including annular cascade experiments and rocket turbo-pump exhaust systems.

Performance characterization of a novel compact dust collector with pleated filter cartridges

A compact dust collector loaded with four pleated filter cartridges is numerically studied by Computational Fluid Dynamics (CFD). The performance of the dust collector is evaluated during filtration and during backwashing, and the predictions are compared with the experimental results. In the filtration operation, the upper right corner entry design effectively guides the airflow to the two sides of the four cartridges through the air opposite-side-wall impacting and rebounding. A relatively poor filtration is found at the right side of the left lower cartridge. The CFD predictions agree well with the experimental data in terms of showing better backwash cleaning performances of the lower cartridges and at the cartridge front positions. During the 50 ms pulse-jet backwashing, the difference between the maximum and minimum air mass flow rates through a total of 16 × 2 inject holes is less than 4.0%. The high pressure of 0.5 MPa draws the air flowing from the normal exit, which acts as the majority of the backwashing air (ca. 73.7%). The hydrostatic pressure acting on the four cartridges is greater than 1750 Pa in 80% of the pulse-jet cleaning time.

Computational fluid dynamics predictions of critical hydrodynamics thresholds in the erodibility of inland waterway bank by ship-induced waves

A three-dimensional numerical simulation method based on the resolution of Navier–Stokes equations is used to determine the causal relationship between the ship-induced waves and the erosion phenomenon of the inland waterway banks, and also to provide the critical conditions for waterway bank protection. The erosion process of the bank is defined here as the criterion in which the movements of the discrete medium blocks and particles that form the banks by the action of ship waves undergo a significant jump beyond the critical value of Froude number (Frc). By investigating the wave elevation drawdowns nearby the waterway banks, it is shown that the normal impact force and tangential shear stress caused by ship-induced waves are the main culprits of bank erodibility. The numerical results show that by approaching the Frc, the increasing rate of drawdown reaches its maximum causing a jump in the force of total pressure, which in turn gives a rise to a peak in the shear stress on the waterway bank.

Amplitude and Wavelength Effects for Wavy Channels

Abstract To improve the efficiency and durability of gas turbine components, advancements are needed in cooling technologies. To accomplish this task, some manufacturers are turning to additive manufacturing (AM), as it offers the ability to both rapidly iterate on component design as well as incorporate unique internal cooling structures directly into parts. As one example, wavy microchannels can be readily integrated into turbine components. This study investigates wavy channels of varying channel amplitude and wavelength through experimental measurements of heat transfer and pressure loss. In addition to experimental testing, computational fluid dynamics (CFD) predictions were made to identify internal flow features that impacted performance. Five channel geometries were integrated into test coupons that were additively manufactured out of Hastelloy-X using direct metal laser sintering. True coupon geometric characteristics and wall roughness values were captured non-destructively using computed tomography (CT) scans. Geometric analyses indicated that coupons were reproduced accurately with minimal deviation from design intent. Experimental results indicated that decreasing the channel wavelength and increasing the channel amplitude resulted in substantial increases in both bulk friction factor and Nusselt number with respect to the nominal case and were scaled using a relative waviness parameter. CFD simulations predicted significant mixing of flow in the cases with the smallest wavelength and greatest amplitude.

Modeling of Thermal-Lag Engine with Validation by Experimental Data

Thermal-lag engines are external combustion engines with a single moving piston. This feature leads to lower manufacturing and maintenance costs than traditional Stirling engines. Although the original concept of thermal-lag engines was invented roughly 35 years ago, the information on thermal-lag engines is still limited. Therefore, this study focuses on thermal-lag engine performance by developing a three-dimensional computational fluid dynamics (CFD) model. The grid independence check and the time step independence check are firstly performed to select the number of elements and size of the time step for simulation. The CFD model is then validated by the experimental data, which were collected by measuring an existing prototype engine. It has been found that the CFD predictions are well fitted to the experimental data over the range of engine speed from 200 to 1600 rpm at temperatures of 1173 or 1273 K. Furthermore, the CFD model predicts that the maximum engine power is 21.1 W while the prototype engine practically generates the highest power of 22.35 W at 1000 rpm and 1273 K. Finally, a further parametric study shows that crank radius, piston diameter, working gas mass, working gas species, and heating temperature significantly affect engine power.

Combined experimental and numerical studies on soot characteristics of diesel sprays with split injection strategies

Experimental and numerical studies on the soot characteristics of Spray A flames were conducted with a focus on the effects of different split injection strategies in high-pressure high-temperature vessels within the engine combustion network. Three high-speed imaging techniques were used to obtain the transient information of flame evolution. Schlieren was applied to identify the spray boundaries and spray penetration. Moreover, a combined extinction and radiation technique was applied to quantify the spatial soot and temperature fields simultaneously. Finally, high-speed OH* chemiluminescence was applied at the baseline condition to measure the temporal high-temperature flame evolution. To substantiate the analysis, the experimental results were compared with the computational fluid dynamics (CFD) predictions from a Reynolds-averaged Navier Stokes approach with Unsteady Flamelet Progress Variable combustion and a two-equation soot-in-flamelet models, which qualitatively reproduced the experimental results.According to the chosen split injection strategy, dwell and first-injection pulse durations were varied. For all cases, the ignition delay (ID) and soot production of the second-injection pulse were shorter and higher than those of the first one, respectively. A slight change in the experimental soot production was observed when the dwell time was changed. The CFD calculations revealed a small reduction in the ID of the second pulse with long dwell times, which was due to variations in the interaction between the newly injected fuel and combustion recession. However, the effect of dwell variation was extremely small to produce significant differences in the evolution of the second pulse, neither in combustion nor in soot production. With regard to the duration of the first injection, no effect on the experimental soot production of the second pulse was observed despite noticeable differences due to the first pulse. The CFD results indicated that because of the quasi-steady flow and combustion process near the nozzle along the first injection, the evolution of the spray in the region close to the nozzle was identical in all first-pulse-duration cases, with differences only occurring at the tip of the first pulse jet. Thus, the second pulse evolution occurred under identical conditions for ignition and soot production. Although the sensitivity to both dwell and first-pulse durations was low, the CFD analysis highlighted the important effects of both end-of-injection wave propagation and combustion recession on the ignition and soot production of the second pulse.

Influence of morphological parameters on hemodynamics in internal carotid artery bifurcation aneurysms

Recent advancements in medical imaging techniques have enabled the accurate identification of unruptured intracranial aneurysms. To facilitate a proper patient management strategy, it is important to develop suitable mathematical models for their rupture prediction. To this end, the development of high-fidelity computational fluid dynamics (CFD) simulations with patient-specific boundary conditions will be useful in providing reliable hemodynamic parameters. In recent review articles, researchers have pointed out that, among several clinical and image-based indicators, morphological parameters, such as aspect ratio (AR) and size ratio (SR) of the aneurysm, correlated consistently with the rupture mechanism. However, it is not clear how these morphological indicators influence the hemodynamics-based CFD predictions. In the present work, the effect of these top-ranked morphological parameters on aneurysm hemodynamics and rupture prediction is investigated. Three patient-specific models have been used for analysis with the patient-specific inlet boundary conditions. We found that with an increase in AR and SR, the maximum value of wall shear stress (WSS) near the aneurysm neck is increased. Oscillatory shear index and relative residence time values are also increased with an increase in AR and SR. Furthermore, it was observed that an aneurysm with a multilobed structure shows complex flow, low WSS, and higher residence time over the secondary lobe. The turbulent kinetic energy and vorticity near the aneurysm neck are also increased with an increase in AR and SR.

Enhancement of turbulent thermal convection in a circular tube with a slotted twisted tape

PurposeThe purpose of this paper, computational fluid dynamics (CFD) work, is to promote turbulent thermal convection in a heated circular tube using a passive scheme of a slotted twisted sheet.Design/methodology/approachThe inventive design uses square-cut and conjugate triangular perforations to diversify the twisted tape for better thermal convection. The current novel passive scheme methodology is accomplished by carving the same square cuts and slitting various sizes of equilateral triangle perforations (side length varies between 8 and 16 mm). The re-normalisation group turbulence model and the semi-implicit method for pressure-linked equation method examine the turbulent thermal convection aspects of all simulations at different Reynolds numbers (6,000, 10,000 and 14,000).FindingsThe analyses of simulations exhibit that the placement of a twisted tape with triangle perforations and equidistant square cuts can effectually promote thermal convection in a circular tube. A larger-sized triangle perforation can increase the thermal convection enhancement and thermal performance factor, but an enlarged perforation may decrease the thermal convection enhancement and thermal performance factor. As a result, compared with the smooth circular tube, the circular tube with the slotted twisted sheet slit by a 10 mm equilateral triangle brings about the maximum improvement ratio of the mean Nusselt number of about 2.8 at Re = 6,000. Under weighing the friction through the circular tube, the tube with the slotted twisted sheet slit by a 10 mm equilateral triangle gains the best thermal performance factor of about 1.36 at Re = 6,000.Research limitations/implicationsThe working fluid is water and its physical features are assumed to be constant. In addition, the fluid is considered a steady flow in this CFD work.Practical implicationsThese CFD predictions will benefit the development of heat exchanger tubes equipped with a slotted twisted sheet to acquire preferable thermal convection enhancement.Social implicationsHigher thermal performance achieved by placing a slotted twisted tape in a heated tube will benefit society in lower energy consumption, machinery maintenance costs and impact on the environment.Originality/valueThis study combined triangle perforations and square cuts on the twisted sheet. This combination can induce the fluid flow across the sheet to disturb the swirling flow and then promote the fluid mixing to increase thermal convection. Therefore, this modified tape can be a profitable passive device for designing a heat exchanger.

Experimental and numerical investigation for PV cooling by forced convection

A photovoltaic (PV) cell is very sensitive to temperature changes where decreasing temperature plays the main role in the increase of PV electrical efficiency and output power. Therefore, researchers used different techniques for cooling PV modules to avoid immoderate heating PV modules and to decrease the panel temperature, resulting in raising power output, energy efficiency, performance, and life of the panel. In the present work, PV panels are cooled by forced convection. Cooling airflow characteristics and panel temperature distribution are examined using computational fluid dynamics (CFD). In order to evaluate CFD predictions, experimental measurements are obtained in a sunny day on 23rd September 2021 for the period from 10 AM to 4 PM. The experimental measurements are performed on three different arrangements of PV panels. The uncooled panel was considered as the reference case. Two different cooling methods were examined: PV panels with forced air-cooling using a lower duct and supplying air using the blower, and PV panels with forced air-cooling using small fans arranged symmetrically on the backside of the PV panels. The temperatures determined by the CFD calculations are compared to the experimentally measured temperatures and it was found to be in good agreement. The results showed that cooling PV using small backside fans can enhance the performance and achieve a maximum total increase of 2.1% in PV panel efficiency with 7.9% saving energy. Using the blower cooling technique achieves a maximum total increase of 1.34% in PV panel efficiency with 4.2% saving energy.

Power generation and thermal stress characterization of thermoelectric modules with different unileg couples by recovering vehicle waste heat

Unileg thermoelectric generators (TEGs) possess a simple structure, high mechanical strength, and lower thermal stress and cost than conventional TEGs. So it is a promising clean and green energy device that directly converts waste heat into power. This study focuses on the performance and thermal stress of unileg thermoelectric modules (TEM) with different leg heights and numbers of legs by harvesting the vehicle's waste heat. This work also combines TEM and computational fluid dynamics (CFD) to analyze the influence of fluid flow. When the hot-side and cold-side temperatures are fixed, the results reveal that the shorter the leg height, the higher the output power, as a consequence of smaller internal resistance. Alternatively, the maximum efficiency is merely reduced by 1.45% when the height decreases from 12 mm to 8 mm. The output power linearly increases with the number of legs, showing no interaction between the unilegs. Compared to the CFD predictions, the results without considering the fluid flows overestimate the maximum output power and efficiency by 80% and 32%, respectively. Because the thermal expansion coefficients of different materials in the TEM are not matched, thermal stress is generated. The thermal stress increases from the center of the TEM outward along the x- and y-axes, yielding maximum thermal stress of 219.48 MPa. Compared with conventional TEM thermal stress (348 MPa) at moderate and high-temperature applications, the maximum thermal stress is reduced by 36.9% from the unileg TEG.

On the influence of axial cylinder distortions on lubricant film and friction force of top piston ring conjunction near to TDC

ABSTRACT The purpose of this paper was to understand how axial cylinder distortions influence the tribological performance of piston ring. The current model contains the effects of axial distortions and ring conformability under mixed to hydrodynamic regimes of lubrication through computational fluid dynamics predictions. The axial cylinder profiles were modelled using series of sinusoidal waves. The effect of axial distortions on ring wear rate was determined using the Archard’s model. The ring friction and minimum lubricant film were predicted for low and high combustion pressures near to top dead centre. The study shows that, when the maximum combustion pressure rises, the different wear identities affect the operation of the ring friction much more limited, while the number of sine waves are more pronounced. Finally, it is proved that, even the increase of oil film thickness, it does not presuppose the reduction of friction when it comes to shaping the surface of the cylinder liner.

Experimental and numerical study on the thermal and hydraulic characteristics of porous-media heat exchangers in cryogenic conditions

The thermal performance of a metal-foam heat exchanger in cryogenic environments was examined through an experiment and a computational fluid dynamics (CFD) simulation. A plate-type heat exchanger with metal foam inserted in the channels was designed and manufactured. The metal foam was 20-pore-per-inch nickel metal foam and brazed to stainless-steel channel plates. Nitrogen gas in cryogenic conditions was supplied to both hot and cold-side channels with a counter-flow configuration. The pressure drop was measured in a wide range of mass-flow rates of 8–48 kg/h. Heat transfer experiments were conducted while adjusting the gas temperature using heating wires in a range of mass-flow rate of 10–20 kg/h. The data obtained through the experiment were compared with those of CFD simulations and previous correlations obtained at room-temperature conditions. The CFD prediction of the pressure drop agreed well with the experimental data with less than 10 % discrepancy. Furthermore, the pressure-drop correlation models at room-temperature conditions agreed well with the experimental results in cryogenic conditions. Among the pressure-drop models, the Dietrich model showed the best agreement. CFD predicts the heat transfer well within 10 % discrepancy from the experiment. However, the heat transfer in cryogenic conditions is 15–20 % higher than that of the previous correlations obtained at room-temperature conditions. This can be explained by the hydraulic performance not being affected much by the temperature difference, while the Nusselt number of the metal-foam-filled channel has higher value due to lower thermal conductivity at a cryogenic environment.

Computational fluid dynamics predictions of draught and trim variations on ship resistance in confined waters

Adjusting a vessel's trim and draught to enhance resistance characteristics is a promising strategy to improve the energy efficiency of maritime transport. However, the vast majority of scientific effort has been directed at such gains in deep, unrestricted waters. Shallow and confined waters modify the flow and pressure distribution around a ship, altering considerably the resistance curve. This study aims to elucidate trim and draught increase effects on a ship's resistance while advancing through a restricted waterway using Computational Fluid Dynamics. The results show that increasing the draught of a benchmark hull magnifies the hydrodynamic resistance by approximately 10% to 15% depending on the ship speed. This added hydrodynamic resistance may be compensated by adjusting the vessels’ trim, but the ability to compensate the added hydrodynamic resistance is sensitive to ship's speed. At low speeds, the numerical model predicts the increase in resistance due to a 10% higher draught can be reduced by varying the trim angle leaving the total resistance 0.87% higher than at the design draught and zero trim angle condition. On the other hand, higher speeds offer a greater potential for resistance reduction through trim.