Unsteady Computational Fluid Dynamics Simulations Research Articles

Effect of volute tongue angle on the performance and flow unsteadiness of an automotive electronic cooling pump

Automotive electronic cooling pump is the core component of the new energy vehicle cooling system, with the exclusive advantages of high efficiency and low vibration which will bring a broad market prospect. However, the rotor–volute interaction between impeller blades and volute tongue is a severe problem which can result in the performance degradation and unsteady flow fields of pumps. The influences of volute tongue angle on hydraulic performance and fluid stability are investigated in this study. The flow loss inside the pump is visualized by means of the entropy production theory, while the pulsations of pressure and radial force of the rotor are compared by unsteady computational fluid dynamics simulation. The results show that entropy production is an effective tool to visualize the loss distribution which allows accurate assessment of pump performance. Specifically, the pressure pulsation intensity near the volute tongue is larger and the main frequency of the radial force is the blade passing frequency. With the increase of volute tongue angle, the hydraulic efficiency under large flow conditions is improved, with an alleviated rotor–volute interaction intensity, and greatly reduced fluctuations of pressure and radial force. The main significance of this study is that it provides a new perspective to investigate the optimization design for automobile electronic cooling pumps.

Numerical and experimental analysis of flow and particulate matter dispersion in indoor environment

Reducing indoor particulate matter (PM) concentration is an issue of concern from an environmental point of view as the world’s population spend only 4% of their time outdoors. Computational fluid dynamics (CFD) is a fundamental tool for predicting indoor pollutant dispersion and improving knowledge on how indoor and outdoor environments interact in terms of pollutant and momentum exchanges. In this paper, an unsteady CFD simulation has been carried out to investigate the airflow and PM concentration in a classroom of the University of Rome “La Sapienza”. Wind velocity and PM concentration acquired during a field campaign conducted within and outside the building of interest have been used as input for the simulation and to test the model performance as well. The results show a reasonable agreement between measured and simulated concentration within the classroom and emphasize the major role played by the micrometeorology in PM concentration. The importance of the boundary conditions at the room openings has been also discussed.

Evaluation of the unsteady aerodynamic forces acting on a vertical-axis turbine by means of numerical simulations and open site experiments

An increasing number of vertical-axis wind turbine prototypes have reached the step in which the theoretically predicted performance needs to be validated in order to move to the next steps of a real commercial project. This step often faces the significant challenges posed by their airfoil aerodynamics that are more complex than those of conventional horizontal-axis wind turbines, and it has also to deal with the lack of fundamental experimental data for robust validation. In this context, an accurate prediction of the real turbine operation is important and the use of computational fluid dynamics (CFD) is imposing itself as the most suitable tool to characterize the unsteady phenomena that are difficult to detect by means of experimental measurements.In the current work, two-dimensional numerical simulations of an H-type three-blade Darrieus turbine have been performed in a wide range of tip-speed ratios (TSRs) from TSR ​= ​1.8 to TSR ​= ​5.0. Unsteady CFD simulations were compared with unique experimental data collected in the field in terms of normal aerodynamic forces acting on the blades during the revolution. Generally, nice agreement was found between simulations and experiments, especially at medium-high tip-speed ratios. The influence of operating conditions on the performance prediction capability of the numerical model was also discussed. This is one of the key points of study since the lack of detailed experimental data often makes numerical analyses doubtful or scarcely effective. Finally, the simulation results were exploited in order to analyze the phenomena occurring during the revolution and to correlate them with the experimental findings.

The Wave-to-Wire Energy Conversion Process for a Fixed U-OWC Device

Oscillating water column (OWC) devices, either fixed or floating, are the most common wave energy converter (WEC) devices. In this work, the fluid dynamic interaction between waves and a U-shaped OWC breakwater embedding a Wells turbine has been investigated through unsteady Computational Fluid Dynamic (CFD) simulations. The full-scale plant installed in the harbor of Civitavecchia (Italy) was numerically modeled. A two-dimensional domain was adopted to simulate the unsteady flow, both outside and inside the U-OWC device, including the air chamber and the oscillating flow inside the conduit hosting the Wells turbine. For the numerical simulation of the damping effect induced by the Wells turbine connected to the air chamber, a porous medium was placed in the computational domain, representing the conduit hosting the turbine. Several simulations were carried out considering periodic waves with different periods and amplitudes, getting a deep insight into the energy conversion process from wave to the turbine power output. For this purpose, the three main steps of the overall energy conversion process have been examined. Firstly, from the wave power to the power of the water oscillating flow inside the U-duct. Secondly, from the power of the oscillating water flow to the air pneumatic power. Finally, from the air pneumatic power to the Wells turbine power output. Results show that the U-OWC can capture up to 66% of the incoming wave power, in the case of a wave period close to the eigenperiod of the plant. However, only two-thirds of the captured energy flux is available to the turbine, being partially dissipated due to the losses in the U-duct and the air chamber. Finally, the overall time-average turbine power output is evaluated showing that it is strongly influenced by a suitable choice of the turbine characteristics (mainly geometry and rotational speed).

Three-Dimensional CFD Simulation and Experimental Assessment of the Performance of a H-Shape Vertical-Axis Wind Turbine at Design and Off-Design Conditions

The paper presents the results of a computational study on the aerodynamics and the performance of a small-scale Vertical-Axis Wind Turbine (VAWT) for distributed micro-generation. The complexity of VAWT aerodynamics, which are inherently unsteady and three-dimensional, makes high-fidelity flow models extremely demanding in terms of computational cost, limiting the analysis to mainly 2D or 2.5D Computational Fluid-Dynamics (CFD) approaches. This paper discusses how a proper setting of the computational model opens the way for carrying out fully 3D unsteady CFD simulations of a VAWT. Key aspects of the flow model and of the numerical solution are discussed, in view of limiting the computational cost while maintaining the reliability of the predictions. A set of operating conditions is considered, in terms of tip-speed-ratio (TSR), covering both peak efficiency condition as well as off-design operation. The fidelity of the numerical predictions is assessed via a systematic comparison with the experimental benchmark data available for this turbine, consisting of both performance and wake measurements carried out in the large-scale wind tunnel of the Politecnico di Milano. The analysis of the flow field on the equatorial plane allows highlighting its time-dependent evolution, with the aim of identifying both the periodic flow structures and the onset of dynamic stall. The full three-dimensional character of the computations allows investigating the aerodynamics of the struts and the evolution of the trailing vorticity at the tip of the blades, eventually resulting in periodic large-scale vortices.

Investigation on the Impact of Air Admission in a Prototype Francis Turbine at Low-Load Operation

Due to significant changes in the energy system, hydraulic turbines are required to operate over a wide power range. In particular, older turbines which are not designed for these environments will suffer under off-design conditions. In order to evaluate whether or not such a turbine could fulfill the new requirements of the energy market, a study about the behavior of a prototype plant in low-load operation is presented. Therefore, prototype site measurements are performed to determine the most damaging operating point by means of acceleration sensors and pressure transducers. Moreover, unsteady computational fluid dynamics (CFD) simulations considering two-phase flow and two hybrid turbulence models are used to analyze the flow conditions inside the turbine. The resulting pressure pulsations are mapped onto the runner blade to obtain stress and further calculate damage factors. Accordingly, the stresses are compared to those obtained by the strain gauge measurement. Moreover, the influence of active flow control by means of air injection on plant behavior and runner lifetime is discussed as well.

Computational fluid dynamics of sulfur dioxide and carbon dioxide capture using mixed feeding of calcium carbonate/calcium oxide in an industrial scale circulating fluidized bed boiler

The sulfur dioxide (SO2) and carbon dioxide (CO2) emissions from fuel combustion in a coal-fired power plant constitute a significant source of damage to the environment. Therefore, SO2 and CO2 should be captured before being released into the atmosphere. However, the competitiveness between SO2 and CO2 for calcium carbonate (CaCO3)/calcium oxide (CaO) solid sorbents is still unclear. In this study, unsteady state computational fluid dynamics simulation in a riser of an industrial scale circulating fluidized bed boiler integrated with heterogeneous combustion, carbonation, calcination, and desulfurization reactions using a mixed feeding of CaCO3/CaO solid sorbents was developed in a two-dimensional model to investigate the competition between SO2 and CO2 capture. Then, the effect of three operating variables, the mixed solid CaCO3/CaO sorbent particle size, feed position, and the proportion of inlet fuel velocity on the SO2 and CO2 capture were evaluated using a 23 factorial experimental design. The CaCO3/CaO solid sorbent particle size had a significant effect on the SO2 capture, while the interaction between CaCO3/CaO solid sorbent particle size and feed position had a significant effect on the CO2 capture. The reaction rate for CO2 capture was higher than that for SO2 capture. For SO2 capture, CaO reacted with SO2 faster than CaCO3 while, for CO2 capture, solid sorbents had higher carbonation rate than calcination rate. In addition, the overall level of SO2 and CO2 capture with a mixed CaCO3/CaO solid sorbent feed was higher than those with the conventional CaO solid sorbent.

Investigation of a ship resonance through numerical simulation

Understanding dynamic stability of a ship at a resonance frequency is important because comparatively smaller external forces and moments generate larger motions. The roll motion is most susceptible because of smaller restoring moments. Most studies related to the failure modes such as parametric roll and dead ship condition, identified by second generation of intact stability criteria (SGISC) are performed at a resonance frequency. However, the nature of resonance, where the model experiences an incremental roll motion, has not been well understood. In this study, nonlinear unsteady computational fluid dynamics (CFD) simulations were conducted to investigate the resonance phenomenon using a containership under a sinusoidal roll exciting moment. To capture the complexity of the phenomenon, simulations were conducted over a range of frequencies to cover the resonance frequency including lower and higher amplitudes. In addition to the resonance frequency, the phase shift between roll exciting moment and roll angle, as well as the phase difference between acceleration and roll angle, were found to have significant effects on the occurrence of resonance.

CFD analysis of variable wall thickness scroll expander integrated into small scale ORC systems

Abstract The studies of constant wall thickness scroll expander have pointed out that geometries with large built-in volume ratios are necessary to achieve high performances in small-sized organic Rankine cycle (ORC) units. The variable wall thickness expander design offers the opportunity of increasing the geometric expansion ratio with the number of scroll turns remaining unchanged to avoid sealing and lubricating issues. In this paper, unsteady and three-dimensional computational fluid dynamics (CFD) simulations of scroll expander using variable wall thicknesses were therefore carried out to investigate the effects of the geometry on the internal flow behaviour. The scroll expander was integrated into an ORC unit fed by R123. The dynamic mesh technology of ANSYS Fluent was applied to generate the deforming mesh in the expander working chambers. The aerodynamic performance analysis yielded how over-expansion phenomena occurred at low pressure ratio while under-expansion phenomena were existing at high pressure ratio which are consistent with the thermodynamic theory of scroll expander. The higher pressure ratio was also contributing to higher temperature drops during the expansion process. Moreover, the occurrence of flank leakages through the radial clearances and its effects on the flow field were pointed out further proving the thermodynamic theory of scroll expander.

Numerical simulation and research on improving aerodynamic performance of vertical axis wind turbine by co-flow jet

The co-flow jet (CFJ) based active flow control method is a new approach for suppressing the flow separation on the blade, delaying the stall, and enhancing the lift. In this study, a detailed numerical analysis of the flow field developing around a Darrieus wind rotor equipped with CFJ is presented in order to explore the effectiveness of CFJ in the efficiency improvement of vertical axis wind turbines (VAWTs). Unsteady Computational Fluid Dynamics simulations of the straight one-bladed and three-bladed rotors with the NACA0015 profile, using the shear stress transport (SST) k-ω turbulence model, were conducted. As a result, the effects of jet operational parameters such as the injection site and the momentum coefficient on the aerodynamic performance of the straight-bladed Darrieus vertical axis wind turbine (VAWT) have been comprehensively studied. The numerical results suggest that CFJ would be capable of recovering some of the power loss due to blade stall and its control effectiveness is more pronounced with the increase in the jet momentum coefficient. In addition, a power saving method for intermittent injection of a CFJ in accordance with the cyclic variation of the angle of attack is proposed and our results show that the stalling characteristics around a VAWT blade especially at low tip speed ratios can be considerably improved using intermittent CFJ which requires much less additional energy input.

Development of a food temperature prediction model for real time food quality assessment

Multilayer food temperature models considering the spatial temperature distribution in the food were developed to predict temporal temperature changes experienced by food during distribution considering the surrounding temperature, initial food temperature, and the predetermined surface heat transfer coefficient. Parameters used in the models were determined using the results of unsteady three-dimensional computational fluid dynamics simulations for the conjugated heat transfer problem between the food and surroundings. Models were validated by comparing model predictions with the food temperature experimentally measured under fluctuating temperature conditions with a mean absolute deviation of less than 1 °C.The developed food temperature model was integrated with a freshness monitoring model to complete the real-time food quality monitoring system. Food quality prediction using the integrated food quality monitoring system was validated for eggs and milk. Quality predictions using the food temperature model showed better accuracy than when using a single point measurement of food temperature.

Poststall Behavior of a Multistage High Speed Compressor at Off-Design Conditions

Stall followed by surge in a high speed compressor can lead to violent disruption of flow, damage to the blade structures and, eventually, engine shutdown. Knowledge of unsteady blade loading during such events is crucial in determining the aeroelastic stability of blade structures; experimental test of such events is, however, significantly limited by the potential risk and cost associated. Numerical modeling, such as unsteady computational fluid dynamics (CFD) simulations, can provide a more informative understanding of the flow field and blade forcing during poststall events; however, very limited publications, particularly concerning multistage high speed compressors, can be found. The aim of this paper is to demonstrate the possibility of using CFD for modeling full-span rotating stall and surge in a multistage high speed compressor, and, where possible, validate the results against experimental measurements. The paper presents an investigation into the onset and transient behavior of rotating stall and surge in an eight-stage high speed axial compressor at off-design conditions, based on 3D Reynolds-averaged Navier–Stokes (URANS) computations, with the ultimate future goal being aeroelastic modeling of blade forcing and response during such events. By assembling the compressor with a small and a large exit plenum volume, respectively, a full-span rotating stall and a deep surge were modeled. Transient flow solutions obtained from numerical simulations showed trends matching with experimental measurements. Some insights are gained as to the onset, propagation, and merging of stall cells during the development of compressor stall and surge. It is shown that surge is initiated as a result of an increase in the size of the rotating stall disturbance, which grows circumferentially to occupy the full circumference resulting in an axisymmetric flow reversal.

Numerical and Experimental Fluid–Structure Interaction-Study to Determine Mechanical Stresses Induced by Rotating Stall in Unshrouded Centrifugal Compressor Impellers

Unshrouded industrial centrifugal compressor impellers operate at high rotational speeds and volume flow rates. Under such conditions, the main impeller blade excitation is dominated by high frequency interaction with stationary parts, i.e., vaned diffusers or inlet guide vanes (IGVs). However, at severe part load operating conditions, sub-synchronous rotating flow phenomena (rotating stall) can occur and cause resonant blade vibration with significant dynamic (von-Mises) stress in the impeller blades. To ensure high aerodynamic performance and mechanical integrity, part load conditions must be taken into account in the aeromechanical design process via computational fluid dynamics (CFD) and finite element method (FEM) analyzes anchored by experimental verification. The experimental description and quantification of unsteady interaction between rotating stall cells and an unshrouded centrifugal compression stage in two different full scale compression units by Jenny and Bidaut (“Experimental Determination of Mechanical Stress Induced by Rotating Stall in Unshrouded Impellers of Centrifugal Compressors”, ASME J. Turbomach. 2016; 139(3):031011-031011-10) were reproduced in a scaled model test facility to enhance the understanding of the fluid–structure interaction (FSI) mechanisms and to improve design guide lines. Measurements with strain gauges and time-resolved pressure transducers on the stationary and rotating parts at different positions identified similar rotating stall patterns and induced stress levels. Rotating stall cell induced resonant blade vibration was discovered for severe off-design operating conditions and the measured induced dynamic von-Mises stress peaked at 15% of the mechanical endurance limit of the impeller material. Unsteady full annulus CFD simulations predicted the same rotating stall pressure fluctuations as the measurements. The unsteady Reynold's Averaged Navier-Stokes simulations were then used in FEM FSI analyses to predict the stress induced by rotating stall and assess the aerodynamic damping of the corresponding impeller vibration mode shape. Excellent agreement with the measurements was obtained for the stall cell pressure amplitudes at various locations. The relative difference between measured and mean predicted stress from fluid–structure interaction was 17% when resonant blade vibration occurred. The computed aerodynamic damping was 27% higher compared to the measurement.

Thermo-hydraulic performance of the U-tube borehole heat exchanger with a novel oval cross-section: Numerical approach

In this paper, a novel U-tube Borehole Heat Exchanger (BHX) with an oval cross-section is proposed for improving the performance of Ground Source Heat Pump systems (GSHP). To study the thermo-hydraulic performance of Borehole Heat Exchanger (BHX) with an oval U-tube: A three-dimensional, symmetric and unsteady state multi-physics Computational Fluid Dynamic (CFD) simulations are carried out by ANSYS FLUENT environment. The conjugate heat transfer and fluid flow are simulated. Borehole thermal resistance, fluid inlet and outlet temperatures, fluid temperature profile along pipe length and pressure drop inside the tube are calculated and analyzed. The oval shape dimensions are adapted, then a comparison between the circle and the oval cross-sections is carried out; four different cases are investigated. As a result, the CFD results show a good agreement with their experimental pairs via Thermal Response Test (TRT) test which was done during September 2017 by our laboratory members. Besides, the results show that the oval shape enhances the heat transfer process and achieves the lowest borehole thermal resistance of 0.125 m K/W, also, it decreases the thermal resistance by 18.47% compared with the circular one. Besides, the oval shape shows a pressure drop of 32% less than the circular tube. Consequently, the oval shape could enhance the coefficient of performance (COP) of GSHP, decrease the borehole length, decrease the operating pressure and cut down initial and operating costs. Also, it fit easily inside tight borehole, so it is preferable in high-densely urban areas.

Validating a Reduced-Order Model for Synthetic Jet Actuators Using CFD and Experimental Data

Synthetic jet actuators (SJA) are emerging in various engineering applications, from flow separation and noise control in aviation to thermal management of electronics. A SJA oscillates a flexible membrane inside a cavity connected to a nozzle producing vortices. A complex interaction between the cavity pressure field and the driving electronics can make it difficult to predict performance. A reduced-order model (ROM) has been developed to predict the performance of SJAs. This paper applies this model to a canonical configuration with applications in flow control and electronics cooling, consisting of a single SJA with a rectangular orifice, emanating perpendicular to the surface. The practical implementation of the ROM to estimate the relationship between cavity pressure and jet velocity, jet velocity and diaphragm deflection and applied driving voltage is explained in detail. Unsteady Reynolds-averaged Navier Stokes computational fluid dynamics (CFD) simulations are used to assess the reliability of the reduced-order model. The CFD model itself has been validated with experimental measurements. The effect of orifice aspect ratio on the ROM parameters has been discussed. Findings indicate that the ROM is capable of predicting the SJA performance for a wide range of operating conditions (in terms of frequency and amplitude).

A computational fluid dynamics study on the solid mineral particles-laden flow in a novel offshore agitated vessel

Most seabeds are unexplored and rich in mineral deposits, making offshore mining a promising activity. However, offshore operation brings in great challenges from technical equipment to physical space. For instance, an offshore agitated vessel is supposed to stabilize the solids concentration from the underwater mining and make little impact on the stability of the platform or ship. For this reason, we proposed a novel offshore agitated vessel. The whole system based on the arrangement of the mineral processing platform and the slurry mix flow rate is obtained from the previous design stage. Large-scale unsteady computational fluid dynamics simulations are performed to calculate its effectiveness. The simulation model equipped with two pitched blade turbines and inlets/outlets is investigated. A classical Eulerian multiphase model and a modification of the standard k-ε eddy-viscosity turbulence model are adopted to simulate the dense solid–liquid suspension dynamics. Computational fluid dynamics results were found to be in satisfactory agreement with the theoretical predictions. The agitated system obtained was found to be effective to stabilize the solid particle concentration. In order to achieve a higher concentration at outlets and lower power consumption, further improvement was made and validated by computational fluid dynamics simulations. The proposed offshore mechanical agitated vessel could be equipped on offshore mining.

Design criterion for asymmetric twin-entry radial turbine for efficiency under steady and pulsating inlet conditions

The internal combustion engine plays an important role in energy conservation and environmental protection. An efficient way to simultaneously reduce the engine fuel consumption and the emissions (NOx) is an asymmetric twin-entry turbine turbocharging system. The asymmetric twin-entry turbine uses a volute that has two scrolls in the axial direction with different throat areas. The smaller area scroll can increase the backpressure to support the exhaust gas recirculation system for lower NOx emissions; meanwhile, the larger area scroll can decrease the backpressure to reduce the exhaust resistance for lower fuel consumption. The area ratio is dependent on the required exhaust gas recirculation rate and fuel consumption. However, there are no guidelines for how to arrange the scrolls, and whether placing the big scroll on the hub side or the shroud side can provide improved results remains unknown. In this paper, two different design types are discussed for the asymmetric turbine, one with the big scroll on the hub side, while the other is on the shroud side. Also, the unsteady computational fluid dynamics (CFD) simulation is under steady and pulsating inlet conditions. The results show that when under steady inlet conditions, the efficiency of the asymmetric turbine with the big scroll on the shroud side is nearly 1.6% higher. However, when facing pulsating inlet conditions, the efficiency of the asymmetric turbine with the big scroll on the hub side is almost 1.1% higher. The difference is due to the different mass flow storage capacities of the two scrolls under pulsating inlet conditions. The design criterion for asymmetric twin-entry turbine states that if the turbocharger is designed for a large volume exhaust manifold, it is better to place the big scroll on the shroud side. If for a small volume exhaust manifold, it is better to set the big scroll on the hub side.

Physics-Based Low-Order Model for Transonic Flutter Prediction

This paper presents a physical low-order model for two-dimensional unsteady transonic airfoil flow, based on small unsteady disturbances about a known steady flow solution. This is in contrast to traditional small-disturbance theory, which assumes small disturbances about the freestream. The states of the low-order model are the flowfield’s lowest moments of vorticity and volume-source density perturbations, and their evolution equation coefficients are calibrated using off-line unsteady computational fluid dynamics simulations through dynamic mode decomposition. The resulting low-order unsteady flow model is coupled to a typical-section structural model, thus enabling prediction of transonic flutter onset for wings with moderate to high aspect ratios. The method is fast enough to permit incorporation of transonic flutter constraints in conceptual aircraft design calculations. The accuracy of this low-order model is demonstrated for forced pitching and heaving simulations of an airfoil in compressible flow. Finally, the model is used to describe the influence of compressibility on the flutter boundary, and its accuracy is demonstrated for the Isogai Case A classical flutter test case.

Comparison of steady and unsteady RANS CFD simulation of a supersonic ORC turbine

This article presents computational fluid dynamics (CFD) simulations of a supersonic flow inside the first stage of an Organic Rankine Cycle (ORC) turbine. The expander considered in this work is the first stage of a three-stage axial turbine that uses hexamethyldisiloxane (MM) as the working fluid. The thermodynamic properties of the working fluid are modelled by a Helmoltz free energy-based equation of state during both the design and simulation steps to accurately account for the non-ideal nature of the fluid under the considered conditions. The high expansion ratio of the turbine leads to a supersonic flow in the stator and rotor blade passages. The design of the stator blade shape is carried out by means of the generalized method of characteristics (MOC). The CFD code used in this work is the commercial ANSYS CFX solver and the simulations are based on the two-dimensional Reynolds-Averaged Navier-Stokes (RANS) equations supplemented by a k-ω turbulence model. Results provided by steady mixing plane simulations are compared to those of unsteady sliding mesh simulations in order to understand the implications of stator/rotor flow interaction on blade loading, torque, entropy generation and flow structure.

Application of a three-step approach for prediction of combustion instabilities in industrial gas turbine burners

Abstract Recently, because of environmental regulations, gas turbine manufacturers are restricted to produce machines that work in the lean combustion regime. Gas turbines operating in this regime are prone to combustion-driven acoustic oscillations referred as combustion instabilities. These oscillations could have such high amplitude that they can damage gas turbine hardware. In this study, the three-step approach for combustion instabilities prediction is applied to an industrial test rig. As the first step, the flame transfer function (FTF) of the burner is obtained performing unsteady computational fluid dynamics (CFD) simulations. As the second step, the obtained FTF is approximated with an analytical time-lag-distributed model. The third step is the time-domain simulations using a network model. The obtained results are compared against the experimental data. The obtained results show a good agreement with the experimental ones and the developed approach is able to predict thermoacoustic instabilities in gas turbines combustion chambers.