Steady Calculations Research Articles

An Efficiency Methodology for Unsteady Full-engine simulation

When carrying out numerical analysis for the entire gas turbine engine, it is necessary to numerically model the flow field, heat conduction, and combustion in the compressor, combustor, and exhaust turbine section, which are its components. The calculation cost tends to increase because it is necessary to consider the complexity of shape, non-stationaryness, and the time scale of each physical phenomenon being largely different. In this paper, we apply approximate solution method of unsteady calculation with using nonlinear harmonic method, and introduce the analysis result of the whole engine including combustion analysis with comparison with the steady calculation result.

Comparison of Power Hardware-in-the-Loop Approaches for the Testing of Smart Grid Controls

The fundamental changes in the energy sector, due to the rise of renewable energy resources and the possibilities of the digitalisation process, result in the demand for new methodologies for testing Smart Grid concepts and control strategies. Using the Power Hardware-in-the-Loop (PHIL) methodology is one of the key elements for such evaluations. PHIL and other in-the-loop concepts cannot be considered as plug’n’play and, for a wider adoption, the obstacles have to be reduced. This paper presents the comparison of two different setups for the evaluation of components and systems focused on undisturbed operational conditions. The first setup is a conventional PHIL setup and the second is a simplified setup based on a quasi-dynamic PHIL (QDPHIL) approach which involves fast and continuously steady state load flow calculations. A case study which analyses a simple superimposed voltage control algorithm gives an example for the actual usage of the quasi-dynamic setup. Furthermore, this article also provides a comparison and discussion of the achieved results with the two setups and it concludes with an outlook about further research.

Unsteady effects of vortex interaction on tip leakage vortex breakdown and its loss mechanism

In an unshrouded high-pressure turbine, tip leakage flow causes a loss of efficiency. The loss mechanisms of tip leakage flow were investigated mostly with steady uniform incoming conditions in previous studies. This paper investigated the unsteady aerodynamic performance of the tip leakage flow in a turbine stage by solving Unsteady Reynolds-Averaged Navier–Stokes (URANS) equations. The unsteady results were also compared with results obtained by the steady simulation. Cases without rotor tip clearances were simulated to estimate the entropy generation due to the tip leakage flows.In the steady calculation, tip leakage vortex breakdown occurs due to its strong streamwise vorticity. Large entropy loss is generated due to vortex breakdown. Caused by blade rotation, the unsteady stator passage vortex evolution through the rotor passage is clearly unveiled. The enhanced stator passage vortex periodically interacts with the flow near the rotor tip and reduces the vorticity magnitude of tip leakage vortex. As a result, the vortex interaction periodically suppresses this vortex breakdown, thus reducing the loss. In the steady simulation, the axial length of tip leakage vortex breakdown is about 25% chord. However, in the unsteady simulation, the minimum axial length of the breakdown vortex is 10% chord. The entropy due to the tip leakage flow is evaluated in both the steady simulation and unsteady simulation. The time-averaged entropy generation of unsteady tip leakage vortex is 18% less than the loss in the steady case, which implies that unsteady vortex interaction improves the aerodynamic performance of tip leakage flow.

Unsteady Methods Applied to a Transonic Aeronautical Gas Turbine Stage

The importance of considering the unsteady effects in aeronautical engine design has brought to the implementation of simplified unsteady CFD models to respect the temporal restrictions of design cycles. A comparison among steady, Non-Linear Harmonic (NLH) and Full-Annulus (FA) methods has been carried out analyzing the transonic turbine stage CT3, experimentally studied at von Karman Institute for Fluid Dynamics. The understanding of the unsteady phenomena is fundamental to increase the engine efficiency and is precluded in steady calculations. As the computational cost of NLH calculations is of the same order of magnitude of steady ones, it represents a valid and competitive option in a turbine design process.

The effects of the rotor-stator interaction on unsteady pressure pulsation and radial force in a centrifugal pump

An unsteady numerical analysis has been conducted to study the strong interaction between impeller blade and volute tongue of a centrifugal pump. The 3-D-URANS equations were solved with the shear stress transport turbulence model for a wide range of flow rates. These unsteady interactions are mostly related to the unsteady radial force due to an imbalance in the pressure field at the impeller periphery. This force represents dynamic load that are one of the most important sources of vibration and hydraulic noise. Based on this phenomenon, this work analyzes and gives a more realistic prediction of the pressure fluctuation and the radial force during steady and unsteady calculation by considering the effect of the change in the pump operating point. Actually, the pressure fluctuations in the impeller and the volute were recorded by mounting nine monitoring points on the impeller and volute casing. The results of the existing analysis has proven that the pressure fluctuation is periodic due to the relative position of impeller blade to volute tongue. The characteristics of the time domain and frequency domain of the pressure pulsation were analyzed under different coupling conditions. Fast Fourier transform was performed to obtain the spectra of pressure pulsation. Besides, the steady and unsteady forces were calculated around the impeller periphery to fully characterize the pump behavior. The obtained pump performance curves were numerically compared with the experimental ones, and the outcome have shown an acceptable agreement between both curves.

Aerodynamic analysis of Ampyx’s airborne wind energy system

Besides classical horizontal and vertical axis wind turbines, alternative ways to harvest wind energy are currently being explored. One promising concept is represented by airborne wind energy systems. Different airborne wind energy concepts have been ideated and investigated over the past few decades. The work reported here focuses on the concept being developed by Ampyx Power that basically generates power using a tethered airplane which drives a generator on the ground. The aim of the work reported here is to develop and compare design calculations for such a power plane. In this study a comparison was made between steady and unsteady RANS CFD calculations from SU2 and OpenFOAM. Subsequently, using the data from the CFD calculation two different power prediction models were developed and compared, one using 2D CFD data with the free vortex code AWSM and the other using the same data with a nonlinear lifting line approach to model the power plane in its figure eight flying trajectory. Overall, comparisons show that for all practical angles of attack, as well as a range of flap angles, that 2D CFD results agree extremely well between both solvers. The 3D CFD results show <20% differences on the computed total forces, despite achieving good agreement on the relative force contributions across the power plane. A very good agreement has also been established between the power prediction models.

COPRA experiment and numerical research on the behavior of internally-heated melt pool with eutectic salt

In-vessel retention (IVR) has been proposed as an effective mitigation strategy during severe accidents in pressurized water reactors (PWRs). To help understand better the application of IVR, the large-scale COPRA facility was used for experimental research and the eutectic salt, i.e. the binary mixture of 50 mol% NaNO3 – 50 mol% KNO3 was selected as the simulant material for corium. Besides, numerical studies were done to capture the detailed flow field information. The experiment studied the effects of a sudden change in boundary condition and the influences of power changes on the transient behaviors and the steady-state characteristics of melt pool. Experimental results show that a sudden introduction of sidewall cooling and the melt pouring are the most threatening factors to the reactor vessel safety, but which can be guaranteed as long as the vessel is kept undamaged during melt pouring phases. Also, power changing won’t lead to rapid variation of sideward heat flux, so the correlations for steady state heat transfer calculation are basically qualified to evaluate the reactor safety in analysis of long time cooling. Besides, the power changing only exerts obvious impacts on the crust growth on the curved sidewall inner surface with polar angle below 30°, contrary to which the sideward heat flux is clearly influenced by different power densities only above the angle of 30°. Finally, the numerical simulation results show that vortexes could form near the region where the crust formed, and lead to a more homogeneous temperature distribution; generally, the convection happens between the lower part of melt pool and cooling boundary, with flow velocity at a magnitude of 0.01 m/s.

Experimental study on natural circulation flow resistance characteristics in a 3 × 3 rod bundle channel under rolling motion conditions

In order to study and analyze natural circulation flow resistance characteristics in a 3 × 3 rod bundle channel under rolling motion conditions, a natural circulation loop was set up and a series of experiments were conducted. The results show that the flow transition Reynolds number for this rod bundle channel in a steady condition is approximately 800. Correlations for rod bundle friction factor and spacer grid local resistance coefficient are fitted. Rolling motion increases the cycle-averaged friction factors when the steady Reynolds number is low. The cycle-averaged local resistance coefficients are generally larger than the steady values. No obvious phase shift is observed between the frictional pressure drop and flow rate and the total pressure drop fluctuation is dominated by the gravity pressure drop. With the inlet fluid subcooling increasing, the absolute pressure drop fluctuation amplitude keeps approximately invariable while the relative fluctuation amplitude increases. Additionally, the relative pressure drop fluctuation amplitude increases with the maximum rolling angular acceleration. The instantaneous friction factor variation rules are dominated by the flow regimes and explained by different variation gradients in different flow regimes. The steady correlation is proved applicative for the instantaneous friction factor calculation in a laminar regime. However, the instantaneous local resistance coefficient fluctuates more drastically than the steady calculation values.

(Invited) Linker Group Effects of Linearly Pi-Extended Porphyrins for Solar Energy Conversion

Linearly pi-extended porphyrins often show significantly broadened and red-shifted absorption bands due to the largely extended pi-system. As such, linearly pi-extended porphyrins appear to be attractive candidates as sensitizers for dye-sensitized solar cells.1-3 Linearly pi-extended porphyrins, such as benzoporphyrins, are generally very difficult to synthesize and functionalize. In this presentation, we present concise and versatile synesthetic methods to access different types of benzoporphyrins. Push-pull porphyrins, in which the push (electron-donating) and pull (electron-withdrawing) groups are installed at the porphyrin beta-positions are also designed and synthesized.4-6 These porphyrins are evaluated as sensitizers for dye-sensitized solar cells. Significant linker effects and push-pull effects were observed for these porphyrins. Systematic investigation on linker groups, electron-donating groups as well as electron-withdrawing groups are performed using UV-Vis spectroscopy, life-time and steady state fluorescence spectroscopy, cyclic voltammetry and DFT calculations. Figure 1

Preliminary Design and Model Assessment of a Supercritical CO2 Compressor

The compressor is a key component in the supercritical carbon dioxide (SCO2) Brayton cycle. In this paper, the authors designed a series of supercritical CO2 compressors with different parameters. These compressors are designed for 100 MWe, 10 MWe and 1 MWe scale power systems, respectively. For the 100 MWe SCO2 Brayton cycle, an axial compressor has been designed by the Smith chart to test whether an axial compressor is suitable for the SCO2 Brayton cycle. Using a specific speed and a specific diameter, the remaining two compressors were designed as centrifugal compressors with different pressure ratios to examine whether models used for air in the past are applicable to SCO2. All compressors were generated and analyzed with internal MATLAB programs coupled with the NIST REFPROP database. Finally, the design results are all checked by numerical simulations due to the lack of reliable experimental data. Research has found that in order to meet the de Haller stall criterion, axial compressors require a considerable number of stages, which introduces many additional problems. Thus, a centrifugal compressor is more suitable for the SCO2 Brayton cycle, even for a 100 MWe scale system. For the performance prediction model of a centrifugal compressor, the stall predictions are compared with steady numerical calculation, which indicates that past stall criteria may also be suitable for SCO2 compressors, but more validations are needed. However, the accuracy of original loss models is found to be inadequate, particularly for lower flow and higher pressure ratio cases. Deviations may be attributed to the underestimation of clearance loss according to the result of steady simulation. A modified model is adopted which can improve the precision to a certain extent, but more general and reasonable loss models are needed to improve design accuracy in the future.

Formation of free round jets with long laminar regions at large Reynolds numbers

The paper describes a new, simple method for the formation of free round jets with long laminar regions by a jet-forming device of ∼1.5 jet diameters in size. Submerged jets of 0.12 m diameter at Reynolds numbers of 2000–12 560 are experimentally studied. It is shown that for the optimal regime, the laminar region length reaches 5.5 diameters for Reynolds number ∼10 000 which is not achievable for other methods of laminar jet formation. To explain the existence of the optimal regime, a steady flow calculation in the forming unit and a stability analysis of outcoming jet velocity profiles are conducted. The shortening of the laminar regions, compared with the optimal regime, is explained by the higher incoming turbulence level for lower velocities and by the increase of perturbation growth rates for larger velocities. The initial laminar regions of free jets can be used for organising air curtains for the protection of objects in medicine and technologies by creating the air field with desired properties not mixed with ambient air. Free jets with long laminar regions can also be used for detailed studies of perturbation growth and transition to turbulence in round jets.

Internal unsteady flow characteristics of centrifugal pump based on entropy generation rate and vibration energy

The transient fluid exciting force induced by unsteady flow in the centrifugal pump is the only exciting force that cannot be effectively eliminated. In order to explore the vibration problem caused by unsteady flow in the centrifugal pump, the steady and unsteady numerical calculations of the internal flow in a centrifugal pump with low specific speed were carried out under different flow rate conditions. With volute circumferential pressure pulsation test, the accuracy of numerical calculations was verified. At the same time, the vibration acceleration sensors were arranged in different positions of the pump body to complete the vibration characteristics experiment under different working conditions. Based on the numerical results, the amount and location of the internal flow loss of the centrifugal pump were predicted by the entropy generation rate method. According to the results of the vibration test, the vibration energy distribution of the centrifugal pump under different working conditions was obtained. In combination with the entropy generation rate and vibration energy distribution, the change rules of flow loss and the vibration energy with the flow condition of the pump were analyzed. By using the frequency-domain analysis method, the pressure pulsation, the unsteady radial fluid exciting force fluctuation and the vibration acceleration were compared and analyzed to study the change rules of the pressure pulsation, the unsteady fluid exciting force and the vibration characteristics with the flow rate. The results show that the internal flow loss is mainly concentrated in the impeller runner near the volute tongue under low flow condition and the internal flow loss under large flow rate condition is mainly concentrated in the volute channel near the pump outlet. The vibration induced by the unsteady flow in the centrifugal pump is mainly low-frequency vibration, which is very sensitive to the change of the flow rate. The vibration energy in the middle- and high-frequency ranges is almost not affected by the working condition. The internal flow loss and the low-frequency vibration energy change with the flow condition, showing similar change rules.

Study on correlation between wind turbine performance and ice accretion along a blade tip airfoil using CFD

In cold-region environments characterized by high wind speeds, low external air temperatures, and supercooled water droplets, wind turbines frequently experience ice formation along the leading edge of the blade tip, which has a higher angular velocity. This ice formation directly and negatively affects the wind turbine's power performance. As such, a clear understanding of the relationship between ice formation and wind turbine power performance is required. In this study, we used computational fluid dynamics (CFD) simulation methods based on the dispersed multiphase (DMP) droplet model to predict ice formation along a blade tip airfoil's leading edge. To validate the ice formation model, the CFD simulation results were compared with the previously published wind tunnel testing results. Following validation, the model was used to predict the ice formation along the leading edge of the National Advisory Committee for Aeronautics (NACA) 64-618 airfoil and estimate its aerodynamic performance under variable external environmental conditions. Steady power calculations for the National Renewable Energy Laboratory's (NREL) 5-MW wind turbine was performed using the BLADED software. The aerodynamic performance of the NACA 64-618 airfoil following ice formation and the power performance of the NREL 5-MW wind turbine were subsequently correlated. The results indicated that ice formation on the blade tip airfoil's leading edge caused wind turbine power performance to decrease by approximately 8–29%.

Minimum steady corona inception voltage calculation method under rain condition

As the most common precipitation form, rain-lead corona problems are inevitably faced by designers of transmission lines. Through long-term observations of corona inception process under rain conditions by ultraviolet camera, a definition of minimum steady corona inception voltage is presented, which is more useful for practical application. To predict the minimum steady corona inception voltage, water droplet deformation model of smooth conductor is promoted based on the principle of minimising energy in this publication, which takes the influence of electric field force into consideration. Critical breakdown voltage and corresponding critical geometric dimensions of specific water droplet are calculated through the deformation model. The accuracy of the presented deformation model is proved to be >95%, by verification tests performed in a corona cage. Corona discharge model presented in the previous work is adopted to predict the water droplet corona inception voltage of given geometric dimensions. Finally, through the calculation results of both models, the defined minimum steady corona inception voltage of smooth conductor is calculated when there exist rain droplets on conductor lower surface. Also, the calculation error of minimum steady corona inception voltage is ∼8.0% when compared with the validation experiments.

MPPT Control System Based on Incremental Conductance and Constant Voltage using Coupled Inductor-Capacitor Zeta Converter in hybrid PV-Wind Turbine System

This paper proposes a method of maximum power point tracking control using Incremental Conductance employed in hybrid photovoltaic-wind turbine for DC micro-grid system. The rapid growth of renewable energy using photovoltaic (PV) and wind turbine due to low operational cost and easy to implementation. Improvement of the efficiency of PV and wind turbine based on incremental conductance is the solution to get maximum power in PV and Wind Turbine. The proposed method uses variable step to achieve maximum power with high speed tracking, high efficiency and low steady state error calculation. The performance of the MPPT technique has been applied in dc-dc boost converter. Moreover, Zeta Converter using Coupled Inductor-Capacitor is added for maintain constant voltage in passive load. This high gain converter can reach 311 Volt DC with 1000 W output power and high efficiency up to 90%. The effectiveness of proposed method is implemented in two hybrid system using PV and wind turbine source.

Numerical investigation of subsynchronous vibration in floating ring bearings

Floating ring bearings are popular among turbochargers due to their simplicity and reliability. The disappearance of subsynchronous vibration with the increase of shaft speed in a low oil-supplied pressure floating ring bearing is reported by Hatakenaka and Yanai. This finding may help eliminate the noise and decrease the loss of turbochargers. This work aims to explain this phenomenon in the low oil-supplied floating ring bearing using computational fluid dynamic. Steady computational fluid dynamic calculation is conducted to validate the effect of air entrainment. Transient computational fluid dynamic calculation method with mesh motion method is established. The subsynchronous vibration of the shaft can be obtained by discrete Fourier transform analysis. The results are validated by comparing them with those in the literature. It is found that the disappearance of the subsynchronous vibration is the result of the change in lubricant properties caused by the air entrainment.

Aerodynamic analysis of nonuniform trailing edge blowing

Purpose Numerical and experimental results for different oncoming base-flow conditions indicate that nonuniform trailing edge blowing (NTEB) can expand the performance range of compressors and reduce the thrust on the rotor, while the efficiency of the compressor can be improved by more than 2 per cent. Design/methodology/approach Relevant aerodynamic parameters, such as total pressure, ratio of efficiency and axial thrust, are calculated and analyzed under conditions with and without NTEB. Measurements are performed downstream of two adjacent stator blades, at seven equidistantly spaced reference locations. The experimental measurement of the interstage flow field used a dynamic four-hole probe with phase lock technique. Findings An axial low-speed single-stage compressor was established with flow field measurement system and nonuniform blowing system. NTEB was studied by means of numerical simulations and experiments, and it is found that the efficiency of the tested compressor can be improved by more than 2 per cent. Originality/value Unlike most of the previous research studies which mainly focused on the rotor/stator interaction and trailing edge uniform blowing, the research results summarized in the current paper on the stator/rotor interaction used inlet guide vanes for steady and unsteady calculations. An active control of the interstage flow field in a low-speed compressor was used to widen the working range and improve the performance of the compressor.

Study on the Blowing Scheme of a Propellant Storage Vessel

The blowing flow field of a propellant storage vessel are simulated using Fluent software. Through steady flow field calculation, some important flow field parameters, such as pressure, temperature, velocity and Streamline, are obtained under three different working conditions. Based on these parameters, the effects of three different working conditions are analysed. According to the analysis, the improved blowing scheme is given, which can effectively improve the efficiency of the blowing process.

Modelling heat loss effects in high temperature oxy-fuel flames with an efficient and robust non-premixed flamelet approach

The non-premixed steady flamelet model is extended by two simple, robust and effective heat loss modelling approaches. The heat release damping (HRD) approach decreases the chemical source term in the energy equation by a constant factor, while the artificial radiation (AR) approach introduces an augmented temperature dependent radiative source term. The models are tested in a simulation of the 0.78 MWth IFRF (International Flame Research Foundation) pilot scale, non-premixed, natural gas/oxygen flame. Both approaches are applied in steady laminar flamelet calculations with detailed chemistry, tabulating the thermo-chemical state as a function of mixture fraction and normalised heat loss. The turbulence-chemistry interaction is modelled using the β-PDF approach. An enthalpy transport equation is solved to keep track of the heat loss, while radiative heat transfer is calculated by the P-1 model. We observe that the major species, temperature, velocities and velocity fluctuations show a good agreement with the available experimental data. The heat loss modelling yields a significant improvement over the adiabatic model. Interestingly, both heat loss models (HRD and AR) show negligible differences in the simulations of the turbulent flame and permit to apply the steady laminar flamelet model to oxy-fuel processes in a simple, robust and user friendly manner.

Effect of swirl on the performance and stability of transonic axial compressor

To investigate the effect of twin swirl and bulk swirl on the performance and stability of a transonic axial compressor, a blade swirl generator was designed and simulated with a transonic single rotor using steady and unsteady numerical calculation methods. The bulk swirl intensity was adjusted by replacing the blades with different camber angles. The twin swirl intensity was decreased by reducing the blade number. The counter-rotating bulk swirl generated a significant drop in both the efficiency and stall margin, and resulted in an increase in the choked mass flow, and total pressure ratio. The co-rotating bulk swirl generated a decrease in the mass flow, total pressure ratio and stable operating range. The counter-rotating bulk swirl resulted in suction surface boundary layer separation beyond 50% of the span-wise height as well as more serious tip leakage blockage. The twin swirl resulted in a decrease in the total pressure ratio, maximum efficiency and stable operating range. The steady and unsteady numerical calculation results were consistent, though some differences were observed in the values. For the steady calculation, the maximum efficiency and choked mass flow decreased by 0.88% and 1.74%, respectively, and the mass flow at the stable boundary increased by 3.92% as compared to the uniform inlet flow at twin swirl intensity of 24°. During the unsteady calculation, the mass flow exhibited an increase of only 2.2% at the stable boundary. Under twin swirl and co-rotating bulk swirl and uniform inlet flow, the leading edge spillage of the tip leakage flow resulted in compressor instability. The counter-rotating bulk swirl changed the mechanism of instability. The characterisation of the swirl distortion presented a difference between the steady and unsteady calculations near the stable boundary. The unsteady calculation exhibited a lower mass flow at the stable boundary point and a higher total pressure ratio.