Research Papers: Gas Turbines: Combustion, Fuels, and Emissions

J. Eng. Gas Turbines Power. 2015;137(8):081501-081501-10. doi:10.1115/1.4029368.

The current paper investigates the nonlinear interaction of the flow field and the unsteady heat release rate and the role of swirl fluctuations. The test rig consists of a generic swirl-stabilized combustor fed with natural gas and equipped with a high-amplitude forcing device. The influence of the phase between axial and azimuthal velocity oscillations is assessed on the basis of the amplitude and phase relations between the velocity fluctuations at the inlet and the outlet of the burner. These relations are determined in the experiment with the multimicrophone-method and a two component laser Doppler velocimeter (LDV). Particle image velocimetry (PIV) and OH*-chemiluminescence measurements are conducted to study the interaction between the flow field and the flame. For several frequency regimes, characteristic properties of the forced flow field and flame are identified, and a strong amplitude dependence is observed. It is found that the convective time delay between the swirl generator and the flame has an important influence on swirl-number oscillations and the flame dynamics in the low-frequency regime. For mid and high frequencies, significant changes in the mean flow field and the mean flame position are identified for high forcing amplitudes. These affect the interaction between coherent structures and the flame and are suggested to be responsible for the saturation in the flame response at high forcing amplitudes.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2015;137(8):081502-081502-8. doi:10.1115/1.4029371.

Understanding the stability of turbulent flames is a key for the design of efficient combustion systems. The present paper reports an experimental study on the effect of the internal geometry of a rectangular orifice on the characteristics/stability of a turbulent methane flame. Three rectangular nozzles with different orifice lengths having an identical exit aspect ratio (AR) of 2 were used. The co-airflow strength was also varied to evaluate its effect on the jet flow emerging from the rectangular nozzle. The experimental data revealed that the jet initial conditions affect both the flow characteristics and the liftoff of turbulent diffusion methane flame. That is, increasing the orifice length of the rectangular nozzle resulted in delaying the occurrence of the axis-switching phenomenon, reducing the length of the jet potential core, and accelerating the liftoff transition of the attached flame. The co-airflow was found to reduce the velocity strain rate in the shear layer, displace the occurrence of axis-switching farther downstream of the jet, and delay flame detachment. The results revealed also that there is a clear interplay between the flame liftoff and the jet near-field molecular mixing and flow characteristics. That is, a rectangular jet which spreads faster and generates higher near-field velocity strain and turbulence intensity causes flame detachment at a lower fuel jet velocity. Based on this, a correlation was found between the flame liftoff velocity, the fuel molecular thermal diffusivity, the stoichiometric laminar flame speed, and the fuel jet strain rate at the nozzle exit. This relationship was shown to successfully predict the liftoff velocity of methane flame as well as other common gaseous hydrocarbons and hydrogen flames.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2015;137(8):081503-081503-11. doi:10.1115/1.4029426.

The atomization characteristics of blends of bioderived camelina hydrogenated renewable jet (HRJ) alternative fuel with conventional aviation kerosene (Jet A-1) discharging into ambient atmospheric air from a dual-orifice atomizer used in aircraft engines are described. The spray tests are conducted in a spray test facility at six different test flow conditions to compare the atomization of alternative fuels with that of Jet A-1. The fuel sprays are characterized in terms of fuel discharge, spray cone angle, drop size distribution, and spray patternation. The measurements of spray drop size distribution are obtained using laser diffraction based Spraytec equipment. The characteristics of fuel discharge and cone angle of alternative fuel sprays do not show any changes from that of Jet A-1 sprays. The characteristics of spray drop size, evaluated in terms of the variation of mean drop size along the spray axis, for the alternative fuel sprays remain unaffected by the variation in fuel properties between the alternative fuels and Jet A-1. The measurements on spray patternation, obtained using a mechanical patternator at a distance 5.1 cm from the atomizer exit, show an enhanced fuel concentration in the vicinity of spray axis region for the alternative fuel sprays discharging from the dual-orifice atomizer.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2015;137(8):081504-081504-10. doi:10.1115/1.4029479.

To meet stringent Euro-6 emission regulations, a lean NOx trap (LNT) catalyst should be considered to effectively abate NOx emissions. This LNT catalyst should be periodically regenerated without deteriorating driving quality and also satisfy emission constraints, such as CO, low particulate matter or smoke, and low O2 during the regeneration phase. As a means of reductant delivery, in-cylinder post fuel injection with a feedforward (FF) control has been applied due to its simple implementation in an engine management system (EMS). However, with this method, it is difficult to satisfy the driving quality and emission constraints during the transition to or out of the regeneration phase. To solve this problem, we propose a novel LNT regeneration control method using an indicated mean effective pressure (IMEP) and a combustion lambda feedback (FB) control combined with the FF control. For the precise FB control of the post injection timing, among the location of the second rate of heat release (ROHR) peak, the magnitude of the second ROHR peak, and IMEP, the IMEP was selected as a control parameter because of its lowest cyclic variation. In addition, the exhaust lambda control was applied for the accurate FB control of the post injection quantity. The proposed method was implemented in an in-house EMS. The performance in several engine tests indicated that the torque fluctuation was minimized and all emission constraints were effectively satisfied. Furthermore, this method was also robust with regard to the thermal disturbance.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2015;137(8):081505-081505-10. doi:10.1115/1.4029623.

In order to analyze the difference between the inverse diffusion flame (IDF) and normal diffusion flame (NDF) under various conditions, the emission spectra of OH* and CH* chemiluminescence in two dimensions measured by hyperspectral and ultraviolet (UV) cameras are described in this article. The results show that CH* mainly appears in the fuel side near the flame front, while OH* distribution can reflect the reaction region of flame. According to the OH* radial distributions in IDF and NDF, the flame can be divided into three parts: the core area of the flame, the transition region of the flame, and the developed region of flame. The peak intensity of CH* in IDF is higher than that in NDF. Moreover, the length of reaction region in NDF increases with O/C equivalence ratio ([O/C]e) until it reaches a steady value, while in IDF the length decreased with the increase of [O/C]e.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Controls, Diagnostics, and Instrumentation

J. Eng. Gas Turbines Power. 2015;137(8):081601-081601-11. doi:10.1115/1.4029309.

The unsteady flow in inlet valves for large steam turbines used in power stations was investigated using the method of computational fluid dynamics (CFD). As the topology of the flow depends on the stroke and the pressure ratio of the valve, the flow was investigated at several positions. Various turbulence models were applied to the valve to capture the unsteady flow field. Basic Reynolds-averaged Navier–Stokes (RANS) models, the scale adaptive simulation (SAS), and the scale adaptive simulation with zonal forcing (SAS-F, also called ZFLES) were evaluated. To clarify the cause of flow-induced valve vibrations, the investigation focused on the pressure field acting on the valve plug. It can be shown that acoustic modes are excited by the flow field. These modes cause unsteady forces that act on the valve plug. The influence of valve geometry on the acoustic eigenmodes was investigated to determine how to reduce the dynamic forces. Three major flow topologies that create different dynamic forces were identified.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Heat Transfer

J. Eng. Gas Turbines Power. 2015;137(8):081901-081901-11. doi:10.1115/1.4029389.

In modern aero-engines, the lubrication system plays a key role due to the demand for high reliability. Oil is used not only for the lubrication of bearings, gears, or seals but it also removes large amounts of the generated heat. Also, air from the compressor at elevated temperature is used for sealing the bearing chambers and additional heat is introduced into the oil through radiation, conduction, and convection from the surroundings. The impact of excessive heat on the oil may lead to severe engine safety and reliability problems which can range from oil coking (carbon formation) to oil fires. Coking may lead to a gradual blockage of the oil tubes and subsequently increase the internal bearing chamber pressure. As a consequence, oil may migrate through the seals into the turbomachinery and cause contamination of the cabin air or ignite and cause failure of the engine. It is therefore very important for the oil system designer to be capable to predict the system’s functionality. Coking or oil ignition may occur not only inside the bearing chamber but also in the oil pipes which carry away the air and oil mixture from the bearing chamber. Bearing chambers usually have one pipe (vent pipe) at the top of the chamber and also one pipe (scavenge pipe) at the bottom which is attached to a scavenge pump. The vent pipe enables most of the sealing air to escape thus avoid over-pressurization in the bearing compartment. In a bearing chamber, sealing air is the dominant medium in terms of volume occupation and also in terms of causing expansion phenomena. The scavenge pipe carries away most of the oil from the bearing chamber but some air is also carried away. The heat transfer in vent pipes was investigated by Busam (2004, “Druckverlust und Wärmeuebergang im Entlueftungssystem von Triebwerkslagerkammern (Pressure Drop and Heat Transfer in the Vent System in an Aero Engine’s Bearing Chamber),” Ph.D. thesis, Logos Verlag, Berlin, Germany) and Flouros (2009, “Analytical and Numerical Simulation of the Two Phase Flow Heat Transfer in the Vent and Scavenge Pipes of the CLEAN Engine Demonstrator,” ASME J. Turbomach., 132(1), p. 011008). Busam has experimentally developed a Nusselt number correlation for an annular flow in a vent pipe. For the heat transfer predictions in scavenge pipes, no particular Nusselt number correlation exist. This paper intends to close the gap in this area. As part of the European Union funded research programme ELUBSYS (Engine Lubrication System Technologies), an attempt was done to simplify the oil system’s architecture. In order to better understand the flow in scavenge pipes, high speed video was taken in two sections of the pipe (vertical and horizontal). In the vertical section, the flow was a wavy annular falling film, whereas the flow in the horizontal section was an unsteady wavy stratified/slug flow. Heat transfer has been investigated in the horizontal section of the scavenge pipe, leaving the investigation on the vertical section for later. Thanks to the provided extensive instrumentation, the thermal field in, on, and around the pipe was recorded, evaluated, and also numerically modeled using ansys cfx version 14. Brand new correlations for two-phase flow heat transfer (Nusselt number) and for pressure drop (friction coefficient) in horizontal scavenge pipes are the result of this work. The Nusselt number correlation has been developed in such a way that smooth transition (i.e., no discontinuity) from two-phase into single phase flow is observed.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Structures and Dynamics

J. Eng. Gas Turbines Power. 2015;137(8):082501-082501-14. doi:10.1115/1.4029272.

Modern aero-engine development requires also a gradual increase in the overall effectiveness of lubrication systems. This particularly applies to bearing chambers where a complex two-phase flow is formed by the interaction of the sealing air and the lubrication oil. It is important to increase the level of understanding of the flow field inside the bearing chamber and to develop engineering tools in order to optimize its design and improve its performance. To achieve this, an experimental and a computational study of the whole front bearing chamber were carried out for a range of shaft rotational speeds and sealing air mass flow. The experimental measurements of the air velocity inside the chamber were carried out using a laser Doppler anemometer (LDA) in two-phase air/oil-flow conditions. The experimental facility is a 1:1 scale model of the front bearing chamber of an aero-engine. Computational 3D modeling of the bearing chamber was performed. The bearing gap and the presence of lubrication oil were modeled as an anisotropic porous medium with functions relating the pressure loss of the air coming through the gap and the tangential component of velocity of the air exiting the gap of the ball bearing with the air-flow rate through the gap and the rotational speed of the shaft. The methodology to obtain the above mentioned functions and the results of the detailed study are given (Aidarinis, J., and Goulas, A., 2014, “Enhanced CFD Modeling and LDA Measurements for the Air-Flow in an Aero Engine Front Bearing Chamber: Part II,” ASME Paper No. GT2014-26062). The enhanced computational model of the chamber implementing the law of pressure drop of the “lubricated” bearing and the function of modeling the tangential velocity of the air exiting the bearing was used to calculate the flow field for the full range of the measurements. A limiting curve dividing the operational map of the bearing chamber into two areas was predicted. Large vortical and swirling structures dominate the flow and they vary in size according to the position of the operation point relative to the limiting curve. Operation above the limiting curve leads to flow classified as type I with air going through the ball bearing while for operation below the limiting curve line the flow is classified as type II, there is no air-flow through the bearing gap.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2015;137(8):082502-082502-15. doi:10.1115/1.4029365.

A detailed computational study of the air-flow through the outer gap of the front bearing of an aero-engine is presented. The reason to carry out this study was to understand the flow through the bearing as a function of the operational parameters of the engine, which was necessary for the modeling of the flow in the whole bearing chamber. The complex geometry and the size of the bearing gap relative to the overall dimensions of the bearing chamber and the need for very precise and detailed information of the effect on the flow within the chamber of the bearing operational parameters, prohibited the solution of the flow through the gap together with the rest of the bearing chamber. A 3D modeling of the flow through the outer bearing gap, which included a section of the ball bearing, was performed. Functions relating the pressure drop of the air coming through the bearing gap and the tangential component of velocity of the air exiting the bearing region, to the mass of air through the gap of the ball bearing and the rotational speed of the shaft were developed. The effect of the lubrication oil within the bearing was modeled as an anisotropic porous medium with a predefined law. In order to acquire in a mathematical form the above relationships a series of computational runs were performed. These relationships, in the form of second order curves, were subsequently introduced to the model of the bearing chamber as described by Aidarinis and Goulas (2014, “Enhanced CFD Modeling and LDA Measurements for the Air-Flow in an Aero Engine Front Bearing Chamber (Part I),” ASME Paper No. GT2014-26060). The constants of the relationships were derived through comparisons of the calculations with the experimental data. From the analysis, it was concluded that the pressure drop across the bearing increases with the square of the rotational speed of the shaft with the mass flow of air through the ball bearing as a parameter and vice versa. For this particular ball bearing, there is a region where, for any combination of rotational speed of the shaft and pressure drop through the bearing, there is no flow of air through the bearing. In this paper the detailed modeling methodology, the computational flow field, the boundary conditions and finally the results are presented and discussed.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2015;137(8):082503-082503-10. doi:10.1115/1.4029372.

Since the mass of the rotor in high energy density magnetically suspended motor (HEDMSM) is always large and there are only three balancing planes on the flexible rotor restricted by the structure of the motor, which means that the second bending mode cannot be balanced using N + 1 planes method which is always applied to balance the flexible rotor. Then, the rotor displacements maybe large and this situation will make the system consume large amplifier currents when the rotor passes the first bending critical speed. Therefore, the mode separation method is proposed to separate the first and the second bending modes in rotor displacement and reconstruct the displacement signal nearby the first bending mode. Then, the original rotor displacement signal used by the digital controller is substituted by the reconstructed displacement signal and the amplifier current is reduced a lot when the rotor passes the first bending critical speed. Finally, the experiment of mode separation is carried out in 100 kW magnetically suspended motor and the experiment results show the effectiveness and superiority of the mode separation method in reducing the amplifier current when the rotor passes the first bending critical speed.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2015;137(8):082504-082504-8. doi:10.1115/1.4029392.

Flexible torsional couplings are used primarily to transmit power between rotating components in industrial power systems, including turbomachinery, while allowing for small amounts of misalignment that may otherwise lead to equipment failure. The torsional coupling lumped characteristics, such as torsional- and flexural stiffness, as well as natural frequencies of vibration are important for design of the entire power system and, therefore, must be calculated or computed with a high degree of accuracy. In this paper, we compare theoretical-, computational-, and experimental methods of characterizing torsional stiffness of a family of metallic disk type flexible couplings. We demonstrate the sensitivity of torsional stiffness to various design parameters and characterization assumptions, including boundary conditions, level of model detail, and material properties of the coupling's components. We also develop a full 3D parametric finite element model of the coupling and report on its experimental validation.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2015;137(8):082505-082505-9. doi:10.1115/1.4029393.

The rated rotational speed of the magnetically suspended motor (MSM) is always above the bending critical speed to achieve high energy density. The rotor will have a dramatic resonance when it passes the critical speed. Then, the magnetic bearing has to provide large bearing force to suppress the synchronous vibration. However, the bearing force is always limited by magnetic saturation and power amplifier voltage saturation. This paper proposed an optimum damping control method which can make effective use of the limited bearing force to minimize the synchronous vibration amplitude of the rotor nearby the critical speed. The accurate rotor model is obtained by theoretical analysis and system identification. The unbalance force response of the bending mode of the rotor is analyzed. The small gain theorem is used to determine the range of the magnitude of the control system. Then, the relationship of the optimum damping varying with the magnitude and phase of the control system nearby the critical speed is analyzed. The run-up experiments are carried out in 315 kW MSM and the results show the effectiveness and superiority of the optimum damping control method.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2015;137(8):082506-082506-5. doi:10.1115/1.4029480.

An understanding of the high temperature mechanics experienced in thermal barrier coatings (TBC) during cycling conditions would be highly beneficial to extending the lifespan of the coatings. This study will present results obtained using synchrotron X-rays to measure depth resolved strains in the various layers of TBCs under thermal mechanical loading and a superposed thermal gradient. Tubular specimens, coated with yttria stabilized zirconia (YSZ) and an aluminum containing nickel alloy as a bond coat both through electron beam-physical vapor deposition (EB-PVD), were subjected to external heating and controlled internal cooling generating a thermal gradient across the specimen's wall. Temperatures at the external surface were in excess of 1000 °C. Throughout high temperature testing, 2D high-resolution XRD strain measurements are taken at various locations through the entire depth of the coating layers. Across the YSZ, a strain gradient was observed showing higher compressive strain at the interface to the bond coat than toward the surface. This behavior can be attributed to the specific microstructure of the EB-PVD-coating, which reveals higher porosity at the outer surface than at the interface to the bond coat, resulting in a lower in plane modulus near the surface. This location at the interface displays the most significant variation due to applied load at room temperature with this effect diminishing at elevated uniform temperatures. During thermal cycling with a thermal gradient and mechanical loading, the bond coat strain moves from a highly tensile state at room temperature to an initially compressive state at high temperature before relaxing to zero during the high temperature hold. The results of these experiments give insight into previously unseen material behavior at high temperature, which can be used to develop an increased understanding of various failure modes and their causes.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2015;137(8):082507-082507-8. doi:10.1115/1.4029481.

The importance of automotive turbocharger performance is continuously increasing. However, further gains in efficiency are becoming progressively difficult to achieve. The bearing friction losses impact the overall efficiency of the turbocharger and accordingly the understanding of bearing systems and their characteristics is essential for future improvements. In this work, a detailed analysis on the mechanical losses occurring in the bearing system of automotive turbochargers is presented. Friction losses have been measured experimentally on a special test bench up to rotational speeds of nTC = 130,000 1/min. Special interest was given to the thrust bearing characteristics and its contribution to the total friction losses. For this, the experiments were split into three parts: first, friction power was determined as a function of turbocharger speed at zero externally applied thrust load. Second, external thrust load up to 40 N was applied onto the turbocharger bearing at fixed rotational speeds of nTC = 40,000, 80,000, and 120,000 1/min. Increasing thrust load was observed to result in increasing friction losses amounting to a maximum of 32%. At last, a specially prepared turbocharger center section with deactivated thrust bearing was investigated. A comparison of these results with the measurement of the conventional bearing system under thrust-free conditions allowed separating journal and thrust bearing losses. The contribution of the thrust bearing to the overall bearing losses appeared to be as high as 38%. Furthermore, a modeling approach for estimating the friction power of both fully floating journal bearing as well as thrust bearing is illustrated. This theoretical model is shown to predict friction losses reasonably well compared to the experimental results.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2015;137(8):082508-082508-9. doi:10.1115/1.4029624.

Experimental results indicating negative direct static stiffness of highly eccentric straight gas annular seals were very recently presented by Childs and Arthur (2013, “Static Destabilizing Behavior for Gas Annular Seals at High Eccentricity Ratios,” ASME Paper No. GT2013-94201). This instability occurred at zero rotation speed and at high eccentricities. Up to then only gas annular seals with zero rotation speed, operating in centered position and with choked exit section were known as being susceptible of developing negative direct static stiffness. The seals and the working conditions presented by Childs and Arthur (2013, “Static Destabilizing Behavior for Gas Annular Seals at High Eccentricity Ratios,” ASME Paper No. GT2013-94201) had clearly no choked exit section. The present work advances a theoretical explanation of results reported by Childs and Arthur (2013, “Static Destabilizing Behavior for Gas Annular Seals at High Eccentricity Ratios,” ASME Paper No. GT2013-94201). The analysis is based on the numerical solution of the bulk flow equations of the flow in the annular seal. Theoretical results show a negative static stiffness at high eccentricities and zero rotation speeds. Other seal geometries and working conditions were tested and showed that the decrease of the direct static stiffness at high eccentricities and zero rotation speeds is a characteristic of all straight annular seals whether the fluid is compressible or not. Nevertheless with increasing rotation speed, the static stiffness becomes again positive and may increase with increasing eccentricity. The negative static stiffness is then limited to very specific working conditions: high eccentricities and zero rotation speed.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Turbomachinery

J. Eng. Gas Turbines Power. 2015;137(8):082601-082601-11. doi:10.1115/1.4029599.

The exhaust hood of a steam turbine is an important area of turbomachinery research as its performance strongly influences the power output of the last stage blades (LSB). This paper compares results from 3D simulations using a novel application of the nonlinear harmonic (NLH) method with more computationally demanding predictions obtained using frozen rotor techniques. Accurate simulation of exhausts is only achieved when simulations of LSB are coupled to the exhaust hood to capture the strong interaction. One such method is the NLH method. In this paper, the NLH approach is compared against the current standard for capturing the inlet circumferential asymmetry, the frozen rotor approach. The NLH method is shown to predict a similar exhaust hood static pressure recovery and flow asymmetry compared with the frozen rotor approach using less than half the memory requirement of a full annulus calculation. A second option for reducing the computational demand of the full annulus frozen rotor method is explored where a single stator passage is modeled coupled to the full annulus rotor by a mixing plane. Provided the stage is choked, this was shown to produce very similar results to the full annulus frozen rotor approach but with a computational demand similar to that of the NLH method. In terms of industrial practice, the results show that for a typical well designed exhaust hood at nominal load conditions, the pressure recovery predicted by all methods (including those which do not account for circumferential uniformities) is similar. However, this is not the case at off-design conditions where more complex interfacing methods are required to capture circumferential asymmetry.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2015;137(8):082602-082602-8. doi:10.1115/1.4029577.

Supercritical CO2 (S-CO2) power cycles have demonstrated significant performance improvements in concentrated solar and nuclear applications. These cycles promise an increase in thermal-to-electric conversion efficiency of up to 50% over conventional gas turbines (Wright, S., 2012, “Overview of S-CO2 Power Cycles,” Mech. Eng., 134(1), pp. 40–43), and have become a priority for research, development, and deployment. In these applications the CO2 is compressed to pressures above the critical value using radial compressors. The thermodynamic state change of the working fluid is close to the critical point and near the vapor–liquid equilibrium region where phase change effects are important. This paper presents a systematic assessment of condensation on the performance and stability of centrifugal compressors operating in S-CO2. The approach combines numerical simulations with experimental tests. The objectives are to assess the relative importance of two-phase effects on the internal flow behavior and to define the implications for radial turbomachinery design. The condensation onset is investigated in a systematic manner approaching the critical point. A nondimensional criterion is established that determines whether condensation might occur. This criterion relates the time required for stable liquid droplets to form, which depends on the expansion through the vapor–pressure curve, and the residence time of the flow under saturated conditions. Two-phase flow effects can be considered negligible when the ratio of the two time scales is much smaller than unity. The study shows that condensation is not a concern away from the critical point. Numerical two-phase calculations supported by experimental data indicate that the timescale associated with nucleation is much longer than the residence time of the flow in the saturated region, leaving little opportunity for the fluid to condense. Pressure measurements in a converging diverging nozzle show that condensation cannot occur at the level of subcooling characteristic of radial compressors away from the critical point. The implications are not limited to S-CO2 power cycles but extend to applications of radial machines for dense, saturated gases. In the immediate vicinity of the critical point, two-phase effects are expected to become more prominent due to longer residence times. However, the singular behavior of thermodynamic properties at the critical point prevents the numerical schemes from capturing important gas dynamic effects. These limitations require experimental assessment, which is the focus of ongoing and future research.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2015;137(8):082603-082603-7. doi:10.1115/1.4029482.

A simplified one-dimensional model for the performance estimation of vaneless radial diffusers is presented. The starting point of such a model is that angular momentum losses occurring in vaneless diffusers are usually neglected in the most common turbomachinery textbooks: It is assumed that the angular momentum is conserved inside a vaneless diffuser, although a nonisentropic pressure transformation is considered at the same time. This means that fluid-dynamic losses are taken into account only for what concerns pressure recovery, whereas the evaluation of the outlet tangential velocity incoherently follows an ideal behavior. Several attempts were presented in the past in order to consider the loss of angular momentum, mainly solving a full set of differential equations based on the various developments of the initial work by Stanitz (1952, “One-Dimensional Compressible Flow in Vaneless Diffusers of Radial or Mixed-Flow Centrifugal Compressors, Including Effects of Friction, Heat Transfer and Area Change,” Report No. NACA TN 2610). However, such formulations are significantly more complex and are based on two empirical or calibration coefficients (skin friction coefficient and dissipation or turbulent mixing loss coefficient) which need to be properly assessed. In the present paper, a 1D model for diffuser losses computation is derived considering a single loss coefficient, and without the need of solving a set of differential equations. The model has been validated against massive industrial experimental campaigns, in which several diffuser geometries and operating conditions have been considered. The obtained results confirm the reliability of the proposed approach, able to predict the diffuser performance with negligible drop of accuracy in comparison with more sophisticated techniques. Both preliminary industrial designs and experimental evaluations of the diffusers may benefit from the proposed model.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2015;137(8):082604-082604-7. doi:10.1115/1.4029600.

The structure of in-cylinder flow field makes significant impacts on the processes of fuel injection, air–fuel interactions, and flame development in internal combustion engines. In this study, the implementation of time-resolved particle image velocimetry (PIV) in an optical engine is presented. Flow field PIV images at different crank angles have been taken using a high-speed double-pulsed laser and a high-speed camera with seeding particles mixed with the intake air. This study is focused on measuring the flow fields on the swirl plane at 30 mm below the injector tip under various intake air swirl ratios. A simple algorithm is developed to identify the vortex structure and to track the location and motion of vortex center at different crank angles. Proper orthogonal decomposition (POD) has been used to extract the ensemble and variation information of the vortex structure. Experimental results reveal that strong cycle-to-cycle variations exist in almost all test conditions. The vortex center is difficult to identify since multiple, but small scale, vortices exist during the early stage of the intake stroke. However, during the compression stroke when only one vortex center exists in most cycles, the motion of vortex center is found to be quite similar at different intake swirl ratios and engine speeds. This is due to the dominant driving force exerted by the piston’s upward motion on the in-cylinder air.

Commentary by Dr. Valentin Fuster

Technical Brief

J. Eng. Gas Turbines Power. 2015;137(8):084501-084501-6. doi:10.1115/1.4029391.

The bypass dual throat nozzle (BDTN) does not consume any secondary injection from the other part of the engine, while it can produce steady and efficient vectoring deflection similar to the conventional dual throat nozzle (DTN). A BDTN model has been designed and subjected to dynamic experimental study. The main results show that: (1) The frequency spectrums of the dynamic pressures are different between each thrust vector state. (2) The variation rates of dynamic vector of the new BDTN can reach as high as 50 deg/s, 40 deg/s, and 34 deg/s under nozzle pressure ratio NPR = 3, 5, and 10, separately. (3)The dynamic hysteresis time is less than 1 ms.

Topics: Pressure , Thrust , Nozzles
Commentary by Dr. Valentin Fuster

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