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Research Papers: Gas Turbines: Manufacturing, Materials, and Metallurgy

J. Eng. Gas Turbines Power. 2013;135(11):112101-112101-8. doi:10.1115/1.4025039.

The goal of this work is to establish simple condition monitoring principles for diagnosing the depth and location of transverse fatigue cracks in a rotordynamic system. The success of an on-line crack diagnosis regimen hinges on the accuracy of the crack model, which should account for the crack's depth and location. Two gaping crack models are presented; the first emulates a finite-width notch typically manufactured for experimental purposes, while the second models a gaping fatigue crack. The rotordynamic model used herein is based upon an available overhung rotordynamic test rig that was originally constructed to monitor the dynamics of a mechanical face seal. Four degree-of-freedom, linear equations of motion for both crack models are presented and discussed. Free and forced response analyses are presented, emphasizing results applicable to condition monitoring and, particularly, to diagnosing the crack parameters. The results demonstrate that two identifiers are required to diagnose the crack parameters: the 2X resonance shaft speed and the magnitude of the angular 2X subharmonic resonance. First, a contour plot of the 2X resonance shaft speed versus crack depth and location is generated. The magnitude of the 2X resonance along the desired 2X frequency contour is then obtained, narrowing the possible pairs of crack location and depth to either one or two possibilities. Practical aspects of the suggested diagnostic procedure are discussed, as well as qualitative observations concerning crack detection.

Commentary by Dr. Valentin Fuster

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

J. Eng. Gas Turbines Power. 2013;135(11):111501-111501-9. doi:10.1115/1.4025043.

In most dry, low-NOx combustor designs of stationary gas turbines, the front panel impingement cooling air is directly injected into the combustor primary zone. This air partially mixes with the swirling flow of premixed reactants from the burner and reduces the effective equivalence ratio in the flame. However, local unmixedness and the lean equivalence ratio are supposed to have a major impact on combustion performance. The overall goal of this investigation is to answer the question of whether the cooling air injection into the primary combustor zone has a beneficial effect on combustion stability and NOx emissions or not. The flame stabilization of a typical swirl burner with and without front panel cooling air injection is studied in detail under atmospheric conditions close to the lean blowout limit (LBO) in a full-scale, single-burner combustion test rig. Based on previous isothermal investigations, a typical injection configuration is implemented for the combustion tests. Isothermal results of experimental studies in a water test rig adopting high-speed planar laser-induced fluorescence (HSPLIF) reveal the spatial and temporal mixing characteristics for the experimental setup studied under atmospheric combustion. This paper focuses on the effects of cooling air injection on both flame dynamics and emissions in the reacting case. To reveal dependencies of cooling air injection on combustion stability and NOx emissions, the amount of injected cooling air is varied. OH*-chemiluminescence measurements are applied to characterize the impact of cooling air injection on the flame front. Emissions are collected for different cooling air concentrations, both global measurements at the chamber exit, and local measurements in the region of the flame front close to the burner exit. The effect of cooling air injection on pulsation level is investigated by evaluating the dynamic pressure in the combustor. The flame stabilization at the burner exit changes with an increasing degree of dilution with cooling air. Depending on the amount of cooling, only a specific share of the additional air participates in the combustion process.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2013;135(11):111502-111502-7. doi:10.1115/1.4025001.

A design for thermo-acoustic stability (DeTAS) procedure is presented that aims at selecting the most stable burner geometry for a given combustor. It is based on the premise that a thermo-acoustic stability model of the combustor can be formulated and that a burner design exists, which has geometric design parameters that sufficiently influence the dynamics of the flame. Describing the flame dynamics in dependence of the geometrical parameters, an optimization procedure involving a linear stability model of the target combustor, maximizes the damping and thereby yields the optimal geometrical parameters. To demonstrate the procedure on an existing annular combustor a generic burner design was developed that features significant variability of dynamical flame response in dependence of two geometrical parameters. In this paper the experimentally determined complex burner acoustics and complex flame responses are described in terms of physics-based parametric models with excellent agreement between experimental and model data. It is shown that these model parameters correlate uniquely with the variation of the burner geometrical parameters, allowing interpolating the model with respect to the geometrical parameters. The interpolation is validated with experimental data.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2013;135(11):111503-111503-8. doi:10.1115/1.4025068.

The turbulent flame speed (ST) is proposed to be an indicator of the flashback propensity for hydrogen-rich fuel gases at gas turbine relevant conditions. Flashback is an inevitable issue to be concerned about when introducing fuel gases containing high hydrogen content to gas turbine engines, which are conventionally fueled with natural gas. These hydrogen-containing fuel gases are present in the process of the integrated gasification combined cycle (IGCC), with and without precombustion carbon capture, and both syngas (H2 + CO) and hydrogen with various degrees of inert dilution fall in this category. Thus, a greater understanding of the flashback phenomenon for these mixtures is necessary in order to evolve the IGCC concept (either with or without carbon capture) into a promising candidate for clean power generation. Compared to syngas, the hydrogen-rich fuel mixtures exhibit an even narrower operational envelope between the occurrence of lean blow out and flashback. When flashback occurs, the flame propagation is found to occur exclusively in the boundary layer of the pipe supplying the premixed fuel/air mixture to the combustor. This finding is based on the experimental investigation of turbulent lean-premixed nonswirled confined jet flames for three fuel mixtures with H2 > 70 vol. %. Measurements were performed up to 10 bar at a fixed bulk velocity at the combustor inlet (u0 = 40 m/s) and preheat temperature (T0 = 623 K). Flame front characteristics were retrieved via planar laser-induced fluorescence of the hydroxyl radical (OH-PLIF) diagnostics and the turbulent flame speed (ST) was derived, accordingly, from the perspective of a global consumption rate. Concerning the flashback limit, the operational range of the hydrogen-rich mixtures is found to be well represented by the velocity gradients prescribed by the flame (gc) and the flow (gf), respectively. The former (gc) is determined as ST/(Le × δL0), where Le is the Lewis number and δL0 is the calculated thermal thickness of the one-dimensional laminar flame. The latter (gf) is predicted by the Blasius correlation for fully developed turbulent pipe flow and it indicates the capability with which the flow can counteract the opposed flame propagation. Our results show that the equivalence ratios at which the two velocity gradients reach similar levels correspond well to the flashback limits observed at various pressures. The methodology is also found to be capable of predicting the aforementioned difference in the operational range between syngas and hydrogen-rich mixtures.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2013;135(11):111504-111504-8. doi:10.1115/1.4025078.

The paper presents a one-dimensional approach to assess the reduction potential of NOx emissions for lean premixed gas turbine combustion systems. NOx emissions from these systems are known to be mainly caused by high temperatures, not only from an averaged perspective but especially related to poor mixing quality of fuel and air. The method separates the NOx chemistry in the flame front zone and the postflame zone (slow reaction). A one-dimensional treatment enables the use of detailed chemistry. A lookup table parameterized by reaction progress and equivalence ratio is used to improve the computational efficiency. The influence of mixing quality is taken into account by a probability density function of the fuel element–based equivalence ratio, which itself translates into a temperature distribution. Hence, the NOx source terms are a function of reaction progress and equivalence ratio. The reaction progress is considered by means of the two-zone approach. Based on unsteady computational fluid dynamics (CFD) data, the evolution of the probability density function with residence time has been analyzed. Two types of definitions of an unmixedness quantity are considered. One definition accounts for spatial as well as temporal fluctuations, and the other is based on the mean spatial distribution. They are determined at the location of the flame front. The paper presents a comparison of the modeled results with experimental data. A validation and application have shown very good quantitative and qualitative agreement with the measurements. The comparison of the unmixedness definitions has proven the necessity of unsteady simulations. A general emissions-unmixedness correlation can be derived for a given combustion system.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2013;135(11):112502-112502-9. doi:10.1115/1.4025065.

One of the main challenges of the present industrial research on centrifugal compressors is the need for extending the left margin of the operating range of the machines. As a result, interest is being paid to accurately evaluating the amplitude of the pressure fluctuations caused by rotating stall, which usually occurs prior to surge. The related aerodynamic force acting on the rotor can produce subsynchronous vibrations, which can prevent the machine's further operation, in case their amplitude is too high. These vibrations are often contained due to the stiffness of the oil journals. Centrifugal compressor design is, however, going towards alternative journal solutions having lower stiffness levels (e.g., active magnetic bearings or squeeze film dampers), which will be more sensitive to this kind of excitation: consequently, a more accurate estimation of the expected forces in the presence of dynamic external forces such as those connected to an aerodynamically unstable condition is needed to predict the vibration level and the compressor operability in similar conditions. Within this scenario, experimental tests were carried out on industrial impellers operating at high peripheral Mach numbers. The dedicated test rig was equipped with several dynamic pressure probes that were inserted in the gas flow path; moreover, the rotor vibrations were constantly monitored with typical vibration probes located near the journal bearings. The pressure field induced by the rotating stall in the vaneless diffuser was reconstructed by means of an ensemble average approach, thus defining the amplitude and frequency of the external force acting on the impeller. The calculated force value was then included in the rotordynamic model of the test rig: the predicted vibrations on the bearings were compared with the measurements, showing satisfactory agreement. Moreover, the procedure was applied to two real multistage compressors, showing notable prediction capabilities in the description of rotating stall effects on the machine rotordynamics. Finally, the prospects of the proposed approach are discussed by investigating the response of a real machine in high-pressure functioning when different choices of journal bearings are made.

Commentary by Dr. Valentin Fuster

Research Papers: Power Engineering

J. Eng. Gas Turbines Power. 2013;135(11):113001-113001-11. doi:10.1115/1.4025077.

Hybrid solar micro gas-turbines are a promising technology for supplying controllable low-carbon electricity in off-grid regions. A thermoeconomic model of three different hybrid micro gas-turbine power plant layouts has been developed, allowing their environmental and economic performance to be analyzed. In terms of receiver design, it was shown that the pressure drop is a key criterion. However, for recuperated layouts, the combined pressure drop of the recuperator and receiver is more important. In terms of both electricity costs and carbon emissions, the internally-fired recuperated micro gas-turbine was shown to be the most promising solution of the three configurations evaluated. Compared to competing diesel generators, the electricity costs from hybrid solar units are between 10% and 43% lower, while specific CO2 emissions are reduced by 20–35%.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Coal, Biomass, and Alternative Fuels

J. Eng. Gas Turbines Power. 2013;135(11):111401-111401-7. doi:10.1115/1.4025128.

Historically, gas turbine fuels have been procured based on availability and low cost criteria. However,in the past few decades, with the growing concern over the negative environmental impacts produced by emissions, alternative fuels have been developed and tested under the objective of reducing such negative effects. The physical properties and broad chemical composition of fuels, including trace elements, may result in engine performance issues found only after extensive operation. This, in turn, results in higher maintenance and operation costs. This paper studies the feasibility of several renewable fuels for microturbine application, identifying key relationships between the physical and chemical properties, thermal stability, materials compatibility, and turbine performance.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2013;135(11):111402-111402-8. doi:10.1115/1.4025040.

In order to produce less emission of greenhouse gases, many studies have been done on the binary-cycle geothermal power plant to obtain better performance. The radial turbo expanders play an important role in the performance of the organic rankine cycle (ORC) for binary-cycle geothermal plants. However, few studies have investigated the effect of parameters of radial turbo expanders on the performance of the ORC. In this paper, a new thermodynamic model of the ORC coupled with the preliminary design of radial turbo expanders is developed. The effects of geothermal water temperature on the ORC performance parameters, such as power output and thermal efficiency are investigated by using the proposed thermodynamic model. The variation of radial turbo expanders’ parameters, such as specific rotational speed with geothermal water temperature is revealed. In the present study, the reasonable efficiency of radial turbo expanders by using the preliminary design is adopted to analyze the performance of the ORC, and an accurate reference about the effect of geothermal source on the parameters of radial turbo expanders is obtained.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Industrial & Cogeneration

J. Eng. Gas Turbines Power. 2013;135(11):112001-112001-8. doi:10.1115/1.4025069.

Equipment sizing decisions in the oil and gas industry often have to be made based on incomplete data. Often, the exact process conditions are based on numerous assumptions about well performance, market conditions, environmental conditions, and others. Since the ultimate goal is to meet production commitments, the traditional method of addressing this is to use worst case conditions and often adding margins onto these. This will invariably lead to plants that are oversized, in some instances, by large margins. In reality, the operating conditions are very rarely the assumed worst case conditions, however, they are usually more benign most of the time. Plants designed based on worst case conditions, once in operation, will, therefore, usually not operate under optimum conditions, have reduced flexibility, and therefore cause both higher capital and operating expenses.

The authors outline a new probabilistic methodology that provides a framework for more intelligent process-machine designs. A standardized framework using a Monte Carlo simulation and risk analysis is presented that more accurately defines process uncertainty and its impact on machine performance. Case studies are presented that highlight the methodology as applied to critical turbomachinery.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2013;135(11):112002-112002-7. doi:10.1115/1.4025037.

A 40 MW-class test facility has been constructed to verify practicability of applying the advanced humid air turbine (AHAT) system to a heavy-duty gas turbine. Verification tests have been carried out from January 2012, and interaction effects between the key components were established. First, water atomization cooling (WAC) was confirmed to contribute to both increased mass flow rate and pressure ratio for the axial-flow compressor. The good agreement between measured and calculated temperatures at the compressor discharge was also confirmed. These results demonstrated the accuracy of the developed prediction model for the WAC. Second, a control method that realized both flame stability and low nitrogen oxides (NOx) emissions was verified. Although the power output and air humidity were lower than the rated values, NOx concentration was about 10 ppm. Finally, a hybrid nozzle cooling system, which utilized both compressor discharged air and humid air, was developed and tested. The metal surface temperatures of the first stage nozzles were measured, and they were kept under the permissible metal temperature. The measured temperatures on the metal surface reasonably corresponded with calculation results.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Aircraft Engine

J. Eng. Gas Turbines Power. 2013;135(11):111201-111201-9. doi:10.1115/1.4025066.

Progress in the development of electrical storage and conversion technology progressively attains focus in aerospace motive power research. Novel propulsion system concepts based on hybrid or even entirely electrical energy sources are seriously considered for aircraft design. To this point, unified figures of merit are required in order to allow for consistent comparative investigations of existing combustion engines and future electrically-based propulsion systems. Firstly, this paper identifies the shortcomings of conventional performance metrics used for nonthermal electrical conversion processes and then approaches exergy-based loss methods as means of metrics extensions. Subsequently, energy source-independent figures of merit based on exergy analysis are derived and embedded into the well-known performance definitions. Finally, the unified metrics are demonstrated through application to a conventional turbofan, a parallel-hybrid turbofan, a novel integrated-hybrid turbofan concept, and an entirely electrical fan concept.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Structures and Dynamics

J. Eng. Gas Turbines Power. 2013;135(11):112501-112501-11. doi:10.1115/1.4024867.

The objective of this work was to assess and understand the effects of a parametric variation performed on a typical overlapping rim seal geometry. The datum geometry has been the focus of a detailed experimental investigation employing a large-scale linear cascade subjected to a range of the mass flow rates and swirl velocities of the leakage air. The parametric variations described in this paper were examined using validated computational fluid dynamics (CFD). As a part of the parametric studies, both the axial and the radial seal clearance between the rotor fin (angel wing) and stator platform were varied as well as the length of the overlap between stator and rotor platforms. In addition, the effects of forward and backward facing annulus steps were also investigated. It has been found that a backward-facing annulus step was detrimental for all conditions considered, while a forward-facing step offered improvements for smaller step heights and/or lower leakage fractions. Tightening of the seal clearances closer to the annulus line improved the sealing effectiveness but often at the expense of increased losses. On the other hand, increasing the overlap length led to improvements in the sealing effectiveness with very small effects on the overall losses. Moving the rim seal away from the blade-leading edges reduced the pressure asymmetry at the rim seal and increased the flow uniformity of the leakage air. However, this led to an increased cross-passage flow (more negative skew) and higher losses at all but lowest leakage fractions. The results presented in this paper highlight the fact that there may not be an optimum rim seal solution that would offer an improvement for the full range of leakage fractions and that, for different rim sealing flows, there may be a different optimum geometry. In addition, rotor disk movements in radial and axial directions at various off-design conditions should be considered as a part of the design process. Based on the presented results, it may be of a benefit to the turbine designer to consider rotor disk designs that would be more biased towards the upstream and outward disk movements, which would result in tightening of the seal clearances and avoidance of a backward-facing annulus step.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2013;135(11):112503-112503-9. doi:10.1115/1.4025033.

Aircraft engine rotors, invariably supported on rolling element bearings with little damping, are particularly sensitive to rotor imbalance and sudden maneuver loads. Most engines incorporate squeeze film dampers (SFDs) as a means to dissipate mechanical energy from rotor motions and to ensure system stability. The paper experimentally quantifies the dynamic forced performance of two end sealed SFDs with dimensions and an operating envelope akin to those in actual jet engine applications. The current experimental results complement and extend prior research conducted with open ends SFDs (San Andrés, 2012, “Damping and Inertia Coefficients for Two Open Ends Squeeze Film Dampers With a Central Groove: Measurements and Predictions,” ASME J. Eng. Gas Turbines Power, 134, p. 102506). In the tests, two journals make for two SFD configurations, both with a diameter D = 127 mm and nominal radial film clearance c = 0.127 mm. One short length damper has film lands with extent L = 12.7 mm, while the other has 25.4 mm ( = 2L) land lengths. A central groove of length LG = L and depth at ¾ L separates the film lands. A light viscosity lubricant is supplied into the central groove via three orifices, 120 deg apart, and then flows through the film lands whose ends are sealed with tight piston rings. The oil pushes through the piston rings to discharge at ambient pressure. In the tests, a static load device pulls the damper structure to increasing eccentricities (maximum 0.38c) and external shakers exert single-frequency loads 50–250 Hz, inducing circular orbits with amplitudes equaling ∼5% of the film clearance. The lubricant feed and groove pressures and flow rates through the top and bottom film lands are recorded to determine the flow resistances through the film lands and the end seals. Measured dynamic pressures in the central groove are as large as those in the film lands, thus demonstrating a strong flow interaction, further intensified by the piston ring end seals which are effective in preventing side leakage. Dynamic pressures and reaction loads are substantially higher than those recorded with the open ends dampers. Comparisons to test results for two identical damper configurations but open ended (San Andrés, 2012, “Damping and Inertia Coefficients for Two Open Ends Squeeze Film Dampers With a Central Groove: Measurements and Predictions,” ASME J. Eng. Gas Turbines Power, 134, p. 102506) demonstrate at least a threefold increase in direct damping coefficients and no less than a double increment in added mass coefficients. Predictions from a physics-based model that includes the central groove, the lubricant feed holes, and the end seals' flow conductances are in agreement with the test results for the short length damper. For the long damper, the predicted damping coefficients are in good agreement with the measurements, while the added masses are under-predicted by ∼25%.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2013;135(11):112504-112504-7. doi:10.1115/1.4025067.

The paper presents results from film thickness measurements in a bearing chamber test rig. The measurements were performed at different operating conditions and with two offtake designs. A discussion of the measurement technique using in situ calibrated capacitive sensors shows good accuracy and repeatability. The film thickness results are first compared to measurements with the same chamber in a vented configuration. The analysis of the measurements at various operating conditions shows a strong influence of the shaft speed, the chamber pressure, and the offtake design. In contrast to that, flow rate and scavenge ratio have only minor influence. Furthermore, the momentum flux of the core air flow is proposed as a suitable parameter with which the influence of shaft speed and pressure can be correlated to the film thickness distribution.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2013;135(11):112505-112505-11. doi:10.1115/1.4025070.

Recent comprehensive experimental data showcasing the force coefficients of commercial size tilting pad journal bearings has brought to rest the long standing issue on the adequacy of the [K,C,M] physical model to represent frequency independent bearing force coefficients, in particular viscous damping. Most experimental works test tilting pad journal bearings (TPJBs) with large preloads, operating over a large wide range of rotor speeds, and with null to beyond normal specific loads. Predictions from apparently simple fluid film bearing models stand poor against the test data which invariably signals to theory missing pivot and pad flexibility effects, and most importantly, ignoring significant differences in bearing and pad clearances due to actual operation, poor installation and test procedures, or simply errors in manufacturing and assembly. Presently, a conventional thermo hydrodynamic bulk flow model for prediction of the pressure and temperature fields in TPJBs is detailed. The model accounts for various pivot stiffness types, all load dependent and best when known empirically, and allows for dissimilar pad and bearing clearances. The algorithm, reliable even for very soft pad-pivots, predicts frequency reduced bearing impedance coefficients and over a certain frequency range delivers the bearing stiffness, damping and virtual mass force coefficients. Good correlation of predictions against a number of experimental results available in the literature bridges the gap between a theoretical model and the applications. Predicted pad reaction loads reveal large pivot deflections, in particular for a bearing with large preloaded pads, with significant differences in pivot stiffness as a function of specific load and operating speed. The question on how pivot stiffness acts to increase (or decrease) the bearing force coefficients, in particular the dynamic stiffness versus frequency, remains since the various experimental data show contradictory results. A predictive study with one of the test bearings varies its pivot stiffness from 10% of the fluid film stiffness to an almost rigid one, 100 times larger. With certainty, bearings with nearly rigid pivot stiffness show frequency independent force coefficients. However, for a range of pad pivot stiffness, 1/10 to ten times the fluid film stiffness, TPJB impedances vary dramatically with frequency, in particular as the excitation frequency grows above synchronous speed. The bearing virtual mass coefficients become negative, thus stiffening the bearing for most excitation frequencies.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2013;135(11):112506-112506-10. doi:10.1115/1.4025079.

Implementing gas foil bearings (GFBs) in micro gas turbine engines is a proven approach to improve system efficiency and reliability. Adequate thermal management for operation at high temperatures, such as in a gas turbine or a turbocharger, is important to control thermal growth of components and to remove efficiently mechanical energy from the rotor mainly. The paper presents a test rotor supported on GFBs operating with a heated shaft and reports components temperatures and shaft motions at an operating speed of 37 krpm. An electric cartridge heater loosely inserted in the hollow rotor warms the test system. Thermocouples and noncontact infrared thermometers record temperatures on the bearing sleeve and rotor outer diameter (OD), respectively. No forced cooling air flow streams were supplied to the bearings and rotor, in spite of the high temperature induced by the heater on the shaft outer surface. With the rotor spinning, the tests consisted of heating the rotor to a set temperature, recording the system component temperatures until reaching thermal equilibrium in  ∼ 60 min, and stepping the heater set temperature by 200 °C. The experiments proceeded without incident until the heater set temperature equaled 600 °C. Ten minutes into the test, noise became apparent and the rotor stopped abruptly. The unusual operating condition, without cooling flow and a too large increment in rotor temperature, reaching 250 °C, led to the incident which destroyed one of the foil bearings. Post-test inspection evidenced seizure of the hottest bearing (closest to the heater) with melting of the top foil at the locations where it rests on the underspring crests (bumps). Analysis reveals a notable reduction in bearing clearance as the rotor temperature increases until seizure occurs. Upon contact between the rotor and top foil, dry-friction quickly generated vast amounts of energy that melted the protective coating and metal top foil. Rather than a reliability issue with the foil bearings, the experimental results show poor operating procedure and ignorance on the system behavior (predictions). A cautionary tale and a lesson in humility follow.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Cycle Innovations

J. Eng. Gas Turbines Power. 2013;135(11):111701-111701-12. doi:10.1115/1.4025127.

The U.S. Department of Energy is currently focused on the development of next-generation nuclear power reactors, with an eye towards improved efficiency and reduced capital cost. To this end, reactors using a closed-Brayton power conversion cycle have been proposed as an attractive alternative to steam turbines. The supercritical-CO2 recompression cycle has been identified as a leading candidate for this application since it can achieve high efficiency at relatively low operating temperatures with extremely compact turbomachinery. Sandia National Laboratories has been a leader in hardware and component development for the supercritical-CO2 cycle. With contractor Barber-Nichols Inc., Sandia has constructed a megawatt-class S-CO2 cycle test-loop to investigate the key areas of technological uncertainty for this power cycle and to confirm model estimates of advantageous thermodynamic performance. Until recently, much of the work has centered on the simple S-CO2 cycle—a recuperated Brayton loop with a single turbine and compressor. However, work has recently progressed to a recompression cycle with split-shaft turbo-alternator-compressors, unlocking the potential for much greater efficiency power conversion, but introducing greater complexity in control operations. The following sections use testing experience to frame control actions made by test loop operators in bringing the recompression cycle from cold startup conditions through transition to power generation on both turbines, to the desired test conditions, and finally to a safe shutdown. During this process, considerations regarding the turbocompressor thrust state, CO2 thermodynamic state at the compressor inlet, compressor surge and stall, turbine u/c ratio, and numerous other factors must be taken into account. The development of these procedures on the Sandia test facility has greatly reduced the risk to industry in commercial development of the S-CO2 power cycle.

Commentary by Dr. Valentin Fuster

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