Successful implementation of sCO 2 power cycles requires high compressor efficiency at both the design-point and over a wide operating range in order to maximize cycle power output and maintain stable operation over a wide range of transient and part-load operating conditions. This requirement is particularly true for air-cooled cycles where compressor inlet density is a strong function of inlet temperature that is subject to daily and seasonal variations as well as transient events. In order to meet these requirements, a novel centrifugal compressor stage design was developed that incorporates multiple novel range extension features, including a passive recirculating casing treatment and semi-open impeller design. This design, presented and analyzed for CO 2 operation in a previous paper, was fabricated via direct metal laser sintering and tested in an open-loop test rig in order to validate simulation results and the effectiveness of the casing treatment configuration. Predicted performance curves in air and CO2 conditions are compared, resulting in a reduced diffuser width requirement for the air test in order to match design velocities and demonstrate the casing treatment. Test results show that the casing treatment performance generally matched CFD predictions, demonstrating an operating range of 69% and efficiency above air predictions across the entire map. The casing treatment configuration demonstrated improvements over the solid wall configuration in stage performance and flow characteristics at low flows, resulting in an effective 14% increase in operating range with a 0.5-point efficiency penalty. The test results are also compared to a traditional fully shrouded impeller with the same flow coefficient and similar head coefficient, showing a 42% range improvement over traditional designs.

Many energy supply systems around the world are currently undergoing a phase of transition characterized by a continuing increase in installed renewable power generation capacities. The inherent volatility and limited predictability of renewable power generation pose various challenges for an efficient system integration of these capacities. One approach to manage these challenges is the deployment of small-scale dispatchable power generation and storage units on a local level. In this context, gas turbine cogeneration units, which are primarily tasked with the provision of power and heat for industrial consumers, can play a significant role if they are equipped with a sufficient energy storage capacity allowing for a more flexible operation. The present study investigates a system configuration which incorporates a heat-driven industrial gas turbine interacting with a wind farm providing volatile renewable power generation. The required energy storage capacity is represented by an electrolyzer and a pressure vessel for intermediate hydrogen storage. The generated hydrogen can be reconverted to electricity and process heat by the gas turbine. The corresponding operational strategy for the overall system aims at an optimal integration of the volatile wind farm power generation on a local level. The study quantifies the impact of selected system design parameters on the quality of local wind power system integration that can be achieved with a specific set of parameters. In addition, the impact of these parameters on the reduction of CO 2 emissions due to the use of hydrogen as gas turbine fuel is quantified. In order to conduct these investigations, detailed steadystate models of all required system components were developed. These models enable accurate simulations of the operation of each component in the complete load range. The calculation of the optimal operational strategy is based on an application of the Dynamic Programming algorithm. Based on this model setup, the operation of the overall system configuration is simulated for each investigated set of design parameters for a one-year period. The simulation results show that the investigated system configuration has the ability to significantly increase the level of local wind power integration. The parameter variation reveals distinct correlations between the main design parameters of the storage system and the achievable level of local wind power integration. Regarding the installed electrolyzer power consumption capacity, smaller additional benefits of capacity increases can be identified at higher levels of power consumption capacity. Regarding the geometrical volume of the hydrogen storage, it can be determined that the storage volume loses its limiting character on the operation of the electrolyzer at a characteristic level. The additional investigation of the CO 2 emission reduction reveals a direct correlation between the level of local wind power integration and the achievable level of CO 2 emission reduction.

One of the most important problems facing gas turbine designers today is the ingestion of hot mainstream gases into the wheel-space between the turbine disc ( rotor ) and its adjacent casing ( stator ). A rim seal is fitted at the periphery and a superposed sealant flow — typically fed through the bore of the stator — is used to prevent ingress. The majority of research studies investigating ingress do so in the absence of any leakage paths that exist throughout the engine’s architecture. These inevitable pathways are found between the mating interfaces of adjacent pieces of hardware. In an environment where the turbine is subjected to aggressive thermal and centrifugal loading these interface gaps can be difficult to predict and the resulting leakage flows which pass through them even harder to account for. This paper describes experimental results from a research facility which experimentally models hot gas ingestion into the wheel-space of an axial turbine stage. The facility was specifically designed to incorporate leakage flows through the stator disc; leakage flows were introduced axially through the stator shroud or directly underneath the vane carrier ring. Measurements of CO 2 gas concentration, static pressure and total pressure were used to examine the wheel-space flow structure with and without ingress from the mainstream gas-path. Data is presented for a simple axial-clearance rim-seal. The results support two distinct flow-structures, which are shown to be dependent on the mass-flow ratio of bore and leakage flows. Once the leakage flow was increased above a certain threshold, the flow structure is shown to transition from a classical Batchelor-type rotor-stator system to a vortex-dominated structure. The existence of a toroidal vortex immediately inboard of the outer rim-seal is shown to encourage ingestion.

This paper presents a numerical investigation on the rationality and reliability of film cooling experiments, in which the realistic temperature ratio (TR) of mainstream to cooling air are substituted by the density ratio (DR) of ambient air to a foreign gas with high density. Today, advanced gas turbines operate at much higher temperatures than the allowable temperature of turbine component materials, which makes it very difficult to achieve real TR in the most of laboratory environments, because the real TRs are usually larger than 2.0. Therefore, the foreign gas (for example CO 2 ) with high density was widely used to obtain a proper density ratio for simulating the mixing process of cooling air with high temperature mainstream thereby. However, the TR effect on film cooling is not completely replaced by DR, because the influence factors of film cooling performances are not only DR, but also the thermal properties of cooling gas and mainstream, such as specific heat capacity, viscosity and conductivity. In this work, a film-cooled endwall is used as specimen, and DRs are controlled by two ways, i.e. changing mainstream temperatures, and using a foreign gas, respectively. The numerical results of film cooling performances at the same DR obtained by the two ways are analyzed and compared. The analysis reveals the difference of film cooling performances between TR and DR on the film-cooled endwall, and the comparison indicated that the errors caused by the substitution of foreign gas are acceptable, only when BR, DR, Re ∞ and T ∞ are all small, but when BR, DR or Re ∞ increases, the relative error cannot be neglected, and it may reach 30% in real running conditions.

Different forms of Reynolds equation are widely used to predict the performances of foil thrust bearings for air cycle machines. When analyzing bearings operating with highly dense CO 2 , computational fluid dynamics yields more accurate results, particularly at the high rotational speed. In addition, the structural deformation of the top and bump foils are also considered. For some applications, the high temperature increase caused by the viscous heating effect are also modelled in literature. The multi-physics effects within foil bearings, including the fluid flow, structural deformation and viscous heating create challenges and modelling complexity to accurately predict its performances. The aim of this paper is to review and compare different modelling approaches for foil thrust bearings with CO 2 at a range of operating conditions, including loads and rotational speed. For steady state performances, results from turbulent Reynolds equation and computational fluid dynamics are in close agreement for foil thrust bearings operating with low load (large rotor to top foil separations). However, considerable differences exist between turbulent Reynolds equation and computational fluid dynamics method at high loads (small rotor to top foil separation). Here the computational fluid dynamics method must be employed, as the centrifugal inertia effect becomes significant. The top foil deflection need to be considered as the corresponding deformation is significant compared to the initial separation between the rotor and the top foil. At the rotational speed larger than 30000 rpm, the results from the fully fluid-structure-thermal simulations differ from other modelling approaches. The additional deformation caused by temperature increase largely alters the separation between the rotor and top foil. For dynamic performance, the top foil deflection again must be considered as the equivalent stiffness and damping are influenced by bump foil structures. This work provides recommendations for the selection of the suitable modelling approaches for bump-type foil thrust bearings operating with supercritical CO 2 .

The primary need of reducing pollutant and greenhouse gas emissions has led to new energy scenarios. The interest of research community is mainly focused on the development of energy systems based on renewable resources and energy storage systems and smart energy grids. In the latter case small scale energy systems can become of interest as nodes of distributed energy systems. In this context micro gas turbines (MGT) can play a key role thanks to their flexibility and a strategy to increase their overall efficiency is to integrate gas turbines with a bottoming cycle. In this paper the authors analyze the possibility to integrate a MGT with a super critical CO 2 Brayton cycle turbine (sCO 2 GT) as a bottoming cycle (BC). A 0D thermodynamic analysis is used to highlight opportunities and critical aspects also by a comparison with another integrated energy system in which the waste heat recovery (WHR) is obtained by the adoption of an organic Rankine cycle (ORC). While ORC is widely used in case of middle and low temperature of the heat source, s-CO 2 BC is a new method in this field of application. One of the aim of the analysis is to verify if this choice can be comparable with ORC for this operative range, with a medium-low value of exhaust gases and very small power values. The studied MGT is a Turbec T100P.

700°C High Ultra Supercritical (HUSC) technology is taken into account as a more efficient clean coal-fired power generation technology which can achieve higher efficiency and less CO 2 emission. With the increase of the main steam and reheat steam temperature, the temperature of regenerative extraction increased accordingly. This not only means higher investment cost and higher unreliability of power plant, but also leads to a great reduction of energy grade efficiency. To solve the above-mentioned problem, we introduce a novel system, called echelon cycle system (EC system). In EC system, a BEST (Backpressure Extraction Steam Turbine) is added, which provides power for feed-water pump and steam for feed-water heaters. The steam source of high temperature regenerative extractions is switched from main turbine to BEST, and the steam source of BEST is cold-reheat. Hence the highest regenerative extraction steam temperature decreased accordingly. EC system has been demonstrated to be a more efficient system by exergy theory[1] and energy grade theory. Three types of EC system are proposed in this paper. Thermal performance calculation of these three types of EC system under rated-load condition and part-load condition is carried out to evaluate and compare the economy of system. In order to obtain a more appropriate thermodynamic system solution, safety and restriction should also be given sufficient consideration. Meanwhile, the matrix solution method for energy grade efficiency of EC system is derived in this paper. Finally, energy grade theory is used to analyze how different schemes cause different hate rate profits.

In this study, research activity concerning the utilization of biogas into a commercial micro gas turbine (MGT) Ansaldo Turbec T100P is presented. The use of biogas in MGT has been already quite explored in literature. However, conflicting and not yet fully comprehensive results highlight the need for further scientific investigations. Although several authors have demonstrated, through numerical studies, the possibility of using biogas with amounts of CO 2 up to 40–50% in volume in commercial MGT, experimental evidences of this condition are not yet present in literature. In fact, the CO 2 concentration limit experimentally validated in literature, after which phenomena of detachment and instability of the flame happen, is considerably lower (∼12%). The utilization of biogas in micro gas turbines generally requires structural adaptations, often invasive and expensive, to the fuel compression unit and control valves. Not infrequently a partial redesign of the combustor is often necessary. Costs associated with the modifications significantly reduce the benefits of a rational and sustainable exploitation of energy resources of lower value as the biogas. Aim of this activity is to overcome these issues, by identifying a non-invasive and cost-effective technical solution to increase the CO 2 concentration in the fuel and to extend the operation domain when low calorific value fuels are used. It has been studied, implemented and tested an innovative management strategy of fuel feeding and combustion. Experimental layout and preliminary results of this novel approach are shown and discussed.

The automotive low pressure exhaust gas recirculation or closed-cycle gas turbine engines are among applications where an alternative fluids to air are used. This paper analyses compressor performance measured while operating with substitute gas. Several compressor performance maps were generated and compared on a steady-state turbocharger test facility run in a closed-loop mode. The automotive centrifugal compressor was mapped with pure air, CO2-air mixtures (3%, 5%, 10% CO2 by mass) and pure CO2. The analysis of the obtained data has indicated that the use of reduced speed parameter and reduced mass flow parameter formulas worked well in the process of the compressor map correction for the CO2-air mixtures. The corrected (with reduced mass flow and speed parameters) compressor performance map generated for the case of pure CO2 has shown a shift in both choke and surge lines and changes in measured pressure ratio and efficiency. Further efficiency and pressure ratio corrections based on the method proposed by Roberts and Sjolander [1] have shown no match between the predicted and measured values of efficiency for CO2. The series of adiabatic 3D CFD simulations indicated that this was likely caused by the heat transfer between the uninsulated turbomachinery components.

In high-pressure turbines, cool air is purged through rim seals at the periphery of wheel-spaces between the stator and rotor discs. The purge suppresses the ingress of hot gas from the annulus but superfluous use is inefficient. In this paper the interaction between the ingress, purge and mainstream flow is studied using unsteady numerical simulations based on the DLR TRACE solver. The computations are compared to experimental measurements from a one-and-a-half stage axial-turbine rig operating with engine-representative blade and vane geometries, and overlapping rim seals. Radial traverses using a miniature CO 2 concentration probe quantified the penetration of ingress into the rim seal and the outer portion of the wheel-space. Unsteady pressure measurements from circumferentially-positioned transducers on the stator disc identified distinct frequencies in the wheel-space, and the computations reveal these are associated with large-scale flow structures near the outer periphery rotating at just less than the disc speed. It is hypothesised that the physical origin of such phenomenon is driven by Kelvin-Helmholtz instabilities caused by the tangential shear between the annulus and egress flows, as also postulated by previous authors. The presence and intensity of these rotating structures are strongly dependent on the purge flow rate. While there is general qualitative agreement between experiment and computation, it is speculated that the under-prediction by the computations of the measured levels of ingress is caused by deficiencies in the turbulence modelling.

Central targets for jet engine research activities comprise the evaluation of improved engine components and the assessment of novel engine concepts for enhanced overall engine performance in order to reduce the fuel consumption and emissions of future aircraft. Since CO 2 emissions are directly related to engine fuel burn, a reduction in fuel consumption leads to lower CO 2 emissions. Therefore improvements in engine technologies are still significant and a multi-disciplinary pre-design approach is essential in order to address all requirements and constraints associated with different engine concepts. Furthermore, an increase in effectiveness of the preliminary design process helps reduce the immense costs of the overall engine development. Within the DLR project PEGASUS (Preliminary Gas Turbine Assessment and Sizing) a multi-disciplinary pre-design and assessment competence of the DLR regarding aero engines and gas turbines was established. The application of modern preliminary design methods allows for the construction and evaluation of innovative next generation engine concepts. The purpose of this paper is to present the developed multi-disciplinary pre-design process and its application to three aero engine models. First, a state of the art twin spool mixed flow turbofan engine model is created for validation purposes. The second and third engine models investigated comprise future engine concepts: a Counter Rotating Open Rotor and an Ultra High Bypass Turbofan. The turbofan used for validation is based on publicly available reference data from manufacturing and emission certification. At first the identified interfaces and constraints of the entire pre-design process are presented. An important factor of complexity in this highly iterative procedure is the intricate data flow, as well as the extensive amount of data transferred between all involved disciplines and among the different fidelity levels applied within the smoothly connected design phases. To cope with the inherent complexity data modeling techniques have been applied to explicitly determine the required data structures of those complex systems. The resulting data model characterizing the components of a gas turbine and their relationships in the design process is presented in detail. Based on the established data model the entire engine pre-design process is presented. Starting with the definition of a flight mission scenario and the resulting top level engine requirements thermodynamic engine performance models are developed. By means of these thermodynamic models, a detailed engine component pre-design is conducted. The aerodynamic and structural design of the engine components are executed using a stepwise increase in level of detail and are continuously evaluated in the context of the overall engine system.

In the present paper, the synergistic combination of inter-cooling with pulsed detonation combustion is analyzed concerning its contribution to NO x and CO 2 emissions. CO 2 is directly proportional to fuel burn and can, therefore, be reduced by improving specific fuel consumption and reducing engine weight and nacelle drag. A model predicting NO x generation per unit of fuel for pulsed detonation combustors, operating with jet-A fuel, is developed and integrated within Chalmers University’s gas turbine simulation tool GESTPAN. The model is constructed using CFD data obtained for different combustor inlet pressure, temperature and equivalence ratio levels. The NO x model supports the quantification of the trade-off between CO 2 and NO x emissions in a 2050 geared turbofan architecture incorporating intercooling and pulsed detonation combustion and operating at pressures and temperatures of interest in gas turbine technology for aero-engine civil applications.

The MEE (More Electric Engine) is a concept for engine system electrification and is an evolutionary step in engine system design that contributes to the reduction of aviation CO 2 emissions. Mifee (Metering and integrated fuel feeding electrification) and the E3M (Engine Embedded Electric Machine) are the key technologies of the MEE. The purpose of engine thermal management is maintaining the balance between heat generation by the engine system and heat dissipation to the outside of the engine. In recent engine system designs, thermal system design has become an issue because of increased heat generation within the system. For example, a recently developed turbo-fan engine system increases the heat generation by introduction of a fan drive gear system that produces a large amount of heat in addition to the conventional heat source, such as engine main bearings and gears. The MEE will have further heat sources within its system, like the E3M, which is a high-power electric machine. In this paper, an investigation approach and the result of a feasibility study of the MEE thermal management system is described. In addition, the perspective of the technology trend from the MEE toward future hybrid propulsion is also discussed. The global requirements for climate protection strongly demand game-changing technology that significantly improves the aircraft’s overall efficiency. A series/parallel partial hybrid propulsion system, in which both a turbo-fan engine and electrical motor-driven fans generate propulsive power, is considered to be one of the most promising approaches for the future commercial aircraft hybrid propulsion system. The MEE and E3M technology evolves until it will be applied in hybrid propulsion system.

In the drive for greater increases in fuel efficiency and reductions in CO 2 emissions from aero engines, an epicyclic reduction gearbox can be used to break the link between the turbine and fan, enabling the engine to run at a higher bypass ratio. However, even small power losses can generate significant amounts of heat, due to the high loads transmitted from the gearbox. A substantial amount of cooling is required to remove this heat and a large part of this is supplied directly to the gear face. Assessing the performance of coolants and minimising the buildup of oil in the system is therefore a critical stage in the design process. Traditionally, finite volume CFD methods have been used to compute flow and heat transfer solutions. More recently, Lagrangian methods such as Smoothed Particle Hydrodynamics (SPH) have also been applied. The Lattice Boltzman Method (LBM) is a mesoscopic particle based method which uses statistical properties of particles based at each point of a lattice to calculate flow properties. This is a fully transient method and allows for a simple and efficient derivation of LES turbulence properties. In this work the Lattice Bolztman Method is used to investigate the impingement of an oil jet on a rotating spur gear. A comparison of LBM simulations is made against published work using other methods such as SPH and CFD — utilising the Volume of Fluid method — as well as a qualitative comparison with published experimental high speed images. These all show an excellent agreement and the simulations take the same order of magnitude of computational power as 3D single phase SPH, but are fully multiphase and have LES turbulence. This method is then used to investigate how changes to the oil feed delivery rate affect the spreading of the oil jet on the gear tooth and the splashing profiles. The potential for applying this method to other scenarios, such as lubricating and cooling meshing gears, is also discussed.

As bypass-ratio in modern aero engines is continuously increasing over the last decades, the radial offset between low pressure compressor (LPC) and high pressure compressor (HPC), which needs to be overcome by the connecting s-shaped intermediate compressor duct (ICD), is getting higher. Due to performance and weight saving aspects the design of shorter and therefore more aggressive ducts has become an important research topic. In this paper an already aggressive design (with respect to current aero engines) of an ICD with integrated outlet guide vane (OGV) is used as a baseline for an aerodynamic optimization. The aim is to shorten the duct even further while maintaining it separation free. The optimization is broken down into two steps. In the first optimization-step the baseline design is shortened to a feasible extent while keeping weak aerodynamic restrictions. The resulting highly aggressive duct (intermediate design), which is shortened by 19 % in axial length with respect to the baseline, shows separation tendencies of low momentum fluid in the strut/hub region. For the second step, the length of the optimized duct design is frozen. By implementing new design features in the process of the optimizer, this optimization-step aims to eliminate separation and to reduce separation tendencies caused by the aggressive shortening. In particular, these features are: a nonaxisymmetric endwall contouring and parametrization of the strut and the OGV to allow for changes in lift and turning in both blade designs. By comparison of the three designs: Baseline, intermediate (separating flow) and final design, it can be shown, that it is possible to decrease length of the already aggressive baseline design even further, when adding a nonaxisymmetric endwall contouring and changes in blade shape of the strut and OGV. Flow separation can be eliminated while losses are kept low. With a more aggressive and therefore shorter duct the engine length and weight can be reduced. This in turn leads to lighter aircrafts, less fuel consumption and lower CO 2 and NO x emissions.

This paper discusses a method for predicting hot gas migration into turbine disk cavities using two incompressible orifices for ingress and egress flows at the main flow path-to-rim seal interface. The method employs a flow network system that involves axisymmetric disk cavity flow solver elements to model inflows and outflows that exist in a turbine disk cavity subject to a simulated main gas path pressure field. A modified lumped-parameter two-orifice model in which ingress/egress discharge coefficients and the corresponding orifice physical areas are explicitly defined is demonstrated for predicting sealing effectiveness as part of a flow network system. This model has been calibrated against CO 2 concentration measurements results previously reported for a 1.5-stage disk cavity research rig at Arizona State University. The boundary pressures for the entire flow network system were defined to be the peak and trough pressures measured. Two fractional variables are introduced in defining the ingress orifice physical area and driving pressure difference between the main gas flow path and the rim cavity. An exponential curve-fit model for the ingress area fraction was developed to compute orifice egress discharge coefficients for a pre-determined ingress discharge coefficient. Excellent agreement with sealing effectiveness data was observed for a radial-gap rim seal geometry for the tested ranges of rotational Reynolds number and non-dimensional purge flow rate. Assumptions employed in developing the model were supported by a 90 degree sector RANS CFD result for the same disk cavity geometry and flow conditions.

In modern lean burn aero-engine combustors, highly swirling flow structures are adopted to control the fuel-air mixing and to provide the correct flame stabilization mechanisms. Aggressive swirl fields and high turbulence intensities are hence expected in the combustor-turbine interface. Moreover, to maximize the engine cycle efficiency, an accurate design of the high pressure nozzle cooling system must be pursued: in a film cooled nozzle the air taken from last compressor stages is ejected through discrete holes drilled on vane surfaces to provide a cold layer between hot gases and turbine components. In this context, the interactions between the swirling combustor outflow and the vane film cooling flows play a major role in the definition of a well performing cooling scheme, demanding for experimental campaigns at representative flow conditions. An annular three-sector combustor simulator with fully cooled high pressure vanes has been designed and installed at THT Lab of University of Florence. The test rig is equipped with three axial swirlers, effusion cooled liners and six film cooled high pressure vanes passages, for a vortex-to-vane count ratio of 1:2. The relative clocking position between swirlers and vanes has been chosen in order to have the leading edge of the central airfoil aligned with the central swirler. In this experimental work, adiabatic film effectiveness measurements have been carried out in the central sector vanes, in order to characterize the film-cooling performance under swirling inflow conditions. The Pressure Sensitive Paint technique, based on heat and mass transfer analogy, has been exploited to catch highly detailed 2D distributions. Carbon dioxide has been used as coolant in order to reach a coolant-to-mainstream density ratio of 1.5. Turbulence and five hole probe measurements at inlet/outlet of the cascade have been carried out as well, in order to highlight the characteristics of the flow field passing through the cascade and to provide precise boundary conditions. Results have shown a relevant effect of the swirling mainflow on the film cooling behaviour. Differences have been found between the central airfoil and the adjacent ones, both in terms of leading edge stagnation point position and of pressure and suction side film coverage characteristics.

Laser additive manufacturing (LAM) is an emerging technology that builds parts in a layer-by-layer process by selectively melting metal powders. This additive manufacturing technique among others can produce very complex geometries, which are not possible using conventional methods. A mock segment of the leading edge of a turbine blade, designed with both internal and external cooling features fabricated by LAM of Inconel powder, is investigated. This design consists of an internal impingement cooling array and an engineered-porous structure. This porous region consists of a lattice of intersecting cylinders that simulates the effect of a transpiration cooled segment or permeable wall with a designed porosity of 0.57. Transpiration cooling is a promising external cooling technique capable of reducing thermal gradients at the surface of the blade by providing a more uniform film than conventional discrete film cooling holes. In this current study, adiabatic film cooling effectiveness is experimentally investigated using pressure sensitive paint (PSP) for blowing ratios ranging between 0.03 and 0.15. Using air as the mainstream, and CO 2 as the coolant source, a density ratio of 1.5 is obtained. Steady state simulations using RANS are analyzed and used to compare against experimental results. All cases result in an increase in effectiveness values with increasing blowing ratio. Highest effectiveness values are associated with high pressure drop. Coolant uniformity is observed downstream the porous region and mixing becomes more significant as blowing ratio is increased.

A numerical method for compressible flows with nonequilibrium condensation is reconstructed for simulating supercritical CO 2 flows with nonequilibrium condensation under high pressure conditions. Thermophysical properties are interpolated from pressure-temperature look-up tables and density-internal energy look-up tables, which are generated using the polynomial equations in REFPROP. We employ the high pressure nonequilibrium condensation model in which the critical radius of a liquid droplet is modified by considering non-ideal gas. We simulate high pressure CO 2 flows through a Laval nozzle, which was experimentally investigated by Lettieri et al. High-pressure CO 2 passes through the nozzle, leading to a decrease in its pressure and temperature. It reaches the supercooled condition near the throat. Nucleation and the subsequent growth of droplets lead to an increase in the condensate mass fraction in the diverging area. The proposed method for real gas reproduced the peak of pressure distribution owing to the release of latent heat, whereas the numerical result assuming ideal gas is different from the experimental result. The nucleation region obtained using the present method is earlier and narrower than that in the case of ideal gas. The early and rapid nucleation leads to the high mass condensate rate at the outlet. These results show that considering the real gas effect and nonequilibrium condensation is crucial for developing the impeller of a compressor for the supercritical CO 2 Brayton cycle.

Supercritical carbon dioxide (sCO 2 ) Brayton power cycles take advantage of the high density of CO 2 near the critical point to reduce compressor power and increase cycle efficiency. However, thermophysical properties of CO 2 vary drastically near the critical point. Concerns of large property variations and liquid formation within the compressor can result in sCO 2 cycle designers selecting compressor inlet operating conditions substantially above the critical point, thereby reducing cycle performance. The Naval Nuclear Laboratory has built and tested the 100 kWe Integrated System Test (IST) to demonstrate the ability to operate and control an sCO 2 Brayton power cycle over a wide range of conditions. Since the purpose of the IST is focused on controllability, the design compressor inlet conditions were selected to be 8.2°F (4.6°C) and 270 psi (18.4 bar) above the critical point to reduce the effect of small variations in compressor inlet temperature and pressure on density. This paper evaluates the effect of design compressor inlet pressure on cycle efficiency for a simple recuperated Brayton cycle and the performance of an operating Brayton power cycle with a fixed design over a range of compressor inlet pressures.