Staged combustion systems like the rich-relaxation-quench-lean (RRQL) offer the potential for low nitrogen oxides (NOx) emissions while burning ammonia (NH3). This process involves rich premixed NH3-air combustion in a primary zone, allowing time for NOx relaxation, followed by a lean secondary combustion zone via air injection. However, improper design of the secondary stage can lead to high NOx and nitrous oxide (N2O) emissions, offsetting the climate benefits of carbon-free fuels. This study investigates the effects of primary zone length and secondary stage geometry on the stability and emissions of a lab-scale RRQL system. Experiments were conducted at atmospheric pressure using a modular axial swirl burner (swirl number = 1.1, 16 vanes), with primary equivalence ratios (phi primary) of 1.13, 1.15, and 1.18. Two quartz lengths (76 and 178 mm) were tested using a five holes (2.03 mm) secondary injection design. Strong flame interaction and elevated NOx were observed for the 76 mm liner. A longer chamber allowed better NOx relaxation and NH3 cracking. Additional tests with 5-, 10-, and 16-holes configurations showed that fewer holes, implying higher momentum flux ratios, yielded lower NOx-N2O emissions, especially at phi primary = 1.13. These effects diminished as phi primary decreased from 1.18 down to the optimum 1.13. Diffusion-like combustion was seen for 0.90 <= phi global <= 1.10, leading to inefficient combustion marked by excess O2 values compared with equilibrium. Optimal performance was achieved with phi primary = 1.13 and 0.70 <= phi global <= 0.90, with estimated burner outlet temperatures between 1720 and 1970 K.
This study provides a design-relevant numerical analysis of ammonia-oxygen (NH3/O-2) combustion, focusing on the effects of inlet preheating (298-520 K), equivalence ratio (phi = 0.7-1.2), and bulk flow velocity (5.2-12 m/s) on flame stability and NOx formation. The simulations reveal that preheating improves flame compactness and stabilization, raising maximum flame temperature by only similar to 30 K (2830 -> 2860 K) but accelerating chemical kinetics. Equivalence ratio is the dominant flame parameter: lean conditions (phi = 0.9) produce the highest flame temperatures (2860 K), while moderately rich conditions (phi =1.2) suppress NO by 86% (9000 -> 1250 ppm) due to OH depletion (45% drop) and enhanced H-2 generation (0.015 -> 0.10 mole fraction). Flow velocity strongly influences aerodynamics and flame stability: low velocity (5.2 m/s) generates weak recirculation, high velocity (12 m/s) elongates and destabilizes the flame, whereas intermediate velocities (7-9 m/s) establish coherent recirculation zones that ensure robust anchoring. NOx emissions remain essentially invariant across the velocity range (3846 ppm), with NO2 and N2O negligible, confirming that inlet velocity affects stability but not equilibrium pollutant levels. The findings offer actionable insights into the design of low-NOx, carbon-free ammonia gas turbine systems.
The sharp rise in energy demand in next-generation aircraft motivates this study, which proposes a novel kerosene-ammonia dual-fuel hybrid system. By integrating an ammonia-air heat exchanger and a solid oxide fuel cell (SOFC) into a turbofan, ammonia serves as both coolant and fuel. System performance was compared across three configurations, including a baseline single fuel system (scheme 0), an ammonia-cooled system without SOFC (scheme 1), and a full hybrid system (scheme 2), with a focus on specific thrust, thermal efficiency, and fuel consumption. The hybrid system achieved a specific thrust of 929.91 N/(kg/s) and thermal efficiency of 45.1%, surpassing the baseline scheme by 3.52% and 4.64%, respectively. The enhancements are attributed to a decrease in turbine cooling air requirements, resulting from ammonia's higher heat sink capacity, and the implementation of direct electrochemical conversion in the SOFC, which overcomes the Carnot cycle limits. Enthalpy entropy analysis confirmed a 3.38% reduction in entropy generation in scheme 2, indicating higher energy utilization quality. Key influencing parameters, including ammonia mass fraction ( delta f) and SOFC flow allocation ratio ( delta sofc), were rigorously analyzed. An increase in delta f resulted in greater thrust, which entailed a corresponding tradeoff with cooling capacity to achieve its maximum, and higher delta sofc improved thermal efficiency. Optimal performance was attained at delta f and delta sofc of 0.237 and 0.644, respectively, under a cooling temperature drop of 250 K, demonstrating substantial improvement in thrust output and energy utilization efficiency.
Experimental evaluation of aero-engine compressor performance under distorted inflow conditions is clearly specified in airworthiness standards. Generally, to quantify the distortion intensity, six to eight multihole comb pneumatic probes were experimentally used to capture the circumferentially distorted flow field. Although the blockage ratio of the probes meets the requirements when designing them, whether the presence of the probes will cause changes in the distorted flow after passing through the probes, then affects the accuracy and reliability of performance evaluation under distorted inflow has not been conclusively determined. To address this issue, a numerical study was conducted on the distortion intensity with and without inlet-measuring pneumatic probes in a two-stage high-speed axial flow fan of an aero-engine. The results show that, compared with the case without pneumatic probes, the installation of inlet pneumatic probes increases the inlet total pressure and temperature distortion intensities by 0.011 and 0.0015, respectively. However, the pneumatic probes can act as guide vanes to reduce the swirl angle, which leads to a significant 0.0411 decrease in the swirl distortion intensity ( tau ct). This reduction in swirl distortion further decreases the angle of attack at the rotor blade leading edge and increases the inlet axial velocity, thereby enhancing the flow capacity and improving the corrected flowrate and pressure ratio by 1.09% and 1.56%, respectively. The optimization of the inlet angle of attack reduces flow separation and increases the efficiency by 1.7%. Accounting for the influence of the probes brings the numerical simulation results closer to the experimental data. It demonstrates that the pneumatic probes exert a dual effect on the distorted flow field in practical applications, necessitating careful balancing during the design and experimental processes.
Ammonia (NH3) is a carbon-free energy source and hydrogen carrier, but its fuel-bound nitrogen can lead to significant nitrogen oxides (NOx) emissions. Staged combustion strategies, such as rich-quench-lean, can achieve low NOx emissions. However, improper design may result in high NOx levels when operating with a rich head end without a sufficient post-flame relaxation time. Previous work has shown that a rich-relaxation-quench-lean (RRQL) configuration can minimize NOx emissions in the rich head end (Cole et al., 2024, "Rich Ammonia Flame Shapes and NO Relaxation: Facility Development and Characterization," ASME Paper No. GT2024-122369). This study focuses on how different swirl geometries affect exhaust emissions and flame morphology in rich, premixed NH3-air flames, for the design of a rich relaxation head end of an RRQL combustor. Experiments were conducted using a modular swirl burner and measuring NOx, N2O, and NH3 emissions, and recording natural flame luminosity and NH2* and OH* chemiluminescence images. Swirler design affects emissions by influencing flame length (with shorter flames allowing for more postflame relaxation time) and flame-wall interactions (influencing NH3 and N2O emissions). A compact flame with minimal wall interactions allows for increased NOx relaxation, while minimal wall interactions prevent heat losses and instabilities, which minimize NH3 and N2O emissions at a richer equivalence ratio, potentially enhancing H2 production during the relaxation phase without increasing overall primary stage NOx emissions, thereby improving system efficiency. Detailed spatial evolution of NOx, N2O, and NH3 emissions further supports the RRQL as a promising combustor design for NH3 combustion, if the rich head end is designed properly.
Hydrogen-fired combustion turbines will play a critical role in the decarbonization of the Energy sector. Retrofittable technologies to reduce carbon emissions from existing gas turbine power plants are desirable to aid with the transition and enable cost-effective interim solutions. Due to the scarcity of Hydrogen in today's market, the low volumetric Energy density of gaseous Hydrogen, and the expensive Energy needed to produce and maintain liquid Hydrogen (LH2), liquid Ammonia (LNH3), a traded commodity, becomes an attractive fuel for storing and transporting vast quantities of Energy across great distances. While direct combustion of Ammonia is producing an unmanageable level of Nitrogen oxides (NOx) from its fuel-bound Nitrogen, equipment may be colocated at a gas turbine site to crack NH3 into Hydrogen and Nitrogen. Additional equipment to obtain a pure Hydrogen stream can be eliminated if the resulting Hydrogen/Nitrogen blend can be directly used. The needed volumetric flow is far greater than that of pure Hydrogen or Natural Gas (NG), which presents design challenges that will be discussed. A novel combustor was designed, and high-pressure rig testing results are summarized in this paper to showcase the fuel flexibility of the FlameSheet (TM) combustion system technology, and its ability to operate with any blend of Natural Gas and cracked Ammonia. Differences in flame stability, emissions, and combustion dynamics will be compared between Natural Gas and cracked Ammonia operation. Considerations are also given to various engine operating concepts.
In aircraft engine gas-path analysis, the number of measurable parameters is often limited and smaller than the number of health parameters to be estimated, resulting in an underdetermined diagnostic problem that compromises the accuracy and reliability of engine degradation assessment. To address this issue, this paper develops a novel gas-path degradation diagnostic framework that integrates data-driven and model-based approaches and proposes a new hybrid gas-path analysis with data-driven priors (HGPA-DDP) method for engine degradation assessment under underdetermined conditions. First, a data-driven model is constructed to perform preliminary diagnostics on measurable parameters, generating prior information on component degradations. Then, this prior information is incorporated into a nonlinear gas-path analysis framework through Tikhonov regularization, enabling iterative optimization under physical-model constraints and achieving an effective fusion of physical mechanisms and data-driven advantages. The proposed method is validated through turbofan engine degradation simulations and on-wing operational data. The results demonstrate that, compared with conventional nonlinear gas-path analysis methods, the proposed HGPA-DDP approach significantly improves the accuracy and reliability of diagnostic results under underdetermined measurement conditions, providing a more reliable diagnostic tool for aircraft engine health management.
To support the transition toward climate-neutral aviation, hydrogen is considered one of the most promising energy carrier candidates. However, hydrogen combustion is inherently prone to high NOx emissions. To address this challenge, the micromix principle has been developed to enable low-NOx, flashback-safe hydrogen combustion in gas turbines. Building upon this principle, this paper presents the development, design methodology, and evaluation of a novel, additively manufactured micromix combustor configuration — the Radial-Spokes (RS)-Burner. Unlike the established Ring-Burner, which distributes hydrogen circumferentially, the RS-Burner employs radial hydrogen distribution through multiple spokes. This new arrangement requires a complete reorientation of the micromix principle. The design is enabled through additive manufacturing (AM) using laser powder bed fusion (LPBF). A structured design methodology is introduced, accounting for design challenges like varying spoke width and injector hole diameters. Numerical simulations were conducted using Star-CCM+ to analyze the combustion performance of the RS-Burner across a range of operating conditions compared to the Ring-Burner. Additionally, results were validated against experimental data using test burner and real engine test data. The results confirm the viability of the RS-Burner as a low-NOx hydrogen combustion chamber. The new concept shows stable combustion, satisfactory flame separation, and high combustion efficiency. NOx emissions were significantly reduced at part-load conditions compared to the reference design. The presented study highlights the feasibility and advantages of combining micromix combustion with additive manufacturing to meet the emission targets of future hydrogen-powered gas turbines.
Nowadays, gas foil bearings (GFBs) have become a key enabling technology for S-CO2 turbomachinery applications, due to their capacity to withstand extreme operating conditions, and without pollution. Furthermore, GFBs eliminate the complicated external facilities and the speed limitations. While extensive research exists on air-lubricated GFBs, a few studies focus on the dynamic performance of CO2-lubricated GFBs under high-pressure and high-temperature conditions, particularly considering the complex thermal and centrifugal deformations. This lack of research impedes the application of GFBs in S-CO2 turbomachinery. Therefore, a thermohydrodynamic (THD) analysis method was proposed and validated in this study for the dynamic characteristics of the third-generation CO2 gas foil bearing (GFB). In this paper, a novel numerical method was presented for THD analysis of the dynamic characteristics of the third-generation CO2 bump-type GFBs, considering thermal and centrifugal deformations. The proposed numerical method was validated by the experimental data of the eight dynamic characteristic coefficients of air-lubricated bump-type GFBs under different rotating speeds (21,600 rpm and 27,600 rpm) and static loads (20 N-40 N). The present method was further validated based on the measured power loss of CO2 bump-type GFBs for different rotating speeds. The dynamic characteristics were calculated and analyzed for the third-generation CO2 bump-type GFBs at different operating conditions, including ambient pressures (1.0-3.0 MPa), ambient temperatures (300-600 degrees C), and eccentricity ratios (0.3-0.9). Calculations were performed for a wide range of perturbation frequencies (dimensionless perturbation frequencies 0.1-4.0) to encompass all possible excitation frequencies encountered in the bearing. The influences of operating parameters (high pressure and high temperature) and static loads on the dynamic characteristics of the third-generation CO2 bump-type GFBs were presented and discussed. The numerical results demonstrate that thermal and centrifugal deformations are critical for accurately predicting the dynamic performance of the third-generation CO2 bump-type GFB at high-pressure and high-temperature conditions. Both dynamic stiffness and damping coefficients decrease with rising ambient temperature. At an ambient pressure of 2.0 MPa and a dimensionless perturbation frequency nu & strns;=2.0, the direct stiffness exhibits 21.1% and 15.8% reductions as the ambient temperature rises from 300 degrees C to 600 degrees C. Meanwhile, the direct damping coefficients Cxx and Cyy decrease by 11.7% and 9.7%, respectively. To maintain sufficient direct stiffness and damping coefficients, a lower ambient temperature is recommended for high-temperature S-CO2 turbomachinery applications. The direct stiffness and damping coefficients increase rapidly with the rising ambient pressure below an ambient pressure of 2.0 MPa, while exhibit a diminished growth rate or decrease above this threshold. Thus, a critical ambient pressure threshold (2.0 MPa) is recommended to optimize rotor-bearing system stability while declining windage losses in the rotor cavity. Furthermore, the third-generation CO2 bump-type GFB has enough direct dynamic stiffness and damping coefficients, and a positive energy dissipation factor for all perturbation frequencies and eccentricity ratios, enhancing the stability of the rotor-bearing system.
In this paper an enhanced thickened flame model (TFM) capable of accounting for both premixed and nonpremixed combustion regimes is leveraged to reproduce the extinction limit of an industrial burner operating at atmospheric pressure with a vitiated oxidizer. CO2 is employed to dilute the air, simulating the effects of the exhaust gas recirculation (EGR). The CO2 content in the oxidizer is progressively increased until the occurrence of lean blow-out (LBO). Due to practical constraints, the numerical procedure consists of an acceleration of the experiment trend of O2 depletion. In this context, a dedicated strategy for the dynamic calculation of the laminar properties of the flame—i.e., the laminar flame speed (LFS) and laminar flame thickness (LFT)—as a function of the local elemental mass fraction is used. This quantity evolves during the simulation in response to the time-dependent boundary conditions. It will be demonstrated that the proposed approach is capable of successfully capturing the flame characteristics of both the stable conditions and the transient phase leading to blow-out. In fact, it is found that the adopted combustion model is able to predict the LBO at the same oxidizer composition detected experimentally. Moreover, as extinction is approached, the numerical model reproduces the same dynamics observed during the reactive test. A detailed comparison between the numerical results and experimental measurements during this phase is presented and discussed.
The paper compares experimentally derived dynamic force coefficients for a tooth on rotor (TOR) labyrinth seal (LS) against predictions obtained from URANS (Unsteady Reynolds Averaged Navier-Stokes) computational fluid dynamics (CFD) simulations and a one control volume (1CV) bulk flow model (BFM). With 14 teeth and radial clearance CS = 0.1016 mm, the test LS has diameter D = 114.3 mm and overall length L = 0.62 D. The seal, supplied with pressurized air at inlet pressure Pin = 50 bar and ambient temperature, operates with rotor speed of 20.2 krpm and is set to an exit to inlet pressure ratio = 0.4. The CFD analysis applies a simultaneous multiple-frequency excitation method to extract the rotordynamic force coefficients. The CFD-predicted force coefficients show good correlation with the experimental force coefficients, albeit the predicted LS leakage is similar to 14.5% greater than the recorded flow. Note the experimental setup includes upstream and downstream uniform clearance sections, which affect considerably the evolution of the circumferential swirl entering the LS section. The upstream and downstream plain annular seal sections do affect the test element (estimated) cross-coupled stiffness (30% of overall) and direct damping coefficients (40% of the overall). The study further compares the results of a 1CV BFM against the CFD predictions and the experimental results. The simple BFM underpredicts the seal cross-coupled stiffness and direct damping even when implementing the static pressure and inlet swirl condition derived from the CFD flow field.
Digital engine twins duplicate the real existing, complex system by a digitally created construct representing its time-dependent characteristics. Based on real-time measured data, they provide information about the system state, which are not accessible by a data analysis. This makes accurate transient temperature measurements critical to digital engine twins. Thermocouples represent an established means of engine temperature measurement. However, they are affected by thermal inertia and conductive heat transfer, which introduce a bias and a time-delay of the sensor response. There are different physical phenomena leading to bias and time delay. Discussion of these phenomena makes it obvious that the transient reading of a thermocouple highly depends on its installation in the engine and the associated thermal environment. It is demonstrated that these distortions obscure the time series of measurements and complicate the distinction between nominal and non-nominal engine state. The time delay can be corrected a priori via a first-order time lag system based on basic physical properties. The potential of calibrating for the complex gas path flow conditions affecting the time lag is estimated using a physics-informed neural network. Matching the temperature readings completely to the model output indicates the amount of transient calibration data required to capture all effects due to the installation of the thermocouple in the engine structure.
Dual-stage brush seal exhibits issues of imbalanced pressure drop at bristle pack between stages and high temperature resulting from frictional heat accumulation at bristle tips. In this paper, a dual-stage brush seal with holes on backing plate (BSHB) was proposed. Theoretical analysis was conducted on flow and heat transfer characteristics of BSHB. A numerical calculation model for porous media of BSHB was established. The flow and leakage, as well as heat transfer characteristics of BSHB with a round hole array and one with an U-shaped hole array were compared. The impacts of structural parameters (i.e., diameter, width and shape of a hole array) on the flow and leakage, as well as heat transfer characteristics of BSHB, were examined. Results showed that the BSHB significantly balanced pressure drop between stages, resulting in a decrease in percentage of second-stage pressure drop of the structure with a round hole array (BSRH) from 59.30% to 49.94%. And the percentage of second-stage pressure drop of the structure with an U-shaped hole array (BSUH) reduced from 59.30% to 51.04%. BSRH more effectively balanced the pressure drop between stages under principle of area transfer because of broader flow path. The hole array on backing plate increased leakage. Moreover, the leakage of BSRH exceeded that of BSUH under principle of area transfer. The hole array on backing plate promoted the convective heat transfer between bristle pack and leakage flow, resulting in a reduction of the temperature of bristle pack. With the increase of flow area, the maximum temperatures of both BSRH and BSUH were decreased. Furthermore, the heat dissipation efficiency of BSRH exceeded that of BSUH under the same area because of more leakage mass flow.
An experimental investigation is conducted to elucidate the trajectory evolution of a liquid jet subjected to a swirling air crossflow under elevated pressure conditions, representative of airblast atomization environments in modern aircraft combustor systems. The swirling flow, generated via a 30 deg axial swirler within an annular passage (swirl number, SN = 0.42), interacts with a liquid jet issuing from a circular orifice. The study systematically examines the influence of momentum flux ratio (2 <= q <= 25) and Weber number (55.2 <= We <= 128.1), achieved by varying the air injection pressure from 2 to 5 bar. A tomographic imaging methodology, incorporating synchronized endoscopic views at multiple azimuthal locations combined with backlight illumination, enables the three-dimensional reconstruction of the jet's trajectory and angular deflection characteristics. Results reveal that increasing the momentum flux ratio significantly enhances the liquid jet's maximum projected penetration, radial penetration, and angular deflection. Furthermore, elevated pressure conditions induce pronounced jet bending and early trajectory curvature compared to atmospheric conditions, driven by intensified aerodynamic loading and swirl-induced shear forces.
To overcome the limitations of one-dimensional calculation and achieve rapid prediction of internal fields in aeroengines, this study adopts a method combining proper orthogonal decomposition and machine learning. The principal orthogonal decomposition (POD) method is applied to reduce the dimensionality of three-dimensional numerical simulation results of an entire aeroengine, and the generated time matrix is then used for machine learning training. This approach successfully establishes models for predicting the Mach number, total temperature, and total pressure field within the operational range of the engine. In tests involving 1,000 operational conditions, the best prediction performance was achieved using the first 20 modes. For the predictions of the Mach number, total temperature, and total pressure fields, the relative root-mean-square error (RRMSE) values were 0.1847, 0.1981, and 0.04252, respectively, while the R2 values reached 0.98267, 0.90155, and 0.99051. Compared to CFD calculations, this method saves 99.7% of the computational time. Simultaneously, this study demonstrates that improving the accuracy of the POD-ML method fundamentally requires enhancing the high-frequency feature extraction and synthesis capabilities within the ML model. This could be achieved, for example, by adopting frequency-band-specific feature extraction methods, or by optimizing the model architecture and training strategies. Furthermore, the high-order POD modes capture high-wavenumber spatial structures associated with strong local gradients. These structures, though energetically minor, govern the accuracy of peak Mach number, temperature overshoot, and pressure recovery predictions, which are critical for engine safety and performance assessment.
Pumped thermal energy storage (PTES), a subcategory of Carnot battery technologies has attracted attention in recent years, mainly because of its ability to use grid arbitrage to convert electricity to be stored as heat (charging), which can be again converted to electric power (discharging). The benefit offered by the heat as the storage medium is the direct supply of heat for industrial or district heating purposes (heat as output) as well as the integration of heat sources to support the cycle performance (heat as input) usually in the charging cycle. The latter configuration is referred to as thermally integrated PTES (TI-PTES). A key advantage of TI-PTES is that it allows the integrated heat to be stored together with electricity during charging and used at a latter time of high demand while removing the constraint of cold storage especially for Brayton cycle-based TI-PTES plants as well. Leveraging on such a concept, the EU SCO2OP-TES project aims at demonstrating a sCO2-based TI-PTES up to TRL 5 through a lab-scale demonstration plant using industrial waste heat (WH) as integrated heat source; the charging and discharging cycles are sCO2-based Brayton heat pump and power cycles, respectively, adopting radial turbomachinery for both the hot compressor (charging) and the hot turbine(discharging). Along with the mean line design and computational fluid dynamics (CFD) analysis of the sCO2 turbomachines, this study explores the influence of their part load operation on the SCO2OPTES TI-PTES demonstration plant. The study elaborates on the effect of partial electric loads considered for hot compressor (high-temperature turbomachine for the charging) and for hot turbine (high-temperature turbomachine for discharging) on the waste heat recovery unit in the charging cycle, thermal energy storage heat exchanger, recuperator, and cooler in the discharging cycle. Separate and combined performance of the charging and the discharging cycles because of variations in thermodynamics and mass flows is evaluated. The part load scenario shows an exceedingly small off design envelop for the charging cycle (mostly limited by the hot compressor operating envelope), while for the discharging cycle, the part load operation benefits from a broader operational range as well as allowable mitigation space. Based on such results, preliminary operational constraints for the proper controllability of the cycles are suggested.
In this study, the dynamics of enstrophy and terms contributing to its amplification and attenuation are examined for a rotating detonation engine (RDE) combustor. The analysis is performed using a dataset obtained from unsteady Reynolds-averaged Navier-Stokes (RANS) simulations, which showed the transition from a sustained single detonation wave to a double corotating detonation wave in good agreement with experiments when mass flow rates of fuel and oxidizer are changed while maintaining the same equivalence ratio. The results show that the presence of detonation waves and inhomogeneities in the reactive flow field leads to the production and dissipation of enstrophy, particularly in the vicinity of the rotating detonation fronts. The mixing efficiency shows an inverse relationship with enstrophy where it increases sharply near the injection region and saturates at downstream locations. In the vicinity of the detonation front, vortex stretching and baroclinic effects contribute positively to enstrophy, whereas dilatation contributes negatively, exhibiting significant spatial variations during the wave mode transition. During the transition phase, the magnitude of all the terms contributing to enstrophy increases compared to instants when sustained single/double detonation fronts are observed.
Diesel oxidation catalysts and selective catalytic reduction systems are two of the most commonly used exhaust after treatment methods, which aid on- and off-road diesel engines to comply with stringent emissions regulations. Effective operation of these devices requires them to reach their light-off temperatures as quickly as possible after engine startup. Late-cycle heat release achieved using different postinjection strategies has been identified as an effective approach to increase exhaust temperatures, which contributes to effective catalyst heating operation. Previous research has identified a tradeoff between exhaust temperatures and hydrocarbon (HC) emissions that limits postinjection retardability, and therefore the maximum achievable exhaust enthalpy. This work explores the effect of fuel properties, including cetane number, distillation characteristics, and oxygen content, on the tradeoffs between exhaust enthalpy and combustion efficiency in catalyst heating operation. Experimental investigations are performed in a single cylinder optical diesel engine for fuels with different cetane numbers and distillation characteristics, and for blends of diesel fuel with 1-octanol and di-butyl ether, with an injection calibration comprising of one pilot, one main and one postinjection. Increasing the cetane number seven units (from 43 to 50) reduces HC emissions nearly by 60% with no significant changes in exhaust heat flux, whereas the HC emissions and exhaust heat flux for fuels with different distillation characteristics show lower sensitivity to postinjection timings. Diesel blends with 20% by vol. of oxygenates (di-butyl ether, polyoxymethylene dimethyl ether, and 1-octanol) show excellent potential to reduce HC emissions even at late postinjection timings, which helps achieve better tradeoffs between exhaust temperatures and engine-out emissions. Simultaneous high-speed visible and infrared imaging techniques are applied to analyze the effects of oxygenated blends on in-cylinder HC formation. Images show that the amount of unburned hydrocarbons formed by the postinjections decreases as the oxygen content and the cetane number of the fuel increases likely because the mixing rate required to burn the fuel efficiently decreases with the oxygen content of the fuel, improving postinjection retardability. The addition of di-butyl ether improves the reactivity of the mixture, which promotes the faster penetration of main combustion in the squish region, which is also beneficial for HC reduction.
Performance testing of a three-stage 9 MW axial CO2 compressor has been successfully completed at the closed-loop CO2 compressor test facility at the University of Notre Dame. The design speed, inlet pressure, inlet temperature, and mass flowrate of the compressor were 19,800 rpm, 2.77 MPa, 98 degrees C, and 125.9 kg/s, respectively. The compressor performance was evaluated from 60% to 100% of the design speed. Each speedline covered operating points from choke to near stall except for at 60% speed, where the compressor was throttled into rotating stall. Blade tip clearance and blade vibrations were measured during the compressor operation to ensure the safety of the compressor. Calibrated capacitance probes were used to establish average running tip clearances of 0.38 mm (1.2% of span), 0.5 mm (1.9% of span), and 0.5 mm (2.4% of span) for stage 1, stage 2, and stage 3, respectively. Blade vibrations were measured using tip timing probes. A six engine-order synchronous blade vibration of the first bending mode was observed at the first stage rotor around 73-82% speed. Stall testing at 60% speed revealed a single stall cell, which extends from the first stage rotor to the third stage rotor, and is rotating at 60.4% of the shaft speed. The compressor stall at 60% caused significant shaft vibration. Hence, the stall line at 70% and higher speeds was extrapolated from stall testing at 60% and lower speed to avoid potential damage of the test rig from stall or surge. The measured peak isentropic efficiency of the compressor was 91.5%, 91.1%, 90.6%, 90.0%, 85.3% at 60%, 70%, 83%, 90%, and 100% speeds, respectively. Considering increases in efficiency with decreases in mass flowrate and that the performance test above 70% was stopped with some safety distance away from the stall point to avoid damage, the peak isentropic efficiency is expected to be above or close to 90% at 100% speed as well. The measured performance successfully demonstrated the performance benefit and usefulness of the axial compressor for grid-scale energy systems such as pumped thermal energy storage (PTES).
The rapidly growing global energy demand and concerns over the environmental impact of fossil-fuel-based energy call for the exploration of reliable carbon-free fuels, including hydrogen, ammonia, or blends of both. Despite its great potential as a hydrogen carrier, burning ammonia is not practical due to low laminar flame speed, narrow stabilization limits, and high NOx emissions. This paper aims to study the effect of ammonia addition on the combustion dynamics and stability characteristics of turbulent diffusion CH4/air flames for typical combustion systems. Experiments are conducted using a practical swirl-stabilized burner to investigate the emissions characteristics and the impact of increasing the ammonia content on the flame liftoff height and macrostructure. Results from OH* chemiluminescence indicate stable combustion using fuel blends with up to 35-40% mole fraction, whereas intermittent flame detachments followed by flame root extinction are observed using higher ammonia content of about 60%. Interestingly, a linear relationship is revealed between the maximum achieved liftoff height and the corresponding global equivalence ratio. Emissions measurements with high ammonia content ( >60%) reveal NOx emissions lower than those produced with pure methane. Macrostructural analysis with respect to stable methane flame shows that the central recirculation zone with 40% ammonia becomes severely distorted such that its ability in anchoring and stabilizing the flame gets weaker with the increase in the ammonia content.
