EXPERIMENTAL AND NUMERICAL INVESTIGATION OF CYCLOPENTANE SPRAYS IN TRANSCRITICAL ENVIRONMENT

In liquid rocket engines or internal combustion engines, increasing the inlet fuels temperature or chamber pressure exceeding its critical point is capable of improving the combustion efficiency. Under these conditions, the thermophysical and transport properties have an important effect on fluids mixing and combustion process. In this study, the fuel of n-heptane injected into a multi-species environment are simulated by large eddy simulations and the performance of the injected fuel temperature and different chamber conditions are compared in con-junction with high accuracy equation of state and transport properties. The results show that as the injected temperature or the chamber pressure increase, the penetration length and density gradient decrease, while the width of mixing layer increase. The results obtained in this investigation indicated that for the single injection condition, by increasing the fuel inlet temperature or chamber pressure, the essence is to reduce the initial density ratio, thereby reducing the density stratification between the jet and environment gas, which is beneficial to the jet mixing and combustion process.

Interaction between self-excited oscillations and fuel–air mixing in a dual swirl combustor

This research investigates spray phase transitions under subcritical and transcritical conditions relevant to high-pressure combustion systems. Using advanced optical diagnostic techniques, including shadowgraphy (SH), Mie scattering (MS), and infrared radiation (IR) measurements, the study focuses on cyclopentane sprays in a high-pressure, high-temperature chamber filled with gaseous nitrogen. Three cases are studied to understand spray behavior under varying injection conditions. SH and MS analyses reveal significant differences across the cases. As the chamber temperature increases, the liquid core length shortens, indicating enhanced vaporization. This is particularly evident in the MS images, where step-like transitions in the axial distribution highlight abrupt phase changes from liquid to gas in subcritical conditions. Such transitions are absent under supercritical conditions, suggesting a smoother phase transition process. IR measurements provide additional insights into the spray dynamics, especially under transcritical conditions. The IR images display complex downstream (at x/D > 125) signal patterns with bimodal distributions. The application of inverse Abel deconvolution highlights negative intensities near the spray axis downstream, indicative of regions near the pseudo-critical temperature. This phenomenon, characterized by blocked background radiation due to critical opalescence, offers indirect evidence of the pseudo-critical temperature and the associated phase transition dynamics. Dynamic Mode Decomposition (DMD) analysis identifies dominant frequencies around 4.5 kHz, corresponding to injection pressure fluctuations. This correlation suggests that injection pressure variations significantly influence spray fluctuations. Differences in mode shapes among the cases are also observed; lateral fluctuations near the injector nozzle exit are present in Cases 1 and 2 but not in Case 3. Axial fluctuations beyond a certain length are consistent in all cases. The lateral fluctuations are related to the chamber pressure and the potential large-scale vortex formation or cavitation in the nozzle. The combined use of SH, MS, and IR methodologies provides a comprehensive understanding of transcritical spray dynamics, emphasizing the effectiveness of combining multiple optical diagnostic techniques. These findings will improve numerical simulations and contribute to the development of more efficient high-pressure combustion systems. The research underscores the complexity of phase transitions in sprays and the critical role of precise diagnostics in the advancement of combustion technology.

The authors have used a nonionic inverse micelle synthesis technique to form nanoclusters of platinum and palladium. These nanoclusters can be rendered hydrophobic or hydrophilic by the appropriate choice of capping ligand. Unlike Au nanoclusters, Pt nanoclusters show great stability with thiol ligands in aqueous media. Alkane thiols, with alkane chains ranging from C{sub 6} to C{sub 18} were used as hydrophobic ligands, and with some of these they were able to form 2-D and/or 3-D superlattices of Pt nanoclusters as small as 2.7 nm in diameter. Image processing techniques were developed to reliably extract from transmission electron micrographs (TEMs) the particle size distribution, and information about the superlattice domains and their boundaries. The latter permits one to compute the intradomain vector pair correlation function of the particle centers, from which they can accurately determine the lattice spacing and the coherent domain size. From these data the gap between the particles in the coherent domains can be determined as a function of the thiol chain length. It is found that as the thiol chain length increases, the gaps between particles within superlattice domains increases, but more slowly than one might expect, possibly indicating thiol chain interdigitation.

Mixing characteristics of multi-chambered contact tank are analyzed employing the validated three-dimensional numerical model developed in the companion paper. Based on the flow characterization, novel volumetric mixing efficiency definitions are proposed for the assessment of the hydrodynamic and chemical transport properties of the contact tank and its chambers. Residence time distribution functions are analyzed not only at the outlet of each chamber but also inside the chambers using the efficiency definitions for both Reynolds averaged Navier–Stokes (RANS) and large eddy simulation (LES) results. A novel tracer mixing index is defined to characterize short circuiting and mixing effects of the contact system. Comparisons of the results of these indexes for RANS and LES solutions indicate that mixing characteristics are stronger in LES due to the unsteady turbulent eddy mixing even though short circuiting effects are also more prominent in LES results. This result indicates that the mixing analysis based on the LES results simulates the mixing characteristics instantaneously, which is more realistic than that in RANS. Since LES analysis can capture turbulent eddy mixing better than RANS analysis, the interaction of recirculation and jet zones are captured more effectively in LES, which tends to predict higher turbulent mixing in the contact system. The analysis also shows that the mixing efficiency of each chamber of the contact tank is different, thus it is necessary to consider distinct chemical release and volumetric designs for each chamber in order to maximize the mixing efficiency of the overall process in a contact tank system.

Understanding the internal solid motion and heat transfer behaviour within rotating drums is paramount for their design and operation across various industries. The discrete element method (DEM) is utilized to elucidate the general flow, mixing, and heat transfer characteristics of particles within rotating drums. Following model validation, this study delves into the mixing behaviour and heat transfer patterns of binary‐size particles in the rotating drum, while also assessing the impact of size ratio and rotating speed. The findings reveal that variations in particle size result in noticeable radial segregation, consequently affecting the heat transfer dynamics of solid phase within the system. Higher rotating speeds enhance mixing and dispersion of solid phase but lead to a decrease in the averaged particle temperature. Furthermore, the heat flux exhibits a negative correlation with particle size. Distinct heat transfer behaviours are observed among particles of different sizes in both active and passive areas, with larger particle size ratios exacerbating segregation, potentially impacting final product quality. In summary, these findings offer crucial insights into heat transfer phenomena in rotating drums, aiding in the design and operation of apparatus.

On the application of Large-Eddy simulations in engine-related problems

In support of efforts to develop improved models of turbulent spray behavior and combustion in diesel engines, experimental data and analysis must be obtained for guidance and validation. For Reynolds-averaged Navier–Stokes (RANS)-based Computational fluid dynamics (CFD) modeling approaches, representative ensemble average experimental results are important. For high-fidelity models such as large eddy simulations (LES)-based CFD, precise individual experimental results are desirable. However, making comparisons between a given experiment and LES is a challenge since local parameters cannot be directly compared. In this work, an optically accessible constant pressure flow rig (CPFR) is utilized to acquire diesel-like fuel injection and reaction behavior simultaneously with three optical diagnostic techniques: rainbow Schlieren deflectometry (RSD), OH* chemiluminescence (OH*), and two-color pyrometry (2CP). The CPFR allows a large number of repeated injection experiments to be performed for statistical analysis and convergence using ensemble-averaging techniques, while maintaining highly repeatable test conditions. Even for stable test conditions, variations in local turbulent fuel–air mixing introduce variability, which manifests as significant differences in OH* and 2CP results. Experimental measurements of characteristic parameters including liquid and vapor jet penetration, liftoff length, soot temperature and concentration, and turbulent flame speed, along with the shot-to-shot variability of each dataset, are presented and discussed. A statistical method is utilized to analyze the extent of this variability, and to identify superlative injections within the dataset for discussion and analysis of shot-to-shot variations.

In recent decades, stringent emission norms have been enforced upon the engine research community and OEMs to encourage them to develop new spark ignition engine technologies, such as variable valve lifts, turbocharging, and direct injection spark ignition (DISI) engines. For further development, greater control of parameters such as in-cylinder air motion, spray characteristics, injection, and ignition events is required. Spray characterizations are crucial for understanding the mixing phenomena in heated and pressurized engine combustion chamber conditions. Spray pattern, fuel injection pressure (FIP), rate shape, and thermodynamic conditions of the combustion chamber play a vital role in the mixture preparation. The present study uses Mie-Scattering techniques to examine spray structures of fuels like methanol and ethanol and compare them to gasoline, which is of great interest to DISI engines. Three different temperatures of 50, 100, and 200°C and two chamber pressures, 4 and 8 bar, are considered to simulate typical engine-cylinder conditions. It is observed that the initial chamber conditions greatly influence the spray structure. Spray collapse is lesser for alcohol than gasoline. Three semi-empirical models for predicting spray penetration are analyzed: Dent, Hiroyasu and Arai, and Arrègle. These models could not differentiate between the test fuels, particularly methanol and ethanol, for predicting spray penetration length. The degree of deviation in predictions is the lowest in the Hiroyasu and Arai model and the highest in the Dent model. Spray penetration length increased with an increasing FIP regardless of ambient conditions; however, the spray penetration length decreased with increasing chamber pressure.

In the present experimental work, the behavior of laminar liquid jet in its own vapor as well as supercritical fluid environment is conducted. Also the study of liquid jet injection into nitrogen (N2) environment is carried out at supercritical conditions. It is expected that the injected liquid jet would undergo thermodynamic transition to the chamber condition and this would alter the behavior of the injected jet. Moreover at such conditions there is a strong dependence between thermodynamic and fluid dynamic processes. Thus the thermodynamic transition has its effect on the initial instability as well as the breakup nature of the injected liquid jet. In the present study, the interfacial disturbance wavelength, breakup characteristics, and mixing behavior are analysed for the fluoroketone liquid jet that is injected into N2 environment as well as into its own vapor at subcritical to supercritical conditions. It is observed that at subcritical chamber conditions, the injected liquid jet exhibits classical liquid jet characteristics with Rayleigh breakup at lower Weber number and Taylor breakup at higher Weber number for both N2 and its own environment. At supercritical chamber conditions with its own environment, the injected liquid jet undergoes sudden thermodynamic transition to chamber conditions and single phase mixing characteristics is observed. However, the supercritical chamber conditions with N2 as ambient fluid does not have significant effect on the thermodynamic transition of the injected liquid jet.

Isothermal measurement and modeling of VLE properties for 2,3,3,3-tetrafluoroprop-1-ene + trifluoromethane + tetrafluoromethane ternary system at temperatures from 253.15 K to 273.15 K

Molecular simulations of benzene and hexafluorobenzene using new optimized effective potential models: Investigation of the liquid, vapor–liquid coexistence and supercritical fluid phases

Oil supply pockets are essential parts for the functionality of journal bearings, because mixing and oil carry-over issues are substantially dominated by these design features. In recent years, the great potential of Computational Fluid Dynamics (CFD) has been recognized. An extensive analysis of the turbulent multiphase flow phenomena including cavitation has been reported for a whole gas and steam turbine journal bearing (GT2012). Furthermore, the complex structure of the turbulent flow in an oil supply pocket has been investigated numerically and validated by Laser-Doppler-Anemometry (LDA) measurements (GT2013). The interaction between oil supply jets with the main flow and the effective mixing behavior remained as challenging tasks. Therefore this study focused on this interaction by means of an up-scaled geometry and a full scale journal bearing. Due to the upscaled geometry, it was possible to assess the flow phenomena experimentally. An extensive CFD analysis including large eddy simulation (LES) was conducted and validated by flow visualizations. The influence of geometric features and performance parameters was investigated. It was found, that even minor changes of the inflow conditions could lead to measurable effects with regard to mixing and oil carry-over behavior.

Aiming at the near-clogging boundary condition of a certain type of transonic compressor rotor, a numerical simulation method is used to systematically study the influence of different droplet diameters on its performance under the condition of moisture content of 3%. The results show that humidifying the intake air of the transonic compressor near the blockage boundary can reduce the exhaust temperature, the mass flow rate, and the total pressure ratio. In the axial direction, the mass fraction of water vapor increases slowly and linearly under the large droplet condition, while the mass fraction of water vapor under the medium and the small droplet conditions presents a two-stage parabolic growth. The mass fraction of water vapor under the three conditions increases by 0.002, 0.0055, and 0.0072 from inlet to outlet, respectively. On the radial section with the axial span = 0.7, the difference in water vapor mass fraction between the shroud and the hub under the small droplet condition is 0.00329, and that under the medium and the large droplet condition are 46.81% and 27.96% of the small droplet condition. The research results can provide theoretical guidance for the analysis of droplet evaporation at different positions in compressor near-plugging conditions.

Optical diagnostic techniques, such as chemiluminescence imaging, are commonly used to study turbulent flames. Inherent to turbulent flames is the spatio-temporal variation of the volumetric distribution of temperature and chemical composition. In consequence, the index of refraction varies accordingly and causes distortion of any optical ray intersecting the turbulent flame. This distortion is well known as beam steering. Beam steering may degrade imaging quality by reducing the overall spatial resolution. Its impact of course depends on the actual specifications of the imaging system itself. In this study a methodology is proposed to tackle this issue numerically and is exemplified for chemiluminescence imaging in a well-known turbulent hydrogen-fueled jet flame. Large-eddy simulation (LES) of this unconfined non-premixed flame is used to simulate instantaneous volumetric distributions of the flow and scalar fields including the local index of refraction. This simulation additionally predicts local concentrations of electronically excited chemiluminescent active species. At locations with significantly high concentrations of luminescent species, optical rays are initiated in the direction of the array detector used for recording single chemiluminescence images. Assuming the validity of geometrical optics, these rays are tracked along their pathways. Their direction of propagation changes according to the local instantaneous distribution of the index of refraction. After leaving the computational domain of the ray tracing code which is fed by the LES, each ray is processed by the commercial code ZEMAX® and imaged onto an array detector. Measured and numerically simulated ensemble-averaged chemiluminescence images are compared to each other. Overall, a satisfying agreement is observed. The primary aim of this paper is the exposition of this method where numerical and experimental results are not any more compared in the flame but where this comparison is shifted to the imaging plane. Future extensions to higher pressures in enclosed combustors or internal combustion engines where beam-steering effects are much more pronounced than in atmospheric jet flames are addressed.