1,721,021 research outputs found
Development of a flexible MAPMT photon-counting read-out system
No full textThe design and the attained performances of a read-out system for a 64-channel Hamamatsu Multi Anode Photo Multipliers (MAPMT) are discussed. The system has been developed in the framework of the Extreme Universe Space Observatory (EUSO) experiment. EUSO is a space telescope devoted to the study of the ultra-high-energy cosmic rays, proposed for accommodation on board the International Space Station (ISS). This paper reports the tests on the read-out system and the optimization of its main parameters. A quick overview of some other possible applications of the system is also given
A novel quasi-one-dimensional model for performance estimation of a Vaporizing Liquid Microthruster
Full text linkThe present work aims to propose a novel quasi-one-dimensional model for the performance estimation of a Vaporizing Liquid Microthruster (VLM). The analytical model was applied to the analysis of a MEMS-based VLM composed of a rectangular inlet chamber, a set of parallel microchannels as heating chamber, and a planar convergent–divergent micronozzle. It combines a steady-state boiling model for the analysis of the heater with a real nozzle flow model for the evaluation of actual thrust force and specific impulse, based on iterative procedure aiming at the convergence of the actual mass flow rate and the heat flux. For the purpose, a set of semi-empirical formulas found among both theoretical and experimental scientific works have been introduced for the estimation of the critical heat flux condition and the local heat transfer coefficient. In addition, the real nozzle flow model predicts the performance and the viscous losses due to the boundary layer growth inside the micronozzle. The last ones are estimated by introducing analytical expressions for the discharge coefficient and the Isp-efficiency into the isentropic nozzle flow theory. The resulting performance predictions of the 1D model referred to the on-design operating conditions. They well agreed with the experimental data, with a maximum estimated error of 7.3% on the thrust and the specific impulse. Furthermore, the analytical model of the micronozzle predicted a reduction of the mass flow rate up to about 8%, as well as thrust losses up to 15% due to the contraction of the cross sectional area. In addition, 2D and 3D computational fluid dynamics (CFD) simulations were performed in order to enforce the analysis of the viscous effects. Predictions of 2D computations overestimated the performances of the microthruster with respect to experiments, up to about 19% of the thrust and 20% of the specific impulse. On the other hand, the 3D predicted thrust approached to the experimental one with an error of about 9.2% below. In addition, a severe reduction of jet thrust in favor of the pressure thrust was observed at the nozzle exit. Furthermore, 3D computations pointed out the influence of the micronozzle depth on the boundary layer growth and the viscous losses. In particular, they revealed the establishment of the nozzle blockage and the thermal chocking of the supersonic flow owing to the subsequent viscous heating
Comparison of numerical predictions of the supersonic expansion inside micronozzles of micro-resistojets
Full text linkThe present work provides a numerical investigation of the supersonic flow inside a planar micronozzle configuration under different gas rarefaction conditions. Two different propellants have been considered, namely water vapor and nitrogen, which relate to their use in VLMs (the former) and cold gas microthrusters (the latter), respectively. Furthermore, two different numerical approaches have been used due to the different gas rarefaction regime, i.e. the typical continuum Navier–Stokes with partial slip assumption at walls and the particle–based Direct Simulation Monte Carlo (DSMC) technique. As a result, under high–pressure operating conditions, both water and nitrogen flows supersonically expanded into the micronozzle without chocking in combination with a linear growth of the boundary layer on walls. However, when low–pressure operating condition are imposed and a molecular regime is established inside the micronozzle, a very rapid expansion occurred close to the nozzle exit in combination with a strong chocking of the flow and a micronozzle quality reduction of about 40%. Furthermore, water exhibited specific higher specific impulse than nitrogen above 60%
Characterization of unsteady cavitating flow regimes around a hydrofoil, based on an extended Schnerr–Sauer model coupled with a nucleation model
No full textThe present work is aimed at the numerical spatio-temporal characterization of the unsteady cavitating flow structures on a hydrofoil by means of Proper Orthogonal Decomposition (POD) and Fast Fourier Transform (FFT) techniques. Three different cavitation regimes have been investigated: bubble cavitation, cloud cavitation and supercavitation. The homogeneous mixture approach has been used, in combination with an extended Schnerr–Sauer cavitation model. The accuracy of the numerical predictions has been improved by means of the implementation of a Density Correction Model of the turbulent viscosity, and a simplified Population Balance Modeling (PBM) which solved the spatial distribution and the temporal evolution of the nuclei. In particular, the PBM has led to a reduction of the intensity of the evaporation inside the vapor cavities and a consequent condensation enhancement at the cavity closure and in the wake downstream. This phenomenon mainly impacted on the vapor cavity dynamics during supercavition by facilitating the formation of the re-entrant jet and the vapor cavity detachment. Also, during supercavitation the nuclei density n b exhibited maximum variations of about 35.6% with respect to the inlet nuclei density. As the cavitation number increased, both the intensity and the extension of oscillations significantly reduced, and in bubble cavitation regime n b fluctuated at amplitudes of about 10% of the inlet nuclei density. The characterization of the cavitation regimes revealed that the bubble cavitation regime had a more stable and periodic dynamics highlighted by a higher hydrodynamic efficiency and a reduced root mean square of the lift force. The cloud cavitation and the supercavitation exhibited a more violent bubble detachment which caused stronger oscillations of the vapor cavity as well as the pressure upstream. This was retrieved in an increase of the average drag coefficient of about the 38% due to the presence of vapor cloud transported downstream, which promoted the surge of the flow. The vorticity analysis underlined that the formation of the re-entrant jet and the bubble detachment were promoted by the baroclinic vorticity, while the dilatation vorticity drove the dynamics of the detached clouds, governed by the phase change phenomena. The FFT analysis of the dynamics of the vapor cavity and the pressure upstream led to the detection of the most representative frequencies and Strouhal numbers of each cavitation regimes, in particular (f s =16.6Hz, St=0.355) for bubble cavitation, (f s =10.74Hz, St=0.358) for cloud cavitation, and (f s =8.79Hz, St=0.300) during supercavitation. The POD analysis allowed for the detection of the most relevant cavitating structures, in relation to the vapor cavity fluctuations and their frequency content. Furthermore, the FFT analysis of the temporal eigenfunctions demonstrated that the first POD mode of the liquid volume fraction described the overall unsteady behavior previously detected. Instead, high order POD modes revealed frequency values well above the overall ones of the main flow previously detecte
Combustion Characteristics of Hydrogen/Air Mixtures in a Plasma-Assisted Micro Combustor
Full text linkThis work performs an analysis of plasma-assisted non-premixed H2-air flames in Y-shaped micro combustors in the presence of field emission dielectric barrier discharge (FE-DBD) plasma actuators. The combustion, flow, and heat transfer characteristics are numerically investigated, and the effect of sinusoidal plasma discharges on combustion performance is examined at various equivalence ratios (φ). A coupled plasma and chemical kinetic model is implemented, using a zero-dimensional model based on the solution of the Boltzmann equation and the ZDPlasKin toolbox to compute net charges and radical generation rates. The estimated body forces, radical production rates, and power densities in the plasma regions are then coupled with hydrogen combustion in the microchannel. Plasma-assisted combustion reveals improvements in flame length and maximum gas temperature. The results demonstrate that FE-DBDs can enhance mixing and complete the combustion of unreacted fuel, preventing flame extinction. It is shown that even in cases of radical and thermal quenching, these plasma actuators are essential for stabilizing the flame
Thrust augmentation of micro-resistojets by steady micro-jet blowing into planar micro-nozzle
Full text linkThe present work investigates the impact of steady micro-jet blowing on the performance of a planar micro-nozzle designed for both liquid micro-thrusters and nitrogen cold-gas micro-resistojets. Two micro-injectors have been placed into the divergent region along the sidewalls, injecting a secondary flow of propellant perpendicularly to the wall where they have been located. The micro-jet actuator configuration is characterized by the dimensionless momentum coefficient cμ. The best performance improvement is retrieved at the maximum cμ for both water vapor (∆%T,jet = +22.6% and ∆%Isp,Tjet = +2.9% at cμ = 0.168) and nitrogen gaseous flows (∆%T,jet = +36.1% and ∆%Isp,Tjet = +9.1% at cμ = 0.297). The fields of the Mach number and the Schlieren computations, in combination with the streamline visualization, reveal the formation of two vortical structures in the proximity of secondary jets, which energize the core flow and enhance the expansion process downstream secondary jets. The compressible momentum thickness along the width-wise direction θxy in presence of secondary injection reduces as a function of cμ. In particular, it becomes smaller than the one computed for the baseline configuration at cμ > 0.1, decreasing up to about and −57% for the water vapor flow at cμ = 0.168, and-64% for the nitrogen gaseous flow at cμ = 0.297
Active control of unsteady cavitating flows in turbomachinery
No full textA preliminary 2D numerical investigation of the active control of unsteady cavitation by means of one single synthetic jet actuator (SJA) is presented. The SJA has been applied to hinder the intrinsic instabilities of a cloud cavitating flow of water around a NACA 0015 hydrofoil with an angle of attack of 8° and ambient conditions. It has been placed inside the inception region at a distance of 16% of the chord from the leading edge. Concerning the numerical approach, a Eulerian homogeneous mixture/mass transfer model has been used, in combination with an extended Schnerr-Sauer cavitation model and a Volume of Fluid (VOF) interface tracking method. The synthetic jet has been modeled by means of a user-defined velocity boundary conditions based on a sinusoidal waveform. A sensitivity analysis has been first performed in order to evaluate the influence of the main control parameters, namely the momentum coefficient Cμ, the dimensionless frequency F+ and the jet angle αjet. By combining the cavitating vapor content and the impact on the hydrodynamic performance, the best performing SJA configuration has been retrieved. Then, a deeper analysis of the vapor cavity dynamics and the vorticity field has been conducted in order to understand the modification of the main flow produced by the synthetic jet. The best SJA configuration was observed at Cμ = 0.0002, F+ = 0.309 and αjet = 90°, which led to a reduction of both the average vapor content and the average torsional load in the measure of 34.6% and 17.8% respectively. A reduction of the average pulsation frequency of the pressure upstream confirmed the beneficial effect of the SJA. The analysis of the coupled dynamics between vapor cavity-vorticity and their POD-based modal structures highlighted that the benefit of the SJA lies on preventing the growth of a thick sheet cavity which tends to cause the development of the highly cavitating cloud dynamics after the cavity breakup. This is mainly due to an additional vorticity close to the hydrofoil surface just downstream the SJA, as well as a local pressure modification close the SJA during the blowing stroke
CFD data of unsteady cavitation around a hydrofoil, based on an extended Schnerr-Sauer model coupled with a nucleation model
Full text linkThe data presented in this article were the basis for the study reported in the research articles entitled “Characterization of unsteady cavitating flow regimes around a hydrofoil, based on an extended Schnerr-Sauer model coupled with a nucleation model” (De Giorgi et al., 2018)[1]. The reference study presented a spatio-temporal characterization of different cavitating flow regimes using Computational Fluid Dynamics (CFD). The authors evaluated the accuracy of an extended Schnerr-Sauer cavitation model. The accuracy of the numerical model has been improved by means of the introduction of a Density Correction Model of the turbulent viscosity, and a simplified Population Balance Modeling (PBM)
Effects on performance, combustion and pollutants of water emulsified fuel in an aeroengine combustor
No full textThe present work provides an experimental investigation of the use of water emulsified fuels to control the combustion performance and reduce nitrogen oxides emissions into a Jet-A1 fueled gas turbine combustor. Experiments have been carried out using a test rig equipped with a 300-kW liquid-fueled swirling burner. Several fuel-to-air ratios have been tested in combination with various water concentrations. Measurements of exhaust emissions have been performed. Furthermore, high-speed cameras in visible and ultraviolet spectral ranges, have been used. The snapshot Proper Orthogonal Decomposition of the flame images of both broadband emission and hydroxyl radical chemiluminescence has allowed to detect the most relevant flame structures, in combination with the modal frequency spectra. Results figured out that, the addition of water in the fuels led to lower combustion temperature and consequently to lower thermal nitrogen oxides than the case of neat fuel. On the other hand, the thermal efficiency significantly dropped in presence of high-water content (5% H2O) and ultra-lean conditions, while it remained acceptable at 2.5% H2O and fuel rich conditions. Furthermore, under the near-lean blowout condition, the flame becomes very unstable and flame oscillations take place in the axial direction. This combines with the increase in the relative energy of the first Proper Orthogonal Decomposition modes. Finally, the phase space analysis of modes 1–2 of the hydroxyl radical chemiluminescence emission defined a criterion for the detection of the establishment of the flame instability, which corresponds to phase angles ranging between -π/6 and π/6
Numerical investigation of nonisothermal cavitating flows on hydrofoils by means of an extended schnerr–sauer model coupled with a nucleation model
No full textThis work aimed to investigate cavitating flows of water, liquid hydrogen, and nitrogen on hydrofoils numerically, using the open source code OPENFOAM. The Eulerian homogeneous mixture approach has been used, consisting in a mass transfer model, which is based on the combination of a two-phase incompressible unsteady solver with a volume of fluid interface tracking method. Thermal effects have been introduced by means of the activation of energy equation and latent heat source terms plus convective heat source term. The dependency of the saturation conditions to the temperature has been defined using Antoine-like equations. An extended Schnerr–Sauer model based on the classical nucleation theory (CNT) has been implemented for the computation of the interfacial mass transfer rates. In order to investigate the nucleation effects, an extension of the CNT has been considered by coupling the population balance equation (PBE)/extended quadrature-based method of moments with the computational fluid dynamics (CFD) model, which has been defined in combination with a transport equation for the nuclei density. Results showed that nucleation determined a nonuniform field of nuclei density so as to produce a reduction of the temperature drop inside the vapor bubbles, as well as a warmed wake downstream the vapor cavity. Unsteady computations also revealed an influence of the nucleation on the dynamics of the vapor cavity and the bubble detachment
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