1,721,072 research outputs found
Propensity Score Matching Procedure on Discharge Records: The Case of Hip Fracture Versus Elective Hip Replacement
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Equity Rights Issue and Dilutive Effect: evidence from Italian Listed Company
Get PDFThis article studies the stock price reaction to Seasoned Equity Offerings (SEOs) through the right issue technique, for Italian listed companies in the period between 2007 and 2016. A few days before the starting date of the capital increase operation, investors are provided with a complete information set of the final characteristics of the equity offerings. The study investigates whether this further information is price sensitive. An event study analysis is performed around two price sensitive dates: the “announcement date” of the equity issue and the “communication date” of its final characteristics. It also focuses on the reasons underlying the offer and on the industry effect. The findings show a significant negative abnormal return at the communication date for the full sample and for companies collecting financial resources for “Corporate Finance Transaction”, for "Capital Adequacy" and for “Restructuring”. A negative market reaction for all sectors is observed as well. Eventually, the article examines the possible causes underlying the negative stock price reaction at the communication date. The results suggest that the dilutive effect is the main explanation to the stock price overreaction
Combined fluid-dynamic modelling of hybrid rocket internal ballistics and nozzle heat transfer
No full textA computational thermo-fluid-dynamic model of the hybrid rocket internal ballistics has been developed in the present work. Numerical simulations of the flowfield in a laboratory 200 N-class hybrid rocket engine, operated with gaseous oxygen and high-density polyethylene or acrylonitrile-butadiene-styrene have been carried out. The objective is twofold: first the prediction of the solid fuel regression rate, which is calculated with an improved gas/surface interface treatment based on local mass, energy and mean mixture fraction balances as well as proper turbulence boundary conditions, along with chamber pressure and combustion efficiency. Second, the detailed study of the discharge nozzle flow and heat transfer. For the validation of the model, data retrieved from two firing tests are compared with the numerical results revealing good agreement of the average regression rates, fuel consumption axial profiles, and of the chamber pressure and combustion efficiency. The output of the motor ballistic simulations are then used for a detailed numerical study of the flow through the nozzle and of the unsteady thermal field inside the nozzle solid block showing different behaviours of graphite compared to ceramic material nozzle, highlighting the severe thermal gradient occurring in the ceramic material
Experimental Investigation of Capillary-Driven Two-Phase Flow in Water/Butanol under Reduced Gravity Conditions
No full textThe capillary flow of water/butanol mixture is investigated in a single groove heat pipe model on board theAirbus A310‘Zero-G’of the European Space Agency. As working fluid for evaporation-based heat transferdevices like conventional or pulsating heat pipes, these kinds of mixtures give more stable behaviour and higherdry-out limit with respect to pure water because of an anomalous behaviour of the surface tension with temper-ature. The groove is embedded in a semi-transparent test cell that allows for the qualitative visualization of theliquid distribution along the channeland is heated and cooled at two opposite sides with an electrical resistanceand a water circulation system. The evaporation/condensation process is regulated changing the power input in arange between 5 and 30 W and the liquid distribution is detected from a top windowusingaCCDcameraandaLED illumination device. The results show that the liquid distribution is affected by the gravity level and thiseffect is normally masked on ground
Computational Evaluation of Aero-Thermo-Dynamic Loads and Effect of Catalyticity in an Arc-Jet Wind Tunnel
No full textWith a view to designing tests in the arc wind tunnel Small Planetary Entry Simulator (SPES) at the Department of Industrial Engineering of the University of Naples "Federico II" on specimens of Ultra-High-Temperature Ceramic (UHTC) materials, computer simulations have been carried out in order to get information about the test conditions, i.e. electrical power supplied to the torch, the nozzle area ratio (exit area/throat area) and the specimen geometry, in order to avoid expensive and time-consuming experimental characterization. Computations have been carried out both by a Computation Fluid-Dynamics (CFD) and a Direct Simulation Monte Carlo (DSMC) code. The computations provided important, quantitative information about flow field as per velocity, temperature, gas composition, etc. and about heat flux and pressure on the specimen. The present computations allowed the authors to define the capabilities of SPES to perform tests on UHTC specimens, identifying a range of potential applications. Computed values of heat flux have been also compared with preliminary measurements performed by means of a copper slug calorimeter. Due to the evident effect of surface catalytic level on the experimental measurements, an experimental/computational procedure has been used to estimate the recombination efficiencies of oxygen and nitrogen on the calorimeter surface. The definition of this procedure is aimed at a future evaluation of surface catalyticity of UHTC specimens. At the conditions tested in the present paper, the procedure indicates that oxygen catalytic effect is higher than that of nitrogen, in agreement with what reported in literature
Computational fluid-dynamic modeling of the internal ballistics of paraffin-fueled hybrid rocket
No full textComputational fluid dynamics is becoming a key tool for reducing the hybrid rocket operation uncertainties and development cost, but numerous challenges, due to the complexity of modeling the solid fuel consumption mechanism and the interaction with the reacting flowfield, have still to be addressed. These latter features are further complicated with paraffins for the melted-fuel entrainment phenomenon. This paper presents a computational thermo-fluid-dynamic model of the internal ballistics of hybrid rockets burning gaseous oxygen and paraffin-based fuel. With the purpose of predicting the local fuel regression rate, the model is coupled with an improved gas/fuel-surface interface treatment based on local mass, energy and mean mixture fraction balances, combined to an additional analytical equation for the calculation of the entrainment fraction of the fuel consumption rate. Parametric analyses are carried out to assess the effect of fuel properties on the regression rate. Several experimental test cases, obtained from static firing of a laboratory-scale rocket, are simulated. Calculated regression rates show an error with respect to the measured data around 10% in the worst case. Chamber pressure is predicted with lower accuracy, with errors less than 20%; the main factor of deviation is shown to be the estimation of the combustion efficiency
A simplified computational fluid-dynamic approach to the oxidizer injector design in hybrid rockets
No full textFuel regression rate in hybrid rockets is non-negligibly affected by the oxidizer injection pattern. In this paper a simplified computational approach developed in an attempt to optimize the oxidizer injector design is discussed. Numerical simulations of the thermo-fluid-dynamic field in a hybrid rocket are carried out, with a commercial solver, to investigate into several injection configurations with the aim of increasing the fuel regression rate and minimizing the consumption unevenness, but still favoring the establishment of flow recirculation at the motor head end, which is generated with an axial nozzle injector and has been demonstrated to promote combustion stability, and both larger efficiency and regression rate. All the computations have been performed on the configuration of a lab-scale hybrid rocket motor available at the propulsion laboratory of the University of Naples with typical operating conditions.
After a preliminary comparison between the two baseline limiting cases of an axial subsonic nozzle injector and a uniform injection through the prechamber, a parametric analysis has been carried out by varying the oxidizer jet flow divergence angle, as well as the grain port diameter and the oxidizer mass flux to study the effect of the flow divergence on heat transfer distribution over the fuel surface. Some experimental firing test data are presented, and, under the hypothesis that fuel regression rate and surface heat flux are proportional, the measured fuel consumption axial profiles are compared with the predicted surface heat flux showing fairly good agreement, which allowed validating the employed design approach. Finally an optimized injector design is proposed
Transient Computational Thermofluid-Dynamic Simulation of Hybrid Rocket Internal Ballistics
No full textA computational thermofluid-dynamic model of hybrid rocket internal ballistics is developed. Numerical simulations of the flowfield in a laboratory small-scale hybrid rocket motor, operated with gaseous oxygen and high-density polyethylene propellants, are carried out with the aim of predicting the solid fuel regression rate experimentally achieved with two different oxidizer injectors. The fuel regression rate is the main parameter for the hybrid rocket design. Here, it is calculated with a detailed gas/surface interface characterization based on local mass and energy balances. The combustion of oxygen and gaseous ethylene injected from the fuel wall is modeled by means of the probability-density-function approach coupled to chemical equilibrium. Two oxidizer-injection configurations, which generate either a two-dimensional axially symmetric or three-dimensional flowfield, are analyzed. The local regression rate is evaluated along both the fuel grain axis and inner circumference in the three-dimensional case, as well as at several stages in the firing, by updating the local port diameter, which is thus not assumed constant. Data retrieved from three firing tests are compared with the numerical results, revealing good agreement between both the average regression rates (maximum deviation less than 5%) and the fuel consumption axial profiles (with maximum deviation of 14%
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