Fabio Cozzi

Combustion mechanism of an RDX-based composite propellant

The co mbustion mechanism of a composite propellant having the nominal composition 84% RDX + 16% HTPB has been investigated experimentally. The following steady combustion parameters were collected over the pressure range 0.1 to 8.21 MPa: burn rates, burn surface temperatures, thermal gradients, heat release in the condensed phase, heat feedback from the gas phase, first and second (or final) flame temperatures. In general, for the tested RDX.HTPB mixture, values smaller than the corresponding AP.HTPB parameters are found, while the burning rate pressure exponent is larger. Likewise, the measured values are smaller than the corresponding pure RDX parameters when HTPB binder is added; a notable exception is the heat release in the condensed phase, which is larger. The obtained dependence of burn rate on burn surface temperature fits the unified gasification law for nitramines. Thus, RDX seems to dictate its macrokinetics law upon the HTPB gasification. The main processes taking place simultaneously at the burn surface are: gasification and thermal decomposition of RDX, as well as oxidation of part of the HTPB binder, probably due to liquid-phase mixing between molten ingredients.

A LES Simulation of a CH4/Air Microcombustor with Detailed Chemistry

The authors compare numerical simulations and experiments carried out on a swirling methane/air cylindrical microcombustor with diameter and height 0.006 m and 0.009 m, respectively. They extend previous work, using LES and Flamelet methodology; here the EDC finite rate model and the GRIMech 1.2 (32 species and 177 reactions) mechanism are used. Gas temperature at the exhaust section, together with CH* chemiluminescence measurements and combustion efficiency analysis are provided. Results reproduce exhaust temperature and combustion efficiency measurements, generally differing by less than 10%. This work should be seen as an advance in the understanding of how to design future microcombustors, presently under rapid development in particular for propulsion (e.g., for UAVs) and microelectrical power generators.

Preliminary characterization of an expanding flow of siloxane vapor MDM

The early experimental results on the characterization of expanding flows of siloxane vapor MDM (C8H24O2Si3, octamethyltrisiloxane) are presented. The measurements were performed on the Test Rig for Organic VApors (TROVA) at the CREA Laboratory of Politecnico di Milano. The TROVA test-rig was built in order to investigate the non-ideal compressible-fluid behavior of typical expanding flows occurring within organic Rankine cycles (ORC) turbine passages. The test rig implements a batch Rankine cycle where a planar converging-diverging nozzle replaces the turbine and represents a test section. Investigations related to both fields of non-ideal compressible-fluid dynamics fundamentals and turbomachinery are allowed. The nozzle can be operated with different working fluids and operating conditions aiming at measuring independently the pressure, the temperature and the velocity field and thus providing data to verify the thermo-fluid dynamic models adopted to predict the behavior of these flows. The limiting values of pressure and temperature are 50 bar and 400 °C respectively. The early measurements are performed along the nozzle axis, where an isentropic process is expected to occur. In particular, the results reported here refer to the nozzle operated in adapted conditions using the siloxane vapor MDM as working fluid in thermodynamic regions where mild to medium non-ideal compressible-fluid effects are present. Both total temperature and total pressure of the nozzle are measured upstream of the test section, while static pressure are measured along the nozzle axis. Schlieren visualizations are also carried out in order to complement the pressure measurement with information about the 2D density gradient field. The Laser Doppler Velocimetry technique is planned to be used in the future for velocity measurements. The measured flow field has also been interpreted by resorting to the quasi-one-dimensional theory and two dimensional CFD viscous calculation. In both cases state-of-the-art thermodynamic models were applied.

Experimental And Numerical Analysis Of Supersonic Blade Profiles Developed For Highly Loaded Impulse Type Steam Turbine Stages

The paper describes the results of a numerical and experimental research program addressing the aerodynamic investigation on the performance of blade profiles specifically developed for application in highly loaded impulse type turbine stages. The industrial requirements driving toward the adoption of highly loaded stage solutions are presented, along with an estimation of the profiles operating parameters. Two stator vanes and one rotor blade profile have been developed and extensively tested by means of flow field measurements and schlieren visualization in a transonic blow-down wind tunnel for linear cascades. Experimental results for the relevant operating conditions are presented, providing validation data for the CFD model used for blade design and evidencing that the main goals of the design optimization procedure have been achieved.

NATURAL GAS BURNERS FOR DOMESTIC AND INDUSTRIAL APPLIANCES. APPLICATION OF THE PARTICLE IMAGE VELOCIMETRY (PIV) TECHNIQUE.

This contribution presents some examples of the application of the particle image velocimetry (PIV) technique to domestic appliances and small-scale burners, with the aim of discussing relevant results together with problems encoun- tered. Combustion efficiency and pollutant emissions of gas burners are strongly influenced by the fluid dynamics of the mixture in both premixed and nonpremixed flames. For these reasons the Combustion Laboratory of Politecnico di Milano started using laser diagnostic techniques (LDV and PIV) many years ago. The first PIV application was on a premixed V-flame attached over a burner plate with rectangular twin slots, developed for domestic appliances. The autocor- relation of a double-exposed photograph was used to define the 2-D velocity flow field and high spatial resolution was obtained with a 1 : 1 magnification and a zoom Nikkor objective. More recently, a crosscorrelation CCD camera with a double-pulse Nd:YAG laser was used to characterize high-swirl flows under both nonreacting and reacting conditions. In the following, special emphasis will be given to the discussion of the most relevant results and the main problems encountered with PIV applications to the investigated cases.

Natural Gas Burners for Domesticand Industrial Appliances.

This contribution presents some examples of the application of the particle image velocimetry (PIV) technique to domestic appliances and small-scale burners, with the aim of discussing relevant results together with problems encountered. Combustion efficiency and pollutant emissions of gas burners are strongly influenced by the fluid dynamics of the mixture in both premixed and nonpremixed flames. For these reasons the Combustion Laboratory of Politecnico di Milano started using laser diagnostic techniques (LDV and PIV) many years ago. The first PIV application was on a premixed V-flame attached over a burner plate with rectangular twin slots, developed for domestic appliances. The autocorrelation of a double-exposed photograph was used to define the 2-D velocity flow field and high spatial resolution was obtained with a 1 : 1 magnification and a zoom Nikkor objective. More recently, a crosscorrelation CCD camera with a double-pulse Nd:YAG laser was used to characterize high-swirl flows under both nonreacting and reacting conditions. In the following, special emphasis will be given to the discussion of the most relevant results and the main problems encountered with PIV applications to the investigated cases.

Errors and Noise in Laser Recoil Measurements

A study of noise and error sources encountered during development of a laser-recoil test capability is reported. The laser-recoil technique is a method of evaluating the response of a burning energetic material to fluctuations of radiant heat flux, usually supplied by a laser. The technique involves measuring the extremely small recoil force imparted as the burning gases leave the surface. This technique offers the advantages of small sample size, reduced cost, and potentially improved accuracy over the existing acoustic T-burner technique but has the disadvantage of high sensitivity to noise because of the small magnitude of the recoil force to be measured. Noise sources identified include electrical ground loops, thermal drift, structural vibrations, acoustics, and low-frequency atmospheric pressure variations. Remedial changes reduced the overall noise level by a factor of five and extended the usable measurement range from 200 to 700 Hz. Tests and analyses of several potential errors are discussed, including demonstration of linear behavior at input heat-flux modulation levels up to 25%.

RADIATION-DRIVEN BURNING OF ENERGETIC SOLID MATERIALS WITH PHASE TRANSITION*

An extension of the Zeldovich - Novozhilov approach to burning of solid energetic materials subjected to a concentrated phase transition is presented. The radiation-driven frequency response function is obtained hi the linear approximation of the problem. Connections with the equivalent FM formulation are discussed. The corresponding previous results are recovered as special cases for no phase transition. The steady burning sensitivity parameters depending on initial temperature, r and k, are deduced by assuming a surface pyrolysis law. Some typical results, with and without radiation penetration, are presented. To validate these theoretical expectations, accurate error estimates of experimental data are needed. NOMENCLATURE