Gianpietro Cossali

An integral model for gas entrainment into full cone sprays

The paper proposes a one-dimensional model for predicting gas entrainment into a non-evaporating full cone steady spray injected into a stagnant gas at uniform pressure. The main outcome is a law relating the entrained mass flow rate to injected mass flow rate, gas properties, mean droplet diameter and axial distance from the nozzle. A comparison with available experimental data is presented. The model allows the apparent inconsistency of the experimental results obtained under different conditions to be eliminated by identifying two new non-dimensional parameters.

Near-Field Entrainment in an Impulsively Started Turbulent Gas Jet

The entrainment characteristics of an impulsively started gas jet injected into quiescent atmosphere was studied by means of two-dimensional visualizations and laser Doppler velocimetry. The focus was on near-field behavior, where the velocity profiles are not self-similar, unlike those far downstream, and the unsteady jet head structure plays a relevant role in the early jet development. In the investigated near-field region, direct measurements of the size of the unsteady leading jet structure shows that its length is larger than 12 nozzle diameters and grows linearly with time

Near-field entrainment in unsteady gas jets and diesel sprays: A comparative study

Entrainment of surrounding gas into diesel sprays and impulsively started, axisymmetric round jets was investigated experimentally to identify the main characteristics of the gas/fuel mixing process. Diesel sprays and gaseous jets were injected into quiescent air under different ambient temperature and density. The entrainment was evaluated by measuring the air velocity normal to a cylindrical control surface surrounding the jet/spray cone. The comparison was performed over two periods: (a) the initial transient period, characterized by a strong variation of the entrainment velocity, due to the passage of the jet-head vortex: (b) the quasi-steady period, after the passage of the jet head, where the entrained gas velocity undertakes only smooth variations because of the varying injection conditions. A strong similarity between gas jets and diesel sprays was observed in the transient period when appropriate dimensionless variables were introduced, whereas differences were observed during the quasi-steady period, which may be related to the different momentum transfer mechanism acting during that period. A simple qualitative model, based on an analogy with the movement of a growing solid sphere, is also provided to interpret the experimental results about the leading vortex of diesel sprays and impulsively started gas jet.

A Fourier transform based data reduction method for the evaluation of the local convective heat transfer coefficient

Abstract A new data reduction technique for measuring the convective heat transfer coefficient is reported. The technique is based on the evaluation of the Fourier transform of simultaneously measured freestream temperature and surface wall temperature or heating power. Any wave shape can be used to heat-up the stream or the wall and the method yields information redundancy on the local heat transfer coefficient. Effects of various uncertainties on the accuracy of the heat transfer coefficient evaluation are considered and quantitatively analysed. A numerical simulation of the effects of noise on the measured temperature signal is also reported and discussed.

A new laser based technique for instability growth rate evaluation in liquid jets

Rayleigh instability of a liquid jet has been extensively studied during the past century and a theoretical solution for the inviscid case was published in 1875 by Rayleigh. Extensions of the solution were proposed by Weber (1931), Levich (1962), Levich and Krylov (1969) and Sleicher (1975), the full linear viscous theory was given by Chandrasekhar. Nonlinear theory has also been used to deal with the problem, a classical paper being that of Bohr (1909); more recent work is that of Wang (1968) and Yuen (1968). Experimental tests of those solutions have been reported and the most comprehensive analysis seems to be that of Donnelly and Glaberson which made use of a photographic technique in order to evaluate the instability growth rate as a function of wave number. More recently other results, making use of a similar technique, have been reported (Sakai et al. 1985). Photographic techniques are quite time-consuming: evaluation of growth rate by direct measurement of the pictures is not always easy and the procedure must be repeated for all the .wave numbers of interest. The technique proposed here allows the growth rate to be evaluated for any wave number by a direct Fourier transform of the signal output from a photodiode on which a laser beam of low power, crossing the liquid jet, has been imaged. The advantage of this technique relies on the rapidity of the procedure and the simple setup needed, whereas the uncercainty does not seem to be greater than that obtained using the conventional technique.

High-speed visualization of interface phenomena: single and double drop impacts onto a deep liquid layer

Single and double drop impacts onto a deep pool were observed with a high-speed camera. The early stage of drop impact was investigated to detect the evolution of the interface between the fluid of the impacting drop and of the target pool. A new flow regime for the single drop impact was detected, due to a superposition of partial coalescence and crater formation. The closure of the crown above the crater was observed for some impact parameters. The submerged flow geometry generated by the simultaneous impact of two identical drops was also reported.Graphical Abstract

Micro-heat-sinks for space applications

During the space missions, the problems related to the thermal conditioning of devices, to the personnel comfort and to the thermo-mechanical stresses are known and important. Furthermore for a space mission certain priorities are stressed, such as the small dimension and the lightness of thermal equipments. Due to the presence of high temperature gradients, which straightforwardly implies significant heating/cooling powers, these characteristics are sometimes difficult to obtain. The decreasing of the satellites payloads in terms of mass and volume has brought to the necessity of a further development of traditional space technologies, such as heat pipes and radiators. A promising technology is the fabrication of micro-heat-sinks for active and passive thermal control systems suitable for the space environment, which is always an important workshop for future progresses. In fact, miniaturized heat sinks will have a terrestrial large industrial diffusion for electronic component cooling, in propulsion and in the power production for satellites, spacecrafts and airplanes, in various biomedical applications and in cloth conditioning in harsh environmental conditions. The present paper intends to introduce the reader to the standard space requirements, to present some new prospective and experiments to present some new prospective and experimental results and to discuss the use of thermal MEMS for micro- and nano-satellites.Copyright

Radiant heat transfer between grey surfaces: an alternative approach

Abstract The paper proposes an alternative approach for reformulating the problem of heat interchange between grey surfaces. An integral Fredholm equation of second kind is still found, as for the usual treatment, but the unknown function depends only on geometry and surface properties, being independent of local surface temperature and heat flux, which can then be related to each other in a simple way by the solution of the integral equation. Examples of the solution of the integral equation by series or by decomposition in a complete set of orthogonal normalised functions are reported.

Moving Boundary Problem for Heating and Evaporation of a Spherical Drop

The process of heat and mass transfer from a spherical liquid drop evaporating into a gas environment is investigated relaxing the commonly used quasi-steady approximation and accounting for the inherent unsteadiness caused by the sudden immersion of a liquid drop in a gaseous environment. The drop radius shrinking due to evaporation settles a moving boundary problem, which is changed to a fixed boundary one by a proper coordinates transformation. The heat and evaporation rates and the drop diameter evolution is quantified by numerical solution of the species and energy conservation equations and the overall mass and energy balances over the drop for different species (water, n-octane, n-dodecane, ethanol). The paper extends the analysis previously reported by the same authors by considering the coupling between the species and energy equations.

Numerical Simulations of Internal Flow in an Aircraft Engine Pressure Swirl Atomizer

The internal flow and the liquid structure at the nozzle exit are investigated in a typical pressure swirl atomizer for aeroengine applications under an isothermal nonreacting environment using a two-phase flow modeling according to the volume-of-fluid numerical methodology. A parametrical analysis is performed to investigate the effect of operating conditions on the injector performances and the characteristics of the liquid jet exiting the nozzle. The liquid film thickness at the nozzle exit and the injector discharge coefficient predicted by the simulations are compared against commonly used correlations available in the literature. The influence of the turbulence model on the injector performances is also analyzed using three different models: the renormalization group k-ϵ model, the Reynolds stress model, and the large-eddy simulation model.