Structure of Evaporating and Combusting Sprays: Measurements and Predictions

Complete measurements of the structure of nonevaporating, evaporating and combusting sprays for sufficiently well defined boundary conditions to allow evaluation of models of these processes were obtained. The development of rational design methods for aircraft combustion chambers and other devices involving spray combustion were investigated. Three methods for treating the discrete phase are being considered: a locally homogeneous flow (LHF) model, a deterministic separated flow (DSF) model, and a stochastic separated flow (SSF) model. The main properties of these models are summarized

Evaluation of a locally homogeneous flow model of spray combustion

A model of spray combustion which employs a second-order turbulence model was developed. The assumption of locally homogeneous flow is made, implying infinitely fast transport rates between the phase. Measurements to test the model were completed for a gaseous n-propane flame and an air atomized n-pentane spray flame, burning in stagnant air at atmospheric pressure. Profiles of mean velocity and temperature, as well as velocity fluctuations and Reynolds stress, were measured in the flames. The predictions for the gas flame were in excellent agreement with the measurements. The predictions for the spray were qualitatively correct, but effects of finite rate interphase transport were evident, resulting in a overstimation of the rate development of the flow. Predictions of spray penetration length at high pressures, including supercritical combustion conditions, were also completed for comparison with earlier measurements. Test conditions involved a pressure atomized n-pentane spray, burning in stagnant air at pressures of 3, 5, and 9 MPa. The comparison between predictions and measurements was fair. This is not a very sensitive test of the model, however, and further high pressure experimental and theoretical results are needed before a satisfactory assessment of the locally homogeneous flow approximation can be made

Experimental verification of computer spray-combustion models

Analytical model formulation, representing performance of spray-combustion device, is based on understanding of atomization, mixing, vaporization, and combustion which occurs in device. Report lists results of correlations of computed values with values obtained from experiments with rocket combustor. Technique offers excellent method for evaluating validity and ranges of applicability of combustion models

Modeling of Spray Combustion under Cryogenic and High Pressure Conditions

The paper concerns both the numerical and experimental investigation of turbulent liquid oxygen/hydrogen spray combustion for elevated subcritical pressure and cryogenic inlet temperature conditions. In particular, the combustion in the single injector combustion chamber is studied where experimental data are obtained for gas phase temperature and both droplet size and velocities. The model uses an Eulerian--Lagrangian formulation for the gas and the liquid phase, respectively. Detailed models for droplet heating and vaporization in a convective flow field are employed, and detailed gas phase reactions are accounted for through use of a flamelet model for turbulent spray combustion. The results show a very good agreement between experimental and computational spray characteristics. The computed gas phase temperature lies somewhat above the experimental values which is associated with CARS single shot measurements and incomplete data for the initial conditions of the combustion process

Effervescent Breakup and Combustion of Liquid Fuels: Experiment and Modelling

Tato práce se zaměřuje na oblast effervescentních sprejů a jejich aplikace na kapalné spalování s důrazem na průmyslové spalovací komory. Oba aspekty – modelování a experiment – jsou řešeny. Práce obsahuje obecný úvod, ve kterém jsou vysvětleny základní jevy rozpadu kapaliny a vířivého spalování a dále je představena effervescentní atomizace. Poté jsou popsány použité experimentální postupy jak pro měření spreje, tak pro měření tepelných toků do stěn při spalování. V následující kapitole jsou popsány numerické modely a jejich podstata je vysvětlena. Jsou zde uvedeny modely pro rozpad spreje, turbulenci a spalování použité během výzkumu. Vlastní výsledky práce jsou uvedeny formou samostatných článků (vydaných nebo přijatých) s dodatečnou částí věnovanou nepublikovaným relevantním výsledkům. Bylo zjištěno, že standardní modely sprejů jsou do jisté míry schopny popsat effervescentní spreje. Nicméně aby bylo možné predikovat plamen kapalného spreje, jsou zapotřebí detailnější modely sprejů, které dokáží přesně zachytit změnu průměrů kapek v radiálním a axiálním směru. Experimentální měření effervescentních sprejů bylo provedeno pomocí navrhnuté metodiky. Výsledky měření byly analyzovány s důrazem na radiální a axiální vývoj průměrů kapek a některé nové jevy byly popsány. Nepřímá úměrnost mezi gas-liquid-ratio a středním průměrem kapek byla potvrzena. Dále by popsán jev, kdy pro různé axiální vzdálenosti které dojde k úplnému převrácení závislosti středního průměru na axiální vzdálenosti. V závěru je uvedeno shrnutí, které rekapituluje hlavní výsledků a závěry. V závěrečných poznámkách je nastíněn možný budoucí postup. Experimentální data pro ověřování budoucích effervescentních modelů jsou poskytnuta.This thesis presents an investigation of effervescent sprays and their application to spray combustion with emphasis on large-scale combustors. Both aspects – modelling and experiment – are addressed. The thesis contains a general introductory part, where underlying phenomena of spray forming and turbulent combustion are explained and effervescent atomization is presented. Then, adopted experimental approaches are described both for the spray measurement and for the measurement of wall heat fluxes during combustion experiments. In the following chapter numerical models and their philosophy is discussed. Models for spray formation, turbulence and combustion adopted during the research are introduced and explained. The actual results of the thesis are presented in form of separate papers (published or accepted for publication) with an additional section devoted to unpublished relevant results. It is found that standard spray models can to some extent represent effervescent sprays. However, in order to predict a spray flame more detailed spray models are needed in order to describe accurately radial and axial variations of drop sizes. Numerous experimental measurements of effervescent sprays are performed using a proposed methodology. Drop size data are analysed with emphasis on radial and axial drop size evolutions and some new phenomena are described. The inverse relationship between gas-liquid-ratio and mean diameter has been confirmed. Moreover a complete reversal in radial mean diameter trends for various axial locations has been described. Finally, a result summary is put forward that recapitulates the main accomplishments and conclusions. In the closing remarks possible future research is outlined. Experimental data for future effervescent model validations are disclosed.

Computational experience with a three-dimensional rotary engine combustion model

A new computer code was developed to analyze the chemically reactive flow and spray combustion processes occurring inside a stratified-charge rotary engine. Mathematical and numerical details of the new code were recently described by the present authors. The results are presented of limited, initial computational trials as a first step in a long-term assessment/validation process. The engine configuration studied was chosen to approximate existing rotary engine flow visualization and hot firing test rigs. Typical results include: (1) pressure and temperature histories, (2) torque generated by the nonuniform pressure distribution within the chamber, (3) energy release rates, and (4) various flow-related phenomena. These are discussed and compared with other predictions reported in the literature. The adequacy or need for improvement in the spray/combustion models and the need for incorporating an appropriate turbulence model are also discussed

Use of fluent for the development of a di-si engine

The recent surge of electric vehicles has put pressure on the development and manufacture of batteries. However, batteries are still expensive, bulky and heavy, creating the need for inboard electricity generation using an internal combustion engine, usually referred as “range extender”. This paper presents the initial development of a DI-SI engine to work as range extender, focusing on the interaction between fuel spray and airflow inside the combustion chamber. To enable efficient combustion of lean and extra lean mixtures, a technique called stratified charge, is used. With direct injection spark ignition (DI-SI) engines it is important, under part load, to direct the fuel spray to the vicinities of the spark plug, enabling a fast and stable combustion of a lean mixture. A rich mixture region is created near the spark plug allowing an easy kernel formation and development. There are three types of systems for “directing” the fuel spray towards the spark plug: wall guided, air guided and spray guided. The developed design is a mixture of wall and air guided systems and the idea is to inject the spray towards the piston crown and to divert it to the spark plug location by the barrel swirl existent within the combustion chamber at this time. The system development was carried out using CFD FLUENT code. The study comprises three parts, the design of the components and its location (combustion chamber, piston crown, intake passage and injector location and aim), the air flow modeling and finally, the two phase modelling. A simple engine geometry and mesh were created in the Ansys CFD software. The air flow was considered to be transient, incompressible, Newtonian and viscous turbulent. The turbulence model used was the standard k-ε model, since it is the most common, simple and well-known model of turbulence. The spray has been simulated using the Discrete Phase Model. The Lagrangian discrete phase model in Fluent™ follows the Euler-Lagrange approach, where the fluid phase is treated as a continuum by solving the time-averaged Navier-Stokes equations, while the dispersed phase is solved by tracking a large number of particles through the calculated flow field. Preliminary results are now being obtained.MIT Portugal, Fundação para a Ciência e a Tecnologia (FCT

Development of a Multiphase Photon Monte Carlo Method for Spray Combustion and its Application in High-pressure Conditions

In this work the development of a multiphase photon Monte Carlo (PMC) method with a focus on resolving radiative heat transfer in combustion simulations is presented. The multiphase PMC solver can account for description of participating media in both Lagrangian and Eulerian frameworks. The solver is validated against exact solutions in several one-dimensional configurations. The developed solver is then applied to Diesel spray combustions, where liquid spray droplets are assumed to be cold, nonemitting, large, and isotropically scattering. Several formulations for radiative properties of the Diesel spray are first explored. The PMC solver has then been coupled with the multiphase spray combustion solver in OpenFOAM and the coupled solver is used for simulations of high pressure Diesel spray combustion. It was found that in typical Diesel spray combustion applications, such as in an internal combustion engine, impact of radiation on the evolution of the liquid spray was insignificant. Although the impact of radiation on the spray was minimal, nongray spectral properties and the assumption of semi-transparency for Diesel spray were found to impact the radiative transfer significantly, while impact of scattering was marginal. Spray radiation was also found not to have much effect on global combustion characteristics in high-pressure engine-relevant configurations. However, a small but noticeable effect on minor species distribution relevant to pollutant formation was observed