SESSION 1

STATIONARY SPRAYS AND GAS COMBUSTION SYSTEMS

ABSTRACT

An experimental study has been made of the ignition process of a blended fuel droplet composed of n-hexane and n-hexadecane subjected to a laminar flat flame propagating through a lean homogeneouþ propane-air mixture at constant pressure. A combustion chamber made of a transparent duct was installed with spark electrodes, a fine quartz fiber to suspend a fuel droplet, and a shutter to allow the burned gas to flow out. A high speed video camera was provided for the photographic observation of the droplet ignited by the propagating flame and for the determination of the ignition delay. The ignition delay showed a peak as a function of initial droplet diameter and the diameter at the peak ignition delay decreased with an increase in the hexane volume concentration. A part of the propagating flame behind the droplet was deformed to be convex toward the unburned gas and the deformation length of it increased with an increase in the n-hexane volume concentration. Also evident was that the ignition delay decreased rapidly with an increase in the n-hexane volume concentration.

ABSTRACT

The objectives of this study were to observe ignition events and to measure ignition times of a droplet array of n-decane placed in a high-temperature low- speed airflow under microgravity field. Due to the difficulty of making droplets of the same size within a short period of time in a drop capsule, imitation droplets made of porous ceramic balls soaked with n-decane were used. Experimental conditions were a droplet diameter of 1 mm, droplet spacing within the range of 0 to 6 mm, airflow velocity of 0 to 10 cm/s, and an airflow temperature of 925 K. According to OH emission images taken by a high-speed camera with an OH band-path filter, ignition occurred around the droplets simultaneously at zero airflow velocity. At higher airflow velocities of more than several centimeters per second; however, ignition was initiated in the wake flow of the droplets and the flame spreads to the forward region of droplets. A range of droplet spacing existed in which ignition times of droplet arrays were less than those of a single droplet and had a minimum ignition time at a certain spacing. The spacing of this minimum ignition time increased with an increase of airflow velocity.

ABSTRACT

Droplet combustion usually involves the burning of liquid fuels in an oxidizing environment. In the large Damköhler number limit, the vaporized fuel reacts in a flame-sheet where the reaction heat is liberated. In this laminar diffusion flame structure, soot particles may be nucleated at the fuel rich side of the flame. Some experiments in droplet combustion have shown the soot accumulated in a narrow shell-shaped region between the droplet and the flame. The purpose of this work is to analyze the transport mechanisms which control the dynamics of the soot particles and may lead to the formation of this soot shell accumulation layer. The influence of thermophoretic and photophoretic drifts of the soot particles is taken into account. Assuming that thermophoresis is the only soot particle diffusive transport, the analysis shows that the soot velocity may vanish at a location between the droplet surface and the flame. However, this shell locus is dynamically unstable. Moreover, for the high temperatures prevailing near the flame, soot radiative heat transfer may be important. The radiation of the individual soot particles generates an overall radial radiative flux. Every particle receives the radiative fluxes produced for the remaining particles except from those shadowed by the presence of the droplet. The incoming radiative flux induces a photophoretic drift of the soot particles. The drift turns out to be in the direction of the droplet. Therefore, a new locus of vanishing soot particle radial velocity may appear near the flame. In this work, the photophoretically modified soot velocity and the conditions for the appearance of this stable soot stagnation locus are evaluated.

ABSTRACT

The three dimensional gas velocity and fuel concentration field in the turbulent mixing zone of a natural gas swirl burner have been investigated in the isothermal case by means of laser Doppler anemometry (LDA) and laser sheet visualisation (LSV). The two techniques enabled useful insight into the fully turbulent, three dimensional swirling flow to be gained for various operating conditions and fuel injection typologies. The effect of swirl number and fuel injection mode, radiat or coaxial, has been quantified on both the flowfield structure and fuel concentration distribution. The results indicate that the unmixedness index, U, may be a significant parameter to judge the mixing efficiency due to swirl strength and fuel-air momentum ratio in a specific burner geometry, at least under isothermal conditions in the near field zone downstream the quarl exit.

ABSTRACT

The oscillatory motion encountered in a laminar spray flame as described in some experimental works is different from any reported previously in the literature. In the published works this phenomenon is associated with gravity or heat loss.

In this paper a simplistic physical model for oscillations in the position of flame zone relative to the combustor is described It is shown that this model is sufficient to demonstrate that the flame position instability may occurs due to fuel stream warming at the entrance to combustion zone.

ABSTRACT

The liquid spray combustion of Toluene has been studied in shock waves at temperatures between 900K and 1800K and pressures up to 20 bar in oxygen/argon mixtures. The ignition delays were found to be dependent on the pressure and or fuel/oxygen ratio and the presence of up to 10% of additive. Two types of combustion were found (i) Type I occurred at lower temperatures and gave a steady combustion where the pressure rises steadily (ii) Type II was detected at higher temperatures and in the presence of certain additives and gave a very sharp 'spiked' rise. This phenomenon may be due to the occurrence of small droplet explosions which are similar to 'knock' in engines. The effect of additives on the ignition delay time depended on both the additive used and the concentration. 1% TBP in the toluene has little effect on the ignition delay but 10% TBP and 5% IPN both reduce this time significantly. The latter was also found to affect the type of combustion and changed it to type II. IPN was more effective than TBP in reducing the ignition delay.

ABSTRACT

A new burner design is presented in this work for highly stabilized partially premixed flames using gaseous fuel. The partially premixed flow is created in a concentric flow tube and the flames are stabilized by a relatively large diverging conical nozzle. The concentric flow with conical nozzle burner (CFCN) creates highly stabilized flames at high Reynolds number, up to 60000. Three versions of the burner have been investigated and a maximum load of about 250 kW with 20 mm diameter nozzle has been achieved in partially premixed flame. Higher load is expected for larger burner size. The stability characteristics of the CFCN burner shows that it is suitable for industrial applications. The burner turndown ratio is between 15 and 20.

Fundamental research studies and modeling of the flames in CFCN are feasible because of the simple flow geometry of the burner. Therefore, four flames have been selected in the present work for detailed thermal reaction zone structure investigation and OH radical distribution using simultaneous two-dimensional I îmaging of Rayleigh scattering and Laser Induced Predissociation Fluorescence (LIPF). The reaction zone structure is relatively thin and may be classified in the thin reaction zones regime. The OH signal correlates well with temperature at the reaction zone. More fine structure in the preheat zone can be observed from the temperature images as compared with those of the OH radical. Stable flame structure with continuous reaction zone at high stretch conditions and no extinction have been observed.

ABSTRACT

The heating and vaporization of a pure cold fluid package in a hot environment of the same fluid has been analyzed. The model applies to subcritical as well as supercritical fluid conditions and relies on the assumption of constant pressure and quasi-steady conditions in the gas phase (in a reference system receding with the cold front). An asymptotic analysis is performed using the ratio of the hot fluid density to the density of the cold fluid package as the smallness parameter. Then, a transcendental equation is obtained which provides the evolution of the cold package radius. For longer times when isothermal conditions are achieved in the cold region, the d²-law is obtained. Some deviations from this law, due to the unsteadiness of the process in the cold region, are calculated and discussed.

ABSTRACT

The Mild Combustion is a relatively novel combustion technique characterized by both an elevated temperature of reactants and an adiabatic flame temperature not higher than 1600K. These features are the results of several technological demands coming form different application fields.

The main advantages derived by Mild Combustion concern both the combustion process itself and its applications. In the former case the pollution reduction, the increase of efficiency process and fuel flexibility has to be considered whereas the latter one is principally related to heat treatment process and wall confinement.

In this paper, characteristics of Mild Combustion are discussed with particular regards towards the aspects of plant design. A general classificafion of the processes, where initial and adiabatic temperature values are different from characteristic ones of classical combustion system, is given. This classification leads to a definition of Mild Combustion process as a direct consequence of technological evolution and practical constrains, especially related to environmental problems. In this sense, the high temperature of reactants and the low adiabatic temperature, principally obtained using exhausts for both heat recovery and dilution cannot be considered independently.

Applications of Mild Combustion in practical systems infers the use of particular solutions because of high temperature of reactants involved. In the final part of the paper, specific solutions utilized in a laboratory mild combustor are presented.

ABSTRACT

A comparative analysis of the stability regimes of three different electrohydrodynamiç atomization (electrospray) systems is presented. Availability of a high resolution imaging system allowed for a direct visualization of the breakup mechanisms. Both liquid flow rate and applied voltage are varied in a large grid of values in order to build up a database of the observable jet breakup regimes. The exploited conditions have been chosen with the purpose of evidencing the relative influences of geometrical, electrical and fluid-dynamical effects at various working conditions.

Experimental results shed light on the complex interaction between the controlling parameters and resulted in the formulation of some necessary conditions for the occurrence of stable jet regimes. The final conclusion of the paper is the recognition of the actual limitation in the mass flow rate range where such regime can be stabilized. Nevertheless some suggestion comes for the exploitation of atomization condition different from the classical Taylor cone jet that could be successfully exploited either in combustion systems or other technological application.

ABSTRACT

This work analyses the simulation potential of two premixed turbulent combustion models based on different combustion mechanism concepts: the Eddy Dissipation Concept based on the volume combustion mechanism, and the Turbulent Flame-speed Closure based on the thickened-wrinkled flamelets combustion mechanism.

Ability of simulating numerically a standard experimental test case (premixed methaneair combustion in a plane channel at high flow velocity) and the influence of flow parameters variation on the combustion process have been tested.

The paper shows that the flamelets model describes the standard experimental data more accurately. Furthermore, comparisons of the two models results obtained varying combustion flow parameters show the presence of quantitatively, and in one case even qualitatively different trends. These results are explained, and potentialities and limits of these models are discussed from an industrial premixed burner applications standpoint.

ABSTRACT

The effect of swirling flow on heat transfer enhancement in a horizontal concentric circular annulus is experimentally investigated. A hub vane-swirler is used to augment the heat transfer at a uniform outer wall heat flux. The axial distributions of the outer wall temperature and static pressure are measured. The radial profiles of air temperature at six axial positions are also measured. The effect of swirl number is studied over a Reynolds number range of 0.7x10⁴-3.2x10⁴. Heat transfer augmentation is presented in terms of Nusselt number. Pressure drop, which is proportional to the pumping power, is also presented. The average Nusselt number and overall wall static pressure drop are correlated in terms of Reynolds and swirl numbers. These correlations are important for heat transfer equipment design, performance and operation. When compared with plain annulus flow results, swirling flow data showed an increase in heat transfer by up to 30% at swirl number of 0.74.

Keywords: Experiments, concentric annulus, swirling turbulent flow, heat transfer enhancement