Atomization and Combustion of a Simple Elettrospray

A simple atomizing device is presented capable of generating controlled spray by using an electric field either in isothermal and burning conditions. The stability operating range of controlling parameters (e.g. liquid flow rate and applied voltage) is determined. The coupling of atomization and combustion processes is analyzed by measuring, using a PDA system, the drop diameter distribution as well as the radial and axial components of drop velocities. Preliminarily the influence of controlling parameters on the droplet size is presented and discussed. The spray structure as well as its evolution in space and time is presented in selected conditions, chosen on the ground of the preliminary analysis. The presence of the flame affects significantly the spray shape and the droplet dimensions. This effect is particularly relevant in the higher liquid flow rate case. A deeper analysis of the measurements allow for the determination of relative importance of different physical effects in the different conditions. The final conclusions of the paper is that the coupling between atomization and combustion processes can be hardly forecasted without a thorough analysis of the spray characteristics in the specific working condition. This poses a severe challenge for the practical application of these atomizing system for the set-up of effective combustion controlling systems that want take advantage of electric field modulation. Introduction Atomization of liquid fuels by means of electro-hydro-dynamically assisted nozzles could represent a very interesting technique to increase the performance of combustion systems. In fact, the fast time response typical of electrical system and the residual electrical charge deposited on the droplets could allow for an effective control of atomization and drop dispersion processes. The potential range of application of such techniques is very wide and span from simple deposition system, used in material synthesis, to internal combustion engines used in power generation system. A large number of paper has been published in recent years on the atomization peculiarities of electro-hydro-dynamically aided systems and the behavior of charged droplets subject to thermal field has been investigated either theoretically or experimentally. On the other hand, the effect of the mutual interaction of combustion and atomization processes has not yet been clarified. Nevertheless, this interaction represents a key point in the assessment of realistic potentialities of electro-hydro-dynamical systems exploitation in combustion systems. The nature of this interaction is intrinsically complex and a thorough description of even its phenomenology is difficult. As matter of fact, the pool of radicals formed during the chemical reactions taking place in the flame regions interacts with the charged droplet in a complicated way and can affect the droplet cloud shape and behavior. On the other hand the electric field itself interacts with the flame and, as a consequence, it can be expected that the flame be influenced by the electric field presence. It can be envisaged that atomization and combustion processes cannot be decoupled and their characterization has to be attempted using a systematic approach. In this paper a systematic characterization of well-defined nozzle configurations operated in the same feeding and applied voltage conditions is presented. This paper aims both to identify the ranges of fuel flow rate and of applied voltage in which a stable combustion condition can be established and to determine the changes in the spray droplets size and velocities induced by the combustion process. Experimental Set-up The atomizer used to this aim is a simple coaxial system, sketched in Fig. 1, in which a stainless steel needle is used to fed the liquid fuel (a mixture of n-heptane and an antistatic additive). During the experimental work different needles with and inner diameter from 0,1 to 0,8 mm have been tested. The needle is hold by a PTFE insulator in the center of a stainless pipe with a 5 mm inner diameter. Around the pipe a large annular duct allows the supplying of an air stream. In the annular duct a bed of spherical glass particles llow for the straightening of an air flow used to prevent the accumulation of liquid and to shield the spray from external perturbations. On the glass balls bed a metallic mesh electrically connected to the external pipe and to the atomizer body acts as the cathode of the electrical system. The external air flow velocity has been kept constant and very low (with respect to the droplet velocities) in all the investigated conditions in order to avoid a significant interaction with the atomization and combustion processes. The liquid fuel was supplied to the needle by means of a peristaltic pump operating in the range from 10 up to some hundreds of cc/hr. A dumping tank and a capillary pipe were inserted in the feeding line in order to smooth away possible oscillations induced by the pump in the fuel flow rate when small amount of liquid were supplied. A purposely realized voltage generator capable of supplying voltages up to 10 KV was used to generate the electric field, In a great part of the tests a positive voltage was applied to the needle while the remaining part of the atomizer was kept at ground level. In this way an intense electric field in the spatial region between the needle and the pipe was generated. Some tests were also conducted inverting the polarization of the electrodes in order to evidence possible effects on the atomization and combustion processes. Figure 1. Schematic of the atomizer. Droplet size distribution as well as axial and radial velocities has been measured by means of a PDA system both along the spray axis and in the radial direction at selected distances from the nozzle. As a complementary technique an high resolution CCD camera with pulsed laser illumination has been used to visualize the instantaneous distribution of droplets in the different conditions. Results and Discussion