Effervescent atomizer: influence of the internal geometry on atomization performance

Our work is being conducted with the aim to develop an effervescent atomizer for industrial burners that will generate a fine and stable spray in large turn-down ratio. The single-hole, plain orifice atomizer is powered with light heating oil and uses air as an atomizing medium in the “outside-in” gas injection configuration. Published design concepts of the effervescent atomizer are described. Based on the published results several design parameters are modified: size and number of aerator holes, their location and diameter of the mixing chamber. Influence of these parameters on spray performance is studied at atomizing pressures 0.1, 0.3 and 0.5 MPa and gas-to-liquid-ratio (GLR) of 2, 5 and 10%. * Corresponding author: [email protected] Proceedings of the 21 ILASS Europe Meeting 2007 Introduction A method of atomization commonly referred to as „effervescent atomization” was developed in the late 1980s by Lefebvre and co-workers [1]. Also some earlier works [2, 3] mention similar concepts. Effervescent atomizers can be categorized as twin-fluid atomizers with internal mixing. In the simplest form of the effervescent atomizer, a gas is injected directly at low velocity into a flowing liquid at some point upstream of the atomizer exit orifice in such a way to create a bubbly two-phase flow. As the liquid flows through the discharge orifice it is transformed by the gas bubbles into thin shreds and ligaments. When the gas bubbles emerge from the nozzle at sufficient pressure drop, they expand so rapidly that the surrounding liquid is shattered into droplets. Effervescent atomizers are becoming more and more commonplace in numerous engineering applications in which a liquid must be fragmented into droplets. Major advantage of effervescent atomizers is their relative insensitivity to fuel physical properties and ability to provide good atomization over a wide range of operating conditions even for less refined fuels. A possibility to vary both the operating pressure and also the ratio of flow rates of gas and liquid, GLR, leads to large atomizer turn-down ratios. As the atomizing gas is utilized by effervescent atomizer in relatively efficient manner, a good atomization can be achieved using very small flow rates of the gas. Another attractive feature is good atomization even when operating at low injection pressures. Furthermore the E-atomizers can have larger orifice than conventional atomizers which alleviates clogging problems and facilitates atomizer fabrication. It also predestinates this type of pneumatic atomizers for atomization of suspensions and slurries [4-8]. Despite of its inherent simplicity the effervescent atomizer gives possibility for wide variety of design configurations. Large amount of literature can be found to describe an influence of geometry of the liquid-air mixing system on performance of effervescent atomizers. It is shown that optimization of the atomizer design can improve the spray characteristics. Surprising diversity of design modifications can be seen in different papers. However it is not fully possible to generalize these results made for different effervescent atomizer concepts and for liquids of different physical properties (typically water). Moreover currently only SMD is often evaluated and other important spray parameters (spray cone angle, velocity profiles, entrainment number and mass flux) are neglected. The SMD varies with spray position and this feature is often neglected. In this study we bring an overview of design concepts investigated by different researchers together with specification and description of important geometric parameters. Our work is being conducted with the aim to develop an effervescent atomizer for industrial burners that will generate fine and stable spray in large turn-down ratio. The single-hole, plain orifice atomizer is powered with light heating oil and uses air as an atomizing medium in the “outside-in” gas injection configuration. Several design parameters are modified: size and number of aerator holes, their location and diameter of the mixing chamber. Influence of these parameters on the spray performance is studied at atomizing pressures 0.1, 0.3 and 0.5 MPa and GLR of 2, 5 and 10%. A near nozzle spray visualization by digital camera illustrates the atomization process at different operation modes. Effervescent Atomizer Configurations Already in some early papers [9] a lot of different designs of effervescent atomizer appeared. We will attempt to make their basic classification here. To simplify our task we focus only on single-hole plainorifice atomizers. Design described in different works can thus be divided into three basic groups, see Fig. 1. Type A: In this configuration the liquid flows through a central tube. Gas is introduced into the liquid