Pressure-Swirl Atomization of Water-in-Oil Emulsions

The atomization of surfactant stabilized and natural, unstable water-in-oil emulsions injected into an ambient environment are investigated experimentally. Fuel flow conditions typical in large-scale gas turbine applications are studied. A specialized setup is used to generate controlled emulsions stabilized by addition of surfactants. The emulsion generation process used allows control over the discrete phase droplet sizes within the emulsions. The spray droplet sizes are measured using laser diffraction and the spray pattern is measured using patternation. The results shown that the surfactants do affect the size of the atomized liquids, but in a manner expected based on the change in physical properties. Emulsification can reduce or increase the size of the spray depending upon the amount of water used. Atomization can change the size distribution of the discrete phase depending upon the initial sizes present and the injection pressure used. For the conditions studied, the stabilized emulsions performed very similarly to natural unstable emulsions as long as the unstable emulsion was produced shortly prior to atomization (i.e., <1 sec) and was produced with sufficient shear as to result in a fine discrete phase droplet size. Finally, it is observed that the atomization process also lead to a slight separation of components, with the denser water found preferentially at the outer edges of the spray. This was observed for both stabilized and unstable emulsions. Overall, the results obtained provide significant new insight into how the characteristics of the emulsion affects atomized spray. 1 Corresponding Author, mcdonell@ucicl.uci.edu ILASS Americas, 22 Annual Conference on Liquid Atomization and Spray Systems, Cincinnati, OH, May 2010 Introduction In many practical combustion systems, water is utilized to lower emissions of NOx and soot, improve thermal management, and increase combustion efficiency [1]. Water can be injected into the system air stream, the combustion zone, or together with the fuel as an emulsion. Injection of liquid water or gaseous steam can control combustion temperatures by absorbing heat in the engine. This helps reduce thermal stress and maintains the hardware’s structural integrity. In addition, oxides of nitrogen formed via the thermal (i.e., Zeldovich) can also be greatly reduced due to the heat removed from the reaction zone [2]. Use water in form of an emulsion offers the opportunity to overcome limitations associated with physical properties of the fuel itself. An example is “Orimulsion” wherein water is added to highly viscous bitumen to allow improved flowability and atomization quality [3]. Introduction of water as a emulsion also minimizes modifications to the fuel injection system compared to discrete water injection which requires additional flow manifolds, atomizers, and injection points. In addition, atomized emulsions can promote the onset of “microexplosions”, where the water component flash vaporizes prior to the fuel within a fuel droplet or globule causing a secondary droplet breakup [4]. This breakup improves the vaporization and mixing of the fuel and oxidizer within the combustion chamber which can increase both the combustion and thermal efficiencies as well as lower the sooting propensity [1,5]. Thus, introduction of water in combustion systems can have benefits and introduction of water as an emulsion offers additional some further advantages. Marcroemulsions, which possess discrete phase droplets larger than 1 μm, are thermodynamically unstable due to immiscibility of the oil and water components [6]. This immiscibility is due to the differences in the molecular forces between components. The strong attraction of hydrogen bonds between the polar water molecules compared to the larger non-polar oil molecules (which only have weak London dispersion forces) causes a grouping of like molecules, and results, in continuum mechanics, the coalescence and separation of these components once the liquids are at rest. This instability is represented by a large interfacial tension between the two liquids, where a significant amount of energy is required to create dispersed droplets within a continuous medium. From a practical perspective, fuel emulsions are generally of the water-in-oil type. Such emulsions allowing moderate amounts of water addition and avoid a substantial loss in heating value. These fuel emulsions can be generated quickly and with a reasonable expenditure of shear (experiencing a strain rate on the order of 10 10 s) [7]. Separation time scales are on the order of seconds, which poses a considerable challenge in managing a fuel emulsion in a practical system as well as studying it experimentally. For detailed study and convenience, the surface stabilizing agents (surfactants) can be added to significantly slow down the separation of water and oil. The addition of these surfactants decreases the surface and/or interfacial tension between the water and oil, slowing down coalescence [6]. Concentrations are generally below 10 percent (but can be substantially larger depending on solvent) to avoid micelle formation and solution saturation. The addition of surfactant does, however, alter the liquid properties of these solvent fluids and may modify the behavior of the emulsion to some extend once formed. It is still not well understood how surfactants modify the breakup of stabilized emulsions during atomization and how this breakup compares with that of natural unstable (surfactant free) emulsions. During atomization, the effect of stabilizing surfactants on the emulsion viscosity and surface tension may be similar to that on an unstable emulsion due to surfactant depleted regions where new interface (liquid liquid) and surface (liquid gas) is rapidly generated resulting in slow rates of diffusion and limiting wetting [8]. Low interfacial tension enhances the formation of smaller emulsion droplets with a narrower droplet size distribution and overall greater kinetic stability [9]. This can lead to a finer atomization of the emulsion during injection potentially resulting in better engine performance [10]. However, surfactants can be expensive and require an additional aspect of complexity associated with mixing. Hence for high volumetric flows needed for large scale power generation a surfactant free system is clearly preferred. Various methods have been documented in the literature regarding emulsion formation. High strain rates present in the piping, which can include valves, strainers, and/or bends in the flow, provide emulsification upstream of atomizers. Dedicated inline mixers can also provide additional shear to drive the water globules/droplets to a smaller droplet size with a more narrow distributions. The current research produces the emulsions in a rotor/stator mixing device which introduces the water and diesel fuel through small passages to create high shear interactions for stabilized emulsions. Unstable emulsions formed through a T-junction and static screen filter (of 150 μm pore size) at a distance of 0.3 m from the exit of the atomizer nozzle. Not unexpectedly, the atomization process itself also provides very high rates of shear for the emulsion, which modifies the nature of the emulsion [11]. The atomization process depends on the type of injector and the fluid properties of the emulsion spray, which are the two main factors examined in the present study. A pressure-swirl atomizer nozzle is used in the current study to atomize the emulsion with a representative droplet Sauter Mean Diameter (SMD). ILASS Americas, 22 Annual Conference on Liquid Atomization and Spray Systems, Cincinnati, OH, May 2010 Lefebvre (1989) formulated a SMD correlation for pressure simplex nozzles defined as 25 . 0 5 . 0 25 . 0 25 . 0 25 . 0 25 . 2 − − Δ = A L L L L P m SMD ρ μ σ & (1) where σL is the fuel surface tension, μL is the fuel dynamic viscosity, ρA is the air density, ΔPL is the injector pressure drop, and L m& is the fuel mass flow rate. Wang and Lefebvre (1989) further expanded the correlation by including the effects of film thickness and spray cone angle.