Full Length ArticleViscosity effect on the pressure swirl atomization of an alternative aviation fuel
Introduction
The pressure-swirl nozzle is known as one of the most widely applicable atomizers in industrial fields, such as stationary and aviation gas turbine combustors. The atomization process of pressure-swirl nozzles has motivated numerous works to investigate parameters by which the transformation of bulk pressurized liquid into small droplets is governed. Injection of bulk pressurized liquid through tangential vanes into the swirling chamber leads it to form an unsteady, turbulent swirling motion with high centrifugal force due to the atomizer geometry [1] and then exit from the discharge orifice into a gaseous medium. The issuing liquid disintegrates first into ligaments and then fine droplets. Harmful emissions, such as smoke, unburned hydrocarbons, nitrogen oxides, and carbon monoxide, are products of combustion, and their reduction is motivated by environmental concerns and can be achieved by improving the fuel injector design [2]. An ideal combustion process in a gas turbine needs uniform target coverage. Spray characteristics, such as spray angle, flowrate, atomization quality, and concentration, depend upon various physical parameters that are well addressed in the literature and documented in [3]. Effective parameters are known to be atomizer geometry, such as Ap, αs, ϑ, d0, L0, Ds, and Ls (see Fig. 2); injection conditions, such as injection pressure (Pinj), ambient pressure (Pamb), fuel temperature (Tf), and ambient temperature (TA) and test fuel physical properties, such as viscosity (νl), density (ρl), and surface tension (γl).
The variation of fuel properties according to temperature was studied by Yoon et al. (2008) [4]. They addressed how fuel density decreases linearly, and the kinematic viscosity decreases exponentially with increasing fuel temperature. The stability analysis, as well as experimental investigations, show that physical properties of the liquid, such as density, viscosity, and surface tension against the ambient medium, influence the drop size resulting from the two-step model breakup process. The breakup length of the sheet and consequently the Sauter mean diameter (SMD), as “the mean diameter of drops whose ratio of volume to surface area is the same as that of the entire spray” [3], were found to be directly under the influence of the surface tension, viscosity and mass flowrates of the liquid while changing inversely with the density and injection pressure of the liquid [5], [6], [7].
Most previous researchers have reported that the viscosity showed the greater influence on SMD compared with the surface tension [7], [8], [9], [10]. The SMD increases as the liquid viscosity does, and its effect is observed for various types of atomizers [11]. In addition, the effects of density are relatively smaller. The experiments of Satapathy et al. (1998) [12], conducted at considerably higher injection pressures than those employed in other studies, showed mean drop size to be strongly dependent on liquid viscosity and to increase markedly with an increase in viscosity. Bremond et al. (2007) [13] showed that the wavelengths of the dominant waves, formed on the surface of the liquid sheet, are influenced by the surface tension and density of the liquid. In non-Newtonian liquids, such as water-in-oil emulsions, it is seen that an increase of the aqueous-phase concentration leads to larger SMD when atomizing liquid jets. Agreed well with the work by (Sheng et al., 2006) [14]. Work by Goldsworthy et al. (2011) [15] also shows how differences between spray characteristics of fuels at various conditions depend on the properties of the fuel. Wang et al. (1988) [16] describes how drop size tends to increase with decreasing fuel temperature and decreasing fuel injection pressure. Jasuja [7] and Lefebvre [17] correlated SMD with the properties of liquid and established empirical correlations for its prediction. A perusal of their equations shows that SMD varies directly with the surface tension, viscosity and mass flowrates of the liquid while changing inversely with the density and injection pressure of the liquid. Later, researchers such as Wang and Lefebvre [6], Couto [18], and Wei and Yong [19] more comprehensively studied the phenomenon of atomization from the pressure-swirl atomizers and produced more elaborate correlations for the prediction of SMD. Their equations show that SMD does not depend on the properties of liquid only but rather is a complex function of the properties of liquid, characteristics of the surrounding environment and geometry of the atomizers.
Spray cone angle tends to decrease with the increase in viscosity due to higher energy losses and reduction of angular momentum of the liquid. The viscosity of the liquid is inversely related, while the density and injection pressure are directly related to the spray cone angle [5], [20]. De Corso et al. (1960) [21] and Guildenbecher et al. (2008) [22] suggested that the spray cone angle tends to decrease and drop size tends to increase with increasing ambient pressure. Park et al. (2004) [23] identified that viscosity and surface tension affect spray atomization quality at cold temperatures.
The effects of liquid viscosity on the size of the air core were revealed by Yong et al. [24]. They showed how an increase in viscosity decreases the air-core diameter and that the air core will gradually disappear when the dynamic viscosity coefficient increases to 29.844 mPa.s. The effect of liquid viscosity on the size of the air core is easy to understand, and the tangential velocity inside the nozzle will decrease with the liquid viscosity. The air core will disappear when the tangential velocity inside the nozzle is small enough. They observed “the internal-external unstable regime” phenomenon. This unsteady, transitional behavior of the atomizer is attributed to the eruption of the air core due to the increase of liquid viscosity at its low temperature. An unstable atomization in low temperature operating condition is also reported in [23].
Park et al. [25] studied the spray characteristics of the kerosene-based fuels. They showed that, for higher fuel temperature, which means lower kinematic viscosity, the inlet Reynolds number increases in both stable and unstable observed regions. They also showed that the spray cone angle increases as a function of the outlet Reynolds number. This is caused by the greater radial component of the liquid-fuel velocity with growing swirling strength when the Reynolds number is increased. They also reported the gradual disappearance of air-core with increasing fuel viscosity. They reported that the nearly freezing fuel at low atmospheric temperatures becomes a source for atomizer instabilities. They classified the spray instabilities into three different regimes: “(1) The external unstable regime, in which the air core disappears due to swirl weakening caused by increased viscosity at low fuel temperatures. Turbulence dominates in this regime and pulsation was observed only externally (T < 260 K). (2) The internal–external unstable regime, in which both internal and external flows experienced the transitional stage due to the eruption of the air core during the fuel’s viscosity change. The temperature range for this transitional stage was found to be approximately 260 K < T < 280 K (3). The stable regime is typical of the pressure-swirl atomizer with the aircore column centered vertically throughout the atomizer’s internal geometry (T > 280 K). Ballester and Dopazo (1994) [26] examined a number of small pressure-swirl atomizers to inject heavy oil with various operational conditions. They reported an increase in spray angle with increasing oil temperature which is attributed to the viscous effect, influencing air-core diameter. At low temperatures, the viscous friction lowers the centrifugal force and prevents the formation of the air core, which causes the liquid to exit as a full jet. Since flowrate is a monotonically increasing function of temperature, as the temperature increases, the tangential velocity of the liquid inside the swirl chamber causes the appearance of the air core, and the liquid emerges as an annular jet. The consequence is the reduction of the effective exit area for higher temperatures.
Kannaian and Sadr (2014) [27] compared the spray characteristics of CSPK-GTL with Jet A-1 and suggested that lower kinematic viscosity and surface tension cause a faster disintegration and dispersion of the droplets in the core region of the spray compared to that of a highly viscous fuel. The viscous drag force acting on an object or liquid is proportional to the viscosity, its velocity, and a characteristic length.
Despite numerous studies on the effects of the liquid properties, including viscosity changes, the effects of the aviation fuel temperature on the spray atomization characteristics are still not clear, particularly in low temperature regions. Therefore, in this study, the effects of fuel temperature and pressure on the atomization characteristics are outlined as the main goal. Low-temperature and high-viscosity fuels have always been a challenge for atomization and ignition, especially in cold-start ignition of vehicles in cold climates and in pilot restart of gas-turbine combustors of aircraft engines [28], [29].
Section snippets
Fuel injection system
The bulk liquid was first pressurized with nitrogen gas in a stabilized pressure tank and then led to a chiller JEIO TECH – Lab. Companion HTRC-30 to control the temperature. The mass flowrate of the fuel was indicated through signals of a Gear-type mass flow meter (KEM Liebigstraβe ZHA02 KL.W.V) with a KEM kuppers VHEF.02 Pulse amplifier. The fuel with adjusted temperature was then generated by a typical pressure-swirl atomizer to the gaseous medium with low humidity atmospheric ambient
Discharge coefficient with fuel temperature
In the case of aviation gas turbines, the fuel injection quantity at a given injection pressure is an important parameter for determining engine thrust. Therefore, the discharge coefficient (Cd), given as Eq. (1) and defined as the ratio of the actual mass flowrate to the theoretical mass flowrate calculated from the applied injection pressure, is widely used in conjunction with flow number (FN) to characterize the flow behavior of the nozzle applied in gas turbine engines.
Cd is the
Conclusion
The atomization characteristics of the aviation fuel spray with the fuel temperature in the range of −30 to +60 °C were experimentally investigated using optical diagnostic methods, such as phase Doppler anemometry and particle image velocimetry. An aviation fuel as the working fluid was injected through a pressure-swirl nozzle with six swirl vanes to investigate the effect of fuel temperature and its physical properties.
The viscosity of the tested fuel, increased exponentially by 26 times from
Acknowledgement
This research were supported by Agency for Defense Development of Korea and by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (Grant number: 2018(2) D1A1B07040902).
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