Break up length on Urea Water Solution jet in hot cross flow

Abstract

Selective Catalytic Reduction (SCR) using Urea-Water Solution (UWS) as an ammonia precursor is considered as one of the best choices to meet the current stringent emission norms for reduction of NOX in diesel engines. UWS sprayed in the engine exhaust line forms ammonia, and this ammonia reduces NOX into nitrogen. The NOX reduction efficiency depends on the mixing and evaporation behavior of the UWS spray in the hot exhaust gas. Spray characteristics decide the evaporation rate and hence the NOX reduction efficiency. The spray structure is closely related to the breakup point and breakup mode of the jet. Hence, in this study, breakup length and breakup mode were investigated by injecting UWS (32.5 % by weight) through a nozzle in a hot air cross flow. A CCD camera and pulsed Nd:Yag laser were used for capturing the images. Experiments were conducted with varying nozzle size (150, 250 and 400 micron), injection pressure (0.5 to 3 bar), temperature (32 °C,150 °C and 200 °C) and air flow rate. The effect of operating parameters (nozzle size, injection pressure, air temperature and velocity) in terms of dimensionless numbers (Weber number and momentum flux ratio) on jet breakup mode and jet breakup length was studied. It was observed that the breakup length for UWS was close to that of water. The jet breakup length increases with momentum flux ratio since a jet having a higher momentum is able to penetrate a larger distance in the cross flow. Increasing the air temperature increases the velocity of the cross flow and hence reduces the breakup length. A correlation for jet breakup length was developed. The effect of inclusion of Weber number in the breakup length correlation, in addition to the momentum flux ratio, was studied. Visual observation shows that droplet sizes obtained from the plain orifice injector without preheating is large. Preheating the UWS before injection is recommended to reduce the droplet size. Keywords UWS, cross flow, breakup length and laser diagnostics. IntroductionDiesel engines deliver a high thermal efficiency, high torque density and good fuel economy. However, emissions produced by diesel engines, especially oxides of nitrogen (NOx), pose a major problem to the environment and to human health. In-cylinder treatment methods like retarding injection timing, Exhaust Gas Recirculation (EGR) and exhaust after treatment methods like three-way catalysts and lean NOX traps are not suitable to meet future US and European emission norms. Urea-SCR after treatment method has the ability to meet these norms [1].