Microscopic Imaging Spray Diagnostics under High Temperature Conditions: Application to Urea–Water Sprays

• Main Authors: Christian Lieber, Rainer Koch, Hans-Jörg Bauer
• Format: Article
• Language: English
• Published: MDPI AG 2019-10-01
• Series: Applied Sciences
• Subjects: shadowgraphy; droplet sizing; particle tracking velocimetry; turbulent dispersion; astigmatism; evaporation; selective catalytic reduction
• Online Access: https://www.mdpi.com/2076-3417/9/20/4403

The quantitative investigation of droplet laden turbulent flows at high temperature conditions is of great importance for numerous applications. In this study, an experiment was set up for investigation of evaporating urea&#8722;water sprays, which are relevant for the effective reduction of nitrogen oxide emissions of diesel engines using Selective Catalytic Reduction. A shadowgraphy setup is pushed to its limits in order to detect droplet diameters as small as 4 <inline-formula> <math display="inline"> <semantics> <mi mathvariant="sans-serif">&#956;</mi> </semantics> <semantics> <mi mathvariant="normal">m</mi> </semantics> </math> </inline-formula> and droplet velocities up to 250 <inline-formula> <math display="inline"> <semantics> <mrow> <mi mathvariant="normal">m</mi> <msup> <mi mathvariant="normal">s</mi> <mrow> <mo>&#8722;</mo> <mn>1</mn> </mrow> </msup> </mrow> </semantics> </math> </inline-formula>. In addition, the operating conditions of the gaseous flow of up to 873 <inline-formula> <math display="inline"> <semantics> <mi mathvariant="normal">K</mi> </semantics> </math> </inline-formula> and <inline-formula> <math display="inline"> <semantics> <mrow> <mn>0.6</mn> </mrow> </semantics> </math> </inline-formula> <inline-formula> <math display="inline"> <semantics> <mi mathvariant="normal">M</mi> </semantics> </math> </inline-formula><inline-formula> <math display="inline"> <semantics> <mi>Pa</mi> </semantics> </math> </inline-formula> are an additional challenge. Due to the high temperature environment, image quality is prone to be compromised by Schlieren effects and astigmatism phenomena. A water-cooled window and an astigmatism correction device are installed in order to correct these problems. The results to be presented include characteristics of the turbulent gas flow as well as detailed spray characteristics at different positions downstream of the atomiser. It is demonstrated that the velocity of the gas can be approximated by the velocity of the smallest detectable droplets with sufficient accuracy. Furthermore, the statistical analysis of velocity fluctuations provides data for predicting the turbulent dispersion of the droplets.