Experimental and numerical study of mini-riser geometry effect on drops residence time in the developing flow

• Mini-riser geometry effect is seen on drops residence time in the developing flow.
• Microscopic high-speed PSV technique is used for visualizing drops motion near wall.
• Simultaneous velocity and concentration measurements in narrow areas are performed.
• Sharper corners and higher aspect ratio increase drops residence time significantly.
• Using triangular shape makes at least 24.8% higher growth for drops residence time.

In this study, velocity distribution and concentration of droplets for mini-riser with different geometries in the developing region is investigated experimentally and numerically to estimate the effects of riser shapes on the drops residence time. Since drops residence time affect on the drops size and consequently total separation efficiency by increasing of the condensation rate for moving droplets in the super-saturated carrier gas. Further, it increases the coagulation in mono dispersed dense gas-particles flow. In the experimental study, an ultrasonic atomizer produces small size water drops. Mini-risers with corners including triangular, rectangular and square shapes with the same hydraulic diameter are examined. Particle shadow velocimetry (PSV) technique based on shadow casting combined with the high-speed microscopic imaging is introduced for visualizing droplets motion and simultaneous velocity and concentration measurements near walls and corners. Such a technique is well suited for small water droplets measurements near surfaces especially in narrow geometries where conventional laser-based PIV methods are hampered by surface glare, interference of scattered lights and optical access. Effectively narrow-depth-of-field optical setup is employed for imaging a two dimensional plane within a flow volume. In addition, the numerical modeling is carried out for estimating general drops residence time in the riser. Results indicate that the drops concentration near wall is very low especially in the corners. Droplets residence time and concentration near the wall and corner is highest for triangular shape. Triangular shape enhances the average residence time of drops 50.6% more with respect to square one. Using of sharper corners and higher aspect ratio for the riser geometry increase droplets residence time in addition ease the separation process.

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