Bulletin of the American Physical Society
71st Annual Meeting of the APS Division of Fluid Dynamics
Volume 63, Number 13
Sunday–Tuesday, November 18–20, 2018; Atlanta, Georgia
Session M31: Jets: Impinging |
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Chair: Shyam Menon, Louisiana State University Room: Georgia World Congress Center B403 |
Tuesday, November 20, 2018 8:00AM - 8:13AM |
M31.00001: 3D vortex structure and wall pressure distribution on a circular cylinder with jet impingement Mirae Kim, Kyung Chun Kim Jet impingement is a direct and efficient way to transfer heat and mass in various applications. In practical applications, most jet flows are impinging on curved surfaces, however, less attention has been given to circular jet impingements on convexly curved surfaces. Interactions of three-dimensional flow structures of a round jet impinging obliquely on a convex circular cylinder was studied using a large eddy simulation and 3D PIV measurements. Quantitative visualization of surface pressure was performed by PSP technique to verify the LES results and explain the Coanda effect on 3D wall jet flow along the cylinder wall. The strong shear layer near the impingement area makes small-scale vortices with high vorticity. These structures distribute throughout the surface and break down to multiple vortex structures with lower vorticity. Large-scale negative vortex structures are moved away from the wall jet and are sustained longer at the edge of the wall jet. |
Tuesday, November 20, 2018 8:13AM - 8:26AM |
M31.00002: Computational Studies of Vortex Rings Interacting with a Constant-Wall-Temperature Heated Wall Hussam H Jabbar, Ahmed M Naguib This study is motivated by understanding and controlling the cooling mechanisms arising from vortex-wall interaction in impinging jets. Three cases involving different configurations of axi-symmetric vortex rings interacting with a flat constant-temperature heated wall are studied computationally using ANSYS FLUENT. The cases represent simplified models of vortex-wall interaction scenarios during the evolution of jet vortices: before (Case I), during (Case II) and after (Case III) vortex pairing. The results show that the strongest wall interaction occurs during pairing (Case II), leading to significant instantaneous enhancement in Nusselt number (Nu) in comparison to that associated with a single vortex interacting with the wall (Cases I and III). However, the strong Nu enhancement in Case II has the shortest persistence in time because of the formation of a very strong secondary vortex, which leads the Nu enhancement to decay faster than in the other two cases. Overall, the results suggest that to gain the full potential of heat transfer enhancement in Case II, control of vortex-induced boundary-layer separation is necessary. |
Tuesday, November 20, 2018 8:26AM - 8:39AM |
M31.00003: Heat transfer characteristics and flow structure of sweeping jet impinging on curved surfaces Dong Ju Kim, Seyoung Joung, Tongil Park, Daegyoum Kim Impinging jet has been widely applied for convective heat transfer because of its superior performance of removing locally concentrated heat. Impinging sweeping jet has drawn attention recently by the virtue of its capability of generating spatially oscillating flow with a simple non-moving structure and noticeable heat transfer enhancement. In this study, the heat transfer characteristics and flow structures of the sweeping jet impinging on the convex or concave surfaces of different curvatures are experimentally investigated. In order to evaluate heat transfer performance, temperature distribution is measured on the curved surface, and local Nusselt number is computed, while applying constant heat flux uniformly on the surface. Flow visualization using particle image velocimetry and modal decomposition analysis of the jet flow field suggest that unsteady flow structures near a jet centreline and unique vortical structures at the downstream are critical in determining the distribution of the local Nusselt number. In addition, the effects of geometric and dynamic parameters such as surface curvature, nozzle-to-plate distance, and Reynolds number are analyzed. |
Tuesday, November 20, 2018 8:39AM - 8:52AM |
M31.00004: Empirical Exploration of Thermal Transport due to Jet Impingement at Superhydrophobic Surfaces Matthew Searle, Dewey Potts, Julie C Crockett, Daniel Maynes We present an empirical exploration of thermal transport due to jet impingement at heated smooth hydrophobic and post-patterned superhydrophobic surfaces. Post patterns with microfeature pitch (w = 8, 16, and 24 µm) and cavity fraction (Fc = 0.56 and 0.85) are explored. Further, the jet Reynolds number varies from 1.1 × 104 to 1.7 × 104 and surface heat flux varies from 2.5 × 104 to 4.9 × 104 W/m2. Experimental results obtained from surfaces with smaller pitch and cavity fraction display a significant decrease in Nusselt number and agree well with predictions by an analytical model. Surfaces with larger pitch and cavity fraction yield a decrease in the Nusselt number, which is less than that predicted by the model. We propose that this phenomenon occurs due to dynamic wetting of the microcavities. |
Tuesday, November 20, 2018 8:52AM - 9:05AM |
M31.00005: Rebound of large jets from hydrophobic surfaces in low-gravity Karl Cardin, Mark M Weislogel When an oblique water jet impacts a sufficiently hydrophobic substrate the resulting landing flow may give rise to a secondary jet. The parametric range of terrestrial jet rebound research is limited due to the presence of gravity. By employing a drop tower in this investigation we have the opportunity to significantly extend regimes, since in such free fall environments brief relative low-gravity levels <10-4g0 are readily achieved. This extended parameter range reveals unique flow structures including fishbone splashing and double-rivulet rebounds. The drop tower test data is used to compile a regime map showing the landing flow structures dependence on the relevant dimensionless groups. We develop simple models predicting landing geometry and the onset of instability that are found to yield good agreement with experiments. |
Tuesday, November 20, 2018 9:05AM - 9:18AM |
M31.00006: Abstract Withdrawn
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Tuesday, November 20, 2018 9:18AM - 9:31AM |
M31.00007: Similarity of the Wall Jet Resulting from Planar Underexpanded Impinging Jets Patrick Fillingham, Igor Novosselov Wall shear stress is mapped across nozzle parameters (stand off height, jet width and nozzle pressure ratio) for two dimensional planar underexpanded impinging jets using existing wall jet similarity theory. Computational fluid dynamics is used to calculate the flow field resulting from impinging jets with height to diameter ratios of 15-30 and nozzle pressure ratios of 1.2-3.0. A wall jet power law similarity solution with source coefficients is assumed for the inner and outer layers of the wall jet. Flow field data is used to fit wall jet characteristics and determine power law exponents. Source coefficient are determined as a function of nozzle parameters using the conjugate gradient method. It is found that normalizing by momentum, rather than characteristic length or source velocity, is optimal for achieving similarity. Power laws for momentum normalized friction velocity and half velocity wall distance as a function of momentum normalized X location are successfully developed. These power laws allow for complete mapping of wall shear stress along the impingement surface for a wide range of nozzle parameters. |
Tuesday, November 20, 2018 9:31AM - 9:44AM |
M31.00008: Effect of forcing by a plasma actuator on impinging-jet heat transfer Basil Abdelmegied, Ahmed M. Naguib Impinging jets have many engineering applications, such as heating, cooling and drying. This work is part of a larger study focused on using different active flow control strategies for enhancement of the cooling effectiveness of impinging jet arrays. Here, we examine the influence of steady forcing, using a circular Single Dielectric Barrier Discharge plasma actuator, on the Nusselt number (Nu) distribution of an axisymmetric jet impinging on a heated flat surface. In the experiments, the plasma actuator was mounted as close as possible to the sharp edge of the jet exit and was actuated sinusoidally at a frequency much higher than that of the jet’s initial instability. The Nu distribution was measured using temperature sensitive paint applied to a heated stretched stainless steel foil. Data were obtained for jet Reynolds number based on jet diameter in the range 5,000 to 20,000, jet-exit-to-plate distance range of 2 to 6 diameters, and different forcing amplitudes. The results illustrate the influence of the control on the jet’s cooling effectiveness, and the dependence of this influence on the flow and the forcing parameters. Time-resolved flow visualization is used to examine associated changes in the flow structure. |
Tuesday, November 20, 2018 9:44AM - 9:57AM |
M31.00009: Flow Physics and Enhanced Mixing in Confined Impinging Jet Mixers Yue Hao, Jung-Hee Seo, Yizong Hu, Hai-Quan Mao, Rajat Mittal Confined Impinging Jet Mixers (CIJMs) are used in chemical processes that requires fast mixing. In this mixing device, two jets from opposite sides, directly impinge on each other, form thin shear layers. These shear layers rapidly break down into small structures, and this enables effective mixing of the species transported by each jet. Interaction of the shear layers with the inner walls can induce self-sustained oscillation of the shear layers, and this can significantly enhance the mixing process. In the present study, we investigate the flow physics, especially the instability and self-sustained oscillation of the impinging jet shear layers. The study employs direct numerical simulation with a sharp interface immersed boundary method (ViCar3D). To better understand the instability of the confined impinging jet, we compare the behavior of confined and unconfined impinging jets. |
Tuesday, November 20, 2018 9:57AM - 10:10AM |
M31.00010: Parametric study of the turbulent flow inside a spherical jet stirred reactor Ghazaleh Esmaeelzade, Kai Moshammer, Ravi Fernandes, Detlev Markus, Ulrich Maas, Holger Grosshans Due to the importance of the kinetics of gas-phase chemical reaction in low-emission combustion, stirred tank reactors have been used for decades as the experimental tool of many investigations. A Jet Stirred Reactor (JSR) can be used to study the evolution of species mole fractions, while it provides efficient mixing by turbulent jets. However, it is important that the JSR is designed such that the concentration field is as homogeneous as possible in order to avoid disturbance of the chemical kinetics by inhomogeneities. In this work, numerical simulations were performed to investigate the effects of the geometrical parameters and the thermodynamic conditions on a JSR's mixing characteristics; i.e., the diameter of the spherical chamber was modified together with the diameter of nozzle. The turbulent structures inside the JSR and the nozzle tubes at microscales are captured using Large Eddy Simulations. The accuracy of the numerical results is demonstrated through a comparison of the residence time distribution (RTD) with a previous experimental study of tracer-decay technique. The results derived by RTD show that using a JSR with a diameter of 40 mm and a nozzle diameter of 1 mm, creates a nearly perfectly mixed composition and accordingly, behaves similar to an ideal reactor. |
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