Droplet Detachment Mechanism in a High-Speed Gaseous Microflow

2013 ◽  
Vol 135 (7) ◽  
Author(s):  
Brian Carroll ◽  
Carlos Hidrovo
Keyword(s):  
Surface Tension ◽  
High Speed ◽  
Air Flow ◽  
Hydrodynamic Pressure ◽  
Surface Tension Force ◽  
Continuous Phase ◽  
Flow Conditions ◽  
Droplet Detachment ◽  
Scaling Arguments ◽  
Hydrostatic Pressure Difference

This paper experimentally investigates the mechanism of water droplet detachment in a confined microchannel under highly inertial (10 < Re < 200) air flow conditions. Experimental observations show that as the Reynolds number of the continuous phase is increased, the droplet transitions from an elongated slug to a nearly uniform aspect ratio droplet. Supporting scaling arguments are then made that examine the relevant forces induced by the continuous phase on the droplet at the point of detachment. The inertial, viscous, and hydrodynamic pressure forces that result as the air flow is confined in the small gap between droplet and channel walls are compared to the surface tension force pinning the droplet at the injection site. The results indicate that the dominant detachment mechanism transitions from the hydrostatic pressure difference to inertial drag as the continuous phase velocity is increased.

An Experimental Investigation of Droplet Detachment in High-Speed Microchannel Air Flow

2009 ◽  
Author(s):  
Brian Carroll ◽  
Carlos Hidrovo
Keyword(s):  
Surface Tension ◽  
Pressure Drop ◽  
High Speed ◽  
Air Flow ◽  
Surface Tension Force ◽  
Tension Force ◽  
Spherical Droplet ◽  
Major Mechanism ◽  
Order Of Magnitude ◽  
Droplet Detachment

This paper experimentally investigates the mechanism of water droplet detachment in a confined microchannel under highly inertial air flow. Experimental observations show that as the Reynolds number in the channel is increased, the droplet transitions from a nearly spherical droplet to a high aspect ratio slug. Scaling arguments are then made to address this behavior in an order of magnitude sense. These results show that although shear stress at the droplet-air interface may contribute to droplet elongation, the major mechanism of droplet detachment appears to be the pressure drop across the droplet. This pressure drop is a result of a highly inertial air flow being squeezed through a narrow gap between the droplet and the adjacent microchannel wall. This pressure difference creates a force imbalance across the droplet that overcomes the surface tension force pinning the droplet at the injection site, thereby leading to droplet detachment. A formulation of a nondimensional number is then presented that relates the pressure force acting across the droplet to the surface tension force at the droplet wall interface, which indicates that detachment occurs when this number is of order 1.

Optimization Design of the Lawn Mowing Vehicle's Blade Based on Aerodynamics

2011 ◽  
Vol 199-200 ◽  
pp. 173-181
Author(s):  
Li Zu ◽  
Lei Zhang ◽  
Hua Kun Wang
Keyword(s):  
High Speed ◽  
Optimization Design ◽  
Air Flow ◽  
Optimized Design ◽  
Motion Model ◽  
Flow Conditions ◽  
Air Movement ◽  
Blade Tip ◽  
Cfd Method ◽  
Stable Flow

In this paper, air movement states within the cutter deck caused by high-speed rotating blades are studied, impacts on the mowing blade by the air movement are analyzed with the consideration of the cutter structure. It will provide important references for the blade optimal design. By Computational Fluid Dynamics (shortened as CFD) method, the high relative speed motion model between the blade and air flow is established, the flow field through the blade tip section are simulated, and the instantaneous movement of air flow around the blade can be known. By the analysis of the aerodynamic influence, the optimized design of the mowing blade can be achieved. The lawn mowing vehicle with the optimized blade can have a good and stable flow conditions when cutting the grass, the performance and the value of the vehicle thereby are enhanced.

scholarly journals Experiments performed with bubbly flow in vertical pipes at different flow conditions covering the transition region: simulation by coupling Eulerian, Lagrangian and 3D random walks models

2012 ◽  
Vol 33 (1) ◽  
pp. 3-39 ◽  
Author(s):  
José Muñoz-Cobo ◽  
Sergio Chiva ◽  
Mohamed El Aziz Essa ◽  
Santos Mendes
Keyword(s):  
Surface Tension ◽  
Random Walks ◽  
Transition Region ◽  
Void Fraction ◽  
Slug Flow ◽  
Bubbly Flow ◽  
Continuous Phase ◽  
Flow Conditions ◽  
Transition Flow ◽  
Carrier Fluid

Experiments performed with bubbly flow in vertical pipes at different flow conditions covering the transition region: simulation by coupling Eulerian, Lagrangian and 3D random walks modelsTwo phase flow experiments with different superficial velocities of gas and water were performed in a vertical upward isothermal cocurrent air-water flow column with conditions ranging from bubbly flow, with very low void fraction, to transition flow with some cap and slug bubbles and void fractions around 25%. The superficial velocities of the liquid and the gas phases were varied from 0.5 to 3 m/s and from 0 to 0.6 m/s, respectively. Also to check the effect of changing the surface tension on the previous experiments small amounts of 1-butanol were added to the water. These amounts range from 9 to 75 ppm and change the surface tension. This study is interesting because in real cases the surface tension of the water diminishes with temperature, and with this kind of experiments we can study indirectly the effect of changing the temperature on the void fraction distribution. The following axial and radial distributions were measured in all these experiments: void fraction, interfacial area concentration, interfacial velocity, Sauter mean diameter and turbulence intensity. The range of values of the gas superficial velocities in these experiments covered the range from bubbly flow to the transition to cap/slug flow. Also with transition flow conditions we distinguish two groups of bubbles in the experiments, the small spherical bubbles and the cap/slug bubbles. Special interest was devoted to the transition region from bubbly to cap/slug flow; the goal was to understand the physical phenomena that take place during this transition A set of numerical simulations of some of these experiments for bubbly flow conditions has been performed by coupling a Lagrangian code, that tracks the three dimensional motion of the individual bubbles in cylindrical coordinates inside the field of the carrier liquid, to an Eulerian model that computes the magnitudes of continuous phase and to a 3D random walk model that takes on account the fluctuation in the velocity field of the carrier fluid that are seen by the bubbles due to turbulence fluctuations. Also we have included in the model the deformation that suffers the bubble when it touches the wall and it is compressed by the forces that pushes it toward the wall, provoking that the bubble rebound like a ball.

Some Developments in Instrumentation for Air-Flow Analysis

1949 ◽  
Vol 161 (1) ◽  
pp. 213-226 ◽  
Author(s):  
K. W. Todd
Keyword(s):  
Wind Tunnel ◽  
High Speed ◽  
Air Flow ◽  
Flow Analysis ◽  
Physical Analysis ◽  
Flow Conditions ◽  
Compressor Cascade ◽  
Confined Spaces

Evolution of research into flow conditions in confined spaces, such as model ducts or blade cascades under high subsonic or transonic air velocities, has necessitated the development of specialized forms of instrumentation for both optical and physical analysis. In this paper some of these special instruments are described, and an example of their application to a compressor-cascade examination in a high-speed wind tunnel is included.

MULTIDIMENSIONAL SIMULATIONS AND TEST FIRES OF A HYDROGEN-FUELED RAMJET WITH AN ANNULAR DETONATIVE COMBUSTOR AT APPROACHING AIR FLOW OF MACH 2 AND 1.5

2020 ◽  
Author(s):  
V. S. IVANOV ◽  
◽  
V. S. AKSENOV ◽  
S. M. FROLOV ◽  
P. A. GUSEV ◽  
...  
Keyword(s):  
Unmanned Aerial Vehicles ◽  
High Speed ◽  
Constant Pressure ◽  
Air Flow ◽  
Thermodynamic Cycle ◽  
Aerial Vehicles ◽  
Mach Numbers

Modern high-speed unmanned aerial vehicles are powered with small-size turbojets or ramjets. Existing ramjets operating on the thermodynamic cycle with de§agrative combustion of fuel at constant pressure are efficient at flight Mach numbers M ranging from about 2 to 6.

scholarly journals Modeling the Excess Velocity of Low-Viscous Taylor Droplets in Square Microchannels

Fluids ◽  
2019 ◽  
Vol 4 (3) ◽  
pp. 162 ◽  
Author(s):  
Thorben Helmers ◽  
Philip Kemper ◽  
Jorg Thöming ◽  
Ulrich Mießner
Keyword(s):  
High Speed ◽  
Process Intensification ◽  
Continuous Phase ◽  
Channel Cross Section ◽  
Optimization Approach ◽  
Mean Flow ◽  
Bypass Flow ◽  
Wall Film ◽  
Proposed Model ◽  
The Mean

Microscopic multiphase flows have gained broad interest due to their capability to transfer processes into new operational windows and achieving significant process intensification. However, the hydrodynamic behavior of Taylor droplets is not yet entirely understood. In this work, we introduce a model to determine the excess velocity of Taylor droplets in square microchannels. This velocity difference between the droplet and the total superficial velocity of the flow has a direct influence on the droplet residence time and is linked to the pressure drop. Since the droplet does not occupy the entire channel cross-section, it enables the continuous phase to bypass the droplet through the corners. A consideration of the continuity equation generally relates the excess velocity to the mean flow velocity. We base the quantification of the bypass flow on a correlation for the droplet cap deformation from its static shape. The cap deformation reveals the forces of the flowing liquids exerted onto the interface and allows estimating the local driving pressure gradient for the bypass flow. The characterizing parameters are identified as the bypass length, the wall film thickness, the viscosity ratio between both phases and the C a number. The proposed model is adapted with a stochastic, metaheuristic optimization approach based on genetic algorithms. In addition, our model was successfully verified with high-speed camera measurements and published empirical data.

scholarly journals Droplet impact onto a spring-supported plate: analysis and simulations

2021 ◽  
Vol 128 (1) ◽  
Author(s):  
Michael J. Negus ◽  
Matthew R. Moore ◽  
James M. Oliver ◽  
Radu Cimpeanu
Keyword(s):  
High Speed ◽  
Flexible Substrate ◽  
Practical Importance ◽  
Hybrid Approach ◽  
Hydrodynamic Pressure ◽  
Analytical Framework ◽  
Canonical System ◽  
Linear Process ◽  
Plate Motion ◽  
The Impact

AbstractThe high-speed impact of a droplet onto a flexible substrate is a highly non-linear process of practical importance, which poses formidable modelling challenges in the context of fluid–structure interaction. We present two approaches aimed at investigating the canonical system of a droplet impacting onto a rigid plate supported by a spring and a dashpot: matched asymptotic expansions and direct numerical simulation (DNS). In the former, we derive a generalisation of inviscid Wagner theory to approximate the flow behaviour during the early stages of the impact. In the latter, we perform detailed DNS designed to validate the analytical framework, as well as provide insight into later times beyond the reach of the proposed analytical model. Drawing from both methods, we observe the strong influence that the mass of the plate, resistance of the dashpot, and stiffness of the spring have on the motion of the solid, which undergo forced damped oscillations. Furthermore, we examine how the plate motion affects the dynamics of the droplet, predominantly through altering its internal hydrodynamic pressure distribution. We build on the interplay between these techniques, demonstrating that a hybrid approach leads to improved model and computational development, as well as result interpretation, across multiple length and time scales.

scholarly journals Comparison of Surface Tension Models for the Volume of Fluid Method

Processes ◽  
2019 ◽  
Vol 7 (8) ◽  
pp. 542 ◽  
Author(s):  
Kurian J. Vachaparambil ◽  
Kristian Etienne Einarsrud
Keyword(s):  
Surface Tension ◽  
Multiphase Flow ◽  
Capillary Rise ◽  
Surface Tension Force ◽  
Surface Force ◽  
Parallel Plates ◽  
Time Step ◽  
Mesh Resolution ◽  
Active Research ◽  
Spurious Currents

With the increasing use of Computational Fluid Dynamics to investigate multiphase flow scenarios, modelling surface tension effects has been a topic of active research. A well known associated problem is the generation of spurious velocities (or currents), arising due to inaccuracies in calculations of the surface tension force. These spurious currents cause nonphysical flows which can adversely affect the predictive capability of these simulations. In this paper, we implement the Continuum Surface Force (CSF), Smoothed CSF and Sharp Surface Force (SSF) models in OpenFOAM. The models were validated for various multiphase flow scenarios for Capillary numbers of 10 − 3 –10. All the surface tension models provide reasonable agreement with benchmarking data for rising bubble simulations. Both CSF and SSF models successfully predicted the capillary rise between two parallel plates, but Smoothed CSF could not provide reliable results. The evolution of spurious current were studied for millimetre-sized stationary bubbles. The results shows that SSF and CSF models generate the least and most spurious currents, respectively. We also show that maximum time step, mesh resolution and the under-relaxation factor used in the simulations affect the magnitude of spurious currents.

Experimental Investigation on the Heat Transfer of a Portable Forced-Convection Solar Air Heater

2010 ◽  
Author(s):  
Mohammad Fakoor Pakdaman ◽  
Pejman Zohorian Izadi ◽  
Mohammad Javadinia Azari ◽  
Amir Lashkari
Keyword(s):  
Forced Convection ◽  
Air Flow ◽  
Bottom Plate ◽  
Solar Air Heater ◽  
Air Flow Rate ◽  
Flow Conditions ◽  
Maximum Enhancement ◽  
Air Heater ◽  
Weather Changes ◽  
Plate Absorber

A cross-corrugated portable forced-convection solar air heater has been designed, fabricated, and developed. A wavelike bottom plate has been positioned crosswise to the air flow while rectangular baffles have been attached to the flat-plate absorber. The relative corrugation height, (e/Dh) ranges between 0.24 and 0.4, and relative baffles distance (l/L) varies between 0.21 and 0.48. The air flow rate in the heater duct has been varied in the range of 0.001 kgs−1 to 0.01kgs−1 (Reynolds number ranges from 350 to 3500), while other thermal specifications such as inlet, outlet, and plate temperatures have varied due to weather changes. Results of this study have been compared with those related to smooth ducts and other literatures, and the maximum enhancement in Nusselt number is observed to be approximately five times of that of the smooth duct under similar flow conditions. Finally, thermal efficiency of the device for different case studies has been determined and compared with other researches.

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