Transitional Flow and Related Transport Phenomena in Complex Microchannels

2009 ◽  
Author(s):  
Norbert Kockmann ◽  
Craig Holvey ◽  
Dominique M. Roberge
Keyword(s):  
Single Channel ◽  
Transverse Velocity ◽  
Transport Phenomena ◽  
Flow Characteristics ◽  
Period Doubling ◽  
Wake Flow ◽  
Chaotic Advection ◽  
Transitional Flow ◽  
Dean Flow ◽  
Vortex Pairs

In microchannels with typical dimensions from 10 μm to few hundreds μm, the flow is dominated by viscous forces, leading often to laminar flow conditions. At the entrance or in bends and curves, where the flow is accelerated or changes its direction, inertial forces generate transverse flow velocities. Due to continuity, compensating transverse velocity components generate vortex pairs, such as Dean flow in circular bends. The flow is still laminar, steady, and shows no statistically distributed fluctuations typical for turbulent flow. This deviation from straight laminar conditions, often in larger channels (100 μm to few mm) or for higher flow rates, is called transitional flow. That embraces the first occurrence of pulsating vortices, period doubling of vortex pairs, flow bifurcation, and regularly fluctuating wake flow or vortex shedding. With increased flow velocity, this process leads to chaotic flow phenomena being first evidence of turbulence. This paper describes the transitional flow characteristics in single channel elements such as bends and T-junction as well as around fins and posts in channels. These elements are used to augment the transport characteristics in microchannels for enhanced heat and mass transfer and for performing chemical reactions in microreactors. The profound understanding of the flow characteristics is fundamental for the understanding of transport phenomena. Additionally, this knowledge can be used to design successful microstructured devices for various applications by knowing how to generate and control vortices in microchannels. Concepts from chaotic advection are presented here to describe vortex flow and related transport characteristics. Though recent advances has shed new light on transport phenomena in complex channel structures, many issues are still unknown and huge potential is hidden in optimized channel devices.

scholarly journals Random pore structure and REV scale flow analysis of engine particulate filter based on LBM

Open Physics ◽  
2020 ◽  
Vol 18 (1) ◽  
pp. 881-896
Author(s):  
Chunrui Wu ◽  
Tiechen Zhang ◽  
Jiale Fu ◽  
Xiaori Liu ◽  
Boxiong Shen
Keyword(s):  
Single Channel ◽  
Flow Analysis ◽  
Flow Characteristics ◽  
Axial Direction ◽  
Particulate Filter ◽  
Random Structure ◽  
Structure Generation ◽  
Inlet Flow ◽  
Random Structures ◽  
Boltzmann Method

Abstract In this article, lattice Boltzmann method (LBM) is used to simulate the multi-scale flow characteristics of the engine particulate filter at the pore scale and the representative elementary volume (REV) scale, respectively. Four kinds of random wall-pore structures are considered, which are circular random structure, square random structure, isotropic quartet structure generation set (QSGS), and anisotropic QSGS, with difference analysis done. In terms of the REV scale, the influence of different inlet flow velocities and wall permeabilities on the flow in single channel is analyzed. The result indicates that the internal seepage laws of random structures constructed in this article and single channel are in accordance with Darcy’s law. Circular random structure has better permeability than square random structure. Isotropic QSGS has better fluidity than anisotropic one. The flow in single channel is similar to Poiseuille flow. The flow lines in the channel are complicated and a large number of vortices appear at the ends of channel with high inlet flow rate. With the increase of inlet velocity, the static pressure in channel gradually increases along the axial direction as well as the seepage velocity. The temperature field in the channel becomes more uniform as the flow velocity increases, and the higher temperature distribution appears on the wall of the porous media.

Heat Transfer and Flow Phenomena in a Swirl Chamber Simulating Turbine Blade Internal Cooling

1998 ◽  
Author(s):  
C. R. Hedlund ◽  
P. M. Ligrani ◽  
H.-K. Moon ◽  
B. Glezer
Keyword(s):  
Heat Transfer ◽  
Turbine Blade ◽  
Flow Characteristics ◽  
Leading Edge ◽  
Internal Cooling ◽  
Vortex Pairs ◽  
Nusselt Numbers ◽  
Swirl Chamber ◽  
Flow Phenomena ◽  
Energy Balances

Heat transfer and fluid mechanics results are given for a swirl chamber whose geometry models an internal passage used to cool the leading edge of a turbine blade. The Reynolds numbers investigated, based on inlet duct characteristics, include values which are the same as in the application (18000–19400). The ratio of absolute air temperature between the inlet and wall of the swirl chamber ranges from 0.62 to 0.86 for the heat transfer measurements. Spatial variations of surface Nusselt numbers along swirl chamber surfaces are measured using infrared thermography in conjunction with thermocouples, energy balances, digital image processing, and in situ calibration procedures. The structure and streamwise development of arrays of Görtler vortex pairs, which develop along concave surfaces, are apparent from flow visualizations. Overall swirl chamber structure is also described from time-averaged surveys of the circumferential component of velocity, total pressure, static pressure, and the circumferential component of vorticity. Important variations of surface Nusselt numbers and time-averaged flow characteristics are present due to arrays of Görtler vortex pairs, especially near each of the two inlets, where Nusselt numbers are highest. Nusselt numbers then decrease and become more spatially uniform along the interior surface of the chamber as the flows advect away from each inlet.

scholarly journals Calibration and validation of the Dynamic Wake Meandering model Part I: Bayesian estimation of model parameters using SpinnerLidar-derived wake characteristics

2021 ◽  
Author(s):  
Davide Conti ◽  
Nikolay Dimitrov ◽  
Alfredo Peña ◽  
Thomas Herges
Keyword(s):  
Wind Farm ◽  
Atmospheric Stability ◽  
Flow Characteristics ◽  
Uncertainty Assessment ◽  
Wake Flow ◽  
Model Parameters ◽  
Wake Field ◽  
Wind Speeds ◽  
Standard Lead ◽  
Estimation Of Model Parameters

Abstract. In this first part of a two-part work, we study the calibration of the Dynamic Wake Meandering (DWM) model using high spatial and temporal resolution SpinnerLidar measurements of the wake field collected at the Scaled Wind Farm Technology (SWiFT) facility located in Lubbock, Texas, U.S.A. We derive two-dimensional wake flow characteristics including wake deficit, wake turbulence and wake meandering from the lidar observations under different atmospheric stability conditions, inflow wind speeds and downstream distances up to five rotor diameters. We then apply Bayesian inference to obtain a probabilistic calibration of the DWM model, where the resulting joint distribution of parameters allows both for model implementation and uncertainty assessment. We validate the resulting fully-resolved wake field predictions against the lidar measurements and discuss the most critical sources of uncertainty. The results indicate that the DWM model can accurately predict the mean wind velocity and turbulence fields in the far wake region beyond four rotor diameters, as long as properly-calibrated parameters are used and wake meandering time series are accurately replicated. We demonstrate that the current DWM-model parameters in the IEC standard lead to conservative wake deficit predictions. Finally, we provide practical recommendations for reliable calibration procedures.

scholarly journals The Effect of Boattail Angles on the Near-Wake Structure of Axisymmetric Afterbody Models at Low-Speed Condition

2020 ◽  
Vol 2020 ◽  
pp. 1-14
Author(s):  
The Hung Tran
Keyword(s):  
Strouhal Number ◽  
Reynolds Shear Stress ◽  
Flow Behavior ◽  
Flow Characteristics ◽  
Wake Flow ◽  
Turbulent Intensity ◽  
Near Wake ◽  
Wake Structure ◽  
Low Speed ◽  
Speed Condition

The effect of a boattail angle on the structure of the wake of an axisymmetric model was investigated at low-speed condition. Four conical boattail models with angles of 0° (blunt-based body), 10°, 16°, and 22° were selected for this study. The Reynolds number based on the diameter of the model was around 1.97×104. Particle image velocimetry (PIV) was used to measure the velocity of the wake flow. The time-averaged flow characteristics including the length of recirculation of the afterbody, turbulent intensity, and Reynolds shear stress were analyzed and compared among those boattail models. The experimental results showed that the length of recirculation decreases with increasing boattail angle to 16°. At a boattail angle above 16°, the flow was fully separated near the shoulder and near-wake structure was highly changed. The turbulent intensity at a boattail angle of 22° showed a similar level to that in the case of the blunt-based body. Flow behavior on boattail surface should be accounted as an important parameter affecting the wake width and drag of the model. Power spectral density and proper orthogonal decomposition (POD) analyses showed that a Strouhal number of StD=0.2 dominated for the boattail model up to 16°. The fully separated flow was dominated by a Strouhal number of StD=0.03−0.06, which was firstly presented in this study.

Experimental Investigations of Laminar, Transitional to Turbulent Gas Flow in a Micro-Channel

2010 ◽  
Author(s):  
Chungpyo Hong ◽  
Toru Yamada ◽  
Yutaka Asako ◽  
Mohammad Faghri ◽  
Koichi Suzuki ◽  
...  
Keyword(s):  
Mach Number ◽  
Flow Regime ◽  
Gas Flow ◽  
Flow Characteristics ◽  
Channel Length ◽  
Transitional Flow ◽  
Experimental Investigations ◽  
Compressibility Effect ◽  
Micro Channels ◽  
Wide Range

This paper presents experimental results on flow characteristics of laminar, transitional to turbulent gas flows through micro-channels. The experiments were performed for three micro-channels. The micro-channels were etched into silicon wafers, capped with glass, and their hydraulic diameter are 69.48, 99.36 and 147.76 μm. The pressure was measured at seven locations along the channel length to determine local values of Mach number and friction factor for a wide range of flow regime from laminar to turbulent flow. Flow characteristics in transitional flow regime to turbulence were obtained. The result shows that f·Re is a function of Mach number and higher than incompressible value due to the compressibility effect. The values of f·Re were compared with f·Re correlations in available literature.

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