Microjet Flow Research Articles

Validation of Numerical Simulation Method for Active Flow Control Technology

The numerical simulation method of active flow control technology was studied in this paper. The simplified mathematical model of the active flow control is established with unsteady velocity boundary condition at the specific location of model surface. The reliability of flow control model was verified by standard cases of CFDVAL2004, and the capability of capturing micro-jet flow characteristics for Spalart-Allmaras (SA) and Menter’s Shear Stress Transport (SST) turbulence model were analyzed. The results showed that the accuracy of SA turbulence model is better than the SST model, and flow control model meet the accuracy requirements for numerical simulation method.

Microjet Flow Control Effectiveness of Cavity Configurations at Low Speeds

An experimental study was conducted to investigate the aeroacoustics of open- and closed-cavity configurations with and without flow control using microjets at low speeds. The cavity models had length-to-depth ratios of 2, 10, and 18, representing open, transitional, and closed cavities, respectively. The investigation was carried out at a flow speed of 16 meters per second. The microjet effectiveness was tested for jet velocities of 8, 16 and 24 meters per second. The vortex shedding aft of the trailing edge was reduced when the microjet velocity was equal to that of the freestream (16 meters per second). The results from the transitional cavity experiment, with a length-to-depth ratio of 10, showed that either higher microjet velocities or different locations of the microjets may be required to have any significant effect on the cavity flow characteristics.

Stability of Subsonic Microjet Flows and Combustion

Results of experimental studies of round and plane propane microjet combustion in a transverse acoustic field at small Reynolds numbers are presented in this paper. Features of flame evolution under the given conditions are shown. Based on the new information obtained on free microjet evolution, new phenomena in flame evolution in a transverse acoustic field with round and plane propane microjet combustion are discovered and explained.

A supersonic broadband microjet actuator using piezohydraulic actuation

A supersonic piezohydraulic microjet flow control actuator has been developed using a 400-µm-diameter micronozzle capable of producing pulsed microjet flow over a broad frequency range from quasi-static to 1.6 kHz. Such flow control actuators are useful in providing ideal control inputs for controlling flow properties to improve aerodynamic performance, for example, reduce or eliminate flow separation and mitigate noise from propulsion systems and other aircraft components. The design of the piezohydraulic microjet is presented in combination with experimental results to quantify the internal system dynamic characteristics in comparison with the microjet flow. The microjet actuator couples a piezoelectric stack actuator and a hydraulic circuit to amplify the 20-µm stack actuator displacement to an amplitude that is necessary to throttle flow through the 400-µm-diameter micronozzle. Unsteady pressure measurements at the microjet exit are correlated with the piezoelectric stack actuator displacement and voltage input to provide comparisons between the internal electromechanical actuation and external pulsed flow behavior. Phase-conditioned micro-Schlieren imagery is also utilized to quantify the flow field. The results illustrate broadband supersonic pulsed microjet actuation frequencies that approach 1 kHz: frequencies that are much higher than currently achieved with broadband actuators with this control authority.

Torch Design Modification Using Micro-jets to Suppress Fluid Dynamic Instabilities in Plasma Arc Cutting

Highly constricted plasma arcs are widely used for metal cutting. One important characteristic of the cutting process is the consistency of the cut edge around the perimeter of the workpiece. Cut edge properties, including surface roughness, edge shape and dross formation, are presumed to depend on the local temperature and chemical composition of the cutting arc adjacent to the cut edge. Fluid dynamic instabilities in the arc boundary leading to entrainment of the low temperature ambient gas can have a strong effect on cutting performance. This paper describes the use of micro-jets to suppress fluid dynamic instabilities in the boundary layer of a plasma cutting arc. Previously developed optical diagnostics and analysis methods are used to characterize the arc boundary layer. Multiple nozzle designs have been investigated to quantify the effects of utilizing micro-jet flow around the arc column, and some relationships between nozzle design and cut quality are presented.

3-D visualization of flow in microscale jet impingement systems

Microjet impingement cooling devices promise high heat removal rates for the development of advanced thermal management solutions. However, understanding of microjet hydrodynamics is needed to optimize cooling performance. In this paper, we combined experiments and modeling to obtain three-dimensional (3-D) microjet flows. We fabricated single-jet and multi-jet arrays with 50 μm diameter orifices and used micron-resolution particle image velocimetry ( μPIV) to capture two-dimensional (2-D) images of the flow field at different imaging planes. The data was subsequently used to obtain the out-of-plane (z-component) velocities, which play an important role in enhancing heat transfer at the impingement surface. The results from the reconstruction of the 3-D flow field offers new insights into the impact region of a single jet and optimized design of microjet cooling devices.

Microjet array single-phase and flow boiling heat transfer with R134a

An experimental study of single-phase and flow boiling heat transfer of a submerged microjet array was conducted with R134a. The staggered array of seventeen 112-μm diameter orifices impinged onto a 1 × 1-mm heater. Single-phase data were gathered over the range 3050 ⩽ Re d ⩽ 10,600 with 53.6 ⩽ Nu d ¯ ⩽ 128 . Boiling experiments were conducted with liquid subcoolings of 10, 20, and 30 °C at jet velocities of 4, 7, and 10 m/s. Boiling was generally found to enhance heat transfer with a maximum heat flux of 590 W/cm 2. However, temperature excursions occurred at low superheats and were found to be related to non-condensable gas content.

Cavitation enhancement of silt erosion: Numerical studies

Cavitation and silt erosion often co-exist causing severe damage The effect that the silt erosion is worsened by the presence of cavitation is here referred as cavitation enhancement of silt erosion A micro-model proposed by Li (2003, 2006) [1,2] describes how the silt particles gain extra-damaging potential from the collapsing bubbles In order to verify this envisaged model, as suggested in Li (2006) [2] we have performed initial numerical studies by employing a simplified model which considers a single silt particle entrained in the micro-jet of a collapsing cavitation bubble Initial CFD simulations have been conducted by using both commercial software Star CCM+ and our simplified analytical approach (written in MATLAB) Results from both methods support the mechanism of the micro-model that the silt particle damage potential to nearby solid boundary is largely enhanced through the acceleration of particle by the micro-jet of collapsing bubble in which the particle entrained. Our calculations further show that given the size of bubble the enhancement is mainly correlating with the mass (Its shape also plays an important role in gaining the drag force from the micro-jet flow, which will be discussed in detail later) of particle and the ratio between the distance from the collapsing bubble to a solid boundary and its radius upon collapse. Though there are more attributions of the enhancement need to be further investigated such as the influence of presence of the particle on the micro-jet flow, etc. (by using a real coupled calculation approach) these initial results do reveal the major features of this phenomenon quantitatively for the first time (C) 2009 Elsevier B V All rights reserved

Peculiar phenomenon of micro-free-jet flow

This is the first time that free microjet flow dynamics is explored experimentally in the open literature. The microjet is issued from a microslot nozzle made by microelectromechanical system techniques. Three different sizes of slot nozzles that have the widths of 50, 100, and 200 μm are fabricated. Careful flow visualization and instantaneous velocity measurements at different locations for these jet flows are made at different Reynolds numbers. The results indicate that the vortex formation and merging processes that occurred in the large-scale macrojet are completely absent in the microjet, and the microjet flow phenomenon is drastically different from that of the macrojet. A microjet can penetrate much deeper than a macrojet. A critical Reynolds number for the breakdown of the microjet is determined. Detailed discussion on the microjet dynamics will be presented in this paper.

Piezohydraulic Actuator Development for Microjet Flow Control

This paper describes the development of a piezohydraulic actuator for broadband microjet flow control applications. The actuator utilizes a lead zirconate titanate (PZT) stack actuator and a hydraulic amplification design to achieve relatively large displacements in a compact actuator to control flow through a microjet orifice. Displacement amplification of 81 times the stack actuator displacement was achieved using a new dual-diaphragm design. The nonlinear and hysteretic field-coupled material behavior, structural dynamics, and fluid dynamics of the actuator are modeled using a system dynamic model and compared with experimental results. The nonlinear and hysteretic piezohydraulic actuator characteristics are shown to be strongly dependent on the nonlinear deformation of the rubber diaphragm and minor loop hysteresis of the PZT stack actuator. The modeling technique provides a design tool for broadband performance predictions and system optimization to facilitate implementation of the actuator in a flow environment.

An open, non-contact cell for laser ablation-inductively coupled plasma-mass spectrometry

A novel, open, non-contact cell for laser ablation, capable of sampling: large planar samples, embedded planar samples, or samples of less than 2 mm mounted on a planar platform, without an outer containment enclosure, is described. This cell, when tested on NIST-613 CRM, exhibited rapid wash-out (<3.6 s for 99% signal reduction), low limits of detection and good signal precision in LA-ICP-MS. The cell uses a dual, annular, micro-jet gas flow array to exclude atmospheric gases and to entrain the ablated sample aerosol. The micro-jet array employed enabled a sampling height, between the sample surface and the lowest plane of the cell, of up to 200 µm. The micro-jet array has the facility to be electrically biased if the application demands it, e.g. extracting a charged plume in Matrix-Assisted Laser Desorption/Ionisation (MALDI) or Desorption Electrospray Ionisation (DESI) experiments. This particular implementation of the cell featured dual sample output channels, which could enable connection of the cell to more than one analyser. A micro jet-pump was coupled to the cell to extract the ablated aerosol from the low-volume inner ablation chamber, provide mixing of the aerosol with the injector flow of the ICP and to isolate the cell from downstream conditions in the injector flow. Potential applications of this cell include: analysis of silicon wafers, gel-plates/membranes, tissue samples, multiple sample analysis in high throughput facilities; and toxic fume removal and analysis in laser scribing, cutting and etching.

Electrochemical printing: in situ characterization using an electrochemical quartz crystal microbalance

Electrochemical printing (EcP) is a multi-scale freeform fabrication method employing localized electrodeposition beneath a rastering microjet print head flying microns above the deposition substrate. As a result of the confined geometry and unusual electrochemical configuration, conventional in situ diagnostics are difficult to implement. We show that EcP microfabrication on an electrochemical quartz crystal microbalance (EQCM) allows in situ characterization of the electrochemical processes occurring locally beneath the microjet print head and over large patterned areas where corrosion occurs. Copper deposition experiments show the microjet print head can produce extraordinary mass transfer limiting currents (in excess of 10 A cm−2). The current efficiency for EcP patterning of copper on gold is limited by oxygen reduction at low applied currents and by hydrogen evolution at high currents. Nonetheless, the maximum current efficiency is >90% over two orders of magnitude change in microjet flow rate; the limiting current varies five-fold over the same flow rates. Calibration of the EQCM local sensitivity factors allows characterization and control of corrosion rates when printing extended patterns.

Analysis of deformation process for cell model including a gas bubble by shock waves

This paper describes the analysis of deformation process for cell model including a gas bubble by shock waves and flow induced by propagating underwater shock waves, in order to study the mechanism and high efficiency of destruction of cells and microcapsules by shock waves and bubbles, such as for drug delivery system (DDS) and for bioprocesses to make energy and for environmental protection. For this analysis, the cells are modeled as liquid droplet including a gas bubble in water. By computation, using the ALE (arbitrary Lagrangian–Eulerian) method, the coupling oscillation is analyzed for particle velocity and pressure wave through the cell. The results show that the effects of the acoustic impedance of the fluid media in water is large for the deformation process and the effects of deformation of droplet boundary is also large for deformation process for a gas bubble inside the cell and for microjet flow by a collapsing bubble.