Václav Tesař

Annular synthetic jet used for impinging flow mass-transfer

An annular synthetic jet was investigated experimentally, both with and without an opposing impingement wall. The experiments involved smoke visualization and mass transfer measurement on the wall by means of naphthalene sublimation technique. Two qualitatively different flow field patterns were identified, depending upon the driving amplitude level. With small amplitudes, vortical puffs maintain their identity for a relatively long time. If the amplitudes are large, breakdown and coalescence of the vortical train is much faster. Also the resultant mass transfer to the impingement wall is then much higher. Furthermore, a fundamental change of the whole flow field was observed at the high end of the investigated frequency range, associated with radical reduction of the size of the recirculation bubble.

Hybrid synthetic jets as the nonzero-net-mass-flux synthetic jets

The “hybrid synthetic jet” combines the zero-net-mass-flux synthetic jet and fluidic pumping through a valveless pump. No fluid is supplied from an external source (blower or compressor). Hot-wire experiments on a demonstration model confirm that the hybrid synthetic jet exhibits a higher extrusion volume flow rate than the ordinary synthetic jet. For the tested configuration, the increase is by 25% at the 1.30 ratio of the extrusion and suction volume fluxes.

New Fluidic-Oscillator Concept for Flow-Separation Control

A novel fluidic oscillator, particularly suitable to serve as the key element in actuators for control of fluid flows, was proposed and experimentally investigated. Its core part, as in known oscillators, is a fluidic bistable jet-deflection diverter amplifier. The unique feature of the new layout is the absence of the closed feedback loop channels. Instead, there is a quarter-wave Helmholtz resonator tube connected to one of the amplifier control terminals. The acoustic waves traveling in it and reflected from its open end cause the phase-delayed switching of the amplifier. The main advantage is high achievable oscillation frequency and no dependence on the flow rate.

Performance of Synthetic Jet Actuators Based on Hybrid and Double-Acting Principles

The design of the discussed fluid jet actuator followed two ideas: a double-acting operation of reciprocating pumps, and a hybrid synthetic jet (HSJ) actuation. The entire actuator consists of two basic parts, namely the front (central) and rear chambers, from which fluid is displaced by the opposite sides of the same diaphragm. The actuator operates in a double-acting (antiphase) regime with air as the working fluid. The central chamber generates the standard (zero-net-mass-flux) axisymmetric synthetic jet (SJ), while the rear chamber generates system of several non-zero-net-mass-flux HSJs arranged around the central SJ. A number of variants of HSJs with different geometry were designed, manufactured and tested. Their behavior was investigated experimentally using the smoke visualization, reaction force measurement, hot-wire anemometry, and naphthalene sublimation technique. The tests confirm the efficiency of the present design.

Fluidic valve for reactor regeneration flow switching

An unusual and in many respects advantageous no-moving-part valve is described, developed for switching fluid flows in a through-flow reactor that requires a periodic regeneration by temporary replacement of the process fluid by another, regeneration fluid. The unusual feature of the valve is that it is axisymmetric, built integrally into the inlet part of the reactor body. The valve operation is based upon a monostable axisymmetric variant of the Coanda effect of jet attachment to a wall. The jet is annular and there are two attachment walls of conical shape. The outer hollow cone is dominant while the auxiliary inner convex cone is small, almost vestigial. Concentrating on the performance in a no-spillover regime, experimental data obtained in cold-air laboratory tests using a full-scale model are compared with numerical flowfield computations, using unusual non-dimensional presentation.

Development of a Microfluidic Unit for Sequencing Fluid Samples for Composition Analysis

A microfluidic sample-sequencing unit was developed as a part of a high-throughput catalyst screening facility. It may find applications wherever a fluid is to be selected for analysis from any one of several sources, such as microreactors operating in parallel. The novel feature is that the key components are fluidic valves having no moving parts and operating at very low sample flow Reynolds numbers, typically below 100. The inertial effects utilized in conventional no-moving-part fluidics are nearly absent; instead, the flows are pressure-driven. Switching between input channels is by high-Reynolds-number control flows, the jet pumping effect of which simultaneously cleans the downstream cavities to prevent cross-contamination between the samples. In the configuration discussed here, the integrated circuit containing an array of 16 valves is etched into an 84 mm diameter stainless steel foil. This is clamped into a massive assembly containing 16 mini-reactors operated at up to 400° C and 4 MPa. This paper describes the design basis and experience with prototypes. Results of CFD analysis, with scrutiny of some discrepancies when compared with flow visualization, is included.

Annular Impinging Jet Controlled by Radial Synthetic Jets

This experimental study focuses on generation and control of annular impinging jets. The annular nozzle used in the investigations was designed with an active flow control system using 12 synthetic jets issuing radially from the central nozzle body. Measurements of the control effects were made on the impingement wall. The data acquisition involved wall pressure and wall mass transfer (by the naphthalene sublimation technique) using air as the working fluid. Also measured was time-mean flow velocity (by a Pitot probe) in the jet flow field. Moreover, flow visualization was carried out. Two main flow-field patterns (A and B) were identified. The patterns differ in the size of the separated-flow recirculation regions that develop attached to the nozzle central body: While pattern A is characterized by a quite small recirculation region (bubble) extending not far from the nozzle exit, pattern B exhibits a large recirculation region, reaching up to the impingement wall, on which it forms a stagnation circle. The control action modifies the flow field, resulting in changes of the corresponding heat/mass transfer distributions. The convective transfer rate on the stagnation circle can be demonstrably enhanced by 20% at a moderate nozzle-to-wall distance, equal to 0.6 of the nozzle outer diameter.

Fluidic Generator Of Microbubbles – Oscillator With Gas Flow Reversal For A Part Of Period

Abstract Paper presents a fluidic device developed for generation of small (less than 1 mm in diameter) microbubbles in a liquid from gas passing gas through small passages. Until now the bubbles are larger than the size of aerator passage exits so that making the passages smaller did not result in obtaining the desirable microbubbles. Analysis of high-speed camera images (obtained with a special lens of large working distance) have shown show that the large bubble size is caused by slow ascent motion of very small bubbles so that they get into mutual contact and grow by conjunction. The solution is to pulsate the supplied gas flow by a no-moving-part fluidic oscillator. The generated small bubble is moved back into the aerator passage where it is for a part of oscillation period protected from the conjunction with other, previously generated microbubbles.