Enhancement of synthetic jets by means of an integrated valve-less pump Part II. Numerical and experimental studies

The paper studies the performance of the new fluid jet actuator based on the novel principle of the generation of fluid jet, which has been presented in [Z. Travnicek, A.I. Fedorchenko, A.-B. Wang, Enhancement of synthetic jets by means of an integrated valve-less fluid pump. Part I. Design of the actuator, Sens. Actuators A, 120 (2005) 232-240]. The fluid jet actuator consists of a synthetic jet actuator and a valve-less pump. The resulting periodical fluid jet is intrinsically non-zero-net-mass-flux, in contrast to the traditional synthetic jet. The numerical results have been compared with the laboratory experiments comprising phase-locked smoke visualization and time-mean velocity measurements. The results have confirmed the satisfactory performance of the actuator

The near wall effect of synthetic jets in a boundary layer

Copyright @ 2007 Elsevier Inc. All rights reserved.An experimental investigation to analyse the qualitative near wall effect of synthetic jets in a laminar boundary layer has been undertaken for the purpose of identifying the types of vortical structures likely to have delayed separation on a 2D circular cylinder model described in this paper. In the first instance, dye visualisation of the synthetic jet was facilitated in conjunction with a stereoscopic imaging system to provide a unique quasi three-dimensional identification of the vortical structures. Secondly, the impact of synthetic jet structures along the wall was analysed using a thermochromic liquid crystal-based convective heat transfer sensing system in which, liquid crystals change colour in response to the thermal footprints of a passing flow structure. Of the different vortical structures produced as a result of varying actuator operating and freestream conditions, the footprints of hairpin vortices and stretched vortex rings revealed a marked similarity with the oil flow pattern of a vortex pair interacting with the separation line on the cylinder hence suggesting that either of these structures was responsible in delaying separation. Conditions were established for the formation of the different synthetic jet structures in non-dimensional parameter space

No-moving-part hybrid-synthetic jet actuator

In contrast to usual synthetic jets, the “hybrid-synthetic jets” of non-zero timemean nozzle mass flow rate are increasingly often considered for control of flow separation and/or transition to turbulence as well as heat and mass transfer. The paper describes tests of a scaled-up laboratory model of a new actuator version, generating the hybrid-synthetic jets without any moving components. Self-excited flow oscillation is produced by aerodynamic instability in fixed-wall cavities. The return flow in the exit nozzles is generated by jet-pumping effect. Elimination of the delicate and easily damaged moving parts in the actuator simplifies its manufacture and assembly. Operating frequency is adjusted by the length of feedback loop path. Laboratory investigations concentrated on the propagation processes taking place in the loop

Free and Confined Buoyant Flows

Flows driven by density differences, whether natural or induced by man, surround us on a wide range of scales. As a density difference is generated by commonly occuring temperature or salinity differences, these flows are ubiquitous. On the largest scales, they transport and mix the water in our oceans and air in our atmosphere. At an intermediate scale, frequently referred to as the mesoscale, the cold outflows which flow downwards from thunderstorms and impact the ground are a common aviation hazard. On a smaller scale, the toxins emitted by fire plumes or dense gas releases are a threat to health. This work focusses on a continuous, localised release of buoyant fluid from a horizontal source whose dimension is significantly smaller than the horizontal and vertical scales of the quiescent, uniform environment into which the flow propagates. We consider both passive releases, which are only driven by the density difference between the source fluid and the denser ambient, and forced releases, where the fluid has source momentum in addition to its buoyancy. In general, these releases give rise to turbulent plumes, a familiar example being the cloud of smoke and water vapour seen rising from a chimney stack on a cold, still morning. The first part of the research presented in this thesis focusses on the freely-propagating plume. Velocity and temperature measurements are presented which contribute considerably to the existing experimental data available in the literature. This data-set is used to validate classical plume theory and make a check of the experimental set-up so that the subsequent results can be presented with confidence. It is also possible that this dataset will be used by other researchers to validate numerical simulations of buoyant flows. The effect of varying the source balance of buoyancy and momentum upon the plume dynamics is investigated. Measurements also reveal the development region or ‘zone of flow establishment’. Frequently, plumes are restricted by some form of confinement, either vertically, horizontally or both, for example the plumes rising from the occupants of a room. Whether this restriction takes the form of a solid wall, free surface or density discontinuity, the disturbance to the flow is typically significant. The simplest confining boundary is arguably a horizontal surface located some distance H from the source of buoyant fluid. The horizontal boundary forces the vertical flow to change direction and propagate radially outwards. This type of semi-confined flow can be frequently observed in the natural world with examples including the impingement of a fire plume against a ceiling and a plume of volcanic ash with the tropopause. An investigation into this type of flow, which we refer to as the ‘impinging buoyant plume’, constitutes the second part of the research. Plume impingement has not been studied as extensively as jet impingement and several key questions remain unanswered. For example, how much energy is lost as the vertical flow is forced to turn and propagate horizontally? What effect does buoyancy have on the horizontal flow? How does the flow evolve with increasing radial distance and what is the effect of changing the source-boundary separation? These are just some of the questions addressed in this thesis guided by the novel application of highly-resolved Particle Image Velocimetry measurements to this relatively low-speed, buoyant, turbulent flow. The free and impinging plume studies both employed similar experimental techniques and analysis methods. Statistics of the steady flow were determined from a highly-resolved data-set. The third part of the research concerns a time-dependent flow and is of a more qualitative nature. The complexity of the impinging plume increases considerably when a radial confinement is added to the geometry. This restricts the radial propagation of the flow produced by the impinging plume. The plume is now effectively enclosed and buoyant fluid begins to accumulate within and thereby fill the enclosure, a configuration known as the ‘filling-box’. While previous work, which we shall go on to review in detail, has contributed analytical solutions for the density profiles in the enclosure after a certain time-scale has elapsed, in many applications, such as the spread of smoke carried by a fire plume in a room, what happens in the early moments of a confined release following impingement with the horizontal and then vertical boundaries, may be critical. This has been overlooked in earlier studies, yet is crucial as it is during these early transients that the fire is best tackled by fire-fighters. Visualisations and velocity measurements of these early filling-box transients are reported. This work provides the first detailed measurements of the velocity field induced in the filling-box by the turbulent plume during the early transients and resolves the turbulent structures that comprise the plume outflow. The experiments which investigated the impinging jet were conducted on thermal air plumes in facilities at the Laboratoire de Mécanique des fluides et de l’Acoustique (LMFA) of École Centrale de Lyon (ECL). Filling-box experiments were performed on brine plumes in fresh water in visualisation tanks in the Department of Civil & Environmental Engineering at Imperial College London (ICL). The set of experiments at ECL used a combination of Particle Image Velocimetry (PIV) and thermocouples to measure flow velocities and temperatures. At ICL, Light-Induced Fluorescence (LIF) enabled visualisation of a plane through the centre of the axisymmetric flow to complement the PIV work. These experiments enabled effective use of the equipment, techniques and expertise available at both institutions. The principal objective of this research was to use experimental measurements to answer questions of importance regarding these impinging flows which remain unresolved in the literature. Using experimental techniques unavailable to earlier researchers, the work presented herein makes a substantial contribution to the existing knowledge of these flows. Free and impinging plumes and the dynamics of the filling-box flow are studied in detail. Notably, the data gathered are of very high spatial resolution and provide a resource for those interested by not only the plume dynamics, but also radial gravity currents and the filling-box

Experimental Investigation of the Oil Jet Heat Transfer for an Aero Engine Gearbox

Geared turbofan engines have the potential to propel future civil aircraft engines more efficiently. A planetary gearbox between the low-pressure turbine and the fan enables the operation of both components at their respective optimum rotational speeds. This makes it possible to achieve higher bypass ratios and thus a better propulsion efficiency. A crucial part of the planetary gearbox design is the cooling and lubrication of the gears. Sufficient heat removal from the gear tooth flanks is necessary to ensure reliable operation without the risk of gear failure through pitting or scoring. Fast rotating and highly loaded gears are cooled with impinging oil jets according to current design guidelines. This impingement cooling process comprises a complex, multi-phase flow with heat transfer. Previous experimental, numerical and analytical investigations have shown that the cooling process depends both on the highly unsteady liquid flow dynamics and on the heat conduction in the oil film formed on the gear tooth flank. In this study, the gear is replaced by a cylinder in order to be able to study the impingement cooling on a rotating surface without the influence of unsteady flow phenomena. A hollow cylinder is instrumented with 42 thermocouples across the surface, which are all connected to a telemetry system. A single oil jet is directed radially onto the outer cylinder surface. The measured temperatures are subsequently corrected using a new algorithm to reduce systematic measurement errors without distorting the data. The corrected temperatures are used to calculate the Nusselt number distribution across the cylinder surface by means of a finite element analysis. A parameter study is performed to identify the influence of the parameters oil flow rate, oil viscosity and rotational speed of the cylinder on the heat transfer. The fundamental results of the present study enable a better understanding of the heat transfer on impingement cooled cylinders and spur gears

Cooling Strategies for Heated Cylinders Using Pulsating Airflow with Different Waveforms

Pulsate flow is an effective technique applied for cooling several engineering systems depending on their pulsate frequency. One very sound external flow pulsation application is heat transfer over heated bodies. In present work, an experimental design and numerical model of controlled pulsating flow according to generated pulsating frequency and wave shape around a heated cylinder were performed. The effects of pulsating frequency, amplitude, and mean velocity on the fluid flow and heat transfer characteristics over a heated cylinder were studied. The wave frequency varied from 2 to 12 Hz, and the amplitude varied from 0.2 to 0.8 m/s. Moreover, different waveforms were investigated to determine their effect on wall cooling. For constant wave frequency and amplitude, the most efficient wave in cooling was the sawtooth wave, with the average wall temperature after 30 s was 1.6 °C cooler than that of the forced convection case, followed by the triangular wave at 1.2 °C less. The heat transfer rate and the flow field were drastically influenced by the variations of these parameters. Optimization was conducted for each wave type to find the optimum wave frequency and amplitude. The optimizing showed that, the most efficient wave was the sawtooth with 12°C temperature reduction compared with that of the forced convection case, followed by the triangular. Furthermore, regression analysis was conducted to estimate the relationships between these variables and surface temperature. It was found that the wave amplitude had a greater role in cooling than that of the frequency

Direct numerical simulation of backward-facing step flow at Ret = 395 and expansion ratio 2

Backward-facing step (BFS) constitutes a canonical configuration to study wallbounded flows subject to massive expansions produced by abrupt changes in geometry. Recirculation flow regions are common in this type of flow, driving the separated flow to its downstream reattachment. Consequently, strong adverse pressure gradients arise through this process, feeding flow instabilities. Therefore, both phenomena are strongly correlated as the recirculation bubble shape defines how the flow is expanded, and how the pressure rises. In an incompressible flow, this shape depends on the Reynolds value and the expansion ratio. The influence of these two variables on the bubble length is widely studied, presenting an asymptotic behaviour when both parameters are beyond a certain threshold. This is the usual operating point of many practical applications, such as in aeronautical and environmental engineering. Several numerical and experimental studies have been carried out regarding this topic. The existing simulations considering cases beyond the above-mentioned threshold have only been achieved through turbulence modelling, whereas direct numerical simulations (DNS) have been performed only at low Reynolds numbers. Hence, despite the great importance of achieving this threshold, there is a lack of reliable numerical data to assess the accuracy of turbulence models. In this context, a DNS of an incompressible flow over a BFS is presented in this paper, considering a friction Reynolds number (Reτ) of 395 at the inflow and an expansion ratio 2. Finally, the elongation of the Kelvin–Helmholtz instabilities along the shear layer is also studied.Postprint (published version

Enhancement of jet shear layer mixing and surface heat transfer by means of acoustic disturbances

Thesis (Master)--Izmir Institute of Technology, Mechanical Engineering, Izmir, 2010Includes bibliographical references (leaves: 57-66)Text in English; Abstract: Turkish and Englishxii, 66 leavesThe objective of this thesis is to investigate how the surface heat transfer of an impinging jet flow can be increased by acoustic actuation of which changes the turbulence characteristics of the flow. This work is built upon experimental studies which includes flow visualization experiments and surface heat transfer measurements. A loudspeaker system which is controlled by means of a function generator is used for the purpose of actuating impinging jet flow. Acoustic waves in different waveforms, frequencies and amplitudes which are generated by the loudspeaker reaches the jet nozzle, resulting in the formation of an oscillating component on the mean nozzle velocity since the actuation itself is in the form of a periodic fluctuation. It is this oscillating component that actuates the shear layer of the jet flow. Reynolds number is kept at 10.000 for all experimental cases. Influence of nozzle geometry is investigated by using sudden and smooth contracting (with a curvature of 5 degree polynomial) nozzles. Dimensionless nozzle-to-plate spacing is adjusted between 2, 4 and 6. Strouhal number, which is the non-dimensional form of actuation frequency is changed between0 < St < 1. The amplifier, which is used for generating sine and square waves, is set for 1.5 and 2 Volts amplitudes

The Effect of Orifice Shape on Convective Heat Transfer of an Impinging Synthetic Jet

A greater heat load due to the miniaturization of electronic products causes the need for a new cooling system that works more efficiently and has a high thermal capacity. A synthetic jet is potentially useful for the cooling of electronic components. This paper reports the results of our experimental studies and the influence of orifice shape for Impinging Synthetic Jet cooling perfomance. The effect of shape of the orifice of an impinging synthetic jet assembly on the apparatus cooling of a heated surface is experimentally investigated. It will be seen that the characteristics of convective heat transfer will occur by moving the piezoelectric membrane. The prototype of the synthetic jet actuator is coupled with two piezoelectric membranes that operate by 5 volt electrical current and create a sinusoidal wave. The orifice shapes considered are square and circular. The results show the significant influence of orifice shape and sinusoidal wave frequencies on the heat transfer rate that were obtained. The temperature drop with a square orifice is found to be larger than that with circular shapes. A square orifice has a larger covered area if compared to the circular orifice at the same radius, thus resulting in a larger entrainment rate that leads to an increase of heat transfer performance