A channel flow affected by a synthetic jet array an experimental study

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1 channel flow affected by a synthetic jet array an experimental study Petra Dančová, Zdeněk Trávníček, Tomáš Vít and Michal Kotek bstract This paper is focused on the interaction of a synthetic jet array with a laminar channel flow. The study is experimental and measurements of the flow field have been performed with water as a working fluid. For a problem investigation, hot wire anemometry (HW), laser Doppler vibrometry (LDV) and particle image velocimetry (PIV) were used. The present results confirmed that the channel flow can be effectively controlled by means of the synthetic jet array. Keywords Laminar channel flow, Laser Doppler vibrometry, Particle Image Velocimetry, Piezoceramic transducer, Synthetic jet array. I. INTRODTION synthetic jet (SJ) is generated by the periodic motion of an actuator oscillating membrane. It is synthesized by the interactions within a train of vortex rings or counter-rotating vortex pairs in either axi-symmetric or two-dimensional geometry, []. The time-mean mass flux of the oscillatory flow in the orifice is zero; hence the other common expression is zero-net-mass-flux (ZNMF) jet. The ZNMF eliminates the requirement of a blower and piping for the fluid inlet. Though the SJ actuator works with ZNMF, the momentum of the resulting SJ is non-zero. The SJ has many significant possible applications, which can be divided into two main groups: Flow control: The SJ can be used for jet vectoring, flow field control in external and internal aerodynamics and for increasing of intensity of mixing. sing of stand-alone SJ or arrays of them: The SJ is used for intensification of the heat and mass transfer. This study focuses on the interaction of a SJ array with a laminar channel flow, with emphasis on low Reynolds Manuscript received May,. This work was supported by the Grant gency S R (Project I768), G R (P//J9 and P//79) and SGS 8. P. Dančová is with the Technical niversity of Liberec, FS / KEZ, Studentská, 6 7 Liberec, zech Republic ( ; fax ; petra.dancova@tul.cz). Z. Trávníček is with the Institute of Thermodynamic, zech cademy of Sciences, v.v.i., Dolejškova 5, 8 Praha 8, zech Republic ( tr@it.cas.cz) T. Vít is with the Technical niversity of Liberec, FS / KEZ, Studentská, 6 7 Liberec, zech Republic ( tomas.vit@tul.cz). M. Kotek is with the Technical niversity of Liberec, FM / RSS, Studentská, 6 7 Liberec, zech Republic ( michal.kotek@tul.cz). numbers. This arrangement can be useful in many micro-scale applications, such as cooling of micro-electronics or the detection of various (biological, biomedical or chemical) species. The flow regime in micro-scale is usually laminar with very small Reynolds numbers. Therefore, the transfer processes such as mixing and cooling are typically based on gradient diffusion. It is worthy to mention here that the present Reynolds numbers (in order ) are too low for an initiation of transition to turbulence. On the other hand, actuation of the laminar channel (Poiseuille) flow can essentially enhance the transport mixing creating the so-called quasi-turbulent flow character, []. Moreover, it should be mentioned that the present subject (laminar channel flow controlled by SJs) is different from SJ in a boundary layer or SJ in a cross flow. II. EXPERIMENTL INVESTIGTION. Problem parameterization SJ is characterized by several independent parameters. onsidering the plug flow model (one-dimensional piston like flow in the actuator orifice), the main parameters are: - The actuator orifice diameter D. - The time-mean orifice velocity,, respectively: T TE u ( t) dt TE u t r d dt T (, ) where T is the time period, i.e. (a, b), T / f and f means the frequency, T E is extrusion time ( T E T / for the sinusoidal waveform or T E T / for the non-sinusoidal waveform), u (t) means the periodical axial orifice velocity. For a sinusoidal waveform u (t) max sin(πft), one can easily derive max π. is the cross-section area of the exit orifice of the actuator and r is the orifice radius. - The stroke length L, L, respectively: L T f L T f (a, b). ISBN:

2 - The Reynolds number of SJ: D Re SJ ν D Re SJ _ ν where ν is the kinematic viscosity [m /s]. (a, b), - The Strouhal number is not an independent parameter; it depends on the above parameters (Eq. (a, b)). For a sinusoidal waveform: f D D St π π L (a, b). f D D St π π L - The Reynolds number of the channel flow (for D simplification): DH H Re (5), ν ν where ( / ) is the time-mean velocity through the max channel, max is the maximum velocity in a channel and H is the channel high. - The strength of the control SJs can be quantified relative to the main channel flow in terms of the ratios of velocities, flow rates, and momentum: c (6), c Q πd n n (7), BH 5π c M n (8), BH where n is the number of control SJs (n for present study), B x H is the cross-section of the channel, width x high, respectively. Fig. a SJ array equipment Fig. b Detailed view of the SJ array of actuators (bottom view) honeycomb plexiglass wall with SJ array B. Experimental setup Two experimental setups were used. First experimental setup is made out from plexiglass (Fig. a), internal dimensions are (xx) mm, length x width x high. The upper wall is bolted with four screws this connection enables to take off this wall. SJ array is made from four SJ actuators, for dimensions see Fig. b. SJ actuator consists of a sealed cavity (diameter D D 6mm), which is equipped with an emitting orifice (diameter D.mm) and with an actuating piezoelectric membrane (piezoceramic transducer - KINGSTTE KPS-). The piezoceramic transducer (PT) was used because it has many significant advantages, e.g. applicability in different fluids, in broad temperature ranges, and in heavy-duty facilities. Other advantages of PT are long an operational lifetime and low power consumption. This setup Fig. Drawing of s plexiglass water channel ISBN:

3 was used to determine a natural frequency of individual SJ actuators, for investigation of PTs behavior and to see influence of SJ array without a channel flow. Second experimental setup is a plexiglass water channel in which is inbuilt a plexiglass wall. Sizes of a channel see on Fig.. omponent of a channel is a pump for water circulation (not drawn in Fig. ). Dimensions of SJ array corresponds with first setup. Dimensions of a plexiglass wall are length (length with beginning) x width x thickness (86 (88) x x ) mm. The wall inlet is carefully rounded and SJ array is put 5mm from the beginning. This setup was used for PIV investigation of affection of a laminar channel flow with SJ array.. Experimental methods Three different experimental methods were used. For SJ actuator frequency characteristic a hot wire anemometry (HW) was used in regime of constant temperature. The sampling frequency and number of samples were 7 khz and 768, respectively. For data analysis the commercial software StreamWare version. and Excel was used. The actuator piezoelectric membrane centre displacement and velocity was measured using laser Doppler vibrometry (LDV). Measurement is based on the Doppler effect, and the portable digital vibrometer Ometron VH--D B&K 88 was used. The main parameters: frequency range.5hz khz; measurement ranges (full scale (peak- peak)) mm/s, mm/s, 5mm/s; spurious free dynamic range (SFDR): > 9dB; and best resolution:. µm/s/hz.5. The response of a PT to a harmonic driving signal for frequency f 5Hz was measured. The LabView software was used to acquire and analyze the signal obtained from the Ometron vibrometer. Software Excel was used for additional calculations and analysis. Behavior and flow field of SJ array and channel flow was measured using particle image velocimetry (PIV). Seeding particles for the PIV experiments were polyamide balls with diameter µm (DNTE) and fluorescent rhodamin coated particles with diameter. µm (Microparticles GmbH). The particles were illuminated by a double pulse laser with maximum 5 mj per ns pulse and repetition rate x 5 Hz. The typical delay time between two pulses was 5 ms. The laser beam was expanded into a light sheet of about mm thick. The image pairs were acquired using the camera (HiSense model MKI, DNTE) with a spatial resolution of 8 x pixels. The resulting vector maps were averaged over PIV records. Velocity vectors were determined by adaptive correlation over interrogation windows x pixels at a 5 % overlap. Data processing used the commercial software DynamicStudio version. (DNTE). dvantage of polyamide balls is identical density with water, which guarantees real measurement of the flow. Disadvantage is the attachment of the particles on channel walls and their consequential high reflection. dvantage of rhodamine coated particles is the negation of the influence of bubbles in water, foreign matters drift with flow and reflection from walls. During measurement the narrow-band filter on camera is used. +p [m/s] y [mm] Fig. Frequency characteristic of SJ actuator t/t [] Fig. a ctuator membrane centers velocity t/t [] Fig. b ctuator membrane centers displacement III. RESLTS SJ array actuators were fed with sinusoidal current. During the experiments, the electric power of the actuators was set on value.8w. Natural frequency (using HW): It is known that SJ actuators work very well near their resonance/natural frequencies. Following this fact, the frequency characteristic of the SJ actuator is the first step of the experiments. If the actuator works with the natural frequency, it has the highest power and velocity. Fig. shows the frequency characteristic. Frequency was measured in range of ( TTL [V] TTL [V] TTL TTL ISBN:

4 Fig. 5 Laminar channel flow Fig. 6a SJ array focused on the orifice axes (in form of velocity magnitude vectors) Fig. 6b: SJ array displaced mm from the orifice axes (in form of velocity magnitude vectors) Laminar channel flow (using PIV): Figure 5 shows channel flow without affection of SJ array. The Reynolds number of channel flow is Re 8 (calculated from equation 5); the time-mean velocity through the channel cross section is about.6m/s. Flow field of SJ array (using PIV): First step of PIV experiments is the synchronization of SJ actuators with laser and camera system. Synchronization procedure is described e.g. in []. Experiments were performed with the upper wall of the equipment (Fig.a). Experimental results on Fig. 6a show whole SJ array in time of full extrusion from the actuators cavities with the upper wall of the equipment in form of velocity magnitude vectors. Monitored plane was aligned to the collective center line of all orifices. It demonstrates the influence of particular SJs. On Fig. 6b is seen SJ array in the same time as Fig. 6a, but the view is displaced mm from the orifice axes. There is visible a small influence between SJs in form of outer SJ bending. ffection of channel flow with SJ array (using PIV): Experiments were performed with full extrusion from the actuators orifices. The interaction of the channel flow with a SJ is shown in Fig. 7 in a form of contours of time-mean velocity magnitude. This experiment clearly demonstrates the SJ bending and its propagation downstream in the main channel flow. Moreover, SJ reaches the opposite wall, where an oblique jet impingement can be identified (see the ellipse mark in Fig.7). Obviously, heat/mass transfer enhancement can be expected in the impingement area this effect is desirable in various applications. IV. ONLSION Fig. 7 Velocity magnitude of the channel flow interacting with SJ (in form of velocity magnitude contours) 6) Hz. Fig. shows that SJ actuator works with the highest voltage, velocity respectively, on 5 Hz, which is the working frequency of SJ array. The far from natural frequency, the more velocity values decrease (Fig. a). SJ actuator has also the second resonance, which follows the Helmholtz resonator frequency, []. Behavior of PTs (using LDV): Figures a and b show the results from LDV measurement. The sampling rate for this measurement was Hz. The period starts with the leading edge of the TTL signal. TTL signal has the same frequency (5 Hz) as the frequency of SJ actuators driving signal from the signal generator. Figure a shows measurement of the membrane centers velocities in time, smooth square curve is TTL signal. There is a visible phase shift between TTL signal and the response of the membrane. Here is also shown, that the SJ actuators work without phase shift, which is important for the general task of SJ array with affection of a channel flow. Figure b shows the membrane centre displacement in time compared with TTL. Velocities are measured with laser Doppler vibrometer and displacements are calculated from measured values. This paper brings results of experimental research of the synthetic jet array and affection of SJ array with laminar channel flow. Investigation is focused on low Reynolds numbers and the working fluid is water. For actuators natural frequency determination HW is used. Behavior of PT membrane using LDV is measured. PIV investigation brings results of SJ array with and without affection of channel flow. REFERENES [] B. L. Smith,. Glezer,, The formation and evolution of synthetic jets, Phys. Fluids, 998, pp [] J.E. ater, J. Soria, The evolution of round zero-net-mass-flux jets, J. Fluid Mech. 7,, pp. 67. [] L. E. Kinsler,. R. Frey,. B.oppens, J. V. Sanders: Fundamentals of coustics, th Ed., Wiley & Sons, Inc., New York,. [] M. Kotek, V. Kopecký, pplication of laser anemometric and visualization methods on research of weft propulsion mechanism, in: Proc. of In. conference Experimental Fluid Mechanics, 9, Liberec, zech Republic, pp [5]. Glezer, M. mitay, Synthetic jets, in: nnu. Rev. Fluid Mech.,, pp [6] V. Timchenko, J.. Reizes, E. Leonardi, n evaluation of synthetic jets for heat transfer enhancement in air cooled micro-channels, in: Int. J. Numer. Methods Heat Fluid Flow, 7(), 7, pp [7] R. Holman, Y. tturkar, R. Mittal, B.L. Smith, L. attafesta, Formation criterion for synthetic jets, I Journal (), 5, pp. -6. ISBN:

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