377 research outputs found

    Aeroacoustics of shear layers in internal flows : closed branches and wall perforations

    Get PDF
    Flow induced pulsations in resonant pipe networks have been observed in many technical applications, such as natural gas transport systems, steam lines of nuclear power plants and re-heat steam lines of boilers. These pulsations, which present a serious threat to the integrity of the systems, have been identifed as self-sustained aeroacoustic oscillations driven by the instability of the flow.The main goal of the proposed research is the prediction of the coupling of acoustic waves with shear layers formed by ow separation in internal flows and the design of remedial measures. We consider here, in particular, the shear layers formed at the opening of closed branches along a pipe and shear layers formed by grazing/bias ow along/through wall perforations.Although there are extensive studies in literature on the pulsations generated by the separating ow along a closed side branch, the configurations with the flow entering or leaving the side branch have not been recognized as source of pulsations. In our study, strong flow induced pulsations have been observed experimentally in configurations with a mean ow entering a side branch or flowing out of a side branch. When flow induced pulsations occur, wall vibrations can be significant amplitude limiting losses. Therefore, we propose an analytical model for the acoustical energy losses due to wall vibrations induced by an oscillating side branch. The study of the aeroacoustics of complex pipe systems was initiated considering a row of closed side branches placed along a main pipe. Systems with up to 15 shallow side branches produces flow induced pulsations in which the shear layer instability couples with a longitudinal acoustic standing wave along the main pipe. The side branches are not resonant. The whistling observed in such a system is similar to that observed in a main steam line along which a row of safety valves is placed. It is also a model for a corrugated pipe as used in risers for natural gas production. Our experiments and theoretical analysis demonstrate that the aeroacoustic sources are located near the acoustic pressure nodes of the longitudinal acoustic modes. A prediction model for the whistling behavior has been proposed, which is based on the \energy balance" between the acoustic sources and the acoustic losses. Experiments carried out on pipe systems with deep closed side branches show that these systems displays strong trapped modes. These systems have been used to test design rules aiming at a reduction of pulsation levels. The most commonly used solution, detuning the side branch length, appears to be inefficient in multiple deep side branch systems. We propose a semi-empirical model for the prediction of the self-sustained oscillations in pipe systems with closed deep side branches with rounded edged T-junctions. It can predict the oscillation amplitude of a system of six deep side branches within 50% and the oscillation frequency within 2%, for the first hydrodynamic mode. It strongly overestimates the amplitude of higher hydrodynamic modes. In car mufflers, liners of the aircraft engine and liners protecting the walls of combustion chambers, perforated walls are used to absorb sound. The sound absorption is due to the interaction of acoustic waves with the shear layers formed by grazing or/and bias flow. The sound absorption depends strongly on the shape of the perforations and on the ratio of bias to grazing flow velocity. In the low Strouhal number limit, the acoustic resistance (real part of the impedance) of a perforation is observed to be proportional to the steady-state resistance. A high value of the steady-state resistance leads to high pressure losses across the perforation. The design of an efficient acoustic damper requires an optimization between acoustic and fluid dynamic performances. Both resistance (real part of the impedance) and reactance (imaginary part of the impedance) due to a grazing-bias flow display an oscillating behavior as function of the Strouhal number. In particular, at high Strouhal numbers, positive (sound absorption) and negative (sound production) values of resistance are observed. The geometry of the perforation determines the whistling behavior at high Strouhal numbers. This operating condition should be avoided in technical applications. Analytical models of the steady flow and of the low frequency aeroacoustic behavior of a two-dimensional wall perforation are proposed allowing a quasi-steady prediction for the sound absorption at low Strouhal numbers. They compare favorably with the experiments

    A review of experiments on stationary bluff-body wakes

    Get PDF
    Experimental studies dealing with the wake of isolated stationary bluff-bodies are reviewed. After briefly recalling the pioneering works in this domain, the paper focuses on recent research conducted with the latest experimental methods and techniques. The review encompasses a range of topics, including, the effects of bluff-body geometry (non-circular cross sections and nonuniformity in spanwise direction), steady and unsteady (periodic and non-periodic) inflow conditions; surface proximity (rigid wall, confinement and water free surface) and non-Newtonian fluids. Focus is brought to the flow physics of the wakes, including especially the complex threedimensional and oscillatory behaviours induced by the periodic vortex shedding phenomenon. The paper aims to offer a critical and systematic review of new knowledge and findings on the subject area, as well as emerging? and the most frequently adopted experimental techniques. The review also helps identifying knowledge gaps in the literature that need to be addressed in future investigations

    Three-Dimensionality in the flow of an elastically mounted circular cylinder with two-degree-of-freedom vortex-induced-vibrations

    Full text link
    The study numerically investigates the three-dimensionality in the flow and two-degree-of-freedom (2 DOF) vortex-induced-vibrations (VIV) characteristics of an elastically mounted circular cylinder. The cylinder is allowed to vibrate in both streamwise and transverse directions. A low value of mass-ratio with the zero damping coefficient is taken for the simulations. The primary aim is to understand the vortex shedding behind the cylinder and the transition characteristics of the wake-flow from two-dimensional (2D) to three-dimensional (3D). The Reynolds number (Re) is varied from 150 (fully 2D flow) to 1000 (fully 3D flow), which lies inside the laminar range. The reduced velocity is varied which covers all three major VIV branches (Initial Branch (IB), Upper Branch (UB), and the Lower Branch (LB)). The oscillating cylinder sweeps the figure-eight trajectory. Two branches (IB, LB) and three branches (IB, UB, LB) amplitude responses are obtained for the low and high Re values, respectively. The wake behind the cylinder with 2-DOF VIV undergoes the mode-C transition of 2D to 3D flow as opposed to the direct mode-B transition observed for transverse only VIV in the literature. The critical Re range of the 2D to 3D transition for the 2-DOF VIV cylinder at a reduced velocity of 6 is around 250, less than the 1-DOF VIV. Also, this range varies with the variation in and the streamwise to transverse oscillation frequency ratio. A map is proposed for the 2-DOF VIV, highlighting the different modes of transition obtained for combinations of reduced frequency and Re

    Improving Flat Plate Heat Transfer Using Flexible Rectangular Strips

    Get PDF
    Many engineering systems involve proper transfer of heat to operate. As such, augmenting the heat transfer rate can lead to performance improvement of systems such as heat exchangers and solar photovoltaics panels. Among the many existing and studied heat transfer enhancement techniques, a well-designed passive turbulence generator is a simple and potent approach to augmenting convective heat transfer. Two of the most recognized passive convective heat enhancers are wings and winglets. Their potency is attributed to the long-lasting induced longitudinal vortices which are effective in scooping and mixing hot and cold fluids. Somewhat less studied are flexible turbulence generators, which could further the heat transfer enhancement compared to their rigid counterpart. In the current study, the flexible rectangular strips are proposed, marrying the long-lasting vortex streets with the periodic oscillation, to maximize heat convection. This study was conducted in a closed-looped wind tunnel with 76 cm square cross-section. The effects of flexible strips on the turbulent flow characteristics and the resulting convective heat transfer enhancement from a heated flat surface are detailed in four papers which are presented as Chapters 3, 4, 5 and 6. In Chapter 3, the effect of the thickness of the strip is detailed. The 12.7 mm wide and 38.1 mm tall rectangular strip was cut from an aluminum sheet with thickness of 0.1, 0.2 and 0.25 mm. The incoming wind velocity was maintained at around 10 m/s, giving a Reynolds number based on the strip width of 8500. It is observed that the thinnest 0.1 mm strip could induce a larger downwash velocity and a stronger Strouhal fluctuation at 3H (strip height) downstream, leading to a better heat transfer enhancement. The peak of the normalized Nusselt number (Nu/Nu0) at 3H downstream of the 0.1 mm strip was around 1.67, approximately 0.1 larger than that of the 0.25 mm strip. In Chapter 4, the height effect of the strip is disclosed. The strip was 12.7 mm wide and 0.1 mm thick, with a height of 25.4 mm, 38.1 mm and 50.8 mm. The Reynolds number in this chapter was also fixed at around 8500, based on the strip width and the freestream velocity. It was found that the shortest, 25.4 mm strip could induce the closest-to-wall swirling vortices, and the largest near-surface downwash velocity toward the heated surface. Thus, the largest heat transfer augmentation was observed. At 9W (strip width) downstream, the 25.4 mm-strip provided the Nu/Nu0 peak of around 1.76, 0.26 larger than that associated with the tallest, 50.8 mm-strip. In Chapter 5, the effect of the transversal space of a pair of strips is expounded. A pair of 0.1 mm thick, 12.7 mm wide, and 25.4 mm tall aluminum rectangular flexible strips was placed side-by-side with a spacing of 1W (strip width), 2W and 3W. The Reynolds number based on the strip width was around 8500. The results showed that the 1W-spaced strip pair induced the strongest vortex-vortex interaction, the largest downwash velocity, and the most intense turbulence fluctuation. These resulted in the most effective heat convection. At Y=0 (middle of the strip pair) and X=9W, the largest Nu/Nu0 value of around 1.50 was identified when using the 1W-spaced strip pair. This was approximately 0.24 and 0.33 larger than that of the 2W- and 3W-spaced strip pairs. Chapter 6 presents the effect of freestream turbulence on the flat plate heat convection enhancement with a 12.7 mm wide, 25.4 mm tall and 0.1 mm thick flexible strip. A 6 mm thick sharp-edged orificed perforated plate (OPP) with holes of 38.1 mm diameter (D) was placed at 10D, 13D and 16D upstream of the strip to generate the desirable levels of freestream turbulence. The corresponding streamwise freestream turbulence intensity at the strip was around 11%, 9% and 7%. The Reynolds number based on the strip width and freestream velocity was approximately 6000. The freestream turbulence was found to diminish the effect of flexible strip in terms of the relative heat transfer enhancement (Nu/Nu0). This is due to the significant increase of Nu0 with the increasing freestream turbulence. In other words, the flexible strip could always improve the heat transfer, and the relative improvement is greatest for the largely laminar freestream case in the absence of the OPP. Chapter 7 summarizes the effect of all the parameters in previous chapters on the convective heat transfer enhancement. The results show that the freestream turbulence intensity (Tu) had the most significant effect in augmenting the averaged Nu/Nu0, and the local Nu/Nu0 correlated best with the local ke. The maximal averaged Nu/Nu0 over 23W downstream, within ±1 and ±4 strip widths cross-stream was found for Tu=7% case and Tu=11% case, respectively. Conclusions are drawn and recommendations are provided in Chapter 8

    The acoustic characteristics of turbomachinery cavities

    Get PDF
    Internal fluid flows are subject not only to self-sustained oscillations of the purely hydrodynamic type but also to the coupling of the instability with the acoustic mode of the surrounding cavity. This situation is common to turbomachinery, since flow instabilities are confined within a flow path where the acoustic wavelength is typically smaller than the dimensions of the cavity and flow speeds are low enough to allow resonances. When acoustic coupling occurs, the fluctuations can become so severe in amplitude that it may induce structural failure of engine components. The potential for catastrophic failure makes identifying flow-induced noise and vibration sources a priority. In view of the complexity of these types of flows, this report was written with the purpose of presenting many of the methods used to compute frequencies for self-sustained oscillations. The report also presents the engineering formulae needed to calculate the acoustic resonant modes for ducts and cavities. Although the report is not a replacement for more complex numerical or experimental modeling techniques, it is intended to be used on general types of flow configurations that are known to produce self-sustained oscillations. This report provides a complete collection of these models under one cover

    Performance evaluation of louvered fin compact heat exchangers with vortex generators

    Get PDF
    Every day large amounts of heat are transferred in many industrial and domestic processes. This heat transfer takes place in a heat exchanger. Any energy savings in heat transfer processes have a significant impact on the fuel consumption and greenhouse gas emissions. More energy efficient heat exchangers help to meet the 20-20-20 climate and energy targets of the European Union. In many applications air is one of the working fluids (e.g. coolers in compressed air systems, heat pumps, air conditioning devices, domestic heating, etc.). When heat is exchanged with air, the main thermal resistance is located at the air side of the heat exchanger. To increase the heat transfer rate, the heat transfer surface area is enlarged by adding fins to the air side of the heat exchanger. When a high compactness is needed, complex interrupted fin surfaces are used. A typical example is the louvered fin design. The main disadvantage of the louvered fins is the high pressure drop. Delta winglets mounted on a heat transfer surface generate vortices which cause an intense mixing of the flow and thin the thermal boundary layers. In contrast to louvered fins, they enhance the heat transfer with a relatively low penalty in pressure drop. The objective of this doctoral work is to evaluate if the thermal hydraulic performance of a louvered fin heat exchanger with round tubes in a staggered layout can be improved by adding delta winglets to the fins. Such compound designs form the next generation of heat exchangers. Both experiments (flow visualizations in a water tunnel and heat transfer and pressure drop measurements in a wind tunnel) and simulations (Computational Fluid Dynamics - CFD) were performed. The louvers affect the main flow, while the delta winglets reduce the wake regions downstream of the tubes. The generated vortices cause three important mechanisms of heat transfer enhancement: a better mixing, a reduction of the thermal boundary layer thickness and a delay of the flow separation from the tube surface. Further, it was found that the vortices do not extend far downstream as they are destroyed by the deflected flow in the downstream louver bank. The compound heat exchanger has a better thermal hydraulic performance than when only vortex generators or only louvers are used. It is shown that for the same pumping power and heat duty, the compound heat exchanger is smaller in volume. Consequently, less space is required, the material cost is lower and (often also) the operational cost is reduced. The combination of louvered fins and vortex generators is mainly interesting for low Reynolds applications, such as HVAC&R applications or in compressed air systems. A well-considered location and geometry of the vortex generators are essential for an improved performance of the heat exchanger

    Sound absorption mechanisms in perforated plates

    Get PDF

    Sound absorption mechanisms in perforated plates

    Get PDF
    corecore