Estimation of the Time Delay Associated With Damping Controlled Fluidelastic Instability in a Normal Triangular Tube Array

2013 ◽  
Vol 135 (3) ◽  
Author(s):  
John Mahon ◽  
Craig Meskell
Keyword(s):  
Time Delay ◽  
Cross Flow ◽  
Fluid Forces ◽  
Fluidelastic Instability ◽  
Order Of Magnitude ◽  
Sample Frequency ◽  
Pitch Ratio ◽  
Parameterized Model ◽  
Semi Empirical ◽  
Insight Into

Fluidelastic instability (FEI) produces large amplitude self-excited vibrations close to the natural frequency of the structure. For fluidelastic instability caused by the damping controlled mechanism, there is a time delay between tube motion and the resulting fluid forces but magnitude and physical cause of this is unclear. This study measures the time delay between tube motion and the resulting fluid forces in a normal triangular tube array with a pitch ratio of 1.32 subject to air cross-flow. The instrumented cylinder was forced to oscillate in the lift direction at three excitation frequencies for a range of flow velocities. Unsteady surface pressures were monitored with a sample frequency of 2 kHz at the mid plane of the instrumented cylinder. The instantaneous fluid forces were obtained by integrating the surface pressure data. A time delay between the tube motion and resulting fluid forces was obtained. The nondimensionalized time delay was of the same order of magnitude assumed in the semi-empirical quasi-steady model (i.e., τ2 = 0.29 d/U). Although, further work is required to provide a parameterized model of the time delay which can be embedded in a model of damping controlled fluidelastic forces, the data already provides some insight into the physical mechanism responsible.

Measurement of the Time Delay Associated With Fluid Damping Controlled Instability in a Normal Triangular Tube Array

2010 ◽  
Author(s):  
John Mahon ◽  
Craig Meskell
Keyword(s):  
Time Delay ◽  
Cross Flow ◽  
Exact Nature ◽  
Fluid Forces ◽  
Fluid Damping ◽  
Order Of Magnitude ◽  
Sample Frequency ◽  
Pitch Ratio ◽  
Parameterized Model ◽  
Semi Empirical

Fluidelastic instability produces large amplitude self-excited vibrations close to the natural frequency of the structure. It is now recognised as the excitation mechanism with the greatest potential for causing damage in tube arrays. It can be split into two mechanisms: fluid stiffness controlled and fluid damping controlled instability. The former is reasonably well understood, although a better understanding for fluid damping controlled instability is required. There is a time delay between tube motion and the resulting fluid forces at the root of fluid damping controlled instability. The exact nature of the time delay is still unclear. The current study directly measures the time delay between tube motion and the resulting fluid forces in a normal triangular tube array with a pitch ratio of 1.32 with air cross-flow. The instrumented cylinder has 36 pressure taps with a diameter of 1 mm, located at the mid-span of the cylinder. The instrumented cylinder was forced to oscillate in the lift direction at four excitation frequencies for a range of flow velocities. Unsteady pressure measurements at a sample frequency of 2kHz were simultaneously acquired along with the tube motion which was monitored using an accelerometer. The instantaneous fluid forces were obtained by integrating the surface pressure data. A time delay between tube motion and resulting fluid forces was obtained. The time delay measured was of the order of magnitude assumed in the semi-empirical models of by Price & Paidoussis (1984, 1986), Weaver and Lever et al. (1982, 1986, 1989, 1993), Granger & Paidoussis (1996), Meskell (2009), i.e. t = μd/U, with μ = O(1). Although, further work is required to provide a parameterized model of the time delay which can be embedded in these models, the data already provides some insight into the physical mechanism responsible.

Equivalent Theodorsen Function for Fluidelastic Excitation in a Normal Triangular Array

2019 ◽  
Author(s):  
Loay Alyaldin ◽  
Njuki Mureithi
Keyword(s):  
Time Delay ◽  
Single Phase ◽  
Cross Flow ◽  
Experimental Tests ◽  
Triangular Array ◽  
Fluid Forces ◽  
Fluidelastic Instability ◽  
Unsteady Fluid ◽  
Two Phases ◽  
Time Delay Effect

Abstract Fluidelastic instability (FEI) remains an important concern to designers of heat exchangers subjected to high flow velocities of gases, liquids or a combination of the two phases. In the present work, experimental tests are conducted to measure the quasi-steady fluid forces acting on a normal triangular tube array of P/D = 1.5 subjected to single-phase cross-flow. The quasi-steady forces together with previously measured unsteady fluid forces are used to estimate the time delay between the central tube motion and fluid forces on itself. The time delay effect for the quasi-steady fluidelastic instability model is derived in the frequency domain in the form of an equivalent Theodorsen function. The results are compared with the Theodorsen function previously obtained for the rotated triangular array. Using the time delay formulation, a stability analysis is carried out to predict the critical velocity for fluidelastic instability in a normal triangular array subjected to single-phase flow.

An Experimental Study of the Phase Lag Causing Fluidelastic Instability in Tube Bundles

2011 ◽  
Author(s):  
Ahmed Khalifa ◽  
David Weaver ◽  
Samir Ziada
Keyword(s):  
Time Delay ◽  
Phase Lag ◽  
Interstitial Flow ◽  
Flow Perturbation ◽  
Pressure Transducers ◽  
Fluid Forces ◽  
Fluidelastic Instability ◽  
Pitch Ratio ◽  
Downstream Flow ◽  
Lift And Drag

The phenomenon of fluidelastic instability forms a major limitation on the performance of tube and shell heat exchangers. It is believed that fluidelastic instability is attributed to two main mechanisms; the first is called the “Damping Mechanism”, while the second is called the “Stiffness Mechanism”. It is established in the literature that in order to model the damping controlled fluidelastic instability, a finite time delay between tube vibration and fluid response has to be introduced. Experimental investigation of the time delay between structural motion and the induced fluid forces is detailed in the present study. A parallel triangular tube array consisting of seven rows and six columns of aluminum tubes is built with a pitch ratio of 1.54. Hot-wire measurements of the interstitial flow perturbations are recorded while monitoring the tube vibrations in the lift and drag directions. Pressure transducers are installed inside the instrumented tubes to monitor the fluid forces. The phase lag between tube vibration and flow perturbation is obtained at different locations in the array. The effect of tube frequency, turbulence level, location of measurements, and mean gap velocity on the relative phase values is investigated. It is found that there are two well-defined regions of phase trends along the flow channel. It is concluded from this study that the time delay between tube vibration and downstream flow perturbation is associated with the vorticity convection downstream, while the time delay for upstream perturbations is associated with the effect of flow separation and vorticity generation which is propagated upstream from the vibrating tube.

A Method for Extracting the Time Delay From the Unsteady and Quasi-Steady Fluidelastic Forces in Two-Phase Flow

2013 ◽  
Author(s):  
Teguewinde Sawadogo ◽  
Njuki Mureithi
Keyword(s):  
Time Delay ◽  
Two Phase Flow ◽  
Phase Flow ◽  
Two Phase ◽  
Fluid Forces ◽  
Void Fractions ◽  
Fluidelastic Instability ◽  
Measured Time ◽  
Unsteady Fluid ◽  
Theoretical Results

The time delay is a key parameter for modeling fluidelastic instability, especially the damping controlled mechanism. It can be determined experimentally by measuring directly the time lag between the tube motion and the induced fluid forces. The fluid forces may be obtained by integrating the pressure field around the moving tube. However, this method faces certain difficulties in two-phase flow since the high turbulence and the non-uniformity of the flow may increase the randomness of the measured force. To overcome this difficulty, an innovative method for extracting the time delay inherent to the quasi-steady model for fluidelastic instability is proposed in this study. Firstly, experimental measurements of unsteady and quasi-static fluid forces (in the lift direction) acting on a tube subject to two-phase flow were conducted. The unsteady fluid forces were measured by exciting the tube using a linear motor. These forces were measured for a wide range of void fraction, flow velocities and excitation frequencies. The experimental results showed that the unsteady fluid forces could be represented as single valued function of the reduced velocity (flow velocity reduced by the excitation frequency and the tube diameter). The time delay was determined by equating the unsteady fluid forces with the quasi-static forces. The results given by this innovative method of measuring the time delay in two-phase flow were consistent with theoretical expectations. The time delay could be expressed as a linear function of the convection time and the time delay parameter was determined for void fractions ranging from 60% to 90%. Fluidelastic instability calculations were also performed using the quasi-steady model with the newly measured time delay parameter. Previously conducted stability tests provided the experimental data necessary to validate the theoretical results of the quasi-steady model. The validity of the quasi-steady model for two-phase flow was confirmed by the good agreement between its results and the experimental data. The newly measured time delay parameter has improved significantly the theoretical results, especially for high void fractions (90%). However, the model could not be verified for void fractions lower or equal to 50% due to the limitation of the current experimental setup. Further studies are consequently required to clarify this point. Nevertheless, this model can be used to simulate the flow induced vibrations in steam generators’ tube bundles as their most critical parts operate at high void fractions (≥ 60%).

A Quasi-Steady In-Plane Fluidelastic Instability Analysis of a Rotated Triangular Tube Array in Two-Phase Cross Flow

2018 ◽  
Author(s):  
Stephen Olala ◽  
Njuki Mureithi
Keyword(s):  
Time Delay ◽  
Degrees Of Freedom ◽  
Dynamic Test ◽  
Cross Flow ◽  
Two Phase ◽  
Unsteady Forces ◽  
Fluidelastic Instability ◽  
Fluid Coupling ◽  
Flexible Tubes ◽  
Experimental Effort

In-plane fluidelastic instability is a dynamic phenomenon requiring fluid coupling of at least two degrees-of-freedom, in this case, at least two flexible tubes. Due to the nature of the mechanism causing streamwise fluidelastic instability, a purely experimental or an unsteady determination would require intensive experimental effort. As a compromise between experimental effort and prediction accuracy, the quasi-steady model is used in the current study. In the present work, previously measured quasi-steady and unsteady forces are used to estimate the time delay first between the displacement of an oscillating tube and the forces generated on itself then the time delay between the displacement of a central oscillating tube and the forces induced on the adjacent tubes. The estimated time delays are then used together with drag coefficients and derivatives to predict the in-plane fluidelastic instability in a rotated triangular tube array of P/D = 1.5 subjected to two-phase flow. The results closely replicate dynamic test results and confirm the predominance of the stiffness controlled mechanism and the potential of the quasi-steady model in accurately predicting streamwise fluidelastic instability in arrays subjected to two-phase flows.

scholarly journals A Theory for Fluidelastic Instability of Tube-Support-Plate-Inactive Modes

1992 ◽  
Vol 114 (2) ◽  
pp. 139-148 ◽  
Author(s):  
Y. Cai ◽  
S. S. Chen ◽  
S. Chandra
Keyword(s):  
Heat Exchangers ◽  
Linear Models ◽  
Cross Flow ◽  
Bilinear Model ◽  
Steam Generators ◽  
Unstable Region ◽  
Fluid Forces ◽  
Support Plate ◽  
Fluidelastic Instability ◽  
Inactive Mode

Fluidelastic instability of loosely supported tubes, vibrating in a tube support plate (TSP)-inactive mode, is suspected to be one of the main causes of tube failure in some operating steam generators and heat exchangers. This paper presents a mathematical model for fluidelastic instability of loosely supported tubes exposed to nonuniform cross flow. The model incorporates all motion-dependent fluid forces based on the unsteady-flow theory. In the unstable region associated with a TSP-inactive mode, tube motion can be described by two linear models: TSP-inactive mode when tubes do not strike the TSP, and TSP-active mode when tubes do strike the TSP. The bilinear model (consisting of these linear models) presented here simulates the characteristics of fluidelastic instability of loosely supported tubes in stable and unstable regions associated with TSP-inactive modes. Analytical results obtained with the model are compared with published experimental data; they agree reasonably well. The prediction procedure presented for the fluidelastic instability response of loosely supported tubes is applicable to the stable and unstable regions of the TSP-inactive mode.

Towards a Validation Database for Simulation of Fluidelastic Instability in Normal Triangular Heat Exchanger Tube Arrays

2008 ◽  
Author(s):  
John Mahon ◽  
Craig Meskell
Keyword(s):  
Surface Pressure ◽  
Cross Flow ◽  
Initial Step ◽  
Pressure Measurements ◽  
Dynamic Head ◽  
Flow Surface ◽  
Tube Arrays ◽  
Fluidelastic Instability ◽  
Large Asymmetry ◽  
Pitch Ratio

Models for fluidelastic instability are usually validated by comparing critical velocity predictions with experimental data. However, the scatter in this data make detailed validation problematic. As an initial step towards providing a validation database for fluidelastic instability, surface pressure measurements are presented for a cylinder in the third row of three normal triangular tube arrays (P/d = 1.32; 1.58; 1.97) with air cross flow. Surface pressure measurements were also made when the cylinder was statically displaced. Forces were calculated from the pressure measurements enabling an understanding of the force generation mechanism. The results show that the fluid force coefficients do not scale with the dynamic head but exhibit a dependency on Reynolds number and pitch ratio. However, no simple parametrisation was found for the lift force. Jet switching was found in P/d = 1.58 even when the tube was displaced. This phenomenon resulted in the large asymmetry observed in the pressure distribution around a static cylinder.

The Effects of Tube Array Geometry on Fluidelastic Instability in Heat Exchanger Tube Arrays in Cross Flow

2017 ◽  
Author(s):  
Marwan Hassan ◽  
David S. Weaver
Keyword(s):  
Experimental Data ◽  
Research Effort ◽  
Cross Flow ◽  
Triangular Arrays ◽  
Small Pitch ◽  
Fluidelastic Instability ◽  
Significant Research ◽  
Current Design ◽  
Pitch Ratio ◽  
Array Geometry

The shut-down of the San Onofre Nuclear Generating Station (SONGS) has been attributed to damaging streamwise Fluidelastic Instability (FEI) of the steam generator tubes, a phenomenon which has traditionally been assumed not to occur. This has generated a significant research effort to better understand this phenomenon and to develop appropriate design criteria for its prevention. Most current design codes are based on Connors criterion for FEI which neglects both streamwise FEI and the effects of tube array pattern and pitch ratio. It is becoming clear that array geometry and pitch ratio are important determining factors in FEI, especially in the streamwise direction. This paper presents an extension of the theory of Lever and Weaver to consider arrays of flexible fluid-coupled tubes which are free to become unstable in both the transverse and streamwise directions. This simplified modelling approach has the advantages of being very tractable for numerical parametric studies and having no need for experimental data input. Previous research by the authors has shown that the predictions of this model agree very well with the available experiments for parallel triangular arrays for both transverse and streamwise FEI. In this paper, the results of such studies are presented for the both transverse and streamwise FEI for square inline and normal triangular arrays and compared with the authors’ previous results for parallel triangular arrays. It is shown that FEI is strongly influenced by array geometry, especially for small pitch ratio arrays operating at low values of the mass damping parameter. The results show good agreement with the available experimental data.

Sign in / Sign up

Export Citation Format

Share Document