In-line Tube Array Research Articles

Heat Transfer Characteristics of Naturally Driven Air Flow in a Horizontal In-Line Tube Array

Heat Transfer Characteristics of Naturally Driven Air Flow in a Horizontal In-Line Tube Array

Effects of horizontal tube arrays on heat transfer in an external heat exchanger

Few studies have addressed the impact of horizontal tube arrays on the local bed-to-wall heat transfer coefficients. Therefore, in this paper, the local heat transfer coefficients were measured in an external heat exchanger with inline and staggered horizontal tube arrays. The corresponding gas–solid hydrodynamics on the tube surface was obtained by using the optical fiber probe. The lateral distribution of the heat transfer coefficients was found to be non-uniform, and the maximum values were achieved at the position of 0.64 times half-width. The impact of the tube arrays on the heat transfer coefficients varied with the lateral locations and the configuration of tube arrays. Compared with cases without tube arrays, the inline tube array reduced the heat transfer coefficients in the bed center but increased those near the column wall. Compared with the inline tube array, the staggered array increased the deviations of heat transfer coefficients both in the bed center and in the wall region. The modified packet renewal model could reflect the trend in the variation of the heat transfer coefficients along the bed width but slightly overestimated the values. The overall average heat transfer coefficients were also compared to the results predicted using common published correlations.

Aeroacoustic Source Distribution in an Inline Tube Array With a Pitch Ratio of 3

The flow induced acoustics in an inline tube bank (P/d = 3) subject to cross flow, indicative of a generic heat exchanger geometry, are examined over a range of flow velocities using particle image velocimetry (PIV) coupled with acoustic modal analysis using finite element analysis (FEA). The objective is twofold: to determine if the method originally developed for tandem cylinders is applicable to more geometrically complex configurations, with more restricted optical access; and hence to investigate the spatial distribution of acoustic sources within the tube array. The spatial and temporal aeroacoustic source distribution has been successfully obtained experimentally for the case of Strouhal acoustic coincidence (i.e., fa = fv). It is found that the acoustic sources are most intense behind the first row due to the spatial compactness of the vortices. However, a strong negative source (i.e., a sink) is also present in this location, so that the net contribution of the first row wake is small. In subsequent rows, the sources are weaker and more dispersed, but the sink is reduced dramatically. The result is that after the first row the remaining rows of the array contributes energy to the acoustic field. It is noted that, for the coincidence case in the tube bundle studied here, the spatial distribution of sources in the region around the first and second row is similar to the precoincidence regime found for tandem cylinders. This apparent contradiction requires further investigation. Nonetheless, it is concluded that the method of combining PIV with FEA to determine the source distribution can be applied to more complex geometries than previously reported.

Modelling of fluidelastic instability in a square inline tube array including the boundary layer effect

Flow-induced vibration (FIV) is a design concern in many engineering applications such as tube bundles in heat exchangers. When FIV materializes, it often results in fatigue and/or fretting wear of the tubes, leading to their failure. Three cross-flow excitation mechanisms are responsible for such failures: random turbulence excitation, Strouhal periodicity, and fluidelastic instability. Of these three mechanisms, fluidelastic instability has the greatest potential for destruction. Because of this, a large amount of research has been conducted to understand and predict this mechanism. This paper presents a time domain model to predict the fluidelastic instability forces in a tube array. The proposed model accounts for temporal variations in the flow separation. The unsteady boundary layer is solved numerically and coupled with the structure model and the far field flow model. It is found that including the boundary layer effect results in a lower stability threshold. This is primarily due to a larger fluidelastic force effect on the tube. The increase in the fluidelastic effect is attributed to the phase difference between the boundary layer separation point motion and the tube motion. It is also observed that a non-linear limit cycle is predicted by the proposed model.

249 管群拘束による流力弾性振動防止の研究 : ピッチ比1.33と2.0の比較

249 管群拘束による流力弾性振動防止の研究 : ピッチ比1.33と2.0の比較

Effect of tube spacing on the vortex shedding characteristics of laminar flow past an inline tube array: A numerical study

The effect of tube spacing on the vortex shedding characteristics and fluctuating forces in an inline cylinder array is studied numerically. The examined Reynolds number is 100 and the flow is laminar. The numerical methodology and the code employed to solve the Navier–Stokes and continuity equations in an unstructured finite volume grid are validated for the case of flow past two tandem cylinders at four spacings. Computations are then performed for a six-row inline tube bank for eight pitch-to-diameter ratios, s, ranging from 2.1 to 4. At the smallest spacing examined ( s = 2.1) there are five stagnant and symmetric recirculation zones and weak vortex shedding activity occurs only behind the last cylinder. As s increases, the symmetry of the recirculation zones breaks leading to vortex shedding and this process progressively moves upstream, so that for s = 4 there is clear shedding from every row. For any given spacing, the shedding frequency behind each cylinder is the same. A critical spacing range between 3.0 and 3.6 is identified at which the mean drag as well as the rms lift and drag coefficients for the last three cylinders attain maximum values. Further increase to s = 4 leads to significant decrease in the force statistics and increase in the Strouhal number. It was found that at the critical spacing there is 180° phase difference in the shedding cycle between successive cylinders and the vortices travel a distance twice the tube spacing within one period of shedding.

Experimental study on convective coefficients in a slurry bubble column

The main objective of this work was to determine the influence of gas and slurry velocities and solid load concentration on experimental convective heat transfer coefficients (h), for a three-phase suspension in cross flow around a square in-line tube array. Experiments were performed in a pilot scale equipment conformed by a column of square cross section. In the central part of this equipment there is a bank of 49 tubes with a S T =S L =4.4 cm. The system used was air, tap water and sieved sand at average particle diameter of 505 μm. The velocities of liquid and gas phases were varied from 0.28 to 0.60 cm/s and from 1.36 to 2.45 cm/s, respectively; and the solid load concentration from 0 to 30% w/w. The results allow to conclude that h for a three-phase system is larger than either for a two-phase or one-phase system. The presence of solid phase enhances the heat transfer, however, this increase is less evident for high values of solid Concentration.

A Study on Aerodynamic Self-Sustained Tone Generated from a Multitube Once-Through Boiler. Investigation of Generation Mechanism and Oscillating Frequency.

A pure tone noise generated from a multitube once-through boiler (test product) at non-combustion testing was investigated in order to clarify the generation mechanism and the oscillating frequency. The acoustic analysis of a boiler, the water tank test and the two-dimensional CFD analysis for an in-line tube array model were conducted. As a result, it was found that the vortex in wake of the tube shed periodically with the same frequency as the acoustic natural frequency of the boiler, namely the aerodynamic self-sustained tone was generated. Moreover, a wind tunnel test of the in-line tube array model with the various diameters and intervals of tubes was conducted. As a result, it was found that the values of Strouhal number St based on the length of separation shear layer were about 0.3∼0.6, and the phenomena of periodic vortex shedding on a boiler were similar to those on a bellows flow and a cavity flow. This paper presents some useful experimental data for low noise design of a boiler.

The Effect of Platen Fins on the Flow-Induced Vibrations of an In-Line Tube Array

This paper presents the results of an experimental study of the effect of streamwise “platen” fins on the fluidelastic instability of an in-line tube array. The fins are fixed parallel to the axis of the tubes in the streamwise direction and effectively prevent any transverse flow between streamwise tube rows. A geometrically identical, dynamically scaled bundle of finless tubes were used as a datum case for comparison. The results showed that, while the fins eliminate vortex shedding, they enhance the fluidelastic coupling between adjacent tubes and substantially reduce the stability threshold.

Confirmation Test on Acoustic Vibration of Multistage Tubular Heat Exchanger.

We experienced unusual acoustic vibration in the operation of in-line array tubular heat exchangers named gas heater. This vibration mode was not one dimensional in the direction perpendicular to gas flow and could not be vanished by inserting baffle plates into the tube bundle along the gas flow. So we had to confirm acoustic vibration characteristics using 3-dimensional air flow scale model. Here, we report this test results about multi-stage gas heater and some countermeasure methods. Especially we discovered that unusual acoustic vibration mentioned above did not appeared in the case of some staggerd tube array having same heat transfer characteristics as in-line tube array.

An Experimental Study of the Effect of Tube-to-Tube Ties on Fluidelastic Instability in an In-Line Tube Array

An experimental study was conducted on the effect of tube-to-tube ties on the fluidelastic instability of an in-line tube array with a streamwise pitch ratio of 1.33 and a transverse pitch ratio of 2.0. Various arrangements of tubes clamped in the streamwise direction were examined. The results showed that clipping two tubes together had a stabilizing effect, but also exhibited substantial hysteresis. On the other hand, longitudinal clipping of three, four, or six tubes together increased the fluidelastic stability threshold by a factor of more than three. [S0094-9930(00)01501-8]

OPTIMUM DIMENSIONS OF THE CONTINUOUS PLATE FIN FOR VARIOUS TUBE ARRAYS

The finite-difference and sector methods are applied to determine the efficiency of the continuous plate fin pierced by a regularly spaced array of circular tubes in staggered or in-line tube arrays. The comparison of the fin efficiency between the present numerical results, approximate solutions obtained from the sector method, and previous results is made. It is found that the sector method underpredicts the efficiency of the hexagonal and rectangular fins. However, the previous method can overpredict the fin efficiency for lower ro/ a values. Results also show that the plate fin with the staggered tube array is more efficient than that with the in-line tube array. A simple expression relating the fin efficiency to the tube radius, tube spacing, fin thickness, and material properties is obtained by using the least-squares fitting of numerical data. The main purpose of the present study is to determine the optimum dimensions of the continuous plate fin with various tube arrays that can give the greatest amount of heat dissipation under the conditions of the given physical quantities.

Effect of Tube-to-Tube Ties on Fluidelastic Instability of Tube Arrays Exposed to Crossflow

Tube-to-tube ties and their arrangement within the tube array are shown to affect the onset of fluidelastic instability. The influence of tie arrangement for a single tube row and for an in-line tube array is obtained by numerical simulation using S. S. Chen’s unsteady flow theory. Maps of dimensionless critical velocities for groups of tubes consisting of two, three, four, and five tubes tied to each other are developed for several design configurations. It is shown that the stability limits can be raised by appropriate choice of tube group and tie location.

Characteristics and Formulation of Fundamental Strouhal Numbers of In-Line Tube Arrays.

A method is proposed to practically formulate Strouhal numbers as a function of non-dimensional tube pitches combining Rosenhead's theory of Karman vortex street in finite breadth channels and the well-known characteristics of the actual free Karman vortex street. Two examples of its actual application are shown. The first is the formulation of fundamental Strouhal numbers which are considered to be those of Karman streets controlling the drag in the steady flow of tube bundles. As the numerical values of the formula were set tentatively using limited data from the literature allowing some uncertainties, the values and range of validity should be further verified reflecting future experiments. The second is the formulation of the well-known Chen's chart of Strouhal numbers measured by the resonance method.

Mean and R.M.S. surface pressure distributions on two in-line tube arrays in cross flow

Mean and R.M.S. surface pressure distributions on two in-line tube arrays in cross flow

Motion-Dependent Excitation Mechanisms in a Square In-Line Tube Bundle Subject to Water Cross-Flow: An Experimental Modal Analysis

A series of experiments has been conducted on a square in-line tube array subjected to water cross-flow. Three configurations have been tested. In the first one, a single flexible tube was positioned in the middle of an array of rigid cylinders; in the second one, a single flexible tube was positioned in the first row of an array of rigid cylinders; in the third one, the first three rows of the array were flexible, the other cylinders being rigid. Tube response measurements were analysed by the IMENE and MEIDEE methods. IMENE is a multi-degree-of-freedom parametric method for modal identification. It is specially suited to flow-induced vibration analysis. It provides modal parameter variations with flow velocity (i.e., both resonance frequency and damping ratio evolutions). Using these results, MEIDEE achieves an indirect estimation of motion-dependent fluid forces. The results of these analyses are compared to previously published results and to theoretical modal parameter variations with flow velocity, obtained with a slightly extended version of the Lever-Weaver analytical model.

The influence of tube layout on flow and mass transfer characteristics in tube banks in the transitional flow regime

Flow and mass transfer characteristics in tube banks are investigated in the transitional flow regime at intermediate Reynolds numbers. The tube arrays considered are a staggered tube array and an in-line tube array with longitudinal and transverse pitch-to-diameter ratios of two. The flow is steady at the entrance of the tube banks, but becomes oscillatory downstream of an onset location of vortex shedding. This location moves upstream with increasing Reynolds number, and the upstream development of flow transition is much faster for the staggered array than the in-line array. Therefore, the row-by-row variation of the mass transfer rate is small for the staggered array but considerable for the in-line array. The mass transfer for the staggered array is sensitive to the unsteady wakes of the preceding row tubes, while the surface shear stress is insensitive to such wakes. However, for the in-line array the mass transfer and shear stress are both sensitive to the unsteady wakes.

Heat-Transfer Mechanisms in an In-Line Tube Bundle Subject to a Particulate Cross Flow

The effect of solid particles in suspension on heat transfer for tubes located within a square tube bundle of pitch-diameter ratio 1.75 has been investigated. Tests conducted at a Reynolds number of 6000 with mean particle diameters of 58 and 127 μm at a mass loading ratio of 0.5 kg particles/kg air indicate that heat transfer is enhanced at all locations by the presence of the particles. However, at a Reynolds number of 12 000 there is a net decrease in the mean Nusselt number at all positions, with the exception of the first row. Assessment of the main mechanisms by which particles modify heat transfer in in-line tube arrays suggests that the enhancement of heat transfer is a consequence of the increased thermal capacity of the suspension, whereas the reduction in Nusselt number is considered to result from a change in the flow structure and turbulence within the array.

Vorticity shedding and acoustic resonance in an in-line tube bundle part II: Acoustic resonance

The excitation mechanism of acoustic resonances in an in-line tube array with intermediate tube spacings is investigated experimentally. The array is tested in air and water flows. The acoustical mode of the wind tunnel is simulated in the water channel by a standing free-surface wave in the transverse direction. Resonances are found to be excited due to the coupling between the resonant sound field and the unstable shear layers which separate from the tubes. This resonance phenomenon is quite distinct from, and therefore not related to, the jet instability which generates the vorticity-shedding excitation in the absence of resonance. The flow structures under resonant and non-resonant conditions are found to be entirely different. Since the jet instability dominates and the shear layer instability is suppressed at non-resonant conditions, the occurrence of resonance necessitates the switching of the flow instability from the jet mode to the shear-layer mode. The transition mechanism as well as the conditions under which this mode switching can be achieved are described in this paper.

Vorticity shedding and acoustic resonance in an in-line tube bundle part I: Vorticity shedding

An in-line tube array with intermediate tube spacings was tested in a wind tunnel and a water channel. Extensive flow visualization and correlation measurements were carried out to reveal the nature of the vorticity-shedding excitation. The instability of the jet issuing between the tubes is found to be the source of the vorticity-shedding excitation. The jet instability occurs at its symmetric mode, whereupon large-scale vortices are formed symmetrically on both sides of each flow lane. Since the jet instability in each flow lane is 180° out of phase with that in the neighbouring lanes, vortices are formed antisymmetrically in the tube wakes.