Normal Triangular Tube Bundle Research Articles

Pressure and shear stress analysis in a normal triangular tube bundle based on experimental flow velocity field

This paper presents the experimental vector velocity field obtained through the spatial filter velocimetry technique in a normal triangular tube bundle test section with tubes of 20 mm O.D. and transverse pitch of 25.2 mm. The experiments were conducted for water flows and Reynolds numbers ranging from 447 to 1842. The pressure field was estimated based on the velocity results by solving the Poisson equation. The shear stress around the tubes was analyzed calculating the viscous and Reynolds stresses. The results for turbulence strength and flow vorticity were calculated and discussed. All the analyses were performed using a second-order finite difference scheme to estimate the partial derivatives. The results indicate that the spatial filter velocimetry technique provides accurate velocity data and is suitable to be applied to complex geometries. In addition, the obtained velocity fields show that the SFV technique is capable of capturing asymmetric flow distributions, which switch from time to time, indicating a suitable spatial and temporal resolution for the experimental conditions evaluated in the present study.

Design guidelines for fluid-elastic instability of tube bundles subjected to two-phase cross flow

Fluid-elastic instability of tube bundles is the main cause of vibration failure of heat exchangers. To establish more reasonable and reliable design guidelines for fluid-elastic instability of tube bundles subjected to two-phase cross flow, we investigated experimentally the effects of the flow conditions of the two-phase flow and the geometrical characteristics of the tube bundles on damping, vibration, and fluid-elastic instability. Moreover, we proposed recommended values of the instability constant based on the conductivity difference measurement (CDM) model and the classification of tube bundle arrangements. The reliability of these values was also verified. The results indicated that the damping ratio in the lift direction was smaller than that in the drag direction and fluid-elastic instability was more prone to occur. The order of stability of the four tube bundle arrangements from high to low was normal triangular, normal square, rotated square, and rotated triangular. Thus, to avoid fluid-elastic instability, the normal triangular tube bundle is recommended for large shell-and-tube heat exchangers subjected to two-phase cross flow. In addition, for normal square and normal triangular tube bundles, the recommended instability constant is 4.0. For rotated square and rotated triangular tube bundles, the recommended instability constant is 1.1 when the mass damping parameter is less than or equal to 0.54, otherwise the value is 1.5.

Experimental Research on Fluid-Elastic Instability in Tube Bundles Subjected to Air-Water Cross Flow

Fluid-elastic instability is the major factor in causing the vibration of tube bundles. Design guidelines on fluid-elastic instability in tube bundles is necessary to avoid damage due to excessive tube vibration. However, the design guidelines on fluid-elastic instability in tube bundles subjected to two-phase cross flow have no consistent conclusions. Accordingly, this technical note researches the vibration characteristics of three tube bundle distributions, namely, normal square tube bundles with pitch-to-diameter ratios of 1.28 and 1.32 and a normal triangular tube bundle with a pitch-to-diameter ratio of 1.32. Comparison of the present fluid-elastic threshold results with previously published data shows good agreement in single-phase flow. The effects of pitch-to-diameter ratio and tube bundle configurations on fluid-elastic instability induced by air-water cross flow were also compared and analyzed by measuring unstable behavior of tube bundles. It was found that fluid-elastic instability is more prone to occur with a decrease of pitch-to-diameter ratio and that the normal square tube bundle is more stable than the normal triangular tube bundle. From the perspective of the tube bundle configurations, it was recommended that the instability constant K in normal triangular and normal square tube bundles be 3.4 and 4.0, respectively.

Validation of turbulence induced vibration design guidelines in a normal triangular tube bundle during two-phase crossflow

The present paper addresses an experimental investigation on flow-induced vibration for air–water flow across a normal triangular tube bundle, with transverse pitch-to-diameter ratio of 1.26, aiming to validate the design guidelines found in the open literature for turbulence-induced vibration. In order to do so, the present experimental approach features a tube mounted in cantilever, instrumented with two accelerometers, perpendicularly aligned, mounted in the free tip. This configuration allows to measure the tube acceleration response under distinct operation conditions. The dynamic parameters necessary to evaluate the design guidelines are extracted from these data. Assuming that turbulence-induced is the dominant vibration mechanism for the range of experimental conditions covered by the present study, the current models used to predict the tube vibration upper bound are reviewed and implemented. The present results are plotted with these envelopes showing good agreement, hence, validating the test bench for future works. Moreover, it is noticed that the loading patterns are strongly dependent on the experimental conditions, specially the local flow pattern. Additionally, a predictive method for tube displacement amplitude is proposed based on the experimental results obtained in the present study.

Two-phase flow patterns across triangular tube bundles for air–water upward flow

This paper presents flow pattern experimental results obtained during two-phase upward flow across a horizontal tube bundle. Experiments were performed for flows across a normal triangular tube bundle with 19 mm OD tubes and transversal pitch of 24 mm. Results were obtained for gas and liquid superficial velocities ranging from 0.13 to 10.00 m/s and 0.02 to 1.50 m/s, respectively. Flow patterns were identified subjectively based on visual observations through side windows, and objectively using the k-means clustering method based on signals of a differential pressure transducer and a capacitive sensor. Bubbles, large bubbles, dispersed bubbles, churn, intermittent and annular flow patterns were identified subjectively. The clustering method satisfactorily identified groups of data corresponding to the distinct flow patterns, which were compared with predictive methods available in the open literature. New predictive methods for transitions between flow patterns are proposed based on the flow patterns identified objectively. The proposed methods predicted accurately the data obtained in the present study as well as experimental results and flow pattern maps available in the open literature for distinct geometries.

Fluidelastic Instability in a Normal Triangular Tube Bundle Subjected to Air-Water Cross-Flow

This paper presents the results of tests on the vibration of a normal triangular tube bundle subjected to air–water cross-flow. The pitch-to-diameter ratio of the bundle is 1.5, and the tube diameter is 38 mm. The tubes were preferentially flexible in one direction. Both the lift and the drag direction were tested. A wide range of void fractions and fluid velocities was tested. Fluidelastic instabilities and tube resonances were observed. The resonances induced significant vibration amplitudes at high void fractions in the lift direction. The results are compared with those obtained with a rotated triangular tube bundle. They show that the normal triangular configuration is more stable than the rotated triangular configuration.

Vibration of a Normal Triangular Tube Bundle Subjected to Two-Phase Freon Cross Flow

This paper presents the results of a test series to study the vibration behavior of a normal triangular tube bundle subjected to two-phase Freon cross flow. A normal triangular tube bundle of pitch over diameter ratio of 1.5 was tested over a broad range of void fractions and mass fluxes. Fluidelastic instabilities, random turbulence excitation, and damping were investigated. The results were compared with those obtained for a similar tube bundle tested in an air-water cross flow and to those for a rotated triangular bundle similarly tested in Freon.