Role of Three-Dimensional Swirl in Forced Convection Heat Transfer Enhancement in Wavy-Plate-Fin Channels

The influence of swirl flow on enhanced forced convection in wavy-plate-fin cores has been investigated. Three-dimensional computational simulations were carried out for steady-state, periodically developed flow of air (Pr ~ 0.71) with channel walls subject to constant-uniform temperature and flow rates in the range 50 = Re = 4000. The recirculation that develops in the wall troughs and grows to an axial helix is scaled by the Swirl number Sw. As Sw increases, tornado-shaped vortices appear in the wave trough region mid-channel height, then extend longitudinally to encompass majority of the flow channel. As shown by the local wall-shear and heat transfer coefficient variations, the boundary-layer thinning upstream of the wave peak assists to intensify the momentum and heat transfer. However, the flow recirculation in wave trough impedes the local heat transfer at low Sw due to flow stagnation but promotes it at high Sw because of swirl-augmented fluid mixing. Swirling flows also create pressure drag that contributes substantively to the overall pressure loss. Its proportion grows as Sw, corrugation severity, and fin spacing increases to as much as 80% of the total pressure drop. The fin-wall curvature-induced secondary circulation nevertheless produces significantly enhanced convection, and more so in flows with higher Sw. It is quantified by Ff (or j), which is seen to increase log-linearly as fin corrugation aspect ratio and/or fin spacing ratio increases; the influence of cross-section aspect ratio is found to be marginal.

Numerical Study of Bamboo Breakwater for Wave Reduction

Flood inundation and shoreline erosion have long occurred in Sayung, Demak area, the northern coast of Central Java Province, Indonesia. The people of Sayung planted mangroves to reduce the flood inundation and shoreline erosion in that area. They built the bamboo array to protect the juvenile mangroves from incoming waves. The bamboo acts as a breakwater and is considered an environmentally friendly permeable structure to reduce wave energy and stimulate sedimentation. This paper discusses three bamboo arrays’ effectiveness in wave reduction using Numerical Wave Tank (NWT). The interaction of regular waves with a permeable structure comprising a single row of vertical circular poles was conducted based on the Smoothed Particle Hydrodynamics (SPH) method. The effect of different waves and structural dimensions on the permeable structure was investigated based on the structure’s transmission coefficient (Kt) performance. The investigations have revealed that structures with the combination of Vertical-Horizontal formation (VH) attenuate more wave energy than Vertical Only (VO) and the combination of Vertical-Diagonal formation (VD). As the wave steepness increases, the transmission coefficient decreases. Likewise, the transmission coefficient (Kt) is decreasing when the wave height is increasing. On the other hand, the transmission coefficient (Kt) increases as the wave period increases. As the structure spacing ratio between end-to-end and center-to-center spacing (e/S) rises, the transmission coefficient (Kt) also increases. The diameter (D) has a slight effect on the transmission coefficient (Kt). However, the center-to-center spacing (S) has a more significant impact than the diameter on the transmission coefficient, affecting an inclination on the transmission coefficient (Kt) when center-to-center spacing (S) goes up.

Flow-Induced Vibrations of Single and Multiple Heated Circular Cylinders: A Review

This study is an effort to encapsulate the fundamentals and major findings in the area of fluid-solid interaction, particularly the flow-induced vibrations (FIV). Periodic flow separation and vortex shedding stretching downstream induce dynamic fluid forces on the bluff body and results in oscillatory motion of the body. The motion is generally referred to as flow-induced vibrations. FIV is a dynamic phenomenon as the motion, or the vibration of the body is subjected to the continuously changing fluid forces. Sometimes FIV is modeled as forced vibrations to mimic the vibration response due to the fluid forces. FIV is a deep concern of engineers for the design of modern heat exchangers, particularly the shell-and-tube type, as it is the major cause for the tube failures. Effect of important parameters such as Reynolds number, spacing ratio, damping coefficient, mass ratio and reduced velocity on the vibration characteristics (such as Strouhal number, vortex shedding, vibration frequency and amplitude, etc.) is summarized. Flow over a bluff body with wakes developed has been studied widely in the past decades. Several review articles are available in the literature on the area of vortex shedding and FIV. None of them, however, discusses the cases of FIV with heat transfer. In particular systems, FIV is often coupled to heat transfer, e.g., in nuclear power plants, FIV causes wear and tear to heat exchangers, which can eventually lead to catastrophic failure. As the circular shape is the most common shape for tubes and pipes encountered in practice, this review will only focus on the FIV of circular cylinders. In this attempt, FIV of single and multiple cylinders in staggered arrangement, including tandem and side-by-side arrangement is summarized for heated and unheated cylinder(s) in the one- and two-degree of freedom. The review also synthesizes the effect of fouling on heat transfer and flow characteristics. Finally, research prospects for heated circular cylinders are also stated.

Flow-Induced Vibrations of a Rotated Square Tube Array Subjected to Single-Phase Cross-Flow

This paper investigates the flow-induced vibration (FIV) and possibility of fluidelastic instability occurrence in a rotated square geometry tube array through a series of experimental tests. All experiments presented here were conducted in water cross-flow. The array pitch spacing ratio of approximately P/D=1.64 is somewhat larger than that commonly found in typical steam generators. The stability of a single flexible tube as well as multiple flexible tubes were investigated. The tubes were free to vibrate purely in the streamwise direction or the transverse direction relative to the upstream flow. A single flexible tube, in the otherwise rigid tube array, was found to undergo large amplitude vibrations (up to 40 % D) in the transverse direction. Tube vibration frequency analysis indicated the presence of two frequency components related to vorticity shedding in the array. This potential vorticity-induced-vibrations (VIV) and potential coupling between VIV and FEI are discussed in the paper. Test results for streamwise flow-induced vibrations are also presented. Results in water flow show a possible effect related to flow periodicity at low velocity. At significantly high flow velocities, the tubes are found to fully restabilize. This restabilization after VIV locking has not been previously reported as an unlocking result. The present results suggest that the flow-induced vibration of tubes in a rotated square array configuration is significantly more complex than in other geometries, particularly for the streamwise vibration case.

Numerical Computations for Flow Patterns and Force Statistics of Three Rectangular Cylinders

In this work, numerical simulations are performed in order to study the effects of aspect ratio (AR) and Reynolds number (Re) on flow characteristics of three side-by-side rectangular cylinders for fixed spacing ratio ( g ), using the lattice Boltzmann method (LBM). The Reynolds number varies within the range 60 ≤ Re ≤ 180, aspect ratio is between 0.25 and 4, and spacing ratio is fixed at g = 1.5. The flow structure mechanism behind the cylinders is analyzed in terms of vorticity contour visualization, time-trace analysis of drag and lift coefficients, power spectrum analysis of lift coefficient and variations of mean drag coefficient, and Strouhal number. For different combinations of AR and Re, the flow is characterized into regular, irregular, and symmetric vortex shedding. In regular and symmetric vortex shedding the drag and lift coefficients vary smoothly while reverse trend occurs in irregular vortex shedding. At small AR, each cylinder experiences higher magnitude drag force as compared to intermediate and large aspect ratios. The vortex shedding frequency was found to be smaller at smaller AR and increased with increment in AR.

Morphologic signatures of autogenic waterfalls: A case study in the San Gabriel Mountains, California

Waterfalls can form due to external perturbation of river base level, lithologic heterogeneity, and internal feedbacks (i.e., autogenic dynamics). While waterfalls formed by lithologic heterogeneity and external perturbation are well documented, there is a lack of criteria with which to identify autogenic waterfalls, thereby limiting the ability to assess the influence of autogenic waterfalls on landscape evolution. We propose that autogenic waterfalls evolve from bedrock bedforms known as cyclic steps and therefore form as a series of steps with spacing and height set primarily by channel slope. We identified 360 waterfalls split between a transient and steady-state portion of the San Gabriel Mountains in California, USA. Our results show that while waterfalls have different spatial distributions in the transient and steady-state landscapes, waterfalls in both landscapes tend to form at slopes >3%, coinciding with the onset of Froude supercritical flow, and the waterfall height to spacing ratio in both landscapes increases with slope, consistent with cyclic step theory and flume experiments. We suggest that in unglaciated mountain ranges with relatively uniform rock strength, individual waterfalls are predominately autogenic in origin, while the spatial distribution of waterfalls may be set by external perturbations.

Rib Spacing effect on Heat Transfer and pressure loss in a Rectangular channel with semicircular ribs

Ribs of various shapes are used for heat transfer enhancement but its performance is significantly depends on geometrical features and flow conditions. This study experimentally find out the influence of rib spacing, semicircular shape ribs with three rib spacing ratio (P/e) = 8, 10 and 12 are studied and located on lower wall of the rectangular channel. Reynolds numbers varied from 10000 to 29,000 and the blockage ratio of the channel (e/Dh) was 0.151.Result show that semicircular rib performed better than plain plate but found more friction. semicircular rib with rib spacing of 50 mm (P/e =10) shows highest thermal performance, enhanced avg. 39 % heat transfer than rib spacing of 40 and 60 mm (P/e=8 & 12). Friction losses observed highest in rib spacing ratio of 8,found average 10 % more friction compared to rib spacing ratio of 10 & 12. Semicircular rib with spacing ratio 8 shows least thermal performances compared to other configurations.

Holes' Parameters Analysis of a Perforated Thin-Walled Lipped Beam Buckled Under a Bending Load

This work studied the effects of holes on the buckling characteristic of an open thin-walled lipped channel beam under a bending load. A nonlinear finite element method was utilised to examine the buckling behaviour of the beam. Experimental works were carried out to verify the finite element simulation. Three factors were chosen to examine their influence on the buckling of the beam. These factors namely, the holes’ shape, perforated ratio (hole length to beam height) and spacing ratio (centre to centre distance between holes to beam height). The finite elements output was analysed by implementing the Taguchi method to distinguish the best group of three parameters collections for optimal strength of buckling. Whereas the analysis of variance technique (ANOVA) method was applied to specify the impact of each parameter on critical buckling load. Outcomes showed that the combination of parameters that gives the best buckling strength is the hole with a hexagonal shape, perforated ratio =1.7 and spacing ratio =1.3, and the holes’ shape is the most effective factor. In addition, the study demonstrated that the hole's shape factor has the greatest influence on the buckling capacity. While the perforated ratio factor is the least influential.

Modeling Boundary-Dominated Flow in Hydraulically-Fractured Wells

This paper presents a simple method to model boundary-dominated flow in hydraulically fractured wells, including horizontal wells with multiple fractures. While these wells are almost always producedat more nearly constant BHP rather than constant rate, use of material-balance time transforms variable-rate production profiles to constant-rate profiles, allowing us to use the pseudo-steady-state (PSS) flow equation for modeling. However, the PSS equation requires use of shape factors in applications, and shape factors available in the literature are available only for square-shaped bounded reservoirs with hydraulic fractures. In this work, we derived shape factors for wells centered in rectangular-shaped drainage areas with different length-to-width aspect ratios. The superposition principle can be used to transform transient radial flow and transient linear flow solutions into bounded reservoir solutions. At large times (when boundary-dominated flow is established), results from these solutions are similar to those obtained from the PSS equation. Therefore, for a pre-defined reservoir geometry, pressure drop values from superimposed transient flow equationscan be substituted back into the PSS equation to calculate shape factors for that reservoir geometry.We used shape factors previously presented by other authors for square drainage areas to validate themethod before applying it to calculate shape factors for more general drainage area configurations. We present shape factors for different fracture half-length to fracture-spacing ratios ranging from 0.2 to 10. Calculated shape factors, when plotted against the fracture half-length to fracture-spacing ratio, produced a smooth curve which can be used to interpolate shape factor values for other fracture configurations. We present applications of this methodology to example low-permeability wells. The use of the PSS equation for wells with vertical fracturescan be extended to multi-fractured horizontal wells (MFHWs) by incorporating the number of fractures in the equation; hence, shape factorsderived for wells with vertical fractures can also be used for MFHWs. Although our results are rigorously correct only for fluids with constant compressibility, use of pseudo-pressure and pseudo-time transformations extend application to compressible fluids, notably gases. Using the PSS equation in production data analysis allows us to calculate contributing reservoir volume and drainage area in a simple manner not requiring use of specialized software.