Complex Secondary Flow Research Articles

Two-Phase Analysis of A Helical Microchannel Heat Sink Using Nanofluids

A three-dimensional helical microchannel heat sink (HMCHS) model is developed to investigate the heat transfer characteristics using Al2O3–water-based nanofluid. The two-phase mixture model with modified effective thermal conductivity and viscosity equations is employed for solving the problem numerically. The model developed is validated by comparing the results of Nusselt number with available experimental and numerical data for a wide range of Reynolds number. The detailed results of the thermal field are reported for the effects of helix radius (0.15–0.30 mm), pitch (0.5–2.0 mm), number of turns (7–10), and aspect ratio (1.5–3.0). The analysis presents a unique fundamental insight into the complex secondary flow pattern in the channel due to curvature effects.

Hydrodynamics in mangrove prop roots and their physical properties

The circulation of water in riverine mangrove swamps is expected to be influenced by mangrove roots, which in turn affect the transport of nutrients, pollutants and sediments in these systems. Geometric properties of mangrove (Rhizophora sp.) prop roots in the field were studied through the use of photogrammetric methods and samples of such roots were harvested to determine their material properties in the laboratory. The field studies were carried out in the mangrove areas along some of the coastlines of Singapore where the Rhizophora genus are located. We found that the prop roots under tidal hydrodynamic loadings in a mangrove environment can be regarded as fairly rigid on account of a large Young's modulus of about 15 GPa. Physical prop root models were fabricated from downscaling based on field observations with porosity values ranging from 0.96 to 0.98. Flume experiments were performed and measurements of mean flow velocities, Reynolds stress and turbulent kinetic energy (TKE) were made. Our results indicate that the prop roots provide blockage effect on the flow and cause complex secondary flow. The turbulence energy can be generated by both wake and shear, with shear-generated turbulence being dominant in the upper open area between prop roots. A force balance analysis was performed in the nearly uniform flow region to investigate the flow resistance caused by the prop root models. The Chézy roughness coefficient C was found to be 10 and the drag coefficient CD was found to be 1.2 in the fully developed flow, which agrees with reported field studies in mangrove swamps.

Aerodynamics Performance of Endwall Film Cooling under the Influence of Purge Flow in High Pressure Turbine Cascade

Modern gas turbine requires sophisticated cooling technologies to avoid thermal failure due to the extreme operating environment. Film cooling is one of the most important cooling technologies used for gas turbine hot-section components, particularly for blade aerofoil surfaces and endwall. Previous research has shown that the endwall region is considerably more difficult to cool than the blade aerofoil surfaces because of the existence of complex secondary flow structures such as horse-shoe vortex, cross flow and passage vortex in the blade passage. Therefore, this study focuses on aerodynamics interaction of the cooling air through the upstream slot with the secondary flow field. Experiments carried out using 5-holes Pitot tube have revealed the secondary flow field at blade downstream of linear cascade of high pressure turbine. A baseline condition without any leakage flows was compared with the leakage ejection case. Finally, both cases were validated by simulations from commercial software, ANSYS CFX.

Thermal analysis of a compressor for application to Compressed Air Energy Storage

In this paper, the topic of Compressed Air Energy Storage (CAES) is discussed and a program in which it is being applied to a wind turbine system for leveling power supplied to the grid is described. Noted is the importance of heat transfer in the design of the compressor and its effect on performance. Presented is a design for minimizing the temperature rise in the compressor during compression. The design requires modeling regenerative heat transfer from the compressed air to solid material inserted in the compression space. Modeling requires characterizing pressure drop through the porous insert, interfacial heat transfer between solid and fluid in the matrix, and thermal dispersion within the porous regions. Computation and experimentation are applied for developing correlations for such terms. Two types of porous media are applied: interrupted plates and open-cell metal foams. Cases with foam inserts are computed and the results are discussed. Discovered in the results are some complex secondary flow features in spaces above the porous inserts.

Flow patterns and preferred sites of atherosclerotic lesions in the human aorta - II. Abdominal aorta.

As in Part I, to elucidate the role of fluid mechanical factors in the localized genesis and development of atherosclerotic lesions in man, here in the abdominal aorta. Flow patterns and preferred sites of atherosclerotic lesions in the aorta were studied in detail using the same isolated transparent aortic trees prepared from humans postmortem and the flow visualization and cinemicrographic techniques as in Part I. Under steady flow simulating mid-systole, the flow was found to be disturbed at the aorto-celiac and aorto-superior mesenteric artery junctions by the formation of complex secondary and adverse flows along the lateral and posterior walls of the abdominal aorta. More complex secondary and adverse flows formed at the branching sites of the left and right renal arteries. Furthermore, considerable interactions occurred between the secondary and adverse flows formed at the branching sites of the above four arteries, resulting in the formation of a large and long recirculation zone along the lateral and posterior walls of the abdominal aorta corresponding to these branches. The velocity profile was almost flattened throughout the entire length of the descending aorta. Atherosclerotic lesions were found mainly at the posterior and lateral walls of the abdominal aorta where slow adverse and recirculation flows formed and where wall shear stress was low.

Numerical Simulation of Turbulent Mist Flows With Liquid Film Formation in Curved Pipes Using an Eulerian–Eulerian Method

Turbulent flows of air/water mixtures through curved pipes are modeled in this work using a Eulerian–Eulerian method. This is motivated by the possibility of using computational fluid dynamics (CFD) as a design tool applied to curved pipes feeding a gas/liquid separator. The question is to identify the curvature of such pipes that can promote film formation upstream of the separator and, thus, precondition the flow without creating a large pressure drop. The performance of the mixture theory with a drift flux model and the “realizable” k-ε closure was evaluated in the simulations. The enhanced wall treatment (EWT) was utilized to resolve the flow in the near-wall region. A qualitative study was first conducted to investigate the flow patterns and the liquid film formation in a 180 deg bend. The numerical results were validated by comparing the computed pressure drop with empirical correlations from the literature. Subsequently, the importance of droplet size and liquid volume fraction was investigated by studying their effect on the flow patterns of the continuous phase, as well as their impact on the secondary flow intensity, the pressure drop, and the liquid film formation on the wall. Various pipe geometries were studied to achieve a low pressure drop while maintaining a high droplet deposition. Results show that a combination of the drift flux model with the realizable k-ε closure and EWT for the near-wall treatment appears capable of capturing the complex secondary flow patterns such as those associated with film inversion. The pressure drop computed for various flows appear to be in good agreement with an empirical correlation. Finally, bends with a curvature ratio around 7 appear to be the optimal for providing a small pressure drop as well as a high droplet deposition efficiency in a U-bend.

Purge flow effect on aerodynamics performance in high-pressure turbine cascade

The endwall and blade film cooling systems are the typical solution adopted within gas turbines to allow further increase of turbine inlet temperature, avoiding critical material thermal damages. Due to the complex secondary flow field in the blade passage, endwall is more difficult to be cooled than blade surfaces. In the matter of fact, in endwall film cooling studies, it is necessary to investigate the interaction between coolant air and the secondary flow. In present study, the flow field of high-pressure turbine in linear cascade has been investigated by 5-holes pitot tube to reveal the secondary flows behaviour under the influenced of purge flows through a slot which is located 0.63 Cax upstream of blade leading edge. Both measurement and numerical simulation presented almost similar trend of aerodynamics performance thus enable the authors to excute the flow visualizations accurately. The presence of newly generated vortical structures was considered to be responsible to the additional loss at higher MFR cases.

Simulations of Multiphase Particle Deposition on a Nonaxisymmetric Contoured Endwall With Film-Cooling

Designing turbine components for maximum aerodynamic performance with adequate cooling is a critical challenge for gas turbine engineers, particularly at the endwall of a turbine, due to complex secondary flows. To complicate matters, impurities from the fuel and intake air can deposit on film-cooled components downstream of the combustor. Deposition-induced roughness can reduce cooling effectiveness and aerodynamic performance dramatically. One method commonly used for reducing the effects of secondary flows on aerodynamic performance is endwall contouring. The current study evaluates deposition effects on endwall contouring given the change to the secondary flow pattern. For the current study, deposition was dynamically simulated in a turbine cascade to determine its effects on film-cooling with and without endwall contouring. Computationally predicted impactions were in qualitative agreement with experimental deposition simulations, showing that contouring reduced deposition around strategically placed film-cooling holes. Deposition reduced cooling effectiveness by 50% on a flat endwall and 40% on an identically cooled contoured endwall. Although 40% is still a dramatic reduction in effectiveness, the method of using the endwall contouring to alter deposition effects shows promise.

Aero-Thermal Performance of Purge Flow in Turbine Cascade Endwall Cooling

The endwall and blade film cooling systems are the typical solution adopted within gas turbines to allow further increase of turbine inlet temperature, avoiding critical material thermal stresses. Due to complex secondary flow field in the blade passage, endwallis more difficult to cool than blade surfaces. In the matter of fact, in endwall film cooling studies, it is necessary to investigate the interaction between coolant air and the secondary flow. In present study, the flow field of high-pressure turbine cascade has been investigated by 5-holes pitot tube to reveal the secondary flows behavior under the influenced of purge flows while the heat transfer measurement was conducted bythermochromic liquid crystal (TLC). Experimental has significantly captured theaerodynamics effect of purge flowat blade downstream close to the endwall region. Furthermore, TLC measurement illustrated that the film cooling effectiveness and heat transfer coefficient contours were strongly influenced by the secondary flow on the endwall.

LES에 의한 PAC용 시로코홴의 3차원 전산유동해석

The computational flow analysis using LES technique was carried out to investigate the flow characteristics of a sirocco fan under the maximum flowrate condition. The commercial SC/Tetra software was used for this unsteady and three-dimensional numerical analysis. In consequence, because a flow is unstable within the range of about 50% of a housing depth from a bellmouth around the cutoff region, the passing flow through the blade cascade occurred on the X-Y plane is a slow or a reverse with approaching to the housing inlet. Also, the secondary flow shows on the radial plane of a housing, and its vortex center exists within about 33% of a housing depth from a bellmouth except the cutoff region. Moreover, the flow occurring on the exit plane of a sirocco fan shows a complex secondary flow.

Flow and Heat Transfer Characteristics in Rotating Two-pass Channels Cooled by Superheated Steam

In a modern gas turbine, using superheated steam to cool the vane and blade for internal convection cooling is a promising alternative to traditional compressor air. However, further investigations of steam cooling need to be performed. In this paper, the three-dimensional flow and heat transfer characteristics of steam are numerically investigated in two-pass square channels with 45° ribbed walls under stationary and rotating conditions. The investigated rotation numbers are 0 and 0.24. The simulation is carried out by solving the Reynolds averaged Navier-Stokes equations employing the Reynolds stress turbulence model, especially considering two additional terms for Coriolis and rotational buoyancy forces caused by the rotating effect. For comparison, calculations for the air-cooled channels are done first at a Reynolds number of 25 000 and inlet coolant-to-wall density ratio of 0.13. The results are compared with the experiment data. Then the flow and heat transfer in steam-cooled channels are analyzed under the same operating conditions. The results indicate that the superheated steam has better heat transfer performance than air. Due to the combined effect of rotation, skewed ribs and 180° sharp turn, the secondary flow pattern in steam-cooled rotating two-pass channels is quite complex. This complex secondary flow pattern leads to strong anisotropic turbulence and high level of anisotropy of Reynolds stresses, which have a significant impact on the local heat transfer coefficient distributions.

Three-dimensional vortex flow near the endwall of a short cylinder in crossflow: Stepped-diameter circular cylinder

The effect of geometry on the flow around a cylinder in crossflow is investigated in this study. Three different stepped-diameter circular cylinders (SDCC s) with varying step heights are used. Extensive flow visualization using the oil-lampblack and smoke-wire techniques and near wake velocity measurements using a hotwire anemometer reveal complex secondary flows on and around the SDCC. Six vortices are observed in the horseshoe vortex system near the cylinder–endwall junction and six additional vortices are found in the step-induced vortex system on the step surface. Based on these experimental results, new secondary flow models are proposed. The step-induced vortices separate from the step surface at both sides and move toward the endwall, washing down the sides of the top/bottom larger diameter cylinders and interact with the separated shear layer and horseshoe vortices. In this process, they modify the near wake flow significantly: they produce an increase in velocity near the endwall region (below the step) and a decrease in velocity near the mid-span region, even altering the oscillatory behavior of the wake.

Computational guidelines and an empirical model for particle deposition in curved pipes using an Eulerian-Lagrangian approach

Computational guidelines are provided for modeling turbulent particulate-laden flows in curved pipes using a Reynolds-Averaged Navier-Stokes (RANS) approach in ANSYS FLUENT. The standard k-ε model and the linear pressure-strain Reynolds Stress Model (RSM) are used to close the RANS equation. Different near-wall treatments associated with the selected turbulence closures are employed to study their impact on estimating pressure drop and particle deposition (grade efficiency). The bends studied had different bend angles, bend curvature ratios, and the flows had various Re numbers. Experimental results available in the literature are employed to validate the computational results. The observed turbulent flows exhibit complex secondary flow patterns at the bend and these patterns are influenced by the bend curvature ratio and the flow Re number. The pressure drop along the curved pipe is well estimated using either closure for all three near-wall treatments with the exception of pressure at the inner and outer walls of the 180° bend, where an error of about 7% is obtained. With respect to particle deposition however, the performance of the RSM outperforms that of the standard k-ε model. Using an Enhanced Wall Treatment (EWT) combined with an RSM can improve the accuracy by as much as 19% in a 90° bend and up to 30% in a 180° bend when compared to other near-wall treatments and closure models. The RSM with EWT should thus be employed when modeling particle deposition in flows with curved streamlines. In addition, a new empirical model is proposed to model particle deposition efficiency in curved pipes. The model extends the range of previous empirical models and accounts for the effects of the Stokes number, the bend angle, and the curvature ratio.

Numerical investigation on the mixed convection and heat transfer of supercritical water in horizontal tubes in the large specific heat region

Numerical simulation is performed in the present study to get further insight into the mechanism of heat transfer phenomena of water in ∅32×3mm horizontal smooth tubes under supercritical pressures. Both the heat transfer enhancement (HTE) and heat transfer deterioration (HTD) in the so-called large specific heat region (LSHR) of supercritical fluids are analyzed based on the numerical results. The governing equations of the fluid are solved on fixed three-dimensional grid systems, and the RNG-k–ε model with enhanced wall treatment method is employed to handle the coupled wall-to-fluid heat transfer. The numerical results are compared with the corresponding experimental data, and a good agreement is achieved, indicating good accuracy and reliability of the numerical method used in this study. Variation of the tube inner wall temperature and the local heat transfer coefficient with a few parameters, such as the local water enthalpy, the water mass flow rate and the heat flux on the tube wall, are obtained under various conditions. It is showed that in the large specific heat region (LSHR), there exists strong non-uniformity in the circumferential distribution of the tube inner wall temperature, and as well, the heat flux on the inner tube wall are distinctly non-uniform along the tube wall circumference. Further analysis of the numerical data shows that the above-mentioned non-uniformity is mainly due to the rapid change in fluid properties, which results complex secondary flows and mixed convection in the supercritical fluid. Based on the numerical results, a critical value is obtained for the parameter Gr/Re2.7, which is believed to be a key parameter affecting the secondary flow in the LSHR. In the present study, the critical value for Gr/Re2.7 is determined to be 1.5×10−5, above which the HTD phenomenon may be observed.

Three-dimensional vortex flow near the endwall of a short cylinder in crossflow: Uniform-diameter circular cylinder

Flow characteristics, around a short uniform-diameter circular cylinder in crossflow, are investigated experimentally. Extensive flow visualization using oil-lampblack and smoke-wire methods have been performed. Near-wake velocity measurements have been performed using a hotwire anemometer. Complex secondary flows are observed on and around the cylinder in crossflow. Multiple vortices are observed in the horseshoe vortex system near the cylinder–endwall junction. Based on this flow visualization and local mass transfer measurement results, a six-vortex secondary flow model has been proposed.

Experimental and numerical investigation on compressor cascade flows with tip clearance at a low Reynolds number condition

High flow rate aeroengines typically employ axial flow compressors, where aerodynamic loss is predominantly due to secondary flow features such as tip leakage and corner vortices. In very high altitude missions, turbomachinery operates at low density ambient atmosphere, and the recent trend toward more compact engine core inevitably leads to the reduction of blade size, which in turn increases the relative height of the blade tip clearance. Low Reynolds number flowfield as a result of these two factors amplifies the relative importance of secondary flow effects. This paper focuses on the behavior of tip leakage flow, investigating by use of both experimental and numerical approaches. In order to understand the complex secondary flow behavior, cascade tests are usually conducted using intrusive probes to determine the loss. However relatively few experimental studies are published on tip leakage flows which take into account the interaction between a rotating blade row and its casing wall. Hence a new linear cascade facility has been designed with a moving belt casing in order to reproduce more realistic flowfield as encountered by a rotating compressor row. Numerical simulations were also performed to aid in the understanding of the complex flow features. The experimental results indicate a significant difference in the flowfield when the moving belt casing is present. The numerical simulations reveal that the leakage vortex is pulled by the shearing motion of the endwall toward the pressure side of the adjacent blade. The results highlight the importance of casing wall relative motion in analyzing leakage flow effects.

Numerical Study of Inlet and Impeller Flow Structures in Centrifugal Pump at Design and Off-design Points

The objective of present work is to use numerical simulation to investigate the complex three-dimensional and secondary flow structures developed at the inlet and impeller in a centrifugal pump at design and off-design points. The pump impeller is shrouded with 6 backward swept blades and with a specific speed of 0.8574. The characteristic of the pump is measured experimentally with straight and curved intake sections. Numerical computation is carried out to investigate the pump inlet flow structures and subsequently the flow field within the centrifugal pump. The numerical results showed that strong interaction between the impeller eye and intake section. Secondary flow structure occurs upstream at the pump inlet has great influence on the pump performance and flow structure within the impeller.

On the secondary flow through bifurcating pipes

The secondary motion induced by flow through curves and bifurcations has been subject to investigation over long time due to its importance in physiological and technological applications. In contrast to the flow in a straight pipe, curvature leads to the formation of secondary flow which is often unsteady. Streamline curvature occurs also in bifurcating pipes leading to some corresponding secondary, unsteady flow. This paper presents a detailed description of the unsteady flow in the daughter branch after a 90° bifurcation. A range of Reynolds and Womersley numbers are investigated. The results show the presence of Dean vortices and additional vortical patterns not reported in the literature. Both the streamwise (axial) and the secondary velocity components change character at larger Womersley numbers, leading to less complex secondary flow. Also, at larger Reynolds numbers, flow instabilities are observed. The secondary flow may lead to the formation of unsteady separation bubbles. This in turn yields peaks in the wall shear stress components. Such wall shear stress variations have often been related in the literature to the development process of atherosclerosis.

The heat/mass transfer analogy for a simulated turbine endwall

Heat transfer measurements in gas turbine cascades are often difficult because of thin boundary layers, complex secondary flows, and large variation in local heat transfer rates. Thus mass transfer techniques have often been used as an alternative method, the heat transfer coefficients being then calculated from the heat/mass transfer analogy. To ensure confidence in the quantitative conversion to the heat transfer coefficients from the mass transfer results, evaluation of the analogy factors is crucial. The present paper examines the validity of the heat/mass transfer analogy, evaluating the analogy factors on a simulated turbine endwall, with separate heat and mass transfer experiments with equivalent flow and geometric conditions. The Nusselt numbers, determined from the heat transfer experiments with a constant wall temperature boundary condition are compared to Sherwood numbers from the mass transfer experiments employing a constant wall concentration boundary condition to evaluate the heat/mass transfer analogy.

Numerical investigation of aerodynamic performance influenced by circumferentially pre-swirling coming flow on the crossover and de-swirling cascade of a multistage centrifugal compressor

Numerical investigation is implemented on aerodynamic performance inside the crossover and de-swirling cascade of a multistage centrifugal compressor. The emphasis is put on the aerodynamic performance influenced by the circumferentially pre-swirling coming flow. The results indicate that flow separation occurs inside the crossover, and the separation area may be changed with different circumferentially pre-swirling coming flow. Decreased pre-swirling intensity of the coming flow may effectively restrain the flow separation and make the outflow from the crossover more uniform, which helps to improve the aerodynamic performance of the successive de-swirling cascade. The flow inside the de-swirling cascade is a non-uniform swirling flow with large separation. The complex secondary flow occurs along the main flow and experiences a process of generating, developing, dissipating and collapsing.