Steady-state Reynolds Averaged Navier-Stokes Simulations Research Articles

Numerical Investigation of Thermal-Hydraulic Performance of U-Tubes Enhanced With Ellipsoidal 45 deg Dimples

Abstract This study aims to numerically investigate thermal–hydraulic performance augmentation of ellipsoidal 45 deg dimpled U-tubes with various bend curvatures subjected to constant external heat flux (q′′=10kW/m2) for a range of Reynolds numbers (5000 ≤ Re ≤ 30,000). Three smooth U bends with curvatures radii of 0.695Dh, 1.5Dh, and 2.0Dh, where Dh is the hydraulic diameter of the smooth tube, are used in both smooth and enhanced tubes. A comparison of thermal-hydraulic characteristics of dimpled and smooth U-tubes is carried out using steady-state Reynolds averaged Navier–Stokes simulations. The analysis shows that the performance of the dimpled U-tube is superior to the smooth tube for all bend curvatures. The swirl flow patterns generated by the dimples induce early flow reattachment in the postbend section of the tube, which enhances its heat transfer rate. The dimpled U-tube having the shortest curvature radius significantly alters Dean vortices, which leads to a substantial improvement in its heat and flow performances. The dimpled U-tube having the shortest curvature radius enhances the thermal–hydraulic performance by 21.4% while for other curvature radii (1.5Dh, and 2.0Dh), the performance augmentations are found to be 10.7% and 8.9%, respectively.

Effect of dimple pitch on thermal-hydraulic performance of tubes enhanced with ellipsoidal and teardrop dimples

This paper aims to numerically investigate the thermal-hydraulic performance of dimpled tubes having various geometric pitches under a constant external heat flux of 10 k W / m 2 for a wide range of Reynolds numbers (Re). The performance of enhanced tubes consisting of ellipsoidal 0°, teardrop, and ellipsoidal 45° dimples of equivalent volumes are compared using steady-state Reynolds-Averaged Navier-Stokes simulations. It is observed that the performance of dimpled tubes vary significantly with dimple pitch (P) and Re variation. Therefore, optimum pitches and working ranges of Re are identified for all dimpled tube configurations by varying 3.2 m m ≤ P ≤ 13.2 m m and 9000 ≤ R e ≤ 40000 . The ellipsoidal 45° and teardrop dimpled tubes have been found to have an optimum pitch of 3.2 mm, and suitable working ranges of 9000 ≤ R e ≤ 30000 and 14000 ≤ R e ≤ 40000 . The ellipsoidal 45° and teardrop dimpled tubes with optimum pitch enhanced thermal-hydraulic performance up to 45.7 % and 31.2 % , respectively. However, ellipsoidal 0° tubes have relatively short working ranges with limited thermal-hydraulic performance improvement. An in-depth analysis of local drag and heat transfer characteristics of individual dimples along with their effects on the overall performance of the tubes are also carried out. Lastly, new correlations of Nusselt number and friction factor are proposed for optimum dimpled tubes.

Impact of morphological parameters on urban ventilation in compact cities: The case of the Tuscolano-Don Bosco district in Rome

Air pollution and heat stress are major concerns associated with the liveability, resilience and sustainability of cities. They directly affect health and comfort and are associated with augmented morbidity and mortality and an increase in the energy demand for building ventilation, air cleaning and cooling. Nevertheless, the detrimental effects of poor air quality may partly be mitigated by increased urban ventilation. This strategy is closely related to the level of urbanization and the urban morphology. Therefore, detailed investigations on the impact of different morphologies on urban ventilation are of paramount importance. Computational Fluid Dynamics simulations have been widely used during the last decades to investigate the effects of the urban morphology on the urban ventilation. However, most of these studies focused on idealized building arrangements, while detailed investigations about the role of real urban morphologies are scarce. This study investigates the ventilation in a compact area in the city of Rome, Italy. 3D steady-state Reynolds-averaged Navier-Stokes simulations are performed to analyze the impact of Morphological Parameters (MP) on the urban ventilation. The results show a considerable worsening of urban ventilation with increasing building density with a reduction in the mean wind velocity up to 62% experienced at the pedestrian level (zp). Correlations between five MPs, e.g., plan area density, area-weighted mean building height, volume density, façade area density, and non-dimensional mean velocity at pedestrian level and at 10 m height are evaluated, and simple models are obtained using linear regression analysis. Among the selected MPs, the building façade area density shows a remarkable correlation with the non-dimensional mean velocity at zp (R2 = 0.82). Such correlations can be valuable tools for practitioners and urban designers, particularly during the first stage of planning, for highlighting areas potentially vulnerable to poor air conditions without running computationally expensive simulations.

Evaluating the loading increment of a generic submarine hull casing

The creation of a database of hydrodynamic loads for a generic submarine hull with a casing is proposed due to a lack of data in the open literature on the loading effects of submarine casings. This work presents a numerical methodology for the eventual creation of such a database. Steady-state Reynolds-averaged Navier–Stokes simulations were conducted for the Joubert BB2 generic submarine hull in translation, with and without a deck casing while deeply submerged, and with and without a casing in a wind tunnel. Simulation results at a Reynolds number of Re = 5.24 × 10 6 provide hydrodynamic load increments due to the submarine deck casing, and are validated against unique experimental data for a range of angles of attack, − 15 ° ≤ α ≤ 15 ° , and angles of drift, − 30 ° ≤ β ≤ 30 ° . The estimated numerical error of the in-plane force and moment were 8.1% and 6.8%, respectively, and 2.2% and 3.7% for the out-of-plane force and moment, respectively. The simulation results were useful in evaluating the interference of the experimental support system and investigating the hydrodynamic loading effects of the casing. It was found that the addition of a casing can alter the in-plane hydrodynamic loads by at least 30%, and can also introduce out-of-plane loads of comparable magnitude. • Establishment of a numerical methodology for estimating load effects of casings. • In-plane and out-of-plane loads had numerical errors estimated to be less than 8.2%. • Validated CFD predictions against unique experimental data for a wide range of flow angles. • Deck casings can alter the in-plane hydrodynamic loads by at least 30%. • Deck casings can also introduce out-of-plane loads of comparable magnitude.

설계조건에서 선형 익렬의 익단 간극 크기가 익단 간극 누설유동에 미치는 영향에 관한 수치해석

A steady-state Reynolds averaged Navier-Stokes simulations was conducted to investigate the effect of tip clearance size on the distribution of tip leakage flow inside a linear cascade. Two different tip clearance size, t=2 mm and 4 mm, were considered under design condition(i.e. inlet flow angle β = 29.3°). As the tip clearance increased, the gradient of the static pressure trough at the endwall became larger and the tip leakage flow rolled-up more strongly in a helical form. In addition, high total pressure loss region corresponding to the distribution of tip leakage vortex was somewhat enlarged in the downstream positions of the blade trailing edge. The flow rate through the tip clearance increased in proportion to tip clearance size, and as a result, the swirling flow became stronger, so that the center of tip leakage vortex moved more from the blade tip to the hub. Additionally, the region with high turbulent intensity where the leakage flow exited the tip clearance in the form of jet, was extended to the inside of the flow path along the endwall. Finally, it was confirmed that there were several indicators to confirm the existence of tip leakage vortex. The defect of axial velocity in the tip region of blade flow path, the dimensionless helicity with a magnitude close to 1, the endwall pressure trough, the region with high total pressure loss, and strong roll-up of the particle trajectory were the typical examples. By taking these indicators as a comprehensive consideration, the existence of tip leakage vortex can be effectively found.

Computational Fluid Dynamic Simulation of a Supercritical CO2 Compressor Performance Map

The performance map of a radial compressor operating with supercritical CO2 is computed by means of three-dimensional steady state Reynolds-averaged Navier–Stokes simulations. The geometry investigated is part of a 250 kW prototype which was tested at Sandia National Laboratories (SNL). An in-house fluid dynamic solver is coupled with a lookup table algorithm to evaluate the fluid properties. Tables are generated using a multiparameter equation of state, which ensures high accuracy in the fluid characterization. The compressor map is calculated considering three different rotational speeds (45 krpm, 50 krpm, and 55 krpm). For each speed-line, several mass flow rates are simulated. Numerical results are compared to experimental data from SNL to prove the potential of the methodology.

Reynolds-Averaged Navier-Stokes Simulation of the Flow and Tracer Transport in a Multichambered Ozone Contactor

AbstractThis paper reports on results from steady-state Reynolds-averaged Navier-Stokes simulation (RANSS) of the flow and passive nonreactive tracer transport inside a multichambered, laboratory-scale ozone contactor using a structured, collocated, finite-volume discretization. Simulations are posed following a physical laboratory experiment. Simulations performed on different grids lead to varying degrees of short-circuiting and dispersion, which ultimately leads to differences in the residence time distribution (RTD) of the tracer (released at the inflow as a pulse) and in particular t10 index. The predicted t10 index depends on grid density and RANS turbulence model. For fine grids that resolve the viscous sublayer, a model with proper behavior within this sublayer has to be applied for accurate prediction of t10. Simulations on all considered grids with proper turbulence model yield cumulative RTD and associated t10 index in good agreement with the experimental data despite the underresolution of the...

Numerical and Experimental Investigation of Turning Flow Effects on Innovative Pin Fin Arrangements for Trailing Edge Cooling Configurations

The effect of the array configuration of circular pin fins is investigated from a numerical and experimental point of view reproducing a typical cooling scheme of a real high pressure aero-engine blade. The airstream enters the domain of interest radially from the hub inlet and exits axially from the trailing edge (TE) outlet section. More than 100 turbulators are inserted in the wedge-shaped TE duct to enhance the heat transfer: A reference array implementing seven rows of staggered pins is compared with an innovative pentagonal arrangement. Investigations were made considering real engine flow conditions: Both numerical calculations and experimental measurements were performed fixing Re=18,000 and Ma=0.3 in the TE throat section. The effect of the tip mass flow rate was also taken into account, investigating 0% and 25% of the TE mass flow rate. The experimental activity was aimed at obtaining detailed heat transfer coefficient maps over the internal pressure side (PS) surface by means of the transient technique with thermochromic liquid crystals. Particle image velocimetry measurements were performed and surface flow visualizations were made by means of the oil and dye technique on the PS surface. Steady-state Reynolds averaged Navier–Stokes simulations were performed with two different computational fluid dynamics (CFD) codes: the commercial software Ansys CFX®11.0 and an in-house solver based on the opensource toolbox OpenFOAM®, to compare the performance and predictive capabilities. Turbulence was modeled by means of the k−ω shear stress transport (SST) model with a hybrid near-wall treatment allowing strong clustering of the wall of interest as well as quite coarse refinement on the other viscous surfaces.

선형 압축기 익렬에서 발생하는 익단 누설 와류내의 레이놀즈 응력 분포 (I) -입구 유동각 변화의 영향-

A steady-state Reynolds averaged Navier-Stokes simulation was conducted to Investigate the distribution of the Reynolds stress tensor Inside tip leakage vortex of a linear compressor cascade. Two different inlet flow angles β=29.3°(design Condition) and 36.5°(off-riesign condition) at a constant tip clearance Size of 1% blade span were considered. Classical methods of sohd inechamcs, applied to View the Reynolds stress tensor In the principal direction system, clearly showed that the high anisotropic feature of turbulent flow field was dominant at the outer part of tip leakage vortex near the suction side of the blade and endwall flow separation region, whereas a nearly Isotropic turbulence was found at the center of tip leakage vortex, There was no significant difference In the anisotropy of the Reynolds normal stresses Inside tip leakage vortex between the design and off-design condition<br/>

축류형 유체기계에서 익단 누설 유동 해석을 위한 난류 모델 성능 평가

It is experimentally well-known that high anisotropies of the turbulent flow field are dominant inside the tip leakage vortex, which is attributable to a substantial proportion of the total loss and constitutes one of the dominant mechanisms of the noise generation. This anisotropic nature of turbulence invalidates the use of the conventional isotropic eddy viscosity turbulence models based on the Boussinesq assumption. In this study, to check whether an anisotropic turbulence model is superior to the isotropic ones or not, the results obtained from the steady-state Reynolds averaged Navier-Stokes simulations based on the RNG k-εmodel and the Reynolds stress model (RSM) are compared with experimental data for two test cases: a linear compressor cascade and a forward-swept axial-flow fan. Through this comparative study of turbulence models, it is clearly shown that the RSM, which can express the production term and body-force term induced by system rotation without introducing any modeling, should be used to predict quantitatively the complex tip leakage flow, especially in the rotating environment.