K-omega SST Model Research Articles

Analysis of natural convection in a representative cavity of a room considering oscillatory boundary conditions: An experimental and numerical approach

Natural convection heat transfer in cavities commonly occurs in various engineering problems. Specifically, the problem of a differentially heated cavity with oscillatory boundary conditions arises in the design of energy-saving strategies or energy evaluations within the building sector. This work addressed numerically and experimentally the natural convection in a full-scale room subjected to diurnal heating and nocturnal cooling. The experiment was instrumented with temperature sensors and an air velocity sensor, inspired by works reported in the literature. The numerical model involved solving the equations governing natural convection, considering turbulent bi-dimensional and three-dimensional flow in a transient state. This research allowed the selection of an appropriate turbulence model and evaluated the performance of the two-dimensional and three-dimensional approaches in simulations. Results showed that the k-omega SST model performed best in predicting velocity, while the standard k-omega model performed best in predicting temperature for the two-dimensional simulation, and the RNG k-epsilon model performed best for the three-dimensional simulation. Additionally, the three-dimensional simulation more accurately reproduced the chaotic fluctuations observed in experimental velocity data, demonstrating up to 30 % better estimation during periods of high velocities. The study identified the reversal of convective cells in the cavity, with an approximate duration of 2 h and 40 min in a real case. This study contributes to improving the understanding of turbulent natural convection in a differentially heated cavity with oscillatory boundary conditions. These findings have significant implications for various environmental and engineering applications, including the design of thermally comfortable buildings.

RANS study of surface roughness effects on ship resistance

Abstract In marine engineering, optimizing the performance of vessels is a key issue, particularly in increasing fuel efficiency, reducing operating costs, and minimizing environmental impact. One of the most critical determinants of vessel efficiency is hull resistance, which directly affects fuel consumption, power requirements, and cruising speed. We aim to investigate how surface roughness affects hull flow resistance, to quantify the drag variation at different surface roughness levels using special wall functions that reproduce boundary layer dynamics. This approach involves the complex interaction between the intricate complexity of roughness and the nonlinear pressure drag effects on the hull. The KVLCC2 ship model is used as a representative prototype in the CFD analysis based on the RANS equations and the k-omega SST model to independently evaluate the roughness effects on individual ship sections. The comparative analysis with empirical data reveals the complex relationship between surface roughness and ship resistance and provides insights for ship design and operational improvement. The study investigates the interaction between drag coefficient and vessel performance to improve hydrodynamic efficiency.

Effects of Heat Transfer Characteristics of R32 and R1234yf with Al2O3 Nanoparticle through U-Bend Tube Evaporator

This study used the Ansys Fluent® computational fluid dynamics code in conjunction with a volume of fluid multiphase model and phase-change model to analyze the flow boiling evaporation heat transfer coefficient and flow patterns of R32 and R1234yf with Al2O3 nanoparticle through the U-bend tube with a curvature ratio for downward-oriented flow. The volume of fluid (VOF) model was used to follow the patterns at the interface, while the SST k-omega model was used to simulate the gas-liquid flow. This work has been validated by utilizing a R134a refrigerant. Simulations were performed at various mass fluxes, vapor qualities, and temperatures to determine the effects of these variables on heat transfer and frictional pressure decrease in the tube. R1234yf shows much better performance than the other pure refrigerants in terms of heat transfer and vaporization. The addition of nanoparticle Al2O3 with the refrigerants R32 and R1234yf significantly improved the heat transfer coefficient and increased the vapor fraction. The frictional pressure drop increases with increasing mass flux and decreases with increasing vapor quality due to a significant decrease in the liquid film thickness. The heat transfer coefficient, on the other hand, increases with increasing mass flux and decreases with vapor quality up to a point. There are certain changes in the heat transfer coefficient at the bend. After the bend, the frictional pressure drop increased at a higher rate than before the bend, and the vapor fraction increased at a higher rate.

Thermo-hydraulic and entropy generation analysis of nanofluid flow with variable properties in various duct cross sections: a 3-D two-phase approach

A three-dimensional computational fluid dynamics (CFD) study is carried out to explore the effect of duct cross section on the thermo-hydraulic performance of various ducts. A finite volume-based scheme with an SST k-omega model and mixture model (two-phase model) was used to obtain more realistic results. A two-phase mixture model was used to consider the movement between base fluid and nanoparticles. Al2O3 nanoparticle having a volume fraction of 0.01% and 42 nm as particle size, the heat transfer and friction factor characteristic are studied for turbulent flow regime (3000 < Re < 9000) with variable thermo-physical properties. A maximum enhancement of 86% in heat transfer rate is obtained for the serpentine duct compared to the conventional circular duct at Re = 4500. Owing to a significantly lower increase in pressure drop, the elliptical duct has the highest thermo-hydraulic performance parameter of 1.54 relative to the circular duct. Further, to analyze the heat transfer quality, the entropy generation rate is studied, and it is observed that the square duct reported the highest with an increase of 60% and the elliptical duct the lowest with a reduction of 54% relative to the circular duct. This study can aid in choosing the duct geometry to enhance the heat transfer rate with nanofluid for applications such as solar-thermal, heat exchangers, etc.

CFD analysis of thermal fluid dynamic parameters inside an organ pipe: influence of air temperature variations

Organ pipes represent a fluid-dynamic challenge since the time of Bernoulli and Reynolds. A complete and reliable description of the fluid behaviour at the basement of the resonator named the “mouth” of the pipe is still absent. In the context of the design of an air-heating system for the attenuation of the inconsistencies between sounding frequencies of organ pipes, computing efforts were carried out focusing on the fluid-dynamic behaviour of the air at the mouth of the pipe at different temperature conditions. Describing the air dynamic and the heat transfer at the mouth level of the pipe, it would be possible to predict the heating conditions within the pipe resonator, which was the main subject of the project. Multiple points on the pipe organ air blowing scheme were considered for simulation results extraction. Temperature, pressure, and audio measurements were performed and no significant influence of the temperature on those parameters was raised from measurements, rising questions about the effective behaviour of the blowed air at the mouth of the pipe. Simulations were performed to predict both the fluid dynamic and the heat transfer. A finite volume approach in Fluent environment was chosen. The SST k-omega model was considered in reliability vs computation time balance. 3D simulations were performed based on a CAD reconstructed model of a real pipe. The results of the simulations confirmed experimental data, and they may prove the absence of a significant mass exchange between the in and outside of the mouth of the pipe. The oscillation of the air membrane at the mouth of the pipe may be cleared from instabilities and turbulence regime phenomena, as usually described in the literature.

Optimization analysis of nuclear thermal coupling for a small nuclear thermal propulsion (SNTP) reactor

As a main structure, tie-tube elements are used both in the traditional and modern LEU nuclear thermal propulsion reactors. This complex structure and its corresponding complex convective heat transfer process have been studied, including CFD calculations. But these studies did not effectively evaluate the applicability of the models and typical problems. In this study, a general small nuclear thermal propulsion (SNTP) reactor model is established by using three-dimensional Monte Carlo code and commercial CFD program Fluent. The applicabilities of material properties, turbulence models and grids are clarified by analyzing various influencing factors. The calculation results show that due to the small enthalpy difference and heat transfer, the convection effect of inner flow based on hydrogen can be ignored. There is no factor that can significantly reduce the maximum fuel temperature, but the porosity of the insulator can significantly improve the heat transfer ratio. In terms of thermal safety and overall performance, 2:1 support ratio is still the best choice. Based on the simulation results, the phenomenon of steep temperature rise of outer tie-tubes is found, and the SST k-omega model with Y + <3 is recommended for simulation of similar problems.

Assessment of turbulence models and mesh types for the simulation of multi-body separation by coupling the CFD and rigid body dynamics

Turbulence models are considered one of the most important sources of numerical errors in the CFD studies on different aeronautical problems. The mesh type can also affect the numerical accuracy and computational efficiency. In this paper, various simulations on the store separation from an aircraft model in the transonic flow have been carried out by coupling the CFD and the rigid body dynamics on the overset mesh. The influence of S-A model, Standard k-epsilon model, SST k-omega model and inviscid model on the simulation results is examined. Different mesh types of both unstructured mesh and structured mesh as component part in overset are also considered. Grid-independence study are first carried out. The effects of different turbulence models and mesh types are then examined by comparing the trajectories, velocities and Eulerian angles. Recommendations are given to get better results for the simulation of multi-body separation problem.

Characterization of pintle engine performance for GO2/kerosene propellants

Liquid rocket engine with a wide range of thrust variations has drawn much attention in recent years. A GO2/kerosene deep-throttling variable thrust engine using a liquid–gas pintle injector is presented and tested with aim of characterizing engine performance, regulating capability, and heat flux distribution. Three-dimensional numerical simulations are conducted to investigate the flame appearance and spray characteristics of the engine. The SST k-omega model is used for modeling turbulence, the Wave model is used for modeling the secondary breakup, and a multi-step quasi-global combustion mechanism and eddy dissipation concept model are used for modeling combustion. The GO2/kerosene pintle engine achieves stable combustion at 0.18 ∼ 4.35 MPa with a thrust of 21.30 ∼ 864.70 N. The thrust ratio of the pintle engine is 40.6:1 indicating a deep-throttling variable thrust capability. The thrust regulation of the pintle engine using the closed-loop control has high accuracy and response speed, as well as high disturbance suppression capability. The effects of combustor pressure, spray characteristics, and reactants concentration distribution on flame appearance and heat flux distribution of the engine are studied in detail. The engine equipped with a liquid–gas pintle injector could ensure a good atomization effect and high combustion efficiency by actively adjusting the propellant injection pressure drop coefficient. This work will provide a reference for the design of the pintle injector and thermal protection system of deep-throttling variable thrust rocket engines.

Numerical analysis of the ostiomeatal complex aeration using the CFD method

We aimed to analyse ostiomeatal complex (OMC) aeration using the computational fluid dynamics (CFD) method of simulation based on human craniofacial computed tomography (CT) scans. The analysis was based on CT images of 2 patients: one with normal nose anatomy and one with nasal septal deviation (NSD). The Reynolds-Average Simulation approach and turbulence model based on linear eddy viscosity supplemented with the two-equation k-omega SST model were used for the CFD simulation. As a result, we found differences in airflow velocity through the ostiomeatal complex in patients with a normal nose and those with NSD. In a patient with NSD, the flow is turbulent in contrast to the normal nose (laminar flow). A faster (more intensive) airflow through the OMC was observed in the wider nasal cavity of the patient with NSD than on the narrower side. In addition, we want to emphasise the higher speed of airflow through the apex uncinate process area towards the ostiomeatal complex during exhalation, which, in the presence of secretions in the nose, predisposes to its easier penetration into the sinuses of the anterior group.

A numerical study on the levitation system for droplet preparation in a fuel-coolant interaction experiment

The MISTEE facility at KTH was designed to investigate the process and phenomena of a molten droplet falling into a water pool that may be encountered in fuel-coolant interactions (FCI) during a severe accident of light water reactors. An aerodynamic levitation mechanism is proposed to hold the molten droplet during its preparation (melting and heating up to a prescribed temperature) in an induction furnace. The crucible is flushed with argon through an injection nozzle at the bottom to prevent the droplet from falling out of the crucible. A numerical simulation of the aerodynamic levitation system is performed in the present study with the objective of determining and optimizing the design. The problem was simplified as an isothermal two-phase flow in an axisymmetric geometry. The simulation is realized through ANSYS Fluent v17 platform, which employs the VOF method to track interfaces between two phases and the SST k-omega model to describe turbulence flow of argon gas. The numerical model is validated against tests performed in the MISTEE facility after mesh sensitivity study. It is then applied to investigate the impacts of various parameters on the facility levitation capability and the droplet stability. According to the simulation results, stable molten droplets can be obtained in the designed experimental setup. The simulation also provides the appropriate values of argon inlet velocity and sample mass at which a stable droplet can be obtained inside the crucible before its discharge. Either higher or lower inlet velocity will destabilize the formation of the droplet. Considering the temperature-dependent melt properties, both surface tension and viscosity affect the movement and deformation of the molten droplet. The wettability of melt on the crucible wall is critical to droplet formation, and it is found that a poor wettability can ensure the levitation of droplet.

Numerical Studies on Fixed Wing Aircraft Aerodynamic Performance using Injection Suction Mechanism

In this current research work, a NACA 6321 flat bottom aerofoil has been altered to study its aerodynamic performance. A co-flow jet (CFJ) method was used with an injection slot located at 20% of chord length and the suction slot situated at 80% of chord length. Simulations have been carried out on the aerofoil models with various angles of attack (AOA) at a Mach number 0.05 to compute the variation in coefficient of lift (CL) and coefficient of drag (Cd) using a k-omega SST model. From the study, it was observed that CL and stalling AOA of an aircraft are increased by 27% and 33% respectively, with an introduction of the additional mass flow rate of 0.01 kg/sec in comparison with baseline aerofoil. It could be concluded that CFJ method enhanced the aerodynamic performance of fixed-wing aircraft due to boundary layer control over an aerofoil which reduces the overall aircraft weight and maintenance cost.

Flow resistance analysis of non-isothermal supercritical CO2

• Supercritical CO 2 turbulent flow in gas-like region was studied. • Friction factor of supercritical CO 2 decreases with the increase of heat flux. • The density and turbulence fluctuation are the key to thermally-induced mechanism. • Proposed dimensionless number can evaluate thermally-induced and inertial forces. • Proposed correlation of supercritical CO 2 friction factor has excellent accuracy. Supercritical carbon dioxide power cycle is considered as one of the promising systems of next generation power cycle. Nevertheless, the hydraulic characteristics of typical heat source components under operating conditions (gas-like region) are not clear, and the present conclusions for flow characteristics of non-isothermal fluids are also not consistent. In this work, the turbulent flow of supercritical CO 2 in a vertical upward tube was studied experimentally and numerically. The range of experiment parameters was set as 750–2800 kg/(m 2 s) of mass flux, 10–28 MPa of pressure, 200–410 kW/m 2 of heat flux and 65–500 °C of bulk flow temperature. Furthermore, the feasibility of SST k-omega model was verified by experiment results. The effects of mass flux and pressure on friction pressure drop under non-isothermal conditions are basically consistent with those under isothermal conditions. However, the increase of heat flux leads to the decrease of friction pressure drop and friction factor. Under high heat flux condition, both the viscous shear stress and Reynolds shear stress decrease. The viscous shear stress contributes slightly to the mechanical energy dissipation, hence the Reynolds shear stress is the crux to the change of friction pressure drop. Further studies show that the decrease of density in near-wall region and the decrease of turbulence fluctuation in core region are critical to the decrease of mechanical energy dissipation. According to the theoretical and dimensional analysis, the dimensionless number Xi that can be qualitatively analyzed for the ratio of thermally-induced force to inertial force was obtained. Finally, a high-precision correlation of friction factor was proposed. The proportion of calculated value within 10% and 30% error ranges is 76.65% and 100%, respectively.

Effect of Diffusers Installation in Inlet Primary Air Coal Pulverizer on Airflow Characteristic and Wear Concentration using CFD Modeling

This paper investigates the effect of installing several diffuser models in the pulverizer ZGM123-GII inlet ducting on air flow characteristics and wear concentration. Results of the internal check found that an area of the wall and the surrounding components were wearing abnormally or faster than usual. This condition will affect the availability of the pulverizer. There are 6 variations model used in this study to solve this problem. The 2 variations in the number of blade, there are 2 and 3 blades, combined with 3 angle variations, namely 30 o , 45 o , and 50 o . In this study, the viscous k-omega SST model was used to simulate airflow from the primary inlet to the area above the throat ring. The results show the contours of velocity of the air and the velocity vector on the pulverizer. Model with an angle of 45 degrees and the number of blade 3, is able to circulate air dominantly on the left side of the primary air inlet ducting, according to the reference study. From all variations, the model 45 o angle with 3 blades and 50 o angle with 3 blades are able to overcome the wear concentration problem.

Energy Characteristics of a Bidirectional Axial-Flow Pump with Two Impeller Airfoils Based on Entropy Production Analysis.

This research sought to determine the spatial distribution of hydraulic losses for a bidirectional axial-flow pump with arc- and S-shaped impellers. The unsteady Reynolds time-averaged Stokes (URANS) approach with the SST k-omega model was used to predict the internal flow field. The total entropy production (TEP) and total entropy production rate (TEPR) were used to evaluate the overall and local hydraulic losses. The results show that the distribution of TEP and TEPR was similar for both impeller cases. Under a forward condition, TEP mainly comes from the impeller and elbow pipe. The high TEPR inside the impeller can be found near the shroud, and it shifts from the leading edge to the trailing edge with an increase in the flow rate due to the decline in the attack angle. The high TEPR inside the elbow pipe can be seen near the inlet, and the area shrinks with an increase in the flow rate caused by a reduction in the velocity circulation. Under the reverse condition, TEP mainly comes from the impeller and the straight pipe. The TEPR of the region near the shroud is obviously higher than for other regions, and the area of high TEPR near the suction side shrinks with an increase in the flow rate. The high TEPR of the straight pipe can be found near the inlet, and declines in the flow direction. These results provide a theoretical reference for future work to optimize the design of the bidirectional axial-flow pump.

Maxillary sinus aeration analysis using computational fluid dynamics

The maxillary sinus aeration using the computational fluid dynamics (CFD) method based on individual adult patients’ computed tomography (CT) scans were analyzed. The analysis was based on CT images of 4 patients: one with normal nose anatomy and three with nasal septal deviation (NSD) and concha bullosa (CB). The CFD simulation was performed using the Reynolds-Average Simulation approach and turbulence closure based on linear eddy viscosity supplemented with the two-equation k-omega SST model. As a result, it was found that the lower part of NSD has the most significant impact on the airflow change within the maxillary sinuses compared to CB and the upper part of NSD. In a healthy nose, the airflow in the sinuses is continuous, while NSD and CB change this flow into pulsatile. Multiple changes in the direction of flow during one respiratory phase were observed. The flow intensity within the maxillary sinus opening is lower on the NSD side. The concept of vorticity measure is introduced to evaluate and compare various patients qualitatively. Typically, the lowest values of such measures are obtained for healthy airways and the highest for pathological changes in the nasal cavity.

Numerical investigation of the slipstream characteristics of a maglev train in a tunnel

High-speed maglev trains operate at higher speeds than conventional high-speed trains. This has implications on intensified aerodynamic issues, such as the transition between open air running and entering into a tunnel. In this paper, numerical simulation of a maglev train entering a tunnel is carried out using IDDES methods (based on SST k-omega model) to analyze the changing slipstream. The peaks and fluctuations of the slipstream are analyzed, together with the transient wake characteristics and TKE (turbulent kinetic energy) distributions. The influence of train nose length on the slipstream and its associated characteristics inside tunnels is also investigated in this paper. It was found that as the maglev train enters the tunnel, the wake slipstream at measuring points close to tunnel entrance increases significantly then decreases slightly with the increase of distance to tunnel entrance. Overall, the fluctuation and magnitude of slipstream inside tunnel is larger than that on open line, more specifically, the maximum TKE generated inside tunnel is. 7.62% larger than that on the open line at contour X = 3 H behind the train tail. Besides it takes longer time for the slipstream inside tunnel to return to the initial condition. These phenomena could be explained by that the scale of vortex structure formed behind the train tail is larger, the developing distance of the wake vortices in the streamwise direction is longer and the TKE generated is more significant inside tunnel. It was also found that increasing the nose length could effectively decrease the spatial scale and TKE of the wake vortices, which resulted on reducing the peak and pulsation of wake slipstream. Comparing to that of 5.4 m, the peak of the wake slipstream of the maglev trains with the 7.4 m and 9.4 m nose lengths at Y = 0.235 m(0.385) is reduced by approximately 23.7%(58%) and 35.9%(82.2%) on open field, and by about 3.6%(4.7%) and 14%(18.5%) inside tunnel. Besides, the maximum TKE at contour X = 2H/3H/5H behind the train tail decreases about 14.4%/10.7%/11.3% and 51%/31.5%/18% as the nose length increase to 7.4 m and 9.4 m respectively.

Research on the thermal performance of a heat exchanger with meso-scale twisted helical tube bundles

Heat exchangers with helical tubes are applied widely in various industrial fields and cryogenic applications. Currently, the most common ones are the spiral wound heat exchangers, which consists of many layers of spiral tubes with different coil diameters in a closed shell. Most of the fluid in the shell-side passes directly through the gap between two adjacent layers, which weakens the turbulence and the associated heat transfer coefficient. A novel twisted helical bundle geometry is proposed in this paper of which the thermal performance is explored using CFD simulation. The heat exchanger uses helium gas as the working fluid and operates between 300 K and 30 K. The curve of each tube in the bundle is generated by different 3D sinusoidal equations, and the tubes at adjacent layers are twisted together thereby producing the same end-to-end length for each tube. By simulating a spiral wound heat exchanger using different turbulent equations and comparing the results with experimental data from literature, the k-omega SST model is found to be the most suitable one to simulate this kind of geometry. Nusselt numbers and friction factors are obtained under different Re values and compared with those of standard spiral wound heat exchangers. The creative twisted geometry can improve the Nu number although also increasing the friction factor. Geometry parameters such as pitch length are being further explored to obtain an optimal thermal performance.

Influence of Negatively Buoyant Jets on a Strongly Curved Open-Channel Flow Using RANS Models with Experimental Data

Experimental and numerical studies of flow structures in a strongly curved 135-degree laboratory flume were carried out to investigate the influence of negatively buoyant jets using the finite volume method. The performance results of three different turbulence models were investigated by comparing the numerical results with the experimental measurements. The present study demonstrates that fully 3D numerical models are capable of simulating the primary flow pattern in a strongly curved channel with the presence of a negatively buoyant jet. The comparison also shows that the k-omega SST model can satisfactorily predict some of the smaller flow features in bend flow, such as the inner bank circulation cell and the overall form of the vorticity distributions. It was found that the flow distribution and the strength of secondary flow vary due to the interaction between the jet mixing behavior and the secondary flow in the channel bend. The presence of negatively buoyant jets attenuated the development of the outer bank cell as salinity increased. In the inner bank region, flow separation was strengthened by the participation of the negatively buoyant jets.

Augmentation of Heat Transfer in a Circular Channel with Inline and Staggered Baffles

This numerical investigation presents the effects of the position of baffles in the shape of a circle’s segment placed inside a circular channel to improve the thermal and flow performance of a solar air heater. Three different baffles’ positions with Reynolds number varying between 10,000 to 50,000 were investigated computationally. The k-omega SST model was used for solving the governing equations. Air was taken as the working fluid. Three pitch ratios (Y = 3, 4, and 5) were considered, while the height of the baffles remained fixed. The result showed an enhancement in Nusselt number, friction factor, j-factor, and thermal performance factor. Staggered exit-length baffles showed maximum enhancement in heat transfer and pressure drop, while inline inlet-length baffles showed the least enhancement. For a pitch ratio of Y = 3.0, the enhancement in all parameters was the highest, while for Y = 5.0, the enhancement in all parameters was the least. The highest thermal performance factor of 1.6 was found for SEL at Y = 3.0.

Simulation and Validation of CFD turbulent airflow at pedestrian level using 3D ultrasonic anemometer in the controlled urban area “Sense-City”

Computational Fluid Dynamics (CFD) is increasingly used to describe the airflow in urban environments. In this study, we aim to simulate and to validate the airflow in a full-scale urban area submitted to controlled climatic conditions through an impressive climatic chamber. This non-standard equipment named “Sense-City” is made up of a 400 m2 realistic district with buildings and street layout. The Sense-City urban area is highly instrumented, which allows notably the validation of physical models and simulations and the test of innovative urban solutions. Using a URANS model, with the k-omega SST model as the closure model, a CFD analysis is made on the turbulent airflow in Sense-City at a reasonable computational cost. The simulated velocities and turbulent kinetic energy are compared with measurements collected at a pedestrian level using a 3D ultrasonic anemometer. We show that the numerical simulations correctly predict the flow direction and flow characteristics such as regions of near-zero velocity. Geometry simplifications, uncertainties on the boundary conditions and the use of a coarse mesh and time discretization to fulfill operational purposes have led to a Root Mean Square Error (RMSE) score of 0.26 m/s on the velocity magnitude.