Three-dimensional Computational Fluid Dynamics Simulations Research Articles

CFD approach for pumped-recirculation mixing strategy in wastewater treatment: Minimizing power consumption, enhancing resource recovery in commercial anaerobic digester

In present study, three-dimensional computational fluid dynamics (CFD) simulation of a real and working sewage treatment plant (anaerobic reactor) was performed by selecting standard k-ε turbulence model to define the turbulent flow. Three geometrical configurations were created for simulation by varying the plane of inlets. Providing two bottom suctions, two bottom inlets and two to four top inlets, and depending upon the configuration employed, recirculation was incorporated. Four different cases of mass flow rates (40, 60, 70, 80 Kg/s) for each suction was studied for first configuration. The results showed that suction mass flow rate of 70 Kg/s was most acceptable mass flow rate as it provided complete mixing condition with savings of 14.3 % power compared to mass flow rate of 80 Kg/s. By choosing multiple planes of inlets instead of single plane; suction mass flow rate of 60 Kg/s was observed to be the acceptable mass flow rate as it saved 14.3 % power compared to 70 Kg/s mass flow rate with single plane inlets. Hence, configuration-3 which had two bottom suctions, four top inlets and two bottom inlets in two perpendicular XZ and YZ-planes was considered the best geometrical configuration for mixing with minimum power consumption.

Study on the Transient Injection Rate of Each Nozzle Hole in the Combustion Process of Diesel Engine

Abstract Based on the measured injection rates obtained from the spray momentum experiment, the three-dimensional computational fluid dynamics simulation study on the effect of injection rate from each nozzle hole on spray characteristics and combustion process was conducted for a one-cylinder diesel engine. The simulation model was successfully verified by the data of the experiment. The results show that at the beginning and mid-stages of injection, the nozzles with a higher transient injection rate exhibit higher jet velocity, bigger spray penetration distance, and wider equivalence ratio distribution. Besides, the disturbance induced by fuel injection on their surrounding gas is higher. Due to the difference in injection rates from each nozzle hole in the cylinder, gas–fuel mixtures are non-uniform. In the case of measured injection rates from each nozzle hole, Hole 4 records the highest instantaneous injection rate. This results in the injection of more fuel during ignition delay. More heat generated from thermal chain reactions raises fuel spray temperatures and quicker ignition of mixtures. In the case of uniform simulated injection rate (injection quantity values are the same as in the previous case), more uniform flow fields and stronger small swirl motions were generated that enhance fuel atomization and mixture formations. At the later stages of injection and combustion, quicker diesel fuel burning rate with a centralized exothermic reaction process occurs due to in-cylinder uniform fuel distribution and air motion. In the case of simulating uniform injection rate from three holes and non-injection from one (same injection quantity values as previous cases), uneven fuel distribution that occurs in the cylinder will result in poor mixture formations and subsequently poor combustion, and more afterburning will occur.

Numerical Investigation of Remote Ignition in Shock Tubes

Highly resolved two- and three-dimensional computational fluid dynamics (CFD) simulations are presented for shock-tube experiments containing hydrogen/oxygen (H2/O2) mixtures, to investigate mechanisms leading to remote ignition. The results of the reactive cases are compared against experimental results from Meyer and Oppenheim (Proc Combust Inst 13(1): 1153–1164, 1971. https://doi.org/10.1016/s0082-0784(71)80112-1) and Hanson et al. (Combust Flame 160(9): 1550–1558, 2013. https://doi.org/10.1016/j.combustflame.2013.03.026). The results of the non-reactive case are compared against shock tube experiments, recently carried out in Duisburg and Texas. The computational domain covers the end-wall region of the shock tube and applies high order numerics featuring an all-speed approximate Riemann scheme, combined with a 5th order interpolation scheme. Direct chemistry is employed using detailed reaction mechanisms with 11 species and up to 40 reactions, on a grid with up to 2.2 billion cells. Additional two-dimensional simulations are performed for non-reactive conditions to validate the treatment of boundary-layer effects at the inlet of the computational domain. The computational domain covers a region at the end part of the shock tube. The ignition process is analyzed by fields of localized, expected ignition times. Instantaneous fields of temperature, pressure, entropy, and dissipation rate are presented to explain the flow dynamics, specifically in the case of a bifurcated reflected shock. In all cases regions with locally increased temperatures were observed, reducing the local ignition-delay time in areas away from the end wall significantly, thus compensating for the late compression by the reflected shock and therefore leading for first ignition at a remote location, i.e., away from the end wall where the ignition would occur under ideal conditions. In cases without a bifurcated reflected shock, the temperature increase results from shock attenuation. In cases with a bifurcated reflected shock, the formation of a second normal shock and shear near the slip line is found to be crucial for the remote ignition to take place. Overall, the two- and three-dimensional simulations were found to qualitatively explain the occurrence of remote ignition and to be quantitatively correct, implying that they include the correct physics.

Numerical Study of Flow and Heat Transfer Characteristics on Flat Fin with Staggered Tube Arrangement in Transitional Turbulent Flow

Three-dimensional computational fluid dynamic simulations were conducted for flow and heat transfer characteristics around flat fin with staggered tube arrangement to fulfill the cooling performance of the tube fin heat exchanger. Fins are generally used to increase the heat transfer area, so the fin material has a high impact on the heat transfer rate. The material wall fin and tube were changed in three steps: aluminum, steel, copper with two different velocities of 8 and 15 m/s flowing between fins. The geometry of the flat fin and tube used staggered tube arrangement using transversal spacing, ST, of 11.8 mm, longitudinal spacing, SL, of 22.2 mm, and flow depth 66.6 mm. GAMBIT 2.6 software was used to meshing the geometry, and FLUENT 18.0 was implemented to simulate flow and heat transfer. The results show that the fin with copper material has a more uniform temperature distribution along the fin than the other materials. This indicates that the copper material has a higher heat transfer rate compared to aluminum and steel. Furthermore, increasing velocity will make the separation point formation farther behind the tube and decrease the recirculation zone. Moreover, 8 m/s has a lower outlet temperature than 15 m/s. As a result, 8 m/s and copper material have the highest effectiveness of 16.47 and efficiency of 88.35 %. The use of copper and aluminum as fin material will also have the relatively same performance in the heat exchanger.

Investigation of Multistage Combustion Inside a Heavy-Duty Natural-Gas Spark-Ignition Engine Using Three-Dimensional Computational Fluid Dynamics Simulations and the Wiebe-Function Combustion Model

Abstract The conversion of existing heavy-duty diesel engines to lean natural-gas (NG) spark ignition can be achieved by replacing the diesel injector with a spark plug and fumigating the NG into the intake manifold. While the original fast-burn diesel chamber will offset the lower NG flame speed, it will result in a two-stage combustion process (a stage inside and another outside the bowl). However, experimental data at more advanced spark timing, equivalence ratio of 0.8, and mean piston speed of 6.5 m/s suggested an additional combustion stage (i.e., three combustion stages). A three-dimensional (3D) computational fluid dynamics (CFD) simulation and a zero-dimensional triple Wiebe-function model were used to better understand the phenomena. While 78% fuel burned inside the bowl, burning rate reduced significantly when the flame approached the squish entrance and the bowl bottom. Moreover, the triple Wiebe-function indicated that the burn inside the squish was also divided into two separate combustion stages, due to the particularities of in-cylinder flow before and after top dead center. The first stage was fast and took place inside the compression stroke. The second took place in the expansion stroke and produced a short-lived increase in the burning rate, probably due to the increasing squish height during the expansion stroke and the increased combustion-induced turbulence, hence the third heat-release peak. Overall, these findings support the need for further investigations of combustion characteristics in such converted engines, to benefit their efficiency and emissions.

CFD simulation of corium flow through an end fitting of a pressurised heavy water reactor

This study investigates the numerical simulation of corium relocation during a hypothetical accident, focussing on the modelling of core melt, its solidification and redistribution. This paper presents results of a high-fidelity computational fluid dynamics (CFD) simulation of an ex-vessel corium flow through a horizontal end fitting geometry, a pressurized heavy water reactor (PHWR) relevant configuration. The objective of this work is to simulate the corium progression within the end fitting of a PHWR to assess the suitability of two viscosity models using a CFD code; Siemens STAR-CCM+. This first-of-a-kind, exploratory study demonstrates the potential of the existing CFD models for modeling corium spreading within the end fitting geometry. Mixed convection turbulent corium flow in the end-fitting has been modeled in the three-dimensional CFD simulations. Transient simulations using the volume-of-fluid approach and melting-solidification models were performed to predict the corium melt spread within the end fitting. Two suitable temperature-dependant models for corium viscosity were tested, one being empirical viscosity model by Ramacciotti and the other mechanistic viscosity model available in STAR-CCM+. The unique flow features such as the Rayleigh-Bénard instability could be observed using the Ramacciotti empirical viscosity modelling approach. A qualitative assessment of the results was undertaken, showing that the CFD model has predicted the corium spreading behaviour that is consistent with the expectations based on engineering judgement and experience. This kind of simulations are expected to serve as a guidance in planning lab-scale experiments and aid in CFD-informed modeling improvements for integral (or system) codes going forward.

Hydrodynamics of tandem flapping pectoral fins with varying stroke phase offsets

We show how phasing between tandem bioinspired fins flapping at high-stroke amplitudes modulates rear fin thrust production and wake characteristics. Load cell thrust measurements show that the rear fin generates 25% more thrust than the front fin when it lags the latter by a quarter cycle, and performs 8% worse when it leads the front fin by the same amount. The flow interactions between the fins responsible for these observations are analyzed using two-dimensional particle image velocimetry measurements and three-dimensional computational fluid dynamics simulations. Distributions of velocity elucidate variations in the effective flow induced on the rear fin for different phase offsets. Vortex structure interactions and particle rakes reveal the contributions of the leading- and trailing-edge vortices shed by the front fin in modulating the suction at the rear fin leading edge. Furthermore, the wake structure far downstream of the fins changes in its coherence, axial and radial extents for the different phase offsets. These findings are relevant for the design and performance optimization of various unmanned underwater vehicles that utilize such tandem systems.

Considering Two-Phase Flow in Three-Dimensional Computational Fluid Dynamics Simulations of Proton Exchange Membrane Water Electrolysis Devices

Simulations help us to understand electrochemical systems and reduce the cost of research and development. Modeling proton exchange membrane water electrolysis (PEMWE) devices, especially in order to screen components such as the flow field and PTL, is challenging due to two-phase flow. However, this offers an opportunity to advance computational fluid dynamics (CFD) modeling of these systems. We employed CFD to simulate a liquid-fed, straight-channel PEMWE device, exploring the viability of an isothermal continuum model. The results agree with observations of two-phase flow recorded in literature.

Investigation of flow characteristics in a single screw expander: A numerical approach

In this paper, a three-dimensional computational fluid dynamics (CFD) simulation is carried out to investigate the performance of a single screw expander by linking its behavior with the dynamic features of the flow field. To study this aspect and other phenomena that can affect expander operation in an actual Organic Rankine Cycle (ORC) system, in this work an approach based on the dynamic analysis of the machine operation with the use of both the overset mesh strategy and real gas modeling is showed. The expansion process is investigated to analyze both flow characteristics and machine performance during on-design operating conditions. The model aims to investigate the SSE operation without the effects of the sealing process of the lubricant oil to highlight the effects of the fluid expansion on the SSE operation. Pressure and flow patterns which resulted from this analysis depend on the design characteristics of the machine, operating conditions, and working fluid properties. The resulting output torque and isentropic efficiency are coherent with the simulated operating conditions.

High spatial endothelial shear stress gradient independently predicts site of acute coronary plaque rupture and erosion.

To investigate local haemodynamics in the setting of acute coronary plaque rupture and erosion. Intracoronary optical coherence tomography performed in 37 patients with acute coronary syndromes caused by plaque rupture (n = 19) or plaque erosion (n = 18) was used for three-dimensional reconstruction and computational fluid dynamics simulation. Endothelial shear stress (ESS), spatial ESS gradient (ESSG), and oscillatory shear index (OSI) were compared between plaque rupture and erosion through mixed-effects logistic regression. Lipid, calcium, macrophages, layered plaque, and cholesterol crystals were also analysed. By multivariable analysis, only high ESSG [odds ratio (OR) 5.29, 95% confidence interval (CI) 2.57-10.89, P < 0.001], lipid (OR 12.98, 95% CI 6.57-25.67, P < 0.001), and layered plaque (OR 3.17, 95% CI 1.82-5.50, P < 0.001) were independently associated with plaque rupture. High ESSG (OR 13.28, 95% CI 6.88-25.64, P < 0.001), ESS (OR 2.70, 95% CI 1.34-5.42, P = 0.005), and OSI (OR 2.18, 95% CI 1.33-3.54, P = 0.002) independently associated with plaque erosion. ESSG was higher at rupture sites than erosion sites [median (interquartile range): 5.78 (2.47-21.15) vs. 2.62 (1.44-6.18) Pa/mm, P = 0.009], OSI was higher at erosion sites than rupture sites [1.04 × 10-2 (2.3 × 10-3-4.74 × 10-2) vs. 1.29 × 10-3 (9.39 × 10-5-3.0 × 10-2), P < 0.001], but ESS was similar (P = 0.29). High ESSG is independently associated with plaque rupture while high ESSG, ESS, and OSI associate with plaque erosion. While ESSG is higher at rupture sites than erosion sites, OSI is higher at erosion sites and ESS was similar. These results suggest that ESSG and OSI may play critical roles in acute plaque rupture and erosion, respectively.

Validation of a RANS 3D-CFD Gaseous Emission Model with Space-, Species-, and Cycle-Resolved Measurements from an SI DI Engine

Reynolds-averaged Navier–Stokes (RANS) three-dimensional (3D) computational fluid dynamics (CFD) simulations of gaseous emissions from combustion engines are very demanding due to the complex geometry, the emissions formation mechanisms, and the transient processes inside the cylinders. The validation of emission simulation is challenging because of modeling simplifications, fundamental differences from reality (e.g., fuel surrogates), and difficulty in the comparison with measured emission values, which depend on the measuring position. In this study, detailed gaseous emission data were acquired for a spark ignition (SI) direct-injection (DI) single-cylinder engine (SCE) fueled with a toluene reference fuel (TRF) surrogate to allow precise comparison with simulations. Multiple devices in different sampling locations were used for the measurement of average emission concentration, as well as hydrocarbon (HC) cycle- and species-resolved values. A RANS 3D-CFD methodology to predict gaseous pollutants was developed and validated with this experimental database. For precise validation, the emission comparison was performed in the exact same locations as the pollutants were measured. Additionally, the same surrogate fuel used in the measurements was defined in the simulation. To focus on the emission prediction, the pressure and heat release traces were reproduced by calibrating a G-equation flame propagation model. The differences of simulation results with measurements were within 4% for CO2, while for O2 and NO, the deviations were within 26%. CO emissions were generally overestimated probably because of inaccuracies in mixture formation. For HC emissions, deviations up to 50% were observed possibly due to inexact estimation of the influence of the piston-ring crevice geometry. The reasonable prediction accuracy in the RANS context makes the method a useful framework for the analysis of emissions from SI engines, as well as for mechanism validation under engine relevant conditions.

An Improved Immersed Boundary Method for Simulating Flow Hydrodynamics in Streams with Complex Terrains

Three-dimensional (3D) computational fluid dynamic (CFD) simulations have gained substantial popularity in recent years for stream flow modelling. The complex terrain in streams is usually represented by a 3D mesh conforming to the terrain geometry. Such terrain-conforming meshes are time-consuming to generate. In this work, an immersed boundary method is developed in an existing terrain-conforming CFD model named U2RANS as an alternative, in which terrains are represented implicitly in the Cartesian background mesh. An improved two-layer wall function is proposed in the framework of the k-ε turbulence model, with the aim of producing accurate and smooth wall shear stress distribution and paving the way for future model development on sediment transport and scour modeling. The improvement overcomes the inherent discontinuity and nonlinearity of the two-layer velocity profile, which causes error in the estimation of shear velocity. The new algorithm utilizes a distance control on the image point in immersed boundary method and a modification of velocity prediction in the laminar layer. The improved immersed boundary method is tested with 1D, 2D, and 3D cases, and comparisons with flume experiments show promising results.

Three-dimensional CFD simulation and experimental validation of particle segregation in CFB riser

PurposeThe purpose of this paper is to perform the computational fluid dynamics (CFD) simulation with experimental validation to investigate the particle segregation effect in abrupt and smooth shapes circulating fluidized bed (CFB) risers.Design/methodology/approachThe experimental investigations were carried out in lab-scale CFB systems and the CFD simulations were performed by using commercial software BARRACUDA. Special attention was paid to investigate the gas-particle flow behavior at the top of the riser with three different superficial velocities, namely, 4, 6 and 7.7 m/s. Here, a CFD-based noble simulation approach called multi-phase particle in cell (MP-PIC) was used to investigate the effect of traditional drag models (Wen-Yu, Ergun, Wen-Yu-Ergun and Richardson-Davidson-Harrison) on particle flow characteristics in CFB riser.FindingsFindings from the experimentations revealed that the increase in gas velocity leads to decrease the mixing index inside the riser. Moreover, the solid holdup found more in abrupt riser than smooth riser at the constant gas velocity. Despite the more experimental investigations, the findings with CFD simulations revealed that the MP-PIC approach, which was combined with different drag models could be more effective for the practical (industrial) design of CFB riser. Well agreement was found between the simulation and experimental outputs. The simulation work was compared with experimental data, which shows the good agreement (<4%).Originality/valueThe experimental and simulation study performed in this research study constitutes an easy-to-use with different drag coefficient. The proposed MP-PIC model is more effective for large particles fluidized bed, which can be helpful for further research on industrial gas-particle fluidized bed reactors. This study is expected to give throughout the analysis of CFB hydrodynamics with further exploration of overall fluidization.

Computational fluid dynamics studies on the induction period of crude oil fouling in a heat exchanger tube

Fouling impairs the hydrodynamic and thermodynamic performance of crude oil pre-heat train exchangers in refineries. The very early stage of fouling, so-called induction period, containing inappreciable levels of fouling is poorly understood due to a lack of experimental data. To better understand this period and seek potential fouling mitigation strategies, a model is implemented in a computational fluid dynamics (CFD) code, which accounts for chemical reaction- and precipitation-driven fouling, turbulent flow, and ageing in a heat exchanger tube. Three-dimensional CFD simulations are carried out under various operating conditions to gauge the relative importance of the two fouling mechanisms, and provide direct predictions of deposition rates. The interaction between chemical and precipitation fouling processes is characterized using an “interference factor”, introduced to evaluate the extent of suppression of the chemical mechanism and enhancement of precipitation. The results provide insights into the interaction of key mechanisms during the induction period (up to three seconds flow time) of fouling in crude oil heat exchangers and possible strategies for prolonging this near nil-fouling period.

CFD Investigation of MHTGR Natural Circulation and Decay Heat Removal in P-LOFC Accident

One of the key postulated accidents in a high-temperature gas-cooled reactor (HTGR) is the pressurized loss of forced circulation (P-LOFC) of the primary loop, which can be triggered by its primary helium circulator trip or turbine trip. If the reactor shutdown cooling system (SCS) fails during a P-LOFC accident, part of the reactor decay heat is absorbed by the reactor core materials and the rest removed by the reactor cavity cooling system (RCCS). In the extended period of P-LOFC accident compound with SCS failure, the core decay heat is supposed to be removed by conduction, natural circulation, convection, and radiation. A three-dimensional (3-D) computational fluid dynamics (CFD) simulation was performed in this research to study the long-term heat removal mechanisms in the General Atomics’ Modular High Temperature Gas-cooled Reactor (MHTGR) design in a P-LOFC accident. The reactor core temperature distribution and flow field were obtained at different decay power levels. The sensitivity of the natural circulation flow to the bypass gap width was investigated. The natural circulation flow intensity is relatively weak but very sensitive to the width of the bypass gaps.

Assessment of scale-adaptive turbulence models for volute-type centrifugal pumps at part load operation

Unsteady three-dimensional (3D) computational fluid dynamics (CFD) simulations are conducted with the open-source software OpenFOAM to assess the scale-adaptive k-ω-SST-SAS turbulence model (SAS) on a radial, volute-type centrifugal pump at part load operation which is characterized by high unsteadiness and flow separation. SAS results are compared to spatially high resolved and ensemble-averaged flow measurement data in terms of flow angle and turbulence intensity (TI) in the rotor–stator interaction region. Differences to simulation results obtained with the statistical state-of-the-art k-ω-SST turbulence model (SST) are highlighted. The flow angle is predicted with a reasonable agreement to measurement data by both, SST and SAS models. In the highly transient flow of strong rotor–stator interaction near the volute tongue, SST results show a significant overprediction of measured TI while the SAS model yields a considerably better agreement to measurement data even with a typical URANS grid resolution. A grid refinement does not further improve the agreement to measurement data. An in-depth analysis of the SAS model on separated flow, i.e., periodic hill test case, together with a large eddy simulation (LES) reference solution is performed and reveals that with successive grid refinement, in contrast to LES, the SAS model in its present form of Egorov and Menter (2008) does not resolve a successively larger portion of the turbulence spectrum, and the modeled part is not successively reduced. For that purpose, a re-calibration or even a re-formulation of the scale-adaption source term in the transport equation of the turbulent dissipation rate may be indispensable, which will be the subject of future studies.

Three-dimensional CFD simulation of pattern formation in a shallow packed-bed reactor for oxidative coupling of methane

Stationary or spatio-temporal patterns of catalytic reactions can cause operating challenges in shallow packed-bed reactors with large diameter to depth aspect ratios. Partial ignition patterns were observed in a pilot scale oxidative coupling of methane (OCM) reactor operating on the ignited branch near the extinction point. A three-dimensional (3D) computational fluid dynamics (CFD) model was developed for the down-flow shallow packed-bed reactor using porous media approach, with user-defined functions for heterogeneous heat transfer and Langmuir-Hinshelwood form reaction kinetics. The CFD simulation results show that, once the operating condition is nearing or beyond the extinction point (at lower feed temperature or shorter space time), 3D temperature patterns form due to either thermo-flow or thermo-kinetic steady-state multiplicity. The simulations provide insights into the pattern locking mechanism that stems from the non-linear hydrodynamic-kinetic-thermal coupling in the bed. Specific pattern observed in the CFD solution depends on perturbation sources such as unstructured space discretization, local heat loss through refractory walls, variations in catalyst activity or porosity of the packed-bed, as well as non-uniform initial conditions such as preheating, among others. The results indicate that the non-uniformity of bed physical and chemical properties (such as packing porosity, catalyst activity, or thermal properties) can dominate the pattern in practical reactor operation. Based on parametric studies, methods to mitigate pattern formation are recommended, such as more uniform incoming flow, uniform catalyst bed, uniformly preheated bed, higher catalyst activity (operating farther from extinction point), and/or higher bed thermal conductivity.

Numerical and Experimental Study of the Hydrostatic Pressure Correction in Gas Thermometry: A Case in the SPRIGT

Single-pressure refractive index gas thermometry (SPRIGT) is a new type of primary thermometry, which needs an extremely stable working pressure (stability < 4 ppm). In practice, the pressure control system at room temperature is located above the cold resonator at 5 K to 25 K, and a long pressure tube is used to connect them, which entails a hydrostatic pressure correction (HPC). To this end, a three-dimensional (3D) Computational Fluid Dynamics (CFD) simulation model of the pressure tube has been developed and compared with experimental results. First, to verify the simulation results, the helium-4 gas pressure in the center of the resonator was measured using a determination of the refractive index by microwave resonance coupled with the knowledge of the temperature. Results of simulation and experiment showed good agreement. Thereafter, based on this CFD simulation, the non-linear temperature distribution in the vertical pressure tube and the uncertainty caused by this non-linear phenomenon were calculated. After this, the validity of the isothermal assumption to simplify the calculation of the HPC was verified. Finally, the effect of heating on the pressure was studied and its impact found to be negligible. To the best of our knowledge, this is the first time experimental and simulation results have been compared for the HPC. The results are expected to be more generally applicable to the accurate determination of pressure in cryostats.

Segregation behavior of a ternary mixture of titanium dioxide particles in a pseudo two-dimensional fluidized bed

The segregation behavior of a mixture of silica-coated titanium dioxide (TiO2) particles of three different sizes in a pseudo two-dimensional fluidized bed was studied experimentally by the freeze–sieving method and numerically by the multi-fluid model (MFM). Three-dimensional computational fluid dynamics (CFD) simulations were carried out to evaluate the effects of the solid wall boundary conditions on particle segregation in terms of specularity and particle–wall restitution coefficients. The quantitative indexes of segregation tendency and segregation degree were used to determine the axial segregation of the mixture in triangular coordinates. The simulation results revealed that the axial segregation increased with the specularity coefficient, whereas the particle–wall restitution coefficient had a minor effect on axial segregation. Comparison of the simulation results with experimental data showed that the appropriate value of the specularity coefficient used in the CFD model depended on superficial gas velocity. The study of the effects of superficial gas velocity on segregation behavior demonstrated that the greatest segregation was obtained at minimum fluidization velocity and the segregation decreased as the gas velocity gradually increased.

Investigation of heat transfer characteristics of high-altitude intercooler for piston aero-engine based on multi-scale coupling method

Altitude affects the heat exchange capacity of a heat exchanger, which in turn affects the performance of an aircraft with piston engines. In order to study this mechanism of influence and the associated principles, a three-dimensional computational fluid dynamics (CFD) simulation of an intercooler is conducted based on multi-scale coupling. The simulation results have a high degree of coincidence with the test data on ground, with maximum error of no more than 10%. The simulation height is then extended to an altitude of 20 km. It is found that before the aircraft enters the stratosphere, the overall efficiency of the intercooler decreases by 1.98% on average for every 1km increase in the altitude and decreases by 3.26% when the height is above 11 km. The heat transfer capacity of the intercooler is gradually enhanced owing to the gradually increasing temperature difference while atmospheric density is relatively large. After entering the stratosphere, the external temperature stops changing; the density decreases to 1/14 of the value at the ground, and the low density becomes the dominant factor, resulting in reduced heat transfer coefficient. Finally, an empirical formula for the overall heat exchange efficiency of the intercooler as a function of the altitude is proposed.