High Turbulence Intensity Research Articles

Effects of turbulence intensity and integral length scale on the surface pressure on a rectangular 5:1 cylinder

This paper presents the LES of the turbulence effects on the flow over a two-dimensional rectangular 5:1 cylinder. The homogeneous and isotropic inflow turbulence fields are generated by the Iterating-Reshaping based Consistent Discrete Random Flow Generation (IRCDRFG) technique. A total of nine numerical cases with the inflow turbulence intensity equal to 5.0%, 7.5%, and 10.0%, and the ratio of integral length scale to cylinder depth (L/D) equal to 1, 2, and 4 are simulated respectively. The LES tool well reproduces both the instantaneous and time-averaged flow structures in the near cylinder area. The sizes of the mean separation bubble on the cylinder surface are noticed to decrease and be closer to the leading edge with the higher turbulence intensity and the lower L/D, which contributes to the shift of location with the minimum mean value and that with the maximum RMS value of the surface pressure. Besides, the consistent drag force results with different inflow conditions are obtained, which have strong connection with the following correlation results of surface pressure in different directions. Additionally, the PSD analysis shows that the low-frequency energy of the fluctuating pressure is significantly increased with the presence of the incoming large-scale turbulence field.

A Direct Numerical Simulation Assessment of Turbulent Burning Velocity Parametrizations for Non-Unity Lewis Numbers

The predictions of turbulent burning velocity parameterizations for non-unity Lewis number flames have been assessed based on a single-step chemistry Direct Numerical Simulation (DNS) database of premixed Bunsen flames for different values of characteristic Lewis numbers ranging from 0.34 to 1.2. It has been found that the definition of the turbulent burning velocity is strongly dependent on the choice of projected flame brush area in the Bunsen burner configuration. The highest values of normalized turbulent burning velocity are obtained when the projected flame brush area is evaluated using the area of the isosurface of the Reynolds averaged reaction progress variable of 0.1 out of different options, namely the Favre averaged and Reynolds averaged isosurfaces of reaction progress variable of 0.5 and integral of the gradient of Favre and Reynolds averaged reaction progress variable. Because of the axisymmetric nature of the mean flame brush, the normalized turbulent burning velocity has been found to decrease as the burned gas side is approached, due to an increase in flame brush area with increasing radius. Most models for turbulent burning velocity provide comparable, reasonably accurate predictions for the unity Lewis number case when the projected flame brush area is evaluated using the isosurface of the Reynolds averaged reaction progress variable of 0.1. However, most of these parameterizations underpredict turbulent burning velocity values for Lewis numbers smaller than unity. A scaling relation has been utilized to extend these parameterizations for non-unity Lewis numbers. These revised parameterizations have been shown to be more successful than the original model expressions. These modified expressions also exhibit small values of L2-norm of the relative error with respect to experimental data from literature for different Lewis numbers, higher turbulence intensity and thermodynamic pressure levels.

Heat Transfer Coefficient and Adiabatic Wall Temperature Measurements on High-Pressure Turbine Nozzle Guide Vanes With Representative Inlet Swirl and Temperature Distortions

Abstract Combustor exit conditions in modern gas turbines are generally characterized by significant temperature distortions and swirl degree, which in turn is responsible for very high turbulence intensities. These distortions have become particularly important with the introduction of lean combustion, as a mean to control NOx pollutant emissions. For this reason, combustor–turbine interaction studies have recently gained a lot of importance. Past studies have focused on the description of the effects of turbulence, swirl degree, and temperature distortions on the behavior of the high-pressure stages of the turbine, both considering them as separated aspects and accounting for their combined impact. Aspects like pressure losses, hot streaks migration, and film-cooling behavior have been widely investigated. Even if some studies have focused on the characterization of the heat transfer coefficient (HTC) on the nozzle guide vane external surface, none of them have addressed this aspect from a purely experimental point of view. Indeed, when inlet conditions are characterized by both swirl and temperature distortions, they represent a severe challenge for the commonly adopted measurement techniques. The work presented in this paper was carried out on a non-reactive, annular, three-sector test rig made by a non-reactive combustor simulator and a nozzle guide vane cascade; it is able to create a representative combustor outflow, characterized by all the flow characteristics described before. A novel experimental approach, which was developed in a previous work, was exploited to experimentally retrieve the heat transfer coefficient and the adiabatic wall temperature distributions on a non-cooled nozzle guide vane. Temperature measurements on the cascade inlet and outlet planes were also used to provide boundary conditions and achieve a better understanding of the investigated phenomena. The results allowed to evidence the effect of the inlet swirl on the heat transfer coefficient distribution, as well as the evolution of the temperature distribution on the vane surface moving through the cascade, constituting the first attempt to evaluate these aspects from a purely experimental point of view.

A case study of wind turbine rotor over-speed fault diagnosis using combination of SCADA data, vibration analyses and field inspection

Reduction of wind turbines down times is paramount in wind energy develop- ment and cost effectiveness. Such reductions require early fault detection and more importantly, a comprehensive Root Cause Analysis (RCA) to completely fix at risk component(s). Given the complexity of wind turbines (and their con– trol systems) and the chaotic environment in which they operate, such a goal is difficult to obtain in many cases. This paper presents a successful case study that combines data analysis approaches with deep knowledge of wind turbine operation. It is shown how occurrence of over speed faults which normally hap- pen during gusty winds (i.e. high turbulent intensity) can be due to a bearing slipping issue in the gearbox. An integrated high-frequency vibration analyses, gearbox oil particle count, and 1 Hz operational and meteorological data min- ing approach, in addition to site inspection create a very suitable path towards establishment of a reliable operational strategy and repairs in a timely manner. A financial and operation strategy that demonstrates the proposed short- and long-term solutions is also presented.

Numerical study on the effect of turbulence intensity on the separation behavior of magnetic particles under different magnet arrangements

ABSTRACT The crucial problem of magnetic separation is improving the ability to capture magnetic particles in complex fluid flow. This paper develops a numerical model of an open gradient magnetic separation (OGMS) system to study the capture process of magnetic particles in laminar flow and turbulent flow under four kinds of magnet arrangement modes. The particle capture efficiencies of four inlet turbulence intensities are analyzed and compared with that of laminar flow. The results show that the particle capture efficiency of turbulent flow is higher than that of laminar flow under the same inlet velocity distribution in a small Stokes number. Moreover, the particle capture efficiency of high turbulence intensity is significantly higher than that of low turbulence intensity. The results also indicate that the axial alternating arrangement (AAA) can provide a stronger magnetic field, which is beneficial to particle capture.

Numerical and experimental assessment of the water discharge segment in a pumped-storage power station

The discharge channel of the pumped storage power station has bidirectional flow characteristics. When the water flow from the conveyance tunnel diffuses to the reservoir through the discharge channel, the instantaneous velocity at the trash rack section varies greatly, and the fluctuation value exceeds twice the time-averaged value. As previously reported, only the time-averaged velocity parameter cannot fully represent instant flow effects. In this paper, the improved delayed detached eddy simulation (IDDES) is proposed to numerically simulate the velocity field inside the discharge channel. The laser doppler velocimetry (LDV) is used to measure the velocity along the segment inside the discharge channel. It is found that the instantaneous velocity of the trash rack section varies significantly with time, the fluctuating velocity is close to or even more than twice the time-averaged velocity, and the average turbulence intensity is 0.72. The probability density distribution of fluctuating velocity is found to be close to normal distribution. During the flow from the conveyance tunnel segment to the reservoir through the diffusion segment and the adjustment segment, the turbulence intensity initially increases and then decreases along the process. The fluctuating velocity in the conveyance tunnel segment is minor, and the average turbulence intensity is 0.27. The turbulence intensity of water flow at the diffusion segment increases by 4.55 times to 1.23, which explains the large fluctuation velocity and high turbulence intensity at the trash rack section. The research results could help to understand the flow characteristics of the discharge channel, optimize the shape parameters of the discharge channel, and analyze the cause of fatigue failure of the trash rack.

A direct numerical simulation study of the dilution tolerance of propane combustion under spark-ignition engine conditions

Modern spark ignition internal combustion (IC) engines rely on highly diluted fuel-air mixtures to achieve high brake thermal efficiencies. To support this, new engine designs have introduced high stroke-to-bore ratios and cylinder head designs that promote high tumble flow and turbulence intensities. However, mixture dilution through exhaust gas recirculation (EGR) is limited by combustion instabilities manifested in the form of cycle-to-cycle variability. Propane has been observed to have superior EGR dilution tolerance than gasoline, which makes it a very competitive low-carbon fuel for the new IC engines without sacrificing efficiency. Two-dimensional direct numerical simulations (DNS) are performed with detailed chemistry to study and contrast the effect of turbulence intensity and dilution on propane and iso-octane premixed flames at high pressure conditions similar to those in-cylinder. A new reduced mechanism for propane consisting of 53 transported species and 17 quasi-steady state species is developed based on a previously published mechanism and used in these simulations. Three levels of turbulence intensity and two levels of exhaust gas dilution are chosen based on conditions relevant to IC engine operation. The DNS results are analyzed based on the evolution of the flame surface area and the statistics of its driving terms, which are found to be similar for both fuels when there is no dilution but considerably different under high dilution. The analysis of the DNS data provides fundamental insights into the underlying mechanisms for improved stability under dilution.

Unsteady characteristics of lubricating oil in thrust bearing tank under different rotational speeds in pumped storage power station

Oil mist accumulation and leakage in hydropower generator bearings are common. However, they undermine not only the operational safety and stability of the generator but also the balance of living organisms downstream of rivers. Most studies on oil flow patterns are based on steady flow field simulations, which may result in potential errors particularly due to initial and unforeseen velocity increases. The unsteady multiphase flow numerical simulation method was used to calculate the motion behavior of lubricating oil in thrust bearing tanks at different rotating speeds. The variations in lubricating oil flow, pressure distribution characteristics, velocity, temperature, oil level, and vorticity at different times were obtained. The results show that the flow and pressure field distributions in an oil tank are mainly affected by the rotation of the thrust head and mirror plate and by the cooling circulation method, respectively. The lubricating oil motion becomes a function of gravity, centrifugal force, and Coriolis force. The oil surface of the volume fraction exhibited periodic motion around the tank and a climbing movement in the oil-retaining ring and oil slinger. The oil-retaining ring performs well at a speed of 500 rpm; however, minute oil droplets and mist rapidly spread in the tank due to violent oil fluctuations. Large vortices with periodic oil surfaces are also observed in regions with high turbulence intensity owing to shear and centrifugal forces. The method and framework presented can be used to optimize the structural design of thrust bearing assembly and reduce the formation of oil and gas mixtures.

Numerical investigation on turbulent mixed convective heat transfer of CO2 in a horizontal miniature tube at supercritical pressure

Mixed convective heat transfer behaviors of turbulent supercritical CO2 (SCO2) flowing in a horizontal miniature tube (di = 2 mm) have been investigated using an in-house code. The computational model adopted was verified against experimentally determined data with a number of operating conditions. The numerical simulations were carried out for pressure of 8 MPa, inlet Reynolds number of 42,521, and wall heat fluxes ranging from 50 to 320 kW m−2. A significant influence of natural convection in horizontal forced turbulent SCO2 flow was discerned, despite the small diameter and high turbulence intensity. The mechanism of mixed convection heat transfer was studied by analyzing the cross-sectional distributions of thermal properties, eddy diffusivity of heat, and secondary flow vectors. The effect of wall heat flux on the non-uniformity in heat transfer was further investigated, covering q/G ratios ranging from 41.67 to 266.67 J kg−1. The regime of heat transfer was identified by comparing the numerical results with the reference Nusselt number calculated using the Dittus-Boelter correlation. A transition from enhanced to deteriorated heat transfer regime, as well as subsequently heat transfer recovery was found with increasing bulk fluid temperature. In addition, the applicability of three existing buoyancy parameters (Buc, BuJ, and BuP) was evaluated. The Petukhov buoyancy criteria agreed best with the simulation results. A new threshold of 6.0 for BuP was determined by performing a comparative study with cases without gravity, above which natural convection would have a considerable influence on forced turbulent heat transfer.

Multi-mode vortex-induced vibration of a long-span bridge under non-uniform flows

An increasing number of modern long-span bridges have been built in complex valley terrain, and the influence of non-uniform flows on the vortex-induced vibrations (VIVs) of bridges should be considered. Therefore, this study investigated the multi-mode VIVs of long-span bridges in both the completed bridge stage and erection stage using a taut-strip model under a uniform flow and two types of non-uniform flow. The non-uniform flows were generated by adjusting spires (both their separation distance and shapes) in wind tunnel. The two non-uniform flows were represented by the linear and parabolic wind velocity flow distributions. The characteristics of the vibration responses and wake flows, mechanism of multi-mode VIVs, and aerodynamic differences between different stages and under different incoming flows were discussed. The results indicated that the occurrence of multi-mode VIVs arose from a wake frequency difference covering two or more modal frequencies owing to variations in the wind profile. The non-uniform flows disturb the regular vortices in the wake due to the variable wind velocity and higher turbulence intensity, leading to suppression of the VIV response.

Numerical Investigation on the Flow underneath a High-Speed Train of Six Coaches Marshalled with Different Bogies

The turbulent flow developed underneath a scaled high-speed train of six coaches featured with both conventional and articulated bogies is investigated based on computational fluid dynamics to establish a contribution to train aerodynamics. The numerical simulations are verified against the experimental measurements. It is found that strong unsteady flow develops around the train, particularly at the regions of nose car, leading bogie, articulated bogies, inter-carriage gaps and tail car, due to flow separations and vortex interactions generated around the geometries. High intensity turbulence appears around the nose, bogies, inter-carriage gaps and tail. Strong vortices and high degree of unsteadiness are produced within the flow between the train underbody and the ground, making the flow developed around the bogie regions of high turbulence. Interaction from the inter-car gap leads to a particularly strong vortical flow around the articulated bogie, causing a large slipstream in the regions close to the airspring and a larger drag. Thus the bogie configuration has a noticeable influence on the train underbody flow.

Numerical study of film cooling effectiveness of a gas turbine blade under variable blowing ratio and turbulence intensity

AbstractThe present paper investigates a three‐dimensional simulation of film cooling on a C3X turbine blade with a single hole at a suction surface. The Reynolds averaged Navier–Stokes approach with k–ε realizable turbulence model and enhanced wall function are used for the numerical simulation. To simulate the jet flows, the length of the jet input approximately 4.5 times the diameter of the hole is added to the geometry so that the jet outlet flow is closer to the actual condition. The density ratio of the cooling flow to the mainstream flow is assumed about 2. The numerical results in four blowing ratios of 0.5, 0.7, 1.0, and 1.4, and at the low turbulence intensity (0.02%), and high turbulence intensity (12%) are extracted and compared for the turbine blade with a single hole. The results show that the turbulence intensity has a dual effect on the film cooling effectiveness and a higher blowing ratio increases the strength of the jet against the cross‐flow. Moreover, it is illustrated that the distribution of the film cooling effectiveness in higher blowing ratios and high turbulence intensity is more uniform than the low blowing ratios and low turbulence intensity.

Use of ultrasound anemometers to study the influence of air currents generated by a sprayer with an electronic control airflow system on foliar coverage. Effect of droplet size

Air assistance and droplet’s characteristics influence on the pesticide deposit and its distribution over the intended target during the spray application process. The present research work is focused on the characterization of the leaf coverage and the overall spray distribution within the whole canopy by combining different settings of air assistance with different nozzles, generating different droplet’s sizes. A new designed air-assisted orchard sprayer was tested, presenting a wireless remote-regulation system to control the airflow rate from the fan by adjusting the blade pitch. In this way, five airflow rates were obtained by combining the gearbox position and the blade pitch. For each one of these five air settings, three droplet sizes (F-Fine, M−Medium, and C-Coarse) were combined and evaluated over a set of artificial apple trees. The evaluation was divided in two parts: a) a whole characterization of the airstream generated by the five air settings was performed for the two sprayer’s sides using a 3D-ultrasonic anemometer and placing the sprayer in front of the artificial trees and b) a coverage tests for each nozzle-air settings combination by using water sensitive papers (WSP) placed at different heights and depths evenly distributed across the canopy, the WSP’s were analyzed by a processing image software. The results demonstrated that ultrasonic anemometers were helpful to characterize the airstream and to analyze its effect on leaf coverage. Experimental data showed that a higher airflow rate and a larger droplet size generated a more homogeneous coverage on both sides of the sprayer. In this line, the turbulence intensity data suggest that larger variations in the air velocities increase the drag coefficient. Which meant that the canopy resistance to the air stream increased. In addition, an increase of turbulence intensity generated higher spray coverage. Therefore, a higher turbulence intensity did not imply a better spray coverage. It was also observed that the droplet size was critical on regulating the airflow influence on spray coverage. Coarse droplet size did not present any relation with the airflow changes. While the fine droplet size presented a high dependency on airflow conditions.

Optical and Numerical Investigation of Flame Propagation in a Heavy Duty Spark Ignited Natural Gas Engine With a Bowl-in-Piston Chamber

Abstract Increasing the natural gas (NG) use in heavy-duty engines is beneficial for reducing greenhouse-gas emissions from power generation and transportation. However, converting compression ignition (CI) engines to NG spark ignition operation can increase methane emissions without expensive aftertreatment, thereby defeating the purpose of utilizing a low carbon fuel. The widely accepted explanation for the low combustion efficiency in such retrofitted engines is the lower laminar flame speed of natural gas. In addition, diesel engine's larger bowl size compared to the traditional gasoline engines increases the flame travel length inside the chamber and extends the combustion duration. Optical measurements in this study suggested a fast-propagating flame developed even at extremely lean operation. A three-dimensional numerical simulation showed that the squish region of the bowl-in-piston chamber generated a high turbulence intensity inside the bowl. However, the flame propagation speed reduced by 55% when transiting from the bowl to the squish region, due to the large decrease in turbulence intensity inside the squish region. Moreover, the squish volume trapped an important fuel fraction, which experienced a slow and inefficient burning process during the expansion stroke. This resulted in increased methane emissions and reduced combustion efficiency. Overall, it was the specifics of the combustion inside a bowl-in-piston chamber not the methane's slow laminar flame speed that contributed to the low methane combustion efficiency for the retrofitted engine. The results suggest that optimizing the chamber shape is paramount to boost engine efficiency and decrease its emissions.

Effects of free-stream turbulence on the near wake flow and aerodynamic forces of a square cylinder

Despite a large volume of preceding studies on the effects of free-stream turbulence (FST) on the aerodynamic forces of a square cylinder, limited knowledge is available regarding the evolutionary characteristics of flow with the change of turbulence integral length scale and turbulence intensity. This paper presents a systematic experimental study of the near wake flow and aerodynamic forces of a two-dimensional (2D) square cylinder in grid-generated turbulent flows and smooth flow. Particular attention is devoted to exploring the respective influences of turbulence integral length scale and turbulence intensity. The effect of FST with a higher-level turbulence intensity than previous investigations is also investigated. The near wake flow and aerodynamic forces are studied by particle image velocimetry (PIV) and synchronous pressure measurements. The proper orthogonal decomposition (POD) analysis is also performed to further understand the effects of FST on the coherent structures of fluctuating pressures around the square cylinder. The results show that the FST with a larger turbulence integral length scale and higher turbulence intensity can cause an increase in the vortex formation length, weaker vortex shedding process and interactions between the oppositely signed vortex rows. These changes lead to significant reductions in Reynolds normal and shear stresses, turbulence kinetic energy and fluctuating lift forces. A completely different flow pattern featured with flow reattachment and reseparation is interestingly captured on the square cylinder in a highly turbulent flow with a turbulence intensity of 20%. This flow pattern results in an obvious mean pressure recovery on the side surfaces and much larger pressure fluctuations near the leading edges and fluctuating torsional moments compared to those in smooth flow and turbulent flows with lower turbulence intensities.

Fate of dissolved inorganic nitrogen in turbulent rivers: The critical role of dissolved oxygen levels

Dissolved inorganic nitrogen (DIN) is considered the main factor that induces eutrophication in water, and is readily influenced by hydrodynamic activities. In this study, a 4-year field investigation of nitrogen dynamics in a turbulent river was conducted, and a laboratory study was performed in the approximately homogeneous turbulence simulation system to investigate potential mechanisms involved in DIN transformation under turbulence. The field investigation revealed that, contrary to NO-3 dynamics, the NH+4 concentrations in water were lower in flood seasons than in drought seasons. Further laboratory results demonstrated that limitation of dissolved oxygen (DO) caused inactive nitrification and active denitrification in static river sediment. In contrast, the increased DO levels in turbulent river intensified the mineralization of organic nitrogen in sediment; moreover, ammonification and nitrification were activated, while denitrification was first activated and then depressed. Turbulence therefore decreased NH+4 and NO-2 concentrations, but increased NO-3 and total DIN concentrations in the overlying water, causing the total DIN to increase from 0.4 mg/L to maximum of 1.0 and 1.7 mg/L at low and high turbulence, respectively. The DIN was maintained at 0.7 and 1.0 mg/L after the 30-day incubation under low and high turbulence intensities (ε) of 3.4 × 10−4 and 7.4 × 10−2 m2/s3, respectively. These results highlight the critical role of DO in DIN budgets under hydrodynamic turbulence, and provide new insights into the DIN transport and transformation mechanisms in turbulent rivers.

Influence of Atmospheric Turbulence on Wind Turbine’s Rotor Teeter Dynamics

Wind energy is one of the fastest-growing sources of renewable energy. The mass and the cost of a wind turbine affect the cost of energy. A two-bladed wind turbine costs less than the most popular three-bladed wind turbine since it has one blade less. Although the less cost, the rotor dynamics of a two-bladed turbine limits its use. Due to the unbalanced loads of a two-bladed turbine, the rotor has an extra system for the rotor teeter. In this work, the rotor teeter dynamics are investigated under the influence of atmospheric turbulence. The AWT-27 two-bladed wind turbine is simulated for two different turbulence intensities of the values 10% and 50% turbulence at a mean wind speed of 12 m/s. The higher turbulence intensity indicates a 400% increase in the rotor mean teeter deflection. The rotor teeter mean velocity and acceleration increase as well by the ratios of 160% and 660% respectively. This massive increase in the teeter dynamics affects the fatigue loading of the rotor and hence reduces the lifetime of the wind turbine.

Interaction of flow turbulence and nitrogen nutrients on the growth of Scenedesmu Quadricanda

The effects of hydrodynamics and nutrients on algae growth have received considerable attention. In this study, the research of the interaction of flow turbulence and nitrogen nutrients on the growth of Scenedesmu Quadricanda was investigated. Scenedesmu Quadricanda was cultured under different turbulence intensities and nutrients conditions. Furthermore, the improved logistic model was used to unify the effect of nutrient flux and hydraulic diffusion flux on algal growth. Algae cell integrity, chlorophyll-a concentration, and polysaccharide content were determined to represent the algal growth. Results showed that the high turbulence intensity (turbulent energy dissipation rate 3.25 × 10−3 fm2/s3–8.93 × 10−3 m2/s3) and total nitrogen (TN) concentration increased algal density and chlorophyll-a. However, there was no significant correlation between the concentration of TN and turbulence intensity and the growth rate of algae. The polysaccharide determination results showed that the increase of TN concentration would reduce extracellular polymeric substances (EPS), which would reduce the aggregation of algal cells. Moreover, high turbulence intensity promoted the production and accumulation of intracellular polysaccharides (IPS) and inhibited the secretion of EPS. The analysis of the improved logistic model verified that the increase of both TN concentration and turbulent intensity increased the production of chlorophyll-a, maximum biomass, and the intrinsic rate of increase. The findings indicate that flow turbulence and nitrogen nutrients have a synergistic effect on algal growth, chlorophyll-a content, polysaccharide secretion, and nutrient uptake, which can foster further investigations on the control of algal bloom and harvest of microalgae biomass.

The impact of the atmospheric boundary layer on the asymmetric wake profile: A bivariate analysis

The complexities of wind turbine wake evolution necessitate the estimation of turbulence and horizontal and vertical wake expansion. The standard approach for developing a Gaussian wake model based on Gaussian similarity assumes that the turbine wake profile is symmetric around the hub height and approximates a normal distribution. The impacts of Atmospheric Boundary Layer (ABL) and ground surface roughness cause the wake to deviate from radial symmetry, resulting in a nonsymmetric wake around hub height. We develop the Bivariate Gaussian (B-Gaussian) Model to estimate the nonsymmetrical wind turbine wake expansion due to ABL effects. The B-Gaussian model factors the correlation coefficient (Ω) to compare the horizontal and vertical expansions. The radial profiles result for the B-Gaussian shows a faster wake decay for the unstable ABL because of faster mixing at higher-turbulence intensities, and slower wake decay is due to slower mixing at lower-turbulence intensities for the stable ABL. The radial wake profiles for the unstable ABL transition from vertical elliptical-shaped in the near wake region to double circular-shaped profiles in the far-wake region as Ω decreases. Finally, compared to preceding Gaussian models, the B-Gaussian outperforms previous Gaussian wake models and has the least overall and downstream entrainment errors.

Effects of thermal stratification and turbulent intensity on auto-ignition and combustion mode transition

Hydrogen engines with zero-carbon emissions have recently drawn attention while they often suffer from pre-ignition and knocking issues. It is generally understood that knocking intensity is closely associated with auto-ignition development modes, which are affected by multiple variables and their interactions. In this work, the role of thermal stratification and turbulent intensity in auto-ignition development modes were numerically investigated, addressing combustion mode transition. Representative end-gas auto-ignition conditions were employed for hydrogen/air mixtures. Meanwhile, the support vector machine (SVM) algorithm was first applied in the predictions of detonation development. The results show that thermal stratification plays a decisive role in auto-ignition development modes, and supersonic auto-ignition deflagration, direct detonation, and subsonic auto-ignition deflagration can be sequentially observed with the increase of temperature gradient. However, affected by multiple hotspot interactions and pressure wave disturbance, deflagration to detonation transition arising from secondary hotspot auto-ignition becomes prevalent. It is noted that, unlike direct detonation that evolves at the entire circumferential directions, deflagration to detonation transition develops only along certain directions with intermediate temperature gradient, and such gradient steepness is much higher than previous results based on simplified models. When turbulence fluctuation is introduced, the distribution of thermal stratification is modified significantly and the local temperature field is broken into multiple smaller regions filled with mixed temperature gradients. Consequently, deflagration to detonation transition is promoted both temporally and spatially, especially at high turbulence intensity. Besides, the SVM classifier is trained using the above results of numerical simulations for detonation predictions. A diagram consisting of the most energetic length scale and temperature fluctuation is proposed, which shows good performance in predicting detonation occurrence. The present study demonstrates that thermal stratification and turbulent intensity can significantly affect the transition behavior of auto-ignition development mode, and machine learning approaches have the potential to achieve good predictions for detonation occurrence in multiple physical fields.