This study investigates the Conditionally Averaged Structure (CAS) of reactive scalars using 3D Direct Numerical Simulation (DNS) datasets of ultra-lean and near-stoichiometric premixed H_2 -air turbulent flames, with effective Lewis numbers Le≈ 0.5 and 0.8 , and Karlovitz numbers Ka∼𝒪(100) and Ka∼𝒪(1000) , respectively. The CAS unravels how turbulent mixing progressively layers surfaces with curvatures from negative to positive in the neighborhood of the zero curvature location of the reference scalar isosurface, triggering differential and preferential diffusion, amplifying specific chemical pathways. These effects enhance density-weighted local displacement speed, S_d , even at zero curvature for ultra-lean, high Ka , H_2 flames. Irrespective of the reactant equivalence ratio and the chosen isoscalar surface, S_d is enhanced at large negative curvatures from localized flame-flame interactions and is explained by the existing interaction model. CAS at zero-curvature captures the reactant side negatively curved self-interactions and product side positively curved structures. The positive curvature generates enhanced heat release driven by differential diffusion. Investigation of species concentration, their production rates, and detailed chemical pathways within the CAS thus reveals the mechanism underpinning the enhancement of heat release rate at zero curvature locations. The curvature organization within the ultra-lean flame CAS promotes enhanced H_2 diffusion from the negatively curved regions to the positively curved ones downstream. This enhances reactive radical (H, O, OH) production, and their back diffusion to increase H_2 consumption rates, heat release rate and consequently S_d , compared to laminar flames, all at zero-curvature locations of these intensely turbulent flames.

Fundamental questions related to the roles of fuel type, combustion parameters, and turbulence transport interactions in the inception and growth of contrails have remained intractable in remote sensing and in-flight measurements. Consequently, we developed a novel laboratory-scale facility for studying the inception, growth and persistence of contrails for aircraft-relevant conditions. The set of exhaust conditions, generated using an inverted co-flow soot generator at a set of global equivalence ratio for two fuels - ethylene and propane, is supplied to the contrail tunnel which then mixes with an ambient flow emulating long-haul aircraft cruise conditions (20.8kPa and 190K). Detailed soot characterization using a scanning mobility particle sizer and transmission electron microscopy is coupled with measurements of instantaneous and averaged scattering intensities from the generated contrails. The experimental results are complemented by numerical simulations of the contrail tunnel using solutions of the Favre-averaged Navier-Stokes (FANS) equation and a two-equation model for handling particulate matter, including soot and ice. Results show the first experimental snapshots of a contrail cross section, highlighting the interaction of turbulent mixing and microphysical growth scales involved in ice nucleation across the shear layers. As expected, the average scattering intensities of contrails increase with soot number concentrations and water vapor content. Comparisons between ethylene and propane exhausts indicate that the scattering propensity of contrails is more sensitive to exhaust water vapor content than to soot concentrations. Finally, depolarization measurements are used to show asphericity in ice crystal habits. Thus, our study present a unique window into contrail formation, theoretical modeling and simulation.

An experimental investigation in a sector ( 20 deg ) of full-scale annular gas turbine combustor is performed. The sector combustor is optically accessible for the flow and flame visualization of the primary and exit zones of the combustor. The distinctive feature of the experimental setup is that it preserves the geometrical details of an annular combustor that includes the casing, dome and combustor liner. The combustor design features a series of primary and secondary dilution holes with multiple film cooling strips on the outer and inner liner. In the present study, the combustor is operated at inlet Mach numbers of 0.02-0.3 at operating absolute pressures of 1-5 bar. Static pressure measurements are performed at multiple locations in the rig to characterize the pressure drop across the combustor. Two-dimensional particle image velocimetry (PIV) is performed to measure the velocity fields of the primary and exit zones of the combustor simultaneously. The results show the presence of a central recirculation zone (CRZ), high-velocity annular jets, and a pair of dilution jets in the primary zone of the combustor. The steady-state flow structures are invariant of inlet Mach number and pressures. The relationship between the relative pressure drop across the combustor and the combustor inlet condition is obtained. Mass flowrate and momentum flux are calculated for the flow through the swirler, central recirculation zone, the primary dilution jets, and the exit zone. The paper shows how the flow structures in a realistic combustor change with variations in global combustor parameters.

Hydrogen is envisioned to be a key decarbonization solution for fossil fuel-dependent power generation and aviation industries. At present, a significant fraction of the generated electrical power is derived from natural gas. As such, the external energy needed for hydrogen generation, often sourced from fossil fuels, results in CO2 emissions, compromising overall carbon neutrality. Instead, the processes of hydrogen generation can be energetically coupled with the combustion process, in situ, to eliminate external energy requirements. To that end, a novel self-decarbonizing combustor (SDC) has been conceptualized, integrating methane pyrolysis with the combustion process that can in principle decarbonize many contemporary power generation technologies. The underpinning methane pyrolysis process enables in situ pre-combustion capture of solid carbon, while simultaneously generating hydrogen. Consequently, CO2 emissions resulting from the combustion of processed, hydrogen-enriched fuel are mitigated. This study provides a comprehensive analysis, delineating the operating principle and the effect of some of the important governing parameters on the performance of the self-decarbonizing combustor. These parameters, including fuel temperature, residence time, pressure, and catalysis, are studied in the context of potentially applying the proposed concept to natural gas-based decarbonized electrical power generation. Investigating fuel chemistry, combustion exhaust, and carbon structure and morphology under varying process parameters enhances our comprehension of the SDC. Additionally, its self-sufficient nature eliminates the need for separate hydrogen production, storage, and transportation infrastructure, highlighting its potential as a scalable and realizable technology.

Understanding how intrinsically fast hydrogen-air premixed flames can be rendered much faster in turbulence is essential for the systematic development of hydrogen-based gas turbines and spark ignition engines. Here, we present fundamental insights into the variation of flame displacement speeds by investigating how the disrupted flame structure affects speed and vice-versa. Three DNS cases of lean hydrogen-air mixtures with effective Lewis numbers (Le) ranging from about 0.5 to 1, over Karlovitz number (Ka) range of 100 to 1000 are analyzed. Suitable comparisons are made with the closest canonical laminar flame configurations at identical mixture conditions and their appropriateness and limitations in expounding turbulent flame properties are elucidated. Since near zero-curvature surface locations are most probable and representative of the average flame geometry in such large Ka flames, statistical variation of the flame displacement speed and concomitant change in flame structure at those locations constitute the focus of this study. To that end, relevant flame properties are averaged in the direction normal to the zero-curvature isotherm locations to obtain the corresponding conditionally averaged flame structures. In the leanest case with smallest Le, the temperature increases beyond that of the standard laminar flame downstream of the zero-curvature regions, leading to enhanced local thermal gradient and flame speeds in the conditionally averaged structure. These result from increased heat-release rate contribution by differential diffusion (Le << 1) in positive curvatures downstream of the zero-curvature locations. Furthermore, locally, the flame structure is broadened for all cases due to a reversal in the direction of the flame speed gradient. This reversal is caused by cylindrical flame-flame interactions upstream of the zero-curvature regions, resulting in localized scalar mixing within the flame structure. The combined effect of these two non-local phenomena defines the conditionally averaged flame structure and the associated variation of the local flame speed of a premixed flame in turbulence. Novelty and Significance Statement The paper presents fundamental discoveries pertaining to the structure and propagation of intensely turbulent, lean premixed hydrogen-air flames, emerging from the analysis of averaged flame structures conditioned to zero-curvature surface locations. These locations are most probable alongside corresponding to the mean of curvature distribution. Analysis of such structures reveals how non-local effects determine the average flame displacement speed, for the first time. The paper shows that non-local effects within the flame structure address long-standing questions on how premixed flames are broadened in turbulence and why local flame displacement speeds of intensly turbulent, ultra-lean hydrogen-air flames are ubiquitously higher than their standard laminar counterpart.

Natural gas, mainly composed of methane, is ubiquitously burned for power generation, heating, manufacturing, transportation, and propulsion, thereby contributing to about 23.49% of the world's primary energy consumption by source. This is achieved at the expense of generating climate-forcing CO2 2 emissions. In this paper, we demonstrate partial decarbonization of methane-air combustion without involving any external energy input. This is accomplished by thermo-chemically coupling a partially premixed, swirl combustor with a thermo-catalytic pyrolysis reactor. The arrangement allows pre-combustion thermal decomposition of methane to generate hydrogen, alongside solid carbon that can be separated. The endothermic pyrolysis process continuously harnesses a fraction of the thermal energy generated from the combustion of the thermochemically processed fuel with significant hydrogen content, while the larger fraction of the generated thermal energy could be used to produce useful work. In particular, we report up to 41.11% molecular hydrogen concentration, by volume, in the processed fuel, while reducing about 24.23% of the CO2 2 emissions in its combustion products compared to stoichiometric methane-air combustion. Additionally, the fuel composition analyses substantiate the chemical pathway of pre-combustion pyrolysis. The structure and morphology study of the separated carbon indicates the underlying mechanism and the type of carbon black produced. Incorporating a conservatively estimated price of the captured carbon black into an energy-cost assessment model shows that the levelized cost of decarbonized heat generated by the proposed system is similar to that of existing natural gas-powered devices operating without carbon capture. This highlights the possible economic advantage of the self-decarbonizing combustor over other energy-equivalent decarbonized thermal power generators. Such an integrated method of decarbonized thermal power generation from combustion of in-situ produced hydrogen could also circumvent challenges of hydrogen storage and transportation, thereby offering possible scalability and realizability towards low-cost decarbonization of natural gas-based applications.

Thermoacoustic instabilities in turbulent combustors have disastrous consequences and present notorious challenges in their modeling, prediction, and control. Such instabilities are characterized by self-excited periodic oscillations, arising from a positive feedback between the acoustic pressure and heat release rate fluctuations. We present a mean-field approach to model thermoacoustic transitions. The nonlinear flame response is modeled using an ensemble of phase oscillators constrained to collectively evolve at the rhythm of acoustic fluctuations. Starting from the acoustic wave equation coupled with the phase oscillators, we derive the evolution equations for the amplitude and phase for acoustic oscillations. The model captures abrupt and continuous transitions to thermoacoustic instability observed in disparate combustors. We also discover that continuous and abrupt transitions happen through paradigmatic continuous and explosive synchronization, respectively. Importantly, our approach explains spatiotemporal synchronization and pattern formation underlying the transition to thermoacoustic instability. The versatility of the model in capturing different types of transitions suggests promising prospects for its extension to encompass a wide range of fluid dynamics phenomena.

We experimentally study the transition from a state of combustion noise to azimuthal thermoacoustic instability in a laboratory-scale turbulent annular combustor. This combustor has sixteen swirl-stabilized burners to facilitate continuous and spatially distributed combustion along the annular region. Our approach involves simultaneous measurement of CH* chemiluminescence emission of the flame using two high-speed cameras and the acoustic pressure fluctuations using eight piezoelectric pressure transducers mounted on the backplane of combustor. We observe that the transition from combustion noise to azimuthal instability occurs through mode shifting, where the system switches from a longitudinal mode to an azimuthal mode as the equivalence ratio is decreased. Throughout this progression, the combustor exhibits various dynamical behaviors, including intermittency, dual-mode instability, standing azimuthal instability, and beating azimuthal instability. These dynamical states are determined from the acquired pressure signals by decomposing the acoustic pressure fluctuations into clockwise (CW) and counterclockwise (CCW) waves, enabling a reconstruction of the amplitude of acoustic pressure fluctuations, nature angle, (anti-)nodal line location, and spin ratio. The global heat release response is then examined during various dynamical states, contrasting their behavior at different non-dimensional time steps by phase-averaging the fluctuations of the heat release rate over the acoustic pressure cycle. Distinctive flame behaviors were observed based on the direction of pressure wave propagation, showcasing characteristic CCW spinning, standing, and CW spinning heat release patterns. Moreover, our examination of relative phase distributions during various dynamical states, computed by analyzing the phase of heat release rate fluctuations across all burners with respect to one burner, reveals the emergence of diverse patterns in the interaction of neighboring flames influenced by acoustic field.

Natural gas, mainly composed of methane, is ubiquitously burned for power generation, heating, manufacturing, transportation, and propulsion, thereby contributing to about 23.49% of the world’s primary energy consumption by source. This is achieved at the expense of generating climate-forcing CO2 emissions. In this paper, we demonstrate partial decarbonization of methane-air combustion without involving any external energy input. This is accomplished by thermo-chemically coupling a partially premixed, swirl combustor with a thermo-catalytic pyrolysis reactor. The arrangement allows pre-combustion thermal decomposition of methane to generate hydrogen, alongside solid carbon that can be separated. The endothermic pyrolysis process continuously harnesses a fraction of the thermal energy generated from the combustion of the thermo-chemically processed fuel with significant hydrogen content, while the larger fraction of the generated thermal energy could be used to produce useful work. In particular, we report up to 41.11% molecular hydrogen concentration, by volume, in the processed fuel, while reducing about 24.23% of the CO2 emissions in its combustion products compared to stoichiometric methane-air combustion. Additionally, the fuel composition analyses substantiate the chemical pathway of pre-combustion pyrolysis. The structure and morphology study of the separated carbon indicates the underlying mechanism and the type of carbon black produced. Incorporating a conservatively estimated price of the captured carbon black into an energy-cost assessment model shows that the levelized cost of decarbonized heat generated by the proposed system is similar to that of existing natural gas-powered devices operating without carbon capture. This highlights the possible economic advantage of the self-decarbonizing combustor over other energy-equivalent decarbonized thermal power generators. Such an integrated method of decarbonized thermal power generation from combustion of in-situ produced hydrogen could also circumvent challenges of hydrogen storage and transportation, thereby offering possible scalability and realizability towards low-cost decarbonization of natural gas-based applications.

This study investigates the effect of a rotating axial swirler on the flame shape and combustion dynamics of premixed methane-air flames in a dual-swirler burner by using simultaneous hydroxyl (OH) planar laser- induced fluorescence (PLIF) and stereoscopic particle image velocimetry (sPIV). The rapid expansion from the nozzle to the combustor induces shear layer vortices that fold the flame, resulting in subsequent flame-flame interaction. This flame-flame interaction results in flame surface annihilation, causing heat release fluctuations that generate coherent pressure fluctuations, which is explored through a phase-averaged analysis. Increasing the rotational speed of the axial swirler diminishes vortex strength at the shear layer and shifts the vortex trajectory away from the central axis. This is caused by an increase in radial velocity and a decrease in the difference in momentum flux between the inside and outside shear layer. As the momentum flux difference decreases, the shear force weakens, leading to a reduction in the strength of the shear layer vortex. The shear layer vortex structure propagates outward and eventually dissipates due to the increased radial velocity with an increase in swirl number. This, in turn, substantially suppresses flame-flame interactions, resulting in the of the fluctuations.

Open-loop control is known to be an effective strategy for controlling self-excited periodic oscillations, known as thermoacoustic instability, in turbulent combustors. Here, we present experimental observations and a synchronization model for the suppression of thermoacoustic instability achieved by rotating the otherwise static swirler in a lab-scale turbulent combustor. Starting with the state of thermoacoustic instability in the combustor, we find that a progressive increase in the swirler rotation rate leads to a transition from the state of limit cycle oscillations to the low-amplitude aperiodic oscillations through a state of intermittency. To model such a transition while also quantifying the underlying synchronization characteristics, we extend the model of Dutta et al. [Phys. Rev. E 99, 032215 (2019)] by introducing a feedback between the ensemble of phase oscillators and the acoustic. The coupling strength in the model is determined by considering the effect of the acoustic and swirl frequencies. The link between the model and experimental results is quantitatively established by implementing an optimization algorithm for model parameter estimation. We show that the model is capable of replicating the bifurcation characteristics, nonlinear features of time series, probability density function, and amplitude spectrum of acoustic pressure and heat release rate fluctuations at various dynamical states observed during the transition to the state of suppression. Most importantly, we discuss the flame dynamics and demonstrate that the model without any spatial inputs qualitatively captures the characteristics of the spatiotemporal synchronization between the local heat release rate fluctuations and the acoustic pressure that underpins a transition to the state of suppression. As a result, the model emerges as a powerful tool for explaining and controlling instabilities in thermoacoustic and other extended fluid dynamical systems, where spatiotemporal interactions lead to rich dynamical phenomena.

School closures were used as strategies to mitigate transmission in the COVID-19 pandemic. Understanding the nature of SARS-CoV-2 outbreaks and the distribution of infections in classrooms could help inform targeted or ‘precision’ preventive measures and outbreak management in schools, in response to future pandemics. In this work, we derive an analytical model of Probability Density Function (PDF) of SARS-CoV-2 secondary infections and compare the model with infection data from all public schools in Ontario, Canada between September-December, 2021. The model accounts for major sources of variability in airborne transmission like viral load and dose-response (i.e., the human body’s response to pathogen exposure), air change rate, room dimension, and classroom occupancy. Comparisons between reported cases and the modeled PDF demonstrated the intrinsic overdispersed nature of the real-world and modeled distributions, but uncovered deviations stemming from an assumption of homogeneous spread within a classroom. The inclusion of near-field transmission effects resolved the discrepancy with improved quantitative agreement between the data and modeled distributions. This study provides a practical tool for predicting the size of outbreaks from one index infection, in closed spaces such as schools, and could be applied to inform more focused mitigation measures.Author summary At the start of the COVID-19 pandemic, there was huge uncertainty around the risks of SARS-CoV-2 spread in classrooms. In the absence of early predictions surrounding classroom risks, many jurisdictions across countries closed in-person education. There is great interest in adopting a more ‘precision’ approach to better inform future interventions in the context of airborne virus risks. For this purpose, we need tools that can predict the probability of the size of outbreaks within classrooms along with the impact of interventions including masks, better ventilation, and physical distancing by limiting the number of students per classroom. To this end, we have developed a robust but practical model that yields the probability of secondary infections stemming from index cases occurring within schools on a given day. During model development, the major underlying physical and biological factors that dictate the disease transmission process, both at long-range and close-range, have been accounted for. This enables our model to modify its predictions for different scenarios - and possibly allows its use beyond schools. Finally, the model’s predictive capability has been verified by comparing its outputs with publicly available data on SARS-CoV-2 diagnoses in Ontario public schools. To our knowledge, this is the first time an analytical model derived from mostly first principles describes real-world infection distributions, satisfactorily. The quantitative match between the theoretical prediction and real-world data offers the proposed model as a possible powerful tool for better-informed precision pandemic mitigation strategies in indoor environments like schools.### Competing Interest StatementThe authors have declared no competing interest.

Dataset and analysis for: Analysing the distribution of SARS-CoV-2 infections in schools: integrating model predictions with real world observations.Arnab Mukherjee, Sharmistha Mishra, Vijaya Kumar Murty, Swetaprovo Chaudhuri For any questions please contact the first author at: arnab.mukherjee@mail.utoronto.ca Contents: school_active_cases_ON.zip: Contains datasets for number of COVID-19 infections reported by public schools in Ontario on ten different dates. The data files have been created based on the raw data in the file named 'covidtesting.csv' that has also been shared. school_active_cases_pdf.m: Matlab code to obtain PDF of secondary infections in schools for a particular date based on the datasets in 'school_active_cases_ON.zip'. To obtain PDF for different dates, the appropriate dataset needs to be loaded. Created in MATLAB R2021b. U_jet2.m: User-defined Matlab function that is required to run the code 'gZ_code.m'. The function simulates the evolution of a simple jet/puff. Created in MATLAB R2021b. gZ_code.m: Matlab code to obtain the analytical PDF of secondary infections due to long-range transmission, near-field transmission, or both. Created in MATLAB R2021b. covidtesting.zip: Contains the data file 'covidtesting.csv' that reports the breakdown of COVID-19 infections in different public schools in Ontario on a daily basis. Data obtained from 'https://data.ontario.ca/dataset/summary-of-cases-in-schools/resource/dc5c8788-792f-4f91-a400-036cdf28cfe8'. Contains information licensed under the Open Government License – Ontario. schoolrecentcovid2021_2022.zip: Contains the data file 'schoolrecentcovid2021_2022.csv' that reports the status of COVID-19 cases in Ontario, obtained from 'https://data.ontario.ca/en/dataset/status-of-covid-19-cases-in-ontario/resource/ed270bb8-340b-41f9-a7c6-e8ef587e6d11'. Contains information licensed under the Open Government License – Ontario.

Starting with an integral formulation of average mass flow rate through an ensemble of isotherms constituting a statistically planar, turbulent premixed flame, a scaling for the corresponding turbulent flame speed is derived without invoking Damköhler’s hypotheses. Major approximations and interim results are validated using a large Karlovitz number, unity Lewis number, Direct Numerical Simulation (DNS) dataset of n-heptane/air mixture, computed with reduced chemistry. A length scale quantifying the fluctuation distance of the isotherms within the premixed flame structure is analysed.

Local flame displacement speed Sd of a turbulent premixed flame is of fundamental and practical interest. For H2-air flames, the interest is further accentuated given the recent drive towards the development of zero-carbon combustors for both power and aircraft engine applications. The present study investigates several three-dimensional Direct Numerical Simulation (3D DNS) cases of premixed H2-air turbulent flames to theoretically model the Sd at negative curvatures, building upon recent works. Two of the four DNS cases presented are simulated at atmospheric pressure and two at elevated pressure. The DNS cases at different turbulence Reynolds numbers (Ret) and Karlovitz numbers (Ka) are generated using detailed chemistry. It has been shown in the previous studies that at atmospheric pressure, the density-weighted flame displacement speed Sd˜ is enhanced significantly over its laminar value (SL) at large negative curvature κ due to flame-flame interactions. The current work justifiably employs an imploding cylindrical laminar flame configuration to represent the local flame surfaces undergoing flame-flame interaction in a 3D turbulent flame. Therefore, to acquire a deep understanding of the interacting flame dynamics at large negative curvatures, one-dimensional (1D) simulations of an inwardly propagating cylindrical H2-air laminar premixed flame, with detailed chemistry at the corresponding atmospheric and elevated pressure conditions are performed. In particular, the 1D simulations emphasized the transient nature of the flame structure during these interactions. Based on the insights from the 1D simulations, we utilize an analytical approach to model the Sd˜ at these regions of extreme negative κ of the 3D DNS. The analytical approach is formulated to include the effect of variable density, convection and the inner reaction zone motion. The joint probability density function (JPDF) of Sd˜ and κ and the corresponding conditional averages obtained from 3D DNS showed clear negative correlation between Sd˜ and κ at all pressures. The obtained model successfully predicts the variation of 〈Sd˜|κ〉 with κ for the regions on the flame surface with large negative curvature (κδL≪−1) at atmospheric as well as at elevated pressure, with good accuracy. This showed that the 1D cylindrical, interacting flame model is a fruitful representation of a local flame-flame interaction that persists in a 3D turbulent flame, and is able to capture the intrinsically transient dynamics of the local flame-flame interaction. The 3D DNS cases further showed that even in the non-interacting state at κ=0, on average Sd˜ can deviate from SL. Sd˜ at κ=0 is a manifestation of the internal flame structure, controlled by turbulence transport in the large Ka regime. Therefore, the correlation of 〈Sd˜〉/SL with the the normalized gradient of the progress variable, 〈|∇c^|c0〉 at κ=0 is explored.

This paper summarizes the design and development of a unique optically accessible sector of a full-scale annular gas turbine combustor. The distinctive feature of the experimental setup is that it preserves the geometrical details of an annular combustor that includes the casing, dome and combustor liner. The combustor design features a series of primary and secondary dilution holes with multiple film cooling strips on outer and inner liner. The details of the facility requirements and instrumentation and controls are provided in the paper. The methodology employed in the design of the optically accessible combustion chamber is elucidated, including quartz window considerations and thermal management of the experimental hardware under extremely high heat loads. The goal is to obtain the evolution of global parameters such as spray patternation, pattern factor, pressure drop and combustion efficiency through measurement of the local flow-flame interactions using advanced optical diagnostics. Rig contains multiple ports for measurements of temperature and pressure. For current isothermal study, simultaneous pressure and mass flow rate data of the rig is acquired during PIV measurements to ensure the state of the system. 2D two component PIV experiments are conducted to measure the velocity inside the combustor under isothermal conditions. Experiments are conducted at 0.1, 0.3 and 0.5 kg/s of inlet flow rate and time averaged flow structures inside the combustor are reported. The major structure of flow such as central recirculation zone (CRZ) and swirl jet deflection angle for the respective cases are elucidated here.

Starting with an integral formulation of mass flow rate of reactants through an ensemble of isotherms within a statistically planar, turbulent premixed flame, a scaling for the corresponding turbulent flame speed is derived. Closure relations for the local flame displacement speed and scalar dissipation rate anomaly are invoked and a new length scale quantifying the fluctuation distance of the isotherms within the premixed flame structure is introduced. Damköhler’s hypotheses are not used at any stage. Eventually, the scaling is extended to expanding turbulent premixed flames.

Comprehensive knowledge of local flame displacement speed, Sd, in turbulent premixed flames is crucial towards the design and development of hydrogen fuelled next-generation engines. Premixed hydrogen-air flames are characterized by significantly higher laminar flame speed compared to other conventional fuels. Furthermore, in the presence of turbulence, Sd is enhanced much beyond its corresponding unstretched, planar laminar value SL. In this study, the effect of high Karlovitz number (Ka) turbulence on density-weighted flame displacement speed, Sd˜, in a H2-air flame is investigated. Recently, it has been identified that flame-flame interactions in regions of large negative curvature govern large deviations of Sd˜ from SL, for moderately turbulent flames. An interaction model for the same has also been proposed. In this work, we seek to test the interaction model's applicability to intensely turbulent flames characterized by large Ka. To that end, we investigate the local flame structures: thermal, chemical structure, the effect of curvature, along the direction that is normal to the chosen isothermal surfaces. Furthermore, relative contributions of the transport and chemistry terms to Sd˜ are also analyzed. It is found that, unlike the moderately turbulent premixed flames, where enhanced Sd˜ is driven by interactions among complete flame structures, Sd˜ enhancement in high Ret and high Ka flame is predominantly governed by local interactions of the isotherms. It is found that enhancement in Sd˜ in regions of large negative curvature occurs as a result of these interactions, evincing that the interaction model is useful for high Ka turbulent premixed flames as well.