arXiv subjects

Yuvraj

Publications and source records attributed to Yuvraj.

3 recordsLinked to original sources

arXiv2024

How "mixing" affects propagation and structure of intensely turbulent, lean, hydrogen-air premixed flames

Understanding how intrinsically fast hydrogen-air premixed flames can be rendered much faster in turbulence is crucial for systematically developing 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 $Le$ from 0.5 to 1 and $Ka$ from 100 to 1000 are analyzed. Suitable comparisons are made with the closest canonical laminar flame configurations at same mixture conditions and their suitability 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, this study focuses on the statistical variation of flame displacement speed and the concomitant change in flame structure at those locations. Relevant flame properties are averaged normal to the zero-curvature isotherm regions to obtain the conditional mean flame structures. In the smallest $Le$ case, downstream of the most probable zero-curvature regions, the temperature exceeds that of the standard laminar flame, leading to enhanced local thermal gradient and flame speed. This is due to increased heat-release rate contribution by differential diffusion 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. These non-local effects, in combination, define the mean flame structure and the associated variation in local flame speed in turbulent premixed flames.

physics.flu-dyn↗

arXiv2023

On flame speed enhancement in turbulent premixed hydrogen-air flames during local flame-flame interaction

Given the need to develop zero-carbon combustors for power and aircraft engine applications, $S_d$ of a turbulent premixed flame, especially for H$_2$-air, is of immediate interest. The present study investigates 3D DNS cases of premixed H$_2$-air turbulent flames at varied pressures for different $Re_t$ and $Ka$ with detailed chemistry to theoretically model $S_d$ at negative curvatures. Prior studies at atmospheric pressure showed $\widetilde{S_d}$ to be enhanced significantly over $S_L$ at large negative $\kappa$ due to flame-flame interactions. 1D simulations of an imploding cylindrical H$_2$-air laminar premixed flame used to represent the local flame surfaces undergoing flame-flame interaction in a turbulent flame at the corresponding pressure conditions are performed to understand the interaction dynamics. These simulations emphasized the transient nature of the flame structure during flame-flame interactions and enabled analytical modeling of $\widetilde{S_d}$ at these regions of extreme negative $\kappa$ of the 3D DNS. The JPDF of $\widetilde{S_d}$ and $\kappa$ and the corresponding conditional averages from 3D DNS showed a negative correlation between $\widetilde{S_d}$ and $\kappa$. The model successfully predicts the variation of $\langle\widetilde{S_d}|_{\kappa}\rangle$ with $\kappa$ for the regions on the flame surface with $\kappa\delta_L \! \ll \! -1$ at all pressures, with good accuracy. This shows the aforementioned configuration to be fruitful in representing local flame-flame interaction in 3D turbulent flames. Moreover, at $\kappa=0$, on average $\widetilde{S_d}$ can deviate from $S_L$, manifested by the internal flame structure, controlled by turbulence transport in the large $Ka$ regime. Thus, the correlation of $\langle\widetilde{S_d}\rangle/S_L$ with $\langle|\widehat{\nabla c}|_{c_0}\rangle$ at $\kappa =0$ is explored.

physics.flu-dyn↗

arXiv2021

Local flame displacement speeds of hydrogen-air premixed flames in moderate to intense turbulence

Comprehensive knowledge of local flame displacement speed, $S_d$, 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, $S_d$ is enhanced much beyond its corresponding unstretched, planar laminar value $S_L$. In this study, the effect of high Karlovitz number ($Ka$) turbulence on density-weighted flame displacement speed, $\widetilde{S_d}$, in a H$_2$-air flame is investigated. Recently, it has been identified that flame-flame interactions in regions of large negative curvature govern large deviations of $\widetilde{S_d}$ from $S_L$, 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 $\widetilde{S_d}$ are also analyzed. It is found that, unlike the moderately turbulent premixed flames, where enhanced $\widetilde{S_d}$ is driven by interactions among complete flame structures, $\widetilde{S_d}$ enhancement in high $Re_t$ and high $Ka$ flame is predominantly governed by local interactions of the isotherms. It is found that enhancement in $\widetilde{S_d}$ 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.

physics.flu-dyn↗