arXiv subjects

S. Sunil Kumar

Publications and source records attributed to S. Sunil Kumar.

5 recordsLinked to original sources

arXiv2025

Impact of Fuel Injection Temperature Dynamics on the Stability of Liquid Oxygen-Methane Supercritical Combustion

A crucial factor in the stability of high-pressure rocket-scale combustors is the temperature at which fuel is injected. This study investigates its effect on the stability of supercritical liquid oxygen (LOx)-methane combustion and highlights the impact of shear layer dynamics in cases with lower injection temperatures. The stability features of a rocket-scale combustor operating with multiple injector elements are investigated using a high-fidelity large eddy simulation (LES) framework. The numerical framework combines a flamelet-generated manifold (FGM) combustion model with complex real gas thermodynamics in a scale-resolving simulation setup. It reproduces the non-equilibrium transcritical injection and supercritical combustion characteristics of supercritical methane-oxygen flames. To ascertain the effect of injection temperature on flame and combustor stability, we perform several LES simulations at various methane injection temperatures and produce a stability map. Our analysis shows extremely unstable flame characteristics at lower fuel injection temperatures that are not seen under typical fuel injection circumstances. Below a specific methane injection temperature, LES captures a high-amplitude, self-sustaining instability. It is determined that the combustor becomes unstable below a specific stability boundary temperature. Detailed spectral and dynamic mode decomposition (DMD) analysis of the stable and unstable cases reveals the onset of longitudinal acoustic waves in the combustor. Our thorough investigation pinpoints the instability mechanism, emphasizing that the leading causes of this self-sustaining instability in the combustor are a reduced velocity ratio, fuel buildup, and fuel cut-off occurrences.

physics.flu-dyn↗

arXiv2025

A fast and streamlined method for the measurement of absolute photodetachment and photodissociation cross-sections

The absolute photodetachment cross-section characterizes the photostability of atomic and molecular anions against photodestruction by neutralization. The measurement of this quantity has been reported only for atomic and simple molecular ions. In 2006, Wester's group introduced a novel ion-trap-based technique to measure the absolute photodetachment cross-section [S. Trippel et al., Phys. Rev. Lett. 97, 193003 (2006)] of OH^-. In the present work, we propose a novel methodology to streamline this technique to reduce the measurement time by several orders of magnitude by combining a single experimental rate measurement with a simulated column density distribution of the trapped ions. We validated our approach by reproducing the cross-section reported for OH^- at 632.8 nm. Using this technique, we report the first such measurement for a molecule of biological interest, deprotonated indole, at a laser wavelength of 403 nm. The proposed scheme is anticipated to have a significant and transformative impact on the development of a comprehensive database for photodetachment and photodissociation cross-sections of molecular ions. Furthermore, these measurements have the potential to drive the development of cutting-edge computational codes for cross-section calculations, enabling an unprecedentedly detailed understanding of electron dynamics in large molecules and the light-matter interaction.

physics.atm-clus↗

arXiv2023

Numerical Investigation of Supercritical Combustion Dynamics in a Multi-Element LOx-Methane Combustor Using Flamelet-Generated Manifold Approach

The article investigates liquid oxygen (LOx)-methane supercritical combustion dynamics in a multi-element rocket scale combustor using large eddy simulation (LES). A complex framework of real gas thermodynamics and flamelet generated manifold (FGM) combustion model is invoked to simulate transcritical oxygen injection and supercritical methane combustion. A benchmark Mascotte chamber, Rocket Combustion Modelling (RCM) Test Case, i.e. RCM-3 (V04)/G2 test case, is used to validate the real gas FGM model in the LES framework. The validation study accurately reproduces experimental flame structure and OH concentration, demonstrating the FGM model's importance in incorporating finite rate kinetics in LOx-methane combustion. Subsequently, the numerical framework investigates a specially designed multi-element combustion chamber featuring seven bidirectional swirl coaxial injectors. The analyses capture the complex hydrodynamics and combustion dynamics associated with multiple swirl injectors operating at supercritical pressure, effectively demonstrating the initiation of transverse acoustic waves and examining the effect of local sound speed on the evolution of acoustic modes in the combustor. The dominant frequency modes shed light on understanding the role of injectors in enhanced combustor dynamics. Spectral analysis reveals the interplay of the upstream injector and chamber acoustics due to possible frequency coupling. The results also highlight the effect of fuel injection temperature on the stability of the combustor, revealing a violent dynamic activity for lower fuel injection temperature associated with the longitudinal acoustic mode of the combustor. The investigation appropriately reproduces self-sustained limit cycle oscillations at lower fuel injection temperatures and corroborates the conventional understanding of combustor instability.

physics.flu-dyn↗

arXiv2022

Laser-Induced Fluorescence Spectroscopy (LIFS) of Trapped Molecular Ions in Gas-phase

This review presents the Laser-Induced Fluorescence Spectroscopy (LIFS) of trapped gas-phase molecular ions. A brief description of the theory and experimental approaches involved in fluorescence spectroscopy, together with state-of-the-art LIFS experiments employing ion traps, is presented. Quadrupole ion traps are commonly used for spatial confinement of ions. One of the main challenges involved in such experiments is poor Signal-to-Noise Ratio (SNR) arising due to weak gas-phase fluorescence emission, high background noise, and small solid angle for the fluorescence collection optics. The experimental approaches based on the integrated high-finesse optical cavities provide a better (typically an order of magnitude more) SNR in the detected fluorescence than the single-pass detection schemes. Another key to improving the SNR is to exploit the maximum solid angle of light collection by choosing high numerical aperture (NA) collection optics. The latter part of the review summarises the current state-of-the-art intrinsic fluorescence measurement techniques employed for gas-phase studies. Also, the scope of these recent advances in LIFS instrumentation for detailed spectral characterisation of a fluorophore of weak gas-phase fluorescence emission is discussed, considering fluorescein as one example.

quant-ph↗

arXiv2017

Radiative rotational lifetimes and state-resolved relative detachment cross sections from photodetachment thermometry of molecular anions in a cryogenic storage ring

Photodetachment thermometry on a beam of OH$^-$ in a cryogenic storage ring cooled to below 10 K is carried out using two-dimensional, frequency and time dependent photodetachment spectroscopy over 20 minutes of ion storage. In equilibrium with the low-level blackbody field, we find an effective radiative temperature near 15 K with about 90% of all ions in the rotational ground state. We measure the J = 1 natural lifetime (about 193 s) and determine the OH$^-$ rotational transition dipole moment with 1.5% uncertainty. We also measure rotationally dependent relative near-threshold photodetachment cross sections for photodetachment thermometry.

physics.atom-ph↗