M. N. Kiran Kumar

Bio: M. N. Kiran Kumar is an academic researcher from Indian Institute of Technology Madras. The author has contributed to research in topics: Premixed flame & Absorption cross section. The author has an hindex of 1, co-authored 3 publications receiving 10 citations.

The dynamics of near limit self-propagating flame over thin solid fuels in microgravity

Abstract: Microgravity experiments have shown that near limit opposed flow flame-spread show unsteady oscillatory behavior marked with formation of flamelets. Here in this work an elaborate 3D numerical model is used to predict this near limit fame-spread behavior of a self-propagating flame over thin solid in microgravity. As the oxygen level is reduced steady, below certain value, flame-spread over wide thin solid fuel becomes unsteady. Two kinds of unsteady flame-spread phenomena were noted. In one a single flame oscillated back and forth near the fuel side edge and in other the multiple flamelets were formed which oscillated laterally. The former occurs at relatively higher oxygen levels and leads to formation of the later at still lower oxygen levels just ahead of extinction. It is noted that as oxygen level is reduced the amplitude of longitudinal oscillation increases leading to the splitting of flame which then exhibits lateral motion. The lateral oscillatory behavior spans over a wider range of oxygen level than that of longitudinal oscillatory flame behavior. The range of oxygen level over which the lateral oscillatory behavior is observed, increases with increase in fuel thickness and fuel width. The number of flamelets formed increases with increase in fuel width with typical flamelet size of 2–4 cm. Some of the flame behavior noted here are remarkably similar to those seen in the short duration drop tower test where heat loss from the flame was artificially enhanced. However, the numerical computations show that such oscillatory flames can exist for considerably long durations.

Fluorescence Features of Dy3+ Doped PbO-Bi2O3 Borophosphate Glasses

Abstract: The heavy metal oxide lead bismuth magnesium borophosphate glass systems (LBMBPD)dysprosium ions doped with the molar compositions of (50-x) PbO -x Bi2O3-25MgHPO4-24B2O3-1Dy2O3 (where x = 10, 20, 30, and 40 mol %) were prepared using conventional melt quenching technique. The spectral data from the optical absorption photoluminescence were utilized to evaluate various spectroscopic parameters. The Judd-Ofelt parameterization employed reflects the covalency and vibration frequencies of the ligands with dysprosium ions. The radiative parameters such as radiative transition probabilities (A), the total radiative transitional probabilities (AT), radiative life times (τR), branching ratios (β) and absorption cross sections (□) were computed for certain lasing levels. The emission cross sections (σe) for the significant lasing transitions 4F9/2→ 6H13/2 and 4F9/2 → 6H15/2 evaluated from the photoluminescence spectra were reported. The radiative properties thus obtained in our investigations reflect their potentialities as good lasing candidates.

A 3D Numerical Study on Opposed Flow Flame-Spread over Thin Parallel Fuel Sheets of Finite Widths in Microgravity

Abstract: Flame spread over an array of fuel sheets of finite width size has been modeled and numerically investigated for opposed, low convective flows in microgravity. As opposed to the previous studies based on 2D models, steady flame spread rates were observed for all separation distances up to the separation distance of flame extinction. The flame spread rate increased with decrease in separation distance up to a point where it was maximum, further reduction in separation distance, reduced the flame spread rate. The flammability map as a function of separation distance was also obtained for different fuel widths. While the extinction map qualitatively matches with the flammability map obtained from the 2D model, the flame extinguished at higher oxygen levels with the decrease in fuel width due to radiation heat losses.

Numerical Study of the Effects of Confinement on Concurrent-Flow Flame Spread in Microgravity

Abstract: The objective of this work is to investigate the aerodynamics and thermal interactions between a spreading flame and the surrounding walls as well as their effects on fire behaviors. A three-dimensional transient computational fluid dynamics (CFD) combustion model is used to simulate concurrent-flow flame spread over a thin solid sample in a narrow flow duct. The height of the flow duct is the main parameter. The numerical results predict a quenching height for the flow duct below which the flame fails to spread. For duct heights sufficiently larger than the quenching height, the flame reaches a steady spreading state before the sample is fully consumed. The flame spread rate and the pyrolysis length at steady-state first increase and then decrease when the flow duct height decreases. The detailed gas and solid profiles show that flow confinement has multiple effects on the flame spread process. On one hand, it accelerates flow during thermal expansion from combustion, intensifying the flame. On the other hand, increasing flow confinement reduces the oxygen supply to the flame and increases conductive heat loss to the walls, both of which weaken the flame. These competing effects result in the aforementioned nonmonotonic trend of flame spread rate as duct height varies. Near the quenching duct height, the transient model reveals that the flame exhibits oscillation in length, flame temperature, and flame structure. This phenomenon is suspected to be due to thermodiffusive instability.

Near limit flame spread over thin solid fuels in a low convective microgravity environment

Abstract: A few microgravity experiments have reported formation of flamelets at near extinction of freely propagating opposed flow flame over solid fuels. Inspired by these observations, about which little is known so far, an established and elaborate numerical model in 3D is used explore opposed flow flame spread phenomena near oxygen extinction limit in quiescent and low convective environment (with flow velocity, U∞ = 5 cm/s). Simulations were carried out for various fuel widths show that 1 cm wide fuel has the least oxygen extinction limit among fuel widths varying between 0.5 cm and the 2D limit even after accounting for the presence of flamelets over wider fuels (except in 2D limit). Unlike quiescent environment where flamelets were found to be inherently unsteady and eventually extinguished, steadily propagating flamelets were also obtained in low convective space environment. The flamelet oscillations which arise due to thermo-diffusive instability were found to have time period close to sum of times scales in gas phase and solid phase. A scaling analysis showed that fuel area density, flow velocity and fuel Lewis number are key factors that influence size of a steadily propagating flamelets. Several of these features have been observed in experiments and predicted in the present simulations.

Downward flame spread along a single pine needle: Numerical modelling

Abstract: In this work downward flame spread over single pine needle of Pinus Sibirica is studied. Pine needles are thin cellulosic charring combustible forest fuel elements. Idealising pine needles to thin cylinders, a 2D axisymmetric numerical model is developed accounting for char formation and char oxidation to investigate the important mechanisms which control the downward spread of flame over a pine needle in normal gravity, atmospheric condition and at various opposed flow conditions. Simultaneous formation of char and pyrolysate during the pyrolysis process was found to significantly reduce the flame spread rate over thin fuel. Presence of char resulted in change in distribution of fuel vapour mass flux above the fuel surface which led to decrease in forward heat feedback to the fuel and hence the flame spread rate. This mechanism is different from char acting as a thermal barrier to heat transfer from the flame in case of thick fuel. Char oxidation had no influence on flame spread rate as char oxidation was found to occur only after passage of flame with the availability of surrounding oxygen diffusing through the hot plume of combustion products. Char oxidation was primarily controlled by oxygen diffusion rate to the charred fuel surface. The flame spread data for quiescent flame spread, and the blow off opposed flow velocity was used to calibrate gas phase kinetics and pyrolysis kinetics. The model predicted flame spread rate variation with opposed flow velocity quite well. The predicted spatial distribution of temperature and species concentration also compared very well with the experimentally determined flame structure.

Flammability limits and near-limit chemistry controlled flame spread over thermally thin paper under sub-atmospheric pressure

Abstract: Understanding material flammability properties and flame spread rates are important to mitigate fire hazards in high altitude areas, aircraft and spacecraft. Limiting oxygen concentration (LOC) has been widely studied in these conditions, but flame spread near extinction limits has been rarely reported. In this work, the experimental study of horizontal near-limit flame spread over paper is investigated in a sub-atmospheric chamber. We first quantified LOCs at a pressure range from 4 to 45 kPa. Then, the near-limit flame spread was studied by increasing 1mol% oxygen concentration at each pressure. Flame image, flame spread rate and preheating length are measured in this study. Results show that LOC is independent of paper sample thickness but dependent on sample width due to lateral mass and heat losses. Near-limit flames show a two-zone structure, including inner blue flame and outer red flame. We find that the near-limit flame spread rate is dominated by preheating length, and it does not obey the previously proposed the power law relationship between flame spread rate and oxygen concentration and pressure. This work studies flammability and near-limit flame spread at various pressure and oxygen conditions, contributing to understanding the fire risk in the environments different from standard atmospheric conditions.

Numerical investigation of flame splitting phenomenon in upward flame spread over solids with a two-stage pyrolysis model

Abstract: Upward flame spread over thin solids is numerically studied using an in-house three-dimensional transient numerical model with a two-stage solid pyrolysis. The simulation results reveal fundamental...