Flameless combustion offers many advantages over conventional combustion, particularly uniform temperature distribution and lower emissions. In this paper, a new strategy is proposed and adopted to scale up a burner operating in flameless combustion mode from a heat release density of 5.4 to 21 MW/m 3 (thermal input 21.5 -84.7 kW) with kerosene fuel. A swirl flow based configuration was adopted for air injection and pressure swirl type nozzle with an SMD 35-37 µm was used to inject the fuel. Initially, flameless combustion was stabilized for a thermal input of 21.5 kW ( =5.37 MW/m 3 ). Attempts were made to scale this combustor to higher intensities i.e. 10.2, 16.3 and 21.1 MW/m 3 . However, an increase in fuel flow rate led to incomplete combustion and accumulation of unburned fuel in the combustor. Two major difficulties were identified as possible reasons for unsustainable flameless combustion at the higher intensities (i) A constant spray cone angle and SMD increases the droplet number density (ii) Reactants dilution ratio ( ) decreased with increased thermal input. To solve these issues, a modified combustor configuration, aided by numerical computations was adopted, providing a chamfer near the outlet to increase the . Detailed experimental investigations showed that flameless combustion mode was achieved at high intensities with an evenly distributed reaction zone and temperature in the combustor at all heat intensities. The emissions of CO, NO x and HC for all heat intensities (Ф=1 -0.6) varied between 11 -41, 6 -19 and 0 -9 ppm, respectively.These emissions are well within the range of emissions from other flameless combustion systems reported in the literature. The acoustic emission levels were also observed to be reduced by 8-9 dB at all conditions.

Formic acid is a promising fuel candidate that can be generated by reacting renewable hydrogen with carbon dioxide. However, the burning characteristics of formic acid/air mixtures have not been extensively studied. Furthermore, due to its low reactivity, the addition of hydrogen to formic acid/air mixtures may help with improving burning characteristics. This paper presents the first extensive study of formic acid/air premixed laminar burning velocities, as well as mixtures with hydrogen and carbon dioxide. Unstretched laminar burning velocities and Markstein lengths of formic acid in air for two different unburnt gas temperatures and equivalence ratios are presented. Measurements of formic acid mixed with various proportions of hydrogen and carbon dioxide in air are also studied as a potential renewable fuel for the future. Experimental results demonstrate the low burning velocities of formic acid and the ability to significantly enhance flame speeds by hydrogen addition. A modified detailed kinetic model for combustion of formic acid and its mixtures with hydrogen is proposed by merging well-validated literature models. The proposed model reproduces the experimental observations and provides the basis for understanding the combustion kinetics of formic acid laminar premixed flames, as well as mixtures with hydrogen. It is shown that the HOCO radical is the principal intermediate in formic acid combustion, and hydrogen addition accelerates the decomposition of HOCO radical thereby accelerating burning velocities.

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The present work reports the measurement of the laminar burning velocity for n-propanol and air mixtures at 1 atm pressure, with the unburnt mixture temperature varying up to 620 K using externally heated mesoscale diverging channels. Planar flames were stabilized in quartz channels using an externally heated mesoscale diverging channel to create a positive temperature gradient along the direction of fluid flow. The laminar burning velocity was extracted using the mass conservation principle at the flame surface and channel inlet. The performance of six recent kinetic mechanisms was evaluated through a comparison of the predictions to present experimental results. A significant disagreement (≈22%) was observed between different mechanism predictions, even at lower mixture temperatures of 335 K. The temperature exponent, α, was extracted using power-law correlations and observed to follow an inverted parabolic pattern with a minimum at a slightly rich equivalence ratio of 1.1, similar to other alcohol fuels.

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Long chain alcohols are potential fuels for engine applications, however, their combustion characteristics need to be adequately investigated compared to short chain alcohols (C 1-C 4), especially at high mixture temperatures, and other conditions relevant to engine temperatures. In the present work, meso-scale diverging channel method has been used to measure the laminar burning velocity of n-pentanol+air mixtures at elevated temperatures due to existence of very limited data at higher mixture temperatures (~ 473 K). The present experiments are carried out at atmospheric pressure with unburnt mixture temperature varying up to 560 K. The dependence of laminar burning velocity on temperature was correlated using the power law: , where α is the temperature exponent. The results show the existence of a minimum value of α for slightly rich mixtures. A reduced kinetic model based *Manuscript Click here to download Manuscript: Pentanol_final_rev3.docx Click here to view linked References on the previous detailed kinetic model of Sarathy (2014) for C 1-C 5 straight-chain alcohols was generated with 199 species and 1427 reactions. Experimental results of laminar burning velocity of n-pentanol+air mixtures at high temperatures were compared with the present model and other kinetic models from the literature. The skeletal model accurately reproduces the measurements at various conditions.

In the present work, measured laminar burning velocities of methyl formate (MF)–air mixtures at atmospheric pressure are presented for high mixture temperatures (up to 500 K) using an externally heated mesoscale diverging channel method. The experiments were performed for equivalence ratios ranging from Φ = 0.6 to Φ = 1.4 with an unburnt mixture temperature range from 350 to 500 K. The results reported in the literature and mechanism predictions of Aramco 2.0 (2016), Dievart (2013), and Dooley (2010) using PREMIX code are then compared with the data obtained from the existing experimental setup. The progressive change of temperature exponent and laminar burning velocity with equivalence ratios is akin to the other gaseous and liquid fuels outlined in the literature. The maxima and minima associated with the laminar burning velocity and temperature exponent (α) respectively is observed at Φ ≈ 1.1 or a slightly richer side. The mechanism predictions of the Aramco 2.0 (2016) detailed kinetic model is used for a detailed analysis of the mixture oxidation to account for the sensitivity of the key reactions on the laminar burning velocity. The overall effect of H-abstraction of methyl formate enhances the laminar burning velocity at 500 K. From reaction pathway analysis, it is observed that the global combustion rate rises when the unburnt mixture temperature changes from 348 to 500 K.

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