Radiative Heat Transfer Modeling and in Situ Diagnostics of Soot in an 80 kW th Propane Flame with Varying Feed-Gas Oxygen Concentration Adrian Gunnarsson,* ,† Johan Simonsson, ‡ Daniel Bä ckströ m, † Manu Naduvil Mannazhi, ‡ Per-Erik Bengtsson, ‡ and Klas Andersson † † Department of Space, Earth and Environment, Chalmers University of Technology, SE-41296 Gothenburg, Sweden ‡ Department of Combustion Physics, Lund University, SE-22100 Lund, Sweden ABSTRACT: This work presents experimental measure- ments of various 80 kW th propane flames, using a swirl burner, and modeling of the radiative heat transfer. The combustion conditions were altered by varying the oxygen concentration in the oxidant within range of 21-32%, while keeping the thermal input and oxygen-to-fuel ratio constant. Temperature, gas composition, and radiative intensity were measured using probes, while the soot volume fraction was quantified using nonintrusive laser-induced incandescence. The radiative intensity and the soot volume fraction increased with an increased oxygen concentration in the flame. When the oxygen concentration exceeded 27% the soot volume fraction was increased more than 14-fold. The results reveal the potential of promoting radiative heat transfer by increasing the oxygen concentration; the total radiative intensity becomes dominated by the soot particle contribution. In addition, laser-induced incandescence was successfully used for instantaneous and spatially resolved soot measurements in this type of furnace being at a technical scale. 1. INTRODUCTION Suspension-fired systems are applied in many industries, with flame radiation being a major contributor to the total heat transfer. In such systems, the formed soot is a significant contributor to the radiative heat transfer. 1 Mehta et al. 2 studied laboratory-scale turbulent jet flames and concluded that as much as 70% of the emitted flame radiation could be attributed to soot particles. Thus, the presence of soot particles influences the efficiency of the combustor. That is, operating a combustor to increase the soot formation, and thereby the radiative heat transfer, could result in fuel savings. 3 However, soot emissions should be avoided, as it reflects an incomplete combustion process, and more importantly, soot particles are detrimental to human health 4 and have negative impact on environment and climate. 5 Efforts to produce more intermediate soot while keeping the soot emissions from the combustor at minimum level is therefore desirable and motivates investigations of the radiative heat transfer in sooty flames under different combustion conditions combining modeling and experiments. Soot formation has been studied in detail in various combustion systems, including small diffusion flames, different reactors and diesel-fired engines, see, for example, the works by Bockhorn 6 and Omidvarborna et al. 7 The generation of soot is initiated by fuel pyrolysis in high-temperature regions with low concentrations of oxygen, which is followed by nucleation, surface growth and particle coagulation. 8 While the soot nucleation step is poorly understood, polycyclic aromatic hydrocarbons (PAHs) have been proven to play an important role, 9 and the formation of PAHs has been shown to be crucially dependent on the flame temperature. 10 Heteroge- neous reactions and coagulation will cause the soot particles to grow, 11 achieving typical primary particle sizes of 20-50 nm, which aggregates to final sizes of several hundred nanometers to micrometers. 1 Due to their small size and the presence of steep temperature gradients in flames, thermophoresis could have an impact on the soot particle trajectories and, possibly, on the particle growth process. 10 As the particles are transported to more-oxygen-rich regions of the flame, they start to oxidize. The amount of soot emitted from a combustor is therefore dictated by a competition between the formation and oxidation of the particles. 8 These factors are dependent on the local temperatures and gas concentrations, which means that mixing and dilution will affect the amount of soot that is emitted. Oxygen-enhanced combustion has been shown to generate higher concentrations of intermediate soot compared to combustion using air, 12 and it can be used advantageously in Received: June 18, 2018 Revised: August 16, 2018 Accepted: August 17, 2018 Published: August 17, 2018 Article pubs.acs.org/IECR Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX-XXX © XXXX American Chemical Society A DOI: 10.1021/acs.iecr.8b02699 Ind. Eng. Chem. Res. XXXX, XXX, XXX-XXX Downloaded via KAOHSIUNG MEDICAL UNIV on September 4, 2018 at 01:13:20 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.