A Study of Thermal Radiation Enhancement in the Homogeneous Combustion

Amongst all the attempts to fulfill the goal of enhancing thermal efficiency and decreasing pollutant emissions in a combustion system, Homogeneous Combustion (HC), or Flameless Oxidization (FLOX) or Moderate Intense Low-oxygen Dilution (MILD) Combustion is thought to be one of the most promising technologies. HC is achieved by strong exhaust gas recirculation, where reactants are intensely diluted and hence reactions occur volumetrically without a visible flame front. Relative to the conventional counterpart, HC increases the in-furnace thermal uniformity and efficiency, and simultaneously suppresses NOx emissions. Thermal radiation enhancement can be one possible approach to further improve the thermal performance of a HC at no or little expense of pollutant emissions, especially when applied in the heating facilities such as industrial furnace. In practical heating systems where thermal radiation is pursued, such as industrial furnace, certain level of soot production in the combustion zone is favored if adequate aftertreatment is available to limit the soot emissions in the exhaust under the regulated values. The present study focuses on the possibility of thermal radiation enhancement through soot formation under Homogeneous Combustion regime through experimental tests implemented in a large-scale furnace (0.84 m in diameter and 1.4 m in length), which can provide direct insights to the development of practical industrial heating systems. The possibility of sooty HC is studied first. Experimental tests, under various concentrations of propane or ethylene in the fuel, oxygen enrichment level and process temperature, are implemented. Results show that sooty HC can be achieved under the conditions of: 1) fuel rich combustion with equivalence ratio of 1.1; 2) fuel mixture with ethylene while the mass fraction of ethylene should be maintained below 20%; 3) process temperature over 1400 K. The characteristics of sooty HC is studied next. Experimental tests under the sooty HC conditions are implemented to measure the soot content (by laser extinction), the thermal radiation heat flux (by a Schmidt-Boelter Gauge), the thermal field (by in-situ temperature probe), the furnace efficiency (by a cooling plate equipped with water jacket) and the pollutant emissions (by various gas analyzers). Results showed an increasing trend in the in-furnace soot content with increasing ethylene content in the fuel. A close-to-linear relationship is found between the measured radiation heat flux and the soot volume fraction in sooty HCs. However, in the test runs with 20% ethylene in the fuel, visible flames can be found and hence this can no longer be identified as sooty HC. To help understand the flow field of the Homogeneous Combustion in the present study, numerical modeling for the turbulent reacting flows in the HC is implemented with CFD simulation. It is found that increasing the air jet momentum will help in achieving Homogeneous Combustion. The Homogeneous Combustion has a large tolerance to the unbalanced air jet distribution while if the disturbance is too large, local flames can appear. The thermal NO is almost negligible in Homogeneous Combustion while the prompt NO can be the most important path in the current fuel rich condition. The oxidation of NO to NO2 is a significant contribution to the consumption of NO while the reduction of NO2 feeds back as a source of NO production.PHDMechanical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/151465/1/caoweiyu_1.pd