The Effect of Gas Distribution in a Straight-Flow Vortex Burner on the Combustion Process in a Limited Furnace Volume

The aim of the study is to optimize the gaseous fuel combustion conditions in a furnace with a volume limited in the flue gas exit direction and equipped with straight-flow vortex burners. The effect the design and operating factors have on the environmental performance (nitrogen oxide emissions) and reliable operation of heating surface tubes directly exposed to the burner flame is investigated. The results of numerical modeling carried out using the ANSYS software package (ANSYS CFX computer program) and field tests at an industrial facility are given. To avoid excessive computation mesh density, the heating surface limiting the flame jet free propagation area was modeled by a filter, the use of which in CFD-modelling programs makes it possible to produce the required aerodynamic pressure drop and heat transfer. It has been found that in modeling the furnace space, the filter makes an effect similar to that of a heating surface and produces an adequate feedback in the medium flow, which determines its use in further calculations. Numerical experiments were carried, which have shown that the areas of the highest temperatures are in the central root zone and at the flame end as a result of fuel afterburning. Increasing the gas supply through the nozzles of the annular peripheral channels causes the fuel combustion process to take a longer time to proceed and shifts the flame high-temperature region to the furnace chamber end. This results in a reduced generation of nitrogen oxides, but in a higher temperature impact on the heating surface tubes. Increasing the gas flow rate to the central part causes the flame to become shorter, shifts the high-temperature zone to the flame root, accelerates the combustion process, and unambiguously leads to an increase in NOx concentrations. It has been found that gas redistribution between the outer and inner rings of the burner peripheral channel does not yield a visible effect. In the course of field tests, it has been quantitatively assessed how the combustion process (flame shape) influences the nitrogen oxide emissions and the temperature of the heating surfaces located at the furnace outlet and directly exposed to the flame. It is recommended to operate the boiler in the nominal mode with the minimum possible gas flow rate through the burner central header, which will provide the most favorable conditions both in terms of its reliability and environmental indicators.