EXPERIMENTAL AND THEORETICAL STUDY OF THE EFFECT OF SECONDARY AIR DIRECTION ON NATURAL GAS DIFFUSION FLAME

The aim of the present paper is to study experimentally and theoretically the natural gas flame characteristics using different secondary air inlet directions for different primary air swirl numbers and different primary air to fuel ratios. Secondary air was introduced into the combustion chamber in four directions; normal, forward, backward and tangential relative to the chamber axis and the main flow direction. For this study, a test rig was constructed including a vertical test combustor with an air swirler. It also includes primary air, secondary air and fuel lines. Four air swirlers were used changeably. These swirlers have different vane angles of 15o, 30o, 45o, and 60o to generate different swirl numbers of 0.23, 0.5, 0.87 and 1.5, respectively. In addition to the base test combustor, four combustors were constructed to ensure the introduction of secondary air in four directions. During tests, the primary air mass flow rate was kept constant, while the fuel mass flow rate was changed to give different primary air/fuel ratios of 30, 40 and 50. Also, the secondary air flow rate was changed to give different secondary/primary air ratios of 0.25-1. A three dimensional model was used to simulate the turbulent reacting flow using computational fluid dynamics package (Fluent 6.3). The comparison between the measured and calculated temperature distribution shows a good agreement. Also, the results show that the introducing of secondary air leads to a decrease in the flame size, and tangential direction gives the shortest flame length for most cases while backward direction gives the longest one. Normal and forward directions produces higher O2 and lower CO and CO2 concentrations