Increased heat transfer to sustain flameless combustion under elevated pressure conditions – a numerical study

Abstract

Sustaining flameless combustion under elevated pressure conditions remains a challenge that hinders its application in gas turbines. Previous attempts succeeded to sustain flameless operation up to 5 atm. In the present work, reaction rate is decreased by increasing heat transfer through combustor walls so that the reaction zone is kept distributed. Pressure is elevated from 1 to 15 atm. and its effect on reaction zone location and spread, and emissions is investigated. Two-dimensional computations are performed using ANSYS fluent package. Under adiabatic combustor wall conditions, increasing the pressure results in a more concentrated reaction zone, and a departure from flameless mode at 3 atm. The calculated Damköhler number is noticed to increase from 0.2 to 1.9. NOx emissions increases from 10 to 78 ppm. Nearly 90 % of the predicted NOx are generated via the thermal route. Under isothermal combustor wall conditions, increased heat transfer is noticed to limit the increase in heat of reaction and temperature with pressure. The calculated Damköhler number is noticed to increase from 0.12 to 0.85. This is nearly half the values encountered under adiabatic conditions, which demonstrate a distributed reaction under all pressures investigated. Finally, NOx emissions are found below 5 ppm for all pressures investigated.