Analyzing lean blow-off limits of gas turbine combustors based on local and global Damköhler number of reaction zone

Abstract

Lean Blow-Off (LBO) is of great significance for propulsion systems, and developing an efficient LBO prediction method can technically support combustor design. In this paper, a hybrid LBO prediction method combining Damköhler (Da) model and numerical simulation is proposed. The time scales of Damköhler number are defined based on Reaction Zone (RZ) which is obtained from the simulated flow field. RZ is divided into a number of blocks, and each block is modeled as a Perfectly Stirred Reactor (PSR). Two kinds of Damköhler numbers are calculated. Local Da (Dal) number is calculated specific to each reactor in RZ, and Dal field is generated to analyze the local flow and chemical characteristics within RZ under different operating conditions. The Dal fields under different LBO conditions show that reactions concentrate near the outlet of the swirl-cup where the flows out of the swirl-cup and the backflows caused by recirculation collide. Global Da number (Dag) is calculated based on the entire flow and chemical characteristics of RZ, and acts as an indicator of LBO. Theoretically, LBO occurs when Dag is smaller than 1. In the current study, though the inlet temperature (T3), inlet pressure (P3), and inlet mass-flow rate (ma) are varied, the Dag under LBO conditions maintains at about 1, and it is about 11 under design condition, which proves that LBO limits can be accurately predicted under wide operating conditions by the method proposed in this paper.