Impact of Steam-Dilution on the Flame Shape and Coherent Structures in Swirl-Stabilized Combustors

Abstract

Humidified gas turbines and steam-injected gas turbines are promising technologies to lower the emissions and increase the efficiency and fuel flexibility of gas turbines. In the current study, the influence of steam-dilution on swirl-stabilized methane and hydrogenfired flames is experimentally investigated at Reynolds numbers in the range of 22,000 to 32,000. Velocity fields and flame positions were measured using high-speed particle image velocimetry and OH * chemiluminescence. An extension of the quantitative light sheet technique was employed to estimate the temperature fields. The combined results reveal strong changes in the flame position, the velocity field, and the temperature field with increasing rates of steam dilution. In particular, three different flow and flame patterns are encountered: At dry conditions, a V-shaped flame stabilizes in a broad inner recirculation zone with low local turbulent kinetic energy; at moderate steam content, the flame changes into a trumpet-like shape; and at very high rates of steam-dilution, the flame detaches and shows an annular shape. The associated coherent flow structures are extracted from the particle image velocimetry data employing proper orthogonal decomposition. The isothermal flow is dominated by a helical instability arising near the combustor inlet. This structure is completely suppressed for the dry flame and reappears for the heavily steam-diluted detached flame with a similar shape and frequency as for the isothermal case. The flow field of the trumpet-like flame at intermediate to high steam dilution rates features a helical instability of lower frequency that is located further downstream than in the isothermal and very wet case. A conceptional explanation is presented that relates the suppression of the helical instability to the specific encountered temperature fields and flame shapes.

Formation and flame-induced suppression of the precessing vortex core in a swirl combustor: Experiments and linear stability analysis

“…The results might be relevant to other swirl combustors where bi-stable flames have been reported [19,[67][68][69][70][71]. E.g., Hermeth et al [71] observe in their LES very similar flame-shape bifurcations as reported here, however not providing an explanation based on flow stability considerations.…”

“…While the density is approximately uniform for the non-reacting case A, the reacting cases B and C exhibit strong density variations due to thermal expansion of the unburned gas. For these cases the density was determined using the quantitative light sheet (QLS) technique [19,40,49]. As the QLS technique derives the density from the Mie scattering signal of the PIV particles, the particle images recorded for the velocity measurements were used for the density estimation.…”

“…In other combustors, the PVC forms for non-reacting cases, while it is suppressed for certain, but not necessarily all reacting cases [14][15][16][17][18]. It was further observed that the suppression of the PVC in a swirl flame can be triggered by changes of steam content and preheat temperature [19], swirl number [20], thermal power [21], pilot flow rate [22], or axial air injection [23]. The suppression of a PVC in a flame is usually accompanied by a major change of the flame shape [20], which in turn affects the combustor performance in terms of thermoacoustics, NO x emissions, flashback or blowout.…”

Experimental Study of Hydrogen Addition Effects on a Swirl-Stabilized Methane-Air Flame

“…Similar suppression effect of combustion on PVC instability was observed in some research [42,43]. More studies indicated that the PVC occurrence and amplitude under the combustion conditions strongly depended on the operating conditions, e.g., equivalence ratios and the confinement levels [44,45]. The effects of equivalence ratio and fuel composition were examined in the current study.…”

Prediction and control of combustion instabilities in real engines

“…Among all helical, shear later disturbances of swirled ows, the most well known is the PVC (precessing vortex core) and it plays a role in many CIs. Experiments [15,[151][152][153] as well as LES [12,14,154,155] showed that PVCs can trigger certain CI modes, disappear when the ame is ignited or appear only for certain ame positions. PVCs can even appear intermittently during CI.…”