Effect of Nozzle Geometry on the Performance of Non-Assist Flares

TL;DR

This work analyzes how nozzle geometry influences combustion efficiency, mixing, and blow-out resistance in non-assist methane flares under crosswind. It employs high-fidelity large-eddy simulations with a flamelet progress variable approach to compare circle, square, LARE, HARE, and diamond nozzles in a jet-in-crossflow setup. Cornered geometries enhance near-field recirculation and mixing, enabling higher efficiency (up to about 5% improvement) and greater resistance to flame liftoff and blow-out, with the square nozzle performing best across wind directions. The results offer a practical, passive design strategy to improve flare performance under real-world crosswind conditions and align with regulatory constraints for emissions reductions.

Abstract

This study employs large-eddy simulations with a flamelet progress variable approach to systematically quantify the influence of nozzle geometry on combustion efficiency, mixing, and blowout resistance in non-assist methane flares. Five canonical nozzle shapes-circle, low aspect ratio ellipse, high aspect ratio ellipse, diamond, and square-were evaluated under relevant industrial flare conditions. Results demonstrate that cornered geometries enhance near-field recirculation, promote mixing, and sustain flame attachment, resulting in up to a 5% improvement in combustion efficiency compared with streamlined nozzles. The square nozzle performed best irrespective of the wind direction (orientation) and maintained a combustion efficiency greater than 96.5% even at the highest tested crosswind velocities, while other streamlined designs exhibited early flame lift-off, reduced recirculation, and efficiency losses. Analysis of mixing and vorticity reveals that sharp-edged nozzles accelerate scalar homogenization and buffer flames against crosswind-induced strain, directly translating to increased blowout resistance.

Figures (16)

• Figure 1: Schematic of the reacting jet-in-crossflow configuration. The white planes represent the domain boundaries where combustion efficiency is measured. The blue plane represents the crossflow inlet.
• Figure 2: Cross-sectional view of the nozzle geometries considered in this study relative to the crossflow direction $U_c$.
• Figure 3: Contours of mixture fraction on the $y-z$ plane at $x/D = 12D$. The contours, ordered from the outermost to the innermost, correspond to $Z=0.01,0.04,0.08,0.1$. Experiments by Salewski et al. salewski2008mixing (top row) and LES (bottom row) for a velocity ratio $r=0.25$.
• Figure 4: Comparison of combustion inefficiency as a function of crosswind velocity for the circle nozzle ($D = 7.62$ cm, $U_j = 3.4$ m/s). LES data (filled red circles). All open symbols represent flare experiment efficiency values from Stolzman et al.stolzman2025experimental (orange squares), Gogolekgogolek2024personal (blue triangles), Burttburtt2014efficiency (green inverted triangles), and Johnson and Kostuikjohnson2002parametric (magenta diamonds).
• Figure 5: Instantaneous flame profiles for various nozzle geometries (a) $U_c = 6.8$ m/s ($r=2$) and (b) $U_c = 13.6$ m/s ($r=4$), both with $U_j = 3.4$ m/s.