RANS vs LES CFD for Gas-Fired Combustion Equipment Analysis

High temperature gas-fired furnaces and gas flares are widely used in the chemical and
petrochemical industries. The US EPA estimates that approximately 3,200 process heaters are used
in the U.S. petroleum refining industry and 1,400 fired heaters are used in the U.S. chemical
process industry [1]. NASA/NOAA estimated that in 2012 there were approximately 7,500 gas
flares worldwide burning approximately 143 billion cubic meters (BCM) natural gas [2]. Past
developments in combustion diagnostics, computational resources and software development have
greatly expanded the application of computational fluid dynamics (CFD) tools for combustion
analysis in the process industries. Effective use of CFD can help improve combustion efficiency,
enhance heat transfer efficiency, reduce pollutant formation, and establish better safety practices.
CFD has been used to develop insights into unique combustion phenomena (i.e., turbulent mixing
effect on reacting flows), evaluate new and modified equipment design (i.e., improved low-NOx
burners), assess operations (i.e., turn-down conditions), guide experimental work (i.e., reduce costs
of full scale testing), and calculate pollutant control impacts (i.e., flare burner tip spacing on soot
formation). However, not all CFD tools equally simulate all combustion processes accurately.
Successful application of CFD requires an understanding of which tools work best for which
applications.
This paper will compare Large Eddy Simulations (LES) to Reynolds-averaged Navier-Stokes (RANS)
based CFD applied to several combustion applications to illustrate issues to consider when applying CFD
to solve industrial problems. This paper will review the basis, strengths and limitations of these two
approaches and provide recommendations on when each is most applicable. An LES based CFD tool
simulates turbulent reaction chemistry coupled with radiative transport in buoyancy driven flames (i.e., gas
flares) and the impact large flames have on surrounding objects (i.e., wind fence, process equipment, etc.).
Validation work comparing LES based CFD simulations to RANS based simulations of a multi-point
ground flare test will be shown to illustrate each approach for this application. Recent LES based CFD
analysis of transient burner operation will also be discussed. Results of this work will be used to review and
discuss how CFD codes may help assess various risk scenarios including wind, % flame coverage, and thermal fatigue for a given geometry. Several examples will be used to illustrate applications where each
technology has worked and failed.