Modelling and Analysis of Flare Stack Thermal Radiation on Offshore Oil and Gas Facilities

Abstract

This research studied the capabilities of RANS-based computational fluid dynamics to model large scale 2-phase propane jet flares with considerations for thermal radiation, soot development, and other key flare properties. The goal of this work was to determine how modest CFD could be used in the generation and evaluation of large scale jet flares that are often only studied experimentally. Measurements of incident thermal radiation to remote targets, soot volume fraction, and relevant flare geometries are reported based on the CFD models produced and compared to various analytical and experimentally derived models.

The CFD study was completed using the commercial suite of tools from ANSYS, namely ICEM CFD 18 and Fluent 18. Emphasis was put on evaluating the pre-existing models within the solver. The standard k-ε turbulence model was used in this study due to its support from literature, and performance in preliminary tests. A probability density function approach was used to model the non-premixed combustion, and the 2-step soot model was used to predict soot production. The P-1 radiation model was used as it is an economical, non-raytracing model that allows for the inclusion of radiation from soot.

The CFD results were compared to a solid flame model as well as empirical models and measurements. The CFD predicted soot volume fraction in the flare between 2.7×10^(-7) and 4.9×10^(-7) depending on the flare boundary temperature selected. Incident thermal radiation was found to agree strongly with a solid flame model; ranging from 1.6 kW/m^2 at a 2m distance, to 0.11 kW/m^2 at 15m. Agreement with experimental radiometer data was weak, with the CFD results predicting much lower levels of thermal radiation. Average surface emissive power of the flare was determined to be 100.3 kw/m^2 using literature based techniques and agreed well with various empirical models from literature. The radiative signature of the flare was also studied for crosswinds of up to 6m/s, showing expected increases in incident radiation to downstream targets - up to 9.1 kW/m^2 in the near field for 6m/s winds.