Experimental Investigation on Burning Characteristics of Ice Cavity Pool Fires Under Cross Airflow

環境風作用下冰腔池火燃燒特性實驗研究

Student thesis: Doctoral Thesis

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Award date24 Jul 2020

Abstract

Cleaning offshore oil spills has always been a worldwide problem, especially in the Arctic and sub-Arctic regions where oil production and transportation activities are increasing frequently. At that special ice-affected places, due to the perennial ice cover and different kinds of naturally formed gaps, cracks, the traditional machinery and physical methods are becoming harder to play its proper role. In-situ burning, a chemical treatment method which burned the released oil directly on the spill medium, nowadays has become the most popular cleanup strategy in ice-affected spill situations. However, as an important oil spill cleanup technique, ISB has not been well explored in scientific depth. Actually most of the previous efforts are basically focused on combustion of liquid fuel on open water in cold regions, very few studies on the fire behavior of pool fires with boundary condition of ice cavities. Meanwhile, the previous studies focused on the burning of these fires only in a quiescent environment, the effect of cross airflow still remains un-investigated. However, in real outdoor oil spill scenario, there will be the presence of cross flows (or under wind conditions). Once cross airflow is involved, it can be anticipated that the ISB will be more complicated. Note that the cross airflow not only influences the heat transfer mechanisms (radiation, convection, conduction) and mass burning rate for a pool fire, but also affects the flame geometry through altering the convection, counteracting the buoyancy of the plume, as well as changing the entrainment and mixing of fuel and air. The relevant combustion behaviors would be changed significantly owing to the deflected flame and altered flow boundary layer condition, which could always be a big challenge for oil spill response in the Arctic regions.

In this thesis, a series of experiments were carried out and a combination of experimental and theoretical methods were employed to investigate the burning characteristics of ice cavity pool fires in cross airflow, in contrast to previous works only limiting to quiescent air. The burning characteristics (ice cavity dynamics, mass burning rate, burning efficiency as well as the flame geometrical characteristics) of n-heptane (a pure fuel with thermo-physical properties similar to crude oil) in ice cavities of various parameters (initial lip height (h) and cavity diameter (D)) and different cross flow air speeds were systematically investigated. Heat feedback mechanisms and scaling behaviors of ice cavity pool fires in cross winds was also discussed compared with classical pan pool fires (confined by rigid walls). Major findings include:

(1) The burning characteristics of ice cavity pool fire in a windless environment were studied and compared with previous studies, which provides basic data and analysis for the next step to understand the burning characteristics of ice cavity pool fire under cross airflow. Moreover, these studies were compared with pool fires burning on open water, burning efficiencies and related heat transfer mechanisms were also discussed in details.

(2) Studies on burning characteristics of various lip height ice cavity pools under cross airflow. The evolution of burning characteristics such as the geometry change of ice cavities, mass burning rate, and burning efficiency of ice cavity pool fires were totally revealed. Three typical burning phases of the mass burning behavior in cross airflow of ice cavity pool fires were identified. The coupled effect of cross airflow and lip height on the heat feedback mechanism and the mass burning rate of ice cavity pool fires was investigated.

(3) Studies on burning characteristics of various size ice cavity pools under cross airflow. The evolution of burning characteristics such as the geometry change of ice cavities, mass burning rate, and burning efficiency of ice cavity pool fires were totally revealed. Significant asymmetric cavity expansion was formed and quantified. A dimensionless fitting formula for ice cavity expansion and mass burning rate under cross airflow was proposed. The physical mechanism of heat feedback in each evolution stage of ice cavity pool fires was analyzed, and the heat transfer model was established. Meanwhile, under relatively strong wind conditions (Fr>1), the hydrocarbon pool fire scale effect model based on stagnant layer solution theory was further extended to ice cavity pool fires.

(4) Studies and quantifies on flame geometrical characteristics for ice cavity pool fires with/without cross airflow. The applicability of the flame height/tilt angle prediction models (for classical pan pool fires) to the ice cavity pool fire were discussed. A modified flame height prediction model suitable for ice cavity pool fire under cross airflow was established, and the application scope of the AGA flame tilt angle prediction model was further extended to ice cavity pool fires.

    Research areas

  • in-situ burning, the Arctic and sub-Arctic regions, Hydrocarbon pool fire, cross airflow, ice cavity, scale effect, combustion characteristics, geometry change, mass burning rate, burning efficiency, heat transfer model, flame geometrical characteristics, flame height, flame tilt angle, lip height