Influence of Turbulence on Fuel Slick Fires with an Immersed Conductive Object

Chang, Li

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Influence of Turbulence on Fuel Slick Fires with an Immersed Conductive Object

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The hazard associated with crude oil spills has been a major environmental concern as the increasing demands of offshore drilling and oil tanker transport in recent decades. The burning of hydrocarbon liquid fuel floating on water has been a problem of great interest, as it forms the theoretical basis of the oil spill clean-up technique called in-situ burning (ISB). To understand the burning behavior of such configuration, this study analyzes the problem of a floating liquid fuel on wavy water by introducing a bench-scale platform capable of simulating ocean-like turbulent conditions in a controllable fashion. Further, this study investigates the application of an immersed conductive metal for burning enhancement under turbulent conditions. A series of lab-scale experiments are designed to systematically investigate the fuel layer burning with the turbulent water sublayer and the immersed metal object. The first set of experiment studies the subcooled nucleate boiling using an electrically heated thin nichrome wire in a dodecane layer that sits on a turbulence water surface. The surface turbulence is generated by a confined-jet cylinder platform. Four stages of bubble behaviors are observed characterizing different regions of the nucleate boiling curve. They are, respectively: sweeping stage, bouncing and merging stage, hanging stage, and film boiling stage. Bubble departure frequency (BDF) and critical heat flux (CHF) increase as a function of turbulence intensity represented by root-mean-square (RMS) velocity. Bubble departure size decreases as a function of turbulence RMS velocity. Bubble propulsion mechanism is dominated by two competing factors: Marangoni force and thermocapillary convection. Both are subject to the impact of turbulent eddies, which facilitates the bubble lift-off, and improves the heat transfer performance. The second set of experiments are conducted to understand the influence of surface turbulence on the crude oil slick burning on a water bath. As the turbulence intensity increases, a mitigated burning process with smaller flame height and boilover behavior is observed. The thermocouple array measurements indicate that the water sublayer cannot develop temperature gradients even at the low level of turbulence intensity at u'=0.017 m/s. The heat reaching the water is instantly taken away because of the turbulent mixing thus facilitating the heat loss of the oil slick, which causes a reduction in MLR up to 34% at u'=0.033 m/s compared to baseline. The convective heat transfer coefficient in the water sublayer is obtained by matching the one-dimensional conduction model to the measurement data and has the values of h_conv= 78, 93, 125 W/m2 corresponding to turbulence intensity u'=0.017, 0.025, 0.033 m/s. The heat and mass transfer characteristics during the oil slick burning are highly related and both are subject to the impact of the presence of turbulence in the water sublayer. The final sets of the experiments study the heptane and dodecane layer burning on a turbulence water surface with an immersed copper rod. Under zero turbulence condition (u^'=0 m/s), the fuel layer burning rate is noticeably affected by the fuel type, or more specifically, the boiling point of the fuel compared to the water boiling point. The dodecane layer (boiling point of 216 °C) burning cases has nucleate boiling on the rod’s surface in the water sublayer, while the nucleate boiling occurs on the rod’s surface within the fuel layer in heptane layer (boiling point of 98 °C) burning cases. The different nucleate boiling region causes suppression and facilitation on the dodecane and heptane burning rate respectively. The heat transfer by bubble generation is estimated by bubble counting using highspeed image processing. While the nucleate boiling is not observed for both types of fuel on the rod’s surface under turbulence conditions (u^'=0.017 and 0.033 m/s), the suppression and facilitation effects by bubbles in no turbulence condition are no longer in consideration, and the convective heat loss at the water interface become a major factor. For each burning configuration, a burning rate formulation coupled with experimental results is used to capture the details in the heat and mass transfer features. Finally, a theoretical analysis on the effects of multiple copper rods placement is studied using the measurement data adopted from the experiments. The results predict an over 300% increase for dodecane burning rate when 4 copper rods with diameter of 10 mm are immersed in the fuel layer under turbulence intensity of u'=0.03 m/s. For heptane burning cases, the burning rate increases to more than 400% the baseline when 3 rods are immersed in the fuel slick. This analysis provides a reasonable engineering basis to determine the number of the copper rods required for the Flame RefluxerTM design to achieve a certain level of enhancement.

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