Publications

@article{hariharan2020PRL,

title = {Effects of circulation and buoyancy on the transition from a fire whirl to a blue whirl},

author = {Sriram Bharath Hariharan, Yu Hu, Michael J. Gollner, and Elaine S. Oran},

url = {http://firelab.berkeley.edu/wp-content/uploads/2021/01/2020-PhysRevFluids.5.103201-effects-of-circulation-and-buouyancy-on-the-transition-from-a-fire-whirl-to-a-blue-whirl_PREPRINT.pdf},

doi = {10.1103/PhysRevFluids.5.103201},

year  = {2020},

date = {2020-10-14},

journal = {Physical Review Fluids},

volume = {5},

number = {103201},

abstract = {The relative influence of circulation and buoyancy on fire whirls (FWs), blue whirls (BWs), and the transition between these regimes of a whirling flame is investigated using a combination of experimental data and scaling analyses. FWs are whirling, turbulent, cylindrical yellow (sooting) flame structures that form naturally in fires and are here created in laboratory experiments. In contrast, a BW is a laminar, blue flame (nonsooting) with an inverted conical shape. Measurements of the circulation and heat-release rate are combined with measurements of the flame geometry, defined by the flame width and the height, to provide characteristic length scales for these whirling-flame regimes. Using these, a nondimensional circulation (Γ∗f) and a heat-release rate (˙Q∗f) were defined and shown to correspond to azimuthal and axial (buoyancy driven) momenta, respectively. The ratio  R∗=Γ∗f/˙Q∗f, a quantity analogous to the swirl number used to characterize swirling jets, was evaluated for FWs and BWs. For FWs, R∗\<1, so that axial momentum is greater than azimuthal momentum and the flame is dominated by buoyant momentum. For BWs, R∗\>1, so that the flame is circulation dominated. This is argued to be consistent with vortex breakdown being an important part of the transition of FWs to BWs. This work presents a basis for predicting when a BW will form and remain a stable regime.

},

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pubstate = {published},

tppubtype = {article}

}

@article{Hariharan2019,

title = {Thermal structure of the blue whirl},

author = {Sriram Bharath Hariharan and Evan T Sluder and Michael J Gollner and Elaine S Oran},

url = {https://linkinghub.elsevier.com/retrieve/pii/S1540748918301160},

doi = {10.1016/j.proci.2018.05.115},

issn = {15407489},

year  = {2019},

date = {2019-01-01},

journal = {Proceedings of the Combustion Institute},

volume = {37},

number = {3},

pages = {4285--4293},

abstract = {textcopyright 2018 Elsevier Ltd. The blue whirl is a recently discovered regime of the fire whirl that burns without any visible soot, even while burning liquid fuels directly. This flame evolves naturally from a traditional fire whirl in a fixed-frame self-entraining fire whirl experimental setup. Here, detailed thermal measurements of the flame structure performed using thermocouples and thin-filament pyrometry are presented. Thermocouple measurements reveal a peak temperature of ~2000 K, and 2-D temperature distributions from pyrometry measurements suggest that most of the combustion occurs in the relatively small, visibly bright, blue vortex ring. Different liquid hydrocarbon fuels such as heptane, iso-octane and cyclohexane consistently formed the blue whirl with similar thermal structures, indicating that blue whirl formation is independent of fuel type, and also that the transition from a fire whirl to a blue whirl may be influenced by vortex breakdown.},

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pubstate = {published},

tppubtype = {article}

}

@article{Hariharan2019a,

title = {The blue whirl: Boundary layer effects, temperature and OH* measurements},

author = {Sriram Bharath Hariharan and Paul M Anderson and Huahua Xiao and Michael J Gollner and Elaine S Oran},

url = {https://doi.org/10.1016/j.combustflame.2019.02.018},

doi = {10.1016/j.combustflame.2019.02.018},

issn = {15562921},

year  = {2019},

date = {2019-01-01},

journal = {Combustion and Flame},

volume = {203},

pages = {352--361},

publisher = {Elsevier Inc.},

abstract = {The blue whirl is a small flame with an inverted conical shape, first observed as it developed from fire whirls formed using liquid fuels burning on a water surface. Here, it is shown that the water surface is not critical for a transition from a fire whirl to a blue whirl, but that the surface over which the whirl is formed must be flat without any obstructions to the incoming flow. This observation highlights the importance of the radial boundary layer formed at the base of the whirl. The transition therefore also occurs over a flat metal plate, over which temperature maps of blue whirls are obtained using thin-filament pyrometry. Visualization of the reaction front, by imaging spontaneous OH* chemiluminescence, shows that a significant fraction of the combustion occurs in the small, visibly bright, blue ring. The temperature maps are consistent with the burning structure seen with chemiluminescence, and they are qualitatively similar for blue whirls formed over both water and metal plates. High frame-rate images of the transition process show the presence of a recirculation zone within the flame. Together, these observations distinguish the blue whirl as a flame structure unique from the fire whirl and present a basis for understanding the physical processes which control blue whirl formation and its structure.},

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pubstate = {published},

tppubtype = {article}

}

@article{Hu2019,

title = {Conditions for formation of the blue whirl},

author = {Yu Hu and Sriram Bharath Hariharan and Haiying Qi and Michael J Gollner and Elaine S Oran},

doi = {10.1016/j.combustflame.2019.03.043},

issn = {15562921},

year  = {2019},

date = {2019-01-01},

journal = {Combustion and Flame},

volume = {205},

pages = {147--153},

abstract = {This paper presents a laboratory study of the relation between blue whirls and fire whirls in terms of circulation (swirl) and energy-release rate. The blue whirl is a small, completely blue, soot-free flame that was originally seen when it evolved from more traditional fire whirls burning liquid hydrocarbons on water. The experimental apparatus consists of two offset quartz half-cylinders suspended over a water surface, with fuel injected onto the water surface from below. The flow circulation is calculated using the diameter of the enclosure and hot-wire velocity measurements made at the inlet gap between the half-cylinders. The heat-release rate was varied by adjusting the volumetric supply rate of liquid n-heptane, and is calculated assuming complete combustion. Results show that stable blue whirls form in a narrow range of circulation and energy-release rate close to a previously cited extinction limit. A scaling law derived from the data, based on the length scale of the enclosure, shows that the transition to a blue whirl depends on the gap size between the half-cylinders of the enclosure.},

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pubstate = {published},

tppubtype = {article}

}

@article{Hariharan2017,

title = {The Structure of the Blue Whirl},

author = {S B Hariharan and Y Hu and H Xiao and M J Gollner and Elaine S Oran},

url = {http://meetings.aps.org/link/BAPS.2017.DFD.D35.3},

doi = {10.1016/j.proci.2018.05.115},

issn = {1540-7489},

year  = {2017},

date = {2017-01-01},

journal = {70th Annual Meeting of the APS Division of Fluid Dynamics},

volume = {000},

pages = {1--9},

abstract = {Recent experiments have led to the discovery of the blue whirl, a small, stable regime of the fire whirl that burns typically sooty liquid hydrocarbons without producing soot. The physical structure consists of three regions -- the blue cone, the vortex rim and the purple haze. The physical nature of the flame was further investigated through digital imaging techniques, which suggest that the transition (from the fire whirl to the blue whirl) and shape of the flame may be influenced by vortex breakdown. The flame was found to develop over a variety of surfaces, which indicates that the formation of the blue whirl is strongly influenced by the flow structure over the incoming boundary layer. The thermal structure was investigated using micro-thermocouples, thin-filament pyrometry and OH* spectroscopy. These revealed a peak temperature around 2000 K, and that most of the combustion occurs in the relatively small, visibly bright vortex rim. The results of these investigations provide a platform to develop a theory on the structure of the blue whirl, a deeper understanding of which may affirm potential for applications in the energy industry. *This work was supported by an NSF EAGER award and Minta Martin Endowment Funds in the Department of Aerospace Engineering at the University of Maryland.},

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pubstate = {published},

tppubtype = {article}

}

@article{Xiao2016,

title = {From fire whirls to blue whirls and combustion with reduced pollution},

author = {H Xiao and M J Gollner and E S Oran},

doi = {10.1073/pnas.1605860113},

issn = {10916490},

year  = {2016},

date = {2016-01-01},

journal = {Proceedings of the National Academy of Sciences of the United States of America},

volume = {113},

number = {34},

abstract = {textcopyright 2016, National Academy of Sciences. All rights reserved. Fire whirls are powerful, spinning disasters for people and surroundings when they occur in large urban and wildland fires. Whereas fire whirls have been studied for fire-safety applications, previous research has yet to harness their potential burning efficiency for enhanced combustion. This article presents laboratory studies of fire whirls initiated as pool fires, but where the fuel sits on a water surface, suggesting the idea of exploiting the high efficiency of fire whirls for oil-spill remediation. We show the transition from a pool fire, to a fire whirl, and then to a previously unobserved state, a "blue whirl." A blue whirl is smaller, very stable, and burns completely blue as a hydrocarbon flame, indicating sootfree burning. The combination of fast mixing, intense swirl, and the water-surface boundary creates the conditions leading to nearly soot-free combustion. With the worldwide need to reduce emissions from both wanted and unwanted combustion, discovery of this state points to possible new pathways for reduced-emission combustion and fuel-spill cleanup. Because current methods to generate a stable vortex are difficult, we also propose that the blue whirl may serve as a research platform for fundamental studies of vortices and vortex breakdown in fluid mechanics.},

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pubstate = {published},

tppubtype = {article}

}