Dissertation > Excellent graduate degree dissertation topics show

Experimental and Theoretical Researches on the Combustion Dynamics of Fire Whirls

Author: LeiZuo
Tutor: FanWeiCheng; LiuNaiAn
School: University of Science and Technology of China
Course:
Keywords: Fire whirl Ambient circulation Inflow boundary layer Burning rate Flame height Temperature distribution Flame revolution Flameprecession
CLC: X928.7
Type: PhD thesis
Year: 2012
Downloads: 165
Quote: 2
Read: Download Dissertation

Abstract


Fire whirl is a special fire phenomenon which may occur in both wildland and urban fires. Fire whirl is characterized by a strongly swirling flame which is induced by the complex interaction between the fire plume and the ambient vorticity field. The flame of the fire whirl involves the complex physical and chemical coupling between swirling flow and combustion. Fire whirl can highly increase the burning rate, flame height, flame temperature and velocity, and is easy to induce fire spotting, and thus the intensity and growth rate of fire can be significantly enhanced. Therefore, study of the combustion dynamics of fire whirl can improve our understanding on special fire phenomena which involve interaction between flow and combustion, and also the developed theories and models can serve practical fire protection by suppression of the onsets of such fire phenomena.This thesis presents a persistent study on the dynamical characteristics of fire whirl, including burning rate, flame height, velocity field and temperature field. The variations of these burning characteristics with parameters such as pool size and ambient circulation are investigated in detail, in order to develop relevant theoretical models or scaling laws. The inclined flame revolution and swirling flame precession are also studied in order to seek the physical mechanism for the flame instabilities involved in fire whirl. The similarities and differences between fire whirls and genral buoyancy fire plumes are examined from flow filed structure, heat field structure, dynamical behavior and models.The data concerning the burning rate, flame height, temperature field, velocity field and flame instabilities were obtained by systematic experiments using the medium-scale fire whirl facility with four walls and slits. The work and results of this thesis are summarized as follows.Ekman entrainment boundary layer theory is introduced to characterize the momentum boundary layer of fire whirl. Previous researches did not consider the specific flow characteristics of inflow boundary layer in fire whirls. In this thesis, the integral solutions for the Ekman momentum boundary layer of laminar and turbulent fire whirls are obtained based on the control equations of fluid mechanics and the Ekman entrainment boundary layer theory. The obtained solutions agree well with the experimental and numerical results. A model of burning rate for liquid fuel fire whirl is established. The heat and mass transfer mechanisms on the liquid fuel surface are extensively studied. It is shown that the mass and heat transfers on the fuel surface are controlled by natural convection in general pool fires, while are strongly enhanced by forced convection in fire whirls. In terms of the solutions for momentum boundary layer, and by using stagnant model or analogy between heat and mass transfer, semi-empirical correlations are established for burning rates of liquid fuels in laminar and turbulent fire whirls, which couple the burning rates with the two key parameters of ambient circulation and pool size. The results show that the inflow boundary layer above the fuel surface consists of an outer reactive region and an inner non-reactive region, which involve different heat and mass transfer mechanisms. The inflow boundary layer changes the flame shape near the fuel surface. Fuel evaporation rate in the outer region is relatively larger than that in the inner region, and a large proportion of fuel evaporates from the outer region. The fuel surfaces are aerodynamic rough surfaces in relatively large pools. The predictions agree well with the data from the present experiments and the literature.A new correlation for flame height of fire whirl coupling heat release rate and ambient circulation is developed. Two dynamical mechanisms are found to play important roles in the increase of fire whirl flame height. The first is by the enhancement of fuel burning rate resulting from the strong inflow in the bottom boundary layer, and the second is by the significant suppression of turbulent mixing between fuel and air near the flame, which is due to the combined effect of rotation and the positive radial density gradient. There is good agreement between the correlation and the experimental flame height data. The flame height is shown to be proportional to the ambient circulation for both laminar and turbulent fire whirls with liquid fuels.The temperature distribution of fire plumes in fire whirls is discussed in detail. A normalized height in the axial direction for fire whirls is defined, thereby the flow at the plume origin and the related centerline excess temperature decay are investigated. It is found that similarly to general buoyancy fire plume, the fire plume of fire whirl also involves three distinct zones:the continuous flame, the intermittent flame and the plume. In the continuous flame, the radial temperature profile of fire whirl is of hump-type, which indicates that the inner core is fuel-rich. For fire whirls, the ratio between the continuous flame height and the maximum flame height, and the maximum centerline excess temperature, are both much larger than those for general pool fires. It is found that the plume is a moderate swirling jet at its origin and finally decays to a free buoyancy plume. As a result, the plume centerline excess temperature decays rapidly with increasing normalized height adjusted by the plume origin, and approaches the theoretical value of-5/3for free buoyancy plume.The inclined flame revolution and swirling flame precession are studied to understand the three dimensional instabilities involved in fire whirls. The concept of flame revolution and precession in fire whirls are definitely described and termed, and the dynamical mechanisms for the two kinds of flame instabilities are discussed. The experimental observations showed that generally the flame firstly inclines and at the same time the whole flame revolves around the geometric symmetrical axis of the facility, with increasing angular velocity, until the full formation of the fire whirl. There is no self-rotation of the flame during this stage. After that, the erect swirling flame starts to deviate from and precess around the geometrical symmetrical axis of the facility with a relatively stable frequency. The frequencies of flame revolution and precession are both proportional to the average inlet velocity, and the corresponding Strouhal numbers are constants of0.42and0.80respectively. The flame revolves and precesses in the same direction as the self-rotation of the fire whirl flame. The flame revolution is related to the periodical fluctuations of inlet flow, while the flame precession is considered to be concerned with the existence of an external recirculation zone in the fire whirl.

Related Dissertations

  1. The Influence of Long-term Regular Exercise to the Distribution and Change Regulation on Human Skin Temperature,G804.2
  2. The Studying of Temperature Distribution and Control Technique during Polymer Sheet Heat Process,TQ320.6
  3. Digital Simulation of Temperature in Modern Greenhouse,S625.51
  4. Study on Microwave Heating Characteristics of Fresh-water Surimi and Surimi-soybean Compound,TS254.1
  5. Experiment Research and Temperature Distribution Simulation of Thermoplastics in Laser Transmission Welding,TQ320.1
  6. Research on the Effect of Microwave Modification on Pulverized Coal Combustion Properties,TF538.63
  7. Study of the Temperature Distribution Model and Designing Method for Efficient Shell-and-Tube Heat Exchangers,TQ051.5
  8. Experimental Investigation on the Estimation of Temperature Distributions and Thermal Radiative Properties in Furnaces,TP391.41
  9. Arc Plasma Temperature Diagnosis of Pulsed Gas Metal Arc Welding,TG403
  10. Hot wire TIG hot wire welding temperature analysis and simulation of temperature field,TG422.3
  11. Study on Brazing of Multiple Parallel Flow Condenser in Controlled Atmosphere Brazing Furnace,TG454
  12. Experimental Research on Pumping and Airblast Style U-Type Radiant Tube,TK172
  13. Study on Surrouding Rock Thermal Couductivity Experiment and Distribution of Temperature Field in Deep Roadway,TD727
  14. Spontaneous Combustion of Coal Pillar Roadway Numerical Simulation of Temperature Distribution,TD752.2
  15. Research on the Law of Temperature Distribution and Heat Transfer of Microwave Reheating of Instant Rice,TS217.1
  16. The Research of Transformer’s Overload Capacity Improvement,TM406
  17. Numerical Investigation on Temperature Distribution of Aerodynamic Heating Surface with Liquid Nitrogen Cooling,TK124
  18. Calculation of Transient Heat Load for Aircraft Cabins,V223
  19. Research on Numerical Calculation of Lubricating Properties of Water-lubricated Thrust Bearing of Submersible Pump,TH133.3
  20. The oxygen-enriched top-blown smelting reduction furnace Numerical Simulation and Experimental Study,TF557
  21. The Heat Transfer Computation of Multilayer Cylinder Unsteady Nature in Steady State,TF061.21

CLC: > Environmental science, safety science > Safety Science > Safety management (labor protection and management ) > Accident investigation and analysis ( work-related injuries and prevention ) > Fire and explosion
© 2012 www.DissertationTopic.Net  Mobile