William M. Pitts

Global density effects on the self-preservation behaviour of turbulent free jets

An experimental investigation was designed to test the hypothesis that all axisymmetric turbulent free jets become asymptotically independent of the source conditions and may be described by classical similarity analysis. Effects of initial conditions were studied by varying jet exit boundary conditions and the global density ratio. The exit velocity profile and turbulence level was changed by using both pipe and nozzle flow hardware. Initial density differences were imposed by using three gases: helium, methane, and propane. The scalar field (concentration) in the momentum-dominated regime of the far field (10 to 60 jet exit diameters downstream) of turbulent free jets was characterized using Rayleigh light scattering as the diagnostic. The results show that regardless of the initial conditions axisymmetric turbulent free jets decay at the same rate, spread at the same angle, and both the mean and r.m.s. values collapse in a form consistent with full self-preservation. The means and fluctuations follow a law of full self-preservation in which two virtual origins must be specified. The two displacements are required to account for the effects of a finite source of momentum and different development of the velocity and mass distributions in the near fields of the jets. The memory of the jet is embodied in these two virtual origins.

Assessment of theories for the behavior and blowout of lifted turbulent jet diffusion flames

Many competing theories have been published to describe the characteristics and blowout of lifted turbulent jet diffusion flames. The assumptions which are made as to the physical processes responsible for these behaviors vary widely. In this paper these assumptions are summarized for each model and compared with the actual turbulent behaviors of unignited fuel jets. As part of this discussion, recent unpublished measurements of real-time concentration fluctuations along a line in a turbulent fuel jet are introduced. To the extent possible, each theory is also assessed as to its capabilities to accurately predict experimentally observed lift off and blowout behaviors. The conclusion of these analyses is that none of the currently-available theories for flame stabilization are satisfactory. Further experimentation is required before the actual physical processes responsible for flame stabilization can be identified and models which are capable of accurate prediction of lift off heights and blowout velocities developed.

The application of laser-induced Rayleigh light scattering to the study of turbulent mixing

This work describes the development and characterization of an experimental system employing laser-induced Rayleigh light scattering with digital data acquisition as a time-resolved quantitative concentration probe in the turbulent flow field of a binary gas mixture. Equations for the expected signal and noise levels are given. Estimates of these parameters for the experimental system used here are in satisfactory agreement with experiment. It is demonstrated that the laser Rayleigh-light-scattering technique provides measurements having high spatial and temporal resolution for various locations within the concentration flow field. Measurements at various positions in the flow field of an axisymmetric methane jet issuing into a slow flow of air are reported and, where possible, compared with appropriate literature results. The statistical properties of the turbulent concentration fluctuations are found to be in good agreement with other independent measurements. Conditionally sampled measurements are also reported and shown to behave in the same manner as the limited number of similar measurements in the literature. The capability of calculating power spectra and correlation functions for the time behaviour of the methane concentration is also demonstrated. Raman and Rayleigh scattering techniques are compared as measurement techniques of scalar values in turbulent flow fields.

The global equivalence ratio concept and the formation mechanisms of carbon monoxide in enclosure fires

Abstract This report summarizes a large number of investigations designed to characterize the formation of carbon monoxide (CO) in enclosure fires—the most important factor in fire deaths. It includes a review analysis of the studies which form the basis for the global equivalence ratio (GER) concept. Past and very recent (some as yet unpublished) investigations of CO formation in enclosure fires are reviewed. Based on the findings, two completely new mechanisms for the formation of CO, in addition to the quenching of a fire plume by a rich upper layer, which is described by the GER concept, are identified. The first is the result of reaction between rich flame gases and air which is entrained directly into the upper layer of an enclosure fire. Detailed chemical-kinetic modeling studies have demonstrated that CO will be generated by these reactions. The second is due to the direct generation of CO during the pyrolysis of oxygenated polymers (such as wood) which are located in highly vitiated, high-temperature upper layers. The findings of these studies form the basis of an analysis that provides the guidelines for when the use of the GER concept is appropriate for predicting CO formation in enclosure fires. It is concluded that there are limited conditions for which such use is justified. Unfortunately, these conditions do not include the types of fires which are responsible for the majority of fire deaths in building fires.

Wind effects on fires

Abstract Urban mass fires are relatively infrequent events which have historically resulted in immense losses of life and property. Mass fires often have occurred as the result of natural disasters or warfare. The development of nuclear weapons has increased the likelihood of urban mass fires due to the high level of thermal radiation generated by a nuclear detonation. There are a large number of wind-fire interactions which are important in the initiation, development, and spread of these large fires. Dramatic examples include the extremely high winds and fire whirls which are often generated by such fires. Other effects such as wind-aided fire spread, fire brand spotting, and the effects of the atmospheric turbulent boundary layer can contribute significantly to the growth and behavior of mass fires. In this review characteristics of the two types of mass fire—fire storm and conflagration—are discussed. Brief histories of urban mass fire and research efforts on this topic are given. Models which have been developed to predict the initiation, development, spread, and behavior of mass fires following the detonation of a nuclear device in an urban environment are summarized. The current understanding of the fire processes which are believed to control mass fire behavior are reviewed. Particular emphasis is placed on the wind-fire interactions mentioned in the last paragraph. This discussion forms the basis for an analysis of the effectiveness of existing models for mass fire growth and behavior. It is concluded that the understanding of the important physical processes is incomplete and that models for mass fire development and behavior are likely to be subject to large and uncharacterized errors. The possibility of improving our understanding of the underlying physical and chemical processes utilizing reduced-scale experiments is assessed.

Modeling of Bare and Aspirated Thermocouples in Compartment Fires

As part of an effort to characterize the uncertainties associated with temperature measurements in fire environments, models of bare bead, single-shielded aspirated, and double-shielded aspirated thermocouples were developed and used to study the effects of varying the gas and average effective surroundings temperatures on the thermocouple error of each configuration. The models were developed for steady-state conditions and hence provide information about error trends rather than about absolute error values. The models indicate that thermocouples respond differently to changes in effective surroundings temperature in a hot upper-layer than in a relatively cooler lower layer of a room fire. In an upper-layer, for a given gas temperature, the thermocouple error is relatively insensitive to surroundings temperature. In a lower layer, errors which increase rapidly with surroundings temperature are possible. The most extreme errors occur in a lower layer when the gas temperature is low and the surroundings temperature is high. Aspirated thermocouples reduce the errors in both the upper and lower layers of a room fire, but do not eliminate them entirely. The present study is intended to provide fire researchers with a methodology for developing working models of thermocouples which are tailored to their own configurations.

Effects of global density ratio on the centerline mixing behavior of axisymmetric turbulent jets

Measurements, utilizing Rayleigh light scattering, of timeaveraged concentration and unmixedness have been made along the centerlines of axisymmetric turbulent jets formed from six pairs of jet and ambient gases. Jet to ambient density ratios range from 0.14 to 5.11. Findings are compared with predictions of an approx. similarity analysis and with extensive previous literature measurements. It is shown that virtual origins for plots of inverse time-averaged concentration are strongly dependent on global density ratio. Unmixedness values first grow with increasing distance from the jet source and then achieve an asymptote. The flow distance required to reach this asymptote is a strong function of density ratio.

Importance of isothermal mixing processes to the understanding of lift-off and blowout of turbulent jet diffusion flames☆

Abstract Many different theoretical analyses have been developed to predict the lift-off and blowout behaviors of axisymmetric, turbulent jet diffusion flames. Past theoretical and experimental work is summarized. It is then shown that these flame stability properties can be predicted using the known time-averaged concentration and velocity profiles of the corresponding nonreacting jet flows of the fuels into air. In contrast to past theoretical treatments of these processes, it is not necessary to consider interactions of the turbulent concentration and/or velocity fluctuations with the combustion process. Possible physical mechanisms that can lead to such a finding are discussed.

Reynolds number effects on the mixing behavior of axisymmetric turbulent jets

Measurements of time-averaged jet fluid mass fraction and unmixedness are reported along the centerlines of axisymmetric jets having Reynolds numbers (Re) covering a range of 3,950–11,880. Jet gases investigated are propane, carbon tetrafluoride, and sulfur hexafluoride. The slopes for the fall off of inverse centerline mass fraction with distance are found to be independent of Re for moderate downstream distances, but virtual origins for the data are shown to move downstream with increasing Re. Unmixedness measurements show that flows with higher Re require longer flow distances to achieve asymptotic behavior. Results of other investigations reported in the literature are discussed which support the conclusions of this work. The relationship between the centerline mixing and entrainment behaviors of these flows is explored.

Greatly enhanced soot scattering in flickering CH4/Air diffusion flames

Abstract Planar images of laser-induced flurescence from OH · radicals and elastic scattering from soot particles are presented in time-varying, laminar CH4/air diffusion flames burning in a co-flowing, axisymmetric configuration at atmospheric pressure. Acoustic forcing is used to phase lock the periodic flame flicker to the pulsed laser system operating at 10.13 Hz. For conditions where the tip of the flame is clipped, the intensity of the light scattered by the soot particles increases dramatically (by more than a factor of 7 for the maximum signals at a point) compared to a steady-state, laminar flame with the same mean fuel flow velocity. Comparison of the scattering signals integrated along the flame radius is carried out in the steady-state and time-varying flames as a function of height above the burner. Time-varying flames exhibit a larger range of combustion conditions than observed in corresponding steady-state flames, including different residence times, temperature histories, local stoichiometries, and strain and scalar dissipation rates. Thus, their investigation promises to yield new insights into a wide variety of chemistry-flowfield interactions which are prominent in turbulent combustion.