On the Significance of a Wall Effect in Enclosures with Growing FiresCOOPER, LEONARD Y.
doi: 10.1080/00102208408923796pmid: N/A
Abstract This paper studies the significance of a wall effect that has been observed during the growth stage of enclosure fires. Relative to the two-layer phenomenon which tends to develop during such fires, the effect has to do with the near-wall downward injection of hot upper layer gases into the relatively cool uncontaminated lower layer. It is conjectured that these observed wall flows are buoyancy driven, and that they develop because of the relatively cool temperatures of the upper wall whose surfaces are in contact with the hot upper layer gases. For a growing fire (growth proportional to t m ; t being time and m≥0) in an enclosed compartment, an analysis of the conjectured mechanism for the wall flow leads to a time-dependent solution for the ratio of wall flow mass ejection rate from the upper layer, the, to the fire plume mass injection rate to the upper layer, ṁ p . The solution indicates that in practical fire scenarios ṁ w /ṁ p can be of the order of “several tenths” even prior to the time that the upper layer interface has dropped to an elevation midway between the ceiling and fire. In other words, the results of the analysis indicate the importance of taking the wall effect into account in two-layer zonal analyses of enclosure fire phenomena.
Nonluminous Radiation in Turbulent Buoyant Axisymmetric FlamesJENG, S-M.; LAI, M-C.; FAETH, G. M.
doi: 10.1080/00102208408923797pmid: N/A
Abstract A theoretical and experimental study of nonluminous radiation properties of turbulent, buoyant, axisymmetric, methane diffusion flames burning in still air is reported. Spectral radiance was measured along radial paths within and near the luminous portion of three flames whose structure was studied earlier in this laboratory. Predictions of spectral radiance were found by solving the equation of radiative transfer along the same paths. A narrow-band model was used with flow structure predicted by the conserved scalar formalism of a k-ε-g turbulence model. The computations were based on mean properties as well as a stochastic method which allowed for turbulent fluctuations. In general, the stochastic method overestimated the measured spectral radiances by 10-30 percent, while the mean property method was in reasonably good agreement with measurements. Therefore, while turbulent fluctuations have a significant influence on non-luminous radiation properties of the present flames, the effect is comparable to uncertainties in current flame structure and narrow-band models.
Calculations of Three Dimensional Buoyant Plumes in EnclosuresBAUM, HOWARD R.; REHM, RONALD G.
doi: 10.1080/00102208408923798pmid: N/A
Abstract A computational model of the three-dimensional buoyant convection and aerosol dynamics induced by a weak volumetric source of heat and mass is presented. The hydrodynamics is directly based on the time-dependent inviscid Boussinesq equations. No turbulence model or other empirical parameters are introduced. The use of Lagrangian particle tracking together with an exact solution of the Smoluchowski equation allows prediction of smoke aerosol transport and coagulation. The combined calculations represent predictions involving five independent variables. Flow features from three different configurations are illustrated with both Eulerian and Lagrangian displays of information. Sample aerosol coagulation results are compared with data reported from a wood fire. The computer resources required are discussed, and an assessment of the current feasibility of large-eddy simulations in fire research is made.
Thermal Actuation of Extinguishing SystemsEVANS, DAVID D.
doi: 10.1080/00102208408923799pmid: N/A
Abstract A brief review of the Response Time Index (RTI) method of characterizing the thermal response of commercial sprinklers and heat detectors is presented. Measured ceiling layer flow tem-perature and velocity histories from a bedroom fire test are used to illustrate the use of RTI in calculating sprinkler operation times. In small enclosure fires, a quiescent warm gas layer confined by the room walls may accumulate below the ceiling before sprinkler operation. The effects of this warm gas layer on the fire plume and ceiling-jet flows are accounted for by substitution of an equivalent point source fire. Relationships are given for the location and strength of the substitute source relative to a point source representation of the actual fire. Encouraging agreement was found between measured ceiling-jet temperatures from steady fires in a laboratory scale cylindrical enclosure put into dimension-less form, based on parameters of the substitute fire source, and existing empirical correlations from fire tests in large enclosures in which a quiescent warm upper gas layer does not accumulate.
Investigation of Completeness of Combustion in a Wall FireSIBULKIN, MERWIN; MALARY, STEVEN F.
doi: 10.1080/00102208408923800pmid: N/A
Abstract The burning of methyl methacrylate on a porous, vertical surface has been studied. The major objective of this investigation was to measure the degree of completeness of combustion in order to determine the actual heat of reaction in the flame. This “effective heat of combustion” is then used in diffusion flame modeling to predict fuel burning rates. Measurements of CO and CO2 concentration profiles were made in O2/N2 mixtures for ambient oxygen concentrations of 0.20 to 0.30. The results show that relatively large concentrations of CO are produced. Total rates of CO and CO2 production were calculated by integrating across the boundary layer using the measured concentrations and flow factors obtained from theory. It is found that up to 40 percent of the fuel carbon does not burn lo CO2 which reduces the effective heat of combustion by 20 percent. Use of this reduced heat of combustion in fire modeling calculations reduces the predicted burning rates by up to 18 percent for free convection diffusion flames on vertical fuel surfaces.
Jet Diffusion Flame Suppression Using Water Sprays—An Interim ReportMcCAFFREY, B. J.
doi: 10.1080/00102208408923801pmid: N/A
Abstract The feasibility of using water sprays for the control of offshore oil/gas well blowout fires has been addressed. Considering the sheer scale of the problem, knowledge from a fundamental viewpoint is going to be required in order to extrapolate laboratory-sized flame studies up to full scale. Available data and appropriate literature concerned with the application of water sprays as a jet diffusion flame suppression/extinguishment agent have been reviewed. Small pneumatic atomizing nozzles using H2 gas, both as the flame source as well as the atomizing driver, have been used to scale high momentum jet flames and to study the effect of water on the flame. Thermodynamic equilibrium was shown to be an effective guide in interpretating the results. The effect of flame temperature reduction due to water sprays has been observed to correlate with a single spray parameter—the median drop diameter. Directions for further study have been indicated.
Data for Room Fire ModelsROCKETT, JOHN A.
doi: 10.1080/00102208408923802pmid: N/A
Abstract Data needs for state-of-the-art single room fire models are discussed using several examples. Three types of data are needed: geometric, thermal and chemical. Needed geometric data generally present no problem and are not discussed. Under thermal data those quantities which determine the transient surface temperature of objects in the room are considered. For inert materials, these should present no problem but few materials are inert. Even materials used for non-combustible walls or ceilings may have thermal properties which charge significantly during the fire development. Chemical properties are the additional data needed to define the burning behavior of materials. These present a more serious problem. Currently, the Harvard simulation needs fifteen properties and pseudo properties (surrogates for complex combinations of more fundamental properties not currently resolved by the models). The importance (relative sensitivity of the simulation predictions) and the availability of useful data are discussed. Some remarks about additional data needs for more detailed simulations currently being considered are also included.
Recent Developments and Trends in Fire Testing for Fire Growth in BuildingsTHOMAS, P. H.
doi: 10.1080/00102208408923803pmid: N/A
Abstract Comparing building material linings by various standard tests has long been known to expose ambiguities and anomalies. The recognition and publicity for this by Howard Emmons was one reason for his advocacy of the expansion of fire research in the USA. The achievement of this research and its international influence are already having a favourable impact on the development of international standards and the constructive relationship between fire modelling and fire testing is becoming widely accepted internationally. Fire tests are beginning to be seen as having to provide data for productive modelling as well as for regulatory processes.