Frederick W. Mowrer

Lag times associated with fire detection and suppression

The effectiveness of fire detection systems and fire mitigation strategies can be related to three distinct time lags associated with building fires: a transport time lag, a detection time lag, and a suppression time lag. The impacts of these lag periods on fire detection and suppression are developed. Transport lag periods are considered in terms of available correlations of fire plume and ceiling jet data, detection lag periods in terms of available heat detector response models that use these data correlations. Suppression lags are developed in terms of expected response times for automatic and manual suppression. Example calculations are presented.

Methods to characterize heat release rate data

Methods to characterize experimentally measured heat release rate data are developed. Exponential and t-squared representations are considered in detail; algorithms for these two representations are presented. These characterizations require the selection of two representative data pairs from a measured heat release rate history and evaluation of the peak heat release rate and the total cumulative heat released by a product. A superposition method is developed for the evaluation of composite products. Comparisons with measured heat release rate data are presented for a number of products. The selection of characterization parameters based on basic material properties remains for the future.

Enclosure Smoke Filling and Management with Mechanical Ventilation

Enclosure smoke filling and management are addressed from the standpoint of the volumetric flow rates commonly used for mechanical ventilation system design. In this context, fire-induced gas expansion is treated as a volumetric source term. A two-layer analysis developed previously for enclosure smoke filling without mechanical ventilation is extended to consider the impact of mechanical ventilation on smoke layer descent rates and conditions within the smoke layer. A spreadsheet-based model of enclosure smoke filling developed in conjunction with the previous unventilated analysis is also extended to consider both mechanical extraction and injection systems. Some implications of mechanical ventilation on the development and descent of a smoke layer in an enclosure fire are discussed.

Measurements of Smoke Characteristics in HVAC Ducts

The characteristics of smoke traveling in an HVAC duct have been observed along with the response of selected duct smoke detectors. The simulated HVAC system consists of a 9 m long duct, 0.45 m in diameter. An exhaust fan is placed at one end of the duct and is capable of inducing airflow rates that range from 0 to 1.5 m3/s. The flow is controlled by means of a manual damper. On the upstream end of the duct there is a square exhaust hood approximately 2.2 m at the bottom and 0.3 m at the top. The bottom of the hood is approximately 2.5 m above the floor a shroud extends down to approximately 1.5 m above the floor. The test section, placed immediately downstream of the hood, is 3.5 m long duct with a square cross section of 0.4 m on a side. The instrumentation includes oxygen, carbon monoxide and carbon dioxide gas analyzers and a load cell to determine the energy release rate of the fires tested. The smoke within the duct is characterized by means of a laser light sheet and charge coupled device (CCD) camera, two white light source and photocell ensembles, a Pitot tube and an array of eight thermocouples placed on the vertical plane of symmetry. A smoke detector was placed at the downstream end of the test section. Two types of detectors were tested, ionization and photoelectric, with a single sampling probe geometry. The fires tested cover a wide range of fuels (propane, heptane, toluene, toluene/heptane mixture, shredded paper, polyurethane foam, wood cribs) with the peak energy release rates up to 800 kW. The smoke detector performance, temperature, flow field, smoke particle size and particle distributions are dependent on the fire characteristics and airflow through the duct. The different measurements could be scaled by means of the fire size and airflow rate but left a strong dependency on the fuel and burning characteristics (i.e., smoldering, flaming). The optical density and mass optical density are analyzed as metrics for characterizing smoke and smoke detector response. Detailed comparisons between the different metrics used are presented throughout this work. Clear evidence of stratification and aging of the smoke along the duct are also presented. The limitations of the present configuration and the need for a larger scale study are also discussed.

A Comparison of Driving Forces for Smoke Movement in Buildings

It has been suggested that automatic shutdown of mechanical ventilation systems upon smoke detection may be superfluous and unnecessary because other driving forces will continue to transport smoke throughout buildings after the ventilation systems have been shut down. To evaluate this hypothesis, an analysis is presented that considers the relative smoke concentrations throughout a building arising from the driving forces of stack effect, wind effect, and buoyancy of combustion gases as well as those arising from mechanical ventilation. This analysis considers a representative 10-story building, but the approach presented can be extended to buildings of different heights and areas. The results of this study indicate that the shutdown of mechanical ventilation may not prevent the smoke contamination of nonfire floors, but it may still be preferable to leaving ventilation systems running unless the systems are specifically designed for smoke management.

Hazard Assessment of Fire in Electrical Cabinets

Abstract Fire in electrical cabinets is of major concern in nuclear power plants. With the need to reduce incoming electrical power from 14 kV to as low as 50 V, and the need to supply power to hundreds of electrical components, there is an abundance of electrical cabinets in nuclear power plants. The combination of fire load and live electrical energy within electrical cabinets has caused fires and explosions. Such fires are of concern as they may disrupt the delivery of electrical power and instrumentation and control in the plant. In addition, the fire can propagate to nearby cabinets and plant components. This paper presents advances in the knowledge and understanding of the conditions inside a cabinet due to fire and ranks fire hazard potential of electrical cabinets. Test results for electrical cabinet fires have been reported by Sandia National Laboratories and by the Technical Research Centre of Finland (VTT). The Sandia tests provide data for fires in control cabinets. The VTT tests provide a model for calculation of burning rates inside a specific electrical cabinet. This research included a site visit to a nuclear power plant to understand variations in electrical cabinet design as well as performing 39 cabinet fire tests with varying burning rates, ventilation openings, and cabinet sizes. Two types of fuels were used for this study: propane gas and heptane liquid. This paper identifies the minimum fire size that can be maintained in a cabinet as a function of ventilation openings, cabinet wall temperatures, and radiation levels, and the characteristics of external smoke and fire plumes. Based on the test results, a one-zone model was developed for mathematical simulation. The model was used to expand on the results of the tests to construct a risk matrix of fire hazards for various cabinets as a function of the cabinet size, fire size, and ventilation openings. Since the test results in this study are based on propane and heptane as the fire load, it is desirable to also test the effect of fire load from electrical components and wiring, given a range of cabinet dimensions and vent conditions. Heat flux measurements should include the external smoke and/or flame plumes. Further studies should analyze the possibility and implications of an explosion within a cabinet, and results should be compared with existing national and international design standard requirements.

A Closed-Form Estimate of Fire-Induced Ventilation Through Single Rectangular Wall Openings

A closed-form method is developed for estimating quasi-steady mass flow rates in room fires ventilated only through a single rectangular wall opening. This method uses lin earized forms of the vent flow and plume entrainment equations. Two forms of plume entrainment equations are considered: axisymmetric plumes and line fires located against walls. Mass flow rates calculated by this method are compared with mass flow rates measured in a series of room fire experiments.

Exploratory research on nonthermal damage to electronics from fires and fire-suppression agents

Electronic equipment is expected to operate reliably under normal conditions as well as under foreseeable abnormal conditions, particularly in life-critical and environmentally sensitive applications. One foreseeable abnormal condition to which electronic equipment may be subjected at least once during its life-cycle is a fire environment. Such an environment may include the thermal and corrosive effects in the immediate vicinity of the fire and the nonthermal effects associated with smoke contamination, humidity and corrosion in remote locations. Direct thermal effects are generally so severe that reasonable remedial actions may not be feasible. Fortunately, such effects are frequently restricted to a fairly small zone, often through the use of automatic fire detection and suppression systems. On the other hand, the thermal decomposition products of smoke and fire suppression agents resulting from even a small fire may permeate a building and cause nonthermal damage to electronic equipment in locations remote from the actual fire. With ever-increasing reliance being placed on electronic equipment in all types of applications and the consequent increase in value concentrations, nonthermal damage from fires and fire suppression agents is a topic of growing interest. The purpose of this exploratory research is to characterize nonthermal damage mechanisms, consequences, and potential preventive and remedial actions using a physics-of-failure approach.

Geometric Effects on the Fire Resistance of Structural Steel Elements

Standard methods have been developed and are used in current design practice to determine by calculation the fire resistance rating of structural steel elements protected with spray-applied fire resistive materials (SFRMs). Th ese calculation methods are based on simplified analysis of heat transfer through the SFRM material to the steel substrate. This analysis assumes one -dimensional heat transfer in Cartesian coordinates. Based on this analysis, the ratio of the volume per unit length to the surface area per unit length, expressed in terms of the “W/D ratio,” is the governing parameter for heating of the steel element. For cylindrical rods and other small structural shapes, this analysis is inappropriate because the surface area of the insulated element increases with increasing insulation thickness. Simplified and detailed numerical heat transfer analyses have been performed in both Cartesian and cylindrical coordinates that demonstrate the reduced level of fire resistance associated with a given thickness of insulation on a cylindrical rod relative to a wide-flange element with the same W/D ratio.