Michelle K. Donnelly

Measurement of 100 nm and 60 nm Particle Standards by Differential Mobility Analysis

The peak particle size and expanded uncertainties (95 % confidence interval) for two new particle calibration standards are measured as 101.8 nm ± 1.1 nm and 60.39 nm ± 0.63 nm. The particle samples are polystyrene spheres suspended in filtered, deionized water at a mass fraction of about 0.5 %. The size distribution measurements of aerosolized particles are made using a differential mobility analyzer (DMA) system calibrated using SRM® 1963 (100.7 nm polystyrene spheres). An electrospray aerosol generator was used for generating the 60 nm aerosol to almost eliminate the generation of multiply charged dimers and trimers and to minimize the effect of non-volatile contaminants increasing the particle size. The testing for the homogeneity of the samples and for the presence of multimers using dynamic light scattering is described. The use of the transfer function integral in the calibration of the DMA is shown to reduce the uncertainty in the measurement of the peak particle size compared to the approach based on the peak in the concentration vs. voltage distribution. A modified aerosol/sheath inlet, recirculating sheath flow, a high ratio of sheath flow to the aerosol flow, and accurate pressure, temperature, and voltage measurements have increased the resolution and accuracy of the measurements. A significant consideration in the uncertainty analysis was the correlation between the slip correction of the calibration particle and the measured particle. Including the correlation reduced the expanded uncertainty from approximately 1.8 % of the particle size to about 1.0 %. The effect of non-volatile contaminants in the polystyrene suspensions on the peak particle size and the uncertainty in the size is determined. The full size distributions for both the 60 nm and 100 nm spheres are tabulated and selected mean sizes including the number mean diameter and the dynamic light scattering mean diameter are computed. The use of these particles for calibrating DMAs and for making deposition standards to be used with surface scanning inspection systems is discussed.

Reduced Gravity Combustion of Thermoplastic Spheres

Abstract A series of low-gravity experiments were conducted to investigate the combustion of supported thermoplastic polymer spheres under varying ambient conditions. The three types of thermoplastic investigated were polymethylmethacrylate (PMMA), polypropylene (PP), and polystyrene (PS). The low-gravity environment was achieved by performing the experiments aboard the NASA DC-9 and the KC-135 Reduced Gravity Aircraft. Spheres with diameters ranging from 2 mm to 6.35 mm were tested yielding Grashof numbers calculated to be less than 0.1. The polymer sphere was supported using a 75-μm-diameter Al/Cr/Fe alloy wire. The total initial pressure varied from 0.05 MPa to 0.15 MPa whereas the ambient oxygen concentration varied from 19% to 30% (by volume). The ignition system consisted of a pair of retractable energized coils. Two CCD cameras recorded the burning histories of the spheres. The video sequences revealed a number of dynamic events including bubbling, and sputtering as well as soot shell formation and break-up during combustion of the spheres at reduced gravity. The ejection of combusting material from the burning spheres represents a fire hazard that must be considered at reduced gravity. The ejection was found to be sensitive to polymer type, but independent of oxygen concentration and pressure. The average value of the ejection frequency was found to be 3 Hz, 5 Hz, and 5 Hz for PMMA, PS, and PP, respectively. The velocities of the ejected material were estimated by tracking the material in two consecutive video frames. For the PP spheres, Va = 2.3 (± 1.2) cm/s (with 60 events observed). The ejected material appeared to decelerate at an average rate of ≈ 40 cm/s2, and traverse an average distance of only 8 mm before burning to completion. The Va for PS and PMMA was not determined because the ejected material was never observed to exist beyond the visible flame of the parent sphere. The average mass burning rates were measured to increase with initial sphere diameter and oxygen concentration, whereas the initial pressure had little effect. The three thermoplastic types exhibited different burning characteristics. For the same initial conditions, the mass burning rate of PP was slower than PMMA, whereas the mass burning rate of PS was comparable to PMMA. The transient diameter of the burning thermoplastic exhibited two distinct periods: an initial period (enduring approximately half of the total burn duration) when the diameter remained approximately constant, and a final period when the square of the diameter linearly decreased with time. A simple homogeneous two-phase model was developed to understand the changing diameter of the burning sphere. Its value is based on a competition between diameter reduction due to mass loss from burning and sputtering, and diameter expansion due to the processes of swelling (density decrease with heating) and bubble growth. The model relies on empirical parameters for input, such as the burning rate and the duration of the initial and final burning periods.

Dispersed Liquid Agent Fire Suppression Screen Apparatus

Abstract : The design, construction, demonstration, and operation of a bench-scale device capable of screening the fire suppression efficiency of liquid agents are described in detail in this report. The apparatus is based on a well-characterized flame, a means to facilitate the introduction of liquid agents, and a way to generate liquid droplets. A Tsuji-type burner, a porous cylinder used in a counterflow diffusion configuration, is used. Both wake and enveloped flames can be maintained over a wide range of fuel and oxidizer flows. The flame is easily observed, and critical stages such as the blow-off limit (abrupt transition from an enveloped flame to a wake flame) can be ascertained with ease and high reproducibility. A small-scale vertical wind tunnel, which allows for the delivery of a uniform flow of oxidizer to the burner at a low turbulence intensity and also assists in the delivery of liquid agent droplets to the flame, is used for the flow facility. Two techniques of generating droplets have been examined: (1) a piezoelectric droplet generator and (2) a small glass nebulizer. The piezoelectric droplet generator was found incapable of handling fluids with high loading of dissolved solid due to frequent clogging of the orifice opening. The nebulizer is used in the current liquid screen apparatus.

An apparatus for screening fire suppression efficiency of dispersed liquid agents

The design, construction, demonstration, and operation of a bench-scale device capable of comparison screening the fire suppression efficiency of liquid agents are described in this paper. The apparatus is based on a well-characterized flame, a means to facilitate the introduction of liquid agents, and a way to generate liquid droplets. A porous cylinder in a counterflow diffusion configuration is used. A small-scale vertical wind tunnel, which allows for the delivery of a uniform flow of oxidizer to the burner and also assists in the delivery of liquid agent droplets to the flame, is used for the flow facility. Droplets are generated by a small glass nebulizer. The performance of the screening apparatus was evaluated using several liquid fire suppressants with different thermophysical properties. A test protocol is also proposed.

Using Sensor Signals to Analyze Fires

Building fire sensors are capable of supplying substantially more information to the fire service than just the simple detection of a possible fire. Nelson, in 1984, recognized the importance of tying all the building sensors to a smart fire panel [1]. In order to accomplish a smart fire panel configuration such as envisioned by Nelson, algorithms must be developed that convert the analog/digital signals received from sensors to the heat release rate (HRR) of the fire. Once the HRR of the fire is known, a multiroom zone fire model can be used to determine smoke layers and temperatures in the other rooms of the building. This information can then be sent to the fire service providing it with an approximate overview of the fire scenario in the building.This paper will describe a ceiling jet algorithm that is being developed to predict the heat release rate (HRR) of a fire using signals from smoke and gas sensors. The prediction of this algorithm will be compared with experiments. In addition, an example of the predictions from a sensor-driven fire model, SDFM, using signals from heat sensors, will be compared with measurements from a full-scale, two-story, flashover townhouse fire.

NIST Calibration Facility for Sizing Spheres Suspended in Liquids

A calibration facility has been developed to measure the peak diameter of particles suspended in liquid using differential mobility analysis (DMA). A description of the facility and the features that contribute to measurements with low uncertainties is included. Analysis of the DMA convolution integral allows correcting for the effects of charging probability, size distribution, and transfer function on the measured peak particle size. Current research using electrospray to aerosolize the particles is aimed at expanding the measurement size interval.

Experimental and modeling study of thermal exposure of a self-contained breathing apparatus (SCBA)

An experimental apparatus designed to study firefighter safety equipment exposed to a thermal environment was developed. The apparatus consisted of an elevated temperature flow loop with the ability to heat the air stream up to 200°C. The thermal and flow conditions at the test section were characterized using thermocouples and bi-directional probes. The safety equipment examined in this study was a self-contained breathing apparatus (SCBA), including a facepiece and an air cylinder. The SCBA facepiece was placed on a mannequin headform and coupled to a breathing simulator that was programmed with a prescribed breathing pattern. The entire SCBA assembly was placed in the test section of the flow loop for these thermal exposure experiments. Three air stream temperatures, 100°C, 150°C, and 200°C, were used with the average air speed at the test section set at 1.4m/s and thermal exposure durations up to 1200 s. Measurements were made using type-K bare-bead thermocouples located in the mannequins mouth and on the outer surface of the SCBA cylinder. The experimental results indicated that increasing the thermal exposure severity and duration increased the breathing air temperatures supplied by the SCBA. Temperatures of breathing air from the SCBA cylinder in excess of 60°C were observed over the course of the thermal exposure conditions used in most of the experiments. A mathematical model for transient heat transfer was developed to complement the thermal exposure experimental study. The model took into consideration forced convective heat transfer, quasi-steady heat conduction through the composite layers of the SCBA cylinder wall, the breathing pattern and action of the breathing simulator, and predicted air temperatures from the thermally exposed SCBA cylinder and temperatures at the outer surface of the SCBA cylinder. Model predictions agreed reasonably well with the experimental measurements.

Validation Experiments of Large Compartment Fires

In cooperation with the fire protection engineering community, a computational fire model, Fire Dynamics Simulator (FDS), is being developed at NIST to study fire behavior and to evaluate the performance of fire protection systems in buildings. The software was released into the public domain in 2000, and since then has been used for a wide variety of analyses by fire protection engineers. An on-going need is to develop and validate new sub-models. Fire experiments are conducted for a variety of reasons, and model predictions of these experiments over the past few decades have gradually improved. However, as the models become more detailed, so must the measurements. The bulk of available large scale test data consist of temperature (thermocouple) measurements made at various points above a fire or throughout an enclosure. While it is useful to compare model predictions with these measurements, one can only gauge how closely the model reproduces the given data. There is often no way to infer why the model and experiment disagree, and thus no way to improve the model. Also, it is difficult to separate various physical phenomena in a large scale fire test so that combustion, radiation and heat transfer algorithms can be evaluated independently. For example, the heat release rate of the fire governs the rate at which energy is added to the system, convective and radiative transport distribute the energy throughout, and thermal conduction drains the system of some of the energy. The measured value of a temperature, heat flux, or gas concentration at any one point depends on all the physical processes, and uncertainties in each phase of the calculation tend to combine in a non-linear way impacting the prediction.Copyright