H.Y. Yuan

Modified ensemble Kalman filter for nuclear accident atmospheric dispersion: Prediction improved and source estimated

Atmospheric dispersion models play an important role in nuclear power plant accident management. A reliable estimation of radioactive material distribution in short range (about 50 km) is in urgent need for population sheltering and evacuation planning. However, the meteorological data and the source term which greatly influence the accuracy of the atmospheric dispersion models are usually poorly known at the early phase of the emergency. In this study, a modified ensemble Kalman filter data assimilation method in conjunction with a Lagrangian puff-model is proposed to simultaneously improve the model prediction and reconstruct the source terms for short range atmospheric dispersion using the off-site environmental monitoring data. Four main uncertainty parameters are considered: source release rate, plume rise height, wind speed and wind direction. Twin experiments show that the method effectively improves the predicted concentration distribution, and the temporal profiles of source release rate and plume rise height are also successfully reconstructed. Moreover, the time lag in the response of ensemble Kalman filter is shortened. The method proposed here can be a useful tool not only in the nuclear power plant accident emergency management but also in other similar situation where hazardous material is released into the atmosphere.

Quantitative assessment of the relationship between radiant heat exposure and protective performance of multilayer thermal protective clothing during dry and wet conditions

The beneficial effect of clothing on a person is important to the criteria for people exposure to radiant heat flux from fires. The thermal protective performance of multilayer thermal protective clothing exposed to low heat fluxes during dry and wet conditions was studied using two designed bench-scale test apparatus. The protective clothing with four fabric layers (outer shell, moisture barrier, thermal linear and inner layer) was exposed to six levels of thermal radiation (1, 2, 3, 5, 7 and 10kW/m(2)). Two kinds of the moisture barrier (PTFE and GoreTex) with different vapor permeability were compared. The outside and inside surface temperatures of each fabric layer were measured. The fitting analysis was used to quantitatively assess the relationship between the temperature of each layer during thermal exposure and the level of external heat flux. It is indicated that there is a linear correlation between the temperature of each layer and the radiant level. Therefore, a predicted equation is developed to calculate the thermal insulation of the multilayer clothing from the external heat flux. It can also provide some useful information on the beneficial effects of clothing for the exposure criteria of radiant heat flux from fire.

Iterative ensemble Kalman filter for atmospheric dispersion in nuclear accidents: An application to Kincaid tracer experiment

Information about atmospheric dispersion of radionuclides is vitally important for planning effective countermeasures during nuclear accidents. Results of dispersion models have high spatial and temporal resolutions, but they are not accurate enough due to the uncertain source term and the errors in meteorological data. Environmental measurements are more reliable, but they are scarce and unable to give forecasts. In this study, our newly proposed iterative ensemble Kalman filter (EnKF) data assimilation scheme is used to combine model results and environmental measurements. The system is thoroughly validated against the observations in the Kincaid tracer experiment. The initial first-guess emissions are assumed to be six magnitudes underestimated. The iterative EnKF system rapidly corrects the errors in the emission rate and wind data, thereby significantly improving the model results (>80% reduction of the normalized mean square error, r=0.71). Sensitivity tests are conducted to investigate the influence of meteorological parameters. The results indicate that the system is sensitive to boundary layer height. When the heights from the numerical weather prediction model are used, only 62.5% of reconstructed emission rates are within a factor two of the actual emissions. This increases to 87.5% when the heights derived from the on-site observations are used.

A Comparison of Simulation and Experiment on Stack Effect in Long Vertical Shaft

This article discusses the numerical study on stack effect in a long vertical shaft, where physical and numerical techniques were employed to determine the effectiveness of simulations in such a system. This article presented numerical predictions obtained by using different computational fluid dynamics (CFD) models and compared the numerical results with the experimental data obtained from tests performed in this project. Finally, results show that common turbulence model for free plume would not fit for stack effect and a new turbulence model with tilted fire plume due to strong air entrainment needs to be studied.

A New Approach for Predicting Flame Spread along Combustible Solid based on Cellular Automaton

In this study, a new approach for predicting flame spread along a combustible solid is developed. This presented approach consists of a two-dimensional flame spread model coupled with a one-dimensional pyrolysis model in the thickness direction. The flame spread model is based on a cellular automaton combined with piloted ignition temperature, spontaneous ignition temperature, and pyrolysis temperature of a combustible solid. In addition, this approach is used to predict two typical kinds of flame spread, i.e., upward flame spread and ceiling flame spread. The existing experimental data including the upward flame spread over plywood and the ceiling flame spread beneath medium density fiberboard (MDF) are used for comparison to validate this approach. The results obtained from numerical simulations using this approach are consistent with these two kinds of experimental tests.

A New Method of Laser Sheet Imaging-based Fire Smoke Detection

Since smoke appears much earlier than combustion flames, smoke detection technologies are becoming more widely applied in early fire detection. In this article, an optical, non-intrusive measuring method is developed to detect fire smoke based on laser sheet imaging technology. A set of experimental apparatus to capture smoke particle images have been designed, comprised of three parts: 1) the Smoke Passage System, 2) the Light Scattering System, and 3) the Particle Imaging System. Images from the experimental apparatus are analyzed by image processing algorithms which monitor particle size and distribution from various fuels, as well as particle shapes are acquired. This method not only provides us with smoke particle morphology, but also with size and distribution, which represents a new method for early fire detection.

Experimental and Numerical Study of Smoke Plumes in Stable Thermally Stratified Environments

To get more information about early smoke movement in thermally stratified environments in large-volume spaces, a detailed study of low density and big density smoke plumes in thermally stratified environments in a small-scale enclosure was undertaken by means of experimental mea- surement and CFD simulation. The reasonably good agreement between the experimental results and the simulated results indicates that a thermally stratified environment intensifies the decrease of the axial temperature and velocity of a fire smoke plume with height until the upward plume movement terminates at a certain height. Comparisons of the maximum plume heights measured experimentally and integral equation results showed that the integral equation underestimates the actual maximum heights of fire smoke plumes, and is also unable to predict the smoke density influences upon the maximum heights. The previous available integral model should be improved for better applications in real fire situations.

Experimental Study On Response Sensitivity Of Smoke Detectors In High Flow Velocity

The objective of this work was to investigate preliminary design criteria of automatic fire alarm systems for some places with high flow velocity, e.g., exits of large underground car parks, the air outlets of large air conditioner etc. In addition, another main motivation of this research is to evaluate the appropriateness of Chinese standard which states that photoelectrics should be used in high flow locations. Two corresponding series of experiments were designed to analyze the response sensitivity of photoelectric and ionization smoke detectors under the conditions of different flow velocities in the Fire Emulator/Detector Evaluator (FE/DE), using smoldering cottons as the smoke source. The combining analysis of results of the two series of experiments indicated that generally speaking, ionization smoke detectors showed better response sensitivity than photoelectric smoke detectors in environments with high flow velocity. Preliminary analysis suggested that the influence of flow velocity on size distribution of the smoke particles generated is the main reason for the results.