Jinsong Hua

A numerical study of the interaction of water spray with a fire plume

Water spray-based fire extinguishing equipment such as sprinklers has been widely used in fire suppression and control. However, the fire extinguishing mechanism in such devices is not well understood due to the complexity of the physical and chemical interactions between water spray and fire plume. Currently, quantitative approaches (e.g. numerical modeling) to estimate the performance and effectiveness of water spray systems have not been developed to a stage where they can be used to optimize the design for different operating environments and types of fire. In the present work, a numerical simulation approach is introduced to provide a quantitative analysis of the complex interactions occurring between water spray and fire plume. The effects of several important factors (namely water spray pattern, water droplet size and water spray flow rate) on the fire suppression mechanism are investigated. The simulations show that the water spray with solid cone pattern and finer water droplet size is more effective in extinguishing fires than the one with hollow cone pattern and coarse water droplet size. To suppress a fire, the water spray flow rate has to be more than a certain critical value. However, using too high water spray flow rate does not increase fire suppression efficiency but only leads to increased operational cost because of the excess water flow rate. In the current paper, the principles of fire suppression with water spray are also discussed, which are useful in designing more effective water spray fire suppression systems.

Coaxial electrohydrodynamic atomization process for production of polymeric composite microspheres.

Polymeric composite microspheres consisting of a poly(D,L-lactic-co-glycolic acid) (PLGA) core surrounded by a poly(D,L-lactic acid) (PDLLA) shell layer were successfully fabricated by coaxial electrohydrodynamic atomization (CEHDA) process. Process conditions, including nozzle voltage and polymer solution flow rates, as well as solution parameters, such as polymer concentrations, were investigated to ensure the formation of composite microspheres with a doxorubicin-loaded PLGA core surrounded by a relatively drug-free PDLLA shell layer. Various microsphere formulations were fabricated and characterized in terms of their drug distribution, encapsulation efficiency and in vitro release. Numerical simulation of CEHDA process was performed based on a computational fluid dynamics (CFD) model in Fluent by employing the process conditions and fluid properties used in the experiments. The simulation results were compared with the experimental work to illustrate the capability of the CFD model to predict the production of consistent compound droplets, and hence, the expected core-shell structured microspheres.

Numerical simulation of bubble-driven liquid flows

The governing equations of mass and momentum conservation for both continuous and dispersed phases in the bubble-driven flow are analysed by a volume-averaging technique. It is found that two additional terms could be added into the classical two-fluid model to improve the predictions of flow behaviour in the gas-phase. Firstly, the gas-phase dispersion is attributed not only to the convective transport but also to the phase diffusion caused by the non-uniform distribution of gas. Therefore, it is essential to include a diffusion term in the mass conservation equation of the gas phase. Secondly, the viscous forces were normally neglected in the momentum equation of the gas phase in previous studies. This has restricted the validity of model predictions on the gas-phase flow behaviour. Hence, the viscous force terms are included in the gas-phase momentum balance to examine the prediction on gas-phase flow characteristics. Both laminar and turbulent flows with bottom gas injection into a liquid bath have been investigated by computer simulation. For the turbulent bubble-driven liquid flows, a two-equation k–e turbulence model is used to examine the contribution of the mean flow of liquid. Comparisons between the predictions and the literature experimental data illustrate that the present model is capable of capturing a reasonable agreement with experimental results for both the liquid and gas phases.

Evaluation of CFD modeling methods for fire-induced airflow in a room

Methods for exploiting the technique of computational fluid dynamics (CFD) to model fires and the associated fundamental processes of combustion and radiation are appraised by applying the CFD model JASMINE to simulate the fire-induced airflow in a room-sized compartment. For a systematic evaluation of these methods describing the fundamental processes and the associated mathematical formulation, a two-stage-verification and validation procedure is adopted here. In the verification stage, a number of important model parameters, e.g., the choice of fire source model (e.g., combustion model vs. volumetric heat source (VHS) model), radiation transport model (e.g., six-flux vs. discrete transfer radiation model), gas property model (e.g., constant absorption coefficient vs. grey gas model), design and resolution of the numerical grid, etc. are assessed by comparing the predicted results against a specific fire scenario selected from the data of Steckler, K.D., Quintiere, J.G. and Rinkinen, W.J. (1982). Flow Induced by Fire in a Compartment, NBSIR B2-2520, National Bureau of Standards, Pittsburgh [2]. The validation stage has then focused on the detailed comparison of the CFD fire model against the comprehensive data set of Steckler et al. [2] for a wide range of fire scenarios covering a range of fire strengths (i.e., heat release rates), fire source locations, the sizes and shapes of the ventilation opening (i.e., door or window). Such a systematic study has demonstrated the predictive capability of the CFD methodology in reproducing detailed thermal and flow field behavior of the enclosure fires, and has also shown that the predictive accuracy of the CFD methodology can be significantly improved by careful selection of the fire models and model parameters. Some guidance on the use of CFD methodology has been provided for performing the fire modeling properly and effectively for performance-based fire safety design.