Nelson K. Akafuah

Role of buoyant flame dynamics in wildfire spread

Significance Wildfires burn millions of hectares per year on every inhabited continent, but the physical mechanism governing spread is not known. Models of wildfire spread are widely used for prediction, firefighter training, and ecological research but have assumed various formulations of known heat transfer processes (radiation and convection) absent a definitive theory of their organization. New experimental evidence reported here reveals how buoyancy generated by the fire induces vorticity and instabilities in the flame zone that control the convective heating needed to ignite fuel particles and produce spread. Large wildfires of increasing frequency and severity threaten local populations and natural resources and contribute carbon emissions into the earth-climate system. Although wildfires have been researched and modeled for decades, no verifiable physical theory of spread is available to form the basis for the precise predictions needed to manage fires more effectively and reduce their environmental, economic, ecological, and climate impacts. Here, we report new experiments conducted at multiple scales that appear to reveal how wildfire spread derives from the tight coupling between flame dynamics induced by buoyancy and fine-particle response to convection. Convective cooling of the fine-sized fuel particles in wildland vegetation is observed to efficiently offset heating by thermal radiation until convective heating by contact with flames and hot gasses occurs. The structure and intermittency of flames that ignite fuel particles were found to correlate with instabilities induced by the strong buoyancy of the flame zone itself. Discovery that ignition in wildfires is critically dependent on nonsteady flame convection governed by buoyant and inertial interaction advances both theory and the physical basis for practical modeling.

New criteria for filament breakup in droplet-on-demand inkjet printing using volume of fluid (VOF) method

A volume of fluid (VOF) numerical study is presented in which new pi number-based criteria are discussed that identify and separate three different regimes for a droplet-on-demand (DOD) print-head system. A trailing filament coalesces into the main droplet while the filament breaks into one or multiple satellite droplet(s). The numerical simulation results are compared with published large-scale experimental results that used a 2 mm diameter inkjet nozzle head, roughly 50 times larger than the actual diameter of inkjet outlets. Liquid filament break-up behavior is predicted using a combination of two pi-numbers, including either Weber (We)-Ohnesorge (Oh) number couplets or Reynolds (Re)-Weber (We) number couplets that are dependent only on the ejected liquid properties and the velocity waveform at the print-head inlet. These new criteria have merit over the currently existing ones that require accurate measurements of actual droplets to determine filament physical features like length and diameter [1].

Automotive Paint Spray Characterization and Visualization

Understanding the automotive paint atomization and how the atomized paint droplets are transferred from the atomizer to the target surface is a necessary step in the continuous improvement of the paint application process. Ensuring increased transfer efficiency and improvement in coated surface quality require continuous improvement in the application process and reformulation of the paint, especially for metallic paints to satisfy consumer demands. The solutions for these problems require multiple approaches—such as new paint formulation, operation optimization, new paint applicator designs, and improved understanding of the paint droplet transfer process. The ability to visualize the internal structures of the paint spray transfer process is vital to understanding the role that the atomization mechanisms have on the evolution of the paint droplets as they travel from the paint applicator to the target surface. This chapter, therefore, seeks to address some of these issues by focusing on the visualization and characterization of the paint spray transfer process.

Effects of automotive paint spray technology on the paint transfer efficiency – a review:

Automotive spray painting is among the most sophisticated and controlled industrial painting operations currently performed. Nevertheless, improvements in it are still sought in efforts to minimize the costs, the energy use and the environmental impacts. One compelling aspect of improvement is the paint transfer efficiency, i.e. the amount of paint that remains on a vehicle relative to the amount supplied to the paint applicator during coating operations, because currently it has been estimated that the overall paint transfer efficiency in the automotive industry is between 50% and 60%. Hence, this review assesses current automotive spray coating technologies with respect to their transfer efficiencies and discusses the fundamental and operational parameters that influence it. A comprehensive characterization of paint spray applicators (air sprayers, high-volume low-pressure sprayers, airless sprayers, air-assisted airless sprayers, rotary bell atomizers, electrostatic sprayers, and effervescent atomizers) is included. Some problems associated with evaluating and improving their paint transfer efficiencies are discussed. Also, the potential of and the technology needs for developing these applicators are considered.

NO formation analysis of turbulent non-premixed coaxial methane/air diffusion flame

Natural gas combustion is one of the primary sources of harvesting energy for various processes and has gained a wide attention during the past decade. One of the most recent applications of natural gas combustion can be found in non-premixed combustion of methane in a coflow burner system. One of the main environmental concerns that arises from the natural gas combustion is the formation of NO produced by thermal NO and prompt NO mechanisms. Current paper is devoted on an examination of a 2D numerical simulation of turbulent non-premixed coaxial methane combustion in air enclosed by an axisymmetric cylindrical chamber to study the effects of species concentrations of reactants on NO formation, their individual contributions, and the chamber outlet temperature. A finite-volume staggered grid method is utilized to solve conservation equations of mass, energy, momentum, and species concentrations. In order to handle radiation heat transfer, discrete transfer method is used to solve radiation equation. Utilizing weighted-sum-of-gray-gases model, based on the newly obtained high-temperature molecular spectroscopic data, local variations of species absorption coefficients are taken into account. To calculate NO concentration, a single- or joint-variable probability density function in terms of a normalized temperature, mass fractions of species, or a combination of both is employed. Plus, published relevant experimental data are used to validate temperature and species concentration fields. It is shown that a decrease in N2 concentration contributes to reducing NO. More importantly for higher O2 mass fraction, thermal NO formation becomes the dominant mechanism responsible for NO emission.

Infrared Visualization of Automotive Paint Spray Transfer Process

An Infrared thermography based visualization technique for automotive paint spray is presented. Two common automotive paint applicators were studied using this technique and the results presented. The paint applicators studied were high-speed rotary bell atomizer and low pressure air atomizer. The technique uses a uniformly heated blackbody emitter as a background. The emitted infrared energy from the background passing through the spray is attenuated by the droplets in the spray. The attenuated intensity is captured by an infrared camera, to form a two-Dimensional image of the spray flow field. From the acquired intensity image, the entire spray flow field structure is visualized and the result discussed.Copyright

Section B Fire and Explosion - A Study of Flame Spread in Engineered Cardboard Fuel Beds Part I: Correlations and Observations of Flame Spread

Wind-aided laboratory fires spreading through laser-cut cardboard fuel beds were instrumented and analyzed for physical processes associated with spread. Flames in the spanwise direction appeared as a regular series of peaks and troughs that scaled directly with flame length. Flame structure in the stream-wise direction fluctuated with the forward advection of coherent parcels that originated near the rear edge of the flame zone. Thermocouples arranged longitudinally in the fuel beds revealed the frequency of temperature fluctuations decreased with flame length but increased with wind speed. The downstream extent of these fluctuations from the leading flame edge scaled with Froude number and flame zone depth. The behaviors are remarkably similar to those of boundary layers, suggesting a dominant role for buoyancy in determining wildland fire spread.

Section B Fire and Explosion - A Study of Flame Spread in Engineered Cardboard Fuel Beds Part II: Scaling Law Approach

In this second part of a two-part exploration into the dynamic behavior observed in wildland fires, time scales differentiating convective and radiative heat transfer are further explored. Scaling laws for the two different types of heat transfer were considered: radiation-driven fire spread and convection-driven fire spread, which can both occur during wildland fires. A new interpretation of the inertial forces introduced a downstream, time-dependent frequency ω, which captures the dynamic, vortex shedding behavior of flames due to the unstable nature of the turbulent flow created in the wake of the fire. Excelsior and paper strip experiments suggest many wildland fire scenarios fall into the convection-driven spread regime.

Ultrasonically Driven Cavitating Atomizer: Prototype Fabrication and Characterization

A new device, ultrasonic cavitating atomizer (UCA), has been developed that uses ultrasonically driven cavitation to produce fine droplets. In the UCA the role of cavitation is explicitly configured to enhance the breakup of the liquid jet exiting the nozzle into fine droplets; the pressure modulation also assists the breakup process. The experimental study involves the fabrication of a prototype and the building of an experimental rig to test the prototype using water as the working fluid. The parameters tested include liquid injection pressure, horn tip frequency and liquid flow rate. The result shows improvement in the atomization of water with the application of ultrasonic cavitation.Copyright