Browsing by Author "Willson, Bryan, advisor"

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Open Access
A fine resolution CDF simulation approach for biomass cook stove development
(Colorado State University. Libraries, 2011) Miller-Lionberg, Daniel David, author; Willson, Bryan, advisor; DeFoort, Morgan, committee member; Sakurai, Hiroshi, committee member; Volckens, John, committee member
More than half of the world's population meets cooking and heating needs through small-scale biomass combustion. Emissions from these combustion processes are a major health hazard and air pollution concern. Simple improvements over traditional cooking fires have been shown to increase combustion and heat transfer efficiency while reducing physically harmful gaseous and particulate matter (PM) emissions. Over approximately 30 years of modern stove development history, designs have largely been based on empirical guidelines, and attempts at improvements have been made through an iterative, trial-and-error approach. Feedback in this design process is typically attained through bulk measurements made during experimental testing of prototypes. While important for assessing the performance of a stove, such testing offers no information on the fine spatial or temporal scales of phenomena within the stove, leaving it a "black box" in the view of the designer. Without higher resolution information, the rate and ultimate level of design improvement may be limited. In response, a computational fluid dynamic (CFD) simulation of a common, production cook stove is conducted using ANSYS FLUENT 13.0 software. Aspects critical to achieving high spatial and temporal resolution flow and temperature field results are included, enabled by necessary simplifications to less important elements. A model for the steady, time-averaged drying and pyrolysis of wood stick fuel is used in conjunction with a consideration for the simultaneous oxidation of the resulting char, to generate gas-phase fuel boundary conditions for the simulation. Fine spatial and temporal resolution are simultaneously possible in an unsteady formulation with the use of the simplified fuel condition, reduced-mass solid boundaries, and abbreviated runtimes. Employment of a large eddy simulation (LES) turbulence model is proposed as necessary to realistically consider the larger scales of gas mixing. Combustion heat release is approximated by reactions dictated by a mixture fraction formulation, assuming equilibrium conditions in a non-adiabatic system, affected by turbulent fluctuations through a probability density function (PDF). Sensitivity studies are conducted on grid parameters, boundary condition assumptions, and the duration of simulation runtime necessary to achieve result significance. A model for particulate emission formation is secondarily explored. A thermocouple-instrumented stove is used in an experiment to generate internal gas temperature profiles for the validation of the CFD simulation through comparable results. Likewise, a heat-exchanger integrated into a cooking pot is employed with the instrumented stove to measure short time-scale heat transfer values that are compared to the CFD simulation results, as well as to benchmark test data from the production stove. Recommendations for future efforts in stove simulation are made.
Open Access
Development and optimization of a stove-powered thermoelectric generator
(Colorado State University. Libraries, 2008) Mastbergen, Dan, author; Willson, Bryan, advisor
Almost a third of the world's population still lacks access to electricity. Most of these people use biomass stoves for cooking which produce significant amounts of wasted thermal energy, but no electricity. Less than 1% of this energy in the form of electricity would be adequate for basic tasks such as lighting and communications. However, an affordable and reliable means of accomplishing this is currently nonexistent. The goal of this work is to develop a thermoelectric generator to convert a small amount of wasted heat into electricity. Although this concept has been around for decades, previous attempts have failed due to insufficient analysis of the system as a whole, leading to ineffective and costly designs. In this work, a complete design process is undertaken including concept generation, prototype testing, field testing, and redesign/optimization. Detailed component models are constructed and integrated to create a full system model. The model encompasses the stove operation, thermoelectric module, heat sinks, charging system and battery. A 3000 cycle endurance test was also conducted to evaluate the effects of operating temperature, module quality, and thermal interface quality on the generator's reliability, lifetime and cost effectiveness. The results from this testing are integrated into the system model to determine the lowest system cost in $/Watt over a five year period. Through this work the concept of a stove-based thermoelectric generator is shown to be technologically and economically feasible. In addition, a methodology is developed for optimizing the system for specific regional stove usage habits.
Open Access
Fiber delivery and diagnostics of laser spark ignition for natural gas engines
(Colorado State University. Libraries, 2008) Joshi, Sachin, author; Yalin, Azer, advisor; Willson, Bryan, advisor
Laser ignition via fiber optic delivery is challenging because of the need to deliver pulsed laser beam with relatively high energy and sufficient beam quality to refocus the light to the intensity required for creating spark. This dissertation presents work undertaken towards the development of a multiplexed fiber delivered laser ignition system for advanced lean-burn natural gas engines. It also describes the use of laser ignition system to perform in-cylinder optical diagnostics in gas engines. Key elements of the dissertation includes: (i) time resolved emission spectroscopy (TRES) of laser sparks in air to investigate the dependence of spark temperatures and electron number densities on ambient gas pressures, (ii) optical characterization of hollow core fibers, step-index silica fibers, photonic crystal fibers (PCFs) and fiber lasers, (iii) development and on-engine demonstration of a multiplexer to deliver the laser beam from a single laser source to two engine cylinders via optical fibers, and (iv) demonstration of simultaneous use of laser sparks for ignition and Laser Induced Breakdown Spectroscopy (LIBS) to measure in-cylinder equivalence ratios in a Cooperative Fuel Research (CFR) engine. For TRES of laser sparks, the ambient gas pressure is varied from 0.85 bar to 48.3 bar (high pressures to simulate elevated motored in-cylinder pressures at time of ignition in advanced gas engines). At later stages (~1μs) of spark evolution, spark temperatures become comparable at all pressures. Electron number densities increase initially with increasing ambient gas pressure but become comparable at pressures greater than ~20 bar. The effects of launch conditions and bending for 2-m long hollow core fibers are studied and an optimum launch f/# of ~55 is shown to form spark in atmospheric pressure air. Spark formation using the output of a pulsed fiber laser is shown and delivery of 0.55 mJ nanosecond pulses through PCFs is achieved. Successful multiplexed laser ignition of a CAT G3516C gas engine via hollow core fibers is shown. LIBS analysis conducted at equivalence ratios from 0.6 to 0.95 in the CFR engine show a linear variation and linear correlation (R2 > 0.99) of line intensity ratio (Hα/O777 and Hα/Ntot) with equivalence ratio.
Open Access
Laser ignition for internal combustion engines via fiber optic delivery
(Colorado State University. Libraries, 2009) DeFoort, Morgan, author; Yalin, Azer, advisor; Willson, Bryan, advisor
In the effort to reduce emissions and improve the efficiency of Otto cycle engines, the ignition system is often a limiting factor. Many "high energy" ignition systems have been developed, but almost all of these are based on traditional electric arc spark plugs. Laser ignition represents a fundamentally different approach to igniting gas mixtures and opens the door to improvements in fuel-lean engine operation and high-pressure combustion environments. Yet the promise of laser ignition remains unexploited, as practical systems have not been developed. In this contribution, we work towards the goal of developing a practical laser ignition system for stationary natural gas engines. Specifically, we focus on fiber optic delivery of the laser beam to the engine, thereby making a significant advance relative to past open-air (free-space) configurations. A combination of modeling and experimentation has been used to develop the needed fiber optic delivery systems, culminating in the first demonstration of fiber-optically delivered laser ignition on an engine.
Open Access
The development of numerical tools for characterizing and quantifying biomass cookstove impact
(Colorado State University. Libraries, 2013) L'Orange, Christian, author; Willson, Bryan, advisor; DeFoort, Morgan, advisor; Marchese, Anthony, committee member; Volckens, John, committee member
Biomass cookstove use can be damaging to both human health and the global climate. In an effort to minimize these impacts, numerous programs are working to disseminate improved biomass cookstoves. However, few programs have achieved extensive success towards improving either climate or health. One reason programs have only resulted in limited improvements has been the sector's inability to quantify cookstove performance. A numeric tool has been developed for characterizing biomass cookstove performance. This dissertation documents the development of that tool. The document is comprised of three components: (i) the critical analysis of the uncertainty associated with current methods for cookstove field-testing, (ii) the development and validation of a probabilistic impact model for biomass cookstoves, and (iii) the application of these numerical tools to quantify cookstove impact. Biomass cookstoves have traditionally been evaluated empirically. Cookstoves are tested in both the field and the laboratory, with each approach having advantages and limitations. Neither laboratory nor field testing are sufficient, however, for quantifying cookstove impact. Field-testing provides invaluable data on cookstove use but is limited by the large variability typically seen in the results. Drawing conclusions from field tests is challenging due to this variability. Many groups attempt to address testing variability by increasing the number of test replicates conducted. A numeric model was developed to determine the number of test replicates required to quantify cookstove performance in field settings. Because of the large number of test replicates required to have statistical confidence in field-based data, an improved method of quantifying biomass cookstove performance is needed. Therefore, to address this need a probabilistic Monte Carlo prediction model was developed to quantify cookstove performance. The intention of the model is to serve as a tool for predicting the impact of various cookstove designs. The model integrates various facets of existing cookstove performance knowledge in more a cohesive fashion. Model simulations were compared to experimental studies to validate this approach. Numeric tools are only valuable if they result in useful information; for example, information that allows informed decisions to be made. The potential of numeric models to provide valuable information for cookstove programs has been demonstrated by simulating the performance of multiple cookstove designs. Three improved cookstoves designs have been compared to a traditional three-stone fire. Each design was evaluated for multiple scenarios, use patterns, and locations. The impact of each design (in regard to climate and health) was then quantified and monetized. This exercise yielded two important findings. First, consideration of location and context is critical when comparing the performance of cookstoves. Second, numeric models can be used as highly informative tools to support decision-making in the cookstove sector. Empirical testing is necessary for most technical programs; this is especially true for cookstoves projects. There are aspects of cookstove designs that can only be evaluated experimentally. Examples include whether an individual likes the cookstove, or if the design is appropriate for the specific cooking requirements of a particular community. Physical testing is needed to answer some basic questions such as: Do users find the cookstove intuitive to use? Do they like the color? However, empirical testing is not well-suited to answer every question related to cookstove performance. For example, comparing the climate impact of different cookstove designs is difficult in the field. The work presented demonstrates the potential of numerical models to provide invaluable information to the cookstove sector. The development and validation of these models has been documented. These models can help quantify the impact of current designs and help guide the development of future cookstove programs.
Open Access
Toward the understanding and optimization of chimneys for buoyantly driven biomass stoves
(Colorado State University. Libraries, 2013) Prapas, Jason, author; Willson, Bryan, advisor; DeFoort, Morgan, advisor; Marchese, Anthony, committee member; Peel, Jennifer, committee member
The vast majority of indoor combustion devices in the developed world make use of stacks (flues, vents, chimneys, smokestacks) to channel flue gases out of the operator space. In the developing world, where indoor air pollution kills several million people every year, the use of chimneys with biomass cooking and heating stoves has been met with limited success and a high level of controversy. Due to a lack of theoretical understanding, design criteria, poorly executed installation practices, and/or insufficient maintenance routines, many chimney stoves have exhibited inadequate indoor emissions reductions in addition to low thermal efficiencies. This work aims (a) shed light on the physical phenomenon of the "stack effect" as it pertains to dynamic, non-adiabatic, buoyancy-driven stoves (b) apply new understanding toward the optimization of two types of biomass chimney stoves: plancha or griddle type stoves popular in Central America and two-pot stoves common in South America. A numerical heat and fluid flow model was developed that takes into account the highly-coupled variables and dynamic nature of such systems. With a comprehensive physical model, parameter studies were conducted to determine how several field-relevant variables influence the performance of stack-outfitted systems. These parameters include, but are not limited to: power/wood consumption rate, chimney geometry, stove geometry, material properties, heat transfer, and ambient conditions. An instrumented experimental chimney was built to monitor relationships between air flow, differential pressure, gas temperatures, emissions, and thermal efficiency. The draft provided by chimneys was found to have a strong influence over the bulk air-to-fuel ratio of buoyantly-driven cookstoves, greatly affecting the stove's overall performance by affecting gas temperatures, emissions, and efficiency. Armed with new information from the modeling and experimental work, two new stoves were designed and optimized to have significant reductions in fuel use and emissions.
Open Access
Towards the systematic identification of low-cost ecosystem-mediated carbon sequestration opportunities in bioenergy supply chains
(Colorado State University. Libraries, 2015) Field, John L., author; Willson, Bryan, advisor; Paustian, Keith, advisor; Bradley, Thomas, committee member; Leach, Jan, committee member; Marchese, Anthony, committee member
Because the dedicated production of terrestrial biomass feedstocks involves the fixation of atmospheric carbon, carefully managed biofuel and bioenergy supply chains are increasingly recognized as an opportunity for carbon sequestration in soils or geological reservoirs in addition to their climate change mitigation value via the displacement of fossil fuel use. Bioenergy involves the coupling of agricultural systems and industrial supply chains, and finding optimal system designs often requires navigating a fundamental tension between maximizing overall system productivity while simultaneously limiting the intensification of feedstock exploitation to sustainable levels. Bioenergy sustainability analyses are further complicated by strong spatial heterogeneities in feedstock production performance, fundamentally different emission mechanisms across the agricultural and industrial phases of the biofuel lifecycle, and the tendency to perform environmental assessments and economic analyses in isolation. Well-designed integrated assessments are necessary to identify the total amounts and time dynamics of sequestration possible in such systems, to put those results in context relative to other supply chain impacts, and to understand tradeoffs between various environmental impact criteria and production costs. This dissertation starts with a thorough review of the bioenergy lifecycle assessment (LCA) literature to identify outstanding climate impact accounting challenges and inform the integration of production cost estimates. Two integrated assessment case studies are then undertaken to identify low-cost opportunities for improving carbon sequestration at different points in the bioenergy supply chain. The first focuses on feedstock production, assessing the potential for increasing soil carbon sequestration in bioenergy landscapes based on the cultivation of perennial grasses. A spatially-explicit landscape analysis system is created around a newly-parameterized version of the DayCent biogeochemistry model, and switchgrass productivity and soil greenhouse gas balance are assessed across gradients of land quality and cultivation intensity in a real-world bioenergy landscape in western Kansas. Integrating these ecosystem simulation results with existing LCA, farm enterprise budget, and biomass transport models allows for the quantification of landscape level cost – mitigation tradeoffs under various system design strategies and policy constraints. The second case study focuses downstream in the supply chain, considering the use of low-value conversion co-products as soil amendments to improve agroecosystem sustainability. The biochar co-product from a hypothetical thermochemical conversion system in the Colorado Front Range is assessed using simplified models of biochar recalcitrance and agronomic benefits as a function of feedstock material and conversion method. Together, these case study results are illustrative of the potential costs of improving ecosystem-mediated carbon sequestration in bioenergy systems, and the ongoing work required for full global supply chain optimization.