Development of a mN level Meso-Scale Thruster

This dissertation focuses on the practical application of heat recirculating combustors as thrust chambers for micro-spacecraft systems, including: design, development, stability and operational characteristics of the thruster in both the steady state and pulsed configurations. Stable combustion was realized with partially premixed methanol/steam/oxygen, non-premixed methanol/steam/oxygen, and nonpremixed kerosene/steam/oxygen. The steam oxygen mixture is a surrogate for the decomposition products of hydrogen peroxide. The effect of channel geometry on the stability and thermal performance has also been conducted in addition to qualitative and quantitative comparisons of fuel/oxidizer injection configurations. In general it was found that non-premixed combustion is favorable in terms of both thermal performance and flame stability due to the predictable extinction characteristics at low flow rates and the absence of lean blow off at high flow rates. A quantitative extinction criterion was developed to predict extinction at the rich extinction limit. Additionally, nozzle discharge characteristics at low Reynolds number were studied and a correlation developed to predict the discharge coefficient from the Reynolds number for both cold and hot flow scenarios. It was found the discharge coefficient decays more rapidly for high temperature flows than low temperature flows due to the effects of temperature and viscosity on the boundary layer displacement thickness. Additionally, a milli-Newton level thrust stand was developed to indirectly measure the thrust level without allowing the thruster to translate, the thrust stand resolution was found to be <1mN. Using this device a study of the thrust characteristics was carried out in both the steady state and pulsed modes. Measurements of the specific impulse efficiency indicate that the conversion efficiency is high and any loss in thermal efficiency from the adiabatic scenario is due to wall heat losses and not incomplete combustion. Experiments conducted with hydrogen peroxide decomposed in the inlet channel of the combustor were used to validate the results taken with the steam/oxygen as the oxidizer and demonstrated that heat recirculation from the products to the exhaust is sufficient to promote efficient decomposition of the hydrogen peroxide

Advanced Low NO Sub X Combustors for Supersonic High-Altitude Aircraft Gas Turbines

A test rig program was conducted with the objective of evaluating and minimizing the exhaust emissions, in particular NO sub x, of three advanced aircraft combustor concepts at a simulated, high altitude cruise condition. The three combustor designs, all members of the lean reaction, premixed family, are the Jet Induced Circulation (JIC) combustor, the Vortex Air Blast (VAB) combustor, and a catalytic combustor. They were rig tested in the form of reverse flow can combustors in the 0.127 m. (5.0 in.) size range. Various configuration modifications were applied to each of the initial JIC and VAB combustor model designs in an effort to reduce the emissions levels. The VAB combustor demonstrated a NO sub x level of 1.1 gm NO2/kg fuel with essentially 100% combustion efficiency at the simulated cruise combustor condition of 50.7 N/sq cm (5 atm), 833 K (1500 R) inlet pressure and temperature respectively and 1778 K (3200 R) outlet temperature on Jet-A1 fuel. Early tests on the catalytic combustor were unsuccessful due to a catalyst deposition problem and were discontinued in favor of the JIC and VAB tests. In addition emissions data were obtained on the JIC and VAB combustors at low combustor inlet pressure and temperatures that indicate the potential performance at engine off-design conditions

Theoretical limits of scaling-down internal combustion engines

Small-scale energy conversion devices are being developed for a variety of applications; these include propulsion units for micro aerial vehicles (MAV). The high specific energy of hydrocarbon and hydrogen fuels, as compared to other energy storing means, like batteries, elastic elements, flywheels and pneumatics, appears to be an important advantage, and favors the ICE as a candidate. In addition, the specific power (power per mass of unit) of the ICE seems to be much higher than that of other candidates. However, micro ICE engines are not simply smaller versions of full-size engines. Physical processes such as combustion and gas exchange, are performed in regimes different from those that occur in full-size engines. Consequently, engine design principles are different at a fundamental level and have to be re-considered before they are applied to micro-engines. When a spark-ignition (SI) cycle is considered, part of the energy that is released during combustion is used to heat up the mixture in the quenching volume, and therefore the flame-zone temperature is lower and in some cases can theoretically fall below the self-sustained combustion temperature. Flame quenching thus seems to limit the minimum dimensions of a SI engine. This limit becomes irrelevant when a homogeneous-charge compression-ignition (HCCI) cycle is considered. In this case friction losses and charge leakage through the cylinder-piston gap become dominant, constrain the engine size and impose minimum engine speed limits. In the present work a phenomenological model has been developed to consider the relevant processes inside the cylinder of a homogeneous-charge compression-ignition (HCCI) engine. An approximated analytical solution is proposed to yield the lower possible limits of scaling-down HCCI cycle engines. We present a simple algebraic equation that shows the inter-relationships between the pertinent parameters and constitutes the lower possible miniaturization limits of IC engines

Extinction Limits of Premixed Combustion Assisted by Catalytic Reaction in a Stagnation-Point Flow

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/76039/1/AIAA-2006-164-151.pd

COMBUSTION AND HEAT TRANSFER IN MESO-SCALE HEAT RECIRCULATING COMBUSTORS

Combustion in small-scale systems faces problems related to time available for chemical reaction to go to completion and the possible quenching of the reaction by the increased effects of interfacial phenomena (thermal quenching and radical quenching) that occur at the combustor walls due to higher surface to volume ratio. Heat recirculation, where in a portion of the energy from the products is fed back to the reactants through structural conduction is one of the strategies employed in meso-scale combustors to overcome the problems of thermal quenching of the flame. When liquid fuels are employed, structural conduction can help pre-vaporize the fuel and thereby removes the necessity for a fuel atomizer. This dissertation focuses on the design, development and operational characteristics of meso-scale combustors employing heat recirculation principle. Self-sustained combustion of propane-air and methanol-air flames were achieved in sub centimeter dimensions (32.6 mm3). The effects of design and operational parameters like wall thermal conductivity, heat exchanger size/channel length, combustion chamber geometry, equivalence ratio, Reynolds number, and external heat transfer (loss) coefficient on the combustor performance were investigated experimentally and numerically. The experimental procedure involved fabrication of combustors with different geometric features employing materials of different thermal conductivities and then obtaining their operating limits. Thermal performance with respect to various flow conditions was obtained by measuring the reactant preheating and exhaust gas temperatures using thermocouples. Numerical simulations were performed for both reacting and non-reacting flow cases to understand the heat transfer characteristics with respect to various design and operational conditions. Both experiments and numerical simulations revealed that wall thermal conductivity is one of the most important parameters for meso-scale combustor design. For typical meso-scale dimensions wall materials with minimal thermal conductivity (< 1W/m-K), especially ceramics would yield the best performance. Results showed that the most thermally efficient operating condition occurs for fuel lean cases at higher Reynolds numbers. Flame dynamics inside the combustor were investigated through high-speed imaging and flame acoustic spectrum mapping. Due to the small length scales involved, hydrodynamic instabilities have negligible effect on meso-scale combustion. Flame was observed to be extremely stable with negligible fluctuations. However, a significant amount of thermoacoustic phenomena is present within the combustion regime. Chemiluminescence imaging was employed to correctly map the flame zone inside the combustor

Swirling combustor energy converter: H2/air simulations of separated chambers

This work reports results related to the “EU-FP7-HRC-Power” project aiming at developing micro-meso hybrid sources of power. One of the goals of the project is to achieve surface temperatures up to more than 1000 K, with a ΔT ≤ 100 K, in order to be compatible with a thermal/electrical conversion by thermo-photovoltaic cells. The authors investigate how to reach that goal adopting swirling chambers integrated in a thermally-conductive and emitting element. The converter consists of a small parallelepiped brick inside two separated swirling meso-combustion chambers, which heat up the parallelepiped, emitting material by the combustion of H2 and air at ambient pressure. The overall dimension is of the order of cm. Nine combustion simulations have been carried out assuming detailed chemistry, several length/diameter ratios (Z/D = 3, 5 and 11) and equivalence ratios (0.4, 0.7 and 1); all are at 400 W of injected chemical power. Among the most important results are the converter surfaces temperatures, the heat loads, provided to the environment, and the chemical efficiency. The high chemical efficiency, > 99.9%, is due to the relatively long average gas residence time coupled with the fairly good mixing due to the swirl motion and the impinging air/fuel jets that provide heat and radicals to the flame

Critical research and advanced technology (CRT) support project

A critical technology base for utility and industrial gas turbines by planning the use of coal-derived fuels was studied. Development tasks were included in the following areas: (1) Combustion - investigate the combustion of coal-derived fuels and methods to minimize the conversion of fuel-bound nitrogen to NOx; (2) materials - understand and minimize hot corrosion; (3) system studies - integrate and focus the technological efforts. A literature survey of coal-derived fuels was completed and a NOx emissions model was developed. Flametube tests of a two-stage (rich-lean) combustor defined optimum equivalence ratios for minimizing NOx emissions. Sector combustor tests demonstrated variable air control to optimize equivalence ratios over a wide load range and steam cooling of the primary zone liner. The catalytic combustion of coal-derived fuels was demonstrated. The combustion of coal-derived gases is very promising. A hot-corrosion life prediction model was formulated and verified with laboratory testing of doped fuels. Fuel additives to control sulfur corrosion were studied. The intermittent application of barium proved effective. Advanced thermal barrier coatings were developed and tested. Coating failure modes were identified and new material formulations and fabrication parameters were specified. System studies in support of the thermal barrier coating development were accomplished

Combustion in microspaces and its applications

PhD research can be divided in three main parts: part one and two related to the development of some of the most important aspects of the catalytic combustion in microspaces, part three related to a possible application of the catalytic combustion in microspaces. Part 1: The combustion of gaseous HC fuels in a small confined space could represent an alternative way to produce thermal and electrical energy. The combustion of CH4 and its lean mixtures with H2 on catalytic monoliths was studied and optimized. 2% Pd/(5% NiCrO4), 2% Pd/(5% CeO2ZrO2), 2% Pd/(5% LaMnO3ZrO2) and 2% Pt/(5% Al2O3) catalysts, suitably developed, were deposited on SiC monoliths via in situ SCS and tested in a lab-scale microreactor by feeding only CH4, only H2, and three lean CH4/H2 mixtures with increased content of H2 and constant thermal power density of 7.6 MWth m-3. Monolith with 2% Pt/(5% Al2O3) was very appropriate for the combustion of only CH4 or H2, but its performance worsen when H2 was added to the reactive mixture. On the contrary, the Pd-based catalysts were most suitable for the combustion of the CH4/H2 lean mixtures, with the best behavior shown by 2% Pd/(5% NiCrO4) followed by 2% Pd/(5% CeO2ZrO2). Monolith coated with 2% Pd/(5% LaMnO3ZrO2), instead, showed the worse performance, both in terms of CH4 combustion only and of the various mixtures; moreover, it displayed quite high CO emissions, not compatible with the environmental issues. In particular, the catalytic reactivity towards CH4 combustion of the Pd- based raised by increasing the H2 content in the reactive mixture. The observed enhancement in reactivity of the mixture when the CH4 fuel was enriched with H2 could be explained by an increase of the OH• radicals in the gas mixture. Part 2: The present work deals with the investigation on the performance of catalyst 2% Pd/ 5% LaMnO3•ZrO2 (PLZ), lined on silicon carbide (SC, with thermal conductivity of 250 W m-1 K-1) or cordierite (CD, with thermal conductivity of 3 W m-1 K-1) monoliths, for the CH4/H2/air lean mixtures oxidation. The bare and coated monoliths were tested into a lab- microreactor designed to provide a favorable environment for microscale combustion of CH4/H2/air lean mixtures to reach high power density (7.6 MWth m-3; GHSV 16,000 h-1). Various CH4/H2 mixtures were tested in heating and cooling phases on the various monoliths, by studying both the homogenous and heterogeneous reactions. The relative percentages of methane and hydrogen were mutually varied (maintaining the sum of the two fuels equal to 100%), in order to always assure a constant power density. The air was always fed with  equal to 2. The main aim of the catalytic combustion tests was to select the best settings to achieve at the minimum temperature full CH4 conversion with the minimum H2 concentration in the reactive mixture, accompanied by the lowest possible CO concentration. Depending on the thermal conductivity of the tested monoliths, the existence of a steady-state multiplicity was verified, mainly when the hydrogen concentration was quite low. Basically, microburners with low wall thermal conductivity (CD monoliths) exhibited shorter ignition times compared to the higher thermal conductivity ones (SC monoliths) due to the formation of spatially localized hot spots that promoted catalytic ignition. At the same time, the CD material required shorter times to reach steady-state. But SC materials assured longer time on stream operations. The presence of the catalyst lined on both monoliths allowed reaching lower CO emissions. The best results belonged to the catalytic SiC monolith, with a low hydrogen concentration in the fed mixtures. Part 3: The idea was to realize an autothermal steam reforming reaction. This was made by coupling a combustion reaction (exothermic), which provided the heat necessary, with a steam reforming reaction (endothermic) in a same specific built micro reactor. The total reagents chosen for the two reactions were methane (used both as fuel and as a reactant for the steam reforming), air and steam (produced by heating water). The main advantage of this system: producing enough energy, for example, to power auxiliary transportation of vehicles, reducing consumption and pollutant emissions; at the same time, because of the overall limited dimensions, reducing the risk of explosion if compared to the hydrogen "on board " storage. The development was a stainless steel reactor consisting of two plates with microchannels, containing the catalyst (Pt/AlO3), in which the reactions took place. These plates were placed in indirect contact, separated by a middle plate made of stainless steel, so to conduct the heat from the combustion side to the steam reforming, and also to avoid the mixing of the fluids. The sealing of both sides were ensured by two ceramic gaskets, suitable to withstand high temperatures. The sizing was performed first theoretically assuming a S / C = 4 (Steam to Carbon), and taking into account the maximum flow rates that could be set to the mass flow controllers. It was then calculated the theoretical thermal power necessary to sustain the steam reforming process, and then calculated the flow of methane and air to be sent to the combustor, to obtain an autothermal reforming. The catalyst used was chosen because of its catalytic activity for both types of reaction. Once it was determined the best side for the steam reforming, it was decided to experiment the coupled reactions. After having reached 900 °C in oven, with complete methane combustion, oven heat was no more provided: combustion was able to be sustained because of a mixture of 7% CH4 in air (inside the flammability limit) and reagents for the steam reforming were sent in a steam/carbon 4:1 replacing nitrogen flow. Results show how the performance of the reactor was affected by thermal dissipation; hence the material used as insulating, in order to wrap up the reactor, plays a key role for performing tests. Tests were carried out increasing thermal power from combustion side to balance the heat dissipations, so to obtain a balance between heat generated and used by the reaction of steam reforming and the heat lost in the environment. It has been showed the way for producing good quality data on coupling combustion and steam reforming reactions in this reactor. In a future, it could be possible using a GC instead of the ABB analyzer in case of new tests with high CH4 not reacted, or of course improving methane conversion choosing a better catalyst for steam reforming, composing a reactor with multiple plates for optimizing the process as shown in Vlachos' simulations, and trying to run flows in either concurrent or countercurren

Operational stability of lean premixed combustion in gas turbines : an experimental study on gaseous alternative fuels

World electricity consumption is drastically increasing. One of the most common ways of producing electricity is to use the chemical energy of fossil fuels. This can be done in thermal power plants in which the chemical energy of fossil fuels such as natural gas is converted to mechanical energy and finally to electricity. Extracting the chemical energy of fuels is done through combustion of the fuel with air. Combustion produces heat, water and carbon dioxide as its main products. The produced heat can be converted to mechanical energy in different ways. In gas turbines, the hot combustion products are directly used to move turbine blades and produce mechanical energy, which is then converted to electricity by means of an electric generator. However, one should bear in mind that electricity is not the only outcome of this process. During this process, we are consuming the very limited reserves of fossil fuels, we are producing pollutants and we are negatively contributing in the climate change by producing carbon dioxide. These negative consequences are becoming increasingly alarming. These concerns have led to a growing interest in the use of alternative fuels such as bio- and electro-fuels with reduced environmental impact for electricity production. Using bio- and electro-fuels in gas turbines provide reliable production of heat and electricity while decreasing the dependency on fossil fuels and contributing to the reduction of greenhouse gas emissions.One of the most promising gaseous bio-fuels for gas turbines is digestion gas or ‘biogas. Biogas contains varying amounts of CH4 and CO2 as its major components. Another alternative fuel that can be considered for gas turbine combustors is synthesis gas (syngas) fuels that can be produced from renewable sources such as lignocellulosic biomass. Syngas may contain H2, CO, and CH4, as well as CO2, N2, H2O, and small amounts of higher hydrocarbons. The composition of these alternative fuels differs from natural gas, which has CH4 as its main component. This means that these fuels have different chemical and physical properties and therefore different combustion properties than natural gas. Therefore utilizing such fuels as the main or part of the fuel mixture in gas turbine combustors may substantially affect their efficiency, operability and emission characteristics. It is thus important to understand and quantify their operational characteristics to make their use in gas turbines viable.One of the most important aspects of combustion that has to be considered in gas turbines when using alternative fuels is operational stability. It means that the combustion needs to take place in the combustor and in a smooth, reliable manner. In other words, the combustion needs to be sustained under all operating conditions. This is particularly important in modern gas turbines, referred to as lean premixed combustors, where fuel and air are mixed before entering the combustor. There are several operability risks that can occur and should be avoided in a lean premixed gas turbine combustor such as: lean blowout (the flame can extinguish due to reactions taking place too slowly), flashback (the flame can travel in to the premixing section), and autoignition (the fuel/air mixture can autoignite in the premixing section and before entering the combustor).In this work, an experimental approach was used to investigate and understand the combustion of various fuel mixtures that can replace natural gas in gas turbines. A model combustor was designed and built that can mimic a real gas turbine combustor. The focus of the experiments was to investigate the combustion stability in the combustor when burning fuels comprising H2, CO and CO2. The combustor featured a quartz glass tube that provided optical access to the flame. Different experimental techniques were used to shed light on how the combustion behavior and operational stability of such fuels differs from natural gas. Various operating conditions and burner characteristics were examined in order to explore the possibility of reaching a fuel-flexible combustor