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Nozari H.,Koc University | Karabeyolu A.,Koc University | Karabeyolu A.,Space Propulsion Group, Inc
Fuel | Year: 2015

Abstract With its high hydrogen density and already existing infrastructure, ammonia (NH3) is believed to be an excellent green fuel that can be used in energy generation and transportation systems. Combustion of ammonia has certain challenges (associated with its low flame speed and fuel bond NOx emissions) that need to be addressed before its widespread use in practical systems. The primary objective of this study is to develop a reduced reaction mechanism for the combustion of ammonia which can be used to expedite the design of effective ammonia combustors through numerical simulations of realistic combustor geometries with accurate kinetics models. First we have investigated the combustion characteristics of NH3/H2/air mixtures at elevated pressure and lean conditions which are encountered in practical systems such as gas turbine combustors. Laminar premixed freely propagating flame model is used to calculate the combustion properties. The results of sensitivity study of total NOx formation with respect to the equivalence ratio indicates the possibility of localized rich combustion as an effective way to reduce the NOx concentration down to levels that are the same order as the modern gas turbine engines. In the second part of the study, by considering a wide range of conditions in terms of pressure, fuel mixture, and equivalence ratio we have developed two reduced mechanisms based on the Konnov mechanism. The reduced mechanisms are capable of predicting the total NOx emission level and the laminar flame speed at an acceptable accuracy over a wide range of conditions. Evaluating the performance of the reduced mechanisms with respect to the full mechanism and experimental data shows that the mechanisms are able to predict the combustion properties almost at the same accuracy level as the Konnov mechanism, but at a nearly five times less CPU time expense. © 2015 Elsevier Ltd. Source


Nozari H.,Koc University | Karabeyoglu A.,Space Propulsion Group, Inc
13th International Energy Conversion Engineering Conference | Year: 2015

In the first section of this numerical study we investigate the combustion characteristics of ammonia-air mixtures at elevated pressure and lean conditions which are encountered in gas turbine combustors. Laminar premixed freely propagating flame and homogenous reactor models are used to calculate the combustion properties. The improvement by hydrogen addition to the fuel mixture in combustion characteristics such as laminar flame speed and ignition delay time is noticeable. Based on ammonia decomposition sensitivity analysis, it is found that the OH radicals have a leading role in controlling the fuel mole conversion and the laminar flame speed. The results of sensitivity study of total NOx formation with respect to the equivalence ratio reveal the possibility of localized rich combustion as an effective way to reduce the NOx concentration down to levels that are the same order as the modern gas turbine engines. In the second part of the study, by considering a wide range of conditions in terms of pressure, fuel mixture, and equivalence ratio we develop two reduced mechanisms based on the Konnov mechanism. The reduced mechanisms are capable of predicting total NOx emission level and laminar flame speed in an acceptable accuracy under wide range of conditions. Evaluating performance of the reduced mechanisms with respect to the full mechanism and experimental data shows that the mechanisms are able to predict the combustion properties with almost the same accuracy as the full Konnov mechanism and with nearly five times less CPU time expense. Source


Grant
Agency: Department of Defense | Branch: Air Force | Program: SBIR | Phase: Phase I | Award Amount: 99.12K | Year: 2008

Space Propulsion Group, Inc (SPG) proposes to conduct investigations on Nytrox/paraffin-based hybrid rocket systems which promise high energy flexible propulsion solutions with high volumetric fuel efficiency while retaining the safety and cost advantages of classical hybrids. The benefits of the proposed hybrid concept result from a combination of two key SPG technologies: high-regression rate paraffin-based solid fuels and high performance Nytrox oxidizers which are refrigerated mixtures of nitrous oxide and oxygen. The high regression rate capability simplifies the fuel grain design, minimizes fabrication costs, improves fuel volumetric loading and reduces the fuel sliver mass fraction to less than 3%. The major advantages of Nytrox oxidizers are 1) higher density, Isp performance and safer operation compared to N2O, 2) partial self pressurization, non-cryogenic operation compared to LOX. Nytrox/paraffin based hybrids are ideal for systems that require operational flexibility due to the possibility of active throttling with very small performance penalty, a direct consequence of the relatively flat c*-O/F curve. Phase I work has two major components: 1) propulsion system design/optimization studies with emphasis on mission flexibility and 2) small scale motor testing to demonstrate the throttling and gas phase combustion with gaseous oxygen at the end of the liquid burn.


Grant
Agency: National Aeronautics and Space Administration | Branch: | Program: SBIR | Phase: Phase I | Award Amount: 99.92K | Year: 2008

Space Propulsion Group, Inc. proposes to conduct systems studies to quantify the performance and cost advantages of Nytrox oxidizers for small launch vehicles. This new class of oxidizers is composed of mixtures of nitrous oxide (N2O) and oxygen (O2) and has significant advantages over the pure oxidizers, some of which can be summarized as 1) higher density, Isp and safer operation compared to N2O, 2) non-cryogenic operation and ease of development of stable and efficient motors compared to LOX. Thus Nytrox is expected to be an important enabling technology for developing low cost, high performance NanoSat launch vehicles. The primary goal of the Phase I effort is to quantify the increase in the payload mass by changing the oxidizer from N2O to Nytrox for the upper stages of a small launch system. In the proposed effort the cost and operational issues associated with producing, transporting and storing the Nytrox oxidizers shall be also be quantified. The planning for the third stage motor development and ground testing that will be conducted in Phase II shall be started in Phase I. Technology Readiness Level ranges of 2-3 and 5-6 are expected at the end of the Phase I and II, respectively.


Grant
Agency: Department of Defense | Branch: Air Force | Program: SBIR | Phase: Phase I | Award Amount: 39.98K | Year: 2005

Space Propulsion Group, Inc (SPG) proposes to conduct studies on paraffin-based hybrid rocket systems that provide high performance, safe, cost effective propulsion solutions for upper-stage and in-space applications. The benefits of the proposed hybrid concept result from high-regression rate paraffin-based solid fuels developed by Stanford and SPG. The high regression rate capability simplifies the fuel grain design significantly and makes hybrids a viable option for the conventional liquid and solid systems. The paraffin-based hybrids enjoy the high specific impulse performance, variable throttling and stop/restart capabilities of liquid systems, where as the system complexity and total liquid mass is reduced compared to a liquid system resulting in a more reliable and cost effective propulsion system. Moreover solid additives (i.e. Aluminum powder) can easily be incorporated in the fuel grain of a hybrid rocket in order to improve the Isp and density impulse performance. The objective of the proposed program is to mature the paraffin-based hybrid technology to the level that they become a safe, affordable and environmentally friendly alternative to the state of the art liquid and solid systems. The Phase I work has three major components: 1) development and lab-scale testing of fuel formulations optimized for space motors, 2) propulsion system preliminary design/optimization studies and 3) planning of the Phase II large-scale testing effort

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