Graduate Thesis Or Dissertation
 

Effects of Residual CO2 and N2 on Detonation Velocity and Detonation Limits

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/4x51hr322

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  • Detonation combustion has gained interest throughout the years because of its potential to increase thermodynamic cycle efficiency when compared to deflagration based cycles. The expected benefits result from a pressure gain during the combustion process instead of a pressure drop, as observed for deflagrations. The implementation of detonations can be observed in devices such as pulse detonation engines (PDEs) and rotating detonation engines (RDEs). In pressure gain devices, such as PDE’s and RDE’s, mixing of combustion products with fresh fuel and oxidizer is often an inevitable process. The mixing of products with reactants is significant because existing literature indicates that detonations can be sensitive to dilution resulting in a velocity suppression or an inability to detonate. Many studies have investigated the effect of inert diluent, specifically Ar and N2 on detonation velocities while research of non-inert diluents is scarce. To utilize hydrocarbons as a detonation fuel source it becomes pertinent to understand how reactive diluents such as CO2 (which is a primary combustion product) change detonation behavior. Such information can be used to help inform the design and evaluation process of future pressure gain combustion devices and provide insights into the conditions which facilitate a successful detonation with CO2 dilution present. Specifically this work will contribute by quantitively identifying the sensitivity between dilution and detonability and detonation velocity. Experiments were performed using a pulse detonation engine equipped with photodiodes to measure velocity. To achieve the objectives of this study, the equivalence ratio of the fuel (C3H8) and oxidizer (N2O) mixture was set at 1.0 while the diluent (CO2 and N2) concentrations were varied from a mass fraction of 0 < Yx < 0.6 to observe changes in propagation velocity and identify detonation limits. In addition, cell sizes were calculated and compared to the detonation limits observed during experiments. It was found that a species such as CO2 can reduce the detonation velocities and detonability more than an inert species such as N2 for mass fractions larger than 0.2. This was supported by the differences observed in experimental detonation velocities and detonation limits as the mass fraction of CO2 and N2 diluent increased. The factor of greatest influence on detonation sensitivity was identified as the changing mass of the diluent, which reduced the specific sensible enthalpy within the flow. Additional parameters that were identified to contribute to the detonation sensitivity of both diluents were changes in gas properties and temperature. In addition, for results with CO2 dilution, a potential chemical sensitivity was detected. Although it cannot be identified how CO2 affects the chemical reaction process of the fuel and intermediate species, existing literature for deflagrations suggest that CO2 reduces the radical production and plays a role in dissociation reactions. When comparing the detonation limits observed in experiments to calculated cell sizes, the results showcased a correlation between calculated cell sizes and the ability to successfully detonate.
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