Browsing by Subject "Superadiabatic combustion"
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Item Numerical simulation of conversion of n-heptane to hydrogen in an inert, porous medium(2007-08) Hull, Charles Bradley; Ellzey, Janet L.The conversion of liquid hydrocarbon fuels, specifically liquid heptane, to a syngas mixture containing hydrogen by means of superadiabatic combustion within a porous inert media is studied computationally over a wide range of rich equivalence ratios, inlet velocities, and in different types of porous material. Parameters of interest include the composition of the exhaust gases and particularly the amount of hydrogen present in the exhaust, conversion efficiency, energy efficiency, reaction front propagation speeds, firing rates, and overall hydrogen production rates. Both a packed bed of 3 mm diameter Al-oxide pellets and a 3.9 pores-per-centimeter reticulated ceramic are studied in depth and compared as the burner material. The computational model used is a one-dimensional, transient code that models the combustion processes as well as the heat transfer processes that occur as the reaction front moves through the porous material. Several different n-heptane chemical kinetics mechanisms are studied and a process is developed that utilizes both the flexibility of a reduced mechanism and the more accurate predictions of a more detailed mechanism. Computational predictions are compared to preliminary experimental data, as well as similar studies performed previously with other fuels types.Item Superadiabatic combustion in counter-flow heat exchangers(2009-05) Schoegl, Ingmar Michael; Ellzey, Janet L.Syngas, a combustible gaseous mixture of hydrogen, carbon monoxide, and other species, is a promising fuel for efficient energy conversion technologies. Syngas is produced by breaking down a primary fuel into a hydrogen-rich mixture in a process called fuel reforming. The motivation for the utilization of syngas rather than the primary fuel is that syngas can be used in energy conversion technologies that offer higher conversion efficiencies, e.g. gas turbines and fuel cells. One approach for syngas production is partial oxidation, which is an oxygen starved combustion process that does not require a catalyst. Efficient conversion to syngas occurs at high levels of oxygen depletion, resulting in mixtures that are not flammable in conventional combustion applications. In non-catalytic partial oxidation, internal heat recirculation is used to increase the local reaction temperatures by transferring heat from the product stream to pre-heat the fuel/air mixture before reactions occur, thus increasing reaction rates and allowing for combustion outside the conventional flammability limits. As peak temperatures lie above the adiabatic equilibrium temperature predicted by thermodynamic calculations, the combustion regime used for non-catalytic fuel reforming is referred to as 'superadiabatic'. Counter-flow heat exchange is an effective way to transfer heat between adjacent channels and is used for a novel, heat-recirculating fuel reformer design. An analytical study predicts that combustion zone locations inside adjacent flow channels adjust to operating conditions, thus stabilizing the process for independent variations of flow velocities and mixture compositions. In experiments, a reactor prototype with four channels with alternating flow directions is developed and investigated. Tests with methane/air and propane/air mixtures validate the operating principle, and measurements of the resulting syngas compositions verify the feasibility of the concept for practical fuel-reformer applications. Results from a two-dimensional numerical study with detailed reaction chemistry are consistent with experimental observations. Details of the reaction zone reveal that reactions are initiated in the vicinity of the channel walls, resulting in "tulip"-shaped reaction layers. Overall, results confirm the viability of the non-catalytic reactor design for fuel reforming applications.