Simulation, design, and experimental characterization of catalytic and thermoelectric systems for removing emissions and recovering waste energy from engine exhaust
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An analytical transport/reaction model was developed to simulate the catalytic performance of ZnO nanowires as a catalyst support. ZnO nanowires were chosen because they have easily characterized, controllable features and a spatially uniform morphology. The analytical model couples convection in the catalyst flow channel with reaction and diffusion in the porous substrate material; it was developed to show that a simple analytical model with physics-based mass transport and empirical kinetics can be used to capture the essential physics involved in catalytic conversion of hydrocarbons. The model was effective at predicting species conversion efficiency over a range of temperature and flow rate. The model clarifies the relationship between advection, bulk diffusion, pore diffusion, and kinetics. The model was used to optimize the geometry of the experimental catalyst for which it predicted that maximum species conversion density for fixed catalyst surface occurred at a channel height of 520 [mu]m. A modeling study of thermoelectric (TE) vehicle waste heat recovery was conducted based on abundant and inexpensive Mg₂ Si[subscript 0.5] Sn[subscript 0.5] and MnSi[subscript 1.75] TE materials with consideration of performance at the system and TE device levels. The modeling study identified a critical TE design space of fill fraction, leg length, n-/p-type leg area ratio, and current; these parameters needed to be optimized simultaneously for positive TE power output. The TE power output was sensitive to this design space, and the optimal design point was sensitive to engine operating conditions. The maximum net TE power for a 29.5 L strip fin heat exchanger with an 800 K exhaust flow at 7.9 kg/min was 2.25 kW. This work also includes two generations of TE waste heat recovery systems that were built and tested in the exhaust system of a Cummins 6.7 L turbo Diesel engine. The first generation was a small scale heat exchanger intended for concept validation, and the second generation was a full scale heat exchanger that used the entire exhaust flow at high speed and torque. The second generation heat exchanger showed that the model could accurately predict heat transfer, and the maximum experimental heat transfer rate was 15.3 kW for exhaust flow at 7.0 kg/min and 740 K.