Combined Diesel Particulate Filter/Heat Exchanger for Engine Exhaust Waste Heat Recovery with Organic Rankine Cycle
Sprouse, Charles Edward, III
University of Kansas
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Diesel Particulate Filters (DPFs) are currently being used to remove Particulate Matter (PM) from compression ignition engine exhaust streams with collection efficiencies approaching 100%. These devices capture soot by forcing the exhaust gases through porous walls, where entrapment of the particulates initially occurs. Eventually, a cake layer begins forming on the inlet channel walls, causing an increased pressure drop through the device and necessitating a soot combustion event to unload the filter. The exothermic nature of these regeneration events serve to enhance the thermal energy content of the exhaust, which already contains approximately one-third of the fuel energy being consumed by the engine. Typically, the energy from both sources is expelled to the atmosphere, destroying the ability to produce useful work from the exhaust heat. However, a novel device described here as a Diesel Particulate Filter/Heat Exchanger (DPFHX) may be coupled to an Organic Rankine Cycle (ORC) to simultaneously provide particulate matter filtration and waste heat recovery. The DPFHX concept is based on the shell-and-tube heat exchanger geometry and features enlarged tubes to contain DPF cores, allowing energy capture from the engine exhaust while preserving the standard technique of PM abatement. Since the working fluid circulating on the shell side collects heat from the exhaust, the DPFHX serves as the organic Rankine cycle’s evaporator. Along with the cycle’s pump, expander, and condenser, the DPFHX forms an ORC capable of transforming exhaust waste heat into supplementary power for the engine. Reducing exergy destruction in this manner meets the two main objectives of engine research; the reduction of fuel consumption and emissions. The degree to which the proposed DPFHX-ORC system achieves these goals is a focus of this dissertation, where the advancement of this technology occurs primarily through theoretical efforts. As precursors to the eventual DPFHX-ORC computer model, individual ORC and DPF models are created. With respect to simulating an ORC, a historical study of the ORC WHR literature informs the design choices associated with building an ORC model. Authors in this research area note that the two dominant factors influencing cycle performance are the working fluid and expander selections. Based on these findings, eight dry fluids (butane, pentane, hexane, cyclopentane, benzene, toluene, R245fa, and R123) compatible with reciprocating expanders are identified for use in an ORC model. By simulating WHR from a Yanmar L100V diesel engine, the component-based ORC constructed illustrates an approximate 10% improvement to the engine’s efficiency across all operating conditions and favors the use of pentane or cyclopentane as the cycle’s working fluid. These results are consistent with reported ORC outputs in the literature and demonstrate the ORC model’s value as a component of the DPFHX-ORC model. The second foundational component is a DPF model, which is developed using the DPF governing equations in area-conserved format. A series of model validation efforts show that the DPF model is capable of generating accurate thermodynamic parameter profiles in the inlet and outlet channels, along with tracking the monolith temperatures and soot combustion. However, extension of the model to include external heat transfer to the ORC working fluid requires the creation of a novel multi-dimensional DPFHX computer model due to the small DPF core size and enhanced heat transfer in a DPFHX. This model does not follow traditional multi-dimensional modeling schemes by allowing heat transfer with and without the DPF cores as intermediaries. Also, the model does not couple inlet and outlet channels, or force all walls bordering an individual channel to have uniform conditions. The DPFHX model provides heat recovery predictions for coupling with the ORC model, allowing power output predictions from the entire system based on the single cylinder Yanmar test cell at the University of Kansas as the waste heat source. By matching the energy leaving the engine exhaust to the heat entering the ORCs working fluid, a DPFHX-ORC model is constructed in MATLAB. At very low engine load (227.3 W), the ORC system generates 156.8 W of power, corresponding to a 69.0% efficiency improvement over the engine alone. At typical engine loads (1726.7 W to 6205.5 W), the DPFHX-ORC system provides an efficiency increase between 9.5-13.7%. Along with the illustrated fuel consumption reduction is a reduction of all emissions by the same amount, following a short warm up period. The reduction of hydrocarbons and carbon monoxide are unaffected by installation of the DPFHX, and conversion efficiencies of nitrogen oxides are maintained by placing the selective catalytic reduction hardware before the DPFHX, alleviating concerns of low-temperature conversion. Due to the energy removal taking place in the DPFHX, PM collection occurs at reduced temperature levels; however, the efficiency of this process remains high due to the mechanical nature of filtration.
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- Engineering Dissertations and Theses 
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