Theses and Dissertations at Montana State University (MSU)
Permanent URI for this collectionhttps://scholarworks.montana.edu/handle/1/733
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Item Green's function simulation method for earth-air heat exchangers(Montana State University - Bozeman, College of Engineering, 2015) Denowh, Chantz Michael; Chairperson, Graduate Committee: David A. Miller; Kevin Amende (co-chair)Earth-air heat exchangers (EAHXs), or earth-tubes, decrease building heating/cooling loads by pre-conditioning supply air in underground pipes. Air circulates underground to exchange heat with the surrounding soil before entering the building. The concept is fairly simple, but the field currently lacks fundamental information of the energy interactions in common EAHX installations. This identifies the need of a model framework inclusive of all EAHX design considerations, time-dependent climate conditions, and EAHX types to support the completion of this fundamental information. The EAHX types in this study include installations under locations free of structures (under yard) and under foundational slabs (under slab). This research develops the groundwork of this framework through a versatile model using the Green's Function method and numeric integration. The Green's Function method incorporates the majority of time-dependent heat transfer mechanisms surrounding EAHXs through long and short time solution components. The long time component calculates the initial soil temperature distribution in the under yard, non-radiant under slab, and radiant under slab installations. The under yard simulations were successfully validated using experimental soil temperature data from around the United States. The non-radiant under slab temperatures produced unrealistic results in some locations, but the novel Green's Function method in this location has significant potential for under slab EAHX applications. Results from the long time solution feed into the short time solution as a space-dependent initial condition. The short time solution uses a finite difference approach to calculate the heat transfer along the EAHX length. This method was validated using computation fluid dynamics with good agreement. The two components work together to quickly simulate a large number of EAHX installations. The research includes an example optimization procedure to demonstrate the framework's versatility. It successfully optimized the EAHX lengths for 95% effectiveness in cooling on the hottest day of the year in 15 locations around the United States.Item Thermomechanical training and characterization of Ni-Ti-Hf and Ni-Ti-Hf-Cu high termperature shape memeory alloys(Montana State University - Bozeman, College of Engineering, 2011) Denowh, Chantz Michael; Chairperson, Graduate Committee: David A. MillerNickel-Titanium (NiTi) is the most commonly used Shape Memory Alloy (SMA) for actuator applications. Typical SMA actuators use changes in temperature to initiate solid to solid phase transformations resulting in macroscopic material deformations; though NiTi is limited to temperature changes below 100°C. This eliminates high temperature applications of NiTi actuators. To expand the design window of SMAs, many high temperature NiTi based SMAs have been developed by adding ternary elements to the NiTi matrix. The additions result in degradation of the shape memory behaviors and their usefulness as actuators is still in question. The purpose of this research is to characterize and train two recently developed high temperature SMAs, NiTi 29.7Hf 20 and NiCu 5Ti 29.7Hf 20, to determine their effectiveness as linear actuators. Shape memory effect (SME) and superelastic tests were used to characterize the materials behavior followed by thermomechanical training at a constant stress. The SME test resulted in no martensite detwinning plateau and a non-linear stress-strain curve implying the simultaneous occurrence of slip and martensite reorientation. The superelastic tests also show an austenitic yield stress above 600 MPa. Thermomechanical training resulted in small amounts of plastic strain growth, and the development of two-way shape memory (TWSM). The TWSM results were fitted using the Bo & Lagoudas model, and is capable of predicting the actuation strains at other stresses. The results support the conclusion that hafnium distorts martensite slip planes, and (Ti,Hf) 2Ni and (Ti,Hf) 3Ni 4 precipitates form during aging and annealing. The distorted slip planes cause slip and martensite reorientation to occur simultaneously. This develops a strong stress field during training within the first few cycles. The stress field develops TWSM, but limits further plastic growth and TWSM development. The precipitate formation increases material strength, as seen in the superelastic loading, but also embrittles the material. Thermomechanical training of an annealed specimen resulted in a brittle failure after several thermal cycles due to the growth of particulate size during annealing. The two alloys are ideally suited for high temperature actuators. TWSM was trained into the material and the transformation temperatures are higher than that of NiTi, but low enough to avoid annealing and problematic creep temperatures.