Modeling of Advanced Heat Pump Cycles and Aerodynamic Design of a Small-Scale Centrifugal Compressor for Electric Vehicles
By Zhenyuan Mei
Unlike conventional vehicles powered by internal combustion engines, electric vehicles do not have enough waste heat to provide sufficient heating to the cabin. Thus, an additional heating system, such as a heat pump, is needed. However, its performance decreases significantly when the ambient temperature is low. The new kangaroo heat pump cycle (KC) is proposed to increase the heating capacity in low-temperature climates. It is an enhanced flash tank-based vapor injection heat pump cycle (FT-VIC). A sub-cycle is added to the system to increase the refrigerant inlet quality entering the flash tank, which leads to a higher refrigerant mass flow rate and heating capacity. Because KC has a higher heating capacity, the heating needed from the electric heater can be reduced, thus reducing energy consumption and increasing the driving distance. In this study, thermodynamic models were developed for the basic heat pump cycle (BC), FT-VIC, and KC. And a new method evaluating the life cycle climate performance (LCCP) of electric vehicle heat pumps based on the SAE J2766 standard was proposed. Results show that KC is effective at low ambient temperatures. At -15°C, KC can save 13.8% energy compared to BC, and save 2.7% energy compared to FT-VIC. However, due to the additional weight, KC has a higher LCCP than other cycles. If the pressure ratio limit is removed and the compressor efficiencies are constant, KC can have a lower LCCP than other cycles in cold climates. Transient models were also developed to assess their performance in urban driving conditions. Results show that at the end of the simulation, the cabin room temperature of KC is 3.6°C and 7.0°C higher than that of FT-VIC and BC, respectively. However, due to the high pressure ratio and refrigerant mass flow rate, the accumulated power consumption of KC is 32.4% higher than FT-VIC and 64.4% higher than BC. Despite its high energy consumption, it is more efficient than adding heat from an electric heater. In addition, a small-scale centrifugal compressor was designed for an electric vehicle to reduce the compressor’s size and weight. Results show that its COP is 6.6% higher than that of the scroll compressor at the design point. However, its efficiency at the off-design point quickly drops. Future studies are needed to improve its off-design point performance.
Development Of Variable Tube Geometry Heat Exchangers Using Adjoint Method With Performance Evaluation Of An Additively Manufactured Prototype
By Ellery Klein
Air-to-refrigerant heat exchangers are a key component for heating, ventilation, air conditioning, and refrigeration (HVAC&R) systems. The performance of these heat exchangers is limited by their air-side thermal resistance. Finless non-round bare tube designs have the potential to improve the air-side thermal-hydraulic performance over their finned counterparts and consequently improve the coefficient of performance (COP) of air-conditioning systems. Previous researchers have used heuristic methods such as multi-objective genetic algorithms (MOGA) with approximation-assisted optimization (AAO) methods utilizing computational fluid dynamics (CFD) based metamodels to shape and topology optimize non-round bare tube heat exchangers. A rather unexplored optimization technique used for heat exchanger optimizations is the gradient based adjoint method. CFD solvers utilizing discrete adjoint methods can be used to shape optimize bare tube heat exchangers and can reveal unintuitive, organic, and potentially superior designs. Additionally, additive manufacturing technology has the capability of building these previously unrealizable heat exchanger designs. The objectives of this dissertation are to experimentally evaluate the performance of shape and topology optimized compact bare tube heat exchangers with non-round bare tubes on a 1) component level, and 2) system level integrated into an air conditioner. Plus, 3) develop new shape optimized variable geometry compact bare tube heat exchangers using discrete adjoint methods for HVAC&R applications.
First, a comprehensive experimental investigation of multiple shape and topology optimized compact non-round bare tube heat exchangers was conducted under dry and wet evaporator, condenser, and radiator conditions. For all heat exchangers, air-side pressure drop and heat transfer capacity were predicted within 37% and 15%, respectively. Next, an experimental test facility capable of evaluating the system level performance of a 7.03-8.79 kW commercial packaged air conditioning unit was designed and constructed. The performance of the air conditioning unit was evaluated before and after its conventional tube-fin evaporator was replaced with a shape and topology optimized bare tube evaporator. Results are presented and discussed. Lastly, an ε-constraint and penalty method optimization scheme was implemented which utilizes a commercial CFD software with a built-in discrete adjoint solver to perform multi-objective shape optimizations of 2D bare tube heat exchangers. Critical solver/mesh set-up to best optimize heat exchangers with 0.5-10.0 mm diameter bare tubes were identified and established. The optimized designs can achieve a 40-50% reduction in air-side pressure drop with at least the same heat transfer capacity compared to the initial circular bare tube geometry. An adjoint shape optimized 500 W bare tube radiator was additively manufactured in polymer and experimentally tested. Air-side pressure drop and heat transfer capacity were predicted within 15% and 10%, respectively. The experimental performance confirms the adjoint method shape optimized designs improve the thermal-hydraulic performance over the initial circular bare tube geometry.
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