title: Thermomechanical analysis of compact high-performance electric swashplate compressor
Creators: name: Arqam, Mohammad
Publisher: name: Griffith University
Year: 2021
Subjects: subject: Compact electric compressors
subject: mobile air conditioning
subject: thermal resistance
subject: minimal computational effort

description: This PhD is sponsored by industry and is part of a project to develop and manufacture smart electric compressor for mobile refrigeration and air conditioning applications on commercial and heavy vehicles including industrial machinery. Compact electric compressors are of great value for the future due to the growth of the electric vehicle market. Recent advancements in the field of mobile air conditioning and refrigeration have witnessed extensive use of the swashplate compressor due to its compact structure, continuous operation, small size, light weight and better thermal comfort inside the vehicle. The design of the swashplate compressor is complex so that it requires considerable contributions from different fields of engineering viz. engineering mechanics, heat transfer and fluid dynamics. The estimate of compressor performance through modelling and experiments at the early stages of design and development serves as a useful tool for the designer. The input power, torque, in-cylinder gas pressure and temperature, flow through valves, and volumetric efficiency are important parameters to characterize the compressor performance. In this thesis, a set of practical thermomechanical models are derived and validated against experiments. An ideal gas based analytical model is developed for a 10 cylinder swashplate compressor with a view to predict its performance in terms of shaft torque and mass flow rate for a given rotational speed requiring minimal computational effort to run. Three sub-models are developed to account for the piston and swashplate kinematics and dynamics through deriving expressions for piston displacement as an explicit function of angle of rotation of swashplate and interactions between forces and moments. The compression process model is formulated to predict in-cylinder temperatures and pressures during one revolution of the swashplate together with refrigerant mass flow rate in and out of the compressor. A complete time-varying model is then developed by combining above three sub-models. Results are obtained in terms of compressor torque and volumetric efficiency and agree well with experiments. Considering the importance of refrigerant flow through reed valves affecting compressor performance, a real-gas, restricted-flow valve model is also developed and compared with the ideal-gas, ideal-valve model. Real gas properties of R134a are evaluated using NIST standard reference database. A minor-loss discharge coefficient approach is used to determine the refrigerant flow rate through reed valves. The model predicts the discharge temperature, refrigerant mass flow rate and volumetric efficiency accurately as a function of rotational speed. The effect of real gas properties, heat transfer to and from the compressor wall during compression and expansion and the valve model are analyzed. The suction side valve model is found to have the largest influence on the compressor performance as a function of rpm whereas heat transfer model has the least. The key contribution of this study is in assembling a practical combination of models that is capable of capturing the essential physics without being overly complex. To the authors’ knowledge this is the first swashplate study that shows clearly the cyclic variation in thermo-physical properties. The literature shows the dynamic characteristics of the compressor are well connected with the start-up transients of the swashplate mechanism and the suction and discharge pressures. To evaluate this, an experimentally validated transient swashplate compressor model is developed including mass moment of inertia of the pistons and swashplate to evaluate the motor torque loading during compressor start-up. The effects of essential parameters such as moment of inertia, bearing torque, viscous resistance to the piston motion, suction and discharge pressures on the compressor performance are presented. The actual start-up behavior is tracked using a high-speed data logger capturing phase currents for the BLDC motor, instantaneous power and rotational speed. The suction and discharge pressures are found to have the largest influence on the starting torque whereas rotational mass moment of inertia has the least. The original contribution of this work is in deriving a transient swashplate compressor model that includes the mass moment of inertia of the swashplate mechanism and clarifying the relative importance of line pressures, viscous losses and bearing resistance on the start-up torque. Since minimizing the size of the compact Brushless DC (BLDC) motor driving the compressor is important, it is worth optimizing the cooling performance of the electric motor. An experimentally validated computational fluid dynamics (CFD) model is developed to investigate the thermal performance of an air-cooled Brushless Direct Current (BLDC) motor driving swashplate compressor. Different fin arrangements on the motor housing are analysed including small protrusions on the fin surface. The findings show greater enhancements can be achieved by adding an extra fin in the cooling flow passage rather than through the inclusion of grooved walls. Thermographs of the motor housing are found to be in close agreement with the model predictions. The key achievement of this thermal investigation is in demonstrating air-cooling is a practical and effective alternative to refrigerant cooling of compact high performance electric swashplate compressors for mobile refrigeration and air conditioning applications. The effect of thermal resistance between the windings and stator core of an air-cooled Brushless DC motor is also investigated. Measurements are found to be in close agreement with predictions. The numerical simulations suggest significant benefits of injecting encapsulation material in the stator core to enhance heat transmission from windings to the surrounding electrical steel. To confirm this, an experimental investigation is carried out by adding thermal resin to the winding slots on 2.5 kW and 4 kW brushless DC motors. The findings show that the potting material can reduce the temperature of the windings by 10 °C to 20 °C for electrical power inputs of 2.4 kW to 3.8 kW. The winding temperature is also found to be sensitive to the winding arrangement in the stator slot. With tighter, more compact windings also leading to significant temperature reductions.