Experimental Investigation of Stirling Engine Modelling Techniques at Reduced Source Temperatures

• Author / Creator

  Miller, David A

• A Stirling engine was studied experimentally with two objectives. The first was to determine the effects of flywheel polar moment of inertia on the transient and steady-state performance of a Stirling engine. The second was to investigate the accuracy of a 2nd order thermodynamic model with a range of thermal source temperatures and determine how to best improve the model for low-temperature difference Stirling engines.
  Experiments were performed on a low-temperature configuration of the ST05G-CNC Stirling engine. It was heated with electric cartridge heaters, cooled with 21°C water, and charged with air at 517 kPa or 621 kPa. Sensors measured the angular position of the crankshaft, torque load on the engine, gas temperatures, coolant temperatures, and gas pressures. Experiments examined the transition from engine startup to free-running, steady-state and loaded, steady-state operation.
  A 2nd order model that was primarily constructed from procedures in literature was studied. The model calculated the reference cycle with an adiabatic model of the engine with imperfect heat transfer in the heater, regenerator, and cooler. Decoupled loss calculations determined the heat exchanger flow friction, appendix gap losses, conduction loss, gas spring hysteresis, and mechanical losses. An iterative correction was added so the model would reflect the experimentally measured gas temperatures in the expansion space and compression space.
  The transient and steady-state behavior of the low-temperature ST05G was studied experimentally with four flywheel polar moments of inertia. Flywheel polar moment of inertia did not change the free-running, steady-state engine frequency, except for one case. This case had a higher steady-state frequency and was from experiments with the smallest flywheel at the lower of two charge pressures. Increased flywheel polar moment of inertia increased the settling time of
  the transient engine frequency and decreased the overshoot of the engine frequency. At loaded, steady-state operation, increased flywheel polar moment of inertia decreased the angular velocity fluctuations. This did not significantly influence the shaft power or thermodynamics. The angular velocity fluctuated twice per cycle, as one would expect from the Fundamental Efficiency Theorem. A method to calculate flywheel size was developed from the Fundamental Efficiency Theorem. The method estimated the order of magnitude of flywheel polar moment of inertia.
  The 2nd order model was compared to experiments at seven thermal source temperatures from 242 °C to 418 °C. The model inaccurately estimated engine performance at all conditions and less accurately estimated engine performance at lower thermal source temperatures. Methods to improve the model were deduced from the inaccuracies. The reference cycle of the model could improve if a semi-adiabatic model that includes leakage replaces the adiabatic model. This change may also improve the accuracy of the mechanical losses. To improve the accuracy of the decoupled power losses, the model could employ a more sophisticated mechanical loss calculation and replace the current gas spring hysteresis correlation. The model would have to be evaluated again to confirm that these changes improved its accuracy.

• Subjects / Keywords

  • Stirling Engine
  • Low Temperature Difference Stirling Engines
  • Angular Velocity Fluctuations
  • Thermodynamic Model

• Graduation date

  Fall 2019

• Type of Item

  Thesis

• Degree

  Master of Science

• DOI

  https://doi.org/10.7939/r3-jvb2-y408

• License

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