Parameter Optimization of a Low Temperature Difference Gamma-Type Stirling Engine to Maximize Shaft Power

• Author / Creator

  Stumpf, Calynn

• An investigation was performed with three main objectives. Determine the configuration andoperating parameter of a low temperature difference Stirling engine (LTDSE) that would result inthe maximum shaft power. Calculate the mean West number for LTDSEs and compare it to themean West number for high temperature difference Stirling engines (HTDSEs). Validate themethods used to estimate the compression ratio of a Stirling engine.Three different LTDSEs were developed: The Mark 1, Mark 2, used for initial development andthe EP-1. The EP-1 used a bellows for its power piston and was the main engine investigated. Atest rig was used that allowed for measurement of temperature, pressure, torque, engine speed, andcrank shaft position. The LTDSEs were operated with a thermal source and sink temperature of95 °C and 2 °C, respectfully, and were heated and cooled with two water flow loops. They usedair as a working fluid and a constant buffer pressure of the atmosphere, which was equal to 92.5kPa.The compression ratio, phase angle, position of the thermal source heat exchanger, and powerpiston connection to the workspace was varied on the EP-1 to determine its maximum shaft power.A compression ratio of 1.206 ± 0.017, phase angle of 90 ± 1°, and the thermal source heatexchanger residing on top with the power piston connected to the expansions space resulted in theconfiguration that produced the maximum local shaft power. This was a shaft power of6.58 ± 0.09 watts that occurred at an operating speed of 67.7 ± 0.3 rpm. A global maximum shaftpower was not found due to limitations of the power piston. The West number for the configurationthat produced the maximum local shaft power when using a power piston swept volume of1835 ± 50ml was 0.238 ± 0.001.The mean West number was calculated from 12 different LTDSEs that used air as the workingfluid, the atmosphere as the buffer pressure, and a temperature difference less than 150 °C, andequaled 0.21. This was lower, but close to the mean West number for HTDSEs of 0.25. Thissuggests that the West number can be used for comparison of Stirling engine that operate at alltemperature differences.Three methods that estimated the compression ratio for Stirling engines were investigated. Thecompression ratios produced by these methods were compared to the compression ratio thatproduced the local maximum shaft power for the EP-1 of 1.206 ± 0.017. The three methods usedwere Kolin’s compression ratio, Egas’ ideal compression ratio, and Senft’s optimum swept volumeratio. Using Kolin’s method, a compression ratio of 1.085 was calculated by. If the experimentalaverage temperature of the working fluid in the compression and expansion space of the EP-1 wasused for Egas’ method, a compression ratio of 1.156 was produced. If the thermal source and sinktemperature was used for Egas’ method, a compression ratio of 1.338 was calculated. Senft’smethod predicted the compression ratio closely when a mechanism effectiveness of 0.75 was used.Based on the compression ratios used by the 12 experimental LTDSEs, a model was proposed toestimate the compression ratio that could produce the maximum shaft power. This model isapplicable to LTDSEs that using air as a working fluid and a buffer pressure of atmospheric. Theshaft power compression ratio was calculated from the temperature ratio of the thermal source andsink temperature. To further validate the model, more compression ratio optimization tests shouldbe performed for various temperature ratios and LTDSEs.

• Subjects / Keywords

  • Additive manufacturing
  • Power piston connection location
  • Low temperature difference Stirling engine
  • Heat exchanger position
  • Compression ratio
  • Phase angle
  • Shaft power optimization

• Graduation date

  Spring 2019

• Type of Item

  Thesis

• Degree

  Master of Science

• DOI

  https://doi.org/10.7939/r3-jb09-7t31

• License

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