Stirling Engine

Proof of concept - Won university competition

Project Role: Design Engineer / Thermal Analyst

Project Goal: To optimise the thermal performance of a Low-Temperature Differential (LTD) Stirling engine by addressing two key areas: 1) enhancing heat dissipation from the top (cold) plate through heat sink redesign, and 2) minimising heat transfer between the hot and cold plates by selecting an appropriate fastener material.

Background:

LTD Stirling engines operate on small temperature differentials, often as low as 3°C. This project's application involved utilising the heat from a mug of hot water (approximately 90°C) as the heat source. Maximising the temperature difference between the hot and cold plates is crucial for efficient engine operation. This required both effective cooling of the cold plate and minimising thermal bridging between the plates.

  1. Top Plate Heat Sink Redesign:

Design Modifications:

The original top plate design was modified to incorporate a finned heat sink to increase surface area and promote convective heat transfer. Key design considerations included:

  • Fin Alignment: Parallel fins were chosen for simplified manufacturing.
  • Manufacturability: Fin spacing and offset from the plate perimeter allowed for efficient material removal using standard milling operations. The "offset" design enabled single-plane machining, reducing complexity. A single cutting tool sise could be used, streamlining the process.
  • This allowed the design to be manufactured with a standard milling machine.

Thermal Analysis (Heat Sink):

A steady-state thermal analysis was conducted using SolidWorks. Boundary conditions included:

  • Bottom Surface Temperature: 40°C constant temperature.
  • Convection Coefficient: 25 W/m²K (natural convection).
  • Ambient Temperature: 22°C.
  • Radiation: Low emissivity (0.05) and a conservative view factor of 0.5 were used. Radiative heat transfer was deemed negligible, justifying the lack of fin tapering.

Results (Heat Sink):

The redesigned heat sink showed a modest improvement. The original design had a maximum temperature of 39.99999237°C, while the redesigned plate had a minimum of 39.80751801°C, a difference of 0.19247436°C. While measurable, this indicated further optimisation might be needed.

  1. Fastener Material Selection (Nylon vs. Steel Bolts):

Thermal Analysis (Fasteners):

A separate steady-state thermal analysis compared the impact of steel and nylon bolts. Boundary conditions were:

  • Bottom Plate Temperature: 90°C (simulating hot water contact, with some assumed heat loss).
  • Ambient Temperature: 22°C.
  • Convection Coefficient: 25 W/m²K (natural convection).
  • Radiation: 0.05 emissivity for metallic components; negligible for nylon.
  • Simplifications: Internal components between plates were neglected to focus on bolt material influence.

Results (Fasteners):

  • Steel Bolts: System temperature range: 90°C to 34.4°C. Steel acted as a thermal conductor, reducing the temperature differential.
  • Nylon Bolts: System temperature range: 90°C to 21.9°C. Nylon's low thermal conductivity significantly reduced heat transfer, maximising the temperature difference.

Overall Conclusion:

The project addressed two critical thermal aspects of the LTD Stirling engine design. The redesigned finned heat sink on the top plate provided a small improvement in heat dissipation, while the selection of nylon bolts over steel bolts dramatically improved the engine's thermal performance by minimising heat leakage between the hot and cold plates. The nylon bolts, acting as thermal insulators, were crucial for maintaining the necessary temperature differential for efficient engine operation. The combination of these design choices contributes to a greater power output and improved overall performance of the LTD Stirling Engine. Further optimisation of the heat sink, potentially through increased fin density or alternative materials, could be considered for future iterations.