Design and Thermodynamic Simulation

Figure 4.1 illustrates the complete assembly drawing and the components of the con­ceived single-phase free-piston Stirling engine design and Figure 4.2 depicts the simplified schematic diagram of the engine representing the pistons and heat exchanger configuration of the system. The gas circuit of the displacer subsystem is closed by means of tubing in order to provide a closed engine chamber. Both displacer and power piston have identical dimensions. In single-phase operation, the power piston interacts with ambient pressure on one side and engine chamber pressure on the other. To facilitate tight clearance sealing,

Heater Cooler

Design and Thermodynamic Simulation

Tubing

Figure 4.2: Simplified schematic diagram of the conceived Stirling engine.

And thus minimal enthalpy loss, the power piston is located on the cold side of the engine.

The engine design parameters are tabulated in Table 4.1. The displacer piston shuttles the working fluid, ambient pressure air, between expansion and compression spaces at the operating frequency of 3 Hz. Furthermore, the mass of the power piston has been chosen such that it resonates with the gas spring approximately at the operating frequency. When loaded, the power piston behaves as a low-Q resonating system. Thus, it does not require high precision in tuning of its resonant frequency. A dynamical simulation has been carried out using the Stirling engine adiabatic model [33]. Figure 4.3 shows the simulation results in terms of temperature and volume variations of the compression and expansion spaces as well as pressure variation of the engine chamber and the p-V diagram of the Stirling cycle. Based on this simulation, with 150 °C difference between hot and cold side average temperatures, the indicated output power of the engine is about 26.9 W at 9.4% thermal efficiency.

In the sequel, the design of the displacer piston, the heat exchanger, and the power piston subsystems are discussed in detail with an emphasis on the loss contribution of each element.

Table 4.1: Engine thermodynamic design parameters.

Operating temperatures

T^: 175 °C Tave: 25 °C

Indicated powers

Schmidt analysis Input: 75.1 W Output: 25.2 W Adiabatic model Input: 253.6 W Output: 23.8 W

Heater

Housing volume: 210 cm3 x 27.5% Hydraulic diameter: 0.6 mm Wetted area: 0.35 m2

Cooler

Housing volume: 210 cm3 x 23% Hydraulic diameter: 0. 5 mm Wetted area: 0.37 m2

Regenerator

Housing volume: 260 cm3 x 63.4% Hydraulic diameter: 0. 2 mm Wetted area: 1.9 m2

Pistons

Diameter: 10 cm Stroke: 15 cm

Tubing

Volume: 500 cm3 Hydraulic diameter: 19 mm

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