Houda Hachem

Comparison Based on Exergetic Analyses of Two Hot Air Engines: A Gamma Type Stirling Engine and an Open Joule Cycle Ericsson Engine

In this paper, a comparison of exergetic models between two hot air engines (a Gamma type Stirling prototype having a maximum output mechanical power of 500 W and an Ericsson hot air engine with a maximum power of 300 W) is made. Referring to previous energetic analyses, exergetic models are set up in order to quantify the exergy destruction and efficiencies in each type of engine. The repartition of the exergy fluxes in each part of the two engines are determined and represented in Sankey diagrams, using dimensionless exergy fluxes. The results show a similar proportion in both engines of destroyed exergy compared to the exergy flux from the hot source. The compression cylinders generate the highest exergy destruction, whereas the expansion cylinders generate the lowest one. The regenerator of the Stirling engine increases the exergy resource at the inlet of the expansion cylinder, which might be also set up in the Ericsson engine, using a preheater between the exhaust air and the compressed air transferred to the hot heat exchanger.

Energetic and Exergetic Performance Evaluations of an Experimental Beta Type Stirling Machine

This chapter focuses on a beta type Stirling receptive machine operating under an atmospheric pressure between two heat sources at constant temperatures. Two receiving modes are studied experimentally (refrigerator mode and heat pump mode). Parameters such as hot end temperature (about 110 °C maximum hot temperature output for the heat pump mode), cold end temperature (about −32 °C minimum cold temperature output for refrigerating mode) and coefficient of performance (COP) are studied under different engine speeds. In order to assess the receptive Stirling machine, we carry out energy and exergy performance assessment studies. Energetic and exergetic coefficients of performance and exergy destructions in the heat pump system and the refrigerating machine are quantified at different engine speeds. The results show that optimal operating mode for refrigerating machine is obtained at about 155 rpm. But, around this speed the heat pump mode deliver the worst COP. The influence of heat sources insulation on the exergy destruction and coefficient of performance of the system is investigated. Furthermore, the usefulness of good heat insulation is emphasized. In fact, it improves the energetic COP of the heat pump from 2.8 to 3.5 as the exergetic COP from 43 to 75.2 % at the same rotational speed.

4.6 Stirling Engines

The Stirling engine is mainly composed by five compartments: two working spaces and three heat exchangers (heater, cooler, and regenerator). The regenerator (porous medium) characteristics, especially material and porosity, are determinant for the whole engine performance. In order to choose the adequate regenerator, numerical and experimental methods can be adopted. Numerical models, i.e., isothermal, adiabatic, and quasi steady, are applied to determine the engine performances considering the regenerator parameters. The most powerful numerical tool is the computational fluid dynamics (CFD) simulation, which allows a detailed examination of flow behavior through the porous media. The one-variate experimental method is generally considered to test regenerator operation in the Stirling engine but the figure of merit (FOM) formulation and the experimental design methodology are more precise and faster to compare several regenerators performances.

A CFD Analysis of the Air Flow Through the Stirling Engine’s Singularities

In this chapter, a computational fluid dynamics (CFD numerical model) of the air flow through a 300 cm3 Beta Stirling engine has been used to characterize the pressure drop and heat transfer through the regenerator. The Stirling engine had two moving parts (i.e. piston and displacer) which were at a certain phase difference but reciprocated at same frequencies. First, particular specific mesh motion strategies were developed using the software STARCCM+, to describe the motion of the power piston and the displacer. Heat-transfer models were implemented by taking into account the presence of two heat sources and the regenerator porous structure. The results are compared with experimental data. Heat transfer between the air flow and the matrix has been considered by varying the hot end temperature from 400 to 1000 K and keeping the wall temperature of the regenerator at 300 K. Regenerator properties such as matrix material and porosity are investigated.