Effect of Square Texture on Tribological Properties of Nano-SiO2/POB-PTFE Composites

In order to study the seal design problem of piston rings in Stirling engine, on the basis of filling PTFE with Nano-SiO2 and POB and preparing the GCr15 contact surface with square texture by HM20-I laser marking machine, experiments were carried out on LSR-2M wear tester by indirect weighing and in-situ observation methods. Optical microscope (OM) and scanning electron microscope (SEM) were used to observe the evolution of Nano-SiO2/POB-PTFE composites’ transfer film on contact surface. The results showed that the square texture would shorten the running-in and transitionary periods of the composites’ tribological process, accelerate into the stationary period. The formation process of the composites’ transfer film on the square textured contact surface was also different from smooth contact surface. Although the square texture would increase wear rate, its ability to store wear debris is more conducive to the formation of reliable, uniform and continuous transfer film with a same friction direction. Obviously, reasonable design of surface texture can effectively improve the wear resistance of sealing parts made of filling modified PTFE composites, thus providing theoretical guidance for the seal design of Stirling engine piston ring.

INVALIDITY OF ISOTHERMAL ANALYSIS IN ESTIMATING THE THERMAL PERFORMANCE OF STIRLING ENGINE AND CRYOCOOLER

The Stirling engine is an external heat engine, which is considered as the best option for extracting work from concentrated solar power applications. The most prominent characteristics of the engine are low noise, vibration, and emissions besides reflexivity of usage with any kind of heat source such as solar, biomass, industrial heat, etc. In the present paper, the STE-1008 gamma-type Stirling engine had been analyzed by using an isothermal model to demonstrate the failure of the model in analyzing the STE-1008 considering it firstly as an engine and secondly as a cryocooler. The energy equation had been used to demonstrate the disability of the isothermal model in achieving a successful thermal analysis for engine performance. In addition, a MATLAB code had been developed to check the credibility of the isothermal model in the estimation of the engine thermal parameters. The findings of the isothermal analysis revealed that the heat exchangers are unnecessary. But, in reality; all the necessary heat transfer occur within the heat exchangers rather than in the working space boundaries. Therefore, that is invalid conclusion. However, Schmidt's theory is capable of capturing the essential engine features superbly. In particular, it is capable of capturing the fundamental interplay between the mechanically restricted movement of the engine components as well as the thermodynamic cycle which is obtained from this theory.

Non-combustion non-solar deployment characterization of a free-piston Stirling engine to integrate with an exothermic reactor

Background: A small 1 kW free-piston β type Stirling engine was tested for its feasibility of integration with an exothermic reactor under the EU funded research project SOCRATCES (GA 727348). The engine’s heat receptor was minimally modified to adapt it to the reactor’s integration needs, introducing, instead of a combustion chamber, a CFD-optimized hooded enclosure. The open-loop configuration also included a small plate heat exchanger acting as a recuperator. The study attempted to investigate the performance of the Stirling engine under these non-combustion non-solar deployment conditions, focusing on conversion efficiency and thermal loss. Methods: A number of tests were run under different temperatures and flowrates to assess the engine’s response. Temperature, power, pressure and flowrate were measured at points of interest. Results: It was found that the engine is able to operate at efficiencies comparable to that of gasoline engines at much lower working fluid temperatures. It was possible to demonstrate, with the aid of a downstream recuperator, that the system in an open-loop configuration can minimize thermal loss significantly, virtually eliminating it in some cases. Conclusions: The Stirling engine appears to be a sound choice, in terms of conversion efficiency, at comparatively low temperatures, to be integrated with an exothermic reactor, at least at small-scale applications.

Waste Heat Recovery of Biomass Based Industrial Boilers by Using Stirling Engine

Industrial boilers by using biomass for electricity generation have received significant attention recent years. However, during the process, a significant fraction of thermal energy is often lost to the environment as flue gas. The exhaust flue gas heat loss which ranges from 150-180°C (423.15-453.15K) has led to discovery of importance of recovering the waste heat of the flue gas to overcome the reliance on fossil fuel. Stirling engine as an external combustion engine with high efficiencies and able to use any types of heat source is the best candidate to recover waste heat of the exhausted gas by converting it into power. Thus, in this study Stirling engine was introduced in order to evaluate the possibility of recovering waste heat from industrial boilers to produce power. For this reason, Computational Fluid Dynamic (CFD) simulation test was performed to design an initial computational model of Stirling engine for low temperature heat waste recovery. The CFD model was validated with the experiment model and shows 4.3% of deviation. The validated model then connected to a lower temperature. It shows that when the heat source is 400K, the work done by the engine is 8.4J compared to when heat source 773K the work done is 17.0 J. The computational model can be used to evaluate the performance of Stirling engine as waste heat recovery of biomass-based industrial boilers for low-grade temperature heat source.