Experimental correlations and CFD model of a non-tubular heater for a Stirling solar engine micro-cogeneration unit

Abstract

A non-tubular heat exchanger for use in a Stirling solar engine micro-CHP unit is being developed by the University of Oviedo and the technological research centre IK4-Tekniker Foundation. In this article, the correlations for the friction factor and Stanton number previously obtained under steady flow conditions are revised and the corresponding experimental data are used to validate a CFD model of the heater. The CFD model enables the estimation of variables whose measurement is practically unviable, as is the case for the spatial distribution of wall and gas temperatures. The conceptual importance of the heater wall temperature for the analysis and design of Stirling engines is highlighted, and some limitations that are inherent in the non-tubular geometry are observed. The CFD model provides a basis for the analysis of engine operation and for subsequent geometric optimization of the heater. To evaluate the engine power and efficiency forecasts under nominal operating conditions, the CFD model is used to complement the analysis procedure based on experimental data from benchmark engines with very different geometries and operating variables. The results predict that the engine will be able to exceed the targets set in the preliminary design stage.

Experimental and Numerical Characterisation of a Non-Tubular Stirling Engine Heater for Biomass Applications

“…The heat transfer rate increase caused by irreversibilities is an under-researched feature of Stirling heaters. In the only known previous analyses of this issue [3,19], it has been found that ∆ . Q e is related to characteristic variables of the gas circuit performance that might intuitively be considered non-influential in external combustion engines.…”

“…In the case of the cylindrical rod heater designed for solar energy conversion, the following correlation was obtained [18,19]:…”

“…The solid materials making up the walls of the heater were meshed to analyse the effects of heat conduction. Since a previous study found that the variation in results was negligible with mesh sizes of more than 1 million cells [19], in this case, the computations were performed with a mesh of approximately 4,200,000 cells, which was considered sufficient to obtain the behaviour of the air near the grooves in detail. The CFD model was created using the commercial CFD software ANSYS 16.…”

“…In this prototype, a non-tubular heater for solar applications was installed, whose geometry consists of a flat surface designed to absorb the direct solar irradiance of a conventional optical concentrator, and transmit energy through almost a thousand cylindrical rods of 10 mm length and 2 mm diameter [18]. The experimental characterisation of this non-tubular heater served as the basis for the subsequent development of a CFD model of the heater's functionality, making it possible to simulate and analyse various operating conditions [19]. Using air as the working fluid, this prototype could develop an indicated power of approximately 700 W at the nominal operating conditions of 6.9 bar mean pressure, 600 to 700 • C heater wall temperature and of the order of 900 rpm engine speed.…”