Experimental and numerical study on radiative cooling of a linear alternator in a free-piston Stirling electric generator

Abstract

Heat rejection of a linear alternator (LA) plays a pivotal role in achieving high performance and a long lifetime. This paper proposes an innovative approach for the first time that utilizes a high emissivity coating formulated from a blend of microsilica and graphene nanosheets to enhance the radiation heat dissipation inside the LAs, offering advantages of structural simplicity and cost-effectiveness. To elucidate the radiative heat transfer characteristics interacted with the oscillating flow within the LA, firstly, a CFD model was developed to analyse the oscillating flow coupled radiative heat transfer within a 1.5-kW LA. The effects of coating emissivity, backdrop temperature, dissipated heat by the components, and mean pressure on the heat transfer of the LA’s electromagnetic components were investigated. Numerical calculations indicate that the high emissivity coating reduces the average temperature of the coil adjacent to the permanent magnet, the permanent magnet itself, and the inner stator by approximately 10.0 K, while the temperature of the coils adjoining the outer housing and the outer stator decreases by about 4.2 K. Additionally, the efficacy of the high emissivity coating in enhancing the cooling of these components was significantly amplified under elevated heat generation conditions. Subsequently, an experimental test rig was built and the results showed that the high emissivity coating reduces the temperature of the coil and outer stator by 1.1 K and 0.2 K, respectively, increases the output electric power by 6.6 W, and improves thermal-to-electric efficiency by 0.3 % under the operating conditions with cooling water temperature of 298 K and mean pressure of 4.5 MPa. This work provides valuable perception into enhancing the internal cooling effect of LAs.