Impingement heat transfer with pressure recovery

A conventional impinging jet is effective at transferring a large heat flux. However a significant pressure loss is also experienced by the free jet of a jet impingement heat transfer device due to rapid expansion because it does not incorporate effective pressure recovery. A novel high-flux impingement heat transfer device, called the Tadpole, is developed to improve the heat transfer and pressure loss (performance) characteristics of the conventional impingement domain by incorporating pressure recovery with a diffuser. The Tadpole is scrutinized through an experimental comparison with a conventional jet impinging on the inner wall of a hemisphere under the turbulent flow regime. The Tadpole demonstrates promising capability by exceeding the performance characteristics of the impinging jet by up to 7.3% for the heat transfer coefficient while reducing the pressure loss by 13%. Multiple dimensional degrees of freedom in the Tadpole’s flow domain can be manipulated for an enhanced heat transfer coefficient, a reduced total pressure loss or a favourable combination of both metrics. A Computational Fluid Dynamics (CFD) model is developed, the Four-Equation Transition SST turbulence model demonstrates satisfactory experimental validation with a deviation of < 5% for the heat transfer coefficient and < 23% for the total pressure loss. The Tadpole is a promising heat transfer device for high-flux applications and is recommended for further work incorporating design improvements and multidimensional optimization.The Solar Thermal Energy Research Group (STERG) at Stellenbosch University.https://link.springer.com/journal/231hj2023Mechanical and Aeronautical Engineerin

Thermal performance characteristics of a tessellated-impinging central receiver

Current central receiver Concentrating Solar Power plants using molten salt as a heat transfer fluid add heat at around 565 °C in a power plant. Adding heat at a higher temperature can improve the thermodynamic performance and may reduce the cost of power. One way to achieve this is by using pressurized air solar receivers. Current receivers have achieved thermal efficiencies of around 80% at an outlet temperature of 800 °C. This paper investigates a novel central receiver technology that makes use of a tessellated array of heat transfer units. The units employ impingement heat transfer within a concave surface. The receiver can be scaled for a desired thermal rating by the number of heat transfer units. The convolution-projection flux modelling approach is used to model and project an incoming flux distribution on the receiver’s surface. This flux distribution is interpreted by a Computational Fluid Dynamics model as a volumetric heat source. Radiative and convective heat losses are considered. An initial performance outlook estimates that an outlet temperature of 801 °C can be reached at a thermal efficiency of 59% and an exterior surface temperature of 1142 °C for an aperture flux of 635 kW/m2. A limitation is an insufficient exterior surface area to absorb the incoming flux which causes a high surface temperature and thermal losses. Similar thermal performance is estimated at high and low pressures, with increased pumping losses at low pressures. The efficiency may be improved by taking advantage of a larger surface area relative to the aperture area.An Erasmus+ mobility grant awarded by Alliance4Universities which made a collaboration at UC3M possible.http://www.elsevier.com/locate/atehj2023Mechanical and Aeronautical Engineerin

Capability of a novel impingement heat transfer device for application in future solar thermal receivers

CSP receivers are designed to permit higher outlet temperatures in order to enable higher theoretical efficiencies of the associated thermodynamic cycles. For pressurized air receivers, it is attempted to increase the operating temperature of metallic pre-heaters to then achieve high air outlet temperatures with cascaded ceramic receivers. Two limitations of metallic pressurized air receivers are cost and material creep at elevated temperatures and pressures. Therefore, it is necessary to maximize heat transfer from the receiver surface to the working fluid while minimizing the material surface temperature. Current research has demonstrated that jet impingement heat transfer devices are appropriate for application in thermal receivers because of the associated desirable heat transfer characteristics. However, it is shown that significant pressure losses are caused by such impinging jets because of the sudden expansion phenomenon. A novel enhanced impingement heat transfer device is presented in this paper. Experimental testing was conducted to investigate the domain comparatively with impinging jet configurations. The device is shown to be capable of delivering an enhanced surface heat transfer coefficient while affecting a lower total pressure loss around the domain when compared with similar impinging jet configurations. The geometry of the device can also be chosen to achieve a favorable combination of heat transfer and pressure loss characteristics. The device is applicable within the SCRAP concept and may be implementable within the SOLHYCO and SOLUGAS receivers. The device may also find an application in a parabolic dish collector. Finally, a novel receiver concept that incorporates the device in a tessellated structure is introduced – the SUNflower.https://aip.scitation.org/journal/apcpm2021Mechanical and Aeronautical Engineerin