Numerical simulation of jet impingement cooling of the inside of a hemisphere with application to SCRAP.

[摘要] ENGLISH ABSTRACT: Conventional concentrating solar power (CSP) plants use Rankine cycles astheir thermal power generation cycle. Recent developments have shown thepotential for combined cycle (CC) CSP plants to achieve higher e cienciesand lower costs than conventional CSP plants. One con guration of theBrayton cycle of a CC plant is to utilise a pressurised air receiver between thecompressor and turbine to offset or omit fuel consumption. The Spiky CentralReceiver Air Pre-heater (SCRAP) concept, categorised as a metallic tubularpressurised air receiver, has been shown to exhibit promising performance forthe purpose of pre-heating the air stream prior to it entering a combustionchamber or cascading secondary receiver.The receiver's absorber assemblies, the so-called spikes, are designed totransfer the incoming solar radiation energy to the pressurised air stream.With the hemisphere of the spike tip exposed to the solar field, it experiencesthe highest flux with the maximum expected at the hemisphere's centre. Jetimpingement is employed here because the elevated local heat transfer aroundthe maximum flux region cools the receiver material, which reduces externalthermal losses. A reduced maximum temperature also permits a wider rangeof materials.This thesis presents further insight into the local heat transfercharacteristics and fluid mechanical properties of the spike tip jet impingement,which is critical to the concept feasibility. Impingement cooling, in the contextof a Brayton cycle, presents a trade-off between the internal pressure drop andthe external heat losses.To analyse the local heat transfer characteristics of the cooling mechanismin the SCRAP receiver, a computational fluid dynamics (CFD) model was developed and validated against experimental data, from literature, of a flowfield of a similar nature. It was found that the three-equation k-! SSTRANS turbulence model, with the intermittency transition extension, performswell at predicting the Nusselt number surface distributions for designs withdimensionless characteristics similar to those of the SCRAP receiver's spiketip. Area-weighted averages of the distributions were predicted within 10% ofthe experimental results from literature.It was identified that adding a nozzle to the spike tip is necessary to achievethe required cooling of the spike tip, which experiences highly concentratedsolar flux. Using the validated CFD model, a detailed parametric analysis wasconducted to characterise the jet impingement cooling capabilities in the spiketip of SCRAP. It was found that the nozzle diameter is the most sensitivegeometric parameter. Decreasing the nozzle diameter drastically increasespressure drop. However, this accelerates the fluid, which significantly increasesheat transfer.The pressure drop and thermal efficiency of a pressurised air receiver bothaffect the Brayton cycle efficiency. For this reason, a method of calculatinga cycle efficiency that considers receiver pressure drop and thermal losseswas suggested. The resulting efficiency is a quantity that permits a trade-offbetween heat transfer and pressure drop. A set of design points with varyingnozzle diameters, d, showed that a maximum cycle efficiency is achieved for10mm d 2mm. The suggested efficiency quantification tool can be usedin further work for design analyses of solarised gas turbines.

[发布机构] Stellenbosch University