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

Abstract

ENGLISH ABSTRACT: Conventional concentrating solar power (CSP) plants use Rankine cycles as their thermal power generation cycle. Recent developments have shown the potential for combined cycle (CC) CSP plants to achieve higher e ciencies and lower costs than conventional CSP plants. One con guration of the Brayton cycle of a CC plant is to utilise a pressurised air receiver between the compressor and turbine to offset or omit fuel consumption. The Spiky Central Receiver Air Pre-heater (SCRAP) concept, categorised as a metallic tubular pressurised air receiver, has been shown to exhibit promising performance for the purpose of pre-heating the air stream prior to it entering a combustion chamber or cascading secondary receiver. The receiver's absorber assemblies, the so-called spikes, are designed to transfer the incoming solar radiation energy to the pressurised air stream. With the hemisphere of the spike tip exposed to the solar field, it experiences the highest flux with the maximum expected at the hemisphere's centre. Jet impingement is employed here because the elevated local heat transfer around the maximum flux region cools the receiver material, which reduces external thermal losses. A reduced maximum temperature also permits a wider range of materials. This thesis presents further insight into the local heat transfer characteristics and fluid mechanical properties of the spike tip jet impingement, which is critical to the concept feasibility. Impingement cooling, in the context of a Brayton cycle, presents a trade-off between the internal pressure drop and the external heat losses. To analyse the local heat transfer characteristics of the cooling mechanism in the SCRAP receiver, a computational fluid dynamics (CFD) model was developed and validated against experimental data, from literature, of a flow field of a similar nature. It was found that the three-equation k-! SST RANS turbulence model, with the intermittency transition extension, performs well at predicting the Nusselt number surface distributions for designs with dimensionless characteristics similar to those of the SCRAP receiver's spike tip. Area-weighted averages of the distributions were predicted within 10% of the experimental results from literature. It was identified that adding a nozzle to the spike tip is necessary to achieve the required cooling of the spike tip, which experiences highly concentrated solar flux. Using the validated CFD model, a detailed parametric analysis was conducted to characterise the jet impingement cooling capabilities in the spike tip of SCRAP. It was found that the nozzle diameter is the most sensitive geometric parameter. Decreasing the nozzle diameter drastically increases pressure drop. However, this accelerates the fluid, which significantly increases heat transfer. The pressure drop and thermal efficiency of a pressurised air receiver both affect the Brayton cycle efficiency. For this reason, a method of calculating a cycle efficiency that considers receiver pressure drop and thermal losses was suggested. The resulting efficiency is a quantity that permits a trade-off between heat transfer and pressure drop. A set of design points with varying nozzle diameters, d, showed that a maximum cycle efficiency is achieved for 10mm d 2mm. The suggested efficiency quantification tool can be used in further work for design analyses of solarised gas turbines.

AFRIKAANSE OPSOMMING: Konvensionele gekonsentreerde sonkrag (GSK) stasies maak gebruik van die Rankine siklus as die termiese kragopwekking siklus. Onlangse ontwikkelings vir gekombineerde siklus (GS) GSK stasies het potensiaal getoon om hoër doeltreffendheid teen 'n laer koste as konvensionele GSK stasies te behaal. Een aspek van die Brayton-siklus van 'n GS-aanleg is om 'n hoëdruk lugontvanger tussen die kompressor en turbine te gebruik om brandstofverbruik te verskuif of vry te spring. Die Stekelrige Sentrale Ontvanger Lug Voorverwarmer (SCRAP) konsep, gekategoriseer as 'n metaalbuis hoëdruk lug ontvanger, het belowende verrigting getoon vir die doel om die lugstroom te voorverhit voordat dit die verbrandingskamer of inlyn sekondêre ontvanger binnegaan. Die ontvanger se absorpsie-samestellings, die sogenaamde spikes, is ontwerp om die inkomende stralingsenergie van die son na die hoëdruk lugstroom oor te dra. Met die halfrond van die spitspunt wat aan die sonveld blootgestel word, ervaar dit die hoogste hitte-vloed met die maksimum wat by die middelpunt van die halfrond verwag word. Straal botsing word hier ingespan omdat die verhoogde plaaslike hitte-oordrag rondom die maksimum vloedgebied die ontvangermateriaal afkoel, wat eksterne termiese verliese verminder. 'n Verlaagde maksimum temperatuur laat ook 'n wyer verskeidenheid materiale toe. Hierdie proefskrif bied 'n verdere insig in die plaaslike hitte-oordrag eienskappe en vloei-meganiese eienskappe van die spitspunt straalbotsing wat krities is vir die konsep haalbaarheid. Straalbotsing verkoeling in die konteks van 'n Brayton siklus bied 'n oorweging tussen die interne drukval en eksterne hitteverliese. Om die plaaslike hitte-oordrag eienskappe van die verkoelingsmeganisme in die SCRAP ontvanger te ontleed, is 'n berekeningsvloeistofdinamika (CFD) model ontwikkel en bevestig teen eksperimentele data uit die literatuur van 'n vloeibare veld van soortgelyke aard. Daar is bevind dat die drievergelyking k-! SST RANS turbulensie model met die afwisseldende oorgang uitbreiding goed vaar met die voorspelling van die Nusselt nommer oppervlakverdelings vir ontwerpe met dimensielose eienskappe soortgelyk aan dié van die SCRAP ontvanger se spitspunt. Gebied-geweegde gemiddeldes van die verspreidings is voorspel binne 10% van die eksperimentele resultate uit literatuur. Dit is vasgestel dat die toevoeging van 'n spuitstuk aan die spitspunt nodig is om die vereiste verkoeling van die spitspunt te behaal wat hoogs gekonsentreerde hittevloed van die son sal ervaar. Met behulp van die bevestigde CFD model, is 'n gedetailleerde parametriese analise uitgevoer om die straal botsing in die spitspunt van SCRAP te beskryf. Daar is bevind dat die spuitdiameter die sensitiefste geometriese parameter is. Vermindering in die spuitstuk diameter verhoog drasties die drukval, maar dit versnel die vloeistof wat hitte-oordrag aansienlik verhoog. Die drukval en die termiese doeltre endheid van 'n hoëdruk lugontvanger het 'n invloed op die Brayton-siklus doeltre endheid. Om hierdie rede is 'n metode vir die berekening van 'n siklusdoeltre endheid voorgestel, wat die ontvanger se drukval en termiese verliese in ag neem. Die gevolglike doeltreffendheid is 'n hoeveelheid wat 'n afwisseling tussen hitte-oordrag en drukval moontlik maak. 'n Stel ontwerppunte met veranderedne spuitstuk diameters, d, het getoon dat 'n maksimum siklusdoeltreffendheid word behaal vir 10mm d 12mm. Die voorgestelde doeltreffendheid kwantifisering instrument kan gebruik word in verdere werk vir ontwerp ontledings van sonkrag gasturbines.