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

From turbo-machines to solar chimneys

Thesis (PhD)--Stellenbosch University, 2012.ENGLISH ABSTRACT: This dissertation is basically a summary, with some interpretation, of published research by the author. The scope is limited to the fields of turbo-machinery, computational fluid dynamics and solar chimney power plants. The main contribution in the field of turbo-machinery in general is in the development of a through-flow method that automatically satisfies mass conservation. Concerning fan design, the contributions are the realization of the importance of the exit kinetic energy in the determination of the efficiency of rotor-only axial flow fans, and the quantification of the effect of off-axis inflow into cooling system fans on their performance. In the field of centrifugal fans and compressors an original, unifying model for the prediction of slip factor was developed. To investigate accident scenarios in closed cycle gas turbine nuclear reactors, all possible operational modes of multi-stage axial compressor operation caused by flow and rotation direction were investigated experimentally and computationally. Spanning the fields of turbo-machinery and solar chimneys, the basic theory of solar chimney turbines was developed, showing that high turbine efficiency was possible. In the field of solar chimneys, an original thermodynamic approach was developed to predict the main relationships that govern solar chimney performance, and to solve the through-flow equations for non-ideal systems with losses. Equations for the accurate determination of all the thermodynamic variables in a solar chimney as dependent on chimney height, wall friction, additional losses, internal drag and area change were derived and solved. Coefficients of wall friction, bracing wheel loss and exit kinetic energy were determined experimentally, and empirical equations were developed to predict the loss coefficient of the collector to turbine transition section and and the turbine inlet flow angle. A simple power law approach allowed the calculation of the optimal turbine pressure drop in solar chimney power plants. A comparison of two sets of equations used to calculate the heat fluxes into, inside and leaving the solar collector, resulted in similar air temperature rises in the collector, and similar produced power. It turned out however that the optimal flow for minimal turbine pressure drop was dependent on the heat transfer models. Investigation of the performance of various solar chimney turbo-generator layouts using analytical models and optimisation techniques showed that the optimal number of turbines varies with plant size, but the individual turbine size, the number of blades and even the efficiency remains close to constant. It was found that the cost of a turbogenerator system, however, varies significantly with size. A joint paper with several German universities and institutions did a comparative cost analysis of solar chimney power plantsAFRIKAANSE OPSOMMING: Hierdie verhandeling is basies ’n opsomming, met interpretasie, van gepubliseerde navorsing deur die outeur. Die omvang is beperk tot die gebiede van turbomasjinerie, berekeningsvloeidinamika en sonskoorsteenkragstasies. Die hoof bydrae op die gebied van turbomasjinerie in die algemeen is in die ontwikkeling van ’n deurvloeimetode wat outomaties massabehoud bevredig. Wat waaierontwerp betref is die bydrae die besef van die belangrikheid van die uitlaat kinetiese energie in die bepaling van waaierbenuttingsgraad, en die kwantifisering van die effek van af-as invloei in verkoelingswaaiers op hulle gedrag. Op die gebied van sentrifugaalwaaiers en -kompressors is ’n oorspronklike, samevattende model vir die voorspelling van glipfaktor ontwikkel. Om ongeluk-scenario’s in geslote kringloop gasturbine kenreaktors te ondersoek is al die moontlike werksmodusse veroorsaak deur vloei en rotasie rigting van ’n multistadium aksiaalkompressor eksperimenteel en numeries ondersoek. As brug tussen turbomasjinerie en sonskoorstene is die basiese teorie van sonskoorsteenturbines ontwikkel met die aanduiding dat hoë turbine benuttingsgraad moontlik is. Op die gebied van sonskoorstene is ’n oorspronklike termodinamies benadering ontwikkel om die hoofverwantskappe te voorspel wat sonskoorsteen gedrag bepaal, en om die deurvloei vergelykings op te los vir nie-ideale stelsels met verliese. Vergelykings vir die akkurate bepaling van al die termodinamiese veranderlikes in ’n sonskoorsteen soos afhanklik van skoorsteenhoogte, wandwrywing, bykomstige verliese, interne sleur en oppervlakte verandering is afgelei en opgelos. Koëffisiënte vir wandwrywing, verstywingswiel-verlies en uitlaat kinetiese energie is eksperimenteel bepaal, en empiriese vergelykings is ontwikkel om die verlieskoëffisiënt van die kollektor-tot-skoorsteen oorgang en die turbine inlaatvloeihoek te bepaal. ’n Eenvoudige magswet benadering het dit mootlik gemaak om die optimum turbine-drukval in sonskoorsteen aanlegte te bepaal. ’n Verglyking van twee stelle vergelykings om warmtevloede in, binne en uit die sonkollektor te bereken het gelei na soortgelyke temperatuurstygings en gelewerde drywing. Die optimale vloei vir maksimum drywing was egter afhanklik van die warmteoordrag modelle. Ondersoek van die gedrag van verskeie turbo-generator uitlegte, deur gebruik van analitiese modelle en optimeringstegnieke het getoon dat die optimale aantal turbines wissel met aanleg grootte, maar die individuele turbine grootte, die aantal lemme en selfs die benuttingsgraad bly feitlik konstant. Daar is egter gevind dat die koste van ’n turbogenerator stelsel beduidend wissel met grootte. ’n Gesamentlike artikel met verskeie Duitse universiteite en instansies het ’n vergelykende koste analise van sonskoorstene gedoen