Experimental investigation into the influence of low mass and heat fluxes and transient perturbances in vapour quality and heat flux on the behaviour of boiling R-245fa in horizontal pipes

Abstract

Die gebruik van sonkrag vir die opwekking van elektrisiteit het in die afgelope paar dekades meer en meer aandag begin kry, veral om die uitermatige steun op fossiel brandstowwe en die antropogeniese effek wat dit op die klimaat het te probeer stuit. Terwyl fotovoltaïese kragopwekking ‘n beduidende rol speel, is gekonsentreerde sonkrag, gekoppel met termiese stoor-metodes, meer gepas om basis opwekking te verskaf. Een van die primêre navorsings-areas wat steeds dringend aandag vereis is die effek wat die verbygaande aard van die son op die krag-omskakelings proses het. Wanneer ‘n vloeistof die kook-proses vir gebruik in termodinamiese siklusse ondergaan in die sons-vesamelings veld, stel dit direkte stoom opwekking in staat. Hierdie opwekkings-metode hou verskeie termodinamiese, asook omgewings en koste, voordele in. Die termodinamiese staat in hierdie vloei-kook scenarios is egter ook hoogs vatbaar vir kort-termyn fluktuasie in die vloeier inlaat toestande en die termiese hitte vloed grenstoestand wat veroorsaak word deur, byvoorbeeld, korstondige wolk bedekking van die versamelings veld. Min gedokumenteerde navorsing vir hierdie toestande is beskikbaar. In ‘n poging om hierdie agterstand aan te spreek, is die eksperimentele werk wat hierdie tesis bevat voltooi. Die studie het veral gefokus op hitte oordrag koëffisiënte en drukval gevalle waar min of geen data beskikbaar is nie. ‘n Eksperimentele fasiliteit is ontwerp en opgerig om verskeie effekte op die hitte oordrag koëffisiënt en druk val in ‘n vloeier te bestudeer. ‘n Horisontale toets-seksie, wat uit ‘n gelykmatig-verhitte koper pyp met ‘n lengte van 800 mm en ‘n binne-diameter van 8.31 mm bestaan het, is gefabriseer. R-245fa is die toets vloeistof wat gekies is omdat dit al voorgestel is vir gebruik in direkte damp opwekking organiese Rankine-siklusse. Vir praktiese redes is ‘n versadigings temperatuur van 35 °C gekies en fokus is geplaas op areas wat nog nie beduidende aandag in literatuur ontvang het nie. Eerstens is die gedrag van gestadigde hitte oordrag koëffisiënte by relatiewe lae massa- en hitte-vloede (onder 100 kg/m2s en 7.5 kW/m2) ondersoek oor die hele reeks damp kwaliteite van 0.05 tot 0.90. Dit is bevind dat ‘n hoër massa vloed altyd tot ‘n hoër hitte oordrag koëffisiënt gelei het vir ‘n gegewe hitte vloed dwarsdeur die damp-kwaliteit reeks. By die laagste hitte-vloed wat ondersoek is, het die hitte oordrag koëffisiënt egter onreëlmatige gedrag getoon. Dit was tot 70% hoër as vir hoër hitte-vloede by damp kwaliteite van 0.15 tot 0.30. Tweedens is die effek wat ‘n skielike verandering in die damp kwaliteit by die inlaat van die toetsseksie op die hitte oordrag koëffisiënt gehad het bestudeer. Massa-vloede van 200 en 300 kg/m2s is ondersoek by damp-kwaliteite van 0.15 tot 0.40, wat gekies is om verskeie vloeipatrone te dek. Vir ‘n stap-verandering van grootte 0.13 in die damp kwaliteit is gevind dat die hitte oordrag koëffisiënt verskil het van die quasi-gestadigde voorspellings. Dit was hoër as die verwagte gedurende die afwaartse versteurings en laer gedurende die opwaartse versteurings. Laastens is die impak wat ‘n versteuring in die hitte-vloed direk in die toetsseksie op die hitte oordrag koëffisiënt en drukval gehad het ondersoek. Hierdie toetse is gedoen teen ‘n massa-vloed van 200 kg/m2s oor ‘n damp-kwaliteit reeks van 0.10 tot 0.85. Gebaseer op feitelike direkte normale bestralings data is ‘n reduksie van 75% in die hitte vloed toegepas oor ‘n periode van 30 s met stap, driehoekige, en sinusgolf versteurings, wat in ‘n beheerde vorm toegepas is. Gedurende die skielike stap-versteuring in die hitte-vloed is gevind dat die gemiddelde hitte oordrag koëffisiënt 30% laer was as die onversteurde waarde. Vir die driehoekige en sinusgolf versteurings was die hitte oordrag koëffisiënt 8% laer as die onversteurde waarde. Die drukval was onaangetas deur enige van die verskillende versteurings.

The use of solar power to generate electricity has gained increasing traction over the last few decades, particularly to offset the excessive reliance on fossil fuels and the anthropogenic effect this has on the climate. While photovoltaic electricity production plays a significant role, concentrated solar power, coupled with thermal energy storage, is more suited for providing baseline grid power. One of the prime areas still needing research, however, is the transient nature of solar irradiation and its influence on the operation of the power conversion process. When a fluid undergoes flow boiling for use in thermodynamic power cycles within the solar field, it enables the implementation of direct steam generation cycles. These offer several thermodynamic, environmental and cost advantages. However, the thermal operating state of these flow-boiling scenarios is more susceptible to short term transient changes in the fluid inlet condition and thermal heat flux boundary condition caused by, for instance, temporal cloud coverage of the collector field. Little documented research into such conditions is available. In order to help address the current lack of data, this experimental investigation was conducted, in which, particularly, the heat transfer coefficient and pressure drop behaviour was studied for cases where little or no data is available. An experimental test installation was designed and constructed to quantify various effects on the heat transfer coefficient of the flow and pressure drop within the fluid. A horizontal test section was considered which consisted of a uniformly heated pipe with a length of 800 mm and an inner diameter of 8.31 mm. R 245fa was selected as testing fluid since it has been proposed for possible use in organic Rankine cycles which can also be operated as direct steam generation cycles. For practical reasons a saturation temperature of 35 °C was used and focus was placed on aspects that have not yet received significant attention in literature. Firstly, the behaviour of the steady-state heat transfer coefficient for relatively low mass fluxes (less than 100 kg/m2s) and relatively low heat fluxes (less than 7.5 kW/m2) was investigated over the entire vapour quality range from 0.05 to 0.90. It was found that a higher mass flux always resulted in a higher heat transfer coefficient for a given heat flux throughout the quality range. However, at the lowest heat flux considered, the heat transfer coefficient exhibited anomalous behaviour in that it was up to 70% higher at vapour qualities of 0.15 to 0.30 than higher heat fluxes. Secondly, the effects of an abrupt change in vapour quality at the inlet to the test section on the heat transfer coefficient was studied. Mass fluxes of 200 and 300 kg/m2s were considered at vapour qualities of 0.15 to 0.40, such that several flow pattern conditions were covered. It was found that for a step size magnitude of 0.13 in the vapour quality, the actual heat transfer coefficient differed from the expected quasi-steady-state heat transfer coefficient during the transient. It was higher than the expected in the downward step, and lower during an upward step. Lastly, the impact of a changing heat flux on the flow boiling heat transfer coefficient was considered. These tests were performed at a mass flux of 200 kg/m2s over a vapour quality range of 0.10 to 0.85. Based on actual DNI data, a reduction of 75% in heat flux over a period of 30 s with a step, triangular and sinusoidal waveform pulse was applied in a controlled fashion. It was found that for an abrupt step reduction in the heat flux, that the average heat transfer coefficient during the perturbance was 30% lower than the unperturbed value. For triangular and sinusoidal pulses, the heat transfer coefficient was 8% lower. During all tests, the pressure gradient through the test section was unaffected by any form of perturbance.