Experimental investigation of flow boiling at high heat flux conditions

Zajec, Boštjan (author), Končar, Boštjan (mentor), Cizelj, Leon (co-mentor)

Flow boiling is an effective heat transfer mechanism, commonly present in nuclear power plants and in other thermal engineering applications. Despite long history of flow boiling research, some underlying phenomena are still not fully understood. Bubbles change in size and shape as they move through the liquid, due to evaporation on the heated wall, condensation in the subcooled liquid, and interactions with other bubbles. This work focuses on experimental determination of the bubble size distribution to capture the combined effect of these mechanisms. To study flow boiling at high heat flux conditions, a unique water heated annular narrow-gap test section was designed and built. The annular tube in which the boiling takes place has an inner diameter of 12 mm and a gap width of 2 mm. The refrigerant R245fa (pentafluoropropane) was used as a boiling fluid. As the test section operates as a heat exchanger, the operation with a temperature-controlled boundary condition is possible, which in principle enables the measurements at high heat fluxes towards the critical heat flux, without the danger of thermal runaway. High-speed camera was used for two-phase flow recording and image processing was applied to analyse flow boiling regimes and acquire the data on bubble size distributions. In addition to the method of manual bubble size recognition, a neural network-based algorithm was developed to partially automate and speed up the bubble recognition procedure. In this thesis, the flow boiling in horizontally positioned annulus was investigated. Two different modes of test section operation were analysed, either with constant inlet conditions of water or refrigerant. The main finding of the thesis is that at low heat flux conditions, the bubble size distributions can be described by a one-parameter Rayleigh distribution. At high heat flux conditions, the shape of the distribution changes to a two-peak (bimodal) distribution. It has also been found that in a temperature-controlled test section, operating at the constant inlet water conditions, two competing effects are present. In this mode of operation, an increased heat flux can only be achieved by increasing the mass flow rate of the refrigerant. While a higher heat flux increases the vapor (void) content in the test section, a higher refrigerant flow rate acts in the opposite direction and lowers the void content. Both effects inherently occur together and affect the amount of total void in the observed part of the test section and an isolated observation of both effects is not possible. As the total void volume is declining steadily with increased refrigerant mass flux, the mass flux effect appears to be the main influencing mechanism and prevails over the heat flux effect.

• Language: English
• Keywords: flow boiling experiment, temperature-controlled test section, horizontal annulus, flow visualization, bubble size distribution, image processing method, heat flux measurement
• Work type: Doctoral dissertation
• Typology: 2.08 - Doctoral Dissertation
• Organization: FMF - Faculty of Mathematics and Physics
• Year: 2023
• PID: 20.500.12556/RUL-150450
• COBISS.SI-ID: 165930499
• Publication date in RUL: 17.09.2023

Secondary language

• Language: Slovenian
• Title: Eksperimentalno opazovanje konvektivnega vrenja pri visokih gostotah toplotnega toka

Abstract

Konvektivno vrenje je učinkovit mehanizem prenosa toplote, ki se pogosto uporablja v jedrskih elektrarnah in ostalih energetskih sistemih. Kljub številnim raziskavam in dolgi zgodovini uporabe, vsi pojavi povezani v vrenjem še niso v celoti znani. Mehurčki, ki nastajajo med vrenjem toka tekočine, med gibanjem v toku spreminjajo velikost in obliko zaradi različnih mehanizmov, kot so uparanje ob greti steni, kondenzacija v prostem toku in interakcije z drugimi mehurčki. Da bi zajeli vse te vplive, smo se v doktoratu osredotočili na eksperimentalno določanje porazdelitve velikosti mehurčkov. Za eksperimentalno opazovanje konvektivnega vrenja pri visokih gostotah toplotnega toka smo zasnovali in izdelali testno sekcijo v obročasti geometriji z ozko režo in vodnim gretjem površine notranje cevi. Notranja cev ima premer 12 mm, širina reže je 2 mm, za delovno oz. vrelno tekočino smo uporabili hladilo R245fa (pentafluoropropan). Ker testna sekcija deluje kot toplotni izmenjevalec, načeloma omogoča tudi obratovanje v bližini kritičnega toplotnega toka, brez nevarnosti pregretja in poškodovanja sekcije. Za opazovanje in analizo dvofaznega toka smo uporabili hitro kamero, tako pridobljene slike pa smo obdelali in jih uporabili za analizo vrenja in pridobivanje podatkov o porazdelitvi velikosti mehurčkov. Poleg analize velikosti mehurčkov z ročnim označevanjem smo razvili tudi računalniški algoritem na osnovi nevronske mreže, ki delno avtomatizira in pohitri postopek prepoznavanja in zaznave velikosti mehurčkov. V disertaciji smo se osredotočili na konvektivno vrenje v horizontalni obročasti geometriji. Raziskali in analizirali smo dva različna režima delovanja testne sekcije, bodisi s konstantnim masnim pretokom grelne vode, bodisi s konstantnim pretokom hladila. Ugotovili smo, da je pri pogojih nizkega toplotnega toka porazdelitev velikosti mehurčkov mogoče opisati z enoparametrično Rayleighjevo porazdelitvijo, pri pogojih visokega toplotnega toka pa se v porazdelitvi pojavi še drugi vrh pri večjih mehurčkih. Ugotovili smo tudi, da sta v načinu obratovanja s konstantnim pretokom grelne vode prisotna dva nasprotujoča si mehanizma. Pri tem načinu obratovanja je mogoče povečati toplotni tok le z večanjem masnega pretoka hladila. Medtem, ko večji toplotni tok poveča količino pare v testni sekciji, večji pretok hladila deluje v nasprotni smeri in zmanjšuje količino pare. Ker sta oba mehanizma prisotna skupaj in hkrati vplivata na količino pare v testni sekciji, ločeno opazovanje mehanizmov ni bilo mogoče. Rezultati kažejo, da količina pare v testni sekciji upada s povečanim masnim pretokom hladila, zato sklepamo da učinek masnega pretoka prevladuje nad učinkom toplotnega toka.

• Keywords: konvektivno vrenje, temperaturno regulirana testna sekcija, horizontalna obročasta geometrija, posnetki toka s hitro kamero, porazdelitve velikosti mehurčkov, algoritem za analizo slik, meritve toplotnega toka