Experimental investigation of flow boiling at high heat flux conditionsZajec, Boštjan (Avtor) Končar, Boštjan (Mentor) Cizelj, Leon (Komentor) flow boiling experimenttemperature-controlled test sectionhorizontal annulusflow visualizationbubble size distributionimage processing methodheat flux measurementFlow 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.20232023-09-17 08:15:35Doktorsko delo/naloga150450sl