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Steady state flow boiling experiments were carried out on several heated tubes with outer diameter 9.14 mm at outlet pressures 120, 200 and 300 kPa, inlet temperatures 64, 78 and 91 °C and approximately 400, 500, 600 and 800 kg/(m2·s) mass flux entering the vertically aligned test annulus until critical heat flux (CHF) was reached. The tubes were made of Inconel 625 with a length of 400 mm. The Inconel tubes were tested in three different modifications as smooth, abraded with 150 grit sandpaper and bead blasted. Multiple experiments were repeated on the same specimen to investigate the effect of surface characteristic changes (i.e., wettability, roughness and oxide layer morphology) on the occurrence of CHF. Despite the changes in initial wettability, the CHF dependency was not clearly observed, however, the changes in roughness led to an increase in CHF. The total number of 115 experimental runs were collected and the results were also compared with other literature experimental data.

Aim of this work was to study flow boiling in an annular channel at low pressure and lowflow on a tube with modified surface roughness. The tube with the outer diameter of 9.14 mm and theheated length of 380 mm was made of Inconel 625 and was manually modified using 150 grit sandpaper.The tube was placed in a glass tube with an inner diameter of 14.8 mm. Outlet pressure was set to 120,200 and 300 kPa with varying mass flow from 400 to 600 kg/(m2·s). A high speed camera was used torecord several experiments to fully understand ongoing phenomena. Surface roughness was analysedusing a confocal laser microscope and the effects of different mass flux and pressure on the CHF valuewere observed. Above all, the optimization of the flow parameters was done from the collected dataand from the observed behaviour of the experimental loop

Steady state flow boiling experiments were conducted on a technically smooth Inconel 625 tube with outer diameter 9.1 mm at inlet pressures 131, 220 and 323 kPa, inlet temperatures 62, 78 and 94 °C and approximately 400, 600 and 1000 kg/(m2.s) mass flow. Water of these parameters was entering into the vertically aligned annulus, where the uniformly heated tube was placed until the critical heat flux (CHF) appeared. The experimental data were compared to estimations of CHF by local PGT tube correlation and Groeneveld’s look-up tables for tubes. The results imply that in the region of low pressure and low mass flux, the differences between calculations and experiments are substantial (more than 50 % of CHF). The calculations further imply that look-up tables and tube correlations should be corrected to the annulus geometry. Here, the Doerffer’s approach was chosen and led to a substantial enhancement of CHF estimation. Yet, a new correlation for the region of low pressure and flow is needed.

Tato diplomová práce se věnuje pojmu krize varu a rezervy do krize varu. V práci je vysvětlen pojem krize varu v jaderných reaktorech a jsou popsány jednotlivé parametry aktivní zóny reaktoru, které mají vliv na hodnotu rezervy do krize varu v jaderném reaktoru. Práce se následně věnuje subkanálové analýze a popisuje matematické a fyzikální modely vybraného subkanálového programu. V práci jsou následně zpracovány pomocí subkanálového programu ALTHAMC12 vlivy vybraných parametrů na rezervu do krize varu. Závěr práce se věnuje vyhodnocení nejlepší a nejhorší vypočtené varianty., This master’s thesis deals with boiling crisis and with departure from nucleate boiling ratio. This thesis explains terms like the boiling crisis in nuclear reactors and the thesis deals with individual parameters of the reactor core, which have an impact on departure from nucleate boiling ratio. After that, the thesis deals with subchannel analysis and describes basic mathematical and physical models of the chosen subchannel program. The thesis then processes, with the ALTHAMC12 subchannel program, the chosen parameters and their impact on departure from nucleate boiling ratio. The conclusion of the diploma thesis deals with the evaluation of the best and worst calculated combination.

Tato diplomová práce se věnuje pojmu krize varu a rezervy do krize varu. V práci je vysvětlen pojem krize varu v jaderných reaktorech a jsou popsány jednotlivé parametry aktivní zóny reaktoru, které mají vliv na hodnotu rezervy do krize varu v jaderném reaktoru. Práce se následně věnuje subkanálové analýze a popisuje matematické a fyzikální modely vybraného subkanálového programu. V práci jsou následně zpracovány pomocí subkanálového programu ALTHAMC12 vlivy vybraných parametrů na rezervu do krize varu. Závěr práce se věnuje vyhodnocení nejlepší a nejhorší vypočtené varianty., This master’s thesis deals with boiling crisis and with departure from nucleate boiling ratio. This thesis explains terms like the boiling crisis in nuclear reactors and the thesis deals with individual parameters of the reactor core, which have an impact on departure from nucleate boiling ratio. After that, the thesis deals with subchannel analysis and describes basic mathematical and physical models of the chosen subchannel program. The thesis then processes, with the ALTHAMC12 subchannel program, the chosen parameters and their impact on departure from nucleate boiling ratio. The conclusion of the diploma thesis deals with the evaluation of the best and worst calculated combination.

Tato diplomová práce se věnuje pojmu krize varu a rezervy do krize varu. V práci je vysvětlen pojem krize varu v jaderných reaktorech a jsou popsány jednotlivé parametry aktivní zóny reaktoru, které mají vliv na hodnotu rezervy do krize varu v jaderném reaktoru. Práce se následně věnuje subkanálové analýze a popisuje matematické a fyzikální modely vybraného subkanálového programu. V práci jsou následně zpracovány pomocí subkanálového programu ALTHAMC12 vlivy vybraných parametrů na rezervu do krize varu. Závěr práce se věnuje vyhodnocení nejlepší a nejhorší vypočtené varianty., This master’s thesis deals with boiling crisis and with departure from nucleate boiling ratio. This thesis explains terms like the boiling crisis in nuclear reactors and the thesis deals with individual parameters of the reactor core, which have an impact on departure from nucleate boiling ratio. After that, the thesis deals with subchannel analysis and describes basic mathematical and physical models of the chosen subchannel program. The thesis then processes, with the ALTHAMC12 subchannel program, the chosen parameters and their impact on departure from nucleate boiling ratio. The conclusion of the diploma thesis deals with the evaluation of the best and worst calculated combination.

Tato diplomová práce se věnuje pojmu krize varu a rezervy do krize varu. V práci je vysvětlen pojem krize varu v jaderných reaktorech a jsou popsány jednotlivé parametry aktivní zóny reaktoru, které mají vliv na hodnotu rezervy do krize varu v jaderném reaktoru. Práce se následně věnuje subkanálové analýze a popisuje matematické a fyzikální modely vybraného subkanálového programu. V práci jsou následně zpracovány pomocí subkanálového programu ALTHAMC12 vlivy vybraných parametrů na rezervu do krize varu. Závěr práce se věnuje vyhodnocení nejlepší a nejhorší vypočtené varianty., This master’s thesis deals with boiling crisis and with departure from nucleate boiling ratio. This thesis explains terms like the boiling crisis in nuclear reactors and the thesis deals with individual parameters of the reactor core, which have an impact on departure from nucleate boiling ratio. After that, the thesis deals with subchannel analysis and describes basic mathematical and physical models of the chosen subchannel program. The thesis then processes, with the ALTHAMC12 subchannel program, the chosen parameters and their impact on departure from nucleate boiling ratio. The conclusion of the diploma thesis deals with the evaluation of the best and worst calculated combination.