Graduate Thesis Or Dissertation
 

An Investigation on Transient Critical Heat Flux and the Prompting Conditions

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/sj1399676

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  • The collective understanding of the critical heat flux phenomenon is crucial to the safety and operational power limits of every nuclear and petrochemical power plant in the world. To define the maximum heat transfer capability in the postulated reactivity-initiated accident in a nuclear reactor, the mechanisms that trigger the departure from nucleate boiling during fast-power transients must be better understood. This research investigates the CHF phenomenon under rapid power excursions (1.5-5,000ms), subcoolings of 15 and 25K, and coolant flow-rates corresponding to Reynolds number up to 5700. This was achieved by using a high-speed camera to record images of a 20x5mm Nickel-201 heater foil in a de-ionized water flow-loop via joule heating . In the observed flow regimes up to Re=5,700, the CHF value is observed to increase towards 10 MW/m2 under power ramps period smaller than ~100ms for both subcoolings. The CHF value is observed up to 1MW/m2 higher for the 25K series than 15K, due to less of a thermal barrier during heating. When comparing the pool-boiling (Re=0) conditions to the Re=5,700 tests, the increased flow will shift the large tau CHF values up to 2MW/m2 higher in slow transients larger than 500ms. In the intermediate flow conditions of Re=2,900 and Re=4,100 there is great repeatability of the CHF value, especially in fast transient cases of tau smaller than 150ms. After analyzing all the burn-out videos, it has been concluded that the period and flow-rate both heavily influence the footprint of the film as well as the location of its generation. For CHF tests ramping power faster than a tau of 40ms, it may be expected for film formation to be almost as large as the center rectangle of the heater. In very slow transient experiments greater than a tau of 100ms, the localized film takes the geometrical shape of a rectangle spanning the 5mm width of the ribbon and a few millimeters length. This thesis gives detail to 41 unique CHF occurrences tracking surface temperature, heat flux, and ultimately the heat transfer coefficient (h) during each boiling crisis. This coefficient was tracked oscillating values between 1,000 and 10,000W/m٠K for all tests, where the onset of CHF would usually occur in the final approach to 10,000.
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