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[en] Under particular conditions, boiling may lead to sudden and catastrophic phenomena called critical heat flux (CHF). Therefore, better CHF predictions by system codes such as RELAP5 are seen to be important for system safety and operations. Hence, a set of 1145 water CHF data points for vertical up flow covering a wide range of different parameters have been used to assess the CHF prediction methods in RELAP5. The RELAP5 code uses two methods: the 1986 AECL-UO critical heat flux lookup table method and the PG-CHF correlations. In addition, results obtained from the recent lookup tables of 1995 and 2006 are included in the plots and used to support the conclusions and to perform a comparative study of the methods used by the code. Compared to the look-up tables and correlations, the present study showed a good agreement with the experimental data for the basic form of the PG-CHF correlations illustrated by a good statistics (99.4% of predictions are within ±20% and with a root mean square error of 6.44%).
[en] With ever increasing power dissipation in electronic chips that are shrinking in size, cooling demands are becoming more severe. Forced air cooling is reaching its operational limits, and single-phase liquid cooling in microchannels has been able to accommodate the rising heat fluxes. Further increases in computing (chip) power suggest that a switch from single-phase to boiling heat transfer will be needed. A major impediment to using boiling or forced convective vaporization for such a cooling application is the limiting critical heat flux (CHF) condition. In this paper, the CHF condition in microchannels is reviewed. Data from the literature are discussed, and new data for a range of operating and geometric conditions are presented. Influencing factors, parametric trends, phenomenological models, and other aspects of the CHF condition are discussed
[en] Highlights: • A theoretical CHF model based on the liquid microlayer dryout mechanism has been proposed for the saturated flow boiling on the downward curved heated surface. • The effects of the mass velocity and orientation angle of the heated wall are considered for the determination of CHF. • This CHF model could be used to assess the CHF limits on the outer surface of the lower head under in-vessel retention. - Abstract: The critical heat flux (CHF) distribution on the outer surface of the lower head is a crucial parameter to assess the coolability limits of the in-vessel retention strategy through external reactor vessel cooling. Several CHF correlations concerning the orientation angle of heated wall have been proposed while the theoretical analysis is relatively insufficient. Based on the liquid microlayer dryout mechanism, a theoretical CHF model for the saturated flow boiling on the downward curved heated surface has been proposed in this work. With a thorough analysis of the vapor blanket behavior in the near-wall region, the effects of the mass velocity and orientation angle of the heated wall on the CHF are considered in present model. The well agreement between the predicted CHF and the experimental data suggests the validity and applicability of present CHF model to assess the CHF limits on the outer surface of the lower head under in-vessel retention.
[en] Surveys have been published of data obtained in experiments with outgassed water and with water in which the gas concentration either was not measured at all or else was measured after outgassing with an inadequate accuracy. It is desirable to estimate the size of the difference between the critical heat loadings calculated and the values found by experiment with nearly zero dissolved gas. For this purpose a system is described having a gas and steam volume compensation system for the coolant, as well as a system for simulating and monitoring the dissolved gas. The error in determining the dissolved gas concentration was not more than /plus or minus/5%. 3 refs
[en] This paper addresses the topic of phenomenological analysis of film dryout in the context of high quality critical heat flux in a nuclear reactor. It seeks to record the major contribution made to the subject by Geoff Hewitt over many decades, to report some recent advances, and to identify areas where improvement is still needed. It closes with a discussion of what future advances are to be expected.
[en] Highlights: • The safety boundary of flow instability and critical heat flux for parallel channels was theoretically investigated. • A unified form of additional forces was derived. • The mechanism of the parallel channels was proposed. • The effects of motion conditions on the safety boundary was analyzed. - Abstract: The safety boundary of flow instability and critical heat flux(CHF) for parallel channels was theoretically investigated in static and motion. For flow instability, a parallel-channel instability mechanistic model was adopted. For CHF, the liquid sublayer dryout mechanism model was used. Also, a unified form of additional forces caused by motion was derived for both flow instability and CHF model. An in-house code was developed combining the flow instability model and the CHF model in motion. The safety boundary of twin rectangular parallel channels with length of 40 mm, width of 2 mm and height of 1 m was calculated in static, inclination, heaving, pitching and rolling motions. The results show that the safety boundary consists of CHF lines and instability boundary. The heating power of the parallel channels is limited by flow instability or CHF depending on the mass flux. All the motions have little influence on the instability boundary. The effects of longitudinal inclination, heaving and pitching motions on the safety boundary are no more than 1%. However, the CHF is obviously reduced in transverse inclination due to flow maldistribution and in rolling motion due to pulsating flow.
[en] The present study presents the details of boiling structures at high heat flux condition and reveals the triggering mechanism of the critical heat flux in a horizontal pool boiling, based on the in-depth observations obtained by applying the visualization techniques of a total reflection, a side view, and a diagonal view in a synchronized manner
[en] A increase of CHF was observed with nano-fluid. The addition of nano-particle helped to increase the wettability. This happens with the decrease in bubble diameter, breakup of bubbles and avoidance of bubble coalescence. CHF increase or decrease depends upon competition between high wettability and high instability. An optimum nano-fluid concentration is needed which must have high crystalline content. When the concentration reaches at a critical value, CHF will tend to a constant value. Deposition of nano-particles increasing the wettability and the rewetting are cause of CHF enhancement. It delay the growth of dry patch by increasing of wettability and lead to CHF enhancement. Now, we must define the wettability of nano-fluids. At case of nano-fluids using metallic particle, the explanation using contact angle using was reasonable. But, at case of nan-fluids using hydrophobic CNT, this explanation can't be acceptable. Moreover, at case of surfactant solution, contact angle was very low. But CHF enhancement was not great. So, wettability about nano-fluids must be defined anew for explanation of CHF enhancement. I suggest the extension of micro layer are acceptable concept for increasing wettability using nano-fluids
[en] You et al. showed that nanofluids containing only 0.005 g/L of Al2O3 nanoparticles cause a dramatic increase (on the order of 200%) in the critical heat flux (CFH) during pool boiling. There has been much research effort directed at CHF enhancement using nanofluids. The mechanism of CHF enhancement in nanofluid pool boiling is the result of nanoparticle deposition on the heating surface, which changes the surface characteristics. This study focuses on the possible CHF enhancement caused by nanofluids during forced convective flow boiling because of the importance of flow boiling conditions in various practical heat transfer applications. The nanofluid used in this study was a very low concentration of Al2O3 (0.01 % vol) dispersed ultrasonically in water