Results 1 - 10 of 12242
Results 1 - 10 of 12242. Search took: 0.044 seconds
|Sort by: date | relevance|
[en] Recent works published in the last decade on pool boiling heat transfer are reviewed with special reference to the heat transfer mechanisms and characteristics in various boiling modes including nucleate boiling, crisis of nucleate boiling, transition boiling and crisis of film boiling, from which the present state of the art and the future needed work will be elucidated. (orig.)
[en] Post-critical heat flux (CHF) heat transfer is of interest to nuclear engineers because of the need to compute the consequences which immediately follow power in excess of the CHF and the need to be able to determine when adequate cooling has been re-established once CHF has been exceeded. The various regimes of post-CHF heat transfer pertaining to pool boiling and flow boiling are delineated. The correlations appropriate to each regime are indicated and an attempt is made to assess their utility. (author)
[en] The effects of the application of an electric field on pool boiling in dielectric fluids were studied in this work.Two different geometries were used: one which is favorable to the bubble detachment (favorable electric field) and other which attract the bubbles toward the heater (adverse electric field).In the favorable electric field experiments, the void fraction and impact rate were calculated from the measured indicator function.Those parameters were obtained varying the probe-heater distance and the power to the heater.The results show a reduction of the void fraction with increasing applied voltage, probably caused by the combination of the dielectrophoretic force and a smaller bubble size due to the electric field application. Also, the impact rate decreases when a voltage is applied and the heat fluxes are close to the critical heat flux (CHF).On the other hand, the impact rate increases with voltage for moderate heat fluxes.Another interesting result is the approximately exponential decay of the void fraction and impact rate with the distance to the heater. Both the void fraction and the impact rate grow with heat flux if the heat fluxes are moderate, with or without applied voltage.For highest heat fluxes the void fraction still grows with heat flux if there are no applied electric fields while decreases with heat flux when there is an applied voltage. Similar behavior is observed in the impact rate.The boiling regimes was measured with adverse electric fields using two techniques.The heat transfer in the nucleate boiling regime was measured on an electrically powered heater.The results in these experiments show a reduction in the CHF of 10 % for saturation conditions and 10 kV of applied voltage, and a reduction of up to 40 % for 20 oC of liquid subcooling.The boiling curve corresponding to the transition and film boiling was performed with quenching experiments.An increase in the heat flux was achieved when an electric field was applied in spite of the geometry of the electric field which was adverse to the bubble detachment
[en] An analytical model for transient pool boiling heat transfer was developed in this study. The boiling curves of transient boiling were obtained based on the microlayer/macrolayer models proposed by us and the mechanism of transition from nonboiling regime to film boiling, i.e., direct transition was theoretically examined. Since the nucleate boiling heat flux is mainly due to the evaporation of the microlayer and the initial thickness of the microlayer decreases rapidly with increasing superheat, the duration of nucleate boiling is markedly decreased as the incipient boiling superheat is increased. It is found that the direct transition is closely connected to the rapid dryout of the microlayer which occupies almost the whole surface at high wall superheat. (author)
[en] Highlights: • Several observations supporting the Dry Spot/Dry Patch models were introduced. • Based on the observation the physical basis for the Dry Spot model was described. • The Dry Spot and Dry Patch models well predict CHF for pool and convective boiling. • Using the concept of the suppression of NSD, the whole boiling curve was predicted. - Abstract: Recently advanced visualization techniques such as total reflection and IR methods have been applied to observe thermally and hydraulically the CHF mechanism at the surface as well as macroscopic bubble dynamics simultaneously. Based on observation Ha and No (1998a,b, 2000), and Choi et al. (2016) developed a Dry spot model and a Dry patch model, which is an extension version of the dry spot model. Experimental observations clearly showed that the production of unquenchable dry patches mainly contributes to the initiation of CHF. Based on the above observatory conclusions and extensive literature survey, we discussed the physical basis of the Dry Spot/Dry Patch model. In the dry spot model, we assume that the dry spot can become unquenchable one when it is surrounded by 5 neighboring dry spots based on the geometrical consideration, which was confirmed by validation process. In the dry patch model, both criteria from the hydraulic and thermal considerations were proposed to estimate the critical size of unquenchable dry patch at CHF. The wall dry area fraction can be calculated by applying a probabilistic concept for the creation of the unquenchable dry patch. We showed that the Dry Spot/Dry Patch model can be extended into CHF predictions in both pool boiling and forced convective boiling. For transition boiling, we proposed models to represent two suppression mechanisms deactivating potential nucleation sites: nucleation site deactivation and non-availability mechanism. For the physical model of the nucleation site deactivation mechanism we introduced the spatial randomness concept. For the non-availability mechanism we proposed the multi-stage calculation method which considers the sequential bubble activation and their interaction. Then, we showed that the dry spot model modified with the current transition model well predicted the whole boiling curve including CHF, nucleate boiling, transition boiling, and film boiling.
[en] A mechanistic model for forced convective transition boiling has been developed to predict transition boiling heat flux realistically. This model is based on a postulated multi-stage boiling process occurring during the passage time of an elongated vapor blanket specified at a critical heat flux condition. Between the departure from nucleate boiling (DNB) and the departure from film boiling (DFB) points, the boiling heat transfer is established through three boiling stages, namely, the macrolayer evaporation and dryout governed by nucleate boiling in a thin liquid film and the unstable film boiling. The total heat transfer rate during the transition boiling is the sum of the heat transfer rates after the DNB weighted by the time fractions of each stage, which are defined as the ratio of each stage duration to the vapor blanket passage time. The model predictions are compared with some available experimental transition boiling data. From these comparisons, it can be seen that the transition boiling heat fluxes including the maximum heat flux and the minimum film boiling heat flux are well predicted at low qualities/high pressures near 10 bar. 8 figs., 1 tab., 32 refs. (Author)
[en] Highlights: ► Molecular geometric factors were found to be important determinants for boiling entropy and thus the boiling temperature. ► Only four molecular geometric factors were used in the study. ► A group contribution method was used to calculate enthalpy of boiling. ► The proposed method is simple and the estimations are in good agreement with experimental values. - Abstract: Boiling related thermodynamic properties are important parameters in research. In this study, a model integrating both additive groups and non-additive molecular geometric factors has been developed for the calculation of boiling enthalpy, entropy and temperature. The calculated values are in good agreement with the measured values of 470 compounds. This model provides a simple and accurate estimation of enthalpy of boiling, entropy of boiling and boiling temperatures with absolute average errors of 0.62 kJ/mol, 1.15 J/K · mol and 7.13 K respectively.