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From a theoretical assessment of extensive critical heat flux (CHF) data under low pressure and low velocity (LPLV) conditions, it was found out that lots of CHF data would not be well predicted by a normal annular film dryout (AFD) mechanism, although their flow patterns were identified as annular–mist flow. To predict these CHF data, a liquid sublayer dryout (LSD) mechanism has been newly utilized in developing the mechanistic CHF model based on each identified CHF mechanism. This mechanism postulates that the CHF occurrence is caused by dryout of the thin liquid sublayer resulting from the annular film separation or breaking down due to nucleate boiling in annular film or hydrodynamic fluctuation. In principle, this mechanism well supports the experimental evidence of residual film flow rate at the CHF location, which can not be explained by the AFD mechanism. For a comparative assessment of each mechanism, the CHF model based on the LSD mechanism is developed together with that based on the AFD mechanism. The validation of these models is performed on the 1406 CHF data points ranging over P=0.1–2 MPa, G=4–499 kg m−2 s−1, L/D=4–402. This model validation shows that 1055 and 231 CHF data are predicted within ±30 error bound by the LSD mechanism and the AFD mechanism, respectively. However, some CHF data whose critical qualities are <0.4 or whose tube length-to-diameter ratios are <70 are considerably overestimated by the CHF model based on the LSD mechanism. These overestimations seem to be caused by an inadequate CHF mechanism classification and an insufficient consideration of the flow instability effect on CHF. Further studies for a new classification criterion screening the CHF data affected by flow instabilities as well as a new bubble detachment model for LPLV conditions, are needed to improve the model accuracy. 相似文献
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According to the flow passage characteristic of narrow rectangular channel and liquid film dry-out mechanics of annular flow critical heat flux (CHF), an annular flow CHF analytical model for narrow rectangular channel has been achieved. This model may be used to predict the CHF behavior of boiling two-phase flow in narrow rectangular channel with gap width of not being less than 0.0005 m (the equivalent diameter of this channel is 0.001 m). Through analyzing and calculating, when the inlet dimensionless gap width of narrow rectangular channel is within 30-85, the enhancement of CHF in channel is obvious. At the same time, according to the characteristic of two-phase flow, the new determinant laws of CHF in boiling two-phase flow system have been derived. Through analyzing and calculating, it is substantial that this determinant laws is appropriate. The best dimensionless gap width of heat flux enhancement has been achieved to be 45-75. 相似文献
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A previously developed semi-empirical model for adiabatic two-phase annular flow is extended to predict the critical heat flux (CHF) in a vertical pipe. The model exhibits a sharply declining curve of CHF versus steam quality (X) at low X, and is relatively independent of the heat flux distribution. In this region, vaporization of the liquid film controls. At high X, net deposition upon the liquid film becomes important and CHF versus X flattens considerably. In this zone, CHF is dependent upon the heat flux distribution. Model predictions are compared to test data and an empirical correlation. The agreement is generally good if one employs previously reported mass transfer coefficients. 相似文献
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Critical Heat Flux (CHF) is an important parameter for the thermal design of any heat generating system, most importantly, nuclear reactors. Owing to the complex mechanisms of CHF there has been a large proliferation of the correlations, each having narrow range of validity, which shows that the empirical correlation is not an appropriate approach for the CHF prediction for a wide range of validity. This limitation has led to the development of the phenomenological approach of the CHF prediction. The film dryout mechanism is applicable to the high quality CHF corresponding to the annular flow pattern in which the progressive depletion of the liquid film leads to dryout. The basic concern in the prediction of dryout is the accuracy in the evaluation of the droplet deposition and entrainment. There are various models for the estimation of the entrainment and deposition of droplets. However, most of these models are based on the air-water data at the atmospheric conditions and hence their applicability to the BWR conditions needs to be confirmed. Some of the models are based on the steam-water data which needs to be validated for the dryout prediction under BWR conditions. In this paper, the film dryout modelling has been carried out for the prediction of CHF using appropriate models for entrainment fraction and deposition coefficient. The results have been compared with the CHF data generated to substantiate the appropriateness of the selected models under BWR conditions. 相似文献
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The Film Dryout Analysis Code in Subchannels, FIDAS, has been developed with the main objective of predicting dryout and post-dryout heat transfer in a channel and in rod bundles. In FIDAS, two-phase flow consisting of continuous liquid film, continuous vapor and entrained droplets is modeled by a three-fluid, three-field representation of 12 field equations, i.e. three continuity, three energy and six momentum equations. FIDAS can predict dryout without any empirical CHF correlations by introducing annular flow modeling and the ‘film dryout criterion’. Experiments on film flow characteristics, subchannel flow and enthalpy distributions, dryout and post-dryout heat transfer in tubes and rod bundles were analyzed to demonstrate the performance of FIDAS. The predictions of FIDAS are in close agreement with the experiments. 相似文献
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液膜蒸干模型在液态金属CHF预测中的应用 总被引:1,自引:1,他引:0
为对圆管中环状流烧干型沸腾临界进行预测,建立了考虑液滴沉积夹带作用的液膜蒸干模型。沉积率、夹带率等相关关系式采用水等常规流体的已有关系式。在一定参数范围内,将模型预测结果与实验数据和经验关系式进行了比较。结果表明:基于常规流体的液膜蒸干模型大体可应用于液态金属,但在临界热流密度(CHF)较大时可能造成较大偏差;钠、钾两种液态金属在CHF较低时预测准确度区别不大,但在CHF较高时区别较为明显。为更加准确预测液态金属CHF,应开发专门的沉积率、夹带率等相关关系式。 相似文献
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The critical heat flux (CHF) is one of the important phenomena limiting the maximum rate of heat transfer and hence power rating of nuclear reactors. The thermal hydraulic phenomena like pressure drop, heat transfer, stability, etc. depends upon the flow pattern in the system. The CHF phenomenon is also closely related to the two-phase flow patterns. It is important to investigate the dependence of CHF on the flow pattern regimes to understand the underlying mechanisms. The present investigation reveals that CHF generally increases with mass flux in the churn/slug region. However, in the annular region the CHF decreases with increase in mass flux. Considering the dependency of the CHF trend on the flow pattern regime, it will be useful to develop CHF models, which are specific to the flow pattern regime. The data of CHF look-up table has been considered in this investigation since this approach is one of the most reliable methods for the prediction of CHF and is being used in several best-estimate thermal-hydraulic system codes, such as RELAP5, CATHARE and CATHENA. The pressure, mass flux and quality have been considered as important thermal hydraulic parameters to characterize the flow pattern during CHF under various operating condition. 相似文献
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A simple one-dimensional three-fluid model is presented for the simulation and analyses of vertical annular and stratified horizontal or inclined two-phase flows. The model has been verified for various experimental data: developing annular flow, momentum transfer in an annular flow, plane flow with a hydraulic jump, flooding in a horizontal pipe, and stratified flow with direct steam condensation. Emphasis has been laid upon several mass, momentum and energy interfacial transfer processes. New correlations are proposed for the droplet entrainment intensity in annular flow and for steam direct contact condensation on the liquid film in a stratified flow. The liquid entrainment in the annular flow is correlated with the liquid film thickness. Direct contact condensation is correlated with the turbulent convective heat transfer in the liquid film. It has been shown that the present model is able to predict all dominant processes in both types of flow. 相似文献
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The influence of periodically varying acceleration on critical heat flux (CHF) of Freon-113 flowing upward in a uniformly heated vertical annular channel has been studied experimentally. The freon loop was oscillated vertically to determine the ratio of CHF in the oscillating acceleration field to the corresponding stationary value. The amplitude of inlet flow oscillation induced by variation of acceleration, which causes early CHF, is proportional to the acceleration amplitude. The dependence of inlet flow rate on the oscillating acceleration decreases with increasing inlet subcooling, and no oscillation of inlet flow is observed in the case of negative exit quality (subcooled boiling). Nevertheless the degradation of CHF is more remarkable in the low quality region. This result suggests the necessity to introduce an other mechanism of early CHF than flow oscillation. 相似文献
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An experimental study on critical heat flux (CHF) has been performed for water flow in vertical round tubes under low pressure and low flow (LPLF) conditions to provide a systematic data base and to investigate parametric trends. Totally 513 experimental data have been obtained with Inconel-625 tube test sections in the following conditions: diameter of 6, 8, 10 and 12 mm; heated length of 0.31.77 m; pressure of 106951 kPa; mass flux of 20277 kg m−2 s−1; and inlet subcooling of 50654 kJ kg−1, thermodynamic equilibrium critical quality of 0.3231.251 and CHF of 1081598 kW m−2. Flow regime analysis based on Mishima & Ishii’s flow regime map indicates that most of the CHF occurred due to liquid film dryout in annular-mist and annular flow regimes. Parametric trends are examined from two different points of view: fixed inlet conditions and fixed exit conditions. The parametric trends are generally consistent with previous understandings except for the complex effects of system pressure and tube diameter. Finally, several prediction models are assessed with the measured data; the typical mechanistic liquid film dryout model and empirical correlations of (Shah, M.M., 1987. Heat Fluid Flow 8 (4), 326–335; Baek, W.P., Kim, H.G., Chang, S.H., 1997. KAIST critical heat flux correlation for water flow in vertical round tubes, NUTHOS-5, Paper No. AA5) show good predictions. The measured CHF data are listed in Appendix B for future reference. 相似文献
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Jianquan Liu Jingda Shi Baomin Sun Huimin Ji 《Journal of Nuclear Science and Technology》2013,50(12):1412-1424
The present paper aims to investigate the critical heat flux (CHF) characteristics of AP1000 reactor based on the experimental and numerical researches, under normal operation and loop fault conditions, respectively. The differences of flow characteristics in these conditions were analyzed. It indicated that the flow features are very complicated in three dimensions and AP1000 has better self-regulation capability to distribute coolant flow compared to conventional reactors. Under normal operation condition, coolant of two loops is distributed along circumference of the reactor annular channel symmetrically. In case that one of the loops fails suddenly and the coolant is partially lost to total loss, the core flow distribution plate and lower grid plate cannot eliminate uneven flow immediately due to loop failure, also the nonuniformity of reactor coolant flow distribution increases gradually, which leads to the heat transfer deterioration easily. In addition, the reactor core departure from nuclear boiling ratio (DNBR) and CHF does not show a certain linear relation, and the DNBR and CHF of AP1000 are greater than that of conventional reactors which not only improve the reactor thermal efficiency, but also obviously reduce the probability of CHF phenomenon appear. 相似文献
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A small-scale experiment using Freon-11 at 54°C and 450 kPa in a transparent annular test section was used to study the occurrence of critical heat flux (CHF) during blowdown with flow reversal. The transients were initiated by simultaneous operation of three air-actuated values. The inner tube of the annulus was uniformly heated over its 0.61-m length while the outer transparent pyrex wall was unheated. Test section instrumentation included seven pressure taps, inlet and outlet capacitance void probes, inlet and outlet turbine flowmeters, inlet and outlet fluid thermocouples, and 22 wall thermocouples. High-speed motion pictures were taken of the lower end of the test section where the intial CHF occurred. From these high-speed pictures, the flow reversal was observed to occur between 60 and 80 ms followed by a rapid thermal excursion at about 400 ms in the lower regions of the test section. Consequently, this measured CHF occurred well after the flow had reversed and re-established itself in the downward direction. At about 300 ms an annuflar flow pattern appeared and was well-developed at 400 ms. Therefore, the early CHF measured were all associated with the transition from bubbly to annular flows. In some cases this early CHF was rewet and the heated section remained in a stable coolable state for a considerable length of time, and experienced a later CHF when the liquid was nearly depleted. This long-term dryout was a function of the liquid volume contained in the system, whereas the early CHF was independent of the system volume. In addition, the early CHF did not show any significant sign of propagation whereas the latter one was observed to propagate smoothly upward. 相似文献
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《Journal of Nuclear Science and Technology》2013,50(11):1013-1022
Following Part (I) of the present paper, which presented the experimental results obtained on the void distribution and average void fraction shown by nearly fully-developed, vertically downward two-phase flow of air-water mixture, this Part (?) covers the flow regime transition criteria among the three basic flow regimes : bubbly, slug and annular flows. The annular flow further was divided into two subregions of falling film flow and annular drop flow. The general situation of the transition criteria is as follows : (1) bubbly-to-slug flow transition occurs when the local void fraction in the central region of the tube is 0.3; (2) slug-to-annular drop flow transition criterion is given as a case which equations giving average void fraction for the slug flow and the annular flow are simultaneously satisfied; (3) slug-to-falling film flow transition occurs when the pressure difference between the crest of large wave and the bottom overcomes the surface tension; (4) the occurrence of liquid droplets from wave crests gives the transition criterion between the falling film flow and the annular drop flow. These criteria were correlated to predict each flow regime boundary respectively considering flow mechanisms or from experimental results. The correlations obtained were compared with published flow regime maps for atmospheric air-water flow and showed satisfactory agreement. 相似文献
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用实验方法对浸没在饱和液体中的倾斜窄长加热套管内的自然对流沸腾临界热流密度进行了实验研究,考察了套管间隙、管长、倾斜角和工质对临界热流密度的影响,并考虑了倾斜角对重力的影响,对用于预测垂直套管内自然对流沸腾临界热流密度的半理论半经验公式进行了修正。修正后的公式能较好地预测本实验和他人实验的结果。 相似文献