Applicability Of Void Fraction Models In Rod Bundles At High Quality

Void fraction is one of the key parameters in two-phase flow and heat transfer. The void fraction distribution is important to predict the thermal-hydraulic behavior of nuclear reactors. As a consequence, the prediction model of void fraction plays an important role in sub-channel code. However, the study on void fraction models and experiment are only focus on the conditions under low quality. The applicability of void fraction model in sub-channel code under high quality conditions is not sufficiently researched and validated. High quality that occurs during reactor starts up, shuts up and severe accident can easily results in coolant flow fluctuation and core power oscillation, which bring big challenges to the reactor safety. Therefore it is highly necessary to investigate the applicability of void fraction model under high quality condition.In this thesis, the investigation of high void fraction model and experiment is performed using the sub-channel code COBRA. Also the void fraction models in sub-channel code COBRA and four void fraction models selected from open literatures are compared with the experiment data. Firstly, this paper evaluates the effect of various parameters on void fraction. Secondly comparison of void fraction distribution calculated by selected void models is conducted to show the difference among correlations. Finally, the experiment data of Herkenrath 4×4 and BFBT Benchmark 8×8 rod bundle is applied to estimate the accuracy of void fraction models. Comparing to the experimental data obtained in a 4×4 rod bundle using water as coolant , the selected void fraction models are assessed by using relative enthalpy rise and relative mass flux in various sub-channels . It was found that the calculated sub-channel parameters with the Dix model and the Slip model (with a slip ratio of 1.5 at Pressure of 7.0MPa) give the best agreement with the experimental data. Comparing to the experimental data of the 8×8 rod bundle, the selected void fraction models are assessed by void distribution in various sub-channels. It was found that the Dix model give the best agreement with the experimental data which is agree on with the result above. At the same time, the application of the select void model in different conditions e.g. different pressure, different flow, different inlet sub-cooling, are illustrated in this thesis. Based on the results achieved above, this paper presents the applicable range of the void models. The sensitivity analysis of models in the sub-channel code such as turbulent mixing and heat transfer models are also carried out in this paper.