| A systematic study on the CHF in complex geometry has been carried out using a dumbbell shaped tube. The dumbbell shaped flow channel consists of two subchannels interconnected via a narrow gap, which simulates the two adjacent subchannels a in nuclear fuel bundle. The study was divided into two parts: mechanistic modeling and experimental investigation.; Over 620 CHF data were obtained in a dumbbell-shaped tube with R-134a as working fluid. A wide range of flow conditions was covered. The gap size of dumbbell shaped tube was varied from 0, 0.15, 0.3, 0.5, 0.7, to 1 mm. This experimental investigation is one of the most systematic researches on the gap effect. It is found that the CHF trends against thermalhydraulic parameters in the dumbbell shaped tube are similar to those in circular tubes. No severe adverse effect was detected in the dumbbell-shaped tube under the test range of this study. The CHF decreases with decreasing gap size. On the average, as gap size decreases from 1 mm to zero CHF drops about 17.2% at a fixed inlet condition and about 11.5% at a fixed local condition.; An analytical model for complex geometry, the partition model, was developed based on the annular flow assumption. The partition model divides a complex geometry cross section into two logical channels. Instead of making a complex two-dimensional description, the partition model calculates the complex geometry CHF by analyzing flow and heat transfer in two interactive one-dimensional flow channels. A new approach for the geometry effect on deposition was developed. A software package CHFA++ was developed based on the partition model to predict the CHF in complex geometries. The partition model predicts the CHF in dumbbell-shaped tubes and the gap effect with good accuracy.; Several recommendations were made for further research on complex geometry CHF, including the optimization of the partition model, extending the application of the partition model to other complex geometries, and conducting experiments on the liquid film distribution. |
