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The effects of interficial shear between liquid film and flowing-steam cannot be neglected during the steam condensation process in vertical tubes, and the Nusselt condensation model should be modified. In this paper, the heat transfer characteristics of steam condensation in vertical tube were investigated by experiments. Both the Nusselt theoretical model and the modified model considering interficial shear effects were adopted and compared with the experimental measurements. The results show that interfacial shear stress have significant influence on the liquid film flow state and thin liquid film thickness, which further results in higher condensation heat transfer coefficient. As the modified model considers interficial shear correctly, and its calculation results agree well with the experimental results.

In the pipeline system of nuclear industry, shock wave pressure in a pipe will be caused by the fast closing check valve after the pumping stops. This phenomenon is known as water hammer, which brings hidden danger to the security and reliability of the pipeline system. Specially, water hammer may cause serious damage on the pipeline system by the valve misoperation, by the valve malfunction, or by other unexpected events. A vortex diode is used as a highly reliable check-valve in nuclear applications, where it mainly benefits from the intrinsic properties of no moving parts and no leakage. In this paper, we proposed a novel method based on a vortex diode to protect water hammer. In the traditional analysis, a simple one-dimensional (1D) model is often used to simulate the water hammer. However, it is difficult to get the transient flow characteristics in a vortex diode using a 1D model. Thus, a three-dimensional (3D) model using computational fluid dynamics (CFD) is proposed to analyze water hammer in a pipeline system with a vortex diode. The 3D model was firstly verified by comparing the numerical results of CFD with experimental results of a water hammer test. Based on the 3D model, the water hammer was simulated at different inlet conditions in a pipeline system with a vertex diode. In order to investigate the vortex diode used as a leaky check-valve, the inlet pressure was decreased by the corresponding value of pump head to simulate the pump stop after the quasi-steady state was achieved in the vortex diode. It is found that the pressure fluctuation of water hammer is comparable to the pump value, which is not varying with initial velocity in the pipeline system. Thus, we have proved that a vortex diode in the pipeline system acts significantly in suppressing pressure fluctuation of water hammer. This study presents a CFD-based numerical method for water hammer and could be useful in protecting water hammer in nuclear industry.

Jet pumps have the advantages in reliable operation, none of moving components and simple construction etc, which makes it applicable in a wide range of industry situations. The cavitation easily occurs within jet pumps and it turns to be a bottleneck for the jet pump design. In this paper, the cavitation flow inside the jet pump would be investigated by 2-D axisymmetric CFD simulations. Firstly, the single liquid phase flow in the jet pump would be simulated and the grid independence test would be carried out. The simulation results show that both the pressure ratio (h) and the efficiency (n) would reach to convergence values with 143,484 meshes, which would be adopted in the following simulations. By comparing with the experimental data of single phase flow in literatures, it was found that the Reynolds stress model offered better predictions. Then the cavitation flow in the jet pump was simulated. It could be found that the cavitation phenomenon trends to occur in the shear layer and the wall detached regions of the throat. At last, the influence of saturated vapor pressure model, the outlet pressure and the nozzle-to-throat clearance would be evaluated. It was found that different from the single phase flow, the optimal nozzle-to-throat clearance would vary with the operation conditions.

A vortex diode is used as a highly reliable check-valve in nuclear applications, where it mainly benefits from the intrinsic properties of no moving parts and no leakage. Its basic principle is similar to the diode in an electric circuit. The typical structure of a vortex diode consists of a chamber with axial and tangential ports. When the fluid is injected through the axial port, a simple radial flow in the chamber leads to a relatively low flow resistance. On the other hand, in the reverse flow mode, a strongly swirling vortex can be set up in the chamber, resulting in a very high flow resistance. Several experimental studies found vortex-induced vibration of a vortex diode in the reverse flow mode, where it indicated that the flow was unstable in the vortex diode. This phenomenon may affect the reliability of the vortex diode. However, the mechanism has not been investigated systematically and profoundly. In this paper, 3-D simulations are carried out to help understand the related flow characteristics in the vortex diode. Standard k-ε model was selected for forward flow, while Reynolds stress model was selected for reverse flow. We have found that the results from transient simulations are in good agreement with experimental data. The transient simulations also capture the periodic pressure fluctuation in the vortex diode. Vortex diodes with different structures and geometrical parameters are simulated at different Reynolds number conditions. It is found that the characteristics of the pressure fluctuation are determined by the structure parameters and working conditions of the vortex diode. The flow instability is mainly caused by the asymmetry of the vortex diode. The work presented in this paper will be useful to give better understanding of flows in vortex diodes and to provide some guidance for optimizing the vortex diode.

In most of the previous studies, the vapor bubble parameters were averaging processed in order to establish the empirical-based correlations or mechanism-based models. While it should be noted that the thermal hydraulic parameters around a bubble in a subcooled flow boiling channel are unsteady, that means the vapor bubbles behaves some stochastic characteristics. In the present research, a high speed visualization experiment was conducted in a subcooled flow boiling narrow rectangular channel to investigate the vapor bubble behavior. The working fluid in the experiment was deionized water. The obtained bubble images were processed by a digital image processing program in order to acquire the bubble parameters. The stochastic features of the bubbles were analyzed based on the experimental results. Two types of bubble behaviors were observed under different working conditions, which results in two types of bubble stochastic features. The results shown that the distribution function of the bubble maximum diameter in a specific nucleation site can be expressed by the normal distribution, while in the whole range of the observation window the distribution of the bubble maximum diameter was expressed by the lognormal distribution. The distribution of the second type bubble diameter depends on the local bubble nucleation site and the upstream bubble behaviors.