Condensation Induced Water Hammer Research Articles

Performances of the ITER Pressure Suppression System during unstable steam condensation regimes

Abstract The paper deals with the qualification of the main safety system of the nuclear fusion reactor ITER: the Vacuum Vessel Pressure Suppression System (VVPSS). This safety system manages the Loss of Coolant Accident (LOCA), that could occur in the Vacuum Vessel (VV), avoiding a significant increase of the internal pressure that could breach the primary confinement barrier. Steam condensation occurs at sub-atmospheric pressure. This condition is original respect the usual applications of steam direct condensation in nuclear fission reactors. An extensive experimental research program, funded by ITER Organization, on reduced scale experimental rigs (scale 1/22 and 1/10) have been carried out at the University of Pisa. Unstable condensation regimes occurred during some tests at about 500 g/s of steam mass flow rate (10 % of the maximum steam mass flow rate foreseen for the relevant accidental scenarios). In these conditions, high intensity vibrations of the whole experimental rig occurred. These events can be classified as ‘Chugging’ or Condensation Induced Water Hammer (CIWH). 
The performed tests have been analysed elaborating the images of the video-cameras located in the pool and comparing the steam jet shape with the acceleration values recorded by an accelerometer mounted on the sparger structure.
These elaborations permitted to evaluate the dynamic of bubble collapse, the volume of the developed external bubbles, the collapse frequency as function of thermal-hydraulic parameters. 
Moreover, the paper emphasizes the beneficial effects of non-condensable gases on the unstable condensation regimes. In fact, the non-condensable gas inhibits the occurrences of Water Hammer inside the sparger and eliminates the thermal stratification inside the water pool increasing the turbulence.

Experimental investigation and modeling of onset of condensation induced water hammer in equal-height-difference natural circulation system

Condensation induced water hammer (CIWH) in equal-height-difference natural circulation systems (EHDNCS) can lead to significant damage to associated equipment. To investigate the mechanism and the prediction of the onset of CIWH in EHDNCS, a small-scale experiment of EHDNCS was conducted under different water temperatures, heat transfer powers, and resistance coefficients of the water-side inlet. This study analyzed the thermal hydraulics, flow regimes, and the onset mechanism of CIWH through temperature and pressure measurement, U-Net image segmentation, and analytical modeling. Four distinct flow regimes were observed in the horizontal pipe: single-phase flow, non-periodic bubble, periodic bubble, and CIWH. Flow regime maps based on water temperature and heat transfer power were obtained under different inlet resistance coefficients. When the steam bubble length is smaller than the pipe length, no CIWH occurs. When the steam bubble length reaches the pipe outlet to form stratified flow, CIWH occurs at a high steam channel height. The occurrence of stratified flow and reverse flow of subcooled water in the steam channel causes the onset of CIWH in EHDNCS. A dimensionless criterion has been developed to predict the onset of CIWH when the saturated steam-water mixture is discharged through a horizontal pipe in EHDNCS. This criterion effectively predicted the transition from periodic bubble to CIWH in the experiment.

Scaling analysis for the condensation induced water hammer effect in horizontal pipe

In nuclear power engineering, researchers pay significant attention to phenomena related to the condensation process involving direct contact between saturated steam and subcooled water, mainly the phenomenon known as Condensation Induced Water Hammer (CIWH). Within the steam pipes of the pressurized water reactor, when saturated steam comes into direct contact with subcooled water, it may form an isolated steam pocket surrounded by water. The condensation of steam within this pocket results in its subsequent collapse, accelerating the surrounding water column to collide with the pipe wall and causing a substantial pressure peak. In this paper, the scaling analysis was conducted on the CIWH phenomenon in the steam line of a small PWR. The research is rooted in a two-fluid, six-equation framework and employs equation-based analysis to investigate the non-dimensional number of CIWH occurring within horizontal pipelines. By analyzing the conservation equations, suitable non-dimensional criteria for thermohydraulic phenomena were determined. These criteria can be employed to ascertain experimental facilities' geometric dimensions and operating conditions, which can study the risks associated with CIWH phenomenon.

Passive mitigation of condensation-induced water hammer by converging-diverging structures for offshore nuclear power plants

The equal-height-difference passive heat removal system (EHDPHRS) utilizes the ocean environment as an infinite heat sink that can better suit ocean nuclear power plants (ONPPs). However, a structure with an equal-height-difference can easily cause reverse flow of subcooled water, leading to serious condensation-induced water hammer (CIWH). In this paper, a fully visual experimental facility with same geometric characteristics as the EHDPHRS was established with the aim of studying the CIWH phenomenon. First, two kinds of CIWH modes were observed by the results of the visual experiment, namely the intermittent CIWH and the continuous CIWH. The typical feature of the intermittent CIWH is that the single-phase natural circulation occurs intermittently. Then, the converging-diverging structure (CDS) was designed to avoid the CIWH events in the EHDPHRS. The effectiveness of CIWH prevention was evaluated employing the velocity field, which was obtained by the PIV experimental system. It was found that the CDS can effectively inhibit the further reverse flow of subcooled water and thus eliminate the triggering mechanism of CIWH events. Finally, a dimensionless number was used to balance CIWH prevention and natural circulation capacity. It was shown that when the dimensionless number is in the range of 0.67–0.80, the CDS can eliminate the CIWH events while ensuring the natural circulation capacity.

Mechanisms and inhibition method investigation on condensation induced water hammer in an equal-height-difference passive heat removal system

The equal-height-difference passive heat removal system utilizes the ocean as an ultimate heat sink for residual heat removal, demonstrating obvious advantages for ocean nuclear power plant applications. However, the structure with a zero-height difference can trigger the reverse flow of subcooled water. This could cause serious condensation induced water hammer events. In this study, a fully visual experimental setup was established to investigate the occurrence features. First, two kinds of formation mechanisms were defined by the visual experimental results. In this regard, the subcooling and the isolated steam slug are the necessary conditions for the formation and development. In addition, the occurrence positions were found to be concentrated in the test pipe section near the water tank. Then, a butterfly check valve was designed to avoid the condensation induced water hammer events based on the formation mechanisms. The experimental results showed that the check valve usage can effectively inhibit the reverse flow of the subcooled water and thus achieve its purpose of eliminating condensation induced water hammer events. Moreover, the check valve transforms the two-phase natural circulation flow from the condensation oscillation mode to the stable boiling two-phase flow mode. As a result, it enhances the two-phase natural circulation capacity.

Modeling of transition from stratified flow to condensation-induced water hammer in horizontal steam-water pipes

Kelvin-Helmholtz (K-H) instability theory is extensively used to predict the transition from stratified flow to slug flow. This study developed a one-dimensional (1-D) model based on K-H theory and considered the variation of axial wave amplitude to predict the transition from stratified flow to condensation-induced water hammer (CIWH). The prediction results of the present model were compared with the experimental data and existing models. The comparison shows that the present model could reasonably predict critical instability, which is the boundary between stratified flow and CIWH. Critical instability is divided into three types according to the wave amplitude of critical instability. Critical instability occurs at the upper limit, middle, and starting point of the exponential growth of wave amplitude under low, medium, and high liquid levels. The effects of major parameters, including liquid level, water temperature, pipe diameter, and pipe length, on the critical inlet steam Froude number and position of critical instability were analyzed. When the liquid level increases, the critical inlet steam Froude number decreases, then increases, and then decreases, and the position of critical instability moves downstream, then upstream, and finally turns to the steam inlet. The effect of water temperature, pipe diameter, and pipe length on the critical instability depends on the liquid level and is complex.

Reverse flow in a pressure suppression system due to condensation instabilities

The Vacuum Vessel Pressure Suppression System (VVPSS) is designed to protect the ITER reactor (Vacuum Vessel, VV) from an overpressurization accident. The VVPSS is mainly composed of four Vapour Suppression Tanks (VSTs) partially filled with water inside which venting pipes (spargers) are immersed to direct the steam in the subcooled pool to condense it. Since the maximum pressure allowed inside the VV is only 0.15 MPa, a sub-atmospheric pressure must be maintained inside the VSTs. During a series of experimental campaigns funded by ITER Organization to study and validate the correct operability of the VVPSS and conducted in a large-scale plant built at the B. Guerrini laboratory of the University of Pisa, pressure instability and reverse flow through the sparger holes have been observed. The analysis of acceleration signals recorded during this pressure inversion, showed large amplitude oscillations at low frequency due to the Condensation Induced Water Hammer (CIWH).

Effect of rolling motion on transient characteristics of condensation induced water hammer in an equal-height-difference passive heat removal system

The investigation of condensation-induced water hammer (CIWH) under ocean conditions is crucial to the system safety of the offshore floating nuclear plants (OFNPs). This paper establishes a CIWH numerical model under rolling conditions based on the computational fluid dynamics (CFD) code combined with user-defined functions (UDF). First, the numerical model is verified by published experimental data. Then, the transient behaviors of CIWH events under various rolling parameters are numerically investigated. The results show that the rolling motion causes a significant steam slug and enhances the steam-subcooled water contact area, which promotes the average condensation rate. The average condensation rate increases with an increase in rolling angle, decreases with an increase in rolling period. Due to the larger average condensation rate and reverse flow caused by rolling motion, the pressure peak generated by the CIWH significantly increases compared to static conditions. Similarly, the pressure peak increases as rolling angle increases, decreases as rolling period increase. Moreover, the pressure peak is proportional to steam inlet velocity, and the location of the pressure peak gradually moves back to the outlet direction as the steam velocity increases. These qualitative conclusions are useful to understand the effects of rolling motion on the CIWH and provide a reference for the CIWH simulations.

Experimental study on transient flow characteristics in an equal-height-difference passive heat removal system for ocean nuclear power plants

The equal-height-difference passive heat removal system (EHDPHRS) utilizes the ocean as the heat sink for infinite heat dissipation and has broad application prospects in ocean nuclear power plants. However, the equal height difference between the heat source and heat sink is susceptible to severe flow instability. In this study, a fully visualized experimental facility was constructed to analyze the transient flow characteristics in the EHDPHRS. The experimental results revealed three typical two-phase natural circulation flow modes in the EHDPHRS. The formation mechanism of each flow mode was analyzed using thermal–hydraulic parameters. The results revealed that the flow mode is determined by the steam–water direct contact condensation and reverse flow of subcooled water, and both factors promote each other. Moreover, condensation-induced water hammer (CIWH) was found to be the most severe flow instability in the EHDPHRS. CIWH events can lead to violent pressure peaks and reduce the natural circulation capacity. In addition, when the occurrence position of CIWH events is concentrated at the pipe outlet, the influence of CIWH events on the two-phase natural circulation flow capacity gradually decreases. These research results aid in the optimal design of the EHDPHRS and prevention and control of CIWH events.

Numerical investigation of condensation-induced water hammer effects in horizontal and vertical cold reheat pipes

Condensation-induced water hammer (CIWH) is an unavoidable phenomenon in power generation facilities and must be properly adjusted for the thermal efficiency and safety of the system. This study simulated CIWH in a realistic steam pipe to analyze its characteristics in horizontal and vertical pipe sections. Additionally, CIWH intensities with respect to key parameter changes were explored for its mitigation. Numerical simulations for the CIWH were performed using an OpenFOAM solver, ICFMCondFOAM, which was validated for multiple phase change computational fluid dynamics (CFD) problems. The geometric and operating conditions of the cold reheat pipe of our partner company were used for CFD simulations. To assess the intensity of the water hammer effect, various inlet water flow rates and steam temperatures were employed and pressure over time was recorded at two measurement points: horizontal and vertical sections. Results of the CFD simulation show that the CIWH phenomenon behaves differently in horizontal and vertical pipes. CIWH strength was also evaluated using the Froude (Fr) and modified Jakob (Ja) numbers. Smaller Fr and larger Ja were found to be desirable for lowering the pressure increase ratios caused by the CIWH in both the horizontal and vertical pipe sections.

Experimental study of the features of the occurrence and development of condensation-induced water hammer in a horizontal pipe

Condensation-induced water hammers (CIWHs) can occur in pipelines when saturated steam comes into direct contact with subcooled water, causing severe damage to related equipment and personal safety. An experimental study of the occurrence and development of CIWHs in a horizontal pipe is carried out at the special test facility developed in Electrogorsk Research and Development Center for NPP Safety. Subcooled water is supplied into the horizontal pipe filled with steam as a result of which CIWH may occur. The test facility has two novel features: 1) a damper cap to stabilize the water flow at the inlet part of the horizontal pipe and 2) a discharge vessel with closed drainage (during the experiment). The first feature allows us to study the effect of the water flow contraction at the inlet section of the pipe on the CIWHs occurrence. The second feature makes it possible to study CIWHs in various hydrodynamic conditions during the experiment. Four stages of the process are identified: 1) initial flow of water through a steam filled pipe; 2) free falling discharge of water; 3) filling the horizontal pipe; 4) filling the upper part of the discharge vessel. The features of the arising CIWHs at each stage are described. The main parameters affecting the occurrence of CIWH are the temperature and flow rate of the supplied water. An increase in water subcooling contributes to the CIWH probability and an increase in CIWH pressure peaks. CFD calculations reveal that at high flow rates of supplied water (3–5 t/h), strong disturbances develop on the interfacial surface due to the new mechanism of steam–water interaction under these conditions, which can contribute to the occurrence of water hammers. On the plane of parameters “water subcooling - Froude number”, a map of CIWHs based on the present study and other works, as well as the theoretical boundary of the occurrence of CIWHs, has been built. Our results shown on this map are in full agreement with the results of other researchers. It is found that in experiments performed under the same conditions, significantly different magnitudes of water hammers can be obtained, which is associated with the stochastic nature of this phenomenon. It has been revealed that the maximum magnitudes of water hammers are achieved when the horizontal pipe is half filled with water, and the area of the interfacial surface on which steam condenses occurs is maximum.

Predicting reverse flow characteristics in equal-height-difference passive heat removal system for ocean nuclear power

The equal-height-difference passive heat removal system (EHDPHRS) via natural circulation driving and utilizes ocean as infinite heat sink for heat dissipation, showing broad application prospect in ocean nuclear power field; but the heat removal pipe normally encounters the reverse flow of cool seawater into the pipe, leading to operation instability and even condensation-induced water hammer. In this study, the integration of volume of fluid model and condensation model based on surface renewal theory is used to simulate the reverse flow characteristics in the heat removal pipe under single-and two-phase natural circulation conditions. The interfacial heat and mass transfer are calculated by user-defined functions (UDF). The numerical results reveal that the buoyancy force (Fb) and condensation driving force (FΔP) both accelerate the reverse flow, while the inertial force (Fi) restrains the reverse flow; the reverse flow length presents monotonous decrease with increasing the flow velocity under single-phase conditions and parabolic-like trend for two-phase flow. A prediction correlation for the reverse flow length is developed using dimensionless Reynolds number, Grashof number and vapor volume fraction, covering single- and two-phase conditions. The predicted reverse flow length by the correlation agrees with 94% of the results within deviations of ±15%.

Computational study of subcooled water injection into steam line: effect of Reynolds number on flow transition to study condensation induced water hammers

Abstract The direct contact condensation (DCC) of steam in subcooled water encounters in wide range of the industrial applications. On one side, it is an efficient and rapid, mass and heat transfer phenomenon. But, on the other side, it may generate condensation induced water hammers (CIWH) events which may cause high pressure peaks resulting in severe damage to the mechanical systems. This computational study intends to explore the underlying physics of CIWH events while injecting subcooled water into steam filled horizontal pipe section. The Reynolds number is varied from, Re w = 60,750 to 646,900, to study the flow regimes (stratified and slug), onset of CIWH and local flooding conditions. The results have been compared with the published data and found in good agreement. It has been observed that for Re w = 182,300, flow remains stratified. However, the flow regime changes from stratified to slug flow at Re w = 303,850–646,900, possibly due to the onset of CIWH. Extensive steam pockets have been observed at Re w = 303,850, which may be considered as onset of CIWH. Local flooding condition is also started at Re w = 303,850 and is observed to be shifted upstream with the increase in Reynolds number. This study is considered to be useful for the safe design and economical operation of the relevant systems in nuclear and other related industry.

Numerical investigation on direct contact condensation-induced water hammer in passive natural circulation system for offshore applications

For the passive natural circulation systems (PNCSs) in offshore floating nuclear plants, the ocean environment is usually regarded as the final heat sink. Condensation-induced water hammer (CIWH) may occur as the saturated steam released into the ocean by natural circulation. This study aims to develop a CFD model to analysis the CIWH phenomenon induced by direct contact condensation (DCC) in the PNCSs. In the model framework, the two-phase interface is tracked explicitly by the volume of fluid method; the interface heat transfer is acquired by the hybrid Heat Transfer Coefficient model based on the Surface Renewal Theory; the condensation heat and mass transfer rate are obtained by user-defined functions (UDF). The condensation model is firstly validated with the PMK-2 experimental data, showing consistency with the measured temperature evolutions. Then, the model is applied to investigate the CIWH events in the horizontal pipe geometry of the passive natural circulation systems. The simulation results indicate that the reverse flow of subcooled water triggers the formation of the CIWH events, by increasing the condensation rate and forming the entrapped bubble. The reverse flow periodically occurs afterwards, inducing multiple CIWHs. The increase of inlet stream velocity lengthens the reverse flow period and the distance, thus strengthening the interfacial condensation and the pressure impulse of CIWH.

Numerical Investigation on Condensation-Induced Water Hammer in an Equal-Height-Difference Passive Heat Removal System Under Rolling Conditions

Numerical Investigation on Condensation-Induced Water Hammer in an Equal-Height-Difference Passive Heat Removal System Under Rolling Conditions

Effect of Rolling Motion on Transient Characteristics of Condensation Induced Water Hammer in an Equal-Height-Difference Passive Heat Removal System

Effect of Rolling Motion on Transient Characteristics of Condensation Induced Water Hammer in an Equal-Height-Difference Passive Heat Removal System

Numerical Investigation of Condensation-Induced Water Hammer Effects in Horizontal and Vertical Cold Reheat Pipes

Numerical Investigation of Condensation-Induced Water Hammer Effects in Horizontal and Vertical Cold Reheat Pipes

Experimental Study of the Features of the Occurrence and Development of Condensation-Induced Water Hammer in a Horizontal Pipe

Experimental Study of the Features of the Occurrence and Development of Condensation-Induced Water Hammer in a Horizontal Pipe

Cfd Based Approach for Modeling Condensation-Induced Water Hammer in Equal-Height-Difference Passive Heat Removal System for Ocean Nuclear Power

Cfd Based Approach for Modeling Condensation-Induced Water Hammer in Equal-Height-Difference Passive Heat Removal System for Ocean Nuclear Power

Effect of Rolling Motion on Transient Behaviors of Condensation-Induced Water Hammer in an Equal-Height-Difference Passive Heat Removal System

Effect of Rolling Motion on Transient Behaviors of Condensation-Induced Water Hammer in an Equal-Height-Difference Passive Heat Removal System