Condensation Induced Water Hammer Research Articles

Research of condensation induced water hammer under filling a horizontal pipe with subcooled water

Experiments at the KGU test facility, devoted to condensation induced water hammers (CIWH) were analysed with the WAHA code. Test section of the KGU test facility is slightly inclined horizontal pipe of 3 m length and of 64 mm inner diameter. Subcooled water was supplied to the pipe inlet, pipe outlet was connected to separator vessel. Upper part of separator vessel was connected to steam source. Experiments were performed for different system pressure, different water subcooling and different mass flow rate of water supply. The performed experimental study of CIWH showed that in the investigated range of parameters, the development of CIWH occurs in different ways. At the stage of water propagation along the filled with steam horizontal pipe, water hammer was not observed. At the stage of free drainage of water from the outlet end of the pipe, water hammer was realized only when the water was sufficiently subcooled. At the stage of the water level rise in the horizontal pipe, water hammer occurred in all tests. At the last stage of filling the upper part in the separation vessel, there were small water hammers due to the collapses of the remaining steam bubbles. Numerical modelling of these experiments with the WAHA code revealed the shortcomings of the interphase heat transfer model developed for this code, which have a noticeable effect on the numerical solution.

Experimental research on the mechanisms of condensation induced water hammer in a natural circulation system

Natural circulation systems (NCSs) are extensively applied in nuclear power plants because of their simplicity and inherent safety features. For some passive natural circulation systems in floating nuclear power plants (FNPPs), the ocean is commonly used as the heat sink. Condensation induced water hammer (CIWH) events may appear as the steam directly contacts the subcooled seawater, which seriously threatens the safe operation and integrity of the NCSs. Nevertheless, the research on the formation mechanisms of CIWH is insufficient, especially in NCSs. In this paper, the characteristics of flow rate and fluid temperature are emphatically analyzed. Then the formation types of CIWH are identified by visualization method. The experimental results reveal that due to the different size and formation periods of steam slugs, the flow rate presents continuous and irregular oscillation. The fluid in the horizontal hot pipe section near the water tank is always subcooled due to the reverse flow phenomenon. Moreover, the transition from stratified flow to slug flow can cause CIWH and enhance flow instability. Three types of formation mechanisms of CIWH, including the Kelvin-Helmholtz instability, the interaction of solitary wave and interface wave, and the pressure wave induced by CIWH, are obtained by identifying 67 CIWH events.

Oscillation characteristics of periodic condensation induced water hammer with steam discharged through a horizontal pipe

Condensation induced water hammer (hereafter, CIWH) phenomenon is widely observed in various industrial operations, causing severe damage to related equipment and personal safety. This study investigated the CIWH regime map and the oscillation characteristics of periodic CIWH under different pipe lengths when the steam was discharged into a water tank through a horizontal pipe. First, the CIWH regime maps were obtained under different pipe lengths, with distinct regimes of periodic CIWH and non-periodic CIWH. A criterion was proposed to predict the transitions from periodic to non-periodic CIWH. It was found that the occurrence frequency of periodic CIWH decreased with the increase in the pipe length. An empirical correlation of Strouhal number, non-dimensionalized occurrence frequency, was developed to predict the occurrence frequency. The correlation could reproduce more than 95 % of the experimental data with the error of -20.2 % to 23.7 %. The pressure oscillation intensity of periodic CIWH was analyzed. The oscillation intensity increased when the pipe length and steam mass flux increased, whereas it increased first and then decreased when pool temperature increased. Finally, the occurrence position of periodic CIWH was found to be nearly constant under the same test condition. The position moved downstream of the steam when the pipe length and Froude number increased, and Jakob number decreased. The average occurrence position was correlated with the error of -17.1 % to 18.4 %.

Experimental and Calculated Studies of Condensation-Induced Water Hammer

Experience gained from the operation of nuclear power plants shows that emergencies or incorrect actions of the personnel may give rise to water hammer phenomena, which exert additional impacts on the pipelines and equipment. Condensation-induced water hammer (CIWH) is one of the most destructive kinds of water hammer. Experimental studies of CIWH were carried out in a number of scientific organizations around the world. The main features of the experimental facilities used in the world are considered. For studying the CIWH phenomena occurring in a horizontal channel, a special experimental test facility was set up in 2018 at the Elektrogorsk Research and Engineering Center for Nuclear Power Plants’ Safety (EREC). The facility’s test section and the parameters of experimental regimes are described. Fifty-two experiments were carried out on the facility under different initial and boundary conditions for three values of the water flowrate supplied to the facility (1, 3, and 5 t/h), two values of water temperature (30 and 60°С), and two values of vapor pressure in the test section (0.6 and 1.0 MPa). In all experiments, CIWH phenomena were recorded. As an example, one experimental regime was considered in detail, and the CIWH occurrence mechanism is described proceeding from the readings of pressure and level gages and temperature sensors. The described experimental regime was modeled using the RELAP5 and WAHA thermal-hydraulic codes. The effect that the size of nodalization diagram cells and the integration time step have on the computation results is analyzed. The significant amount of experimental points obtained from the study can be used for setting up a database for validating computer codes.

Analysis of experiments on condensation induced water hammers at KGU test facility by WAHA code

Experiments at the KGU test facility, devoted to steam induced water hammers were analysed with the WAHA code. Test section of the KGU test facility is slightly inclined horizontal pipe of 3 m length and of 64 mm inner diameter. Subcooled water was supplied to the pipe inlet, pipe outlet was connected to separator vessel. Upper part of separator vessel was connected to steam source. Experiments were performed for different system pressure, different water subcooling and different mass flow rate of water supply. The WAHA code has been developed for simulation of condensation induced water hammers in pipelines. It is based on two-fluid approach, WAHA numerical scheme is based on Godunov numerical methods. Numerical simulations of the KGU tests revealed disagreement between numerical and experimental occurrences of water slug formations and water hammers events. Methodological calculations with variation of heat transfer to subcooled water showed, that this value significantly influences on water slug and water hammer formation.

Modeling and simulation of onset of condensation-induced water hammer

In view of the importance of the condensation-induced water hammer in various engineering fields, this study performed an analytical modeling and numerical simulation to obtain the map of the onset of the condensation-induced water hammer. Two linear boundaries were experimentally observed for the transitions between no condensation-induced water hammer and non-periodic condensation-induced water hammer and between non-periodic condensation-induced water hammer and periodic condensation-induced water hammer. The analytical modeling based on the energy balance equation was conducted to derive the onset of the condensation-induced water hammer. The analytical model suggested that the map of the onset of the condensation-induced water hammer could be displayed by two essential non-dimensional parameters, such as modified Jakob number and Froude number. The model indicated that the slope of the boundary of the onset of the condensation-induced water hammer was the product of modified Stanton number, non-dimensional interfacial area concentration, and non-dimensional temperature difference. The model could predict the boundaries between no condensation-induced water hammer and non-periodic condensation-induced water hammer and between non-periodic condensation-induced water hammer and periodic condensation-induced water hammer satisfactorily. The one-dimensional nuclear thermal-hydraulic system analysis code, RELAP5, was used for simulating the onset of the condensation-induced water hammer. The obtained boundary corresponded to the experimentally obtained boundary between non-periodic condensation-induced water hammer and periodic condensation-induced water hammer.

Experimental study on flow modes and transient characteristics in low-pressure equal-height-difference natural circulation system

As nuclear power plants tend to be miniaturized and integrated, equal-height-difference natural circulation systems (EHDNCSs) have potential applications in floating nuclear power plants. This work experimentally investigated the flow modes of an EHDNCS at low pressure. It has observed four quintessential flow modes in the experiments. The transient characteristics of each flow mode were analyzed by the visualization results. In addition, the dispersion and amplitude characteristics of flow rate were compared in different flow modes based on 134 experimental cases. The experimental results reveal that the flow oscillation is related to unsteady flow pattern for intermittent and condensation oscillations. The condensation induced water hammer (CIWH) is found in the condensation oscillation. According to the stability map, it is found that the regions of stability and CIWH can be well divided by equilibrium quality of −0.23% and 0.47%, and the unstable range is enlarged by increasing the inlet temperature and resistance.

Experimental and numerical study of the interaction of subcooled water with saturated steam in a horizontal pipe

Interaction of subcooled water with saturated steam in horizontal tube can produce so-called condensation induced water hammers, which may be dangerous for integrity of NPP equipment. This phenomenon has been investigated at the special test facility developed at Electrogorsk Research and Engineering Center. Test section of the test facility is horizontal tube of inner diameter of 64 mm and a length of 3 m. It has been performed several experimental runs, where subcooled water was supplied to the end of the tube filled by saturated steam. Another end of the tube was connected to separator vessel. System pressure, water subcooling and water flow rate were variable parameters. There were a lot of condensation induced water hammers, observed during experiments. Experimental data have been analysed with thermal-hydraulic codes WAHA and RELAP5, based on two-phase, two-fluid model.

Experimental research on characteristics of condensation induced water hammer in natural circulation systems

Abstract Condensation induced water hammer (CIWH) is a common phenomenon possibly appearing in industrial pipelines, which may cause pipeline oscillation and even serious damage to the components. Previous research on the CIWH mostly focuses on the steam-water counterflow under forced flow conditions. For new floating nuclear power plants proposed recently, the CIWH may occur in some passive natural circulation safety systems. Nevertheless, the CIWH in natural circulation systems (NCS) has never been reported publicly yet. In this paper, a series of experiments are performed to investigate the flow characteristics during the CIWH in a NCS. The basic flow and temperature characteristics of the CIWH caused by counterflow of saturated steam-water mixtures and subcooled water in the NCS are studied in detail. Thereinto, the steam condensation induced flashing (SCIF) along with CIWH is firstly observed and analyzed. Moreover, the influences of CIWH on the flow behavior of the system are discussed under natural circulation and forced circulation conditions. The experimental results indicate that the occurrence of CIWH can result in further reverse flow of the subcooled water, which causes a sudden decrease in the flow rate of NCS. Moreover, the flow rate is more unstable under natural circulation than under forced flow.

Characteristics of entrapped bubbles of periodic condensation-induced water hammer in a horizontal pipe

Steam condensation-induced water hammer (CIWH) can cause great pressure oscillation, and the knowledge of CIWH is important to pipes and relevant equipment. In this work, an experiment was conducted to investigate the condensation process of entrapped bubbles for periodic CIWH and its pressure oscillation in a horizontal pipe. Results indicated that CIWH was caused by the condensation of entrapped bubbles, and the initial volume of entrapped bubbles of the periodic CIWH kept nearly unchanged when the test conditions were constant. The initial bubble volume increased with the increases in steam mass flux and water temperature. The pressure oscillation intensity of the CIWH increased with the increase in initial bubble volume but decreased with the increase in water temperature. Furthermore, the change rate of bubble volume gradually increased with the condensation of the entrapped bubbles. The final change rate of bubble volume increased with the increase in initial bubble volume but decreased with the increase in water temperature. Finally, a theoretical correlation, that is, a linear relationship between the pressure oscillation intensity and the final change rate of bubble volume, was proposed to calculate the pressure oscillation intensity of the CIWH. The predicted deviation ranged from −44% to 64%.

Effect of throttling on the two-phase flow stability in an open natural circulation system

Low pressure natural circulation systems fit the concept of passive safety systems as useful feature to bring a nuclear power plant to a safe and stable state during a postulated accident without active systems. The operation is characterized by low mass fluxes and passing the known unstable two-phase region from activation to a continuous heat removal in the stable two-phase region. The occurring mass flow oscillations and the condensation induced water hammer phenomenon limited the performance of heat removal. These effects are highly undesirable because they may effect destruction of entire apparatus. The GENEVA test facility as an open natural circulation system represents the containment cooling condenser of the KERENATM reactor concept in the main dimensions. Initiated by the steam supply, the mass flow results from induced heat flux and is limited only by the internal pressure drops: the local, the frictional, the acceleration and the gravitational pressure drop across the flow path. The influence of the flow resistance on its unstable two-phase region and their oscillatory behavior are experimentally studied in detail. Its variation is summarized in stability maps plotted in the parameter plan of the heat flux and the subcooling number at the inlet of the heated section. Thereby, the dynamical evolution of the boiling boundary indicates the initiation of two-phase flow oscillations and the condensation induced water hammer phenomenon. The increasing local pressure drop at the inlet of the heated section reduces the velocity amplitudes but a clear reduction of the pressure surges could not be determined although these should be more pronounced due to decreasing buoyancy and increasing back flow.

Numerical Analysis of Condensation Induced Water Hammer Mechanisms

Numerical Analysis of Condensation Induced Water Hammer Mechanisms

Modeling and analysis of condensation induced water hammer

Direct contact of steam and subcooled water under certain situations may cause immense steam condensation at the two-phase interface and can lead to the generation of fast and violent pressure surges which is often termed as condensation induced water hammer (CIWH) or direct contact condensation (DCC) driven water hammer. The present work aims at the exploration of the underlying physics of the CIWH phenomenon in a horizontal two-phase flow scenario using a dedicated 1D, compressible in-house code which is formulated based on the two-fluid modeling approach (six-equation based model). The developed code is verified against the benchmark two-phase shock tube problem (Reimann problem) and it is observed that it is capable to capture the shock wave, rarefaction wave and contact discontinuity satisfactorily. A comparative assessment between present in-house code, RELAP5 and WAHA3 against the PMK-2 CIWH experimental data shows that the pressure peak amplitude predicted by our in-house code is more accurate in comparison to WAHA3 and RELAP5 simulation. In this work, emphasis is also given on the detailed investigation to study the effect of inlet water subcooling (20–80 °C), water inflow rate (corresponding = 0.1 and 0.7) on the pressure peak amplitude (along with its occurrence time and location), phase distribution, temperature history and interfacial condensation rate during CIWH. Observation reveals that with the decrease in inlet water temperature, pressure peak magnitude increases. It is also found that the pressure peak amplitude increases with the increase in inlet water flow rate.

Experimental investigation on the phenomenon of steam condensation induced water hammer in a horizontal pipe

The condensation induced water hammer (CIWH) phenomenon may cause serious damage to the pipes and related system, which often occurs during the steam-water direct contact condensation process. In this paper, an experimental investigation was performed to study CIWH phenomenon caused by steam-water direct contact condensation in a horizontal pipe. The entire CIWH process was captured by a high speed video camera and its pressure fluctuation was synchronously measured. Four typical flow patterns were observed during CIWH process: stratification flow, wave flow, slug flow and bubble collapse. Bubble collapse would generate a high pressure peak. Based on different variations of steam-water phase interface and pressure fluctuation signals, three types of CIWH were defined: non-periodic CIWH, periodic CIWH and no CIWH. A CIWH region map was given considering the effect of steam mass flux and water temperature. In periodic CIWH region, the generation frequency of CIWH was found to range from 0.19 Hz to 0.39 Hz, which decreased with the rise of steam mass flux and water temperature. A dimensionless correlation was obtained to predict Strouhal number of CIWH generation frequency. Predicted values corresponded well to the experimental data with the deviation in the range of −16% to +23%.

Experimental and numerical investigations on the direct contact condensation phenomenon in horizontal flow channels and its implications in nuclear safety

Abstract The complex direct contact condensation phenomenon is investigated in horizontal flow channels both experimentally and numerically with special emphasis on its implications on safety assessment studies. Under certain conditions direct contact condensation can act as the driving force for the water hammer phenomenon with potentially local devastating results, thus posing a threat to the integrity of the affected NPP components. New experimental results of in-depth analysis of the direct contact condensation phenomena obtained in Kaunas at the Lithuanian Energy Institute will be presented. The German system code ATHLET employing for the calculation of the heat transfer coefficient a mechanistic model accounting for two different eddy length scales, combined with the interfacial area transport equation will be assessed against condensation induced water hammer experimental data from the integral thermal-hydraulic experimental facility PMK-2, located at the KFKI Atomic Energy Research Institute in Budapest Hungary.

Experimental study on steam chugging phenomenon in a vertical sparger

Unstable direct contact condensation called ``Chugging'' that occurs in certain conditions in the pressure suppression pool of Primary Containment Vessel of Boiling Water Reactors (BWRs) was studied experimentally. The mechanisms of every phase of the chugging was described, and experimental results useful for the development and validation of more accurate CFD models were provided. The experiment was conducted with a transparent pool and a transparent polycarbonate pipe or a stainless steel pipe with inner diameter of 27mm under the conditions of the steam mass flux of 5.5–19.5kg/m2s and the pool temperature of 19–46.5°C. Pressure pulses were measured and synchronized with a high speed video camera for images acquisition. It was identified that the bubble implosion occurred while the pressure in the bubble quickly decreased. This condition might establish instability in the interfacial area which grew abruptly causing the implosion. Moreover the transparent apparatus allowed to interpret and relate internal condensations, generating pressure spikes of around 1.2MPa because of the condensation-induced water hammer. Finally, the chugging condensation regime map was created from the experimental data.

A numerical analysis on the effect of inlet parameters for condensation induced water hammer

Direct contact condensation (DCC) is almost an inevitable phenomenon during accidental condition for all LWRs. Rapid condensation caused by the direct contact of steam and subcooled water can lead to condensation induced water hammer (CIWH). The present work explores the underlying physics of CIWH phenomenon in a horizontal pipe under different inlet conditions such as inlet water temperature, pressure difference between steam and water section, steam superheating, steam quality and duration of valve opening using RELAP5/Mod 3.4. This work emphasises on the prediction of pressure peak magnitude in conjunction with its location of occurrence under different parametric conditions. The stratified to slug flow transition is presented in terms of the ‘flow regime map’ which is identified as the primary cause for pressure wave generation. The strongest pressure wave amplitude due to CIWH is found to be 116.6bar for ΔP=10bar. Observation reveals that peak pressure location shifts towards the subcooled water injection point for higher inlet water temperature. For the lowest inlet water temperature (Tin=20°C), the peak pressure is found at a distance of 47.5cm away from the water inlet whereas, for the high water temperature (Tin=120°C), peak pressure is observed at 6.25cm away from the injection point. It is also observed that the duration of valve opening significantly affects the location of peak pressure occurrence. This study also reveals that the presence of superheated or wet steam could possibly avoid the occurrence of CIWH.

Theoretical study of steam condensation induced water hammer phenomena in horizontal pipelines

AbstractSteam condensation induced water hammer (CIWH) phenomena are investigated and new theoretical results are presented. We use the WAHA3 model based on two-phase flow six first-order partial differential equations that present one dimensional, surface averaged mass, momentum and energy balances. A second order accurate high-resolution shock-capturing numerical scheme was applied with different kind of limiters in the numerical calculations. The applied two-fluid model shows some similarities to RELAP5 which is widely used in the nuclear industry to simulate nuclear power plant accidents. This model was validated with different CIWH experiments which were performed in the PMK-2 facility, which is a full-pressure thermohydraulic model of the nuclear power plant of VVER-440/312 type in the Energy Research Center of the Hungarian Academy of Sciences and in the Rosa facility of the Japan Atomic Energy Agency. In our present study we show the first part of a planned large database which will give us the upper and lower flooding mass flow rates for various pipe geometries where CIWH can happen. Such a reliable database would be a great help for future reactor constructions and scheming.

Online hydraulic calculation and operation optimization of industrial steam heating networks considering heat dissipation in pipes

Constructing industrial parks with DH (district heating) systems has become a main way to develop modern industry, which requires strict security and reliability of heating networks. Industrial steam heating networks are usually ring-shaped with multiple heating sources, and the working conditions would be changeable due to the high frequency and a wide range of load variation of heating consumers. Under a specific working condition, low steam velocity for a long time (namely “steam stagnation”) in certain pipes will result in CIWH (condensation-induced water hammer) which will threaten the security of the whole DH system. In this paper, a hydraulic calculation model is built to study the steam flow regime considering heat dissipation and condensation in pipes, an operation optimization method is proposed to help eliminate steam stagnation through optimizing the heat load distribution of each heating source, a general software system entitled “HEATNET” is presented to realize online hydraulic calculation and operation optimization for arbitrary structured heating networks. The practical application of HEATNET in Shanghai chemical industry zone shows that heat dissipation and condensation in pipes would influence the overall hydraulic calculation of steam heating networks and it can prevent CIWH and improve the security and reliability of steam heating networks.

Investigations on the stochastic nature of condensation induced water hammer

Sudden condensation procedures represent a severe safety issue in numerous industrial applications. One of the most destructive forms of appearance is the so called condensation induced water hammer (CIWH), where subcooled water is brought into contact with saturated steam in an enclosed area. A few examples are the emergency core cooling of nuclear reactors, the start-up process of steam circuits in chemical, oil and gas plants or the hot gas defrost in ammonia-cooled frozen food plants. In each case saturated steam is brought into contact with subcooled liquid through a sufficiently large exchange surface, resulting in a two-phase flow pattern which encourages the entrapment of rather large steam pockets. If these pockets collapse the surrounding water slugs collide, which can create a pressure rise up to several times the operation pressure and cause the failure of pipes and connecting instruments.This paper describes an experimental setup which allows the detailed investigation of CIWHs in horizontal pipes with 1.4° declination. On basis of 185 experiments, conclusions regarding the probability of CIWHs, the height of pressure peaks and the position of nucleation sites of steam implosion are presented. Important results are the identification of critical parameter combinations (injection flow rate and degree of subcooling) for CIWH incidents with high probabilities and high pressure values as well as the determination of a criterion for exclusion. Further the distribution regarding the origin of water hammer effects, respectively the location of slug collision, is determined in dependency on the degree of subcooling. The accurate description of the experimental setup and experimental outcome provides a reliable basis for future simulations and calculations. In the first example a model concept is presented, which focuses on the underlying phase distribution. This model is the first step to a more reliable prediction and prevention of condensation induced water hammers in the future.