Water Hammer Experiments Research Articles

Analysis of Anomalies in Water Hammer Experiments With Partially Open Valves

Abstract This study investigated the water hammer pressure due to the sudden closure of the partially open valve experimentally and analytically. Because the partially open valve could produce local nonuniform flow, a supplementary Joukowsky's water hammer equation was derived based on the assumption of the local nonuniform flow and the kinetic energy equation. A physical model was set up to measure the maximum water hammer pressure of the first positive wave due to the sudden closure of the partially open valve under different conditions, including various water heads, flow velocities, pipe diameters, and valve types. The results showed that Joukowsky's equation obtained by the momentum theorem in the uniform flow field was applicable to the uniform flow field with the valve fully open. The experimental results of the partially open valve-closure water hammer pressures were 3.5–21% larger than Joukowsky's equation, which consisted of the theoretical analysis of the supplementary Joukowsky's water hammer equation. This phenomenon had repeatability and was unrelated to the water head, the inlet flow velocity, the pipe diameter, and the valve type. This study could guide water hammer protection in hydropower and pump stations.

Influence of cavitation on the acoustic boundary conditions in water hammer experiments

A water hammer event is created by stopping a flow in a pipeline using a fast closing valve. The pressure is measured at three different positions upstream the valve. Cavitation occurs close to the valve and is detected by vapour pressure in the pipeline. Frequencies in the liquid part of the pipe were measured and statistically evaluated in order to analyse whether cavitation acts as a boundary condition of acoustics.Without cavitation one side of the pipe is an open end, while the other side is a closed end. In presence of cavitation the cavity should act like an open end due to the high change of acoustic impedance from liquid to gaseous phase. Therefore it is expected that the oscillating behavior is changing from an open/close to an open/open system. The measured frequencies indeed show a change towards an open/open configuration. It is shown that cavitation acts as a boundary condition in this system.

Study on Judgment and calculation method of two water column separation water hammer

Based on two water hammer experiments, a method of judging two water column separations in a pipeline and calculating the boosting pressure are put forward. By using the momentum theorem and the elastic water column theory of water hammer calculation, the calculation formula of the heave pressure in the case of equal and unequal water column velocities at both ends of the cavity is deduced.

Pressure and wall shear stress in blood hammer – Analytical theory

We describe an analytical theory of blood hammer in a long and stiffened artery due to sudden blockage. Based on the model of a viscous fluid in laminar flow, we derive explicit expressions of oscillatory pressure and wall shear stress. To examine the effects on local plaque formation we also allow the blood vessel radius to be slightly nonuniform. Without resorting to discrete computation, the asymptotic method of multiple scales is utilized to deal with the sharp contrast of time scales. The effects of plaque and blocking time on blood pressure and wall shear stress are studied. The theory is validated by comparison with existing water hammer experiments.

Filtering Effects of Periodic Structure in Water Hammer

In this study, we conducted water hammer experiments in the tube which was periodically supported by various numbers of clamps, named periodic structure, initiated by a projectile impact. The parts of the polycarbonate (PC) tube supported by 1-7 steel clamps make the tube stiffer and heavier than the original PC tube and are expected to cause a filtering effect of the frontal frequency components in the water hammer. According to our experimental observations, we confirmed that higher frequency components more than 1 kHz in the wave front were attenuated and the peak strains in circumferential direction of the tube were decreased 20% from the original PC tube. Moreover, we conducted numerical simulations of the water hammer wave similar to the experimental setup. Numerical results also revealed that frontal peak is attenuated 22% through periodic structure.

Effects of Polydispersity in Bubbly Flows

This thesis concerns the dynamics of bubbly flows with a distribution of equilibrium bubble sizes. The main goal is to formulate the physical and numerical models of continuum bubbly flows that enable us to efficiently compute the average mixture dynamics. Numerical simulations are conducted to quantify the effects of bubble size distributions on the averaged dynamics for several model flows. First, the ensemble-averaged conservation laws for polydisperse bubbly flows are derived. One-way-coupled flow computations are conducted to illustrate that the different-sized bubbles can oscillate with different frequencies. The resulting phase cancellations can be regarded as an apparent damping of the averaged dynamics of polydisperse flows. A high-order-accurate finite-volume method is then developed to compute the flow, paying special attention to issues of wave dispersion and stiffness. Next, computations of one-dimensional shock propagation through bubbly liquids are performed. The numerical experiments reveal that the bubble size distribution has a profound impact on the averaged shock structure. If the distribution is sufficiently broad, the apparent damping due to the phase cancellations can dominate over the single-bubble-dynamic dissipation (due to thermal, viscous, and compressibility effects) and the averaged shock dynamics become insensitive to the individual bubble dynamics. One-dimensional cloud cavitation caused by fluid-structure interaction is also solved to investigate the collapse of cavitation clouds with both monodisperse and polydisperse nuclei. The phase cancellations among the cavitation bubbles with broad nuclei size distributions are found to eliminate violent cloud collapse in the averaged dynamics. Finally, shock propagation through a bubbly liquid-filled, deformable tube is considered. The quasi-one-dimensional conservation law that takes into account structural deformation is formulated and steady shock relations are derived. The results are compared to water-hammer experiments; the present shock theory gives better agreement with the measured wave speeds than linear theory. This indicates that the gas-phase nonlinearity needs to be included to accurately predict the propagation speeds of finite-amplitude waves in a deformable tube filled with a bubbly liquid.v

Direct contact condensation induced transition from stratified to slug flow

Selected condensation-induced water hammer experiments performed on PMK-2 device were numerically modelled with three-dimensional two-fluid models of computer codes NEPTUNE_CFD and CFX. Experimental setup consists of the horizontal pipe filled with the hot steam that is being slowly flooded with cold water. In most of the experimental cases, slow flooding of the pipe was abruptly interrupted by a strong slugging and water hammer, while in the selected experimental runs performed at higher initial pressures and temperatures that are analysed in the present work, the transition from the stratified into the slug flow was not accompanied by the water hammer pressure peak. That makes these cases more suitable tests for evaluation of the various condensation models in the horizontally stratified flows and puts them in the range of the available CFD (Computational Fluid Dynamics) codes. The key models for successful simulation appear to be the condensation model of the hot vapour on the cold liquid and the interfacial momentum transfer model. The surface renewal types of condensation correlations, developed for condensation in the stratified flows, were used in the simulations and were applied also in the regions of the slug flow. The “large interface” model for inter-phase momentum transfer model was compared to the bubble drag model. The CFD simulations quantitatively captured the main phenomena of the experiments, while the stochastic nature of the particular condensation-induced water hammer experiments did not allow detailed prediction of the time and position of the slug formation in the pipe. We have clearly shown that even the selected experiments without water hammer present a tough test for the applied CFD codes, while modelling of the water hammer pressure peaks in two-phase flow, being a strongly compressible flow phenomena, is beyond the capability of the current CFD codes.

Analysis of the capability of system codes to model cavitation water hammers: Simulation of UMSICHT water hammer experiments with TRACE and RELAP5

The capabilities of the nuclear system transient codes TRACE and RELAP5 to model coupled two-phase flow and pressure wave propagations in a pipe are assessed by analyzing the UMSICHT PPP cavitation water hammer experiments 329 and 135 after valve closure. Time-dependent pressure, flow behaviour, and the generation and collapse of vapor bubbles at the valve and the first bridge are discussed. We show that both codes are able to model the flow behaviour of the water hammer for the high pressure and high temperature case 329 (initially 10–13 bar and 420 K), however condensation heat transfer for the base case needed to be increased in order to accurately model the magnitude of the first pressure excursion. The experimental broadening and damping of the subsequent pressure peaks by Fluid-Structure Interaction (FSI) phenomena arising from the interaction of the flow with the vibrations of the piping structure are not considered in the modeling results. For the lower pressure and temperature case 135 (initially 1–4 bar and 294 K), the TRACE code provides a good approximation of the propagation of the pressure wave and the void fraction behaviour, already with base case conditions, while RELAP5 overpredicts the vapor generation along the pipe and, as a result, considerably underpredicts the pressure amplitudes and overpredicts the water hammer frequency.

Experimental Investigation of Waterhammer in Simplified Feed Lines of Satellite Propulsion Systems

Operation of spacecraft propulsion systems is regularly adversely faced with waterhammer. The presence of very low pressures or vacuum complicate the classically known waterhammer due to various multiphase phenomena such as cavitation, absorption and desorption of a pressurizing gas, boiling, etc. This complex behavior is hard to model due to a lack of understanding of the physical processes taking place. To build up an experimental database on waterhammers in multiphase confined environments for validation of physical models, a literature survey was first undertaken on detailed waterhammer experiments. Only a few papers with well-documented experiments were found. None of the experiments found matched all the specifications needed for proper simulation and validation in satellite and spacecraft hardware and physical configurations. To complement the literature database, a detailed experimental investigation of the waterhammer phenomenon in a confined environment was required. This investigation was carried out using a simplified setup with two inert fluids, ethanol and acetaldehyde, and an actual propellant, monomethylhydrazine. Numerous tests were performed that confirmed excellent reproducibility of the results. The experiments showed that ethanol can be used instead of toxic monomethylhydrazine for estimating the waterhammer amplitude. The initial pipe pressure has a great influence on the waterhammer amplitude, but it would appear that the fluid vapor pressure is not a physical boundary with respect to waterhammer characteristics. The waterhammer phenomenon produced in a pipe with a low initial pressure exhibited a complex evolution over time and slight differences in amplitude and features according to the pipe geometry.

Investigation of the mechanisms responsible for the breakdown of axisymmetry in pipe transient

Existing transient pipe flow models assume that the flow field is stable and axisymmetric. Recent experimental and theoretical researches show that water hammer flows can become unstable and these instabilities lead to the breakdown of the axisymmetric assumption and to significant changes to the flow held and the magnitude and distribution of wall shear stresses. In addition, previous experiments revealed that in some cases the flow asymmetry developed in a time scale smaller than the water hammer time scale (ratio of pipe length to wavespeed) and in other cases the instability developed in a time scale of the order of the water hammer time scale. The current stability analysis is consistent with previous experimental observations. In particular, it is shown that there are two distinct mechanisms of instability, namely, a transient growth mechanism which develops in time scale shorter than the water hammer time scale and an exponential growth mechanism which develops in time scale of the order of the water hammer time scale; and the instability is indeed of asymmetric nature. Perturbations with large streamwise length scale (i.e. large streamwise wavelength) are more susceptible to the transient instability while perturbations with small streamwise length scale are more susceptible to exponential instability. The exponential instability may become active only if ihe flow “survives” the instability associated with the transient growth. The transient flow instability (sometimes referred to as bypass instability) can arise even when the flow is deemed to be stable by the traditional modal stability analysis. Such instability is known to occur in other Hows and lands support to the experimental finding that instabilities can occur shortly after the passage of the first water hammer wave. Physically. Ihe bypass instability is associated with the tilting of base flow vortices by the velocity fluctuations. Plots of the flow lield clearly show the breakdown of the axisymmetric assumption and produce asymmetric velocity profiles consistent with those observed in water hammer experiments.

Investigation of waterhammer in piping networks with voids containing non-condensable gas

A waterhammer code, PTRAN, has been developed for analyzing hydraulic pressure transients in complex piping systems and for prediction of waterhammer loads for use in design assessment of piping and associated supports. The calculation scheme uses the two-equation method of characteristics. PTRAN had been validated against a series of waterhammer experiments and theoretical calculations of sample problems. To further improve and extend the prediction capability, a void/non-condensable gas model was incorporated into PTRAN to account for the effects of air content in the voids. A set of void collapse experiments were carried out in a nominal 50 mm diameter test rig to investigate the waterhammer phenomena. The test conditions covered three sizes of initial void volumes and a selected range of air content between 0 and 100%. This paper presents the comparison of the PTRAN predictions and the experimental results for the waterhammer caused by void collapse when the void contains a known fraction of non-condensable gas. An application of the model to study the waterhammer transients for a typical high pressure emergency core cooling (ECC) system is also discussed. Considering non-condensable gas in initial voids is shown by a PTRAN simulation to be beneficial in the reduction of waterhammer loads for piping systems.