The authors have been developed and designed a flexible experimental apparatus for investigation of water hammer and column separation in unsteady friction dominated pipeline. The apparatus has been already tested for a number of steady and unsteady flow conditions. Transient cavitation and column separation phenomena have been observed in a number of experimental runs. Water hammer has been triggered by both closing and opening of electro-pneumatic (EV) and hand operated valve (HV). The experimental data are compared with results given from in-house numerical code written in Visual Fortran based on the method of the characteristics (MOC) with convolution based unsteady friction model (CBM) included. The column separation and transient cavitation phenomena are modelled by a discrete gas cavity model (DGCM). There is a good agreement between the experimental and numerical results. Moreover, the model is robust, and therefore, it is recommended for engineering practice. In addition, the influence of variations of the pressure wave speed and the uncertainty in flow rate measured by the electromagnetic flow meter are also investigated.
COBISS.SI-ID: 13773595
This paper presents the results from detailed experiments of the two-phase pressurized flow behavior during the rapid filling of a large-scale pipeline. The physical scale of this experiment is close to the practical situation in many industrial plants. Pressure transducers, water-level meters, thermometers, void fraction meters, and flow meters were used to measure the two-phase unsteady flow dynamics. The main focus is on the water–air interface evolution during filling and the overall behavior of the lengthening water column. It is observed that the leading liquid front does not entirely fill the pipe cross section; flow stratification and mixing occurs. Although flow regime transition is a rather complex phenomenon, certain features of the observed transition pattern are explained qualitatively and quantitatively. The water flow during the entire filling behaves as a rigid column as the open empty pipe in front of the water column provides sufficient room for the water column to occupy without invoking air compressibility effects. As a preliminary evaluation of how these large-scale experiments can feed into improving mathematical modeling of rapid pipe filling, a comparison with a typical one-dimensional rigid-column model is made. At this stage, the steady-state unsteady skin friction model has been incorporated into the model.
COBISS.SI-ID: 13678107
The research refers to the experimental study of cavitation phenomena when water was passing through the slot orifice. The contribution brought up by this article is the detailed time and space dependent analysis of bubbly structures identified using high speed video, microresistivity probes, optical fibre pressure probe, and hot film probe. Two principle experiments were carried out to study cavitation in a confined geometry. The first one was designed to study an induced single bubble cavitation when the water pressure was reduced below the atmospheric pressure, but was still high enough so that there was not any saturated pressure in the slot region. The second experiment was undertaken at a reference pressure which was sufficient to produce a massive cavitation in the slot region. The following flow regimes were identified and analyzed in detail: the so-called detachment regime where bubble breakup was observed in the case of the individual bubble cavitation; and in the case of the large scale cavitation, the regime of macroscopic bubbles clustering into bubble cloud and the regime of collapsing bubbles. The reserarch contributes to better understaning of cavitating transient flow phenomena in general (cavitational regimes, possible dissipation mechanisms).
COBISS.SI-ID: 13255963
This paper investigates the effects of transient vaporous cavitation caused by the closure of the downstream end-ball valve against the discharge. Numerical results are compared with the results of measurements in the simple reservoir-horizontal steel pipeline-valve apparatus. Pressures measured at the end points and at two equidistant positions along the pipeline are compared with computational results as piezometric heads. Novel numerical transient cavitating model includes effects of either of steady or unsteady skin friction and entrapped gas pockets. Comparisons between the results of two distinct water column separation tests and numerical simulations using an advanced discrete gas cavity model showed good agreement when convolution-based unsteady friction model is used. This is not the case when the steady state friction model is used. Column separation phenomenon is relatively rapid transient event characterized by a number of flow situations (accelerating and decelerating flows). Finally, two distinct column separation runs include active and passive column separation cases.
COBISS.SI-ID: 14435355
Trapped air pockets may cause severe operational problems in hydropower and water supply systems. A locally isolated air pocket creates distinct amplitude, shape and timing of pressure pulses. This paper investigates dynamic behaviour of a single trapped air pocket. The air pocket is incorporated as a boundary condition into the discrete gas cavity model (DGCM). DGCM allows small gas (air) cavities to form at computational sections in the method of characteristics (MOC) numerical grid. The behaviour of the pocket and gas cavities is described by the water hammer compatibility equation(s), the continuity equation for the cavity volume, and the equation of state of an ideal gas. Isentropic behaviour is assumed for the trapped gas pocket and an isothermal bath for small gas cavities. A convolution-based unsteady skin friction model is incorporated in DGCM. Experimental investigations have been performed in a laboratory pipeline apparatus. The apparatus consists of an upstream end high-pressure tank, a horizontal steel pipeline (total length 55.37 m, inner diameter 18 mm), four valve units positioned along the pipeline including the end points, and a downstream end tank. A trapped air pocket is captured between two ball valves at the downstream end of the pipeline. The transient event is initiated by rapid opening of the upstream end valve; the downstream end valve stays closed during the event. Predicted and measured results for a few typical cases are compared and discussed.
COBISS.SI-ID: 15149083