Thermodynamic effects on Venturi cavitation characteristics

Abstract

In this paper the thermodynamic effect is systematically studied by Venturi cavitation in a blow-down type tunnel for the first time, using water at temperatures up to relatively high levels, and at controlled dissolved gas contents in the supply reservoir (measured by dissolved oxygen, DO). The mean attached cavity length cav L   is chosen to reveal the thermodynamic effect, and the cavitation characteristics are analyzed from the experiments. With an increase of the thermodynamic parameter *  , a decrease of cav L   vs. the pressure recovery number κ is observed, which is consistent with suppression of cavitation by the thermodynamic effect, but the decrease is related not only to this effect. Based on the experimental results, a model is presented of the attached cavity cloud that develops from the Venturi throat. It is found that either the length of this cloud oscillates stably around a mean value, or the cloud breaks regularly at some upstream position, allowing that a detached cavity cloud is shed, flows downstream and collapses, while the remaining attached cloud regenerates. Applying this model to experimental results obtained first with cold water, then with hot water, we find that when the mean length of the attached cavity cloud oscillates stably, temperature increase causes reduction of the mean cavitation length. This is interpreted to be a consequence of the thermodynamic effect. When detachment of large cavity clouds occurs, the mean length is increased at temperature increase. This is a consequence of cloud configuration changes being superposed on changes due to the thermodynamic effect. These observations explain conflicting results reported for attached cavity clouds in relation to the thermodynamic effect. The gas content in the water is found to be without significance within the range of dissolved oxygen tested.

Analysis of the cavitation instabilities with time-resolved stereo and multiplane particle image velocimetry

“…As shown in figure 29, cloud cavitation is generally divided into four main stages: Growing, up-lift, cloud detachment, and cloud convection. At all steps, consistently with the recirculation cell model 13 , the re-entrant jet plays a crucial role, since its structure and momentum trigger the flow evolution to the different behaviors.…”

“…The falling stream divides into two parts flowing parallel to the wall. One forms the so-called re-entrant jet, which moves upstream and contributes to the cavity break-off and cloud shedding 2,3,8,12,13 . The other one makes the flow re-attach to the wall 14 .…”

Investigation of cavitation and vapor shedding mechanisms in a Venturi nozzle

“…27,28 In addition, the current work clarifies the shedding mechanisms for internal three-dimensional geometries of circular cross section, instead of simplified, often twodimensional geometries, such as the frequently used wedge. 17,20,[29][30][31] Further to that and for the first time on this geometry, average vapor fraction slices of the CFD are compared with those from the experimental computed tomography (CT) and show good agreement overall. Finally, details have been exposed, which cannot be extracted from the experiments and provide additional insight into the shedding mechanisms.…”

Influence of Step Casings on the Cavitation Characteristics of Inducers

“…This severe problem has attracted a great deal of interest in recent decades [1][2][3]. Among the complex cavitation instabilities, rotating cavitation (RC) is considered to make a primary contribution to the disastrous damage to inducers, leading to performance drop, severe vibrations and noise [4][5][6]. One prominent characteristic of RC is the strong interaction between the tip leakage flow and the main flow in the inter-blade flow passages, leading to imbalance in the radial force exerted on the inducer [7,8].…”