Study of the influence of gas inclusions on non-stationary processes during hydraulic transients in pipelines

Abstract

Introduction. The relevance of the work is determined by the need to improve the accuracy of water-hammer prediction in pipelines where local gas cavities are present and dissolved gas release may occur under pressure reduction. The gas phase changes the local compressibility of the medium, the structure of the wave process, and the conditions for the formation of low-pressure zones. Therefore, its consideration is important for assessing dynamic loads and cavitation risks. The purpose of the research: to develop and numerically implement a model of a transient process in a pressurised pipeline with a discrete gas cavity and a threshold mechanism of dissolved gas release. Research methods and materials. The study is based on one-dimensional equations of unsteady liquid flow transformed by the method of characteristics. The computational scheme successively includes a single-phase reference model, a discrete gas-cavity model, and a coupled model taking into account gas inventory growth. The numerical implementation was carried out in MATLAB. The analysis used the parameters of a typical pressurised pipeline and a one-factor scanning procedure for the saturation head, valve closure time, gas release coefficient, initial volume fraction of the gas cavity, and its relative position. Research results. The single-phase model was verified using the Joukowsky estimate; the relative deviation of the first pressure-head peak was about 1.37 %. It is shown that a single gas cavity under weak gas-content conditions has almost no effect on the first global pressure peak, but forms a pronounced local response associated with cavity-volume variation and discharge imbalance. Discussion of research results. It was found that the saturation head determines the activation threshold of gas release, the valve closure time controls the depth of the local pressure decrease, the gas release coefficient affects the growth rate of the gas inventory, and the gas-cavity position determines the spatial sensitivity of the system. Conclusion. The proposed model describes the causal chain “pressure decrease – gas release – gascavity expansion – local feedback”. Conclusions. The results can be used to analyse gas-containing transient processes, identify sensitive pipeline sections, and select rational valve-closure regimes.