Unsteady Flow with Cavitation in Viscoelastic Pipes

Abstract

The current paper focuses on the analysis of transient cavitating flow in pressurised polyethylene pipes, which are characterized by viscoelastic rheological behaviour. A hydraulic transient solver that describes fluid transients in plastic pipes has been developed. This solver incorporates the description of dynamic effects related to the energy dissipation (unsteady friction), the rheological mechanical behaviour of the viscoelastic pipe and the cavitating pipe flow. The Discrete Vapour Cavity Model (DVCM) and the Discrete Gas Cavity Model (DGCM) have been used to describe transient cavitating flow. Such models assume that discrete air cavities are formed in fixed sections of the pipeline and consider a constant wave speed in pipe reaches between these cavities. The cavity dimension (and pressure) is allowed to grow and collapse according to the mass conservation principle. An extensive experimental programme has been carried out in an experimental set-up composed of high-density polyethylene (HDPE) pipes, assembled at Instituto Superior Técnico of Lisbon, Portugal. The experimental facility is composed of a single pipeline with a total length of 203 m and inner diameter of 44 mm. The creep function of HDPE pipes was determined by using an inverse model based on transient pressure data collected during experimental runs without cavitating flow. Transient tests were carried out by the fast closure of the ball valves located at downstream end of the pipeline for the non-cavitating flow and at upstream for the cavitating flow. Once the rheological behaviour of HDPE pipes were known, computational simulations have been run in order to describe the hydraulic behaviour of the system for the cavitating pipe flow. The calibrated transient solver is capable of accurately describing the attenuation, dispersion and shape of observed transient pressures. The effects related to the viscoelasticity of HDPE pipes and to the occurrence of vapour pressures during the transient event are discussed.

Fluid–structure interaction with pipe-wall viscoelasticity during water hammer

“…With the inclusion of two or more of these items in the analysis, eleven possibilities are offered from which some combinations already have been studied and some have not. The combinations of VE and UF (Covas et al, 2004a(Covas et al, , b, 2005Soares et al, 2008;Duan et al, 2010), CS and UF (Bughazem and Anderson, 2000;Bergant et al, 2008a, b), FSI and UF (Elansary et al, 1994), FSI and CS (Fan and Tijsseling, 1992;Tijsseling and Vardy, 2005) and VE and CS (Soares et al, 2009;Hadj-Taïeb and Hadj-Taïeb, 2009 ;Keramat et al, 2010) have already been investigated. The remaining combination of two, namely FSI and VE, is the scope of this article.…”

How severe can transients be after a sudden depressurization?

“…The head at the upstream valve drops to the cavitation pressure three times, each of decreasing duration, after which the head oscillates around zero approximately 10–12 times before disappearing after about 25 s. Sensor C shows an upsurge and then repeated oscillations down to almost the cavitation pressure. The damping process produces waves, first of triangular form, and then to what appears to be nearly sinusoidal form, typical of visco‐elastic effects of the medium‐density polyethylene pipes used in the experimental rig (Soares et al, 2008; Covas et al, 2005a, 2005b).…”

Numerical Methods of Visco-elastic Segments on Water Hammer Pressures

“…According to the modeling method, creep functions of a viscoelastic pipe are calibrated by conducting an unsteady flow test. The forgoing method was later revised by Soares [3]. In a comprehensive research Keramat [4] assessed numerical and experimental impacts concerning separation of the liquid column of viscoelastic pipes.…”