Change search
CiteExportLink to record
Permanent link

Direct link
Cite
Citation style
  • apa
  • harvard1
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf
Numerical Investigation of the Pressure-Time Method
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.ORCID iD: 0000-0001-7599-0895
Center of Excellence in Design and Optimization of Energy Systems (CEDOES), School of Mechanical Engineering, College of Engineering, University of Tehran.
Center of Excellence in Design and Optimization of Energy Systems (CEDOES), School of Mechanical Engineering, College of Engineering, University of Tehran.
2017 (English)In: Flow Measurement and Instrumentation, ISSN 0955-5986, E-ISSN 1873-6998, Vol. 55, p. 44-58Article in journal (Refereed) Published
Abstract [en]

The pressure-time method is a discharge measurement technique commonly used to estimate the flow rate in hydro-power plants to assess the turbine hydraulic efficiency. In this paper two-dimensional Reynolds-averaged Navier-Stokes equations with the low-Reynolds k-ω SST turbulence model are used to model and evaluate the pressure-time method. The flow rate decrease is modeled by valve closure and use of the dynamic mesh technique. The results are compared with experimental data showing the close trend. Moreover, the affecting parameters in the pressure-time method including compressibility, calculation of the losses and finding the upper time limit of integration of the pressure-time diagram are studied in detail. Furthermore, the flow characteristics with both compressible and incompressible assumptions are investigated. The obtained results demonstrated the compressibility effect even before the complete valve closure. The physics of the flow, in the closed conduit in the pressure-time method, before the complete valve closure is studied based on the time variation of the wall shear stress and the γγ parameter. The γγ parameter represents the difference between the turbulence structure in a transient accelerating or decelerating flow and the one in the quasi-steady condition. It is demonstrated that for the pressure-time method, some part of the flow decrease excursion can be characterized as quasi-steady and the rest is unsteady. The dominance of inertia and turbulence dynamics is investigated to evaluate the wall shear stress in the part of the excursion with the unsteady assumption. It is found that the inertia has a dominant effect during the excursion. The flow rate is calculated by evaluating all the terms of losses, e.g., wall shear stress, normal stresses, Reynolds stresses and density variation. Based on the relative magnitude of each term the results indicate that the wall shear stress is a good approximation of the losses for the flow rate calculation and the other terms can be neglected. Due to the primary role of losses in the flow rate calculation other methods in losses calculation are also considered for comparison (e.g., quasi-steady friction or steady-state friction factor). The available methods to find the upper time limit of the flow rate approximation integral still have significant errors. The flow rate reduction is calculated at each time step before complete valve closure using CFD methods.

Place, publisher, year, edition, pages
Elsevier, 2017. Vol. 55, p. 44-58
National Category
Fluid Mechanics and Acoustics
Research subject
Fluid Mechanics
Identifiers
URN: urn:nbn:se:ltu:diva-63250DOI: 10.1016/j.flowmeasinst.2017.05.003ISI: 000403861200005Scopus ID: 2-s2.0-85019139193OAI: oai:DiVA.org:ltu-63250DiVA, id: diva2:1093200
Note

Validerad; 2017; Nivå 2; 2017-05-16 (andbra)

Available from: 2017-05-05 Created: 2017-05-05 Last updated: 2019-01-11Bibliographically approved
In thesis
1. Development of the Pressure-Time Method for Flow Rate Measurement in Hydropower Plants by Numerical Simulation
Open this publication in new window or tab >>Development of the Pressure-Time Method for Flow Rate Measurement in Hydropower Plants by Numerical Simulation
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Hydropower is a clean and sustainable energy resource developed since the late 19th century. To specify the hydraulic performance characteristics of hydraulic turbines, the volumetric flow rate as one of the few basic quantities should be determined. Discharge represents the most difficult quantity to measure. A good measurement accuracy and estimation is difficult to estimate compared to the power and head, especially in low head machines. Despite the developments in discharge measurement techniques, this part of the hydraulic machine performance tests is often a major challenge. The pressure-time method is one of the discharge measurement techniques, which is studied in this PhD thesis. Most of the researches, to improve the accuracy of this method, are performed experimentally, whilst limited one-dimensional numerical simulations are done on this method. Therefore, detailed investigation of this method has been difficult. The studies conducted in this thesis are divided in two experimental and numerical parts. Because the flow physics in the pressure-time method is a combination of decelerating flow with variable rate and water hammer phenomenon, at the first experimental studies are performed considering unsteady constant rate decelerating and accelerating flows. The results helped to better understanding the studies in the second part concerned with numerical simulations. In the second part, the physical phenomenon behind the water hammer and constant rate decelerating and accelerating flows is studied. Then the physical characteristics of the flow in the pressure-time method is investigated in detail based on the time variation of the wall shear stress and the γ parameter. The γ parameter represents the difference between the turbulence structure in a transient accelerating or decelerating flow and the one in the quasi-steady condition. It is demonstrated that for the pressure-time method, part of the flow decrease excursion can be characterized as quasi-steady and the rest is unsteady. The dominance of inertia and turbulence dynamics is investigated to evaluate the wall shear stress in the part of the excursion with the unsteady assumption. It is found that the inertia has a dominant effect during the excursion. The evaluation of the effective forces in the flow rate calculation in a straight pipe showed that the wall shear stress is a good approximation of the losses calculation and the other terms can be neglected. To extend the applicability of this method outside the limitations of the IEC41 standard, variable pipe cross-section and different friction loss calculation are also studied. A new method for the loss calculation in the penstocks with variable cross section is proposed.  The errors induced by the proposed method are in an acceptable range provided that the contraction angle is less than ϴ=10°. The evaluation of the important forces showed that the variation of the momentum flux is the most significant term in the flow rate estimation in a pipe with a contraction. Then, the wall shear stress is the second most significant. 

Place, publisher, year, edition, pages
Luleå: Luleå University of Technology, 2019
Series
Doctoral thesis / Luleå University of Technology 1 jan 1997 → …, ISSN 1402-1544
National Category
Fluid Mechanics and Acoustics
Research subject
Fluid Mechanics
Identifiers
urn:nbn:se:ltu:diva-72001 (URN)978-91-7790-284-3 (ISBN)978-91-7790-285-0 (ISBN)
Public defence
2019-02-01, Lulea university of Technology, Lulea, 09:00 (English)
Opponent
Supervisors
Available from: 2018-12-12 Created: 2018-12-11 Last updated: 2019-03-15Bibliographically approved

Open Access in DiVA

No full text in DiVA

Other links

Publisher's full textScopus

Authority records BETA

Saemi, SimindokhtCervantes, Michel

Search in DiVA

By author/editor
Saemi, SimindokhtCervantes, Michel
By organisation
Fluid and Experimental Mechanics
In the same journal
Flow Measurement and Instrumentation
Fluid Mechanics and Acoustics

Search outside of DiVA

GoogleGoogle Scholar

doi
urn-nbn

Altmetric score

doi
urn-nbn
Total: 53 hits
CiteExportLink to record
Permanent link

Direct link
Cite
Citation style
  • apa
  • harvard1
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf