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Numerical Investigation of the Endpoint of Integration and Headloss in the Pressure-Time Method
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.ORCID iD: 0000-0003-2746-1416
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.ORCID iD: 0000-0001-7599-0895
(English)In: Flow Measurement and Instrumentation, ISSN 0955-5986Article in journal (Other academic) Submitted
Abstract [en]

Hydraulic efficiency is a crucial parameter in estimating the performance of hydraulic turbines. However, the flow rate makes such estimation challenging. Several methods have been developed over the years to measure the flow rate. The pressure-time method is an accurate and inexpensive alternative for flow rate estimation, based on transforming momentum into pressure during the deceleration of a liquid mass. The steady-state flow rate is obtained by integrating the differential pressure and the pressure losses history between two cross-sections. 

Two challenges in the pressure-time method application are identified to substantially influence the results:  the endpoint of integration and estimation of the head loss due to friction. In the present work, three-dimensional (3D) computational fluid dynamics (CFD) analyses are performed to investigate these challenges in detail. The valve movement is modelled with an immersed solid method, which is less expensive and more stable than the dynamic mesh method applied in previous CFD studies. The numerical results are validated with experimental data and compared with the dynamic mesh method.

Three different methods for defining the endpoint of integration are compared, and a new methodology is proposed to eliminate any related uncertainty. Furthermore, the losses are investigated and compared to the assumptions of constant, quasi-steady and unsteady friction factors. The calculated flow rate is not found to be precisely related to the initial pressure drop. Therefore, a friction factor correction coefficient is implemented, decreasing the error. 

Keywords [en]
Pressure-time method, head loss calculation, endpoint determination, CFD
National Category
Fluid Mechanics
Identifiers
URN: urn:nbn:se:ltu:diva-92512OAI: oai:DiVA.org:ltu-92512DiVA, id: diva2:1687903
Available from: 2022-08-17 Created: 2022-08-17 Last updated: 2025-02-09
In thesis
1. Development of the pressure-time method: final integration point and head losses
Open this publication in new window or tab >>Development of the pressure-time method: final integration point and head losses
2022 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Hydropower is an efficient renewable energy source able to regulate electrical grid fluctuations. However, many hydropower plants were built decades ago, and now it is the time for a major refurbishment. The turbine's efficiency is essential and needs to be determined before and after refurbishment. To this end, the flow rate needs to be determined. Amongst different discharge measurement methods, the pressure-time method is relatively inexpensive and easy to perform compared to other methods. In this method, the flow rate is estimated by the integration of the measured differential pressure and the pressure loss due to friction between two cross-sections in a conduit during the deceleration of the liquid mass by closing a valve or guide vanes. The pressure-time method's accuracy depends on how accurate the head loss and the integration endpoint are estimated. Furthermore, the pressure-time method has limitations specified in IEC-60041, which make it challenging to apply on low-head turbines due to the short water passages when the flow is developing.

The main focus of work is to improve the accuracy of the pressure-time method and extend its validity for low-head turbine conditions. Numerical simulation and experimental study have been acquired. A CFD model is developed to investigate the effects of the endpoint of integration and friction models on the method's accuracy. The effect of different boundary conditions is studied in the CFD model, and the result is validated with available experimental data. Different frictional models used with the pressure-time method are compared with CFD simulation for the developing and developed flows. A new parameter is suggested to improve deviation related to the flow status; developing and developed. Furthermore, a new methodology is presented, where the flow rate is estimated with the pressure-time method function of several endpoints.

Then, experimental investigations of the pressure-time method outside IEC-60041 recommendations for conditions similar to low-head hydropower are presented. A laboratory setup is designed and built to test the pressure-time method. The method is applied for cases with shorter length, smaller UxL, pipe with variable cross-section and shorter distance to irregularity than IEC-60041 recommendations. Different assumptions for calculating the pressure loss and dynamic pressure variation are studied. Moreover, the quasi-steady assumption's accuracy on the head loss estimation and the difference in dynamic pressure are compared with constant values for their coefficients. The systematic uncertainty of the pressure-time method is also calculated based on the Monte Carlo Method (MCM).

Place, publisher, year, edition, pages
Luleå University of Technology, 2022
Series
Licentiate thesis / Luleå University of Technology, ISSN 1402-1757
Keywords
Pressure-time method, head loss calculation, endpoint determination
National Category
Fluid Mechanics
Research subject
Fluid Mechanics
Identifiers
urn:nbn:se:ltu:diva-92515 (URN)978-91-8048-127-4 (ISBN)978-91-8048-128-1 (ISBN)
Presentation
2022-10-18, F1031, Luleå tekniska universitet, Luleå, 15:00 (English)
Opponent
Supervisors
Available from: 2022-08-17 Created: 2022-08-17 Last updated: 2025-02-09Bibliographically approved

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Kalantar Neyestanaki, MehrdadJonsson, PontusCervantes, Michel

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