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).

Hydropower has stood as a clean and sustainable energy source since the late 19th century. Many turbines were built 50 to 70 years ago and require refurbishment. It is important to assess the efficiency of turbines before and after refurbishment to meet performance guarantees .However, the flow rate makes such estimation challenging. Moreover, determining the volumetric flow rate is crucial to specify the hydraulic performance characteristics of hydraulic turbines. The pressure-time method allows measuring the flow rate in hydraulic turbines, according to the IEC 60041 standard, based on transforming momentum into pressure during the deceleration of a liquid mass. The flow rate is obtained by integrating the differential pressure and the pressure loss history between two cross-sections.

This method assumes a one-dimensional flow (1D) and is limited to straight pipes with a uniform cross-section and specific restrictions on length (L>10 m), velocity (U.L>50 m2s-1) and distance between the measurement sections from any irregularities in the pipeline. However, challenges arise when applying this method in low-head hydropower plants due to the short lengths, irregularities like bends, variation in cross section and developing flows in the intake. This thesis aims to improve the performance of the method out of IEC standards for conditions similar to low-head conditions.

The thesis is divided into the numerical simulation of the fluid during the pressure-time method transient, experimental measurement, and a combination of both. The physics in the pressure-time method is studied to compare different assumptions to estimate the viscous losses for both developed and developing flow. Moreover, a test rig has been developed to extend the method’s applicability. The test rig is designed to study the pressure-time method for developing flow conditions, small measurement lengths, variable cross-section and the presence of bend close to measurement sections, which could be similar to low-head turbine conditions.

Finally, the data are evaluated using the new approach combining the 1D pressure-time method and three-dimensional computational fluid dynamics (3D CFD).