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Illustration of the experimental test rig [6]. The flow is driven by gravity from the upstream tank 2 O to the downstream tank 6 O, while being counteracted by the losses in the components of the system. The downstream valve 5 O is closed and opened

Illustration of the experimental test rig [6]. The flow is driven by gravity from the upstream tank 2 O to the downstream tank 6 O, while being counteracted by the losses in the components of the system. The downstream valve 5 O is closed and opened

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Article
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Despite the increase in computational power of HPC clusters, it is in most cases not possible to include the entire hydraulic system when doing detailed numerical studies of the flow in one of the components in the system. The numerical models are still most often constrained to a small part of the system and the boundary conditions may in many cas...

Contexts in source publication

Context 1
... is because the average velocity, u avg , is the summation of the volumetric face fluxes divided by the patch area, hence a small volumetric flow equals a small error. Term VI is the velocity far from the patch and it is calculated from Eq. (13). The variable UFarScale_ in Listing 3 is the relation in hydraulic diameter of the patch, d h,p , and far, d h,Far , to the power of four, (d h,p /d h,Far ) 4 . ...
Context 2
... flow in the system is thus driven by the head difference between the upstream and downstream tanks, and counteracted by the losses in the components of the system. A transient sequence was studied experimentally, where To validate the headLossPressure boundary condition a small 1D computational domain is placed in the rectangular pipe between markers 4 O and 5 O in Figure 3. The computational domain is shown in Figure 4. Being a 1D computational domain, it has no losses and does therefore not contribute to the system balance. ...
Context 3
... valve opening curve in Figure 5 (left) has a smooth transition at times 10 s (small upper right plot) and at 18 s (small lower right plot) for numerical stability, which yields a corresponding smooth transition for the minor loss variation. The discharge into the upstream tank ( 2 O in Figure 3) remained constant at 50 l/s during the entire sequence. Thus, the free surface in the upstream tank was rising during the time that the downstream valve was not fully open. ...
Context 4
... is because the atmospheric pressure is the same at the free surfaces of the upstream and downstream tanks, and hence just a reference pressure. The test rig shown in Figure 3 indicates no water levels. However, the generic system shown in Figure 1 has a similar layout as the experimental test rig. ...
Context 5
... the inlet, Listing 4, the head HFar has a value of 3 m, as this is the head of the upstream tank, 2 O in Figure 3, when the system initially is in balance. A flowRate of 50 × 10 −3 m 3 /s and a free surface area Ar of 1.27 m 2 is specified. ...
Context 6
... are two minor loss factors, minorLossFactors, supplied at the inlet, both with the hydraulic diameter of 0.222 m. The first entry, expData, has a minor loss coefficient of 45.9 and includes the bend and the upstream valve, 3 O in Figure 3. It was reported from the lab that the upstream valve was partly closed, hence the large value of the expData minor loss coefficient. ...
Context 7
... the outlet, Listing 5, the same atmospheric pressure pFar is used as for the inlet. The head of the downstream tank, 6 O in Figure 3, was at the experimental tests 0.5 m, and the same value is thus given as the head HFar in the numerical simulation. As mentioned, the same hydraulic diameter dP is used everywhere. ...

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Citations

... The flow rate is thus part of the solution. The inlet and outlet boundary conditions for pressure is handled with the novel headLossPressure boundary condition developed by Fahlbeck et al. [21]. This special boundary condition allows the user to specify the head of the system and it also considers head losses up-and downstream of the computational domain. ...
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A larger part of the electricity is today from intermittent renewable sources of energy. However, the energy production from such sources varies in time. Energy storage is one solution to compensate for this variation. Today pumped hydro storage (PHS) is the most common form of energy storage. Usually, it requires a large head, which limits where it can be built. In the EU project ALPHEUS, PHS technologies for low- to ultra-low heads are explored. One of the concepts is a contra-rotating pump-turbine (CRPT). The behaviour of this design at time-varying load conditions is today scarce. In the present work, the impact of the startup time for a CRPT is analysed through computational fluid dynamics (CFD) simulations. The analysis includes a comparison between a coarse and a fine CFD model. The coarse model produces acceptable results and is 50 times cheaper, this model is thus used to assess the startup time. It is found that longer startup times generate lesser loads and peak values. A startup time of 10 s may be a sufficient alternative as the peak loads are heavily reduced compared to faster startups. Furthermore, there is not much difference between a startup time of 20–30 s.