| 1. |
- Cervantes, Michel, et al.
(författare)
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Reformulation of the pressure-time method for application without flow rate cut-off
- 2022
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Ingår i: Measurement. - : Elsevier. - 0263-2241 .- 1873-412X. ; 188
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Tidskriftsartikel (refereegranskat)abstract
- The paper presents a reformulation of the standard pressure-time method for flow rate measurement in closed conduits. According to the IEC 60041 standard, the method is used with turbines flow rate cut-off. Its current formulation requires complete closure of the turbine shut-off device, generally guide vanes for reaction turbines and valves for impulse turbines, which may cause wear and tear on the machine. Any leakage through the closed shut-off device also needs to be measured. In the present work, a new formulation of the pressure-time method is derived, extending the actual use. The newly formulated method allows the determination of the flow rate at any instant of time from a load variation and, thus, the initial, incremental, and final flow rate. The load variation may be positive or negative. Measurements of any leakage flow are eliminated. Numerical and experimental cases are used to assess the validity and applicability of the proposed formulation. The numerical results demonstrate the correctness of the derivation. The experimental results exemplify the applicability of the method. Limitations of the derived methodology are discussed.
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| 2. |
- Kranenbarg, Jelle, 1996-, et al.
(författare)
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Modeling an axial turbine: a parametric study
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Numerical simulations of axial hydraulic turbines away from the best efficiency point are challenging. Especially previous studies show difficulties predicting the tangential velocity at part-load operating conditions where the swirl is high. Moreover, there is a lack of knowledge about the ability of eddy viscosity turbulence models to predict the runner inter-blades flow. Therefore, a parametric study is performed to address these points. Specifically, the effects of the blade clearance, blade angle, flow rate, and different turbulence models are studied on these issues. Results are analyzed by comparing the predicted axial and tangential velocity profiles and torque to experimentally obtained values. In addition, the physical phenomena responsible for head losses are studied in detail. Results show that the model can predict the flow relatively well at optimal flow conditions with low swirl but has problems at part load; the tangential velocity between the runner blades is underestimated by ~20%. The undervalued head losses are the root cause. They result in an overestimation of the axial velocity and an overestimation of the torque which is connected to the runner extracting too much swirl from the flow, hence the low tangential velocity. Furthermore, the blade clearance is found to be important as high-velocity jets that originate from the clearances affect the flow field, especially close to the hub. The torque prediction is also affected. Therefore, the clearance should be carefully measured during an experimental campaign, especially at the leading and trailing edge. A small modeling error of 0.5° in the blade angle and a change of 3% in the flow rate significantly affect the tangential velocity and torque prediction. The studied parameters must be considered carefully when building a numerical model. However, part of the deviation can also be explained by the insensitivity of eddy viscosity models to anisotropy, swirling flow with streamline curvature and runner mesh density.
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| 3. |
- Kranenbarg, Jelle, et al.
(författare)
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Sensitivity analysis of a swirling flow to the GEKO model
- 2022
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Ingår i: 31st IAHR Symposium on Hydraulic Machinery and Systems 26/06/2022 - 01/07/2022 Trondheim, Norway. - : Institute of Physics Publishing (IOPP).
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Konferensbidrag (refereegranskat)abstract
- Accurate numerical models for hydraulic turbine applications are highly coveted. They must be able to correctly capture the swirling flow found at off-design operating conditions in the turbine draft tube. The GEKO model is a relatively fast and flexible eddy viscosity turbulence model with adjustable coefficients to tune the model to different flow scenarios. In this study, the GEKO model is tested on a swirling flow inside a diffusor similar to the flow conditions found at part-load operation of a propeller turbine. The diffusor investigated corresponds to the Porjus U9 draft tube cone section, including the runner cone. Results showed that the near-wall coefficient, with a value of 2, increased the wall shear stress and moved the separation point from the runner cone further downstream. Moreover, with a value of 0.7, the separation coefficient increased the eddy viscosity, which also moved the separation point from the runner cone further downstream. Both coefficients gave velocity profiles closer to experimental values and increased the swirl number at the outlet of the diffusor by up to 36.9 % compared to the GEKO default model. Overall, the near-wall coefficient with a value of 2 gave the best results. The GEKO model provides an opportunity to tweak numerical models to swirling flow.
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| 4. |
- Kranenbarg, Jelle, et al.
(författare)
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Uncertainty in the numerical prediction of the tangential velocity in axial turbines at part load operations: A parametric study
- 2023
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Ingår i: Energy Reports. - : Elsevier. - 2352-4847. ; 10, s. 2597-2611
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Tidskriftsartikel (refereegranskat)abstract
- Numerical simulations of axial hydraulic turbines away from the best efficiency point are challenging. Previous studies especially show difficulties predicting the tangential velocity at Part Load (PL) operating conditions, where the swirl is high, in comparison to experiments. This is a reoccurring problem, and it is essential to understand, as the high tangential velocity is a fundamental characteristic of the flow in hydraulic turbines and is directly related to the swirling flow stability and the turbine's power output. The objective of this study is to numerically investigate and understand the origin of the tangential velocity deviation from experimental results by performing simulations with the finite volume method of an axial turbine operated at PL. A parametric study is performed to address the abovementioned. Specifically, the effects of the blade clearance, blade angle, flow rate, and different turbulence models are studied on this issue. Results are analyzed by comparing the predicted axial and tangential velocity profiles and torque to experimentally obtained values. Primarily the runner inter-blades flow is studied as there is a knowledge gap. In addition, the physical phenomena responsible for head losses are studied in detail. Results show that the model can predict the flow relatively well at optimal flow conditions with low swirl but has problems at part load; the tangential velocity between the runner blades is underestimated by ∼20%. The undervalued head losses are the root cause. They result in an overestimated torque and an underestimated tangential velocity as the runner extracts too much energy from the fluid. A small modeling error of 0.5° in the blade angle and a change of 3% in the flow rate significantly affect the tangential velocity and torque prediction. The studied parameters must be considered carefully when building a numerical model.
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| 5. |
- Abazov, V. M., et al.
(författare)
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Measurement of the ratios of the Z/gamma*+ >= n jet production cross sections to the total inclusive Z/gamma* cross section in p(p)over-bar collisions at root s=1.96 TeV
- 2008
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Ingår i: Physics Letters B. - : Elsevier BV. - 0370-2693 .- 1873-2445. ; 658, s. 112-
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Tidskriftsartikel (refereegranskat)abstract
- We present a study of events with Z bosons and associated jets produced at the Fermilab Tevatron collider in p p collisions at a center of mass energy of 1.96 TeV The data sample consists of nearly 14000 Z/gamma* -> e(+)e(-) candidates corresponding to an integrated luminosity of 0.4 fb(-1) collected with the D circle divide detector. Ratios of the Z/gamma*+ >= n jet cross sections to the total inclusive Z/gamma* cross section have been measured for n = 1-4 jets, and found to be in good agreement with a next-to-leading order QCD calculation and with a tree-level QCD prediction with parton shower simulation and hadronization.
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| 6. |
- Abazov, V. M., et al.
(författare)
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Measurement of the t(t)over-bar production cross section in p(p)over-bar collisions at root s=1.96 TeV
- 2008
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 100:19:100, s. 192004-
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Tidskriftsartikel (refereegranskat)abstract
- We measure the t (t) over bar production cross section in p (p) over bar collisions at root s = 1.96 TeV in the lepton + jets channel. Two complementary methods discriminate between signal and background: b tagging and a kinematic likelihood discriminant. Based on 0.9 fb(-1) of data collected by the D0 detector at the Fermilab Tevatron Collider, we measure sigma(t (t) over bar) = 7.62 +/- 0.85 pb, assuming the current world average m(t) = 172.6 GeV. We compare our cross section measurement with theory predictions to determine a value for the top-quark mass of 170 +/- 7 GeV.
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| 7. |
- Abazov, V. M., et al.
(författare)
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Observation of the B-c meson in the exclusive decay B-c -> J/psi pi
- 2008
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 101:1, s. 012001-:101, s. 012001-
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Tidskriftsartikel (refereegranskat)abstract
- A fully reconstructed B-c -> J/psi pi signal is observed with the D0 detector at the Fermilab Tevatron p (p) over bar collider using 1.3 fb(-1) of integrated luminosity. The signal consists of 54 +/- 12 candidates with a significance that exceeds 5 standard deviations, and confirms earlier observations of this decay. The measured mass of the B-c meson is 6300 +/- 14(stat)+/- 5(syst) MeV/c(2).
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| 8. |
- Abazov, V. M., et al.
(författare)
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Search for W ' boson resonances decaying to a top quark and a bottom quark
- 2008
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 100:21, s. 211803-:100, s. 211803-
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Tidskriftsartikel (refereegranskat)abstract
- We search for the production of a heavy W ' gauge boson that decays to third generation quarks in 0: 9 fb(-1) of p (p) over bar collisions at root s = 1: 96 TeV, collected with the D0 detector at the Fermilab Tevatron collider. We find no significant excess in the final-state invariant mass distribution and set upper limits on the production cross section times branching fraction. For a left-handed W ' boson with SM couplings, we set a lower mass limit of 731 GeV. For right-handed W ' bosons, we set lower mass limits of 739 GeV if the W ' boson decays to both leptons and quarks and 768 GeV if the W ' boson decays only to quarks. We also set limits on the coupling of the W ' boson to fermions as a function of its mass.
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| 9. |
- Abazov, V. M., et al.
(författare)
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Search for W ' bosons decaying to an electron and a neutrino with the D0 detector
- 2008
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 100:3, s. 031804-:100, s. 031804-
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Tidskriftsartikel (refereegranskat)abstract
- This Letter describes the search for a new heavy charged gauge boson W-' decaying into an electron and a neutrino. The data were collected with the D0 detector at the Fermilab Tevatron p (p) over bar Collider at root s=1.96 TeV, and correspond to an integrated luminosity of about 1 fb(-1). Lacking any significant excess in the data in comparison with known processes, an upper limit is set on sigma(')(W)xB(W-'-> e nu), and a W-' boson with mass below 1.00 TeV can be excluded at the 95% C.L., assuming standard-model-like couplings to fermions. This result significantly improves upon previous limits and is the most stringent to date.
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| 10. |
- Abazov, V. M., et al.
(författare)
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Search for ZZ and z gamma* production in p(p)over-bar collisions at root s=1.96 TeV and limits on anomalous ZZZ and ZZ gamma* couplings
- 2008
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 100:13, s. 131801-:100, s. 131801-
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Tidskriftsartikel (refereegranskat)abstract
- We present a study of mu mu mu mu, eeee, and mu mu ee events using 1 fb(-1) of data collected with the D0 detector at the Fermilab Tevatron p (p) over bar Collider at root s = 1.96 TeV. Requiring the lepton pair masses to be greater than 30 GeV, we observe one event, consistent with the expected background of 0.13 +/- 0.03 events and with the predicted standard model ZZ and Z gamma(*) production of 1.71 +/- 0.15 events. We set an upper limit on the ZZ and Z gamma(*) cross section of 4.4 pb at the 95% C.L. We also derive limits on anomalous neutral trilinear ZZZ and ZZ gamma(*) gauge couplings. The one-parameter 95% C.L. coupling limits with a form-factor scale Lambda = 1.2 TeV are -0.28 < f(40)(Z)< 0.28, -0.31 < f(50)(Z)< 0.29, -0.26 < f(40)(gamma)< 0.26, and -0.30 < f(50)(gamma)< 0.28.
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