Pump | EPFL Graph Search

A pump is a device that moves fluids (liquids or gases), or sometimes slurries, by mechanical action, typically converted from electrical energy into hydraulic energy. Mechanical pumps serve in a wide range of applications such as pumping water from wells, aquarium filtering, pond filtering and aeration, in the car industry for water-cooling and fuel injection, in the energy industry for pumping oil and natural gas or for operating cooling towers and other components of heating, ventilation and air conditioning systems. In the medical industry, pumps are used for biochemical processes in developing and manufacturing medicine, and as artificial replacements for body parts, in particular the artificial heart and penile prosthesis. When a pump contains two or more pump mechanisms with fluid being directed to flow through them in series, it is called a multi-stage pump. Terms such as two-stage or double-stage may be used to specifically describe the number of sta

Hydrodynamics of High Specific Power Pumps for Off-Design Operating Conditions

Stefan Berten

Part load operation of high energy centrifugal pumps is associated with increased vibration levels which may adversely affect the machines operational safety. The main sources of vibrations in centrifugal pumps are of hydrodynamic nature. The interaction between rotating and stationary flow fields yields spatially distributed pressure fluctuation patterns, which excites mechanical vibrations of the rotating and stationary pump components. As virtually all modern, high-energy machines are operated using variable speed drives, an operation, where the excitation by rotor-stator interaction pressure fluctuations matches one of the natural frequencies of the impeller or the pumps stationary components, appears to be unavoidable during the impeller life span. This may yield, provided these vibrations are not sufficiently damped, structural damage to the centrifugal pump components. Moreover, at off-design operation of centrifugal pumps, further phenomena are superposed to the Rotor-Stator interaction effects. Local flow separations affect the flow structure in the hydraulic components, modifying existing and creating supplementary hydraulic forces as excitations for deformations and vibrations at additional frequencies. In the present work, experimental investigations of part-load hydrodynamic phenomena as sources of mechanical excitations in a conventional, high-energy centrifugal pump stage are presented. These investigations include unsteady pressure fluctuation measurements in the rotating and stationary elements of the investigated pump stage, performed at different rotational speeds and operating points. The unsteady pressure measurements were accompanied by measurements of impeller deformations using strain gauges embedded in the impeller shroud wall. The acquired data have been completed by measurements of shaft and bearing housing vibrations. The measurements at part-load operation unveiled the existence of stationary and rotating instabilities in the diffuser of the pump stage, referred as stall. Stationary stall, which expresses itself as a non-rotating high pressure zone in the Rotor-Stator interface, yields additional pressure fluctuations at the rotational frequency and its harmonics. At specific relative flow rates, this high pressure pattern begins to rotate around the impeller circumference. This slowly rotating high pressure zone has a dramatic impact on the mechanical behavior of the pump stage. On the one hand, it forms the highest contribution to the impeller shroud strain, generating deformations several times higher than the ones generated by rotor-stator interaction pressure fluctuations. The perturbations in the circumferential pressure distribution yield further a radial net force, slowly rotating around the impeller circumference. This effect has been identified using the shaft vibration measurements, where it can be shown, that the shaft centerline displacement directly follows the radial force direction. This can negatively affect the rotor system dynamic stability but can also be used to detect rotating stall during the operation of the pump, as external pressure fluctuation measurements not always allow the detection of rotating stall. The impeller blade passage through the stall zone strongly affects the flow in the impeller side rooms. The entrainment of fluid with low circumferential velocity reduces strongly the rotation of the flow in the side rooms and thus affects the steady pressure distribution and the axial net force. The blade passage through the detachment zone adds periodic variations of the axial force acting on the impeller, which can be identified in the axial shaft vibration signature, allowing a detection of stationary stall by a thorough analysis of the axial shaft vibrations.

EPFL2010

Improved methodology for analysis and design of Molten Salt Reactors

Rodrigo Gonzalez Gonzaga De Oliveira

Molten Salt Reactors is a class of advanced reactors that is typically characterised by the presence of a molten inorganic salt of a fissile material as fuel. At the moment, this class of reactors is considered one of the promising options in the long run of nuclear power.In the context of safety assessment of the MSFR, a task on analysis of selected transients was planned by the SAMOFAR project, to which this thesis should contribute. In addition, fluids with high melting temperature bring with them challenges regarding solidification of the fluid and possible flow blockage, to which a method was implemented during this thesis to evaluate the consequences. The open core cavity with curved shape adds another issue where the core is highly turbulent and requires the use of unstructured mesh codes for routine analysis, adding uncertainty and computational burden to the analytical workflow. Using ATARI, the tool developed during this thesis, we:1. Analyse the MSFR together with partners in support of a safety assessment task.2. Consider the impact and how to include solidification/melting in an analytical methodology.3. Evaluate options to manage/reduce uncertainties and the burden of routine use of high-fidelity codes.After a careful derivation of the conservation laws and assembling the code main algorithms, ATARI undergoes verification in a limited scope using the Stefan problem, manufactured solutions and energy balances.The nuclear cavity benchmark was performed between SAMOFAR partners showing equivalency of the codes within the scope of the benchmark. Then the participating institutions perform analysis of the MSFR for the safety assessment task, which ATARI is unable to followup with the transient analysis. The reason for failure is identified as the requirement of appropriate acceleration schemes by OpenFOAM-based multiphysics solvers, and the lack of one in ATARI.In the lack of accurate data regarding the heat exchangers, the impact of solidification is evaluated in a few simplified cases of a pipe with square cross-section and heat exchanger. For different salt compositions and different boundary conditions, the heat exchanger is found to recover from blockage if pump maintains its momentum and some cross-flow is allowed. The recovery of cases with abrupt solidification is questionable due to the required pressure head.The use of flow baffles is investigated as an option to structure the flow inside the reactor and reduce uncertainties originating from turbulence modelling while allowing the use of structured mesh codes for analysis. The concept is tested in a chloride based MSR, which retains its breed and burn mode while showing a quasi-1D flow appropriate for modelling with legacy codes.We recommend that the nuclear cavity benchmark should be expanded to include a more demanding transient case, representative of MSFR transient analysis requirements. A case is made for homogenised and economical models as used in ATARI to enable the practical use of coupling methods that promote the simultaneous solution of equations. A niche for ATARI is found as a special-mission code for analysis of challenging cases with deforming structure and flow field that justifies the use of unstructured meshes and the added computational burden. Due to design uncertainties and their nature, it is recommended to consider solidification implicitly when analysing the MSFR fuel circuit.

EPFL2021

ENHANCED GEOTHERMAL SYSTEMS (EGS) - NUMERICAL PREDICTION OF THE MODE AND LOCATION OF FRACTURE INITIATION

Mohamad Zaarour

Geo-energy is a comprehensive term used to describe any form of energy that comes from the Earth. This includes hydrocarbons such as gas, oil, and coal, but also geothermal energy (shallow and deep). The focus of this thesis is on Enhanced Geothermal Systems (EGS), which are renewable sources of energy. In EGS, hydraulic stimulation is the technique employed to enhance the permeability e.g. of the granite basements at great depths, which might exceed 5 kilometers. Rocks have natural fractures which affect the behavior of the rock mass. The mechanical goal of hydraulic stimulation is to open these pre-existing fractures. Due to the generally great depths, only indirect information such as in-situ stress, pumping records, and micro-seismicity, are available. Many issues in the field of reservoir geothermal (but also petroleum) production, are attributed to the lack of proper understanding of how these fractures interact with each other. The main goal of this study is to provide an insight into how the pre-existing fractures in rocks interact with each other. Specifically, this research answers the question of how pre-existing flaws of different geometrical parameters contained in rock specimens subjected to various loading configurations, affect the mode and the location of the mechanism of crack initiation. We develop a finite element (FE) linearly elastic MATLAB Partial Differential Equation (PDE) code. This numerical model predicts the mode and the location of the initiation of the local fractures at the inner tips of two interacting pre-existing flaws in Barre granite in the context of EGS. We investigate different combinations of double-flaw geometries (flaw inclination angle β, bridging angle α, and ligament length L) contained in specimens subjected to various loading scenarios (Combination of Vertical Load VL, Lateral Load LL, and Internal Flaw Pressure FP) likely to induce first local tensile and shear fractures, and we determine their corresponding predicted locations around the inner flaw tips. For a given double-flaw pair geometry, we subject the specimen to a certain loading configuration (Combination of VL, LL, and FP). We then retrieve the state of stress (maximum and minimum principal stresses 𝜎1 and 𝜎3, and maximum shear stresses 𝜏𝑚𝑎𝑥) at the Finite Element (FE) corner nodes on the boundary of the inner tips, from which we plot the corresponding Mohr’s circles. We study the interaction of these Mohr’s circles with the Griffith-Coulomb failure envelope. Since the main objective is to predict the first local failure’s mechanism and location, we iteratively vary the magnitude of the loading component until the first Mohr’s circle touches the failure envelope. Depending on where it does, the mechanism of the local fracture is inferred. If the maximum principal stress (𝜎1) of this Mohr’s circle is equal to the tensile strength of the granite (𝜎𝑇=7.5𝑀𝑃𝑎), a local tensile fracture is numerically predicted. If this Mohr’s circle touches the linear shear Coulomb portion of the envelope, it is a predicted local shear failure. The corresponding location of the FE node is then determined at the inner tip of the flaw, and this is where for the given geometry and loading scenarios, the first local fracture’s initiation is numerically expected. We follow this procedure for several double-flaw geometrical configurations. We compare the numerical predictions with the previously obtained experimental results in the MIT CEE Rock Mechanics Laboratory. This allows us to validate our set-up numerical model. 4 This study provides a solid ground to the understanding of the initiation and the interaction of pre-existing fractures in EGS. This thesis comes as an answer to a problem of major practical importance in the field of rock mechanics, and beyond. Typical geotechnical problems, such as rock slope stability and tunnel support, can also be better addressed when clearly understanding the process of crack initiation in rocks. This study also enables the correlation between numerical models and experimental findings, a challenge that has always existed in the research world. This thesis shows how our developed FE code can be used as a tool to predict the location and the mode of fractures in the laboratory, but also in the real world.

2020