Numerical simulation of unsteady flow in hydraulic turbomachines

Turbines and pumps dealing with incompressible flow are examples of hydraulic turbomachines. In most cases the flow is highly turbulent and time-dependent, caused by the rotation of the impeller in a stationary casing. The geometry, with doubly curved surfaces, adds even more to the complexity. It all leads to a flow which is difficult to model. Yet, to optimize turbomachines it is necessary to analyze the flow in detail. Flow simulations using Computational Fluid Dynamics (CFD) can be a very helpful tool. The software solves the discretized partial differential equations for mass and momentum conservation on a grid that covers the flow domain. Two basic discretization schemes can be distinguished: collocated and staggered. When a collocated scheme is used, the solution suffers from odd-even decoupling. In practice this is suppressed with artificial measures which either decrease the accuracy of the simulation or increase the calculation time for an unsteady incompressible flow. Using a staggered scheme, accurate discretization is more difficult, but odd-even decoupling is avoided. In this thesis a CFD code is developed which is based on a staggered, blockstructured grid scheme. It is suited for the calculation of time-dependent fluid motion in turbomachines. The CFD code, named DEFT, is originally developed by the group ofWesseling at Delft University of Technology. The first extension in the current work was an interpolation procedure implemented to handle non-matching grids for more flexibility in grid generation. Furthermore, a sliding interface to connect the rotating grid in the impeller and the stationary grid was developed. Coriolis and centrifugal forces for calculations in the rotating frame of reference, were mplemented in two ways: using a conservative formulation and using source terms. An adaptation of the pressure equation proved necessary to reduce calculation time for computations involving a sliding interface. Although the conceptual ideas behind these extensions are applicable in 3D, they have been implemented in 2D and verified with the simulation of a number of relatively simple flows. DeFT was validated with the simulation of the flow through a cascade of blades which is a model of an axial-flow pump. The blade surface pressure and the total force on the blade are calculated. There is good agreement between values calculated with DeFT, Fluent, values from experiments, and other CFD calculations obtained from literature. The flow through a centrifugal pump with a vaned diffusor is simulated using the staggered discretization in DeFT and the collocated discretization in Fluent. The calculated time-averaged pressure and velocity along the pitch of a rotor channel show good correspondence. The agreement with results from experiments and other CFD calculations obtained from literature is more qualitative. The calculation time needed by DeFT and Fluent is approximately equal, despite the use of a large number of blocks in DeFT and its lack of a convergence enhancing multi-grid method which is used by Fluent

Numerical analysis of pressure fluctuation in a multiphase rotodynamic pump with air–water two-phase flow

International audiencePressure fluctuation in single-phase pumps has been studied widely, while less attention has been paid to research on multiphase pumps that are commonly used in the petroleum chemical industry. Therefore, this study investigates the pressure fluctuation for a multiphase rotodynamic pump handling air–water two-phase flow. Simulations based on the Euler two-fluid model were carried out using ANSYS_CFX16.0 at different Inlet Gas Void Fractions (IGVFs) and various flow rate values. Under conditions of IGVF = 0% (pure water) and IGVF = 15%, the accuracy of the numerical method was tested by comparing the experimental data. The results showed that the rotor–stator interaction was still the main generation driver of pressure fluctuation in gas–liquid two-phase pumps. However, the fluctuation near the impeller outlet ascribe to the rotor–stator interaction was weakened by the complex gas–liquid flow. For the different IGVF, the variation trend of fluctuation was similar along the streamwise direction. That is, the fluctuation in the impeller increased before decreasing, while in the guide vane it decreased gradually. Also, the fluctuation in the guide vane was generally greater than for the impeller and the maximum amplitude appeared in the vicinity of guide vane inlet

Multiple Heat Exchanger Cooling System for Automotive Applications – Design, Mathematical Modeling, and Experimental Observations

The design of the automotive cooling systems has slowly evolved from engine-driven mechanical to computer-controlled electro-mechanical components. With the addition of computer-controlled variable speed actuators, cooling system architectures have been updated to maximize performance and efficiency. By switching from one large radiator to multiple smaller radiators with individual flow control valves, the heat rejection requirements may be precisely adjusted. The combination of computer regulated thermal management system should reduce power consumption while satisfying temperature control objectives. This research focuses on developing and analyzing a multi-radiator system architecture for implementation in ground transportation applications. The premise is to use a single radiator during low thermal loads and activate the second radiator during high thermal loading scenarios. Ground vehicles frequently use different radiators for each component that needs cooling (e.g., engine blocks, electronics, and motors) since they have different optimal working temperatures. The use of numerous smaller heat exchangers adds more energy-management features and alternative routes for carrying on with operation in the event of a crucial subsystem failure. Moreover, despite cooling systems being designed for maximum thermal loads, most vehicles typically operate at a small fraction of their peak values. To study and examine the planned multi-heat exchanger cooling system concepts, various computer simulations and experimental tests were performed. A nonlinear state space model, featuring input and output heat flow paradigms, was developed using a multi-node resistance-capacitance thermal model. The heat removal rate from the radiator(s) was estimated using the -NTU method as downstream fluid temperatures were not required. The system performance was studied for two driving cycles proposed by the Environmental Protection Agency (EPA) – urban and highway driving schedules. The computer simulation was validated using the laboratory setup in the High Bay Area of Fluor Daniel Engineering Innovation Building. The configuration features computer controlled variable speed electric motor driven coolant pump and independent variable speed fans for each radiator to provide desired fluid flow rates. The pump and fan power consumptions are approximately 0.8-1.2 kW and 0.4-3.2 kW, which corresponds to coolant and air flow rates of 0.2-1.5 kg/s and 0.5-1.75 kg/s, respectively. Two servo motor-controlled gate valves limit the coolant outlet from each radiator. Various thermocouples and a magnetic flow sensor record test data in real time using a dSpace DS1103 data acquisition control system. Designing and analyzing a nonlinear control architecture for the suggested system was the last phase in the study process. A nonlinear controller equipped TMS should offer higher energy efficiency and overall system performance. Three controllers—sliding mode, stateflow, and classical—were designed and implemented in Matlab/Simulink and placed onto the dSpace hardware. The sliding mode controller is recommended for high performance applications since it offers steady temperature tracking, 5oC, an acceptable response time, 120 sec, but suffers from frequent changes in fan speed. The stateflow controller exhibited the fewest fan speed oscillations, the fastest response time, 88 sec, and the smallest temperature offset, 3oC, it is advised for use in common passenger vehicle applications. Both controllers need around six minutes to warm up. The traditional controller, meanwhile, had the quickest warmup, 600 sec, but the slowest response time, 215 sec. Nonlinear cooling systems are essential for maintaining component temperatures which will enable vehicle reliability, and maximize performance given the focus on hybrid and electric vehicles

Developing Process Control Experiments for Undergraduate Chemical Engineering Laboratories

It is the intent of this work to develop a process control apparatus and series of experiments that will help students visualize the PID (Proportional-Integral-Derivative) control of a process and enhance their understanding of the subject. The apparatus is a computer-controlled PID mixing system that responds quickly to set point changes and process disturbances which are directly observable. The system can easily be simulated with a transfer function model in Matlab\u27s Simulink, so that the controller can be optimized for the desired system response. Four experiments can be conducted with this system including: exploration of system modeling and controller optimization in MatLab, set point tracking and disturbance rejection, the destabilizing effect of a time delay, and variable pairing in MIMO systems using the relative gain array (RGA). Several controller tuning methods are discussed, with both simulations and process performances reported and analyzed

Improving LNG Facility Reliability and Operability Via OEM Integrated Compressor Controls

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