Blade row interaction in radial turbomachines

A computational study has been performed to investigate the effects of blade row interaction on the performance of radial turbomachines, which was motivated by the need to improve our understanding of the blade row interaction phenomena for further improvement in the performance. High-speed centrifugal compressor stages with three settings of radial gap are configured and simulated using a three-dimensional Navier-Stokes flow method in order to investigate the impact of blade row interaction on stage efficiency. The performance predictions show that the efficiency deteriorates if the gap between blade rows is reduced to intensify blade row interaction, which is in contradiction to the general trend for stage axial compressors, hi the compressors tested, the wake chopping by diffuser vanes, which usually benefits efficiency in axial compressor stages, causes unfavourable wake compression through the diffuser passages to deteriorate the efficiency. Similarly, hydraulic turbine stages with three settings of radial gap are simulated numerically. A new three-dimensional Navier-Stokes flow method based upon the dual-time stepping technique combined with the pseudo-compressibility method has been developed for hydraulic flow simulations. This method is validated extensively with several test cases where analytical and experimental data are available, including a centrifugal pump case with blade row interaction. Some numerical tests are conducted to examine the dependency of the flow solutions on several numerical parameters, which serve to justify the sensitivity of the solutions. Then, the method is applied to performance predictions of the hydraulic turbine stages. The numerical performance predictions for the turbines show that, by reducing the radial gap, the loss generation in the nozzle increases, which has a decisive influence on stage efficiency. The blade surface boundary layer loss and wake flow mixing loss, enhanced with a higher level of flow velocity around blading and the potential flow disturbances, are responsible for the observed trend

Performance comparison between planar and pyramidal microdiffuser for valveless micropump

This paper was presented at the 3rd Micro and Nano Flows Conference (MNF2011), which was held at the Makedonia Palace Hotel, Thessaloniki in Greece. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, Aristotle University of Thessaloniki, University of Thessaly, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute.The microdiffuser is the most important component of the valveless micropump and its design plays a role in the valveless micropump performance to direct the flow in a proper direction. A planar microdiffuser valveless micropump has been compared with a pyramidal microdiffuser valveless micropump using 3-D CFD simulations. Both planar and pyramidal microdiffuser has a throat hydraulic diameter of 0.6865mm and diffuser half angle of 6.65 degrees. The dynamic mesh was applied under different actuation frequency of the micropump diaphragm (8, 50, 100, 200, 500, 1000, and 2000Hz). The net flow rate and the rectification efficiency were calculated for the two valveless micropumps. The results showed that the pyramidal microdiffuser performance was better than the planar microdiffuser for frequency f ≥ 200Hz as the net flow rate generated by pyramidal microdiffuser was higher than that by planar microdiffuser. The highest net flow rate of 18.3μL/min was achieved by the pyramidal microdiffuser at rectification efficiency of 0.35% and actuation frequency of 2000Hz

LES 및 URANS를 이용한 원심펌프 내부의 난류 유동해석

학위논문(박사) -- 서울대학교대학원 : 공과대학 기계항공공학부, 2021.8. 최해천.원심펌프는 가장 널리 이용되는 펌프로서 저압에서 다양한 산업 분야에서 많은 유량을 이송하기 위해 사용되고 있다. 원심펌프는 다양한 범위의 압력 상승 및 유량 조건을 만족하기 위해 설계 조건뿐 아니라 탈설계 조건에서도 흔히 작동한다. 탈설계 조건에서는 펌프 내부에서 더 복잡한 난류 유동 특성이 발달하여 널리 사용되는 비정상 Reynolds 평균 Navier-Stokes 난류 모델(unsteady Reyonolds Navier-Stokes: URANS)가 부정확한 결과를 도출할 수 있다는 것이 잘 알려져 있다. 따라서 이러한 탈설계 조건 해석을 위해서는 큰에디모사(large eddy simulation: LES)와 같은 보다 정확한 수치 해석 방법이 요구된다. 1장에서는 원심펌프 내부 난류 유동 해석을 위해 LES를 수행하고 설계 조건 및 탈설계 조건에서의 유동 특성을 분석하였다. 임펠러 블레이드의 압력 및 흡입면 모두에서 유동 박리가 발생하였고, 특히 탈설계 조건에서는 블레이드 압력면의 박리 기포가 비정상 특성을 나타내며 볼루트 혀 부근에서 더 크게 발달하였다. 블레이드 회전에 따라 블레이드 후단으로부터 와류 구조가 발생하였고 탈설계 조건에서는 이들이 다음 블레이드 후단 와류와 강하게 상호 작용하여 볼루트 내부에서 더 강한 와도장을 생성하였다. 임펠러 주변 압력 섭동을 살펴보기 위해서 삼중 분해를 수행하였다. 압력의 난류 섭동은 볼루트 혀 부근에서 크게 증가하였고 특히 탈설계 조건에서는 주기적 섭동보다 강하게 발달하였다. 볼루트 혀에서는 유동 박리가 발생하였다. 특히 탈설계 조건에서는 많은 유량이 출구 파이프로 흐르지 않고 임펠러-볼루트 간극을 따라 볼루트 상류로 누설되었다. 이는 블레이드 압력면에 강한 역압력 구배를 형상하여 박리기포의 재부착을 지연시키고 비정상적인 유동박리 현상을 생성하였다. 또한 볼루트에서의 높은 압력은 축 방향으로의 압력 구배를 형성하고 반경 방향 간극으로 누설 유동을 야기하였다. 탈설계 조건에서는 볼루트 내부에서 더 높은 압력이 형성되어 누설 유동이 강하게 발달하며 볼루트 내부에서의 이차 유동의 발달에 기여하였다. 이러한 원심펌프 내부의 다양한 손실유동은 임펠러-볼루트 상호작용에 의해 영향을 받아 탈설계 조건에서 더 크게 발달하였다. 2장에서는 URANS 해석을 수행하고 이를 LES에서의 유동 특성과 비교하였다. LES는 두 유량 조건에서 펌프의 압력 상승 및 효율을 잘 예측하였지만, URANS는 이들을 과다 예측하였다. URANS의 블레이드 후단 와류 구조는 LES의 순간 유동 구조를 잘 나타내지 못 하였고, 오히려 LES의 상평균 유동 구조와 유사한 와도장을 나타내었다. 설계점에서 LES 및 URANS는 블레이드 표면을 따라 유사한 압력 및 마찰항력 분포를 나타내었다. 하지만 탈설계점에서는 LES는 볼루트 혀 부근 블레이드 압력면을 따라 박리기포의 재부착이 지연되어 더 큰 박리기포가 형성되었는데, URANS는 이를 예측하지 못 하였다. 임펠러 주변에서의 압력 섭동은 URANS가 주기적 섭동 성분은 비교적 잘 예측하지만, 난류 섭동을 잘 예측하지 못 하는 것을 보여주었다. 따라서 난류 섭동이 중요해지는 탈설계점에서는 URANS를 통한 압력 섭동 예측이 부정확한 결과를 나타내었다. 또한 설계점에서는 LES 및 URANS가 볼루트 내부를 따라 유사한 전압 분포를 나타내었다. 하지만 탈설계점에서는 볼루트 혀에서 발생하는 유동박리가 손실을 야기하고 볼루트 상류에서 전압이 크게 감소하였다. URANS는 이러한 손실을 잘 예측하지 못 하였지만 이외 영역에서는 전압 분포를 잘 예측하였다. 또한 본 연구에서의 펌프에서는 토출 파이프의 곡률 및 면적 증가로 인해 강한 와류 유동이 발생하였다. 이러한 와류 유동은 큰 손실을 야기하고 이는 설계 조건에서 높은 유량으로 인해 더 크게 발달하였다. LES는 이러한 와류 유동 및 손실을 잘 예측하여 실험에서의 압력 상승 및 효율을 잘 예측하였지만, URANS는 이를 예측하지 못 하여 펌프 성능을 과다예측하였다.Centrifugal pumps, which are the most common type of pumps, are widely used in various industrial applications. Centrifugal pumps often operate at off-design conditions as well as at the design condition to meet various ranges of pressure rise and flow rates required. At off-design conditions, more complex and unsteady flows develop inside pumps making an accurate prediction of the flow challenging. Commonly used Reynolds-averaged Navier-Stokes (RANS) turbulence model often inaccurately predict turbulent flow inside centrifugal pumps at off-design conditions. Therefore, more accurate numerical method like large eddy simulation (LES) is demanding. In part 1, we perform LES to investigate turbulent flow inside a volute-type centrifugal pump for the design and off-design condition. Along the pressure and suction sides of impeller blades, separation bubbles are generated. At the off-design condition, the blade pressure side near the tongue contains a larger separation bubble with highly unsteady characteristics due to the impeller-volute interaction. The trailing vortices shed from rotating blades strongly interact with those from the following blade at the off-design condition, generating stronger vorticity field in a wider region inside the volute. On the other hand, this mutual interaction of vortices shed from consecutive blades is weak at the design condition. Triple decomposition of pressure fluctuations along the impeller periphery demonstrates that turbulent fluctuations are small at the design condition, whereas they become significant at the off-design condition especially near the tongue. Flow separation also occurs at the volute tongue. At the off-design condition, a large amount of volute flow does not follow the main stream to the discharge pipe but re-enters into the volute upstream near the tongue. This pressurized fluid forms a high adverse pressure gradient on the blade pressure side, resulting in strong unsteady separation there. Also, a high pressure gradient in the axial direction at the radial gaps is formed especially near the tongue, creating the leakage into the cavities. Inside the volute, secondary vortices grow along the volute passage. A secondary motion induced by these vortices also significantly affects the leakage to the cavities. All of these flow losses show unsteady features that are strongly influenced by impeller-volute interactions, especially at the off-design condition. In part 2, we conduct unsteady Reynolds-averaged Navier-Stokes (URANS) simulation and compare the flow characteristics with that by LES. URANS overpredicts the head coefficient and efficiency of the pump, whereas LES shows very good agreement with experiments. Vorticity fields inside the impeller and volute show that URANS does not resolve the instantaneous nature of turbulent flows. Rather, URANS displays similar magnitude and distribution of vorticity to phase-averaged fields by LES, indicating it provides phase-averaged flow features to some degree. Inside the impeller passage, for the design condition, LES and URANS show homogeneous characteristics for pressure and skin friction between five blades. On the other hand, along the blade pressure side for the off-design condition, LES reveals that higher pressure is induced near the tongue than the other four blades, delaying the reattachment of the separation bubble there. However, URANS does not show this larger separation bubble near the tongue. Along the impeller periphery, pressure fluctuations by LES and URANS are compared. For both flow conditions, URANS predicts periodic fluctuations well, whereas turbulent fluctuations are largely underestimated. Therefore, total fluctuations by LES and URANS exhibit satisfactory agreement at the design condition, whereas those at the off-design condition shows significant difference because of increased turbulent fluctuations for the latter condition. The time-averaged total pressure coefficient by LES and URANS shows good agreement at the design condition inside the volute. However, at the off-design condition, total pressure decreases at the volute upstream due to flow separation at the tongue. URANS does not predict these losses, overestimating total pressure there. Inside the discharge pipe for the present pump, strong flow separation and swirls are observed by LES due to the curvature and area expansion of the pipe. The losses by these swirls are larger for the design condition because of the higher mean flow rate. However, URANS does not capture strong swirls and subsequent losses, resulting in overprediction of the pressure rise and efficiency.Part I Large eddy simulation of turbulent flow in a centrifugal pump 1 1 Introduction 2 2 Numerical details 5 2.1. Pump specifications and operating conditions 5 2.2. Governing equations and computational setup 6 2.3. Resolution studies and comparison to experiments 8 3 Results and discussions 15 3.1. Overall flow structures in a centrifugal pump 15 3.2. The flow characteristics inside the impeller 17 3.2.1. Relative eddy and equations of motion in a noninertial frame 17 3.2.2. Qualitative analysis of flow structures inside the impeller 18 3.2.3. Quantitative analysis of flow characteristics inside the impeller 21 3.3. Impeller-volute interaction 23 3.3.1. Trailing vortices shed from blades and flow separation at the tongue 23 3.3.2. Pressure fluctuations along the impeller periphery 25 3.3.3. Leakage through the radial gaps 27 4 Summary and concluding remarks 43 Part II LES vs. URANS: turbulent flow in a centrifugal pump 46 1 Introduction 47 2 Numerical details of URANS 53 3 Results and discussions 55 3.1. Resolution studies and comparison to experiments 55 3.2. Flow near the interface between the impeller blade and volute 57 3.3. Flow characteristics inside the impeller 59 3.4. Radial thrust and pressure fluctuations along the impeller periphery 62 3.5. Flow features inside the volute 65 3.6. Flow characteristics inside the discharge pipe 68 4 Summary and concluding remarks 86 References 90 A Modified volute casing to reduce the leakage to cavities 98 B Circumferential grooves to reduce the leakage to the pump inlet 101 Abstract (in Korean) 104박

3D numerical simulation of pump cavitating behavior

The quasi-steady cavitating behavior of three pumps was investigated by 3D unsteady viscous computations. The numerical model is based on the commercial code FINE/TURBO™, which was adapted to take into account the cavitation phenomenon. The resolution resorts to a time-marching algorithm initially devoted to compressible flows. A low-speed preconditioner is applied to treat low Mach number flows. The vaporization and condensation processes are controlled by a barotropic state law that links the void ratio evolution to the pressure variations. A radial pump, a centrifugal pump, and a turbopump inducer were calculated and the cavitating behaviors obtained by the computations were compared to experimental measurements and visualizations. A reliable agreement is obtained for the two pumps concerning both the head drop charts and the extension of the vapor structures. A qualitative good agreement with experiments is also observed in the case of the turbopump inducer. The accuracy of the numerical model is discussed for the three geometries. These simulations are a first attempt to simulate the complete 3D cavitating flows in turbomachinery. Results are promising, since the quasi-steady behaviors of the pumps in cavitating condition are found quantitatively close to the experimental ones. A continuing effort is pursued to improve the prediction accuracy, and to simulate unsteady effects observed in experiments, as, for example, rotating cavitation

Numerical Simulation and Erosion Prediction for an Electrical Submersible Pump

Electrical Submersible Pumps (ESP) are widely used in the oil industry to lift oil and gas at the same time with high efficiency. Many ESPs operate with multiphase flow – liquid, gas and a low concentration of sand, thus having problems of pressure degradation and erosion. To investigate these problems, a numerical method can be used, which provides details of the inner flow field. Using the commercial software ANSYS Fluent, 3D transient multiphase simulations are conducted for a Baker Hughes made ESP MVP-G400. The simulations focus on three parts: secondary flow path in multiphase flow, pressure degradation due to gas volume fraction and erosion prediction. Aside from the main flow path, the clearances and balancing holes inside the ESP create a secondary path which enables the flow to recirculate. Although the volume flow rate in this path is low compared with the main flow path, the erosion in the secondary path cannot be neglected and can result in pump failure. In this research, a water-air-sand three phase simulation is performed on dual stages of the ESP, with all secondary path included. Second part of this research focuses on the pressure degradation due to the presence of a gas phase, especially at the first stage near the pump inlet. The compressibility of gas and the bubble break-up and coalescence effects are considered using the Population Balancing Module in ANSYS Fluent. The last part is the erosion prediction. A low concentration of sand is often inevitable during the operation of an ESP, causing erosion and reducing the life span of the ESP. This erosion becomes more severe with the existence a of gas phase. The three-phase simulations with both Eulerian multiphase and particle tracking explain the erosion and the role of gas in this process, giving a reasonable qualitative prediction on the erosion of the ESP

Enhancement of synthetic jets by means of an integrated valve-less pump Part II. Numerical and experimental studies

The paper studies the performance of the new fluid jet actuator based on the novel principle of the generation of fluid jet, which has been presented in [Z. Travnicek, A.I. Fedorchenko, A.-B. Wang, Enhancement of synthetic jets by means of an integrated valve-less fluid pump. Part I. Design of the actuator, Sens. Actuators A, 120 (2005) 232-240]. The fluid jet actuator consists of a synthetic jet actuator and a valve-less pump. The resulting periodical fluid jet is intrinsically non-zero-net-mass-flux, in contrast to the traditional synthetic jet. The numerical results have been compared with the laboratory experiments comprising phase-locked smoke visualization and time-mean velocity measurements. The results have confirmed the satisfactory performance of the actuator

Experimental rotordynamics and flow visualization approach for periodically reversed flows of a Francis - Type Pump - Turbine in generating mode at off - design operating conditions

A non-conventional tufting visualization method along with an image processing development and specific applied technique adapted to the flow conditions is proposed and implemented on a reduced scale model of a Francis-type reversible pump-turbine in three different turbine stages such as turbine mode, runaway mode and turbine break mode, in order to visualize rotating stall phenomenon -- Fluorescent monofilament wires along with high speed image processing and pressure sensors were installed in the narrow and vane less gap between the impeller blades and guide vanes -- Pressure fluctuations were analyzed along with tuft visualization to describe the flow with and without rotating stal