Abraham Engeda

A numerical investigation on the volute/diffuser interaction due to the axial distortion at the impeller exit

A numerical simulation is performed on a single-stage centrifugal compressor using the commercially available CFD software, CFX-TASCflow. The steady flow is obtained by circumferentially averaging the exit fluxes of the impeller. Three runs are made at the design condition and off-design conditions. The predicted performance is in agreement with experimental data. The flow details inside the stationary components are investigated, resulting in a flow model describing the volute/diffuser interaction at design and off-design conditions

Experimental and numerical investigation of the flow in a centrifugal compressor volute

This paper presents the experimental and numerical investigation of an outward volute of rectangular cross section. The investigation is carried out at the level of stage performance, volute performance, and detailed flow field study at selected peripheral positions for various operating points. The objective of the investigation was to gain further knowledge about the flow structure and loss mechanism in the volute. Simultaneously with the experimental investigation, a numerical simulation of the flow in the volute was carried out. A three-dimensional Euler code was used in which a wall friction term and a tuned artificial dissipation term account for viscous effects. A reasonable agreement between the experimental and numerical results is observed. As a result a good and detailed knowledge about the pressure recovery and loss mechanism in the volute is obtained.

The influence of inlet flow distortion on the performance of a centrifugal compressor and the development of an improved inlet using numerical simulations

Abstract The performance of centrifugal compressors can be seriously affected by inlet flow distortions due to the unsatisfactory nature of the inlet configuration and the resulting inlet flow structure. Experimental tests have been carried out for the comparison of centrifugal compressor stage efficiency with two different inlet configurations, one of which is straight with constant cross-sectional area and the other a 90° curved pipe with nozzle shape. The comparative test results indicated significant compressor stage performance difference between the two different inlet configurations and the details are discussed to understand the performance behaviour of the compressor exposed to the distorted flow from the bend inlet configuration. The experimental investigation motivated the need for a new inlet design as well as a clear picture of the detailed flow field in the existing inlet design using numerical simulations. Two design approaches are reported in this paper, one of which is the location of vanes and the other the length of the curvature radius. For a more effective design method, a generalized formula is developed for the optimum position and number of vanes in such a way that each divided flow passage with vanes shares the same pressure gradient in radial direction. Numerical simulation results are presented and discussed in terms of mass-averaged parameters and flow structures, based on the comparison of flow properties at the pipe exit cross-sectional area of each design. Finally, new designs of different inlet systems are proposed to reduce the secondary flow and to provide flow as uniform as possible for a compressor.

The Inlet Flow Structure of a Centrifugal Compressor Stage and Its Influence on the Compressor Performance

The performance of centrifugal compressors can be seriously degraded by inlet flow distortions that result from an unsatisfactory inlet configuration. In this present work, the flow is numerically simulated and the flow details are analyzed and discussed in order to understand the performance behavior of the compressor exposed to different inlet configurations. In a previous work, complementary to this present work, experimental tests were carried out for the comparison of a centrifugal compressor stage performance with two different inlet configurations: one of which was a straight pipe with constant cross-sectional area and the other a 90-deg curved pipe with nozzle shape. Steady-state compressor stage simulation including the impeller and diffuser with three different inlets has been carried out to investigate the influence of each inlet type on the compressor performance. The three different inlet systems included a proposed and improved inlet model. The flow from the bend inlet is not axisymmetric in the circumferential and radial distortion, thus the diffuser and the impeller are modeled with fully 360-deg passages

A modified theory for the flow mechanism in a regenerative flow pump

Abstract The regenerative flow pump (RFP) and regenerative flow compressor (RFC) are turbomachines capable of developing high pressure ratios in a single stage. They are also known by other names, such as peripheral, side channel, turbine, traction and vortex compressor/pump. Even though the efficiency of RFP/RFC is usually less than 50 per cent based on past design experience, they have found wide applications in automotive and aerospace fuel pumping, booster systems, water supply, agricultural industries, shipping and mining, chemical and food stuffs industries, and regulation of lubrication and filtering. RFCs have been proposed for use in hydrogen gas pipelines and as helium compressors for cryogenic applications in space vehicles. RFTs are used as accessory drives on aircraft and missiles. With the aim of improving the performance and efficiency of an RFP, this paper proposes an improved and modified theoretical model that can explain the change in the circulatory velocity caused by variation in channel area. All previous works concentrated on the fully developed flow region in the RFP and this work expands consideration to the developing region. Furthermore, in order to make the above-suggested model a closed problem, several loss models were assumed and the results of predictions were compared with experimental and CFD data.

Experimental and numerical investigation of the performance of a 240 kW centrifugal compressor with different diffusers

Abstract Eight low solidity vaned diffusers (LSVD1–LSVD8) were designed for a 240 kW centrifugal compressor. All eight LSVDs and for comprison purposes along with two standard and high performing vaneless diffusers and one conventional vaned diffuser were tested downstream of the same impeller. The objective was to understand the pressure recovery phenomena in each of the three types of diffusers, and the effect of design parameters on performance. Also, the results were compared with the results from numerical simulation. The design parameters include the solidity, turning angle, vane setting angle, and the number of vanes. The experimental investigation was performed at three different rotational speeds ( M u =0.69, 0.88, 1.02). The experimental results proved the superior merits of the LSVDs relative to the vaneless and vaned diffuser. The LSVDs fulfilled the high expectations, since they seemed to combine the advantages of the conventional diffusers by providing a good pressure recovery over a wide flow range.

Experimental study of steam turbine control valves

Abstract Because of the converging-diverging configuration of the valve passage, venturi valves have been widely used in large turbines to regulate inlet flow as turbine governing valves for about half a century. From the 1960s, a number of valve failure incidents have been reported. Improvement to current designs was strongly demanded but, owing to the complicated nature of the fluid-structure interaction mechanisms, the basic mechanism causing valve failure is still far from being fully understood. Experimental investigations on a half-scale valve were performed here. The study confirmed that asymmetric unstable flow is the root cause of valve problems, such as noise, vibration and failure.

A centrifugal compressor stage with wide flow range vaned diffusers and different inlet configurations

Abstract In most cases, the diffuser system of a centrifugal compressor comes in the two general categories of a vaneless or a vaned diffuser. The vaned diffuser can generally be subdivided into two types depending on channel geometry (straight or curved channel) or depending on solidity. For the three different vaned diffusers of a centrifugal compressor stage, the design procedure is presented and experimental data are compared. The primary objective of the diffuser design was to achieve substantial improvement, compared with the stage currently used for the intended application, for the stable operating flow range as well as for the head rise from design to surge flow. The design goals also included achieving competitive efficiency levels, which required a significant efficiency improvement. Test results showed a much wider stable operating range than had been expected. This stage operated well beyond the expected vaned diffuser stall limit. The inlet configuration and inlet flow structure are known significantly to affect the compressor performance. Each of the three diffusers is tested for two different types of inlet configuration, and the relative performances are assessed.

Experimental and numerical investigation of the flow in a vaneless diffuser of a centrifugal compressor stage. Part 1: Experimental investigation

Abstract The flow in a radial vaneless diffuser downstream of a centrifugal compressor is highly complex, as the flow is turbulent, unsteady, viscous, and three-dimensional. Depending on the initial state of the end-wall boundary layers and the diffuser length, the flow may become fully developed or may separate from one of the walls. Therefore, to improve the diffuser performance, it is important to understand the flow field in the diffuser in detail. As the diffuser width is generally very small for most radial stages and an adverse pressure gradient exists, secondary flows are generated, making the flow fields more complicated. In addition, skewed boundary layers form on the wall surfaces. As flowrate is reduced, the flow field becomes more complicated and leads to rotating stall. This article presents detailed flow measurements in a vaneless diffuser of a centrifugal compressor stage with a very high flow coefficient radial impeller. Usually, centrifugal compressors with radial impellers are designed in the flow coefficient (ϕ) range ϕ = 0.01 - 0.16. Often, the need arises to design higher flow coefficient, ϕ, radial stages. Detailed measurements were carried out in the vaneless diffuser at seven radial positions downstream of a radial impeller designed for a very high flow coefficient of ϕ = 0.2. The experimental investigation was carried at four rotational speeds 13 000, 15 500, 18 000, and 20 500 r/min, but only the result of 20 500 r/min at near-design-point flowrate (5.11 kg/s) is reported in this article.

Venturi valves for steam turbines and improved design considerations

Abstract As a turbine governing valve, the venturi valve has been widely used in large turbines to regulate inlet flow for about 40 years. It is favoured in terms of low total pressure loss because of the converging-diverging configurations of the valve passage. However, as turbines become larger and larger, a number of valve failure incidents have been reported, and there is a great demand for improved designs. Yet, because of the complicated nature of the fluid—structure interaction mechanisms, the basic mechanism causing valve vibration and failure is still far from being fully understood. Most of the available literature relies heavily on experiments before the 1980s. There are several improved designs by the trial and error method, but governing rules, or even a clear direction for improvement, are almost non-existent. There has still seen no published investigation using computational fluid dynamics (CFD) tools. As CFD is increasingly recognized as a powerful tool for understanding complicated fluid phenomena, a two-dimensional numerical investigation was performed in the present work. The study revealed that valve plug vibration is due to hydraulic forces acting on the plug at its balanced position and fluid-induced excitation as the plug vibrates in the lateral and vertical directions. All this relates to unexpected asymmetric flow patterns. By changing the plug shape, the flow patterns can be made much more symmetric, which reduces the intensity of steady forces and fluid plug interaction.