Numerical Investigation of Negative Flow Characteristics in a Centrifugal Compressor with Vaned Diffuser and Internal Volute

Negative flow from the outlet through the volute, diffuser, and impeller to the inlet of the centrifugal compressor can occur continuously as a result of system accidents. A physical comprehension of negative flow dynamics is crucial in evaluating the compressor characteristics under abnormal working conditions, and is also important in exploring the compressor aerodynamics over the entire flow range. However, limited research on the negative flow dynamics in centrifugal compressors, particularly with the consideration of vaned diffusers and volutes, can be found. This study aims to determine the compressor characteristics, including the negative flow rates of a centrifugal compressor, and to clarify the negative flow mechanism under the interaction of the volute, diffuser, and impeller. The last stage of a four-stage centrifugal compressor, including an internal volute, a vaned diffuser, and a closed impeller was simulated under both positive and negative flow conditions using a computational fluid dynamics (CFD) model. The results show that the pressure ratio-negative flow characteristic is almost matched with a parabolic curve. At negative flow rates, the backflow generated on the hub and shroud sides in the impeller expands upstream and causes flow separation in the diffuser. The negative flow enters the impeller at a large incidence angle and results in jet wall impingement on the pressure surface, flow spillage over the trailing edge, and flow separation near the suction surface. The impeller partially acts as a turbine impeller and performs negative work on the fluid. This work is of scientific significance to enrich the compressor aerodynamics in accident scenarios and of engineering value to improve the advanced design of compressor protection systems.

Surge Limit Prediction for Automotive Air-Charged Systems

Compressor surge has been investigated and predicted since the early days of turbomachinery research. Experimental testing of turbomachinery applications is still needed to determine whether stable compressor operation is possible in the expected application regime. Measuring compressor maps and operating ranges on hot gas test stands is common. The test benches are designed and optimized to ensure ideal inflow and outflow conditions as well as low measurement uncertainty. Compressor maps are used to match turbocharger and application. However, a shift in surge limit, caused by the piping system or application, can only be adequately addressed with full engine tests. Ideal measurements use the corresponding piston engine in the charged-air system. This can only take place in the development process, when surge detection is unfavorable from an economic perspective. The surge model for turbochargers presented here is an extension of the Greitzer’s surge model, which considers the effect of inlet throttling. Application components, such as air filters, pipe elbows and flow straighteners, reduce pressure in front of the compressor and flow conditions might differ from those in laboratory testing. Experimental results gathered from the hot gas test stand at TU Darmstadt indicate strong variation in surge limit, influenced by inlet throttling. An extension to the surge model is developed to explain the observed phenomena. The model was validated using extensive experimental variations and matches the experienced surge limit shift. Additional measurements with a piston engine downstream of the turbocharger demonstrated the validity of the surge model. The results also show that surge is a system-dependent phenomenon, influenced by compressor aerodynamics and boundary conditions.

Low-Cost Rotating Experimentation in Compressor Aerodynamics Using Rapid Prototyping

With the rapid evolution of additive manufacturing, 3D printed parts are no longer limited to display purposes but can also be used in structural applications. The objective of this paper is to show that 3D prototyping can be used to produce low-cost rotating turbomachinery rigs capable of carrying out detailed flow measurements that can be used, among other things, for computational fluid dynamics (CFD) code validation. A fully instrumented polymer two-stage axial-mixed flow compressor test rig was designed and fabricated with stereolithography (SLA) technology by a team of undergraduate students as part of a senior-year design course. Experiments were subsequently performed on this rig to obtain both the overall pressure rise characteristics of the compressor and the stagnation pressure distributions downstream of the blade rows for comparison with CFD simulations. In doing so, this work provides a first-of-a-kind assessment of the use of polymer additive technology for low-cost rotating turbomachinery experimentation with detailed measurements.

Undergraduate Project in Compressor Rig Design, Fabrication, and Testing for Complete Engineering Training

An ambitious project in propulsion was introduced as part of the final-year integrator project offerings of the mechanical and aerospace engineering programs at École Polytechnique de Montréal in 2011–2012. It has been running successfully for the past three academic years. The project consists in the design, fabrication, and placement into service of a functional instrumented multistage compressor test rig, including the compressor, for research in compressor aerodynamics. A team of 15–17 senior-year undergraduate engineering students is given a set of design and performance specifications and measurement requirements, an electric motor and drive, a data acquisition system, and some measurement probes. They must complete the project in two semesters with a budget on the order of Can$15,000. The compressor is made from rapid prototyping to keep production cost and time reasonable. However, the required rotation speed of 7200 rpm stretches the limits of the plastic material and presents the same structural challenges as industrial compressors running at higher speeds. The students are split into subteams according to the required disciplines, namely, compressor aerodynamics, general aerodynamics, structures, dynamics, mechanical design and integration, instrumentation, and project management. For the initial phase, which covers the first two months, the students receive short seminars from experts in academia and industry in each discipline and use the knowledge from fundamental engineering courses to analytically model the different components to come up with a preliminary design. In the second phase, covering three to six, the students are trained at commercial simulation tools and use them for detailed analysis to refine and finalize the design. In each of the first two phases, the students present their work in design reviews with a jury made up of engineers from industry and supervising professors. During the final phase, the compressor is built and tested with data acquisition and motor control programs written by the students. Finally, the students present their results with comparison of measured performance with numerical and analytical predictions from the first two phases and hand over their compressor rig with design and test reports as well as a user manual and an assembly/maintenance manual. This complete project allows the students to put into practice virtually all the courses of their undergraduate engineering curriculum while giving them an extensive taste of the rich and intellectually challenging environment of gas turbine and turbomachinery engineering.

Undergraduate Project in Compressor Rig Design, Fabrication and Testing for Complete Engineering Training

An ambitious project in propulsion was introduced as part of the final-year integrator project offerings of the mechanical and aerospace engineering programs at École Polytechnique de Montréal in 2011–2012. It has been running successfully for the past three academic years. The project consists in the design, fabrication and placement into service of a functional instrumented multi-stage compressor test rig, including the compressor, for research in compressor aerodynamics. A team of 15–17 senior-year undergraduate engineering students are given set of design and performance specifications and measurement requirements, an electric motor and drive, a data acquisition system and some measurement probes. They must complete the project in two semesters with a budget on the order of Can$15,000. The compressor is made from rapid prototyping to keep production cost and time reasonable. However, its required rotation speed of 7200 rpm stretches the limits of the plastic material and presents the same structural challenges as industrial compressors running at higher speeds. The students are split into sub-teams according to the required disciplines, namely compressor aerodynamics, general aerodynamics, structures, dynamics, mechanical design and integration, instrumentation and project management. For the initial phase, which covers the first two months, the students receive short seminars from experts in academia and industry in each discipline and use the knowledge from fundamental engineering courses to analytically model the different components to come up with a preliminary design. In the second phase, covering months three through six, the students are trained at commercial simulation tools and use them for detailed analysis to refine and finalize the design. In each of the first two phases, the students present their work in design reviews with a jury made up of engineers from industry and supervising professors. During the final phase, the compressor is built and tested with data acquisition and motor control programs written by the students. Finally, the students present their results with comparison of measured performance with numerical and analytical predictions from the first two phases and hand over their compressor rig with design and test reports as well as a user manual and an assembly/maintenance manual. This complete project allows the students to put into practice virtually all the courses of their undergraduate engineering curriculum while giving them an extensive taste of the rich and intellectually challenging environment of gas turbine and turbomachinery engineering.

Influence of Tip Clearance on the Turbulent Aerodynamics of Axial Flow Fan under off Design Conditions

Compressor is a dynamic machine with complicated 3d aerodynamics. Dynamics creates an uncertain environment and induces the flow with instabilities, resulting in reduced performance. Motion being circumferential, flow is also subjected to rotational accelerations. Added to these complications are the tip gap and related vortex aerodynamics in the tip region, which also influence the passage flow of the rotor and thus complicates the flow field. The result of these implications is the generation of the turbulence in the flow field. Turbulence is a fluctuating characteristic of the flow, which extracts its energy from the mean flow field. Energy consumed by the turbulent nature of flow is a waste. Therefore it is very much important to understand about the influence of the turbulence and related kinetic energy compressor aerodynamics. In this paper work is presented to understand about the turbulence under such varying geometry conditions of flow, as well as blade. Results of nature of the turbulence and its growth are discussed for varying mass flow rates and different tip gaps of acceptable range.

The Influence of Compressor Aerodynamics on Pressure Probes: Part 2 — Numerical Models

This paper presents the second part of an investigation into the influences of the aerodynamics of compressor blade rows on measurements made using steady state pneumatic pressure probes. In part one, the in rig calibrations of the probes in the low and high speed compressors showed that the wind tunnel derived calibration in yaw could be reproduced with good accuracy in the compressor, despite the flow in the compressor being unsteady, and in the case of the high speed compressor, of a different Reynolds number. In this part, CFD simulations of the flow about a probe, both within a low speed compressor and a steady, uniform flowfield are presented. The influence of the pressure gradient existing within the stators in which the probe is positioned was found to be small, as was the effects of unsteady flow. The major contribution to measurement errors appears to lie within the probe blockage effect.