Centrifugal Compressors

  Centrifugal Compressor Copyright: © RWTH | IST

Centrifugal compressors are used in process plants in the chemical and petrochemical industries to convey and compress gases with a wide variety of compositions. In the field of drive technology, centrifugal compressors are used as exhaust gas turbochargers both in maritime applications and in motor vehicles to boost the internal combustion engine. In addition, centrifugal compressors are also used in small aircraft gas turbines of the lower thrust class as the final stage of an upstream axial compressor. In the course of reducing CO2 emissions from the transport sector, electrically driven centrifugal compressors are of interest for charging a fuel cell on the cathode side.

 

Technical Introduction

Centrifugal compressors are used in a wide variety of applications. In addition to their relatively simple design and low cost, their advantages include their robustness with regard to varying operating conditions and pumped media, their high power density, and their compact axial design.

The generally lower efficiencies compared to axial compressors are mainly due to the strong secondary flow in the impeller, which is unavoidable due to the small height-to-edge ratios and the Coriolis force that sets in as a result of the axial-radial deflection.

 

Innovation with History

The centrifugal pump was invented by Denis Papin around 1689.

 

Research Trends

Acoustics Simulation Copyright: © RWTH Aachen | IST

Acoustics in pressure lines

The project builds on the previous project "Acoustics in pressure lines". The aim is to develop and experimentally validate a robust, flexible and cost-effective acoustic measurement method to record the sound power radiated by a centrifugal compressor on the pressure side as accurately as possible. Compared to complex and cost-intensive rotating duct sections, the measurement system used here will make do with a small number of measurement sensors.

While the development of the method took place in the first part of the project, the aim of the second project phase applied for here is to prove the experimental applicability of the method under industrial conditions and to compare the measurement data obtained with the results of the numerical simulations of the first project phase. In addition, the underlying models are to be extended to include the industrially relevant application of bladed diffusers.

With the aid of the newly developed measurement system, companies in the turbomachinery sector will be able to measure the sound field radiated by centrifugal compressors over the entire characteristic map width under industrial conditions and thus evaluate low-noise centrifugal compressor designs and optimally dimension secondary noise control measures.

Fuel cell compressor design

Hydrogen fuel cell powertrains are particularly attractive for long-distance passenger car, commercial vehicle and rail applications, as they enable long ranges with short refueling times. The polymer electrolyte membrane (PEM) fuel cells used in powertrains are charged cathode (air) side. Increasing the fuel cell stack operating pressure primarily serves to increase the stack efficiency or its power density. Due to their advantages in terms of required installation space, cost, noise emissions and service life, electrically driven centrifugal compressors are used.

However, centrifugal compressors for PEM fuel cells must achieve relatively high pressure ratios even at very low mass flows. Further challenges are the limited compressor speed due to the electric drive and converter as well as the increased requirements for efficiency and structure-borne noise emissions compared to passenger car exhaust turbochargers. Especially due to the speed limitation, the design of centrifugal compressors for PEM fuel cells can only be based on existing designs from exhaust gas turbocharging to a limited extent.

Therefore, within the scope of this research project, a guideline for the targeted design of fuel cell air compressors will be developed and experimentally validated on the basis of an exemplary geometry.

 

Circumferentially inhomogeneous centrifugal compressor flow

  Copyright: © RWTH Aachen | IST
 
 

Within the scope of the project, three centrifugal compressor stages with spiral outlet housings will be used to investigate the extent to which circumferential inhomogeneities at the diffuser outlet influence the flow within the compressor stage. Such circumferential inhomogeneities can be caused, for example, by the spiral outlet housing in off-design operation.

The research objective is to qualitatively and quantitatively evaluate the effects of a circumferential inhomogeneous pressure field on the characteristics of the compressor and its flow.

Industrial compressor with broad characteristic field

At the IST, the industrial compressor stage has been established within the framework of previous research projects. The aim of the present project is to investigate the instability phenomena which limit the operating range of the machine and thus the usable map width. To this end, the instabilities occurring in the compressor stage are identified and localized by means of transiently measuring pressure sensors.

On the basis of the experimental data and with the aid of numerical flow simulations, the prediction models and methods for instabilities described in the literature are to be checked for their suitability and accuracy. The aim of these investigations is to gain knowledge about the physical phenomena occurring at the instability boundary and to understand their interaction.

Based on this, a simulation tool will be developed to predict these instabilities and to evaluate them qualitatively as well as quantitatively. These results will form the basis for deriving design measures to dampen or suppress the identified flow phenomena.

  Heat Flux Copyright: © RWTH Aachen | IST

Centrifugal compressor model validation

Within the scope of this project, 2D/3D calculation methods for the conceptual design of centrifugal compressors are to be further developed. In order to achieve the highest possible accuracy, existing methods will be verified on the basis of a broad database.

At the same time, prediction and scaling rules of centrifugal compressors with a wide stability range will be developed.

Centrifugal compressors in flexible operation

This project aims to provide a fundamental understanding of the interaction between the impeller, diffuser and scroll casing at design and off-design operating points in order to be able to operate future centrifugal compressors at flexible operating modes. For this purpose, the influence of different scroll sizes and diffusers on the steady-state and unsteady-state aerodynamics of the compressor stage as well as on the discharge side sound radiation at off-design operating points is investigated experimentally and numerically.

The analyses relate to the comparison of two scroll cases and two diffuser geometries (unbladed and bladed). As further influencing variables on the operating behavior, the inflow angle can be varied by means of variable inlet guide vanes, the rotational speed and the back pressure. Thus, an experimentally and numerically validated knowledge base on the interaction of the components is generated as a basis for a flexible design philosophy.

To achieve this goal, a new, operationally optimized scroll casing is designed, engineered and manufactured. In addition, a bladed diffuser from the previous FVV project will be designed and fabricated. Above all, the newly generated and experimentally verified knowledge about the geometry-dependent interaction of the individual components at operating points far from design, together with the open test case, represents a major benefit.