Investigation Of Flow Control Methods To Extend The Stable Range Of A 8.0 Pressure Ratio Centrifugal Compressor

In modern helicopter turboshaft engines,space and weight are of great importance as well as a high efficiency.A single stage high pressure ratio centrifugal compressor design is especially preferred due to its simplicity and compactness.The difficulty that arises from using such a high pressure ratio is that the stable range of the compressor dramatically decreases with increasing pressure ratio.Therefore additional measures have to be taken to enhance the range of the compressor.Two possible technologies,Variable Inlet Guide Vanes(VIGV)and Casing Treatment(CT)are studied in this thesis by means of computational fluid dynamics simulation.For the VIGV technology,5 different stagger angles from-20° to +20° have been studied.At 100% design speed,the-10° stagger angle case showed very promising results with an increased peak pressure ratio from 8.03 to 8.28,an increased range from 4.1% to 6.8% and an increased choke mass flow from 2.68 kg/s to 2.75 kg/s.Efficiency was slightly decreased for this case from 79.6% to 78.3% compared to the 0° case.The-20° stagger angle case however couldn’t further enhance these results.For increased stagger angles,range decreases significantly,while the speed line is moved to lower mass flows.For part speed(90% and 80% design speed),the influence of stagger angle on stable operating range is much lower.However,it is found that the peak efficiency will be increased with increasing stagger angle from-20° to +10°.For CT technology,a geometry parameter study with 5 different parameters and in total 20 different cases was conducted.At 100% speed,the results indicate the width of the upstream channel as the most important parameter,with the best setup being the smallest tested width.This case showed an increase in range of roughly 225%(from 4.4% to 9.9%),but the efficiency dropped from 79.9% to 78.4%,which was still the highest efficiency of any CT cases.Since the stall mainly happens at the diffuser of the compressor and not at the impeller,CT showed only limited results.The position and width of the downstream channel were the most important parameters at part speeds.When operating at 90% speed,the CT device worked best,because the surge margin under these conditions was defined by impeller wall stall,which could be treated by the CT.Under these conditions,an increase in range from 10.3% to 17.6% for a downstream channel width of 5.4 mm could be achieved with only slightly decreased efficiency.A position of the downstream channel closer to the leading edge resulted in higher efficiency with the best result at similar efficiency level as the base case.At 80% speed,the effectiveness of the CT is limited as the stall in the impeller is larger and reaches further towards the middle of the channel.The results showed that an increase in efficiency was possible(from 77.2% to 79.1%),but the achieved range was similar or even slightly decreased.Best results were obtained for the position closest to the leading edge and the greatest width of the downstream channel.In general,this work provides a guide for application of the two technologies to high pressure ratio centrifugal compressors.The influence of both technologies on stable operating range was shown at different speeds.Reasons were found to explain the different results and an in detail understanding of the working principle was gained.