Investigation And Analysis Of Vaneless Diffuser Stall And System Surge Precursor

Centrifugal Compressor is a kind of general purpose machinery for improvinggas pressure, which is widely used in many fields, plays an important role and con-sumes huge amount of energy. In recent years, the energy situation also puts forwardstringent requirements on the design of centrifugal compressors, i.e., higher pressureratio, higher efficiency,and wider working range. In these factors, the last one is themost prominent one, if possible, which can decrease the matching difficulties of au-tomotive turbochargers and increase the heating and electricity ratio of CHP system.Besides choke, stall and surge are the main causes for the limitation of stable oper-ation, hinderance of stage matching of centrifugal compressors. If not properly con-trolled, they even can lead to catastrophic disasters. However, until now, there are stillno theoretical breakthroughs especially for radial compressors. Thus, there are stillneeds to do further research until the final a final solution was found. In the followingpart, the author sums the related works and contributions to this filed which were doneduring his study for pursuing doctor's degree.1.Making new explanations about the occurrences of rotating stall. Researchmethods include numerical simulation and comparison between numerical results andexperimental results. Taking the limitations of computational resources and quanti-ties of adjusting parameters, two-dimensional simulation was adopted to investigaterotating stall from the point of core ?ow instability. The results indicate that theredoes exist a kind of stall mechanism which has little relationship with boundary layer.After the confirmation, the authors also studied the inffuences of geometry and ?owparameters on stall inception and mature stall characteristics.2. Establishing a neural network with genetic optimized initial weights and an-alyzing the experimental data on rotating stall in open literature. It was found thatdiffuser geometric parameters have distinct effects on rotating stall when the parame-ter b3/D2≈0.05. In this section, the in?uences of downstream component was alsoinvestigated with data from Florence University, Italy. It is shown that the in?uencesof volute should be considered with impeller outlet width to outlet diameter ratio lessthan a certain value. 3. Adopting radial grooves on the diffuser walls for passive control of rotatingstall. Firstly, detailed numerical study was adopted to study the in?uences of grooveson the ?ow field in vaneless diffuser; then the groove ?ow was modeled theoretically;lastly, the author made a guideline for choosing groove parameters. Fortunately, theguidelines and the groove method were put into real industry turbocharger for test inEcole Centrale de Nantes. The initial findings agree well with our theoretical results.4. Analyzing mutual interactions between impeller and volute on the inception ofrotating stall and surge. Research tools include CFD (computational ?uid dynamics),PIV and hot-wire anemometry. Through the research, the author not only answerthe question that why the optimized impeller configuration is the one with narrowestworking range but also the abnormal phenomenon that the efficiency of back-curvedimpeller configuration was even lower than the forward-curved one.5. Surge characteristics and pathology were analyzed through the pressure sig-nals acquired from different positions on a two-stage centrifugal compressor. Thefrequency response characteristics were also studied experimentally and theoretically.Based on the former research and understanding of the system, the author put forwarda new surge detection variable called coefficient of standard deviation which couldfind surge precursor and advance the time of warning.The above five sections investigated rotating stall and surge in centrifugal com-pressor. The research tools include experiments and theoretical modeling. Even itcouldn't solve the problems completely, it does enrich the scenario of rotating stalland surge and is helpful for guiding the design of centrifugal compressor with vane-less diffuser and its production.