| Steady method is commonly used in current studies on the performance of the turbocharger turbine. But the exhaust process is a periodic non-steady state process while engine is being operation, and the waste gas from exhaust pipes flows into the turbine in the form of periodical pulse wave with the crank angle. Therefore, the steady methods used in present can not reflect the actual work process of the turbine.In this paper, the results of numerical investigation on dual entry turbines performance under unsteady flow conditions are presented by ANSYS CFX software. And the results of the diesel engine bench test are also shown as the indirect validation method of the unsteady simulation of the turbines.Firstly, the best dual entry turbine structure is found by the simulation studies under steady flow conditions. After having verified the feasibility of the simulation model, based on the twin-entry turbine that is the original one and its every entry supplies the whole360°of the turbine periphery but only half of the span, a double-entry turbine which its volute throat section area is same with the twin-entry turbine and each entry supplies a180°section of the turbine wheel, is designed. The performances of the two structures are analyzed in all operating conditions by the simulation study under steady flow conditions. The results show that the efficiency of the double-entry turbine is higher than that of the twin-entry turbine, and the double-entry turbine is relatively higher about10percent in some operating conditions. Three kinds of double-entry turbines that have different volute throat section area are designed to realize the influence of the volute throat section area on the turbine performances. After the comparison and analysis of the performances of the three double-entry turbine structures in all operating conditions by the simulation under steady flow conditions, the results reveal that the turbine that has the smallest volute throat section area has the highest efficiency, which is the best double-entry turbine structure (Structure S1for short).Secondly, the turbine unsteady characteristics of the best double-entry turbine structure (structure S1) is studied. A method is put forward to eliminate the time lag of the unsteady parameters. To solve the abnormal phenomenon that the turbine unsteady efficiency is over1or negative in present studies, in this paper, the calculation method on turbine unsteady efficiency is also improved. The working process of the studied diesel engine is simulated by AVL BOOST software to get the turbine inlet unsteady boundary conditions. The turbine unsteady characteristic circles are obtained by numerical simulation and the corresponding relations between the turbine unsteady characteristics and the inlet pulse. The filling and emptying effects included in the turbine unsteady characteristics and the influence of pulses entering from the inner and outer limbs of the turbine on the turbine unsteady performances are explained.Thirdly, the impacts of turbine speed and pulse amplitude on the double-entry turbine unsteady performances are explored. The study states that the turbine speed has little impact on the mean turbine pulse performances, and the working condition that has higher pulse amplitude has a higher efficiency, a higher output power and smaller flow loss in the scope of the study. To reveal the change law of the turbine in the work process, the turbine unsteady performances under different engine external characteristic conditions are studied. In the research range, the mean turbine pulse efficiency, the mean turbine pulse output power and the mean turbine pulse flow parameter are rising gradually with the increase of the diesel engine working conditions, while it has a decreasing trend in the overall entropy.Fourthly, the numerical research of performance comparison for the twin-entry turbine and the three kinds of double-entry turbines mentioned above is carried out under the same boundary conditions. The work indicates that the mean turbine pulse efficiencies are approximately equal for the twin-entry turbine and the double-entry turbine that have same volute throat section area, which there is no significant difference. For the three kinds of double-entry turbines that have different volute throat section area, the turbine that has the smallest volute throat section area (structure S1) has the highest mean turbine pulse efficiency, and the turbine that has the maximum volute throat section area has the lowest. But the three double-entry turbines have no significant difference in the overall entropy, which the maximum difference is less than0.3%.Furthermore, the diesel engine bench tests installing the four turbines mentioned above are also carried out. The tests show that different turbine structure has different turbine speed under a same engine speed condition. Compared with the original turbine-the twin-entry turbine, the double-entry turbine that has the maximum volute throat section area has no obvious advantage, and the double-entry turbine that has same volute throat section area with the twin-entry turbine has advantages in above1700r/min engine external characteristic conditions. The turbine that has the smallest volute throat section area (structure S1) has advantages in all engine external characteristic conditions compared with the original turbine, which the maximum increase of engine torque and power can be up to11.5%, and the maximum decrease of fuel consumption rate is10.2%. The results of the tests have indirectly verified the validity of the unsteady simulations on the turbines.Lastly, based on the results of the engine bench tests, the inlet boundary conditions of the four kinds of turbines are improved to explore the unsteady performances of the turbines under actual operation... |
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