Helicopter lobed nozzle optimization with differential evolution method

Bayat, Akay
The main purpose of this study is to develop a methodology for the optimization of an ejector system geometry that is used for the helicopter engine bay cooling. The performance of an ejector system is directly related to the entrainment ratio, which is the ratio of the secondary flow rate to the primary flow rate, and the turbine exit backpressure. In the literature, it is observed that the nozzle geometry used in the ejector system has a significant effect on the performance and that the most efficient geometry is reported to be the lobed shape. For this reason, the optimum lobed nozzle geometry is aimed to be obtained by changing the number of lobes, the exit diameter of the lobe, and the lobe tangent radius in this study. In addition to these parameters, the outlet diameter of the center body that is included in the nozzle structure is also considered as an optimization variable. With the help of a code written in Python, the geometry of the helicopter engine compartment was modeled with "CATIA," and a solution network was created with "Pointwise." The flow analysis of geometry is performed with "Ansys Fluent," and the entrainment ratio and turbine exit back pressure values are calculated. The differential evolution (DE) method is used as an optimization approach. As a result of the optimization study, the best geometry is obtained with the minimum lobe number. Similarly, the lobe tangent radius is close to the lower bound of the given range. The nozzle exit diameter gets approximately the middle vi value of the given range while the cone exit diameter having the maximum value of the given range for the best geometry. In order to assess the effect of nozzle shape on flow physics and efficiency, the best and worst geometries in all optimization processes are compared. It is observed that the swirl effect that comes from the engine turbine is highly eliminated for the best configuration. Therefore, the cooling flow can be entrained more easily. However, this decreases the mixing efficiency of the flow.The turbine exit back pressure, on the other hand, is higher for this model since the nozzle exit diameter is relatively small. In addition to the analysis of optimum geometry, effects of all optimization parameters on the performance are examined one by one. It is found that the most influential parameter in the optimization process is the lobe number. It is seen that the nozzle geometry with low-lobe numbers has better performance. On the other hand, the higher cone exit diameter affects the performance well. There is no net effect of the other parameters on the system performance.
Citation Formats
A. Bayat, “Helicopter lobed nozzle optimization with differential evolution method,” M.S. - Master of Science, Middle East Technical University, 2021.