Development of helium turbine loss model based on knowledge transfer with Neural Network and its application on aerodynamic design

Helium turbines are widely used in the Closed Brayton Cycle for power generation and aerospace applications. The primary concerns of designing highly loaded helium turbines include choosing between conventional and contra-rotating designs and the guidelines for selecting design parameters. A loss model serving as an evaluation means is the key to addressing this issue. Due to the property disparities between helium and air, turbines utilizing either as working fluid experience distinct loss mechanisms. Consequently, directly applying gas turbine experience to the design of helium turbines leads to inherent inaccuracies. A helium turbine loss model is developed by combining knowledge transfer and the Neural Network method to accurately predict performance at design and off-design points. By utilizing the loss model, design parameter selection guidelines for helium turbines are obtained. A comparative analysis is conducted of conventional and contra-rotating helium turbine designs. Results show that the prediction errors of the loss model are below 0.5% at over 90% of test samples, surpassing the accuracy achieved by the gas turbine loss model. Design parameter selection guidelines for helium turbines differ significantly from those based on gas turbine experience. The contra-rotating helium turbine design exhibits advantages in size, weight, and aerodynamic performance

Beyond Positive Scaling: How Negation Impacts Scaling Trends of Language Models

Language models have been shown to exhibit positive scaling, where performance improves as models are scaled up in terms of size, compute, or data. In this work, we introduce NeQA, a dataset consisting of questions with negation in which language models do not exhibit straightforward positive scaling. We show that this task can exhibit inverse scaling, U-shaped scaling, or positive scaling, and the three scaling trends shift in this order as we use more powerful prompting methods or model families. We hypothesize that solving NeQA depends on two subtasks: question answering (task 1) and negation understanding (task 2). We find that task 1 has linear scaling, while task 2 has sigmoid-shaped scaling with an emergent transition point, and composing these two scaling trends yields the final scaling trend of NeQA. Our work reveals and provides a way to analyze the complex scaling trends of language models.Comment: Published at ACL 2023 Finding

Large-Eddy Simulation on the Aerodynamic and Thermal Characteristics in a Micropipe of the Hypersonic Engine Precooler

The precooling air-breathing technique has become a study focus in the aerospace field. Research on the internal flow and heat-transfer mechanism of the precooler is important for design and optimization. A large-eddy simulation was used to study the aerodynamic and thermal characteristics in a micropipe of the hypersonic engine precooler with supercritical methane as coolant and fuel. Under the effect of buoyancy, the high-temperature and low-density fluid near the wall in the circumferential direction gradually accumulate to the top wall. The accumulation of low-density fluid enhances the thermal acceleration effect at the top wall, which intensifies the local turbulent relaminarization and forms an M-shaped velocity distribution, resulting in the weakening of the heat transfer. On the other hand, the high-density fluid gathers to the bottom wall under the influence of gravity, the local thermal acceleration effect is weakened, and the flow heat transfer is enhanced. The influence of the relationship between the turbulent burst and the turbulent heat transfer under the effect of buoyancy is analyzed. It is found that the low-speed ejection events and high-speed sweep events are strengthened at the bottom wall, especially the low-speed ejection. However, the occurrence of these events at the top wall is restrained to a certain extent

FLOW ANALYSIS OF A SINGLE-STAGE AXIAL COMPRESSOR WITH A SPLITTER ROTOR

ABSTRACT Unsteady and time-averaged flow fields of a single-stage axial compressor with a splitter rotor working in design conditions are simulated and analyzed with 3-D unsteady CFD code. The results show that strong unsteady flow exists in the rotor passage. Splitter reconstructs the pressure distribution in the rotor passage, changes load distribution of the rotor, and controls the flow near the rotor-blade. Splitter affects the unsteady static pressure distribution and fluctuation on principle blade, splitter and stator. INTRODUCTION Application of splitter rotor to axial compressors to raise the stage loading of compressor is one effective method of raising stage loading of fans/compressors. The major design ideology is to add splitter locally on the rear end of the passage between blades, where flows separation most likely happens. The splitter vane can alleviate or erase the flow separation on the backside of the principle blade suction surface, and also can avoid blocking, efficiency decreasing and weight increasing caused by adding full length blades. In the 70s, Dr. Wennerstrom applied the splitter aerodynamic design to avoid large deviation on the outlet of high-loading rotors caused by the stage pressure ratio of more than 3.0, which is based on the traditional design. However the result of the experiment is far from the design value, the reason is that there is a large-scale separation on the blade suction surface. To solve this problem, full 3-D CFD method is required, which is unavailable at that time. Until several decades later, when computer technology and full 3-D numeric simulation programs were developed greatly, by the aid of IHPTET, Textron Lycoming Co. resumed the aerodynamic design with splitter vanes, and great development is obtained. At the end of 90s, based on new design method and full 3-D steady viscous computation, Che

Effect of surface roughness on the aerodynamic performance of turbine blade cascade

The effect of surface roughness on the boundary development and loss behavior of turbine blades is investigated with different Reynolds numbers in this paper. The result shows that the velocity profile in boundary layer is plumper on rough surface than on smooth blade. The aerodynamic loss is lowered at low Reynolds number, but becomes significantly large at high Reynolds number. The total pressure loss coefficient of cascade can reach a top increase of 129% for rougher blades comparing with smooth blades at Re=300000

Conjugate heat transfer investigation of cooled turbine using the preconditioned density-based algorithm

The preconditioned density-based conjugate heat transfer (CHT) algorithm was used to investigate the heat transfer characteristics of a cooled turbine vane. Fluid domain provided boundary heat flux for solid domain and obtained boundary temperature from it for the coupling strategy. The governing equations were solved by the preconditioned density-based finite-volume method, with preconditioning matrix, improved Abu-Gharmam Shaw (AGS) transition model, matrix dissipation scheme and four kinds of turbulence models. The grid system is multi-block structured grids for fluid domain and unstructured grids for solid domain, with full-matched grids at the fluid–solid interfaces. The effects of turbulence model, outlet Mach number, outlet Reynolds number, inlet turbulence intensity and the temperature ratio of blade surface/gas on the local heat transfer performance were studied. Results indicate that the k–ω shear-stress transport (SST) and AGS model can predict the conjugate heat transfer better than others. The Mach number and Reynolds number have relatively obvious influences on the heat transfer, while the turbulence intensity and temperature ratio only have slight influences. Comparisons with experimental data demonstrate the applicability and accuracy of the numerical algorithm