Session: 15-02 Numerical Studies of Internal Cooling 1

Paper Number: 151493

Computational and Experimental Investigation of Discrete “W” Turbulators for sCO2 Turbine Internal Cooling 

Direct-fired supercritical CO₂ (sCO₂) cycles are of particular interest for power generation due to the potential for higher thermal efficiencies with inherent CO₂ capture. Direct fired sCO₂ turbines must withstand high temperatures and the density of the working fluid is much higher than conventional Brayton cycles. In addition to thermal barrier coatings, active cooling approaches will be required to achieve the desired efficiency and cost targets. Based on conventional Brayton cycle applications, internal serpentine cooling channels with ribbed turbulator strips are a common approach to achieve the required cooling, while minimizing coolant utilization. Most of the previous studies of coolant channel heat transfer enhancements have focused on serpentine channels using air as the cooling media at Reynold’s numbers typical of Brayton cycles. This paper investigates the Nusselt number augmentation of coolant channels with no features, ribbed turbulators, and discrete “W” turbulators with surface roughness effects included and CO₂ as the coolant. The numerical results are compared to experimental results using CO₂ as the cooling media at Reynold’s numbers ranging from 100,000 to 300,000, as well as, prior publications. The results show the Nusselt number augmentation plateaus with increasing Reynolds number, in contrast to the decay observed in prior publications. Additionally, the CFD results indicate the discrete “W” geometry provides a 10% increase in Nusselt number augmentation relative to the angled ribbed turbulators.

This study carried out computational fluid dynamics (CFD) simulations for three serpentine pathway turbulator geometries (plain, ribbed, and discrete “W”), at Reynolds numbers ranging from 100,000 to 300,000. The settings for these simulations were supported by values obtained from literature review as well as those obtained from experimental investigation. These simulations were validated against existing CFD simulations in the literature and were found to be comparable. Once these simulations were completed, values were obtained from the results and Nusselt number augmentation, frictional losses, and thermal performance for each variation were calculated and plotted. These results were compared to experimental results obtained for the similar geometries.

There are several interesting findings for the discrete “W” geometry when compared to other geometries, as well as to the same geometry with air as the working fluid. First, unlike with air as the working fluid, with sCO₂ as the working fluid and the discrete “W” geometry we see Nusselt number augmentation plateaus rather than decays with increasing Reynolds number. Additionally, the CFD results indicate the discrete “W” geometry may yield up to a 10% increase in Nusselt number augmentation. Overall, the findings of this study suggest improvements for internal techniques can be achieved by the investigation of novel turbulator geometries for use in sCO₂ conditions.

 

Presenting Author: Owen Grabowski National Energy Technology Laboratory

Presenting Author Biography: Attended West Virginia University obtaining a B.S. in Mechanical Engineering and a B.S. in Aerospace Engineering, after which accepted a position as a research scientist with the National Energy Technology Laboratory having contributed to several projects with topics ranging from super critical CO2 turbine cooling to geothermal heating and cooling system design.

Authors:

Owen Grabowski National Energy Technology Laboratory
Matthew Searle National Energy Technology Laboratory
Douglas Straub National Energy Technology Laboratory