Award Date

December 2023

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Mechanical Engineering

First Committee Member

Yi-Tung Chen

Second Committee Member

Hui Zhao

Third Committee Member

Huang Chen

Fourth Committee Member

Melissa Morris

Fifth Committee Member

Jichun Li

Number of Pages

230

Abstract

Supercritical carbon dioxide (sCO2) has emerged as a promising working fluid for the Brayton power cycle, offering enhanced efficiency due to the high density and low viscosity of carbon dioxide (CO2) near the critical point. A critical component of the sCO2 Brayton cycle is the turbomachinery, namely the compressor, which significantly contributes to the efficiency enhancement in the sCO2 Brayton cycle owing to reduced compression work requirements. Its primary function involves elevating the fluid's pressure, effectively priming it for the subsequent heat addition process from a thermal energy source. However, despite its advantages, challenges persist that need to be addressed before the adoption of the technology. These challenges stem from factors such as elevated operational pressures and the complex real gas behavior of CO2. While CO2 in its supercritical state boasts favorable properties, its thermophysical and transport properties exhibit increased sensitivity to variations in temperature and pressure, particularly in proximity to the critical point. This not only introduces complexities in the operation of sCO2 compressors but also in their analysis. All these considerations must be carefully addressed during the compressor's design to accommodate the diverse behaviors that sCO2 can manifest under both design and off-design conditions.

This study aims to numerically investigate the aerothermodynamic phenomena inherent in compressors utilizing sCO2 within the framework of sCO2 Brayton power cycles. This effort is prompted by the demand to develop novel and adaptable power cycles that align with the requisites of sustainable energy generation. Traditional analyses of sCO2 compressors have predominantly encompassed meanline analysis and computational fluid dynamics (CFD) evaluation. Although meanline methodologies offer crucial preliminary design insights, transitioning to comprehensive CFD analysis faces challenges due to complex physical phenomena at operational conditions. This is particularly notable in scenarios where compressors exhibit enhanced efficiencies, demanding computationally intensive numerical analysis.

To address this complexity, this dissertation places its focus on the development of a comprehensive throughflow model tailored for sCO2 compressor analysis. The model is designed to adeptly manage the intricate fluid dynamics and thermodynamics of sCO2. Enhanced and efficient numerical throughflow models for sCO2 compressors can advise on the development of more efficient compressors with enhanced aerothermodynamic performance. This study presents the application of a two-dimensional (2D) streamline curvature-based throughflow method, considering real gas thermophysical properties, to analyze a centrifugal compressor operating with sCO2 at near-critical onditions. The method's validity is established through comparisons of the predicted performance characteristics with those of an sCO2 compressor available in open literature and by comparing the performance and flow field predictions from CFD analysis. The streamline curvature-based throughflow method demonstrates its effectiveness by achieving significant agreement between the flow field and performance characteristics, both at design and off-design conditions. Where pressure ratio and isentropic efficiency predictions were within 0.3522% and 6.63% , respectively, compared to CFD simulations.

Furthermore, a study on the impact of the operational conditions of the compressor emphasized the importance of accounting for variations in intake conditions, as even small changes in temperature and pressure near the critical point of CO2 can significantly affect the compressor's performance and operability. An investigation of the impact of compressor intake conditions on the aerodynamic performance of compressors is then conducted employing the throughflow analysis method, considering various intake conditions including subcritical vapor, supercritical vapor, supercritical liquid, and near-critical conditions. This consideration is essential for both sCO2 compressor design, and safe and stable operation, especially when operating at near-critical conditions.

While accounting for the impact of intake conditions is crucial, an interesting trend emerged regarding the computational resources needed during numerical analysis in the near-critical region, holding significant numerical implications. Specifically, at near-critical conditions, there was a maximum increase of 3201% in the computational run-time required for CFD-based computations compared to superheated conditions. In contrast, throughflowbased computations experienced a maximum increase of only 164% in run-time, primarily due to the numerical under-relaxation needed. This underscores the efficiency of the throughflow analysis, particularly for the early design phase of sCO2 turbomachinery, where conducting numerous CFD studies at near-critical conditions may not be practical due to higher computational resource requirements.

Additionally, a non-equilibrium condensation model based on classical nucleation theory was adapted and coupled to the throughflow model to account for the condensation of CO2 during expanding conditions in a converging nozzle to analogize the acceleration and expansion experienced by sCO2 to conditions similar at compressor blade leading edges.

The streamline curvature-based throughflow analysis method proved to be a valuable tool for predicting sCO2 compressor behavior, particularly under near-critical conditions. While challenges and limitations exist, this approach, combined with experimental and CFD analyses, offers a comprehensive framework for advancing designs and understanding of sCO2 compressors in the context of sCO2 based power cycles.

Keywords

Compressor; Streamline curvature; Supercritcal carbon dioxide; Throughflow; Turbomachinery

Disciplines

Aerodynamics and Fluid Mechanics | Mechanical Engineering | Thermodynamics

File Format

pdf

File Size

9680 KB

Degree Grantor

University of Nevada, Las Vegas

Language

English

Rights

IN COPYRIGHT. For more information about this rights statement, please visit http://rightsstatements.org/vocab/InC/1.0/

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Available for download on Sunday, December 15, 2024


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