Simulating S-CO2 Power Cycles with Fidelity Pointwise

A team of engineers at Combustion Research and Flow Technology, Inc. (CRAFT Tech) set out to model an experimental supercritical carbon dioxide compressor configuration. CRAFT Tech needed a solution that would resolve flow gradients with a minimum number of grid points and be flexible enough to provide the best grid for each region of the machine that they were modeling. They found both these solutions in Fidelity Pointwise.

Introduction

Supercritical carbon dioxide (S-CO2) power cycles were among the candidates that were chosen for Gen IV nuclear reactors to increase the efficiency of nuclear power plants. With gas-like behavior and liquid-like density, systems using supercritical CO2 as working fluid can be extremely compact and are valuable to the design of high-efficiency components. A few advantages of using the S-CO2 cycle are a small physical footprint, compact turbomachinery and heat exchangers, a thermal efficiency close to theoretical Carnot efficiency in the turbine inlet region, and reduced release of greenhouse gases.

Figure 1. Applications of the S-CO2 power cycle.

The Brayton Cycle using S-CO2 has gained attention for its power conversion efficiency but its ability to analyze these systems in off-design conditions is limited. By using Fidelity CFD meshing software, it is possible to model the flow transitions between liquid and gas phases with high resolution and a minimum number of grid points

Challenges

The off-design conditions that impact the performance of an S-CO2 Brayton cycle are:

·       Subcritical operations in solar thermal applications where temperature drops can result in fluid reaching the subcritical regime.

·       In start-up circumstances, the temperature and pressure have not yet worked up to full design conditions.

·       Situations where contaminants such as water impact the system performance.

A numerical framework that allows analysis in a broad range of subcritical operating conditions and fluid phase change compositions will help in addressing the off-design conditions. Further, a meshing technology that can resolve the flow gradients and use mixed grid types to suit the grid requirements for each region of the system.

User Benefits of Fidelity CFD

Fidelity CFD offers innovative customer-centric CFD solutions with a flexible end-to-end workflow that helps in accelerating the design process and in optimizing system solutions. Using Fidelity Pointwise mesh generation, the regions away from the Brayton system can be meshed using regular hexahedra to improve the solver’s convergence rate and the accuracy of the solution.

Achieving a good resolution at the leading edge of the blades is crucial, especially in off-design conditions and by using Fidelity Pointwise's T-Rex meshing technique, layers of hexahedra or prisms can be generated to study phase change around the blades. The overall cell count for the system can be reduced significantly, which in turn helps to reduce the time spent on processing the solution.

Figure 2. High-resolution areas of the grid include 1: blade leading edges, 2: fillet features at the hub, and 3: the tip gap clearance between the blade tip and the shroud.

Summary

Several designs of S-CO2 Brayton cycles need to be analyzed before opting for a design that offers an efficiency close to the Carnot cycle. Using the structured, unstructured, near-body, and off-body meshing technologies in Fidelity Pointwise, the flow transitions in the power cycle can be modeled with high resolution to accurately capture the flow physics and reduce the overall cell count of the system solution.

To learn more about Pointwise, request a free trial license.