Simulation and development of dry gas seal for supercritical CO2 carbon dioxide

Abstract

The supercritical CO 2 closed loop Brayton cycle operates at high pressure to achieve higher energy conversion efficiency. One of the important components in turbomachinery of the power cycle plant is the dry gas seals. Dry gas seals are gas-lubricated, mechanical, non-contacting, end-face seals, consisting of a mating (rotating) ring and a primary (stationary) ring. Low leakage dry gas seals are considered as a key enabling technology for achieving the improved thermodynamic cycle efficiency in the supercritical CO 2 power cycle. Alternate seal types, for example, labyrinth seals, suffers from high leakage. Even so there is a growing interest and importance of applying the small length scale dry gas seal in the small to medium scale supercritical closed loop Brayton cycle (1-20 MWe), there are still uncertainties for their operation at supercritical CO 2 conditions. These include the real gas effects near the critical points and the methods of minimizing the deformation of the supercritical CO 2 dry gas seal. In the supercritical region in the vicinity of the critical point (304 K, 7.4 MPa), CO 2 behaves as a real-gas, exhibiting significant and abrupt nonlinear changes in fluid and transport properties and high densities.Comprehensive analysis is performed to simulate the supercritical CO 2 dry gas seal. First, an isothermal simulation assuming rigid sealing ring walls in the gas film is performed using ANSYS Fluent to study the influences of real gas effect on performances of the dry gas seal. Then, conjugate heat transfer simulation is used to optimize the face geometry for a small to median scale supercritical closed loop Brayton cycle (1-20 MWe) using ANSYS Fluent. Finally, the pressure and the thermal outputs from the conjugate heat transfer analysis are used as boundary conditions for one way coupling fluid-structurethermal simulations using ANSYS Static Structural to study the effect of deformation of the sealing rings under applied pressure-loads, thermal-loads, and centrifugal effect.Finding from the simulation results shows, close to critical point the real gas effect is significant, whereas far from the critical point the supercritical fluid resembles an ideal gas. The centrifugal effect is enhanced by the higher density due to the real gas effects, causing a reduction of average pressure in the dam region hence reduces the opening force, and seal leakage.Increasing the groove radius decreases the opening force whereas increasing the spiral angle, increases the opening force. However, the variation is small for all tested cases studies with variation up to 6%. Increasing the groove radius decreases the leakage rate whereas increasing the spiral angle, increases the leakage rate. The variation in changing groove radius is more significant than the spiral angles. The variation in leakage iii is up to 29.2% for all tested cases. In term of the film stiffness, there is no clear pattern when changing the groove radius but the impact of the spiral angle is significant, up to 46.6% for all tested c...

Multiphysics Numerical Investigations of Hot Components in a sCO2 Power Plant Turbine

“…This was externally evaluated by computing the Joule-Thomson coefficient µ JT using the COOLPROP library to calculate the fluid thermodynamic properties. In the test bench analyzed the fluid temperature difference across the seal is almost zero (isothermal transformation) since the J-T cooling is balanced by the friction effect: this is in line with the results found by Zakariya et al [13].…”

“…It was observed that by reducing the film gap the friction heat generated increases. Therefore, Zakaria [13] further investigates the DGS thermal behaviour in sCO 2 with CHT simulations, validating the results through experimental data with air.…”

“…This aspect, combined with a low mass-flow rate makes the fluid almost isothermal through the gap since most of the generated heat is transferred to the solid. Zakariya et al [13] showed how the fluid held only 5% of friction heat across the gap. Furthermore, real gas Joule-Thomson (J-T) leakage cooling shall be considered for a proper fluid temperature calculation.…”

Theoretical Expression and Screening of Real Gas Effect of Spiral Groove Dry Gas Seal

“…When far from the critical point, supercritical carbon dioxide can be regarded as an ideal gas. After, Z M Fairuz [12] performed isothermal simulations using ANSYS Fluent to investigate the effect of real gas effects on the performance of dry gas seals. The seal ring deformation was then analyzed by one-way coupling, and it was found that the thermal deformation had the most significant effect on the dry gas seal.…”