Instabilities of Compressible Flows with Internal Heat Addition

Paxson

Journal of Engineering for Gas Turbines and Power

et al. 2017

Carbon capture and storage could significantly reduce carbon dioxide (CO2) emissions. One of the major limitations of this technology is the energy penalty for the compression of CO2 to supercritical conditions. To reduce the power requirements, supercritical carbon dioxide compressors must operate near saturation where phase change effects are important. Nonequilibrium condensation can occur at the leading edge of the compressor, causing performance and stability issues. The characterization of the fluid at these conditions is vital to enable advanced compressor designs at enhanced efficiency levels but the analysis is challenging due to the lack of data on metastable fluid properties. In this paper, we assess the behavior and nucleation characteristics of high-pressure subcooled CO2 during the expansion in a de Laval nozzle. The assessment is conducted with numerical calculations and corroborated by experimental measurements. The Wilson line is determined via optical measurements in the range of 41–82 bar. The state of the metastable fluid is characterized through pressure and density measurements, with the latter obtained in a first-of-its-kind laser interferometry setup. The inlet conditions of the nozzle are moved close to the critical point to allow for reduced margins to condensation. The analysis suggests that direct extrapolation using the Span and Wagner equation of state (S–W EOS) model yields results within 2% of the experimental data. The results are applied to define inlet conditions for a supercritical carbon dioxide compressor. Full-scale compressor experiments demonstrate that the reduced inlet temperature can decrease the shaft power input by 16%.

Section: Introductionmentioning

confidence: 99%

Characterization of Nonequilibrium Condensation of Supercritical Carbon Dioxide in a de Laval Nozzle

Paxson

Journal of Engineering for Gas Turbines and Power

et al. 2017

“…As fluid property measurements in rotating machines are challenging, converging-diverging nozzles are often the method of choice for the characterization of states in multiphase flows. Gyarmathy [2], Schnerr [5], Ryzhov [6], Guha [7], Duff [8], Nakagawa et al [9], and Bier et al [17][18] investigated the rapid expansion of several fluids in nozzles, including CO 2 . Expansion rates of 1 o C/µs are common in turbomachinery applications and can be reproduced in converging-diverging nozzles.…”

Section: Introductionmentioning

confidence: 99%

“…Expansion rates of 1 o C/µs are common in turbomachinery applications and can be reproduced in converging-diverging nozzles. The work by Schnerr [5] looked at the high speed condensation of steam with subcooling as much as 40 K and nucleation rates of ~1022-1025 m -3 s -1 . Gyarmathy [2], Bier et al [17][18] and Duff [8] determined the onset of condensation with static pressure measurements.…”

Section: Introductionmentioning

confidence: 99%

Characterization of Non-Equilibrium Condensation of Supercritical Carbon Dioxide in a de Laval Nozzle

Paxson

Volume 9: Oil and Gas Applications; Supercritical CO2 Power Cycles; Wind Energy

et al. 2017

On a ten-year timescale, Carbon Capture and Storage could significantly reduce carbon dioxide (CO2) emissions. One of the major limitations of this technology is the energy penalty for the compression of CO2 to supercritical conditions, which can require up to 15% of the plant’s gross power output. To reduce the power requirements supercritical carbon dioxide compressors must operate at reduced temperatures and near saturation where phase change effects are important. Non-equilibrium condensation can occur in the high-speed flow at the leading edge of the compressor, causing performance and stability issues. The characterization of the fluid at these conditions is vital to enable advanced compressor designs at enhanced efficiency levels but the analysis is challenging due to the lack of data on the metastable fluid properties. In this paper we assess the metastable behavior and nucleation characteristics of high-pressure subcooled carbon dioxide during the expansion in a Laval nozzle. The assessment is conducted with numerical calculations, supported and corroborated by experimental measurements. The Wilson line is determined via optical measurements in the range of 41 and 82 bar and near the critical point. The state of the metastable fluid is fully characterized through pressure and density measurements, with the latter obtained in a first of its kind laser interferometry set up. In a systematic analysis the inlet conditions of the nozzle are moved close to the critical point to allow for large gradients in fluid properties and reduced margin to condensation. The results of calculations using a direct extrapolation of the Span and Wagner equation of state model are compared with the experimental measurements. The analysis suggests that the direct extrapolation using the Span and Wagner model yields results within 2% of the experimental data, with improved accuracy at conditions away from the critical point. The results are applied in a pre-production supercritical carbon dioxide compressor and are used to define inlet conditions at reduced temperature but free of condensation. Full-scale compressor experiments demonstrate that the new inlet conditions can reduce the shaft power input by 16%.

“…Condensation due to rapid expansion in high-speed flows usually occurs at nonequilibrium conditions [5]. Due to the rapid expansion rate, the fluid can reach pressures and temperatures below saturation without condensation.…”

Section: Introductionmentioning

confidence: 99%

“…This is often referred to as the Wilson line or the spinodal limit. Schnerr [5] indicates that an expansion rate of 1 C/ls, typical of high-speed flows in converging-diverging nozzles, leads to subcooling of up to 30-40 K to metastable conditions and onset of condensation at maximum nucleation rates of $10 22 -10 25 m À3 s À1 . The fluid eventually reverts to a state of equilibrium, reaching condition E. The difference in temperature between states E and C, which is the state the fluid would reach in a quasi-steady expansion under equilibrium condensation, is what leads to the mentioned thermodynamic wetness losses.…”

Section: Introductionmentioning

confidence: 99%

An Investigation of Condensation Effects in Supercritical Carbon Dioxide Compressors

Yang

Journal of Engineering for Gas Turbines and Power

2015

Supercritical CO2 (S-CO2) power cycles have demonstrated significant performance improvements in concentrated solar and nuclear applications. These cycles promise an increase in thermal-to-electric conversion efficiency of up to 50% over conventional gas turbines (Wright, S., 2012, “Overview of S-CO2 Power Cycles,” Mech. Eng., 134(1), pp. 40–43), and have become a priority for research, development, and deployment. In these applications the CO2 is compressed to pressures above the critical value using radial compressors. The thermodynamic state change of the working fluid is close to the critical point and near the vapor–liquid equilibrium region where phase change effects are important. This paper presents a systematic assessment of condensation on the performance and stability of centrifugal compressors operating in S-CO2. The approach combines numerical simulations with experimental tests. The objectives are to assess the relative importance of two-phase effects on the internal flow behavior and to define the implications for radial turbomachinery design. The condensation onset is investigated in a systematic manner approaching the critical point. A nondimensional criterion is established that determines whether condensation might occur. This criterion relates the time required for stable liquid droplets to form, which depends on the expansion through the vapor–pressure curve, and the residence time of the flow under saturated conditions. Two-phase flow effects can be considered negligible when the ratio of the two time scales is much smaller than unity. The study shows that condensation is not a concern away from the critical point. Numerical two-phase calculations supported by experimental data indicate that the timescale associated with nucleation is much longer than the residence time of the flow in the saturated region, leaving little opportunity for the fluid to condense. Pressure measurements in a converging diverging nozzle show that condensation cannot occur at the level of subcooling characteristic of radial compressors away from the critical point. The implications are not limited to S-CO2 power cycles but extend to applications of radial machines for dense, saturated gases. In the immediate vicinity of the critical point, two-phase effects are expected to become more prominent due to longer residence times. However, the singular behavior of thermodynamic properties at the critical point prevents the numerical schemes from capturing important gas dynamic effects. These limitations require experimental assessment, which is the focus of ongoing and future research.