Low-Flow-Coefficient Centrifugal Compressor Design for Supercritical CO2

Abstract

This paper presents a design strategy for very low flow coefficient multistage compressors operating with supercritical CO2 for carbon capture and sequestration (CCS) and enhanced oil recovery (EOR). At flow coefficients less than 0.01, the stage efficiency is much reduced due to dissipation in the gas-path and more prominent leakage and windage losses. Instead of using a vaneless diffuser as is standard design practice in such applications, the current design employs a vaned diffuser to decrease the meridional velocity and to widen the gas path. The aim is to achieve a step change in performance. The impeller exit width is increased in a systematic parameter study to explore the limitations of this design strategy and to define the upper limit in efficiency gain. The design strategy is applied to a full-scale reinjection compressor currently in service. Three-dimensional, steady, supercritical CO2 computational fluid dynamics (CFD) simulations of the full stage with leakage flows are carried out with the National Institute of Standards and Technology (NIST) real gas model. The design study suggests that a nondimensional impeller exit width parameter b2* = (b2/R)ϕ of six yields a 3.5 point increase in adiabatic efficiency relative to that of a conventional compressor design with vaneless diffuser. Furthermore, it is shown that in such stages the vaned diffuser limits the overall stability and that the onset of rotating stall is likely caused by vortex shedding near the diffuser leading edge. The inverse of the nondimensional impeller exit width parameter b2* can be interpreted as the Rossby number. The investigation shows that, for very low flow coefficient designs, the Coriolis accelerations dominate the relative flow accelerations, which leads to inverted swirl angle distributions at impeller exit. Combined with the two-orders-of-magnitude higher Reynolds number for supercritical CO2, the leading edge vortex shedding occurs at lower flow coefficients than in air suggesting an improved stall margin.

An Investigation of Nonlinear Flow Oscillations in a High-Pressure Centrifugal Pump

“…Even so, the RANS computations predicted the total-tostatic head rise coefficient to within 0.1% of experimentally determined values. A similar level of agreement with experimental data was demonstrated in a centrifugal pump operating with supercritical carbon dioxide [9]. The real gas computations at supercritical state and very high Reynolds numbers ($10 Â 10 6 ) in the pump were within 0.2% of measured efficiency and pressure coefficient.…”

“…One way to stabilize such pumps is to increase the recirculation at the impeller outlet by widening the flow path at impeller exit, as suggested in Gulich [7]. Other research on low flow coefficient pumps [8,9] suggests to reduce the throat area of the volute/diffuser and to make use of vaned diffusers so as to improve stability.…”

“…The narrower diffuser removes the separation and reduces the inlet flow angle by approximately 4 deg compared to the datum design. A similar methodology for defining the best nondimensional impeller width parameter was adopted in the design of low flow coefficient supercritical carbon dioxide compressors [9] and validated experimentally, demonstrating an improvement in efficiency for a single stage of up to 3.5% points.…”

“…A rigorous parametric analysis defines the best flow path width to maintain performance while increasing the stable operating range. In this regard, a useful parameter to define the best flow path width is the ratio of the nondimensional impeller outlet width to the flow coefficient defined in Lettieri et al [9] …”

“…The disadvantages are high inlet flow angles, radial and circumferential flow nonuniformities, and secondary flows, which develop during operation at off-design conditions. To increase the range of stable operation while maintaining pump performance, one design strategy is to use an impeller with a wider flow path and a vaned diffuser with reduced throat area, as outlined in Gulich [7], Casey et al [8], and Lettieri et al [9]. This approach increases the flow passage width at impeller exit, but reduces the width at diffuser inlet, effectively reducing the flow angle and potentially increasing the stable operating range.…”