Carbon capture and storage (CCS) is broadly recognised as having the potential to play a key role in meeting climate change targets, delivering low carbon heat and power, decarbonising industry and, more recently, its ability to facilitate the net removal of CO2 from the atmosphere. However, despite this broad consensus and its technical maturity, CCS has not yet been deployed on a scale commensurate with the ambitions articulated a decade ago. Thus, in this paper we review the current state-of-the-art of CO2 capture, transport, utilisation and storage from a multi-scale perspective, moving from the global to molecular scales. In light of the COP21 commitments to limit warming to less than 2 °C, we extend the remit of this study to include the key negative emissions technologies (NETs) of bioenergy with CCS (BECCS), and direct air capture (DAC). Cognisant of the non-technical barriers to deploying CCS, we reflect on recent experience from the UK's CCS commercialisation programme and consider the commercial and political barriers to the large-scale deployment of CCS. In all areas, we focus on identifying and clearly articulating the key research challenges that could usefully be addressed in the coming decade.

/pdf/carbon-capture-and-storage-ccs-the-way-forward-11rgsqyer0.pdf

Carbon capture and storage (CCS): the way forward

http://www.pvv.org/~stm/research/thermofluid.pdf

CO2 transport: Data and models – A review

While the wake of a circular cylinder and, to a lesser extent, the normal flat plate have been studied in considerable detail, the wakes of elliptic cylinders have not received similar attention. However, the wakes from the first two bodies have considerably different characteristics, in terms of three-dimensional transition modes, and near- and far-wake structure. This paper focuses on elliptic cylinders, which span these two disparate cases. The Strouhal number and drag coefficient variations with Reynolds number are documented for the two-dimensional shedding regime. There are considerable differences from the standard circular cylinder curve. The different three-dimensional transition modes are also examined using Floquet stability analysis based on computed two-dimensional periodic base flows. As the cylinder aspect ratio (major to minor axis) is decreased, mode A is no longer unstable for aspect ratios below 0.25, as the wake deviates further from the standard Benard–von Karman state. For still smaller aspect ratios, another three-dimensional quasi-periodic mode becomes unstable, leading to a different transition scenario. Interestingly, for the 0.25 aspect ratio case, mode A restabilises above a Reynolds number of approximately 125, allowing the wake to return to a two-dimensional state, at least in the near wake. For the flat plate, three-dimensional simulations show that the shift in the Strouhal number from the two-dimensional value is gradual with Reynolds number, unlike the situation for the circular cylinder wake once mode A shedding develops. Dynamic mode decomposition is used to characterise the spatially evolving character of the wake as it undergoes transition from the primary Benard–von Karman-like near wake into a two-layered wake, through to a secondary Benard–von Karman-like wake further downstream, which in turn develops an even longer wavelength unsteadiness. It is also used to examine the differences in the two- and three-dimensional near-wake state, showing the increasing distortion of the two-dimensional rollers as the Reynolds number is increased.

/pdf/low-reynolds-number-wakes-of-elliptical-cylinders-from-the-463b64cw63.pdf

Low-Reynolds-number wakes of elliptical cylinders: from the circular cylinder to the normal flat plate

Carbon capture and utilization (CCU) has attracted increased attention as a means to mitigate and adapt to climate change. CCU technology regards CO2 as a raw material and reduces CO2 emissions. Ho...

Carbon Capture and Utilization Technology without Carbon Dioxide Purification and Pressurization: A Review on Its Necessity and Available Technologies

Analysis and prediction of vortex-induced vibrations of variable-tension vertical risers in linearly sheared currents

The complex wake behind two side-by-side flat plates placed normal to the inflow direction has been explored in a direct numerical simulation study. Two gaps, and , were considered, both at a Reynolds number of 1000 based on the plate width and the inflow velocity. For gap ratio , the biased gap flow resulted in an asymmetric flow configuration consisting of a narrow wake with strong vortex shedding and a wide wake with no periodic near-wake shedding. Shear-layer transition vortices were observed in the wide wake, with characteristic frequency 0.6. For , two simulations were performed, started from a symmetric and an asymmetric initial flow field. A symmetric configuration of Karman vortices resulted from the first simulation. Surprisingly, however, two different three-dimensional instability features were observed simultaneously along the span of the upper and lower plates. The spanwise wavelengths of these secondary streamwise vortices, formed in the braid regions of the primary Karman vortices, were approximately and , respectively. The wake bursts into turbulence some – downstream. The second simulation resulted in an asymmetric wake configuration similar to the asymmetric wake found for the narrow gap , with the appearance of shear-layer instabilities in the wide wake. The analogy between a plane mixing layer and the separated shear layer in the wide wake was examined. The shear-layer frequencies obtained were in close agreement with the frequency of the most amplified wave based on linear stability analysis of a plane mixing layer.

Turbulent wake behind side-by-side flat plates: computational study of interference effects

The transition phenomena in the wake of an inclined flat plate at angle of attack 25 degrees are investigated numerically. The Reynolds number, based on the free-stream velocity, plate width and kinematic viscosity, between 275 and 800 has been considered. The Strouhal number versus Reynolds number curves were plotted and compared with two-dimensional simulation data. In the present three-dimensional simulation results, for Reynolds number above 350, the Strouhal numbers converge to a constant value and multiple basic frequencies are detected at certain Reynolds numbers. The spanwise wavelength of secondary structure is estimated by using the autocorrelation method. In the range of Reynolds numbers investigated the spanwise wavelengths, non-dimensionalized by the plate projected width, have a constant value which is consistent with the second instability wavelength detected in the case with the plate normal to the flow.

https://iopscience.iop.org/article/10.1088/1742-6596/318/3/032044/pdf

Three-dimensional transition characteristics in the wake of an inclined flat plate

This paper reports occurrence of vortex shedding behind bluff-bodies in gas explosions, methods to suppress them using passive flow control techniques, and their overall impact on explosion overpressures. The pressure-time histories from a series of explosion tests, using an initially quiescent propane-air mixture in a vented channel of dimensions 1.5 m × 0.28 m × 0.3 m, are presented. Selected high-speed video frames visualizing the flame propagation are also presented. Three different bluff-obstruction scenarios are considered: 1) a reference case with a single smooth circular cylinder of diameter D = 0.0157 m, 2) a single cylinder identical to that in the reference case, mounted with a splitter plate of varying length from 5.13D to 0.26D, width 17.8D and thickness 0.06D, and 3) a single helically wired cylinder with wire diameter 0.1D and pitch 4D or 8D. All circular cylinders had a length of 17.8D and were mounted normal to the direction of the flow, spanning the channel cross-section 0.5 m downstream of the ignition point. The obstructions were inserted in the rig using a unique experimental setup. The peak overpressure generated by the explosion is of main interest. Both vortex shedding suppression techniques 2) and 3) yielded significant reduction in maximum overpressures when compared to the reference cylinder case 1). While all splitter plate configurations successfully reduced the maximum explosion overpressure, the splitter plates with length 1.02D and 0.51D were the most efficient, with an average reduction in overpressure of 32 ± 3%. The helical steel wire configurations also had a significant effect, with 25 ± 3% and 20 ± 3% reduction in the maximum overpressure for pitch 4D and 8D, respectively. The high-speed video visualization further buttressed the quantitative findings in the pressure measurements and clearly showed vortex shedding suppression. The current observations imply that the contribution from vortex shedding, i.e. apart from turbulence effects, to the overpressure generation in gas explosions is significant. The modelling community must consider this while preparing their simulators.

Suppression of vortex shedding and its mitigation effect in gas explosions: an experimental study

The three-dimensional transition to turbulence in the wake of a tapered circular cylinder with the taper ratio 75:1 has been analyzed by performing direct numerical simulation. The Reynolds number based on the uniform inflow velocity and the diameters at the wide and narrow ends were 300 and 102, respectively. The same Reynolds number range was previously studied by Parnaudeau et al. (J. Turbulence, 2007) but with a different taper ratio 40:1. The effect of taper ratio on the transition to turbulence was investigated in the present study. It was found that the Strouhal number versus Reynolds number curves nearly collapse, thereby indicating that a change in the taper ratio by a factor of two has only a modest effect on the Strouhal number. However, there still exists a significant contrast in the cellular shedding pattern. Flow-visualization of instantaneous λ2-structures and the enstrophy ǀ ω ǀ revealed that the mode A instability appeared around Re ≈ 200 and mode B around Re ≈ 250.

Direct numerical simulation of vortex shedding behind a linearly tapered circular cylinder

Unsteady RANS (Reynolds Averaged Navier Stokes) computations of turbulent trailing-edge flow have been carried out at a Reynolds number of 1000 (based on the free-stream quantities and the trailing-edge thickness) using a 3D Finite Volume code, in which a Low Reynolds Number (LRN) model and model were implemented. The realizability constraints suggested by Durbin was also implemented in the models to study their effect on the unsteady vortex shedding. Central differencing scheme was used for all the computations, but van-Leer scheme was also tried in order to study the influence of numerical schemes on unsteady vortex shedding. Results from a Direct Numerical Simulation (DNS) of the same flow are available for comparison and assessment of turbulence models used in the URANS code. Two-dimensional URANS calculations are carried out with turbulence mean properties from the DNS used at the inlet; the inflow boundary-layer thickness is 6.42 times the trailing-edge thickness, close to typical turbine blade flow applications. The LRN model failed to capture unsteady vortex shedding, but when realizability constraint was implemented, the model captures flow unsteadiness. Many of the key flow features observed in DNS are also predicted by the modelling; the flow oscillates in a similar way to that found in bluff-body flow with a von Kármán vortex sheet produced downstream. The recirculation bubble predicted by unsteady RANS has a similar shape and a length close to DNS. It was found that the unsteadiness plays an important role in the near wake, comparable to the modelled turbulence. A spectral analysis applied to the lift and pressure drag coefficients show that a Strouhal number based on the trailingedge thickness is 0.104, very close to DNS value. It was found that van-Leer scheme being a bounded scheme and dissipative in nature, produces higher resolved kinetic energy compared to central difference scheme; the reason for which has to be investigated.

/pdf/unsteady-rans-simulation-of-turbulent-trailing-edge-flow-15sgwwc0ye.pdf

Vagesh D. Narasimhamurthy

Papers

Turbulent wake behind side-by-side flat plates: computational study of interference effects

Three-dimensional transition characteristics in the wake of an inclined flat plate

Suppression of vortex shedding and its mitigation effect in gas explosions: an experimental study

Direct numerical simulation of vortex shedding behind a linearly tapered circular cylinder

Unsteady-RANS Simulation of Turbulent Trailing-Edge Flow