Ultraviolet spectroscopy of pressurized and supercritical carbon dioxide

Carbon dioxide (CO2) is prevalent in planetary atmospheres and sees use in a variety of industrial applications. Despite its ubiquitous nature, its photochemistry remains poorly understood. In this work we explore the density dependence of pressurized and supercritical CO2 electronic absorption spectra by vacuum ultraviolet spectroscopy over the wavelength range 1455-2000 Å. We show that the lowest absorption band transition energy is unaffected by a density increase up to and beyond the thermodynamic critical point (137 bar, 308 K). However, the diffuse vibrational structure inherent to the spectrum gradually decreases in magnitude. This effect cannot be explained solely by collisional broadening and/or dimerization. We suggest that at high densities close proximity of neighboring CO2 molecules with a variety of orientations perturbs the multiple monomer electronic state potential energy surfaces, facilitating coupling between binding and dissociative states. We estimate a critical radius of ~4.1 Å necessary to cause such perturbations.

Phase transitions may explain why SARS-CoV-2 spreads so fast and why new variants are spreading faster

The novel coronavirus SARS CoV-2 responsible for the COVID-19 pandemic and SARS CoV-1 responsible for the SARS epidemic of 2002-2003 share an ancestor yet evolved to have much different transmissibility and global impact1. A previously developed thermodynamic model of protein conformations predicted that SARS CoV-2 is very close to a thermodynamic critical point, which makes it highly infectious but also easily displaced by a spike-based vaccine because there is a tradeoff between transmissibility and robustness2. The model identified a small cluster of four key mutations of SARS CoV-2 that promotes much stronger viral attachment and viral spreading. Here we apply the model to two new strains (B.1.1.7 and B.1.351)3 and predict, using no free parameters, how the new mutations can further enhance infectiousness.

Numerical and Experimental Study of Abnormal Thermo-Mechanical Effects in Supercritical Fluid

At least three regions can be distinguished on the phase diagram where one-phase fluid has abnormal thermo-mechanical properties: (1) the region above the thermodynamic critical point, strictly the supercritical fluid (SCF); (2) the region close to the coexistence curve; and (3) the region where (d2p/dv2)S<0. This chapter is devoted to the experimental and numerical study of abnormal effects in (1) and (3) regions. The calculations are carried out with the use of equations of Navier-Stokes, mass conservation, energy balance, and van der Waals or perfect gas equation of state. Two basic characteristic features of the SCF near the critical point, the accelerated temperature rise in the bulk heated from boundaries (so-called “piston effect”) and very slow relaxation of the generated inhomogeneities of thermodynamic parameters are studied. For region (3) the abnormal regimes of propagation of the shock and rarefaction waves after decay of a discontinuity are found.

Amplification of Operational Uncertainty Induced by Nonideal Flows in Supersonic Turbine Cascades

In high-temperature transcritical organic Rankine cycles (ORCs), the expansion process may take place in the neighborhood of the thermodynamic critical point. In this region, many organic fluids feature a value of the fundamental derivative of gas dynamics Γ that is less than unity. As a consequence, severe nonideal gas-dynamic effects can be possibly observed. Examples of these nonideal effects are the nonmonotonic variation of the Mach number along an isentropic expansion, oblique shocks featuring an increase of the Mach number, and a significant dependence of the flow field on the upstream total state. To tackle this latter nonideal effect, an uncertainty-quantification strategy combined with Reynolds-averaged flow simulations is devised to evaluate the turbine performance in presence of operational uncertainty. The results clearly indicate that a highly nonideal expansion process leads to an amplification of the operational uncertainty. Specifically, given an uncertainty in the order of 1% in cycle nominal conditions, the mass flow rate and cascade losses vary ±4% and ±0.75 percentage points, respectively. These variations are four and six times larger than those prompted by an ideal-like expansion process. The flow delivered to the first rotating cascade is severely altered as well, leading to local variations in the rotor incidence angle up to 10 deg. A decomposition of variance contributions reveals that the uncertainty in the upstream total temperature is mainly responsible for these variations. Finally, the understanding of the physical mechanism behind these changes allows us to generalize the present findings to other organic-fluid flows.

Interferometric Study of the Heat Transfer Phenomena Induced by Rapid Heating of Nickel Sheet

Visualization of the heat transfer phenomena induced by the rapid heating of nickel sheets was carried out using a Mach–Zehnder interferometer (MZI) and a high-speed camera. This phenomenon may be an important factor in heat transfer phenomena when the working fluids reach the thermodynamic critical point. The effect of heat transfer on the heating conditions of a nickel sheet was quantified by finite fringe analysis. The results show that isotherms near the heating surface with rapid heating are generated, and the induced isotherms are moved upward with similar patterns for different heating conditions. In addition, it is confirmed that the local Nusselt number decreases to the relationship of a secondary function if the thickness of the metal specimen is very thin and the time to reach the highest temperature is very short. Moreover, it decreased according to the increase of heating energy because the heat transfer mainly occurred by conduction and radiation rather than by convection, because the expansive force and compressive force between the fluid layers on the wall increased due to an increase in the heating energy in the beginning.

Analysis of Control Rod Drop Accidents for the Canadian SCWR Using Coupled 3-Dimensional Neutron Kinetics and Thermal Hydraulics

The Canadian Supercritical Water-cooled Reactor (SCWR), a GEN IV reactor design, is a hybrid design of the well-established CANDU™ and Boiling Water Reactor with water above its thermodynamic critical point. Given the batch fueled design, control rods are used to manage the reactivity throughout the fuel cycle. This paper examines the consequences of a control rod drop accident (CRDA) for the Canadian SCWR. The asymmetry generated by the dropped rod requires an accurate 3-dimensional neutron kinetics calculation coupled to a detailed thermal-hydraulic model. Before simulating the CRDAs, the proper implementation of the 3D reactivity feedback was verified and various sensitivity studies were performed. This work demonstrates that the proposed safety systems for the SCWR core are capable of terminating the CRDA sequence prior to exceeding maximum sheath and centerline temperatures. In one instance involving a rod on the periphery of the core, the proposed trip setpoint (115% FP) was not exceeded and a new steady state was reached. Therefore it is recommended that the design also include provisions for a high-log rate and/or local Neutron Overpower Protection (NOP) trips, similar to existing CANDU designs such that reactor shutdown can be assured for such spatial anomalies.

A Compressible Thermohydrodynamic Analysis of Journal Bearings Lubricated With Supercritical CO2

In this study, a compressible thermohydrodynamic (THD) model was developed to examine the flow dynamics of a journal bearing lubricated with supercritical carbon dioxide (sCO2). This model employed a general form of the Reynolds equation governing compressible lubricant flows with a well-known analytical equation of state, and a viscosity model that depends on both pressure and temperature. In order to verify the model, we first compared the results of this compressible Reynolds equation to the full Navier-Stokes solutions. The accuracy of the model was found to be reasonable when the operating condition is sufficiently far from the thermodynamic critical point. Additionally, the numerical solution suggests that different temperature boundary conditions give slight different results due to the small variations of density and temperature across the gap. Finally, the general approach presented in this study introduces a new single parameter — effective bulk modulus that characterize both the frictional and thermodynamic fluid response. The model developed in this study can provide guidance for further studies of CO2-lubricated bearings operating in the full supercritical regime.

Study on Specifics of Forced-Convective Heat Transfer in Supercritical Carbon Dioxide

The appropriate description of heat transfer to coolants at the supercritical state is limited by the current understanding. Thus, this poses one of the main challenges in the development of supercritical-fluids applications for Generation-IV reactors. Since the thermodynamic critical point of water is much higher than that of carbon dioxide (CO2), it is more affordable to run heat-transfer experiments in supercritical CO2. The data for supercritical CO2 can be later scaled and used for supercritical water-based reactor designs. The objective of this paper is, therefore, to discuss the basis for comparison of relatively recent experimental data on supercritical CO2 obtained at the facilities of the Korea Atomic Energy Research Institute (KAERI) and Chalk River Laboratories (CRL) of the Atomic Energy of Canada Limited (AECL). Based on the available instrumental error, a thorough analysis of experimental errors in wall- and bulk-fluid temperatures and heat transfer coefficient was conducted. A revised heat-transfer correlation for the CRL data is presented. A dimensional criterion for the onset of the deteriorated heat transfer in the form of a linear relation between heat flux and mass flux is proposed. A preliminary heat-transfer correlation for the joint CRL and KAERI datasets is presented.

Phase Change Mechanisms in Pulsed Laser-Matter Interaction

ABSTRACTLaser micro-machining is finding many applications in materials processing and manufacturing. Various laser techniques are being used to fabricate micro-electronics, optics, and medical components. This paper will mainly deal with the fundamental issues involved in laser-matter interaction. Our studies are focused on laser induced thermal and thermomechanical phenomena and phase change mechanisms that determine the materials removal process during laser micro-machining. It is shown that during nanosecond laser machining, explosive phase change could occur, during which the liquid is superheated to close to the thermodynamic critical point, followed by an explosive, homogeneous phase transformation. On the other hand, it is observed in the experiment that the time needed for nucleation during laser induced phase explosion is on the order of one nanosecond. Thus, when a laser with a pulsewidth of the order of picosecond or less is used, it is likely that the material can be heated above the critical point, and another type of phase change, spinodal decomposition is possible. Molecular dynamics studies showed that with the use of a femtosecond laser pulse, it is possible to superheat the material to above the critical point, and spinodal decomposition is the dominant mechanism for materials removal.