Near Critical Point Research Articles

A micro-channel cooling system with two-phase looped thermosyphon for a supercritical CO2 Brayton cycle

The supercritical Carbon Dioxide (sCO2) closed Brayton cycle is a promising power generation technology, while the efficient cooling of CO2 and precise control of Compressor Inlet Temperature (CIT) is crucial for the high cycle efficiency and stable compressor operation due to the acute variation of thermophysical properties in the near-critical region. In conventional indirect cooling scheme, an intermediate single-phase water circuit is used to transfer heat from CO2 to water at the precooler, and then from water to the environment at the cooling tower, having the disadvantage of large pumping work consumption and high thermal resistance. In this work, a self-driven two-phase looped thermosyphon that significantly enhances the heat transfer by internal evaporation and condensation, is proposed to replace the water circuit. An experimental ultra-compact cooling system, consisting of a looped thermosyphon combined with micro-channel evaporator and condenser, filled with R134a coolant, is designed, fabricated, and tested. Visualized observation of the two-phase flow pattern and simultaneous measurement of the temperature, pressure and mass flow rates are conducted. A nodal analysis method is adopted, and a MATLAB code is developed for analyzing the internal fluid flow and the coupled sCO2-R134a-Air heat transfer, which is validated by experiment data. The results show that, the CO2 temperature could be accurately maintained at a specified near-critical point with a fluctuation of less than 1 K, and the average heat-releasing temperature can be reduced, while considerable pumping work, usually accounting for 2–5 % of the rated power output can be saved, thus contributing to increased cycle efficiency and system compactness.

Brain Network Modularity and Resilience Signaled by Betweenness Centrality Percolation Spiking

Modularity and resilience are fundamental properties of brain network organization and function. The interplay of these network characteristics is integral to understanding brain vulnerability, network efficiency, and neurocognitive disorders. One potential methodology to explore brain network modularity and resilience is through percolation theory, a sub-branch of graph theory that simulates lesions across brain networks. In this work, percolation theory is applied to connectivity matrices derived from functional MRI from human, mice, and null networks. Nodes, or regions, with the highest betweenness centrality, a graph theory quantifier that examines shortest paths, were sequentially removed from the network. This attack methodology led to a rapid fracturing of the network, resulting in two terminal modules connected by one transfer module. Additionally, preceding the rapid network fracturing, the average betweenness centrality of the network peaked in value, indicating a critical point in brain network functionality. Thus, this work introduces a methodological perspective to identify hubs within the brain based on critical points that can be used as an architectural framework for a neural network. By applying percolation theory to functional brain networks through a network phase-transition lens, network sub-modules are identified using local spikes in betweenness centrality as an indicator of brain criticality. This modularity phase transition provides supporting evidence of the brain functioning at a near-critical point while showcasing a formalism to understand the computational efficiency of the brain as a neural network.

Performance enhancement of quantum Brayton engine via Bose-Einstein condensation.

Bose-Einstein condensation is a quintessential characteristic of Bose systems. We investigate the finite-time performance of an endoreversible quantum Brayton heat engine operating with an ideal Bose gas with a finite number of particles confined in a d-dimensional harmonic trap. The working medium of these engines may work in the condensation, noncondensation, and near-critical point regimes, respectively. We demonstrate that the existence of the phase transition during the cycle leads to enhanced engine performance by increasing power output and efficiencies corresponding to maximum power and maximum efficient power. We also show that the quantum engine working across the Bose-Einstein condensation in N-particle Bose gas outperforms an ensemble of independent single-particle heat engines. The difference in the machine performance can be explained in terms of the behavior of specific heat at constant pressure near the critical point regime.

Experimental study on spray characteristics of RP-3 kerosene under the near critical condition with different injection conditions

In an aero-engine, the effect of oil-gas mixing directly affects the working condition of the combustion chamber, and the mixing process of RP-3 kerosene near the near-critical point is more complicated because its physical properties are more strongly affected by its own temperature and pressure changes. The injection process of RP-3 kerosene near the near-critical point was photographed under different injection conditions using high-speed ripple shadowing technology. It was found that the increase in air injection pressure and the distance from the nozzle outlet to the cyclone outlet would reduce the jet expansion angle and jet width. The former would reduce the jet’s self-excited oscillation frequency, and the latter would increase it.

Mechanistic Understanding of Delayed Oil Breakthrough with Nanopore Confinement in Near-Critical Point Shale Oil Reservoirs

Recently, significant breakthroughs have been made in exploring northeast China’s shale part of the Q formation. On-site observation reveals that the appearance of oil at the wellhead is only seen a long time after fracturing in many wells. Strong coupling between phase behavior and relative permeability curves in the reservoir with the near-critical point initial condition restricts the efficient development of this kind of shale oil. A series of compositional models are constructed to address the issues to reveal the cause of the late oil breakthrough. Nanopore confinement is checked by including this phenomenon in the numerical model. Before the simulations, the work gives detailed descriptions of the geology and petrophysics background of the target formation. Simulation results show that the delayed oil breakthrough is highly related to the coexistence of three phases at the beginning of production, which is not seen in common reservoirs. The extended period of purely water production complicates subsurface flow behavior and hinders the increase of medium- and long-term oil production. Early-time production behavior in such reservoirs is associated with the gas–liquid relative permeability curves and initial water saturation. Oil–water relative permeability curves affect the water-cut behavior depending on wetting properties. The potential oil-wet property slows down oil breakthroughs. Conceivably, purely gas and water phases exist due to the nanopore confinement of crude oil phase behavior; thus, the late oil production is barely related to the gas–liquid relative permeability curves.

Current status of research on heat transfer in forced convection of fluids at supercritical pressures

Investigation of heat transfer at supercritical pressures began as early as the 1930s with the study of free-convection heat transfer to fluids at a near-critical point. In the 1950s, the concept of using supercritical “steam” to increase thermal efficiency of fossil-fired thermal power plants became an attractive option. Germany, USA, the former USSR, and some other countries extensively studied supercritical heat transfer during the 1950s till the 1980s. This research was primarily focused on bare circular tubes cooled with SuperCritical Water (SCW). However, some studies were performed with modeling fluids such as SC carbon dioxide and refrigerants instead of SCW. Currently, the use of supercritical “steam” in fossil-fired thermal power plants is the largest industrial application of fluids at supercritical pressures.Near the end of the 1950s and at the beginning of the 1960s, several studies were conducted to investigate a potential of using SCW as a coolant in nuclear reactors. However, these activities were abandoned for some time and regain momentum in 1990s. In support of development of SuperCritical Water-cooled nuclear-Reactor (SCWR) concepts, first experiments have been started in annular and various bundle-flow geometries. At the same time, more numerical and CFD studies have been performed in support of our limited knowledge on specifics of heat transfer at SC Pressures (SCPs) in various flow geometries.As the first step in this process, heat transfer to SCW in vertical bare tubes can be investigated as a conservative approach (in general, heat transfer in fuel bundles will be enhanced with various types of appendages, i.e., grids, end plates, spacers, bearing pads, fins, ribs, etc.). New experiments in 1990s–2000s were triggered by several reasons: 1) thermophysical properties of SCW have been updated from 1950s to 1970s, e.g., a peak in thermal conductivity in the critical/pseudocritical points was “officially” introduced in 1990s; 2) experimental techniques have been improved; 3) in SCWRs various bundle flow geometries will be used instead of bare-tube geometry; and 4) in SC steam generators of thermal power plants larger diameter tubes/pipes (20–40 mm) are used, however, in SCWRs hydraulic-equivalent diameters of proposed bundles will be within 5–12 mm.A comparison of selected SCW heat-transfer correlations has shown that their results may differ from one to another by more than 200%. Based on these comparisons, it became evident that there is a need for a reliable, accurate and wide-range SCW heat-transfer correlation to be used. Therefore, the objective of this paper is to summarise in concise form the work performed concerning with the heat transfer at SCPs, based mainly on examples of water, carbon dioxide, and R-12.

Planck scale black hole dark matter from Higgs inflation

We study the production of primordial black hole (PBH) dark matter in the case when the Standard Model Higgs coupled non-minimally to gravity is the inflaton. PBHs can be produced if the Higgs potential has a near-critical point due to quantum corrections. In this case the slow-roll approximation may be broken, so we calculate the power spectrum numerically. We consider both the metric and the Palatini formulation of general relativity. Combining observational constraints on PBHs and on the CMB spectrum we find that PBHs can constitute all of the dark matter only if they evaporate early and leave behind Planck mass relics. This requires the potential to have a shallow local minimum, not just a critical point. The initial PBH mass is then below 106 g, and predictions for the CMB observables are the same as in tree-level Higgs inflation, ns=0.96 and r=5×10−3 (metric) or r=4× 10−8 ṡ 2 × 10−7 (Palatini).

Centrifugal Compressor Design for Near-Critical Point Applications

The supercritical CO2 (sCO2) Brayton cycle has been attracting much attention to produce the electricity power, chiefly due to its higher thermal efficiency with the relatively lower temperature at the turbine inlet compared to other common energy conversion cycles. Centrifugal compressor operating conditions in the supercritical Brayton cycle are commonly set in vicinity of the critical point, owing to smaller compressibility factor and eventually lower compressor work. This paper investigates and compares different centrifugal compressor design methodologies in close proximity to the critical point and suggests the most accurate design procedure based on the findings. An in-house mean-line design code, which is based on the individual enthalpy loss models, is compared to stage efficiency correlation design methods. Moreover, modifications are introduced to the skin friction loss calculation to establish an accurate one-dimensional design methodology. Moreover, compressor performance is compared to the experimental measurements.

Critical Heat Flux of Flowing Water in Tube for Pressure Up to Near Critical Point—Experiment and Prediction

Critical heat flux (CHF) experiment with uniform heating was performed in a tube of 8.2 mm in inner diameter and 2.4 m in heated length. The water flowed upward through the test section. The pressure covered the range from 8.6 to 20.8 MPa, mass flux 1157 to 3776 kg/m2s, inlet quality −2.79 to −0.08 (subcooling 19–337 °C), and local quality −0.97 to 0.53. For the pressure close to the near-critical point, the CHF decreased substantially with the pressure increasing. For the subcooling larger than a certain value, the CHF was related to the local condition. But for low subcooling and saturated condition, the CHF was related to the total power. The present results were in agreement with the previous experiment for the same local subcooled condition. Based on the present experimental results with subcooled and saturated conditions an empirical relation of the CHF was presented.

Effect of petrophysical matrix properties on bypassed oil recovery from a matrix-fracture system during CO2 near-miscible injection: Experimental investigation

The effects of petrophysical matrix properties such as porosity and permeability on bypassed oil recovery were investigated during CO2 injection in fractures at different miscibility regimes (first-contact miscibility, near-miscibility, and immiscibility). A special experimental setup was designed for this purpose and a series of CO2 injection experiments were performed using two different types of porous media, sandstones and carbonates. To confirm the analysis, some tests were repeated in the presence of irreducible water saturation. In addition, dimensional analysis was used to capture the dominant forces and mechanisms.The results demonstrated that the highest oil recovery was achieved within near-miscible regime for the both rock types. Furthermore, in all miscibility regimes, the oil recovery factor decreased with the increase of the rock complexity and frequency of dead-end pores, whereas it declined as the permeability decreased. However, differences in recovery factors of near-critical and super-critical tests grew. Considering the analytical calculations and the results of experiments including initial water saturation, it can be concluded that near-critical point wetting and the number of dead-end pores have significant effects on variations of the oil recovery factor. With near-critical point wetting, maximum recovery was achieved at near-critical state, and the presence of dead-end pores caused the role of this mechanism to be more noticeable. As a result, differences in the recovery factor of near-critical and super-critical tests grew.

Critical Heat Flux with Subcooled Flowing Water in Tubes for Pressures from Atmosphere to Near-Critical Point

Critical Heat Flux with Subcooled Flowing Water in Tubes for Pressures from Atmosphere to Near-Critical Point

Study of near-critical states of liquid-vapor phase transition of magnesium

Study of thermodynamic parameters of magnesium in the near-critical point region of the liquid-vapor phase transition and in the region of metal-nonmetal transition was carried out. Measurements of the electrical resistance of magnesium after shock compression and expansion into gas (helium) environment in the process of isobaric heating was carried out. Heating of the magnesium surface by heat transfer with hot helium was performed. The registered electrical resistance of expanded magnesium was about 104-105 times lower than the electrical resistance of the magnesium under normal condition at the density less than the density of the critical point. Thus, metal-nonmetal transition was found in magnesium.

Similarities and contrasts between critical point behavior of heavy fluid alkalis and d-dimensional Ising model

Abstract A brief summary is first given of recent progress in establishing the near-critical point behavior of the fluid alkalis Rb and Cs. Departure from the law of Rectilinear Diameters is emphasized, along with its consequences for theories emphasizing homogeneity and scaling. The behavior as the critical points of Rb and Cs are approached is compared and contrasted with the d -dimensional Ising model.

Influence of working fluid evaporation temperature in the near-critical point region on the effectiveness of ORC power plant operation

Abstract In the paper presented are definitions of specific indicators of power which characterize the operation of the organic Rankine cycle (ORC) plant. These quantities have been presented as function of evaporation temperature for selected working fluids of ORC installation. In the paper presented also is the procedure for selection of working fluid with the view of obtaining maximum power. In the procedure of selection of working fluid the mentioned above indicators are of primary importance. In order to obtain maximum power there ought to be selected such working fluids which evaporate close to critical conditions. The value of this indicator increases when evaporation enthalpy decreases and it is known that the latent heat of evaporation decreases with temperature and reaches a value of zero at the critical point.

Specifics of thermophysical properties and forced-convective heat transfer at critical and supercritical pressures

Investigation of heat transfer at supercritical pressures began as early as the 1930s, with the study of free-convection heat transfer to fluids at the near-critical point. In the 1950s, the concept of using supercritical “steam” to increase the thermal efficiency of fossil-fired power plants became an attractive option. Currently, using supercritical “steam” in fossil-fired power plants is the largest industrial application of fluids at supercritical pressures.

A New Model Approach for the Near-Critical Point Region: 1. Construction of the Generalized van der Waals Equation of State

To date, it has been considered that all classical equations of state (EOS) have failed to describe the properties of fluids near the critical region, where the density fluctuations have a significant influence on fluid properties. In this paper, we suggest a newly constructed equation for fluid states, the generalized van der Waals (GvdW) EOS with the highly simplified Dieterici's form P = [RT/(V - b)] - a(b/V)c by a new model potential construction describing intermolecular interactions. On the basis of the model potential construction, it is shown that the a, b, and c parameters have physical interpretations as an internal pressure, a void volume, and a dimensionless value that represents an inharmonic intermolecular cell potential, respectively. As an illustration of our model approach, we initially apply it to near the critical point (cp) region, where all classical EOS descriptions have been incorporated with experimental thermodynamic data, and we obtain a table of three parameters for 12 pure normal fluids, which precisely describes thermodynamic critical values. On the basis of the basic relations between pressure and volume at the critical point, we express the corresponding EOS in terms of the c parameter, and by this means, we also obtain a theoretical vapor-liquid equilibrium (VLE) line, which closely coincides with the experimental data for several pure normal fluids near the critical region. As a result, we show that thermodynamic properties near the critical region can be described analytically by only three parameters. In addition, to validate our EOS for the temperature-differential derivatives, we show that the calculated isochoric heat capacity (Cv) of saturated argon closely coincides with the experimental data. Moreover, the possibility of a precise description with respect to the entire fluid region is also argued, in terms of the physical cases from the triple point to the ideal gas region.