Heat Front Research Articles

Numerical design in a two-step joint design process of parallel packed bed reactors for CLC of MSW-generated syngas

This paper presents a two-step joint design process for the effective and rational design of the parallel packed bed reactors for chemical looping combustion, fueled by the syngas from municipal solid waste gasification. It integrates phenomenological and numerical design, combining their advantages of low computational cost (the cost is reduced to 5–10 % compared with alone numerical design) and high accuracy. Based on it, the phenomenological design schemes of the 1.5 MWth reactor are firstly obtained, then these schemes are systematically validated and optimized through numerical design. In detail, comprehensive analyses of the distribution and evolution of bed parameters, which are affected by the propagation of the reaction and heat front, are performed, along with an assessment of the reactor’s operational stability. Additionally, the heat/mass regulation strategies for the unit are studied, offering insights into effective heat management. Finally, the design schemes are optimized to achieve a better performance and flexible operational compatibilities (fitting for 2/3 parallel reactors, cocurrent/countercurrent gas feeding mode, inlet gas temperature of 400–600 °C)

Heat front propagation in shale oil reservoirs during air injection: Experimental and numerical studies

Air injection technique for developing shale oil has gained significant attention. However, the ability of the heat front to consistently propagate within the shale during air injection remains uncertain. To address this, we investigated the heat front propagation within oil–detritus mixtures, shale cores, and fractured shale cores using a self-designed combustion tube (CT) and experimental schemes. By integrating the results obtained from high-pressure differential scanning calorimetry and CT, we developed a comprehensive reaction kinetics model to accurately analyze the main factors influencing the heat front propagation within fractured shale. The findings revealed that in the absence of additional fractures, the heat front failed to propagate within the tight shale. The flow of gases and liquids towards the shale core was impeded, resulting in the formation of a high-pressure zone at the front region of the shale. This pressure buildup significantly hindered air injection, leading to inadequate oxygen supply and the extinguishment of the heat front. However, the study demonstrated the stable propagation of the heat front within the oil–detritus mixtures, indicating the good combustion activity of the shale oil. Furthermore, the heat front successfully propagated within the fractured shale, generating a substantial amount of heat that facilitated the creation of fractures and enhanced gas injection and shale oil flow. It was important to note that after the heat front passed through the shale, the combustion intensity decreased. The simulation results indicated that injecting air into the main fracturing layers of the shale oil reservoir enabled the establishment of a stable heat front. Increasing the reservoir temperature (from 63 to 143 °C) and oxygen concentration in the injected gas (from 11% to 21%) promoted notable heat front propagation and increased the average temperature of the heat front. It was concluded that temperature and oxygen concentration had the most important influence on the heat front propagation, followed by pressure and oil saturation.

A unified theory of the self-similar supersonic Marshak wave problem

We present a systematic study of the similarity solutions for the Marshak wave problem in the local thermodynamic equilibrium (LTE) diffusion approximation and in the supersonic regime. Self-similar solutions exist for a temporal power law surface temperature drive and a material model with power law temperature dependent opacity and energy density. The properties of the solutions in both linear and nonlinear conduction regimes are studied as a function of the temporal drive, opacity, and energy density exponents. We show that there exists a range of the temporal exponent for which the total energy in the system decreases, and the solution has a local maxima. For nonlinear conduction, we specify the conditions on the opacity and energy density exponents under which the heat front is linear or even flat and does possess its common sharp characteristic; this characteristic is independent of the drive exponent. We specify the values of the temporal exponents for which analytical solutions exist and employ the Hammer–Rosen perturbation theory to obtain highly accurate approximate solutions, which are parameterized using only two numerically fitted quantities. The solutions are used to construct a set of benchmarks for supersonic LTE radiative heat transfer, including some with unusual and interesting properties such as local maxima and non-sharp fronts. The solutions are compared in detail to implicit Monte Carlo and discrete-ordinate transport simulations as well gray diffusion simulations, showing a good agreement, which highlights their usefulness as a verification test problem for radiative transfer simulations.

A phenomenological design method of the parallel packed bed reactors for chemical looping combustion of gas fuels

The parallel packed bed reactor (PPBR) is a promising route for industrial application of chemical looping combustion. This paper introduces a phenomenological approach for the rational design of PPBR, serving as a manner to efficiently obtain comprehensive design results. Based on the macroscopic propagation of heat and reaction fronts, it elucidates the dynamic heat/mass transfer within the bed. Subsequently, it is applied to design a 1.5 MWth reactor. The risk of carbon deposition in the bed is firstly investigated from a thermodynamic perspective. Then, the study reveals that the height-to-diameter ratio of the reactor must meet certain constraints to ensure stable and cost-effective operation, and these constraints are extended to the scale-up process. Sensitivity analysis is utilized to investigate the effect of key parameters on the design schemes and performance. Effects of particle size on intra-particle gas diffusion is analyzed. Finally, detailed design schemes and performance of the reactor are determined.

Intensifying the particle deposition from a colloidal solution for the purpose of liquid purification: Comparison of deposition mechanisms and rates

The deposition of particles from a layer of colloidal solution (water and microparticles) has been experimentally studied in the presence of laser heating. Instantaneous temperature and velocity fields are measured using a thermal imager and the Particle Image Velocimetry and the Particle Tracking Velocimetry methods. The unstable behavior of the velocity field in the presence of heating and a free liquid surface are assumed to be determined by the Bond number (Bo). Of interest is also an additional mechanism for the instability formation when the Peclet number (Pe) exceeds the critical value and when the ratio of characteristic velocities VT/Vavr > 1, where VT is the heat front velocity at the initial moment of laser heating, and Vavr is the average convection velocity. A new mechanism of particle deposition due to the use of local laser heating is proposed for the first time. The control over local heating and local deposition increases the particle deposition rate by 2–3 orders compared with that without convection. The particle deposition rate and the deposit area increase with increasing height of the liquid layer, reach maximum values at a critical height, and then decrease sharply with increasing height. The dependence for determining the area and mass of the deposit is proposed to take into account the deposition time, the height of the liquid layer and the rate of convection. The resulting particle deposition method and calculation method may be used in technologies related to the purification of liquid from solid impurities in small volumes, as well as in problems where deposition in a given direction is required to form an ordered nanocoating.

Design and experimental analysis on a single tank energy storage system integrated with a cooking unit using funnel system

The paper presents a new innovation in small scale thermal energy storage system suitable for cooking application. A single tank thermal energy storage system integrated with a cooking unit has been developed and the performance analysed. The system consists of a heat storage tank, a heating chamber referred to as a funnel, and the cooking unit. The heating chamber is a pipe made of cast iron, and on top of the pipe is a cylindrical container made of Aluminium. A heating element of 1.5 kW 220 V was inserted inside the funnel and was used for heating the oil. The top of the funnel allows a cooking pot to be inserted inside it. The funnel can be raised or lowered using screws connected to the top of the tank; this regulates the temperature at which oil begins to overflow into the heat storage tank. The heat storage tank is made of stainless-steel, and contains sunflower oil as a heat storage medium. During charging, a small volume of oil about 2 L was heated inside the funnel, and the temperature rose rapidly to about 150 °C in less than 15 min. The system allows for cooking to begin immediately after charging has been started. As heating continues, the hot oil overflows into the storage tank, and cold oil at the bottom of the tank flows into the funnel where it is heated. The storage tank had 18 L of sunflower oil. During discharge, the direction of oil flow reverses; hot oil flows from the top of the tank into the funnel. The hot and the cold oil do not mix during discharge, and therefore, the heat front is preserved. Cooking tests were demonstrated during charging and discharging cycles. About 453.6 kJ of energy was used in boiling 1.5 L of water in 10 min, and 1.04 MJ was used in boiling 1 kg of dry beans in about 2 h.

Analysis of the influence of the thermophysical parameters of the reservoir and fluid on the process of cyclic steam stimulation

Currently, the existing models of cyclic steam stimulation do not allow to research the development of the heat front at the stage of steam injection into the reservoir, considering the possible influence of both convective and conductive flows, as well as the properties of the rock and fluid. Knowledge of the dynamics of the development of fronts allows to determine the features of the ongoing physical processes at specific deposits and can be used to optimize the production process. The aim of the work is to analyze the development of the thermal field in rocks with different thermophysical parameters. A hydrodynamic model of the development of the thermal field was developed and calculated on the tNavigator software package for rocks with various thermophysical properties. The model considering the three-dimensional propagation of heat, the properties of reservoir and injected fluids, as well as heat loss into the top and the bottom of the reservoir. The comparison of the maximum dimensions of the heated area calculated according to the developed hydrodynamic model with the results obtained by the integral model is carried out. The dependence of the advance of the thermal front on the specific heat capacity and thermal conductivity of the rock is determined. The thermal front moves much further in rocks with a lower specific heat capacity and thermal conductivity with the same injection volume. It is shown that the convective component of heat exchange weakly depends on the specific heat capacity and the shape of the heat front remains the same in all cases.

How Can (or Why Should) Process Engineering Aid the Screening and Discovery of Solid Sorbents for CO2 Capture?

ConspectusAdsorption using solid sorbents is emerging as a serious contender to amine-based liquid absorption for postcombustion CO2 capture. In the last 20+ years, significant efforts have been invested in developing adsorption processes for CO2 capture. In particular, significant efforts have been invested in developing new adsorbents for this application. These efforts have led to the generation of hundreds of thousands of (hypothetical and real) adsorbents, e.g., zeolites and metal-organic frameworks (MOFs). Identifying the right adsorbent for CO2 capture remains a challenging task. Most studies are focused on identifying adsorbents based on certain adsorption metrics. Recent studies have demonstrated that the performance of an adsorbent is intimately linked to the process in which it is deployed. Any meaningful screening should thus consider the complexity of the process. However, simulation and optimization of adsorption processes are computationally intensive, as they constitute the simultaneous propagation of heat and mass transfer fronts; the process is cyclic, and there are no straightforward design tools, thereby making large-scale process-informed screening of sorbents prohibitive.This Account discusses four papers that develop computational methods to incorporate process-based evaluation for both bottom-up (chemistry to engineering) screening problems and top-down (engineering to chemistry) inverse problems. We discuss the development of the machine-assisted adsorption process learning and emulation (MAPLE) framework, a surrogate model based on deep artificial neural networks (ANNs) that can predict process-level performance by considering both process and material inputs. The framework, which has been experimentally validated, allows for reliable, process-informed screening of large adsorbent databases. We then discuss how process engineering tools can be used beyond adsorbent screening, i.e., to estimate the practically achievable performance and cost limits of pressure vacuum swing adsorption (PVSA) processes should the ideal bespoke adsorbent be made. These studies show what conditions stand-alone PVSA processes are attractive and when they should not be considered. Finally, recent developments in physics-informed neural networks (PINNS) enable the rapid solution of complex partial differential equations, providing tools to potentially identify optimal cycle configurations. Ultimately, we provide areas where further developments are required and emphasize the need for strong collaborations between chemists and chemical engineers to move rapidly from discovery to field trials, as we do not have much time to fulfill commitments to net-zero targets.

Gas Production Potential in Geothermal-Energy-Enhanced CH4–CO2 Swapping Processes

Carbon capture and storage has become a practice to reduce the greenhouse effect of carbon dioxide (CO2) on the global climate. Recent studies have generated increasing concerns about CO2 leakage from underground structures. This has called for more research on CH4–CO2 swapping in natural gas hydrate (NGH) reservoirs to lock CO2 in a solid state in underground structures. Because the CH4–CO2 swapping is too slow to be efficient, this study proposes to use geothermal energy to accelerate the process. This paper presents a technical feasibility analysis of using geothermal energy to assist CH4–CO2 swapping for simultaneously storing CO2 in NGH reservoirs and producing the dissociated natural gas. Mathematical models were developed to compute heat transfer from geothermal zones to NGH reservoirs. A case study was carried out using the data from an NGH reservoir in the Shenhu area, Northern South China Sea. The result of the case study indicates that heat conduction dictates the heat transfer process when the heat convection flow rate is less than 0.01 m3/s over a heat-releasing borehole length of 2,000 m. Heat convection can significantly accelerate the heat transfer inside the gas hydrate reservoir. The 15°C (designed gas hydrate dissociation temperature in the studied case) heat front will propagate to the upper and lower boundaries of the gas hydrate reservoir (39 ft or 12 m) in 220 days by heat conduction only. This time can be shortened to 140 days with the aid of a fluid convection rate of 0.005 m3/s. Geothermal heating can significantly increase the initial productivity of wells in heated gas hydrate reservoirs in CO2 swapping processes. When the gas hydrate reservoir is heated from 6 to 16°C, the fold of increase is expected to exceed five in the studied case. This study shows that CH4–CO2 swapping process using geothermal stimulation is a promising method for producing natural gas and locking CO2 permanently in NGH reservoirs. Further studies should first focus on investigations of the effect of CO2-hydrate formation on the CO2 mass transfer inside reservoirs.

Feedback in the Extremely Violent Group Merger NGC 6338

The galaxy group NGC 6338 is one of the most violent group–group mergers known to date. While the central dominant galaxies rush at each other at 1400 km s−1 along the line of sight, with dramatic gas heating and shock fronts detected, the central gas in the BCGs remains cool. There are also indications of feedback from active galactic nuclei, and neither subcluster core has been disrupted. With our deep radio uGMRT data at 383 and 650 MHz, we clearly detect a set of large, old lobes in the southern BCG coinciding with the X-ray cavities, while the northern and smaller BCG appears slightly extended in the radio. The southern BCG also hosts a smaller younger set of lobes perpendicular to the larger lobes, but also coinciding with the inner X-ray cavities and matching the jet direction in the parsec-resolution VLBA image. Our spectral analysis confirms the history of two feedback cycles. The high radio frequency analysis classifies the compact source in the southern BCG with a power law, while ruling out a significant contribution from accretion. The radio lightcurve over three decades shows a change about 10 yr ago, which might be related to ongoing feedback in the core. The southern BCG in the NGC 6338 merger remains another prominent case where the direction of jet-mode feedback between two cycles changed dramatically.

Theoretical model of radiation heat wave in two-dimensional cylinder with sleeve

A semi-analytical model is constructed to investigate two-dimensional radiation heat waves (Marshak waves) in a low-Z foam cylinder with a sleeve made of high-Z material. In this model, the energy loss to the high-Z wall is regarded as the primary two-dimensional effect and is taken into account via an indirect approach in which the energy loss is subtracted from the drive source and the wall loss is ignored. The interdependent Marshak waves in the low-Z foam and high-Z wall are used to estimate the energy loss. The energies and the heat front position calculated using the model under typical inertial confinement fusion conditions are verified by simulations. The validated model provides a theoretical tool for studying two-dimensional Marshak waves and should be helpful in providing further understanding of radiation transport.

Can the container-dielectrics control heating patterns for microwave assisted material processing? A finite element based introspection

This work investigates the effect of acrylic and glass containers on microwave heating of a representative low lossy dielectric material enclosed in the container and exposed to uniform plane microwaves from one side. Microwave heating characteristics are determined via detailed mathematical models for the electric field and temperature distributions within the semi-infinite sample–container assembly. An in-house FORTRAN based algorithm involving Galerkin finite element method is developed to solve the governing equations and determine the heating characteristics (power and temperature distributions) for various sample dimensions as a function of container thickness. It has been found that the acrylic container can be safely used without any significant alteration of the heating characteristics of the dielectric samples. On the other hand, the glass container can lead to a significant alteration of the heating characteristics as a function of container thickness. The power absorption, heating pattern (location of the heating fronts) and spatial heating uniformity within dielectric samples are found to vary significantly with the thickness of the glass container. The results indicate that the glass container can play the critical role to alter the heating characteristics based on the processing requirement (uniform heating or targeted heating at varying locations). These results are expected to play important roles for material processing and the role of the glass containers towards optimal microwave processing applications is revealed for the first time. These results are also important for further investigations on the containers (materials, shapes etc.) and their potential applications for the enhanced, controlled or targeted microwave processing of materials.

Redox cationic frontal polymerization: a new strategy towards fast and efficient curing of defect-free fiber reinforced polymer composites.

Frontal polymerization of epoxy-based thermosets is a promising curing technique for the production of carbon fiber reinforced composites (CFRCs). It exploits the exothermicity of polymerization reactions to convert liquid monomers to a solid 3D network. A self-sustaining curing reaction is triggered by heat or UV-radiation, resulting in a localized thermal reaction zone that propagates through the resin formulation. To date, frontal polymerization is limited to CFRCs with a low fiber volume percent as heat losses compromise on the propagation of the heat front, which is crucial for this autocatalytic curing mechanism. In addition, the choice of suitable epoxy monomers and thermal radical initiators is limited, as highly reactive cycloaliphatic epoxies as well as peroxides decarboxylate during radical induced cationic frontal polymerization. The resulting networks suffer from high defect rates and inferior mechanical properties. Herein, we overcome these shortcomings by introducing redox cationic frontal polymerization (RCFP) as a new frontal curing concept. In the first part of this study, the influence of stannous octoate (reducing agent) was studied on a frontally cured bisphenol A diglycidyl ether resin and mechanical and thermal properties were compared to a conventional anhydride cured counterpart. In a subsequent step, a quasi-isotropic CFRC with a fiber volume of >50 vol%, was successfully cured via RCFP. The composite exhibited a glass transition temperature > 100 °C and a low number of defects. Finally, it was demonstrated that the redox agent effectively prevents decarboxylation during frontal polymerization of a cycloaliphatic epoxy, demonstrating the versatility of RCFP in future applications.

Heat Transfer Modeling on High-Temperature Charging and Discharging of Deep Borehole Heat Exchanger with Transient Strong Heat Flux

High-temperature charging and discharging by deep borehole heat exchanger is typical of a large heat exchange temperature difference and transient strong heat flux. Simulation of this problem is not only computationally expensive, but it is also challenging in terms of robustness and stability for numerical methods. This paper formulates a generic and efficient heat transfer model with two distinctive novelties: Firstly, it highlights unsteady- and quasi-steady-state modeling strategies for heat transfer outside and inside a borehole. Secondly, this model provides analytical solutions for the heat front propagation and heat flux density distribution for unsteady-state heat transfer in the rock zone. These analytical formulations prove to be generic and critical to relieve computational effort in the face of strong heat flux. This model is validated by a typical high-temperature heat storage case from the literature, as well as the pilot demonstration project in China. It was discovered that a large prediction error of the heat transfer model only exists in very short operation days during the initial unsteady stages of charging and discharging. Both relative errors under charging and discharging phases are within 5% during the steady-state period. A comparison of the simulation cost with OpenGeoSys software demonstrates its high efficiency. It proves that this heat transfer model achieves an acceleration ratio of 30 times relative to the fully numerical method. In general, the heat transfer model has four advantages: generic applicability, good accuracy, easy implementation, and high efficiency, but it is limited to the heat transfer of a single deep borehole heat exchanger under pure heat conduction.

The evolution of curvature in planar, photoionization-driven heat fronts

Photoionized plasmas are common in astrophysics and cosmology, especially in space near compact objects, and there are effects from photoionization in high-energy-density plasmas due to the large radiation fields present. Photoionized plasmas are an active area of laboratory research and there are currently experiments to study photoionization-supported heat fronts. These photoionization fronts differ from the physics of diffusive radiation waves, commonly called Marshak waves, that are also an active area of research. This work uses a geometric argument to describe the expected evolution of the photoionization front curvature, in a planar geometry. It then compares this curvature to that of a Marshak wave as a method of diagnosing a heat front experiment. It is found that while the curvature of a planar Marshak wave increases in time, it decreases for a photoionization front. A comparison of radiation energy and electron heat fluxes through the container for the heat front propagating medium demonstrates that the geometric argument for the photoionization front curvature is sufficient. This comparison also demonstrates that wall losses are not significant in a photoionization front because the post-front region is very optically thin. A discussion of the implication this work has on material choice in the targets for an experiment follows.

Математическое моделирование динамики тепловых ударных волн в нелинейных локально-неравновесных средах

We performed a mathematical simulation of heat transfer in a local non-equilibrium medium whose transfer characteristics are functions of the temperature distribution. A homogeneous polynomial of arbitrary degree represents nonlinearities in thermal conductivity and thermal diffusivity. The mathematical model consists of a hyperbolic nonlinear heat transfer wave equation, initial conditions and nonlinear boundary conditions of the second and first kind. To solve this problem, we used a conservative homogeneous finite-difference scheme along the upper time grid line (implicitly). We then used the tridiagonal matrix algorithm of the second order in the spatial variable and of the first order in time to solve the resulting system of linearised algebraic equations. A periodic series of rectangular temperature or heat flux pulses form the boundary conditions of the first and second kind. Computation results reveal ultimate propagation rates of temperature and heat fronts featuring pronounced first-kind discontinuities with attenuating magnitudes. As the process unfolds, the initial pulses heat the region between the boundary and the heat wave front, while the subsequent pulses traverse this region at a higher velocity due to thermal diffusivity being a function of temperature, their fronts "catching up" with the previous fronts, increasing the discontinuity magnitude at the initial pulse front, that is, forming a thermal shock wave front similar to that of a shock wave in gas dynamics. We obtained such thermal shock waves for boundary conditions of both the first and the second kind. We also analysed kinematic and dynamic characteristics of thermal waves

Oxidization characteristics and thermal miscible flooding of high pressure air injection in light oil reservoirs

Physical modeling, numerical simulation and field case analysis were carried out to find out the subsurface thermal oxidation state, thermal oxidation front characteristics and production dynamic characteristics of high pressure air injection thermal oxidation miscible flooding technology. The lighter the composition and the lower the viscosity of the crude oil, the lower the fuel consumption and the combustion temperature are. The thermal oxidation front of light oil and volatile oil can advance stably, and a medium-temperature thermal oxidation stable displacement state can be formed in the light oil reservoir under high pressure conditions. With strong thermal gasification and distillation, light oil and volatile oil are likely to form a single phase zone of gasification and distillation with thermal flue gas at the high-temperature and high-pressure heat front, finally, an air-injection thermal miscible front. In light oil reservoirs, the development process of high-pressure air-injection thermal miscible flooding can be divided into three stages: boosting pressure stage, low gas-oil ratio and high-efficiency stable production stage and high gas-oil ratio production stage. Approximately 70% of crude oil is produced during the boosting pressure stage and low gas-oil ratio high-efficiency and stable production stage.

The bitumen formation and Re-Os characteristics of a CO2-rich pre-salt gas reservoir of the Kwanza Basin, offshore Angola

Here we present a study on the formation process and Re–Os systematics of the bitumen discovered in a CO2-rich pre-salt gas reservoir, south Kwanza Basin, offshore Angola. The magmatism-derived CO2 may have expanded downwards from the top of the reservoir, creating CO2 concentration and temperature gradients within the reservoir, and leading to the gradients of asphaltene precipitation and thermal cracking. The heat front may have been behind the CO2 front due to the heat exchange with surrounding rocks. Thus, asphaltene precipitation has likely occurred before the thermal cracking. Such bitumen formation process is consistent with the observed decreasing bitumen content and Rock-Eval Tmax with depth, and also the characteristics of the whole rock and bitumen Re and Os concentrations.Five closely spaced bitumen samples define a Re–Os age of 116 ± 29 Ma that is identical to the timing of CO2 charging and thus may indicate the timing of bitumen formation. The heterogeneous 187Os/188Os and lack of mobility of the asphaltene deposit, and the relatively low extent of thermal cracking (Tmax of 425–470 °C) may account for the lack of homogenization of the bitumen initial 187Os/188Os.Given the highly radiogenic initial 187Os/188Os (ca. 1.2–1.8) of the bitumen and limited 187Os/188Os ingrowth within the ca. 15 million years from the deposition of potential source rock (Barremian-Aptian Red and Grey Cuvo formations) to the bitumen formation, the source rock should possess a highly radiogenic initial 187Os/188Os (>1). This is consistent with that of the equivalent lacustrine strata in the conjugated Brazilian marginal basins. Thus, the Os isotope composition indicates that the lacustrine Red and Grey Cuvo formations could be the source rock for the bitumen of the studied CO2-rich pre-salt gas reservoir in the south Kwanza Basin.

Transient dynamics in the outflow of energy from a system in a nonequilibrium stationary state.

We investigate the thermal relaxation of an ideal gas from a nonequilibrium stationary state. The gas is enclosed between two walls, which initially have different temperatures. After making one of the walls adiabatic, the system returns to equilibrium. We notice two distinct modes of heat transport and associated timescales: one connected with a traveling heat front and the other with internal energy diffusion. At the heat front, which moves at the speed of sound, pressure, temperature, and density change abruptly, leaving lower values behind. This is unlike a shock wave, a sound wave, or a thermal wave. The front moves multiple times between the walls and is the dominant heat transport mode until surpassed by diffusion. We found that it can constitute an order 1 factor in shaping the dynamics of the outflow of internal energy. We found that cooling such a system is quicker than heating, and that hotter bodies cool down quicker than colder ones. The latter is known as the Mpemba effect.

A Misfired Outburst in the Neutron Star X-Ray Binary Centaurus X-4

We report on a long-term optical monitoring of the neutron star X-ray binary Centaurus X-4 performed during the last 13.5 yr. This source has been in quiescence since its outburst in 1979. Our monitoring reveals the overall evolution of the accretion disk; we detect short-duration flares, likely originating also in the disk, superimposed with a small-amplitude (<0.1 mag) ellipsoidal modulation from the companion star due to geometrical effects. A long-term (∼2300 days) downward trend, followed by a shorter (∼1000 days) upward one, is observed in the disk light curve. Such a rise in the optical has been observed for other X-ray binaries preceding outbursts, as predicted by the disk instability model. For Cen X-4, the rise of the optical flux proceeded for ∼3 yr, and culminated in a flux increase at all wavelengths (optical–UV–X-rays) at the end of 2020. This increase faded after ∼2 weeks, without giving rise to a full outburst. We suggest that the propagation of an inside-out heating front was ignited due to a partial ionization of hydrogen in the inner disk. The propagation might have stalled soon after the ignition due to the increasing surface density in the disk that the front encountered while propagating outward. The stall was likely eased by the low-level irradiation of the outer regions of the large accretion disk, as shown by the slope of the optical/X-ray correlation, suggesting that irradiation does not play a strong role in the optical, compared to other sources of emission.