Reactive Fluid Research Articles

Formation of Olivine Veins by Reactive Fluid Flow in a Dehydrating Serpentinite

AbstractMany exposed high‐pressure meta‐serpentinites comprise a channelized network of olivine‐rich veins that formed during dehydration at depth and allowed the fluid to escape from the dehydrating rock. While previous studies have shown that chemical heterogeneities in rocks can control the formation of olivine‐enriched vein‐like interconnected porosity networks on the sub‐millimeter scale, it is still unclear how these networks evolve toward larger scales and develop nearly pure olivine veins. To explore this, we study the effect of reactive fluid flow on a dehydrating serpentinite. We use thermodynamic equilibrium calculations to investigate the effect of variations in the bulk silica content in serpentinites on the dehydration reaction of antigorite + brucite = olivine + fluid and the silica content of this fluid phase. Further, we develop a numerical model that combines the effects of intrinsic chemical heterogeneities with reactive transport with dissolved silica as metasomatic agent. Our model shows how reactive transport can lead to vein widening and olivine enrichment within a vein in an antigorite‐rich matrix, such as observed in the veins of the Erro Tobbio meta‐serpentinites. This is a critical step in the evolution toward larger‐scale vein systems and in the evolution of dynamic porosity, as this step helps account for the chemical feedback between the dehydrating rock and the liberated fluid.

Effect of temperature on sandstone acidizing using non-Newtonian fluids

Rock formation is subjected to acid stimulation to increase oil reservoirs productivity. Acidizing of sandstone rocks is a common method to remove damage and recover the oil flow to the wellbore. A matrix acid stimulation will always require additives depending on mud acid (HF/HCl) components, which cause non-Newtonian fluid behavior. In this study, the 3-D mathematical model of sandstone matrix acidizing in radial flow has been presented by a modified two-parameter model to consider non-Newtonian fluids and temperature effects. To this end, the Carreau-Yasuda equation has been employed to predict dissolution patterns created by non-Newtonian reactive fluids in sandstone formations. Meanwhile, the energy equation has been solved in the porous media to estimate the temperature in each time step, which affects the dissolution patterns in matrix acidizing. Three main phenomena are involved in dissolution patterns: surface reaction, diffusion and convection. Acidizing simulation indicates the acid-rock reaction constant in the formation network is temperature sensitive, and increases exponentially by increasing the reservoir temperature. In this work, the effects of various heterogeneity magnitude and porosity in the porous media, injection rate, reaction rate constant, power-law index (n) and temperature are considered by the various presented dissolution patterns of sandstone acidizing. The results show that, to reach a certain penetration length, the pore volume (PV) of injected acid decreases sharply with reservoir temperature, but after the critical temperature, it decreases slowly. The model discretized the Darcy, mass and energy balance equations by finite difference method and pore evaluation by Karman-Cozney correlations, and then employed the Tri-Diagonal matrix algorithm (TDMA) method to achieve the solution of the model and analysis of dissolution patterns in various acidizing regimes and rheological properties. All mathematical estimations including Darcy correlations, mass and energy balance and pore evaluation have been solved in FORTRAN program by parallel processing to accelerate 3-D calculations.

GENERIC framework for reactive fluid flows

AbstractWe describe reactive fluid flows in terms of the formalismGeneral Equation for Non‐Equilibrium Reversible‐Irreversible Couplingalso known as GENERIC. We present the thermodynamical and mechanical foundations for the treatment of fluid flows using GENERIC and present a framework for noninvertible transformations, that is in particular applied to transformations of variational approaches between Lagrangian and Eulerian coordinates. We bring the abstract framework to life by providing many physically relevant examples for reactive compressible fluid flows.

Reactive fluids for intensified thermal energy conversion

Reactive fluids for intensified thermal energy conversion With the aim to effectively increase the performances of power plants, refrigeration systems and heat pumps, this project proposes the use of equilibrated reactive working fluids instead of inert ones. It applies an original methodology that integrates thermodynamic and kinetic predictive tools to discover and characterise suitable reactive fluids, allowing for the quantification of the effects of reaction features on system performance and optimal architecture.

Thermo-convective Arrhenius reactive fluid flow between two parallel plates

A nonlinear thermo-convective Arrhenius Casson fluid flow between two vertical parallel plates is modeled under the actions of exothermic chemical reaction, linear and nonlinear volumetric thermal and concentration expansions. The non dimensional coupled system of ordinary differential equations are solved numerically by using Parametric Continuation Method (PCM). The numerical results of (PCM) method are graphically justified with the solutions of [Formula: see text] package, which presented an excellent agreement with each other. For further validation of (PCM) method, the obtained results are also compared with previously published work and found an accuracy upto two decimal places. It has been found that Frank-Kameneskii’s parameter, depends on Arrhenius kinetics, has the tendency to enhanced the exothermic chemical reaction, which give rise in internal heat generation and heat transfer rate. The findings of the present study may be used for making energy source materials and in biological systems. Furthermore, the increasing values of [Formula: see text] are responsible to drop the exothermic reaction level within the flow system, due to which the mass transfer is reduced and make a reduction in concentration profile.

Design and performance of a variable gap system for thermal conductivity measurements of high temperature, corrosive, and reactive fluids

High-temperature fluids such as molten salts, liquid metals, and gasses are being proposed for many advanced energy systems including thermal energy storage devices, concentrating solar plants, and advanced nuclear reactor designs. However, the chemical behavior and thermophysical properties of many of these fluids have not been well characterized, which hinders the design, modeling, safety analysis, and deployment of these systems. Thermal conductivity is a property that is especially limited by existing measurement capabilities, which are subject to errors caused by convection, material interaction, radiative heat transfer, and instrument degradation. Therefore, there is a lack of standard, systematic measurement techniques for high-temperature, reactive, and corrosive fluids. In this work, the development of a variable gap thermal conductivity measurement system is detailed. The system is designed to measure the thermal conductivity of highly corrosive and reactive fluids, and survive operation between 100 °C and 800 °C. The effects of convection are minimized by limiting the thickness of the specimen to thin sizes (<0.3 mm). Corrections for radiative heat transfer were included in the working equations to consider specimens with varying optical properties. The design, construction, instrumentation, operating principles, and data analysis techniques are discussed in detail. The system was tested up to 500 °C using helium gas and molten KNO3NaNO3 to verify the measurement technique and determine the sources of error. At 300 and 400 °C KNO3NaNO3, results showed maximum relative error of 6% when compared to results in the literature. The helium results were within 13% of those in the literature at 300 and 400 °C. Higher errors were observed at 500 °C for both fluids, and the sources of these errors are discussed.

Global existence of classical solutions for a reactive polymeric fluid near equilibrium

In this paper, we study a new micro-macro model for a reactive polymeric fluid, which is derived recently in [Y. Wang, T.-F. Zhang, and C. Liu, \emph{J. Non-Newton. Fluid Mech.} 293 (2021), 104559, 13 pp], by using the energetic variational approach. The model couples the breaking/reforming reaction scheme of the microscopic polymers with other mechanical effects in usual viscoelastic complex fluids. We establish the global existence of classical solutions near the global equilibrium, in which the treatment on the chemo-mechanical coupling effect is the most crucial part. In particular, a weighted Poincar\'e inequality with a mean value is employed to overcome the difficulty that arises from the non-conservative number density distribution of each species.

Heat and mass transfer analysis of chemically reactive non-Newtonian fluid with the implementation of generalized Fourier’s and Fick’s laws over a vertical sheet

This article focuses on the 2D nonlinear mixed convective MHD Casson fluid flow influenced by the gyrotactic microorganism boundary layer and nonlinear thermal radiation across the vertical stretching sheet. The solutal and thermal energy analysis is established applying the Cattaneo–Christov theory and thermophoretic impact. The stratification boundary condition and injection/suction effects are imposed on the surface of the boundary. The developed flow model PDEs are assimilated into nonlinear ODEs using appropriative conversion. The built-in Bvp4c Matlab approach is used on the converted nonlinear equations. It is analyzed from the graph that the stronger estimation of magnetic and Casson fluid parameters declines the fluid velocity by the occurrence of resistive forces. More heat transfer in the fluid was due to a larger estimation of radiation parameter. Additionally, it is found from the numerical values of tables that the improvement occurs in the skin friction with the augmentation of buoyancy ratio and mixed convection parameter.

Thermolab: A Thermodynamics Laboratory for Nonlinear Transport Processes in Open Systems

AbstractWe developed a numerical thermodynamics laboratory called “Thermolab” to study the effects of the thermodynamic behavior of nonideal solution models on reactive transport processes in open systems. The equations of the state of internally consistent thermodynamic data sets are implemented in MATLAB functions and form the basis for calculating Gibbs energy. A linear algebraic approach is used in Thermolab to compute Gibbs energy of mixing for multicomponent phases to study the impact of the nonideality of solution models on transport processes. The Gibbs energies are benchmarked with experimental data, phase diagrams, and other thermodynamic software. Constrained Gibbs minimization is exemplified with MATLAB codes and iterative refinement of composition of mixtures may be used to increase precision and accuracy. All needed transport variables such as densities, phase compositions, and chemical potentials are obtained from Gibbs energy of the stable phases after the minimization in Thermolab. We demonstrate the use of precomputed local equilibrium data obtained with Thermolab in reactive transport models. In reactive fluid flow the shape and the velocity of the reaction front vary depending on the nonlinearity of the partitioning of a component in fluid and solid. We argue that nonideality of solution models has to be taken into account and further explored in reactive transport models. Thermolab Gibbs energies can be used in Cahn‐Hilliard models for nonlinear diffusion and phase growth. This presents a transient process toward equilibrium and avoids computational problems arising during precomputing of equilibrium data.

Mixture model for thermo-chemo-mechanical processes in fluid-infused solids

This work presents a new thermodynamically consistent model for thermo-chemo-mechanical processes in open systems comprised of nonlinear elastic solids that are infused with reactive fluids and are undergoing large local strains. The interactions between different constituents of the material system are captured through locally homogenized mixture theory. The development of the formulation starts from constituent-wise balance equations for mass, momentum, and energy. The constitutive relations are derived to enforce non-negative entropy production and satisfy the second law of thermodynamics for the open system. The deformation due to thermal and chemical effects is carried out through a multiplicative split of the deformation gradient. Further assumptions on the fluid and solid material responses are introduced to specialize the model to a class of thermo-chemo-mechanical processes that arise in thermal oxidation of metallic and ceramic materials, lithiation of battery anodes, and other related problems. The model is implemented in the standard Galerkin finite element method using eight node hexahedral elements. A set of test cases involving fully coupled thermo-mechanical, chemo-mechanical, and thermo-chemical effects are analyzed. Numerical results obtained are in reasonable agreement with the experimental data for thermo-chemo-mechanical problems concerning the thermal oxidation of silicon carbide and FeCrAlY bond coat material in thermal barrier coatings.

Chemistry-mediated Ostwald ripening in carbon-rich C/O systems at extreme conditions

There is significant interest in establishing a capability for tailored synthesis of next-generation carbon-based nanomaterials due to their broad range of applications and high degree of tunability. High pressure (e.g., shockwave-driven) synthesis holds promise as an effective discovery method, but experimental challenges preclude elucidating the processes governing nanocarbon production from carbon-rich precursors that could otherwise guide efforts through the prohibitively expansive design space. Here we report findings from large scale atomistically-resolved simulations of carbon condensation from C/O mixtures subjected to extreme pressures and temperatures, made possible by machine-learned reactive interatomic potentials. We find that liquid nanocarbon formation follows classical growth kinetics driven by Ostwald ripening (i.e., growth of large clusters at the expense of shrinking small ones) and obeys dynamical scaling in a process mediated by carbon chemistry in the surrounding reactive fluid. The results provide direct insight into carbon condensation in a representative system and pave the way for its exploration in higher complexity organic materials. They also suggest that simulations using machine-learned interatomic potentials could eventually be employed as in-silico design tools for new nanomaterials.

Influences of Soret and Dufour numbers on mixed convective and chemically reactive Casson fluids flow towards an inclined flat plate

AbstractThe purpose of this study is to examine the magnetohydrodynamic mixed convection Casson fluid flow over an inclined flat plate along with the heat source/sink. The present flow problem is considered under the assumption of the chemical reaction and thermal radiation impacts along with heat and mass transport. The leading nonlinear partial differential equations of the flow problem were renovated into the nonlinear ordinary differential equations (ODEs) with the assistance of appropriate similarity transformations and then we solved these ODEs with the employment of the bvp4c technique using the computational software MATLAB. The consequences of numerous leading parameters such as thermophoretic parameter, local temperature Grashof number, solutal Grashof number, suction parameter, magnetic field parameter, Prandtl number, chemical reaction parameter, Dufour number, Soret number, angle of inclination, radiation parameter, heat source/sink, and Casson parameter on the fluid velocity, temperature, and concentration profiles are discoursed upon and presented through different graphs. Some important key findings of the present investigation are that the temperature of the Casson fluid becomes lower for local temperature Grashof number and solutal Grashof number. It is initiated that the Casson fluid parameter increases the velocity of the fluid whereas the opposite effect is noticed in the temperature profile. Higher estimation of Prandtl number and magnetic parameter elevated the Casson fluid concentration. Finally, the skin friction coefficient, Nusselt number, and Sherwood number are calculated and tabulated. It is also examined that the Nusselt number is weakened for both the Dufour number and Soret number but the skin fraction coefficient is greater for both the Dufour number and Soret number.

A unified diffuse interface method for the interaction of rigid bodies with elastoplastic solids and multi-phase mixtures

This work outlines a new multi-physics-compatible immersed rigid body method for Eulerian finite-volume simulations. To achieve this, rigid bodies are represented as a diffuse scalar field, and an interface seeding method is employed to mediate the interface boundary conditions. The method is based on an existing multi-material diffuse interface method that is capable of handling an arbitrary mixture of fluids and elastoplastic solids. The underlying method is general and can be extended to a range of different applications including high-strain rate deformation in elastoplastic solids and reactive fluid mixtures. As such, the new method presented here is thoroughly tested through a variety of problems, including fluid–rigid body interaction, elastoplastic–rigid body interaction, and detonation–structure interaction. Comparison is drawn between both experimental work and previous numerical results, with excellent agreement in both cases. The new method is straightforward to implement, highly local, and parallelizable. This allows the method to be employed in three dimensions with multiple levels of adaptive mesh refinement using complex immersed geometries. The rigid body field can be static or dynamic, with the tangent of hyperbola interface capturing reconstruction method being used to keep the interface sharp in the dynamic case.

Coreflood Effluent and Shale Surface Chemistries in Predicting Interaction between Shale, Brine, and Reactive Fluid

Summary Field and laboratory observations to date indicate that the efficiency of hydraulic fracturing, as it relates to hydrocarbon recovery, depends significantly on geochemical alterations to rock surfaces that diminish accessibility by partial or total plugging of the pore and fracture networks. This is caused by mineral scale deposition, such as coating of fracture surfaces with precipitates, particle migration, and/or crack closure, because of dissolution under stress. In reactive flow-through experiments, mineral reactions in response to acidic fluid injection significantly reduced system porosity and core permeability. The present study focuses on changes to fluid chemistry and shale surfaces (inlet and fracture walls) resulting from shale-fluid interactions and integrates these findings for an improved estimate of transport-related consequences. The pre- and post-reaction shale surfaces were examined by spatially resolved scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) analysis. Importantly, inductively coupled plasma-mass spectrometry/optical emission spectroscopy (ICP-MS/OES) was utilized to probe the chemical evolution of the coreflood effluents. The three study cores selected from the Marcellus formation represent different mineralogies and structural features. In flow-through experiments, laboratory-generated brine and HCl-based fracture fluid (pH = 2) were injected sequentially under effective stress (up to 500 psi) at reservoir temperature (80°C). SEM-EDS results confirmed by the ICP concentration trends showed significant Fe hydroxide precipitates in the clay- and pyrite-rich outcrop sample because of partial oxidation of Fe-bearing phases in the case of intrusion of low salinity water-based fluids. Porosity reduction in the Marcellus Shale Energy and Environmental Laboratory (MSEEL) carbonate-rich sample is related to compaction of cores under stress because of matrix softening with substantial dissolution, and pore filling by hydroxides, as well as secondary barite and salts. Despite the same fluid compositions and experimental conditions used for both MSEEL samples, barite precipitation was much more intense in the MSEEL clay-rich sample because of its greater sorption capacity and additional sulfate source as well as its fissile nature with multiple lengthwise cracks. ICP tests revealed time-resolved concentration behavior in produced brine and reactive fluids that in turn complemented the pre/post-reaction SEM-EDS observations. The greatest release of metal ions into brine was in clay-rich systems indicating the importance of chemical compatibility between in-situ shale and nonequilibrated injection solutions. A thorough examination of surface and effluent data pointed to the substantial influence of formation brine in the shales, mixing of brine with fracture fluid during flow, and shale mineralogy on mineral dissolution and scale formation that significantly affect flow efficiency.

Analytical treatment of MHD flow and chemically reactive Casson fluid with Joule heating and variable viscosity effect

This manuscript studies the impact of heat transfer in the context of their valuable applications. There has been a lot of interest in using non-Newtonian fluids in biological and engineering disciplines. With such a considerable interest in non-Newtonian fluids, we aim to examine the MHD flow of chemically reactive Casson liquid by a permeable stretching surface by considering the heat source and viscous dissipation effects. The impression of current conductivity as a linear function of the temperature field is subjected to temperature-dependent viscosity fluctuation. A mathematical model simulates the arisen nonlinear partial differential equations (PDEs). By using the suitable transformations, the system of PDEs is then transformed to a nonlinear system of ordinary differential equations (ODEs). The impacts of the pertinent parameters on the velocity profile, energy, and concentration distribution have been discussed. The fundamental dimensionless partial differential flow laws are analyzed using an efficient and validated analytical homotopic (HAM) technique. The ongoing investigation has been converged, according to a stability and convergence analysis. The impression of innumerable dominant physical parameters on the momentum boundary layer, thermal boundary layer, and concentration profile has been made realistically by plotted graphics.

Chemically Induced Evolution of Morphological and Connectivity Characteristics of Pore Space of Complex Carbonate Rock via Digital Core Analysis

AbstractWe study the impact of reactive flow on a complex carbonate rock whose pore space was resolved by X‐ray μ‐CT. A small core plug was imaged in the original and chemically altered states. The morphology of the resolved pore space was quantified using various approaches, such as sample‐spanning cluster analysis, capillary drainage transform (CDT), and Minkowski functionals. In particular, we carefully evaluated the changes in macropore connectivity by calculating the fraction of percolated cluster, the Euler characteristic of macroporous phase, and the critical pore diameter in pre‐ and post‐alteration states. We observed that the reactive flow mainly affected larger pores in the macropore region of the sample. More importantly, a huge increase in macropore connectivity, as indicated majorly by the CDT measurements and the Euler characteristic, was observed such that the poorly connected original macropore phase was converted to well‐connected ones after the reactive flow. In addition, the increases in the macropore surface area and integral mean curvature indicated that the oomoldic and interooidal pores of the rock were dominantly enlarged by the reactive fluid. Based on direct numerical simulations on the main flow paths (i.e., the macro‐connected pore phase), strong correlations were found between changes in transport properties and the evolution of connectivity and length scales of the macropore phase, such that simple power‐law models could be used to predict with acceptable accuracy. This suggests that the evolution of transport properties of the rock was mainly controlled by the changes in the macro‐connected porosity and the critical pore diameter.

Evolution of earth pressure coefficient of sand undergoing varying rate of dissolution

In fluid-saturated granular materials, the physicochemical interaction between pore-fluids and grain minerals alters packing conditions, which in turn leads to stress change, deformation and, in extreme cases, even collapse. Chemical weathering, either naturally occurring or induced by human activities, is among such processes. This paper presents an experimental study illustrating the major effects of chemical weathering on the deformation and stress state of granular materials, emphasising particulate systems entirely made by highly soluble carbonate grains. Laboratory experiments are conducted by subjecting granular assemblies to acidic environments under oedometric conditions. The reaction rate is controlled by regulating various testing parameters, such as acid concentration and pore fluid flow rate. Experiments revealed that the lateral earth pressure steadily reduces in some cases, while others exhibit non-monotonic evolution. From a macroscopic standpoint, the rate of the chemical reaction was critical to determine the emergence of either of these trends. Such findings are relevant for any particulate system in which the stress conditions are controlled by multi-physical processes proceeding at different rates, such as waste products within bioreactors, gouge materials within faults and natural deposits subjected to the injection/extraction of reactive fluids.

Analysis of Hydromagnetic Double Exothermic Chemical Reactive Flow with Convective Cooling through a Porous Medium under Bimolecular Kinetics

In this study, the analytical solution of steady hydromagnetic double exothermic combustible reaction fluid flow in a porous medium with convective cooling wall is presented. The viscous heating reactive liquid is totaling exothermic without consumption of material. The combustion reaction of the fluid takes place in a Poiseuille device, and it is been propelled by pressure gradient and pre-exponential bimolecular kinetics. The device is exposed to convective cooling to keep the reactive hydromagnetic fluid from distortion. The weighted residual method (WRM) is analytically used to get the numerical values for the dimensionless nonlinear governing equations. The solution to temperature and velocity distribution is carried out and the result is graphically depicted. The Nusselt number and skin friction coefficient is also showed for some significant parameters engrained in the flow and the solution obtained is compared with numerical method. As obtained in the study, the second exothermic reaction term increases the combustion process; hence the term will assist in reducing toxic discharge from the engines that pollute the environment. The Frank-Kamenetskii parameter contributes highly to system thermo-fluid destruction; as such it must be monitored.

Metal mobilization and precipitation in a Sn-W skarn system, Gejiu Sn district, China

In the Gejiu Sn-Cu polymetallic district, skarns with or without mineralization developed both at the contact zones between a granitic intrusion and carbonate-rich wall rocks and within carbonate-rich wall rocks. We analyzed the chemical and Sr, Nd, Pb, Li, and B isotopic compositions of mineralogically different zones in two hand specimens, i.e., a banded carbonate and a distal skarn with distinct reaction zones, to interpret chemical and isotopic variation in terms of (i) temporal compositional changes of skarn-forming fluids and (ii) reaction-induced exchange between fluids and carbonate wall rocks. The banded carbonates have seen little fluid-mediated exchange and therefore the compositions of the various bands reflect a two-component system of carbonates and detrital silicates. The compositional range of the banded carbonates represents the compositional heterogeneity of the protoliths of the distal skarn that experienced a pronounced impact by metasomatic fluids. The studied distal skarn is characterized by the vein-like aquifer (Zone I), reaction zones (Zone II and Zone III), and the carbonate aquitard (Zone IV). As evident from the distal skarn, the chemical and isotopic compositions of skarn-forming fluids change over time due to (i) processes in the fluid sources, (ii) variable contributions of different fluid sources, and (iii) reaction-induced exchange with the rocks bordering the aquifer, leading to selective addition or removal of elements from the fluid. During early interaction between aquifer fluids and its wall-rocks, the aquitard is partially consumed and the chemical and isotopic signature of the newly-formed minerals in the reaction zone represent a mixture of material derived from either fluids or wall rock. Once formed, the reaction zones largely protected the aquitard from further interaction with fluids. The Sr-budget of zones I to IV is dominated by Sr derived from the carbonate aquitard and redistributed during the consumption of older reaction zones. The Pb, Li, and B budgets of zones I to III are dominated by materials derived from the fluids. In contrast, the Nd isotopic compositions in zones II to III fall between the values of the carbonate aquitard and the aquifer. The ore elements occur in different zones, i.e., (i) scheelite, sphalerite, pyrite, and chalcopyrite precipitate in the aquifer and (ii) Sn substitutes in calc-silicates at the reaction front between zones II and III and is present in late fractures in Zone III. Thus, ore elements may have been transported by different skarn-forming fluids. The chemical and isotopic signatures of reaction zones do neither strictly reflect the nature of the reactive fluid nor reflect simple bulk mixing between fluid and wall rock, as (i) reaction-induced volume decrease may allow later fluids to infiltrate into earlier reaction zones, (ii) the fluid may be depleted in elements that have been scavenged from the fluid before, (iii) some elements having low compatibility in minerals of the reaction zones may have strongly partitioned into the fluid and are lost from the system. Although reaction zoning provides information about the nature of the reactive fluids, the mineralogically-controlled selective behavior of trace elements, i.e., most ore elements, does not allow to use the ore element content in skarn as a vector to mineralization.

Study of Rayleigh–Bénard convection in a chemically reactive fluid using a generalized Lorenz model and the cubic–quintic Ginzburg–Landau equation

Analytical study of linear and weakly nonlinear stability analyses of Rayleigh–Bénard convection in a chemically reactive two-component fluid system is reported by considering physically realistic and idealistic boundaries. Analytical expression for the eigenvalue as functions of the chemical potential and the relaxation parameter is reported in the cases of stationary, oscillatory, and subcritical instabilities. The preferred stationary mode at onset is attributed to the assumption of a chemical reaction. The possibility of subcritical motion in a specific range of values of the relaxation parameter is reported and the threshold value of this parameter at which the transition from the subcritical to the critical motion takes place is documented for different values of the chemical reaction rate. The higher-order generalized Lorenz model leads to the reduced-order cubic–quintic, Ginzburg–Landau equation (GLE), and using its solution, the heat transport is quantified in steady and unsteady convective regimes. The drawback of the cubic GLE and the need for the cubic–quintic GLE for studying the heat transfer in the case of subcritical regime is explained. The possibility of having pitchfork and inverted bifurcations at various values of the relaxation parameter is highlighted. The steady Nusselt number plots clearly show that in the case of a supercritical bifurcation, a smooth transition takes place from unity in the conduction state to a higher value as the Rayleigh number exceeds its critical value predicted by linear theory. In the case of subcritical bifurcation, however, a discontinuous transition is observed.