Two-dimensional Axisymmetric Simulations Research Articles

Experimental and numerical investigations of the effect of pressure and oxygen concentration on combustion characteristics of Al/Mg fuel-rich propellants

Experimental and numerical investigations are carried out to analyse the effect of pressure and oxygen concentration on combustion characteristics of Al/Mg fuel-rich propellants. In the present study, a small-scale sealed laser ignition experimental system is set up to obtain the experimental measurements in the combustion process of Al/Mg fuel-rich propellants. The result shows that, as the pressure and oxygen concentration increase, the combustion wave temperature increases and presents two platforms. It is conducive to the convective heat transfer and radiant heat feedback of the flame to the burning surface, thus leading to an increase in the burning rate. When the percentage of Al is increased and the percentage of Mg is reduced, the burning rate is increased. In order to understand the distribution of combustion products, a two-dimensional axisymmetric simulation program is developed. The simulated results show that the diffusion of the pyrolyzed gas is inhibited as the pressure increases, resulting in the rapid consumption of the oxidizing gas, a lower oxygen content and a higher cyanide content nearing burning surface. Due to the higher temperature required, carbides and alumina are produced in the far field region of the flow field. Conversely, the oxygen concentration has a small effect on the combustion wave temperature, but has a greater influence on the mass distribution of combustion products.

Effects of the die parameters on the self-piercing riveting process

To facilitate the die selection for new self-piercing riveting (SPR) joint configurations, it is necessary to find out how the critical die geometric parameters influence the SPR process. In this study, a two-dimensional (2D) axisymmetric simulation model was developed to numerically study the riveting process. The influences of the die type, the die diameter, the die depth, and the die pip height on the deformation behaviour of the rivet and sheets were systematically studied. Moreover, the flared rivet shank radius and the thickness at the centre of the bottom sheet during the SPR process were first monitored using the developed simulation model. The simulation results revealed that these die parameters have significant influences on the deformation behaviour of the rivet and sheets. The flared rivet shank radius showed an increasing trend with the increment of the die diameter and the die pip height, while it decreased with the increment of the die depth. Furthermore, it was also found that the flaring speed of the rivet shank depended heavily on the filling condition of the die cavity underneath the rivet cavity. A rapid flare of the rivet shank was observed when this space was fully filled.

Study of Voltage Distribution and Radial Electric Field under Different Pollution Conditions of 500kV MOA

The surface of the porcelain sleeve will be polluted by local environmental pollution and weather factors such as frost, rain and snow, because of the 500kV MOA is generally arranged outdoors. The surface flashover problem of the porcelain sleeve surface caused by contamination is similar to that of the insulator surface. It is a resistor column with three sections connected in series inside the arrester. The influence of surface contamination on the voltage distribution of the internal resistor and the radial electric field between the resistor and the porcelain sleeve remains to be studied. In this paper, ANSYS is used to establish a two-dimensional axisymmetric simulation model for the surface contamination of 500kV zinc oxide arrester. The electric field distribution of the surface of the arrester is cleaned, evenly polluted and the local drying zone appears. The results show that the uniform contamination is inside the arrester. The electric field has little influence, and the local drying zone will form a locally excessively high radial electric field between the resistor and the porcelain sleeve. Under the reference voltage, the radial electric field exceeds the corona starting field and causes corona discharge, and the acceleration resistor Deterioration of the sheet and the dielectric; the voltage of the resistor piece near the local drying zone is increased to a certain extent, which also accelerates the aging of the resistor chip and jeopardizes the safe and stable operation of the arrester

Coupled combustion characteristics of the base-bleed propellant and the igniter under transient depressurization based on detailed chemical kinetics

The coupled combustion process of AP/HTPB propellant and ignition jet in the base-bleed device is studied and analyzed by numerical simulation under transient depressurization. Two-dimensional axisymmetric simulation was established using Navier-Stokes equations, Realizable k-ε turbulence model and finite-rate chemical reaction model. The chemical reaction process of AP/HTPB, using 37 components and 127 chemical reaction equations, simulates the actual process of AP/HTPB combustion. The temperature field, velocity field and concentration field of components were obtained by solving, the analysis of the development process of the ignition jet realized the coupled combustion process of the AP/HTPB with ignition, an annular recirculation zone formed by multi-component blending was observed, and the interaction between the AP/HTPB combustion products and the ignition jet made the maximum Mach number reach to 1.2 near the nozzle of base-bleed device during the process of transient depressurization. The flow development process of H2 and CO2 under the action of the ignition jet is analyzed. It is known that H2 mainly flows near the AP/HTPB burning surface, while most of the CO2 is mixed in the ignition jet. After analyzing the reactants, products and intermediates in the H2 and CO2 reaction, the reason why the H2 and CO2 components are almost completely opposite is explained: the main combustion products H2 and CO are concentrated near the burning surface, while CO2 mainly reacts reversibly with CO. Therefore, the concentration distribution of CO2 is basically opposite to CO. Through the calculation and analysis of the transient depressurization process in the combustion chamber of the base-bleed device under multiple working conditions, it is known that the initial combustion pressure has a great impact on the depressurization in the combustion chamber before t = 1 ms. After t = 2 ms, the effect of initial combustion pressure on the depressurization is almost negligible.

Flow field and injector heat characteristics of hybrid rocket motor with annular-gap injector

A throttleable annular-gap axial injector, which could automatically adjust the injection gap area, is proposed to decrease injection pressure drop variation amplitude during wide range thrust regulation process in hybrid rocket motor. With the annular-gap injector adopted, liquid oxidizer is unhomogeneously injected into combustion chamber through a narrow annular injection gap. In conventional hybrid rocket motor, the oxidizer is usually homogeneously injected through full-inlet injection method. The injection area covers the whole fuel grain port. Difference of the injection methods greatly influences the oxidizer flow characteristics. Consequently, combustion and heat transfer characteristics of the motor are significantly changed. This paper is aimed to analyze the two-phase combustion flow field and coupled injector heat transfer characteristics of a lab-scale hybrid rocket motor with annular-gap injector through two-dimensional axisymmetric steady numerical simulations. The motor adopts 98% hydrogen peroxide and polyethylene as the propellants. Numerical analysis reveals that position of the injection gap influences the regression rate distribution in the first half of solid fuel grain but has little effect on that in the second half. The regression rate is relatively high when the injection gap is close to the fuel inner surface. In addition, the flowing liquid hydrogen peroxide in the injector could cool the chamber head. Sharp turns that produces vortex in oxidizer flow channel decreases the cooling effect and increases the overheating risk. Smooth bend could improve the injector heat transfer characteristics and eliminate the risk.

OpenFOAM based conditional moment closure (CMC) model for solving non-premixed turbulent combustion: Integration and validation

This article presents a direct integration of conditional moment closure (CMC), an advanced combustion model with a computational fluid dynamics (CFD) package, OpenFOAM (Open-source Field Operation And Manipulation) to solve non-premixed turbulent combustion. Earlier our group (Roy and Sreedhara, 2016) had coupled CMC code with FLUENT to model non-premixed turbulent combustion, but it was through manual two-way coupling method where CFD and CMC codes were sequentially solved one after the other until the steady state is reached. The extensible and flexible framework of OpenFOAM encouraged us to build a CFD-CMC solver called cmcFoam. This fully-coupled solver solves CMC equations internally in OpenFOAM along with flow-field equations. Major advantages of this solver are (1) it is built in the open-source framework, (2) it can predict the transient behavior of flames, and (3) one can benefit from all features of OpenFOAM. The validity of the proposed solver has been thoroughly demonstrated by accurately predicting experimental data published for the Sandia syngas (CO/H2/N2) flame. Two-dimensional axisymmetric simulations were performed with a modified k-ε turbulence model and using a chemical kinetic scheme having 21 species and 84 chemical reactions (DRM 19). Results predicted by the solver showed an excellent agreement with the published data, both in mixture fraction and physical space. This benchmarked coupled code may be useful to analyze fairly complex industrial problems.

Prediction of water velocities in circular aquaculture tanks using an axisymmetric CFD model

Computational fluid dynamics (CFD) software was used to construct two-dimensional axisymmetric simulations of the turbulent flow inside 1.5, 5, 9.15 and 10 m tanks containing rotating water. The water rotation was induced mathematically by either placing a rotating cover on the surface of the water or locating a tangential water inlet near the top of the side wall. Central and side drains evacuated the water from the tank when using an inlet. The rotating cover produced a forced vortex in the tanks similar to the one observed in actual rearing tanks when the majority of the flow is leaving via the side drain. The predicted flow structure in tanks with an inlet can be divided into three regions: a narrow region along the side wall where the rotating water is moving down toward the floor, a thin boundary layer along the floor where the rotating water is moving radially inward, and a large region in the bulk of the tank where radial and axial velocities are small and the tangential velocity is independent of elevation but varies with radial position. Calculations were performed with the k-ε (RNG) and Reynolds stress turbulence models. Converged solutions were easier to obtain with the k-ε (RNG) model but only the Reynolds stress model could predict the strong vortex which is experimentally observed to form in the central portion of the tank as the flow leaving via the center drains is increased. The simulations predict that the thickness of the floor boundary layer is proportional to the tank diameter and that the radial velocity within the floor boundary layer is maximum at an elevation equal to about 10% of the boundary layer thickness. At any given radial position, the maximum value of the radial velocity next to the floor is between 15 and 45% of the tangential velocity in the bulk of the tank. Predicted shear stresses along bounding surfaces were used to obtain correlations for the side-wall and floor friction coefficients in terms of the Reynolds number. These correlations were in turn substituted in an overall moment balance to obtain an analytical model for predicting the maximum tangential velocity Vθw near the side wall of multi-drain rearing tanks. The predictions of the analytical model are in excellent agreement with Vθw values reported in the literature for tanks with diameters ranging from 1.5–15 m and indicate that the drag from inlet structures is non-negligible. The results demonstrate that the axisymmetric CFD model can simulate the main features of the rotating flow inside circular tanks and provide valuable boundary-layer information that is difficult to obtain experimentally.

The numerical analysis of controllability of EAST plasma vertical position by TSC

Vertical position controllability with EAST vertical instability fast control system in current mode is analyzed using Tokamak Simulation Code (TSC). TSC is a non-linear two-dimensional axisymmetric free boundary simulation code. Maxwell’s and MHD equations are used to describe the evolution of magnetic field in a rectangular domain for the plasma. In 2016, the model for VDE free drift and control has been benchmarked against experiments. In this paper, the power supply model for fast VDE control is extended to one with limited maximum control current. VDE controllability with the present system capability is studied. The maximum controllable displacement dZmax is numerically analyzed, for both ordinary experimental discharge and equilibria with typical βp and li in EAST. It is found that dZmax inversely grows with βp and li. Rigorous conditions of li=1, βp<0.3 or βp=0.5, li<0.8 are required for dZmax reaching the safe control level. For operation safety, system capability enhancement is needed. Effects of system delay time, the maximum control current and system current ramp time on VDE controllability are discussed.

Dynamics and motion of a gas bubble in a viscoplastic medium under acoustic excitation

We investigate the dynamics of the buoyancy-driven rise of a bubble inside a viscoplastic material when it is subjected to an acoustic pressure field. To this end, we develop a simplified model based on the Lagrangian formalism assuming a pulsating bubble with a spherical shape. Moreover, to account for the effects of a deformable bubble, we also perform detailed two-dimensional axisymmetric simulations. Qualitative agreement is found between the simplified approach and the detailed numerical simulations. Our results reveal that the acoustic excitation enhances the mobility of the bubble, by increasing the size of the yielded region that surrounds the bubble, thereby decreasing the effective viscosity of the liquid and accelerating the motion of the bubble. This effect is significantly more pronounced at the resonance frequency, and it is shown that bubble motion takes place even for Bingham numbers (Bn) that can be orders of magnitude higher than the critical Bn for bubble entrapment in the case of a static pressure field.

Numerical Analysis of Aerodynamic Characteristics of Hyperloop System

The Hyperloop system is a new concept that allows a train to travel through a near-vacuum tunnel at transonic speeds. Aerodynamic drag is one of the most important factors in analyzing such systems. The blockage ratio (BR), pod speed/length, tube pressure, and temperature affect the aerodynamic drag, but the specific relationships between the drag and these parameters have not yet been comprehensively examined. In this study, we investigated the flow phenomena of a Hyperloop system, focusing on the effects of changes in the above parameters. Two-dimensional axisymmetric simulations were performed in a large parameter space covering various BR values (0.25, 0.36), pod lengths (10.75–86 m), pod speeds (50–350 m/s), tube pressures (~100–1000 Pa), and tube temperatures (275–325 K). As BR increased, the pressure drag was significantly affected. This is because of the smaller critical Mach number for a larger BR. As the pod length increased, the total drag and pressure drag did not change significantly, but there was a considerable influence on the friction drag. As the pod speed increased, strong shock waves occurred near the end of the pod. At this point, the flows around the pod were severely choked at both BR values, and the ratio of the pressure drag to the total drag converged to its saturation level. At tube pressures above 500 Pa, the friction drag increased significantly under the rapidly increased turbulence intensity near the pod surface. High tube temperatures increase the speed of sound, and this reduces the Mach number for the same pod speed, consequently delaying the onset of choking and reducing the aerodynamic drag. The results presented in this study are applicable to the fundamental design of the proposed Hyperloop system.

Numerical investigations on bubble-induced jetting and shock wave focusing: application on a needle-free injection.

The formation of a liquid jet into air induced by the growth of a laser-generated bubble inside a needle-free device is numerically investigated by employing the compressible Navier-Stokes equations. The three co-existing phases (liquid, vapour and air) are assumed to be in thermal equilibrium. A transport equation for the gas mass fraction is solved in order to simulate the non-condensable gas. The homogeneous equilibrium model is used in order to account for the phase change process between liquid and vapour. Thermodynamic closure for all three phases is achieved by a barotropic Equation of State. Two-dimensional axisymmetric simulations are performed for a needle-free device for which experimental data are available and used for the validation of the developed model. The influence of the initial bubble pressure and the meniscus geometry on the jet velocity is examined by two different sets of studies. Based on the latter, a new meniscus design similar to shaped-charge jets is proposed, which offers a more focused and higher velocity jet compared to the conventional shape of the hemispherical gas-liquid interface. Preliminary calculations show that the developed jet can penetrate the skin and thus, such configurations can contribute towards a new needle-free design.

Role of porous metal foam on the heat transfer enhancement for a thermal energy storage tube

Thermal energy storage technology has attracted extensive attentions due to its remarkable energy-saving benefits. However, the low thermal conductivity of phase change materials seriously limits the energy storage efficiency, which put forward more stringent requirements for heat transfer enhancement. In this study, a two-dimensional axisymmetric simulation model with natural convection was established for the shell-and-tube thermal energy storage unit. Open-cell metal foam with a porosity of 0.94 and pore density of 15 pore per inch was employed to be arranged in either heat transfer fluid or phase change materials domains. The effects of the metal foam location and the metal foam porosity on the heat storage performance were studied. The numerical method was verified by experimental measurement, achieving good agreement. Results demonstrated that metal foam can significantly enhance heat transfer due mainly to the reduction of thermal resistance in heat transfer fluid. The case that both domains for heat transfer fluid and phase change materials were embedded in porous media can provide the best heat transfer enhancement. Compared with smooth tube without metal foam, the full melting time for this case was reduced by 88.548%; meanwhile, temperature response rate, heat flux and j-factor was increased by 834.27%, 774.90%, 5186.91% respectively. Besides, embedding metal foam into phase change materials can improve the temperature uniformity of phase change materials.

Theoretical study on particle velocity in micro-abrasive jet machining

Micro-abrasive jet machining (AJM) is an advanced subtractive machining technology with ample opportunities to form regular micro-patterns on freeform surfaces. AJM removes material mainly through erosion and abrasion, which transform kinetic energy to fracture and deform substrates. The kinetic energy of a solid particle is tightly connected to its velocity, which is the most significant source of error in precise prediction of a machined feature. The present study involves both theoretical analysis and two-dimensional axisymmetric numerical simulation of particle velocity fields at the lower end of the micro-scale. The developed model represents the finest particles in a cylindrical nozzle down to an inner diameter of 100 μm. The computed results agree well with the experimental data. It is shown that, due to viscous friction, such nozzles are significantly less efficient in terms of particle saturation with kinetic energy. The study highlights the effects of nozzle diameter and length, air pressure, particle size and density on particle velocity development through the jet field. Finally, practical recommendations and multiple regression models of maximum particle velocity, location from the nozzle exit and simplex velocity profile approximation are offered for management of particle kinetic energy.

Numerical investigation of the impact of tailored driver gases and driver inserts on shock tube flows

Presented is a numerical study of the effects of imperfect driver gas tailoring and the use of a driver section insert on the flow field within the shock tube. Two-dimensional axisymmetric simulations of a shock tube are conducted with pure argon as the driven gas and four $$\hbox {He/N}_2$$ mixtures representing different tailoring conditions. The second part of the study investigates the use of a profiled driver insert with two $$\hbox {He/N}_2$$ driver gas mixtures to minimize the pressure rise in the test section such as that resulting from over-tailored driver mixtures. The chemical kinetic implications of imperfectly tailored driver gas mixtures on the interpretation of shock tube experiments are explored through simulations of propanol ignition with pressure and temperature changes prescribed based on the simulation results. The simulations show that imperfect tailoring toward under-tailored mixtures is more detrimental to kinetic studies in the post-reflected shock region than over-tailoring. The least compromising situation in test conditions is observed for a mildly over-tailored condition with 25% of nitrogen as opposed to the tailored value of 20%. The numerical results further show that the combination of the driver insert and slightly over-tailored driver mixture leads to longer test times. The study seeks to address problems of available test times and accuracy of shock tube reactor conditions that are crucial to the use of shock tube data for validation of chemical kinetic models.

Experimental and numerical study of laminar flame extinction for syngas and syngas-methane blends

ABSTRACTExtinction strain rates of syngas and syngas–methane blends at atmospheric pressure were investigated in laminar counter-flow diffusion flames as a function of nitrogen dilution through both experiment and two-dimensional axisymmetric full-domain numerical simulations. Three representative compositions of syngas were examined along with two syngas–methane blends for a range of fuel mole fractions. An opposed-jet burner configuration with straight-tube fuel and air nozzles was used in the experiments and an advanced, computational, solution algorithm was used to obtain the corresponding simulation results. Very good agreement was found between predicted and experimentally observed flame structure as well as global strain rate extinction limits for all syngas fuel and syngas–methane blends over the full range of flow rates and fuel mole fractions considered. Additionally, the local strain rate near extinction was shown to correlate with the global strain rate and an explanation of the differences between these two values was provided by the predicted changes in the nozzle exit plane velocity profiles as a function of flow rate. In particular, non-zero gradients in the axial center-line velocity were predicted at the nozzle exit plane for higher flow rates due to the pressure field created by the opposed-jet flows. The syngas extinction limits as a function of fuel composition were examined. While the amount of hydrogen was found to have a dominant effect on the extinction strain limits, methane and carbon monoxide were also found to induce early extinction and increase reactivity, respectively, if present in sufficiently large quantities.

Theoretical study of the flow in a fluid damper containing high viscosity silicone oil: Effects of shear-thinning and viscoelasticity

The flow inside a fluid damper where a piston reciprocates sinusoidally inside an outer casing containing high-viscosity silicone oil is simulated using a Finite Volume method, at various excitation frequencies. The oil is modelled by the Carreau-Yasuda (CY) and Phan-Thien \& Tanner (PTT) constitutive equations. Both models account for shear-thinning but only the PTT model accounts for elasticity. The CY and other generalised Newtonian models have been previously used in theoretical studies of fluid dampers, but the present study is the first to perform full two-dimensional (axisymmetric) simulations employing a viscoelastic constitutive equation. It is found that the CY and PTT predictions are similar when the excitation frequency is low, but at medium and higher frequencies the CY model fails to describe important phenomena that are predicted by the PTT model and observed in experimental studies found in the literature, such as the hysteresis of the force-displacement and force-velocity loops. Elastic effects are quantified by applying a decomposition of the damper force into elastic and viscous components, inspired from LAOS (Large Amplitude Oscillatory Shear) theory. The CY model also overestimates the damper force relative to the PTT, because it underpredicts the flow development length inside the piston-cylinder gap. It is thus concluded that (a) fluid elasticity must be accounted for and (b) theoretical approaches that rely on the assumption of one-dimensional flow in the piston-cylinder gap are of limited accuracy, even if they account for fluid viscoelasticity. The consequences of using lower-viscosity silicone oil are also briefly examined.

A computational study of the effects of DC electric fields on non-premixed counterflow methane-air flames

Two-dimensional axisymmetric simulations for counterflow non-premixed methane-air flames were undertaken as an attempt to reproduce the experimentally observed electro-hydrodynamic effect, also known as the ionic wind effect, on flames. Incompressible fluid dynamic solver was implemented with a skeletal chemical kinetic mechanism and transport property evaluations. The simulation successfully reproduced the key characteristics of the flames subjected to DC bias voltages at different intensity and polarity. Most notably, the simulation predicted the flame positions and showed good qualitative agreement with experimental data for the current–voltage curve. The flame response to the electric field with positive and negative polarity exhibited qualitatively different characteristics. In the negative polarity of the configuration considered, a non-monotonic variation of the current with the voltage was observed, along with the existence of an unstable regime at an intermediate voltage level. With positive polarity, a typical monotonic current–voltage curve was obtained. This behavior was attributed to the asymmetry in the distribution of the positive and negative ions resulting from ionization processes. The present study demonstrated that the mathematical and computational models for the ion chemistry, transport, and fluid dynamics were able to describe the key processes responsible for the flame-electric field interaction.

Earth Reentry Flow over a Phenolic Aeroshell in the X2 Expansion Tube

The effects of phenolic decomposition on shock layer radiation in air were investigated in the X2 expansion tube. By subjecting a carbon phenolic aeroshell to a flow with a velocity of , decomposing resin on the model surface was allowed to react with the boundary-layer gases. Emission spectroscopy was used to measure radiation emitted by the gas in parts of the ultraviolet, visible, and near-infrared spectrum and then compared with control measurements taken with a steel model. With the composite model in place, measured spectral radiance was seen to increase for CN violet band radiation but remained unchanged for atomic emissions in the visible and near-infrared spectrum. Recorded data were then compared with spectra produced by two-dimensional axisymmetric computational fluid dynamics simulations parsed to the NEQAIR radiation solver. Both the applied chemistry models overpredicted CN violet band radiation, but excellent comparison was found for the band. Good comparisons were also achieved for the O triplet (777 nm) and atomic N line intensities, between 740 and 750 nm. However, peak intensities were underestimated by the numerical simulations in the visible spectrum. Background radiation and broadening of the spectral wings were not accurately accounted for by simulations.

Venus Entry Flow over a Decomposing Aeroshell in X2 Expansion Tube

The effects of phenolic decomposition on shock-layer radiation were investigated experimentally in X2 expansion tube for a Venus entry flow. A carbon–phenolic composite aeroshell was subjected to a flow with a flight equivalent velocity of . Emission spectroscopy was used to measure boundary-layer radiation in parts of the ultraviolet (380–480 nm) and visible (620–700 nm) spectrum, and it was compared to control measurements taken with a cold steel model. With the composite model in place, the calibrated spectral radiance measured was seen to increase for the O (645.598 nm) atomic line and the (391.22 nm) band head, but it decreased for the Swan band (420–480 nm). The recorded data were then compared to numerical spectra produced by two-dimensional axisymmetric computational fluid dynamic simulations coupled to a radiation solver. Both of the applied chemistry models overpredicted CN violet band radiation, which demonstrated strong self-absorption at these conditions. Better comparisons were achieved for the Swan band radiation. At visible wavelengths, peak intensities were underestimated by the numerical simulations. Several possible reasons were hypothesized for these discrepancies.

Influences of aspect ratio on natural convection of power-law fluids in cylindrical annular space with differentially heated vertical walls

Natural convection of power-law fluids in cylindrical annular spaces has not been analysed in detail in spite of its industrial importance in food and chemical processing, solar collectors and cooling of electronics. Therefore, laminar natural convection of power-law fluids in rectangular cross-sectional cylindrical annular spaces with differentially heated vertical walls subjected to the both constant wall temperature (CWT), and constant wall heat flux (CWHF) boundary conditions has been investigated in this study. Two-dimensional axisymmetric steady-state numerical simulations have been conducted for a range of values of normalized internal radius 0.125⩽ri/L⩽16 (where ri internal cylinder radius, L is the difference between outer and inner cylinder radius), aspect ratio 0.125⩽AR⩽8 (AR=H/L, where H is the enclosure height), power-law index (i.e. 0.6⩽n⩽1.8) and nominal Rayleigh number (i.e. Ra=103-106) for a single value of nominal Prandtl number: Pr=103. It is found that the mean Nusselt number based on the inner periphery of the annular space Nu‾i increases with an increase in Ra due to the strengthening of buoyancy forces. By contrast, Nu‾i increases with a decrease in n due to the weakening of viscous resistance. The mean Nusselt number Nu‾i decreases with an increase in ri/L before approaching the mean Nusselt number for a rectangular enclosure in the limit of ri/L→∞. By contrast Nu‾i normalized by the corresponding Nusselt number for pure conduction (i.e. Nu‾i/Nucond) increases with an increase in ri/L. The contribution of convection to overall thermal transport strength increases with an increase in ri/L, since the Nusselt number for pure conductive transport Nucond decreases with an increase in ri/L for cylindrical annular spaces. Additionally, it has been found that Nu‾i exhibits a non-monotonic variation with an increase in AR for same set of values of Ra,Pr,ri/L for shear thinning (n<1), Newtonian (n=1) and shear thickening (n>1) fluids in the CWT configuration, while Nu‾i increases monotonically with an increase in AR in the CWHF configuration irrespective of the value of n. The completion between the strengthening of thermal convection and the weakening of conductive thermal transport with an increase in AR is responsible for the non-monotonic AR dependence of Nu‾i in the CWT boundary condition. A scaling analysis is utilised to explain the influences of normalized radius, aspect ratio, nominal Rayleigh and Prandtl numbers, power-law index on Nu‾i for natural convection of power-law fluids within rectangular cross-sectional cylindrical annular spaces. Finally, new correlations have been proposed for Nu‾i for both CWT and CWHF boundary conditions, which have been shown to provide satisfactorily predictions of Nu‾i for the range of the parameters considered in this analysis.