Isothermal Cold Wall Research Articles

Laminar premixed hydrogen/air flame dynamics with/without sidewall interactions

This paper investigates laminar premixed hydrogen/air flame-wall interactions (FWI) subject to flame stretch (curvature and strain rate). Direct numerical simulations of finite-rate chemical reactions and in-depth molecular diffusion are used to numerically study two-dimensional (2D) V-shaped flames, with one side functioning as an isothermal cold wall that interacts with the flame and the other as a symmetrical center for free-propagating flame. An isometric grid with a grid resolution of 10 μm is used to resolve the FWI zone. It is discovered that the wall at the flame tip experiences increased negative stretching and enhanced local preferential diffusion which reduces the intensity of local combustion. This effect is especially noticeable at low equivalent ratios. Additionally, there is a close relationship between quenching behavior and flame dynamics. The present results show that the Markstein numbers for both the sidewall quenching flame and the freely propagating flame (MaSWQ and MaFF) are negative in lean hydrogen flames and positive in rich hydrogen flames. Cold wall has less impact on Ma. Furthermore, an intersection in the equivalence ratio is observed in the lean-fuel hydrogen/air flame, located at Φ = 0.8–1.0, with a change in the sign of Ma.

Simulation of Natural Convection around an Inclined Heating Triangular Cylinder Using Lattice Boltzmann Method: On the Stabilization of the Oscillatory Regime by Using Nanoparticles

Natural convection heat transfer in an inclined outer square cylinder confining a heated triangular cylinder is studied numerically using the Lattice Boltzmann method. The working fluid (pure water or Al2O3-water nanofluid) is confined in the space between the heated triangular body and an outer square cylinder. The cooling process of the system is ensured through two opposite isothermal cold walls of the external cylinder. The parameters governing the present problem are the Rayleigh number (103 ≤ Ra ≤ 106), the inclination of the configuration (0° ≤ θ ≤ 180°), and the volume fraction of nanoparticles (0 ≤ φ ≤ 0.04). The results obtained are presented in terms of streamlines, isothermes, intensities of the flow cells, and Nusselt numbers. This study shows that the inclination of the studied configuration has an important effect on the flow structure, the flow cells’ intensities, the amount of heat evacuated through each of the two cold walls and on the steadiness of the final state of the flow (steady or unsteady). It is shown that the inclination of the system has a moderate effect on the global quantity of heat leaving the block ( undergoes a maximum variation of 12.2% when the inclination is varied in its range). In addition, we show that it is possible to eliminate the Hopf’s instabilities and bring the flow to the steady regime by adding Al2O3 nanoparticles with certain volume fraction in the base fluid.

Analysis of Wall–flame Interaction in Laminar Non-premixed Combustion

ABSTRACT The study is aimed at demonstrating a methodology for the time-scale characterization of the chemistry-wall-heat-transfer interaction. The driving chemical time-scale is estimated by means of the tangential stretching rate, and a proper thermal timescale for the temperature-time variation due to wall heat flux is presented. A thermal Damköhler number, Dath , is proposed as the ratio of the two. The methodology is applied on a prototypical laminar methane-oxygen diffusion flame impinging on an isothermal cold wall. Non-adiabatic effects are described qualitatively and a CSP-TSR analysis is performed to obtain topological information and physical insights. The thermal Damköhler number field is computed and discussed to highlight the interplay between chemical and diffusive processes and to a-priori assess the accuracy of the steady laminar flamelet assumption under non-adiabatic conditions.

Premixed flame–wall interaction in a narrow channel: impact of wall thermal conductivity and heat losses

The flow physics controlling the stabilisation of a methane/air laminar premixed flame in a narrow channel (internal width $\ell _{i}=5~\text{mm}$) is revisited from numerical simulations. Combustion is described with complex chemistry and transport properties, along with a coupled simulation of heat transfer at and within the wall. To conduct a thorough analysis of the flame–wall interaction, the steady flame is obtained after applying a procedure to find the inlet mass flow rate that exactly matches the flame mass burning rate. The response of the premixed flame shape to various operating conditions is then analysed in terms of flame propagation velocity and flow topology in the vicinity of the reactive front. We focus on the interrelations between the flame speed, the configuration taken by the flame surface, the flow deviation induced by the heat released and the fluxes at the wall. Compared to an adiabatic flame, the flame speed increases with edge-flame quenching at an isothermal cold wall in the absence of a boundary layer, decreases with a boundary layer, to increase again with heat-transfer coupling within the wall. A regime diagram is proposed to delineate between flame shapes in order to build a classification versus heat-transfer properties. Under a small level of convective heat transfer with the ambient air surrounding the channel, the larger the thermal conductivity in the solid, the faster the reaction zone propagates in the vicinity of the wall, leaving the centreline reaction zone behind. The premixed flame front is then concave towards the fresh gases on the axis of symmetry (so-called tulip flame) with a flame speed higher than in the adiabatic case. Increasing the heat loss at the wall through convection with ambient air, the flame shape becomes convex (mushroom flame) and the flame speed decreases below its adiabatic level. Scaling laws are provided for the flame speed under these various regimes. Mesh resolution was calibrated, with and without heat loss, from simulations of one-dimensional detailed chemistry flames, leading to mesh resolution of $12.5~\unicode[STIX]{x03BC}\text{m}$ for detailed chemistry and $25.0~\unicode[STIX]{x03BC}\text{m}$ with a skeleton mechanism. The quality of the resolution was also assessed from multi-physics budgets derived from first principles, involving upstream-flame heat retrocession by the wall leading to flow acceleration, budgets bringing physical insights into flame/wall interaction. Additional overall mesh convergence tests of the multi-physics solution would have been desirable, but were not conducted due to the high computing cost of these fully compressible simulations, hence also solving for the acoustic field with low convective velocities.

3D Numerical Simulation of a Laminar Experimental SWQ Burner with Tabulated Chemistry

Flame-wall interaction (FWI) plays an important role in enclosed combustion systems. For avoiding the complexity of close to reality combustors, in this study an atmospheric premixed V-shaped flame interacting with an isothermal cold wall in a side wall quenching (SWQ) configuration is investigated. A stoichiometric methane/air mixture is used as fuel. A three-dimensional (3D) numerical simulation, which resolves all flow structures is combined with a tabulated chemistry approach (flamelet generated manifold, FGM). Results are compared with experimental data and two-dimensional simulations. The FGM approach is a suitable trade-off between computationally expensive detailed chemistry simulations and over simplified single step mechanisms. 2D simulations are used to investigate the influence of the uncertainty of the wall temperature, to show that the resolution in 3D is sufficient and that the influence of the flame thickening on the wall heat fluxes can be determined. Our results show that the 3D FGM approach is in close agreement to experimentally obtained flow and temperature fields. The dimensionless wall heat flux and Peclet number matches the expected values of 0.16 and 7, respectively. However, during FWI the measured CO mole fractions are not reproduced accurately showing that the transported variables in the present approach of tabulated chemistry do not recover premixed flame structures near walls.

DNS Analysis of Wall Heat Transfer and Combustion Regimes in a Turbulent Non-premixed Wall-jet Flame

Understanding the heat-release effects on the wall heat transfer in turbulent reacting flows, i.e. heat transfer with or without significant density variation, is essential for a wide variety of industrial flows, especially combustion problems. The present study focuses on the wall heat transfer and the near-wall reaction characteristics. The heat-release effects on the wall heat transfer and skin-friction coefficients are investigated using three-dimensional direct numerical simulations of a turbulent reacting wall-jet flow with and without heat release. Reductions in the skin-friction coefficient are observed in the exothermic case, compared to the isothermal one, and the underlying mechanism is explained. The absolute wall heat flux also increases, while the corresponding Nusselt number decreases with increasing heat release. Furthermore, the wall effects on the near-wall average burning rate are assessed. It is found that the isothermal cold wall results in an appreciable decrease of the burning rate in the exothermic cases. We observed indications that the wall increases the chances for the development of the premixed mode and its occurrence is very fast in the wall-normal direction.

Influence Of Relative Size Of The U-Shaped Silicon Rod On Conjugate Heat Transfer In The Regime Of Gas Buayancy Induced Convection

The influence of relative cross-section size of an electrically heated U-shaped silicon rod which is placed in a gas-filled rectangle container with isothermal cold walls on conjugate heat transfer in the regime of buoyancy induced convection was numerically studied in three-dimensional formulation. The natural convection equations in the Boussinesq approximation in term temperature, velocity vortex and velocity vector potential were solved by the finite element method. The spatial form of convective flow and temperature fields in liquid and solid body were studied. It was show that spatial form and intensity of convective flows is significantly depends on the cross-section size of U-shaped silicon rod. This is has strong influence on the temperature field in a solid body.

Conjugate Heat Transfer U-Shaped Silicon Rod with the Environment with Gas Thermogravitational Convection

The conjugated natural convective heat transfer of electrically heated U-shaped silicon rod which placed in a rectangular container filled with gas with isothermal cold walls was numerically studied in a three-dimensional formulation of the problem. The temperature was maintained constant at the control point which placed on a surface of the U-shaped rod by selection of voltage. Temperature difference between the control point and the cold walls is 70 K. The equations of natural convections in Cartesian coordinates with Boussinesq approximation in terms of temperature, vortex and vector potential of velocity field were solver by FEM. Convective flows spatial form and temperature fields inside gas and the solid were investigated

Numerical analysis of effect of ignition methods on flame behavior during passing through a sudden contraction near the quenching conditions

The present study numerically investigates the propagation of a laminar premixed propane–air flame in various tubes with a sudden contraction and isothermal cold walls in the stoichiometric mixture. Two different ignition methods are examined: plane and round shaped ignition. These investigations show that for relatively small tubes, the method of ignition does not influence the final flame shape in the tube – it is mushroom-shaped. For wider tubes and the plane ignition method, the flame transforms to a tulip-shaped one. The numerical results show that a flame can enter a smaller diameter tube from a wider one and propagate in a stable manner in all configurations of diameters of both tubes as long as the diameter of the narrow tube is far from the quenching diameter. Decreasing the diameter of the narrow tube, we approach the conditions where the dimension of the wide tube determines if the flame will propagate or quench. This is associated with a temporarily increased ratio of heat losses to thermal energy generation caused by a greater heat loss area. It is also shown that a tulip-shaped flame occurring at large diameters of the wide tube is particularly exposed to quenching during propagation through the sudden contraction.

Numerical Analysis of Flame Behavior Near the Quenching Conditions During Passage from Wider to Narrower Tube Diameters

The present study numerically investigates the propagation of a laminar premixed flame front in two-dimensional tubes with a sudden contraction and isothermal cold walls. Navier–Stokes equations and species equations are solved with a single-step overall reaction model for a stoichiometric propane/air mixture. The finite volume method is applied in investigating flame behavior during the passage through a sudden contraction in tubes of various diameters. These investigations show that a flame can enter a smaller diameter tube from a wider one and propagate in a stable manner in all configurations of diameters of both tubes as long as the diameter of the narrow tube is far from the quenching diameter. By decreasing the diameter of the narrow tube, we approach conditions where the dimension of the wide tube decides if the flame will propagate or quench. This is associated with a temporarily increased ratio of heat losses to thermal energy generation caused by a greater heat loss area. It is also shown that a tulip-shaped flame occurring at large diameters of the wide tube is particularly exposed to quenching during propagation through the sudden contraction. This is due to a transition from a tulip-shaped flame to a mushroom-shaped one, which takes place at the entrance to the narrower tube.

Effects of Heat Losses on Flame Shape and Quenching of Premixed Flames in Narrow Channels

Unsteady premixed flames in narrow channels of different heights with adiabatic or isothermal walls are investigated numerically. It is assumed that the flame chemistry is modeled by an one-step overall reaction which simulates the reaction of a stoichiometric acetylene-air mixture. The numerical results show that, In the case of plane ignition, both channel height and thermal boundary condition affect flame structure formation. The increase of channel height is favor of the formation of tulip-shaped flame. Under a given channel height, a transition from the tulip-shaped flame to the mushroom-shaped flame occurs as the flame propagates from one end of channel to the other for isothermal cold walls. In addition, some parameters such as quenching temperature, quasi-extinction space and heat-supply rate are introduced to analyze the intrinsic relationships between flame propagation and quenching in narrow channels, which is called the criterion of flame propagation and quenching in narrow channels.

Propagation and quenching of premixed flames in narrow channels

The shape and propagation of unsteady premixed flames in narrow channels with adiabatic and isothermal walls are numerically investigated in the present study. The flame chemistry is modelled by an one-step overall reaction, which simulates the reaction of a stoichiometric acetylene-air mixture. The numerical results show that both ignition methods and thermal boundary conditions affect flame formation and its shape. The flame keeps a single tulip shape in the whole process of propagating through the channel if plane ignition is used and a single mushroom shape if spark ignition is used. For isothermal cold walls, the flame cannot keep a single tulip shape or mushroom shape all the time. Under plane ignition, a transition from a tulip-shaped flame to a mushroom-shaped flame occurs as flames propagate from one end of channel to the other. Under spark ignition, the process is just the contrary. It is also shown that the heat loss at the cold walls has a dual effect on the formation of a tulip-shaped flame. Flame propagation and quenching in narrow channels of different heights are analyzed systematically, and a criterion is proposed to judge the flame states of partial extinction and total extinction. It is called the criterion of flame propagation and quenching in narrow channels.

Van Driest transformation and compressible wall-bounded flows

The transformation validity question utilizing resulting data from direct numerical simulations (DNS) of supersonic, isothermal cold wall channel flow was investigated. The DNS results stood for a wide scope of parameter and were suitable for the purpose of examining the generality of Van Driest transformation. The Van Driest law of the wall can be obtained from the inner-layer similarity arguments. It was demonstrated that the Van Driest transformation cannot be incorporated to collapse the sublayer and log-layer velocity profiles simultaneously. Velocity and temperature predictions according to the preceding composite mixing-length model were presented. Despite satisfactory congruity with the DNS data, the model must be perceived as an engineering guide and not as a rigorous analysis.

Influence of Aperture Height and Width on Interzonal Natural Convection in a Full-Scale Air-Filled Enclosure

The topic of this paper is the influence of aperture height and width on interzonal high Rayleigh number, natural convection heat transfer. Experiments were conducted in an 8 ft. air-filled cube divided into two zones by a vertical partition which was centered between a constant flux hot wall and an isothermal cold wall. The partition was configured to form doorway-like apertures. The aperture height relative to test cell height range from 1/8 to 1 and the aperture width relative to test cell width ranged from 0.009 to 1. The zone-to-zone temperature difference and the overall Nusselt number were determined experimentally, and correlated with the overall Rayleigh number, aperture, and enclosure geometry, using a series resistance model for the enclosure. A turbulent boundary layer resistance was used to represent the hot and cold boundary layer flow, while an orifice resistance was used to represent the aperture flow. For flux Rayleigh numbers between 5*1011 and 5*1012, the enclosure Nusselt numbers ranged between 15 and 165, with a strong dependence on aperture height.

Natural Convection in Collector Cavities With an Isoflux Absorber Plate

The boundary condition at the hot absorber plate in a solar collector will influence the natural convection in the enclosure. For the isoflux boundary condition and an isothermal cold wall an experimental and numerical study has been made for Ra numbers from 105 to 107 and inclinations from 20 to 90 deg with the horizontal. For vertical enclosures the heat transfer by natural convection was about 19 percent above that for an isothermal hot plate. This decreases with angle of inclination, to 9 percent at 20 deg. For solar collectors it means that for cases where the absorber plate is not isothermal the convective losses can be about 10 percent above the usually expected values.

An analytical solution for solidification of a moving warm liquid onto an isothermal cold wall

Successive solutions for the instantaneous frozen layer thickness and temperature profile in the solidified layer were generated by an analytical iteration technique. Each iteration was more accurate than the preceding one and the succession of solutions converged rapidly. Analytical expressions obtained were of a simple form and agreed with numerical and approximate solutions of other investigators.