Effect Of Radiative Heat Loss Research Articles

The dependence of the diffusion coefficient on the mixing length in Kato's overstable zone

The treatment of the overstable zone by Langer, Sugimoto &38; Fricke is extended so that the effects of radiative heat loss from convective elements and of the mixing length are included. As the overstability investigated by Kato arises because of the radiative heat loss from the convective element, this loss may play a role in the heat flux. In the formulation presented in this work, the diffusion coefficient depends on the mixing length when the mixing length is very small. In calculations with the method of Langer et al., the efficiency factor of the diffusion coefficient of chemical species is sometimes much smaller than 1. This exceedingly small efficiency factor may be explained by the dependence of the diffusion coefficient on the mixing length. We find that the neutral condition of Kato's overstable zone is ∇ rad = ∇ ad.

Numerical Investigation of CH4/CO2/Air and CH4/CO2/O2 Counterflow Premixed Flames with Radiation Reabsorption

Effects of radiative heat loss on temperatures and extinction characteristics of CH4/CO2air and CH4/CO2/O2 counterflow premixed flames were numerically investigated by using the detailed chemistry and transport properties with emphasis on assessing the importance of radiation reabsorption. Radiative transfer was calculated using the discrete ordinate method along with the grey gas assumption. Results show that radiation reabsorption has little influence on the temperatures and extinction limits of CH4/ air counterflow premixed flames of low equivalence ratio. However the radiation reabsorption has a significant effect on the flame temperatures and extinction limits of CH4/CO2/air and CH4/CO2/O2 counterflow premixed flames becomes smaller as the fraction of Co2 added to air increases. Radiation extinction of CH4/CO2/O2 flames occurs at higher stretch rates than that of CH4/air flames.

Radiative effects in space-based methanol/water droplet combustion experiments

The results from methanol/water droplet combustion experiments conducted on two separate spacebased investigations are analyzed and compared against the predictions of a detailed numerical model. These two investigations (Fiber Supported Droplet Combustion Investigations 1 and 2) were conducted aboard Space Shuttle Columbia missions STS-73 and STS-94 in November 1995 and July 1997, respectively. In these experiments, pure methanol droplets and methanol/water droplets of 2–7 mm were suspended on a silicon carbide fiber and ignited with a hot wire. In some cases, droplets burned for over 40 s, and flame extinction was observed at small but finite droplet diameters. In other cases, droplets ignited but the flames rapidly extinguished. Each of the experiments is compared against the results of a detailed numerical model that simulates the time-dependent, spherically symmetric combustion of a multicomponent liquid droplet in an infinite oxidizing environemnt. The model includes detailed chemistry, multicomponent molecular diffusion and radiative heat transfer. The model quantitatively reproduces (without modifications or adjustments) measured burning rate, flame position, and extinction diameter for a wide range of initial diameters and initial water content. The results show that the effect of radiative heat loss is significant in experiments with initial diameters greater than 3 mm. Specifically, decreased burning rate and a nonlinear increase in extinction diameter are observed with increasing initial diameter. Moreover, a critical radiative extinction diameter of 6 mm, predicted in a previous study for methanol droplet combustion in 1-atm air, is verified experimentally.

Radiation extinction limit of counterflow premixed lean methane-air flames

The application of the laminar flamelet concept to turbulent flame modeling requires a detailed understanding of stretched laminar flames. In this study, we used numerical methods, including are-length continuation, to simulate the extinction characteristics of counterflow premixed fuel-lean, methane-air flames. Attention was primarily paid to the effect of radiative heat loss on the extinction characteristics of these flames. The results show that at medium to low values of the stretch rate, the radiative heat loss has a particularly strong impact on the counterflow premixed fuel-lean, methane-air flames. It was also found that, in addition to the stretch extinction limit at a high stretch rate, there exits a radiation extinction limit at a low stretch rate. Furthermore, the relationship between these two extinction limits and the equivalence ratio is obtained.

The Effect of Non-Luminous Thermal Radiation in Microgravity Droplet Combustion

The effect or radiative heat loss from isolated droplet flames is usually assumed to be negligible. For the small droplet sizes studied in most isolated droplet combustion experiments conducted to dale (< 1.0 mm), this assumption has been shown to be reasonable. For example, by neglecting radiation, a detailed numerical model accurately predicts the burning rate, flame position and extinction diameter for 1 millimeter-sized methanol/waler droplets. However, recent space-based 3 to 5 millimeter methanol/water droplet combustion data show an increase in extinction diameter and a decrease in burning rate with increasing initial diameter. These results suggest that, at larger initial droplet diameters, the effect of radiative heat loss cannot be neglected. By including a radiation sub-model, the modified numerical model predicts that at droplet diameters greater than about 1 mm the effect of radiation results in a decrease in burning rate and a non-linear increase in extinction diameter with increasing initial diameter. At very large initial diameters (<6 mm in atmospheric air), the model predicts that the droplet flame will ignite, but quickly self-extinguish due to excessive radiative heat loss.

Effects of Radiation Heat Loss on the Flammable Region of Counterflow Premixed Flames at a Large Lewis Number.

The extinction limits and flammable regions of a nonadiabatic counterflow premixed flame at a large Lewis number are investigated numerically. It is found that interaction between the radiative heat loss and the Lewis number effect results in four kinds of flame bifurcations. Furthermore, the results show that there exist four different kinds of flame regimes : normal flames, radiation-stretch-induced weak flames, radiation-Lewis number effect-induced weak flames and unified weak flames at typical equivalence ratios. An increase in the radiation intensity reduces the extinction limit of a normal flame but extends the flammable region of weak flames to large stretch rates. A general graph showing the extinction limits and the flammable regions is obtained. It is found that the G-shaped curve obtained at a low Lewis number is reduced to a K-shaped curve at a large Lewis number. This G-K transition presents a good explanation for the experimental results obtained in microgravity experiments.

Combustion of a stretched carbon monoxide ligament with radiative heat losses

This article concerns a theoretical study of combustion in a stretched, one-dimensional carbon monoxide (CO) strip surrounded by air. The analysis includes the effects of radiative heat transfer and time-dependent strain. Solution of mass, species, and energy equations for a single-step, finite-rate reaction between CO and O 2 is obtained for an externally imposed in-plane strain motion with strain rates up to 1000 s −1 and for several strain rate histories. The radiation heat loss from the flame zone to its surroundings is incorporated into the solution using the optically thin gas radiation approximation. The results indicate that unstretched flames are significantly influenced by the presence of radiative heat loss, resulting in thermal quenching. Although, CO consumption rates in positively stretched flames are unaffected by the effects of radiative heat loss, the structure of the flame zone is somewhat altered. The CO oxidation behavior is found to be dependent on the strain history. Although low-frequency (of the order of inverse diffusion time scale) fluctuations in the strain rate enhance the CO oxidation significantly, the flame does not respond to fast oscillations. Radiative heat losses can promote quenching during negative (compressive) strain periods in the strain history.

The effects of radiative heat loss from the convective element, and of helium flux, on overshooting in stellar convective cores

The non-local mixing-length theory of stellar convective cores by Maeder is extended to include the effects of radiative heat loss from the convective element, and of helium flux. The radiative heat loss tends to enlarge the overshooting region, although it barely changes the size of the overshooting core for the ordinarily accepted size of the convective element. The helium gradient seems negligible in the overshooting region

Influence of through-window radiation on the horizontal Bridgman process for rectangular shaped GaAs crystals

Si-doped GaAs crystals with a rectangular cross section have been grown using the modified two-temperature horizontal Bridgman (M2T-HB) technique [1]. A viewing window was opened on top of the furnace to monitor the growth process of rectangular GaAs crystals. By controlling the position and shape of the solid/liquid interface underneath the viewing window, lineage defects were eliminated and crystals of low dislocation density were successfully grown. Numerical analysis based on a semi-implicit finite difference scheme was performed with a radiative boundary condition imposed on the surface of solid crystal. The effect of radiative heat loss through the viewing window is discussed.

Radiative condensation instability in an inhomogeneous magnetized plasma

A general dispersion relation for the radiative condensation instability in an inhomogeneous magnetized plasma is derived from the modified Braginskii two-fluid equations including radiative heat loss effects. It is found that for parameter values relevant to the marfe regions of current Tokamaks the radiative thermal instability can be linearly coupled to the ion acoustic and drift waves. The dispersion relations and instability characteristics of the coupled modes are presented.

A theoretical analysis of the extinction limits of a methane-air opposed-jet diffusion flame

A theoretical analysis is described for a methane-air diffusion flame stabilized in the forward stagnation region of a porous metal cylinder in a forced convective flow. The analysis includes effects of radiative heat loss from the porous metal surface and finite rate kinetics but neglects the effects of gravity. The theoretically predicted extinction limits compare well with experimentally observed extinction limits from the literature. After the predicted limits compared well with the experimental limits, a parametric study of the effect of fuel surface emissivity and Lewis number was conducted with the numerical model. It was found that the computed blowoff limit is independent of radiative heat loss for high fuel blowing velocities but is a strong function of Lewis number. At low fuel blowing velocities, the extinction limit varies with both radiative heat loss and Lewis number. It is discovered, however, that even if thermal losses from the fuel surface are absent, the flame can extinguish at the fuel surface independently of Lewis number due to excessive reaction zone thinning.

Models for the remnants of recurrent novae - II. Dynamical effect of radiative heat loss

Models for the remnants of recurrent novae - II. Dynamical effect of radiative heat loss

Thermal overstability of hydromagnetic surface waves

We investigate the effects of radiative heat losses and thermal conductivity on the hydromagnetic surface waves along a magnetic discontinuity in a plasma of infinite electrical conductivity. We show that the effects of radiative heat losses on such surface waves are appreciable only when values of the plasma pressure on the two sides of the discontinuity are substantially different. Overstability of a surface wave requires that the medium in which it gives larger first-order compression should satisfy the criterion of Field (1965). Possible applications of the study to magnetic discontinuities in solar corona are briefly discussed.

Effect of Radiative Heat Loss on the Subsonic Region of the Hypersonic Shock Layer

An analytic, uniformly valid solution for the subsonic region of the hypersonic shock layer is found when the gas is losing energy by radiation. For simplicity the gas is assumed to be perfect and the density ratio across the shock is assumed to be large. Use is made of the von Mises transformation, the transformed equations being solved by interchanging the roles of an independent and a dependent variable. The solutions for the sonic line and a stream line are compared with a nonradiative solution. The comparison of the shock stand-off distance in the stagnation region with previous numerical results shows favorable agreement.

The effect of radiative heat loss on steady cellular convection

In the atmosphere there may be layers undergoing cellular convection with a much larger heat flux through the base of the layer than through the top. This may be either because there is a steady loss of heat by radiation from the body of the fluid or because the temperature is everywhere rising. In this latter case the temperature gradients could remain constant so that the mechanics would be the same as if the heat were being lost and the temperature kept steady. The fluid is considered incompressible as in the classical theory of cellular convection, and we determine the critical Rayleigh number for the onset of convection and the width to height ratio of the cells as functions of the heat loss. The problem, is in some respects analogous to that of the motion of a viscous fluid between rotating cylinders but in this case there are two non-dimensional-numbers-the Rayleigh number (g α βh 4/K v) and a number representing the ratio of the heat loss by radiation to the heat flux. It is found that the critical Rayleigh number is decreased and the cells widened as had already been found for the case of a fluid with transfer coefficients having a spatial variation, with free boundaries, but the cells are made more narrow if the boundaries are rigid.

Effect of radiative heat loss on steady hypersonic flow past a blunt body

Using the grey gas approximation, the effect of radiative heat loss on axially symmetric flows is studied. Using an expansion procedure about the axis of symmetry, a numerical solution for the stagnation region is found taking the shock to be spherical. The results of this calculation are compared with the results of Lighthill's non-radiative constant density solution.