Numerical Simulation on the Instability of Cylindrically Expanding Premixed Flames With Radiative Heat Loss at Low Lewis Numbers

2011 ◽  
Author(s):  
Takafumi Kusakai ◽  
Satoshi Kadowaki
Keyword(s):  
Heat Loss ◽  
Thermal Instability ◽  
Radiative Heat ◽  
Premixed Flames ◽  
Lewis Number ◽  
Radiative Heat Loss ◽  
Low Gravity ◽  
Gravity Condition ◽  
Quenching Condition ◽  
Reactive Gases

The instability of cylindrically expanding premixed flames with radiative heat loss was studied by two-dimensional unsteady calculations of reactive gases, based on the diffusive-thermal model equation. When the Lewis number was unity, instability phenomena were not observed. When the Lewis number was sufficiently low, on the other hand, cellular-shaped fronts on adiabatic and non-adiabatic cylindrical flames were observed, which was due to diffusive-thermal instability. As radiative heat loss increased, the behavior of cellular fronts became more unstable. This indicated that the radiation promoted the unstable behavior of flame fronts at low Lewis numbers. When radiative heat loss was much large compared with the quenching condition of a planar flame, cylindrical flames were broken up and several small flames appeared. This was in qualitative agreement with the experimental results on the dynamic behavior of lean hydrogen-air premixed flames with radiative heat loss under the low gravity condition. Several small flames appeared on the grounds that large curvature of flame fronts was necessary to keep high temperature against radiative heat loss.

Effects of the Lewis number and radiative heat loss on the bifurcation and extinction of CH4/O2-N2-He flames

1999 ◽  
Vol 379 ◽  
pp. 165-190 ◽  
Author(s):  
YIGUANG JU ◽  
HONGSHENG GUO ◽  
FENGSHAN LIU ◽  
KAORU MARUTA
Keyword(s):  
Heat Loss ◽  
Radiative Heat ◽  
Lewis Number ◽  
Critical Value ◽  
Radiation Heat ◽  
Absorption Coefficients ◽  
Radiative Heat Loss ◽  
Radiation Heat Loss ◽  
Counterflow Flame ◽  
Counterflow Flames

Effects of the Lewis number and radiative heat loss on flame bifurcations and extinction of CH4/O2-N2-He flames are investigated numerically with detailed chemistry. Attention is paid to the interaction between radiation heat loss and the Lewis number effect. The Planck mean absorption coefficients of CO, CO2, and H2O are calculated using the statistical narrow-band model and compared with the data given by Tien. The use of Tien's Planck mean absorption coefficients overpredicts radiative heat loss by nearly 30 % in a counter flow configuration. The new Planck mean absorption coefficients are then used to calculate the extinction limits of the planar propagating flame and the counterflow flame when the Lewis number changes from 0.967 to 1.8. The interaction between radiation heat loss and the Lewis number effect greatly enriches the phenomenon of flame bifurcation. The existence of multiple flames is shown to be a physically intrinsic phenomenon of radiating counterflow flames. Eight kinds of typical patterns of flame bifurcation are identified. The competition between radiation heat loss and the Lewis number effect results in two distinct phenomena, depending on if the Lewis number is greater or less than a critical value. Comparisons between the standard limits of the unstrained flames and the ammability limits of the counterflow flames indicate that the ammability limit of the counterflow flame is lower than the standard limit when the Lewis number is less than the critical value and is equal to the standard limit when the Lewis number is higher than this critical value. Finally, a G-shaped curve and a K-shaped curve which respectively represent the ammable regions of the multiple flames for Lewis numbers lower and higher than the critical value are obtained. The G- and K-shaped curves show a clear relationship between the stretched counterflow flame and the unstrained planar flame. The present results provide a good explanation of the physics revealed experimentally in microgravity.

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

1967 ◽  
Vol 29 (3) ◽  
pp. 485-494 ◽  
Author(s):  
M. I. G. Bloor
Keyword(s):  
Numerical Solution ◽  
Heat Loss ◽  
Blunt Body ◽  
Radiative Heat ◽  
Constant Density ◽  
Axially Symmetric ◽  
Radiative Heat Loss ◽  
Stagnation Region ◽  
Expansion Procedure ◽  
Axis Of Symmetry

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.

scholarly journals Analysis of MHD Forced Convective Flow of Variable Fluid Properties over a Saturated Porous Medium with Thermal Radiation Effect

2016 ◽  
Vol 9 ◽  
pp. 47-65 ◽  
Author(s):  
Kolawole Sunday Adegbie ◽  
Adeyemi Isaiah Fagbade
Keyword(s):  
Porous Medium ◽  
Heat Loss ◽  
Convective Flow ◽  
Radiative Heat ◽  
Saturated Porous Medium ◽  
Fluid Velocity ◽  
Radiative Heat Loss ◽  
Variable Fluid Properties ◽  
Fluid Properties ◽  
Forced Convective Flow

The present paper addresses the problem of MHD forced convective flow in a fluid saturated porous medium with Brinkman-Forchheimer model, which is an important physical phenomena in engineering applications. The paper extends the previous models to account for effects of variable fluid properties on the forced convective flow through a porous medium in the presence of radiative heat loss using bivariate spectral relaxation method (BSRM). The dynamic viscosity and thermal conductivity of the newtonian fluid are assumed to vary linearly respectively, with temperature whereas the contribution of thermal radiative heat loss is based on Rosseland diffussion approximation. The flow model is described and expressed in form of a highly coupled nonlinear system of partial differential equations. The method of solution BSRM as proposed by Motsa [25] seeks to decouple the original system of PDEs to form a sequence of equations that can be solved in a computationally efficient manner. BSRM is an approach that applies spectral collocation independently in all underlying independent variable is executed to obtain approximate solutions of the problem. The proposed algorithm is supposed to be a very accurate, convergent and very effective in generating numerical results. The results obtained show a significant effects of the flow control parameters on the fluid velocity and temperature respectively. Consequently, the wall shear stress and local heat transfer rate of the present paper are compared with the available results in literatures. Remarkable impacts and a good agreement are found.

Mechanisms of thermal stability during flight in the honeybee apis mellifera

1999 ◽  
Vol 202 (11) ◽  
pp. 1523-1533 ◽  
Author(s):  
S.P. Roberts ◽  
J.F. Harrison
Keyword(s):  
Thermal Stability ◽  
Heat Loss ◽  
Heat Production ◽  
Radiative Heat ◽  
Carbon Dioxide Production ◽  
Metabolic Heat Production ◽  
Evaporative Heat Loss ◽  
Radiative Heat Loss ◽  
Convective Heat Loss ◽  
Metabolic Heat

Thermoregulation of the thorax allows honeybees (Apis mellifera) to maintain the flight muscle temperatures necessary to meet the power requirements for flight and to remain active outside the hive across a wide range of air temperatures (Ta). To determine the heat-exchange pathways through which flying honeybees achieve thermal stability, we measured body temperatures and rates of carbon dioxide production and water vapor loss between Ta values of 21 and 45 degrees C for honeybees flying in a respirometry chamber. Body temperatures were not significantly affected by continuous flight duration in the respirometer, indicating that flying bees were at thermal equilibrium. Thorax temperatures (Tth) during flight were relatively stable, with a slope of Tth on Ta of 0.39. Metabolic heat production, calculated from rates of carbon dioxide production, decreased linearly by 43 % as Ta rose from 21 to 45 degrees C. Evaporative heat loss increased nonlinearly by over sevenfold, with evaporation rising rapidly at Ta values above 33 degrees C. At Ta values above 43 degrees C, head temperature dropped below Ta by approximately 1–2 degrees C, indicating that substantial evaporation from the head was occurring at very high Ta values. The water flux of flying honeybees was positive at Ta values below 31 degrees C, but increasingly negative at higher Ta values. At all Ta values, flying honeybees experienced a net radiative heat loss. Since the honeybees were in thermal equilibrium, convective heat loss was calculated as the amount of heat necessary to balance metabolic heat gain against evaporative and radiative heat loss. Convective heat loss decreased strongly as Ta rose because of the decrease in the elevation of body temperature above Ta rather than the variation in the convection coefficient. In conclusion, variation in metabolic heat production is the dominant mechanism of maintaining thermal stability during flight between Ta values of 21 and 33 degrees C, but variations in metabolic heat production and evaporative heat loss are equally important to the prevention of overheating during flight at Ta values between 33 and 45 degrees C.

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