Ahmet Selamet

Entropy production in flames

Abstract Thermodynamic foundations of the thermal entropy production are rested on the concept of lost heat, ( Q T ) δT. The thermomechanical entropy production is shown to be in terms of the lost heat and the lost work as δS 8 = 1 T Q T δT+δW l where the second term in brackets denotes the lost (dissipated) work into heat. The dimensionless number Πs describing the local entropy production s‴ in a quenched flame is found to be Πs∼(Ped0)−2, where Π s = s‴l 2 k , l = α S u 0 (a characteristic length), k thermal conductivity, α thermal diffusivity, Su0 the adiabatic laminar flame speed at the unburned gas temperature, Pe d 0 = S u 0 D α the flame Peclet number, and D the quench distance. The tangency condition ∂ Pe d 0 ∂θ p = 0 , where θ b = T b T b 0 , Tb and Tb0 denoting, respectively, the burned gas (nonadiabatic) and adiabatic flame temperatures, is related to an extremum in entropy production. The distribution of entropy production between the flame and burner is shown in terms of the burned gas temperature and the distance from the burner.

Entropy production in boundary layers

The development of the entropy production in moving media entails consideration of momentum, energy, and entropy balances. The radiation-affected forced convection over a flat plate is presently investigated in terms of a thin gas, with a view to evaluating the distribution of entropy within and outside the radiation-affected thermal boundary layer. The retained nonlinearity of temperature in the entropy production leads to an extremum in this production within the boundary layer, rather than on the boundary. 14 refs.

Buoyancy-driven turbulent diffusion flames

Abstract A fundamental dimensionless number for pool fires, ∏ β = σ β 1+σ β Ra β , in proposed. Here σ β and Ra β denote a flame Schmidt number and a flame Rayleigh number. The sublayer thickness of a turbulent pool fire, η β , is shown in terms of Π β to be η β l ∼∏ β − 1 3 , where l is an integral scale. The fuel consumption in a turbulent pool fire expressed in terms of η β ( Π β ) and correlated by the experimental data leads to m′ ρD Ra 1 3 = 0.15B (1+0.05B) 1 3 (1+B) 1 3 . where ϱ is the density, D the mass diffusivity, Ra the usual Rayleigh number, and B the transfer number. The model agrees well with a previous model based on the stagnant film hypothesis.

Rayleigh limit—Penndorf extension

generated for the Rayleigh and Penndorf limits for c< = 0.3, 0.5, and 0.7 in the 1.5 ~< n ~ 2.5 and 0.5 ~< k ~< 1.5 domain which covers the range of soot properties. The practical significance of the Penndorf correction is demonstrated in terms of optical diagnostics and radiative heat transfer. Also, the Planck and Rosseland mean absorption coefficients based on the Penndorf expansion are shown to yield relative to those based on the Rayleigh limit P R

Radiation Affected Liquid Fuel Burning on Water

Abstract A model for the radiation affected liquid fuel burning on water is developed which improves substantially on an existing model by including the turbulent fire plume and the explicit effects (hotness and optical thickness) of fire radiation. In achieving these objectives, the experimental literature on flame height and velocity is utilized. The fire plume above the slick is divided into three regions: the continuous flame; the intermittent flame; and the thermal plume. To describe the fire plume, Taylors entrainment model is used which assumes top-hat profiles for the radial velocity and temperature and relates the entrainment and vertical velocities at a given height. The contributions from gaseous combustion products, mainly CO2 and H2O, as well as from particulate matter to radiative heat transfer are also accounted for. Both linear and nonlinear solutions are obtained and compared to examine the accuracy of the usual approach of linearization in the radiative transport equations. The effect o...

Monochromatic absorption of luminous flames

The wavelength dependence of the absorption coefficient for soot, particularly in the visible range of the spectrum, was studied. It was shown that the empirical relation used in the literature to represent this dependency could be replaced by a fundamental relation based on the Penndorf expansion. Through this new expression, higher order wavelength dependency to the Rayleigh limit as well as the particle size effect were introduced

Visible and infra-red sensitivity of Rayleigh limit and Penndorf extension to complex refractive index of soot

The present study makes an improved assessment of the foregoing optical property range for soot by employing the Lorentz dispersion theory [9, IO] combined with the experimental data. The above range is found to fail to cover infra-red radiation adequately and needs to be extended to 1.5 < n < 4.0 and 0.5 < k < 3.0 to incorporate the soot property variation up to about 20 pm. Accordingly, the primary objective of the study is to produce the error contours representing the percent deviation involved with the Rayleigh limit and the Penndorf expansion relative to the Lorenz-Mie theory for the range 1.5 < n < 4.0 and 0.5 < k 6 3.0 as a continuous function of n and k, thus providing a complete coverage for property range of particulate thermal radiation. The contours exhibit a distinguished error dependency on n with the dependency on k being of secondary importance. It is shown that for optical properties associated with infra-red radiation, Rayleigh deviation may well exceed 40% for a = 0.3 while that of Penndorf remains within 5%. Likewise, for a = 0.7, Rayleigh deviation reaches about 60% while that of Penndorf remains about within 10%. Thus. a basis is provided here for judging the accuracy of Rayleigh limit and Penndorf extension in the continuous n-k domain for the particulate radiation. The next section utilizes dispersion theory in computing n and k against wavelength and generates the extinction error contours for both Rayleigh and Penndorf approximations as a continuous function of m and discrete function of a. The study is concluded with some final remarks. 2 + _ n,!!L_