Articles written in Sadhana

    • Differential interferometry in heat transfer

      V Rammohan Rao C B Sobhan S P Venkateshan

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      In this paper the optical methods in heat transfer applications are described briefly. In particular, interferometric methods are discussed in detail as applied to the study of natural convection heat transfer. Of the most popular interferometers, viz. the Mach Zehnder Interferometer (mzi) and the Differential Interferometer (di), thedi is considered in detail. The theoretical and constructional features of adi are outlined. The procedures of heat transfer measurement and the temperature profile estimation are explained. The advantages and limitations of the two interferometers are compared with the help of a sample calculation. The experimental analyses of three cases, viz. (1) isothermal vertical flat plate, (2) single vertical fin attached to a heated horizontal base, and (3) horizontal fin array, are described. Typical heat transfer and temperature profile results are presented. Using the estimated temperature profiles isotherms are plotted. It is possible to get fairly good heat transfer and temperature profiles using adi. The instrument is specially useful in applications involving short optical path lengths because of its superior resolution as compared tomzi.

    • Onset of natural convection in a differentially heated layer of gray and non-gray gas mixtures


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      The onset of convection instability in a differentially heated layer consisting of gray and non-gray gaseous mixtures is studied numerically. The conditions investigated cover a wide range of Planck number values (Pl = ¼ KkT/ 4sT3 0 ), from the conduction-dominated regime of Pl > > 1 to the radiation-dominated regime of Pl < < 1. The linear stability theory is applied to mass, momentum and energy balance equations and the resulting linear stability equations are solved by Chebyshev spectral collocation method. The divergence of radiative flux is solved by the finite-volume-based discrete ordinates method. The Spectral Line Weighted sum of gray gas (SLW) model is used to represent the fine spectral variation of absorption coefficient for a non-gray gas medium. The results indicate that the critical Rayleigh number (Rac) for the onset of convection increases with mean temperature (T0) in the conduction-dominated regime at low values of T0. In the radiation-dominated regime (Pl << 1), Rac decreases with T0 for gray media. If the medium is non-gray, the critical Rac reduces to even lower values (as compared with those of gray gases) due to the dependence of gas absorptivity on temperature T0. A reduction in the wall emissivity value increases the stability of the fluid layer due to reflection of radiation from the wall, in the radiation-dominated regime. The reverse trend is seen for Pl >> 1. The critical parameters also significantly depend on the concentrations of radiatively participating gases in the mixture. The temperature profile in the fluid layer transforms from a linear profile in conduction regime to a stratified profile with steep gradients near the walls, in the presence of non-gray participating gases

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