Attenuation Of Instabilities Research Articles

Hydrodynamic instabilities of flows involving melting in under-saturated porous media

The process of melting in partially saturated porous media is modeled for flow displacements prone to hydrodynamic instabilities due to adverse mobility ratios. The effects of the development of instabilities on the melting process are investigated through numerical simulations as well as analytical solution to unravel the physics of the flow. The effects of melting parameters, namely, the melting potential of the fluid, the rate of heat transfer to the frozen phase, and the saturation of the frozen material along with the parameters defining the viscous forces, i.e., the thermal and solutal log mobility ratios are examined. Results are presented for different scenarios and the enhancement or attenuation of instabilities are discussed based on the dominant physical mechanisms. Beside an extensive qualitative analysis, the performance of different displacement scenarios is compared with respect to the melt production and the extent of contribution of instability to the enhancement of melting. It is shown that the hydrodynamic instabilities tend in general to enhance melting but the rate of enhancement depends on the interplay between the instabilities and melting at the thermal front. A larger melting potential and a smaller saturation of the frozen material tend to increase the contribution of instability to melting.

Determining potential of subcooling to attenuate hydrodynamic instabilities for steam–water two phase flow

Hydrodynamic instabilities are regarded as important but are undesirable occurrences for the systems used in process industries, which involve steam. These instabilities affect to a great extent the life line of the safe operation of their systems by inducing thermal stresses and steam induced water hammers. On the basis of system operational analysis, it was found that condensation induced hydrodynamic instabilities were responsible for one third of the destructive events in steam driven systems and their attributes in power and process industry. Thus it becomes vital to investigate the influence of critical parameter such as sub-cooling to curb the destructive effects due to hydrodynamic instabilities on to the process equipment. Here for steam water two phase flows, the attenuation of hydrodynamic instabilities due to sub-cooling and inlet pressure has been investigated. It was found that sub-cooling has more pronounced and notable effect on the attenuation of these instabilities.

Laser deep penetration welding simulation based on a wavelength dependent absorption model

Today laser sources for laser material processing are available at different wavelengths. In this paper it is depicted that the wavelength dependent Fresnel absorption of the laser at the keyhole front is responsible for a different propagation and amplification or attenuation of instabilities at the keyhole front and the keyhole dynamics. To understand the physics of laser deep penetration welding and the interaction of the melt pool dynamics with these instabilities a three dimensional transient simulation of laser keyhole welding process has been developed. The results obtained by this simulation help to understand the different aspects of the formation of instabilities in the keyhole front at different wavelengths. To investigate the implication of the wavelength dependent absorption effects a CO2-laser and an Yb:YAG-laser are simulated. The presented model includes the calculation of the heat flux in the solid and molten phase and a temperature dependent melting and solidification model. The 3D velocity field inside the melt pool and the keyhole is calculated. The free surface of the melt pool is modelled using the Volume of Fluid method.

Attenuation of Instabilities in Propulsion System Combustors

A theoretical analysis of nonlinear, large-amplitude, acoustic power dissipation by screens and porous liners with and without superimposed uniform flow is presented. Screen and liner acoustic energy dissipation is described in terms of various steady flow drag coefficients which are appropriate at the large amplitudes and low frequencies characteristic of combustion instabilities in ram burners. The limiting assumption is that particle displacement is large relative to typical screen dimensions. A parallel analysis of acoustic power dissipated through a choked system exit also is derived. This analysis was utilized to estimate acoustic power production attending a large amplitude, 45-50 Hz longitudinal instability in a simulated ramjet combustor. A porous liner which incremented system dissipation by less than 5% was subsequently installed and found to be totally effective in extinguishing the observed instability.