Kittinan Maliwan

INFLUENCE OF VIBRATION ON SORET-DRIVEN CONVECTION IN POROUS MEDIA

Two-dimensional thermosolutal natural convection with Soret effect under the simultaneous action of vibrational and gravitational accelerations is investigated. We consider a porous cavity saturated by a binary mixture and adopt the time-averaging formulation. For an infinite horizontal layer, a stability analysis is performed from which the threshold of stability is determined. Numerical simulations, using a pseudo-spectral Chebyshev collocation method, are performed to describe the convective motion. The problem is investigated for different aspect ratios with various directions of vibration. It is concluded that, for both the stationary and the Hopf bifurcation, the vertical vibration has a stabilizing effect while the horizontal vibration has a destabilizing effect on the onset of convection.

EFFECT OF VIBRATION ON THE ONSET OF DOUBLE-DIFFUSIVE CONVECTION IN POROUS MEDIA

Abstract In this chapter we consider the instability of double-diffusive convection in porous media under the effect of mechanical vibration. The so-called time-averaged formulation has been adopted. This formulation can be effectively applied to study the vibrational induced thermo-solutal convection problem. The influence of high-frequency and small-amplitude vibration on the onset of thermo-solutal convection, in a confined porous cavity with various aspect ratios and saturated by a binary mixture has been presented. Linear stability analysis of the mechanical equilibrium or quasi-equilibrium solution is performed. A theoretical examination of the limiting case of the long-wave mode in the case of Soret driven convection under the action of vibration has been carried out. The 2D numerical simulations of the problem are presented which allow us to corroborate the results obtained from the linear stability analysis for both stationary and Hopf bifurcations.

Heat Transfer Enhancement of Impinging Jet from Pulse Jet Combustor

The aims of this study are to investigate flow characteristics in pulse jet combustor and heat transfer characteristics of impinging jet from pulse jet combustor. The combustor is Helmholtz type which has single combustor chamber and single tailpipe. The inner diameter (D) of tailpipe was 47 mm and tailpipe length was about 16D. The effect of number of air inlets: single inlet, double inlets with 90o apart, double inlets with 180o apart and triple inlets with 90o apart on flow and heat transfer characteristics were studied. A water cooled heat flux sensor was applied to measure heat transfer rate on the surface at stagnation point. The jet-to-plate distance was varied at L=1D, 2D, 4D, 6D and 8D. Two of pressure transducers were mounted on the wall of combustion chamber and on the wall of tailpipe at 4D from tailpipe outlet to measure pressure simultaneously. It is found that the variation of pressure near the tailpipe outlet is strongly depended on air inlet configurations. The pressure variations in pulse jet combustor could be preliminary related to the temperature and velocity of jet from tailpipe and heat transfer rate on jet impingement surface.

A Dynamical Model for Studying the Stability of Thermo-Solutal Convection Under the Effect of Vertical Vibration

This paper concerns the thermal stability analysis of porous layer saturated by a binary fluid under the influence of mechanical vibration. The linear stability analysis of this thermal system leads us to study the following damped coupled Mathieu equations: B H + B(π2 + k2) + 1 H +(π2 + k2) − k2k2 + π2 RaT(1 + R sin ω* t*) H =k2k2 + π2 (NRaT)(1 + R sin ω* t*) Fe* B F + B π2 + k2Le + e* F +π2 + k2Le − k2k2 + π2 NRaT(1 + R sin ω* t*) F =k2k2 + π2 RaT(1 + R sin ω* t*) H where RaT is thermal Rayleigh number, R is acceleration ratio (bω2 /g), Le is the Lewis number, k is the dimensionless wave-number, e* is normalized porosity and N is the buoyancy ratio (H and F are perturbations of temperature and concentration fields). In the follow up, the non-linear behavior of the problem is studied via a generalization of the Lorenz model (five coupled non-linear differential equations with periodic coefficients). In the presence or absence of gravity, the stability limit for the onset of stationary as well as Hopf bifurcations is determined.Copyright

Active Control of the Onset of Convection in Porous Medium by Mechanical Vibration

Theoretical studies of Rayleigh-Benard convection subjected to sinusoidal acceleration modulations have been conducted by several workers. Linear and weakly nonlinear stability analyses have been developed by Gresho and Sani [4], Clever et al. [2] and by the Russian workers Gershuni and Lyubimov [3]. As shown by these studies, the Rayleigh number for the onset of synchronous convection increases with the frequency of vibration for a layer oscillating with constant vertical amplitude until a certain frequency of modulation is reached and at which the onset is in the form of subharmonic motions.