J. K. Platten

Coupled buoyancy and Marangoni convection in acetone : experiments and comparison with numerical simulations

This paper presents a study of the convection in acetone due jointly to the thermocapillary (Marangoni) and thermogravitational effects. The liquid (acetone) is submitted to a horizontal temperature difference. Experiments and numerical simulations both show the existence of three different states : monocellular steady states, multicellular steady states and spatio-temporal structures. The results are discussed and compared with the linear stability analysis of Smith & Davis (1983).

Benchmark values for the Soret, thermal diffusion and diffusion coefficients of three binary organic liquid mixtures

With the aim of providing reliable benchmark values, we have measured the Soret, diffusion and thermal diffusion coefficients of the three binary mixtures of dodecane, isobutylbenzene and 1,2,3,4 tetrahydronaphthalene for a concentration of 50 wt% at a temperature of 25C. The experimental techniques applied by the five participating laboratories are transient holographic gratings, annular and parallelepipedic thermogravitational columns, and vertical parallelepipedic columns with velocity amplitude determination by laser doppler velocimetry. The systems have also been studied in a annular thermogravitational column filled with a porous medium in the gap. There is a good agreement between the different experiments with deviations of the order of a few per cent in most cases (8.5% at most). The numerical values are tabulated in the paper.

Surface properties of aqueous solutions of l-leucine

The surface tension, sigma, of solutions of L-leucine (CH3)2CHCH2CH(NH2)COOH in water, as well as in aqueous solutions of NaOH and HCl were measured in the temperature range between 278 and 308 K using the Wilhelmy plate method. L-Leucine was found to be a very weak surfactant, which can be understood if assuming strong interactions of this solute with the water structure. Striking differences were observed in the surface entropy of L-leucine solutions in water, 0.5 M HCl and 0.5 M NaOH. Moreover, surface activity of the solute is much lower than that supposed taking into account the hydrophobicity of this amino acid. It was concluded that the observed phenomena are caused by the water structure changes close to the side chain of leucine, caused by enforced hydrophobic hydration, i.e. formation of clathrate-like hydrates.

Measurements of molecular and thermal diffusion coefficients in ternary mixtures

Thermal diffusion coefficients in three ternary mixtures are measured in a thermogravitational column. One of the mixtures consists of one normal alkane and two aromatics (dodecane-isobutylbenzene-tetrahydronaphthalene), and the other two consist of two normal alkanes and one aromatic (octane-decane-1-methylnaphthalene). This is the first report of measured thermal diffusion coefficients (for all species) of a ternary nonelectrolyte mixture in literature. The results in ternary mixtures of octane-decane-1-methylnaphthalene show a sign change of the thermal diffusion coefficient for decane as the composition changes, despite the fact that the two normal alkanes are similar. In addition to thermal diffusion coefficients, molecular diffusion coefficients are also measured for three binaries and one of the ternary mixtures. The open-end capillary-tube method was used in the measurement of molecular diffusion coefficients. The molecular and thermal diffusion coefficients allow the estimation of thermal diffusion factors in binary and ternary mixtures. However, in the ternaries one also has to calculate phenomenological coefficients from the molecular diffusion coefficients. A comparison of the binary and ternary thermal diffusion factors for the mixtures comprised of octane-decane-1-methylnaphthalene reveals a remarkable difference in the thermal diffusion behavior in binary and ternary mixtures.

Linear stability of mixed convection flows in horizontal rectangular channels of finite transversal extension heated from below

We present a study of the linear hydrodynamic stability of purely conductive Poiseuille flow in three-dimensional horizontal rectangular channels uniformly heated from below. Its originality is to modelize the flow using realistic conditions on all the boundaries of the channel, in particular, no-slip conditions on the vertical lateral boundaries. The aim of this paper is to provide quantitative results for the values of the critical dimensionless parameters. When the Rayleigh number Ra is higher than the critical value Ra*, the instabilities are shown to be three-dimensional horseshoe-shaped transversal rolls for Reynolds numbers Re smaller than the critical value Re*, and longitudinal rolls for Re > Re*. The results are given for a wide range of the Prandtl number (10-6 ≤ Pr ≤ 105) and of the transversal aspect ratio B of the duct (0.1 ≤ B ≤ 5.4 and B → ∞). We present a study of the linear hydrodynamic stability of purely conductive Poiseuille flow in three-dimensional horizontal rectangular channels uniformly heated from below. Its originality is to modellize the flow using realistic conditions on all the boundaries of the channel, in particular, no-slip conditions on the vertical lateral boundaries. The aim of this paper is to provide quantitative results for the values of the critical dimensionless parameters. When the Rayleigh number Ra is higher than the critical value Ra*, the instabilities are shown to be three-dimensional horseshoe-shaped transversal rolls for Reynolds numbers Re smaller than the critical value Re*, and longitudinal rolls for Re > Re*. The results are given for a wide range of the Prandtl number (10-6 ≤ Pr ≤ 105) and of the transversal aspect ratio B of the duct (0.1 ≤ B ≤ 5.4 and B → ∞).

Oscillatory motion in Bénard cell due to the Soret effect

The period of oscillations for the Benard problem in a two-component system taking into account thermal diffusion is given. Schmidt-Milverton plots are presented for water-methanol and water-isopropanol systems. Anomalous heating curves are observed. Thermocouple responses are given for some heating powers and show oscillations in the temperature field. A qualitative agreement exists with the predicted values of the period given by the theory.

Soret effects in ternary systems heated from below

Abstract We have investigated the stability of an horizontal liquid layer heated from below, in the Rayleigh-Benard configuration, on a water-isopropanol-ethanol mixture. Neglecting the cross-diffusional effects, we have developed a linear stability analysis of the rest state and compared it with experiments. We did not find sufficient agreement. However, incorporating the cross-diffusion in the theory, it is possible to recover the critical parameters for the onset of convection.

Influence of interfacial tension gradients on thermal convection in two superposed immiscible liquid layers

This paper considers mainly the theoretical aspect of thermal convection in two superposed immiscible (or partially miscible) fluid layers in a rectangular cavity with differentially heated end walls. We develop a model valid for layers whose lateral extent is much larger than depth, deriving the horizontal velocity profile in each layer as a function of the various parameters such as expansion coefficients, viscosities, depth of each layer, and the interfacial tension gradient along the interface. Results are discussed, taking into account the results of experiments that we presented recently [1].

Stability of a Two‐Component Fluid Layer Heated from Below

In connection with recent experimental results, the stability of a two‐component fluid layer heated from below is examined taking into account the concentration gradient due to thermal diffusion. With the use of a variational method (local potential technique) developed by Glansdorff and Prigogine, an approximate solution is proposed for dilute solutions. The critical Rayleigh number increases for negative thermal diffusion factors and decreases for positive ones.

THE INSTABILITY OF A LIQUID LAYER HEATED FROM THE SIDE WHEN THE UPPER SURFACE IS OPEN TO AIR

When a liquid layer is heated from the side, a monocellular flow develops immediately, no matter how small the temperature difference is. If the temperature gradient between the side walls is increased, this flow becomes unstable. Laser Doppler velocimetry measurements are reported here in an attempt to describe the main features of both the basic flow and the instability modes. It is found that before the appearance of traveling waves (the most dangerous mode as predicted by the theory), stable rolls with their axes perpendicular to the temperature gradient, span over the whole liquid layer, starting from the hot side, even if the aspect ratio (the length of the layer divided by its thickness) is very high. This unexpected situation modifies the basic flow. A further increase of the temperature gradient leads to the appearance of a time periodic motion.