Vasilis Bontozoglou

Experiments on laminar film flow along a periodic wall

Experimental results are reported on the structure of gravity-driven film flow along an inclined periodic wall with rectangular corrugations. A fluorescence imaging method is used to capture the evolution of film height in space and time with accuracy of a few microns. The steady flow is found to exhibit a statically deformed free surface, as predicted by previous asymptotic and numerical studies. Though usually unstable, its characteristics determine much of the subsequent non-stationary dynamics. Travelling disturbances are observed to evolve into solitary multi-peaked humps, and pronounced differences from the respective phenomena along a flat wall are noted. Finally, a remarkable stabilization of the flow at high Reynolds numbers is documented, which proceeds through the development of a three-dimensional flow structure and leads to a temporary decrease in film thickness and recession of solitary waves.

Air–water two-phase flow and heat transfer in a plate heat exchanger

Abstract A plate heat exchanger is tested under two-phase flow conditions by using an air/water mixture as the cold stream. Visual observations recorded by a high-speed video camera lead to the construction of a flow regime map. The heat transfer coefficient of the air/water stream is measured as a function of air and water superficial velocities. The flow regime with a gas-continuous phase covering the core of the channel and liquid flowing in the form of rivulets inside the furrows shows particularly favorable heat transfer characteristics.

Decomposition of NH3 on Pd and Ir Comparison with Pt and Rh

The unimolecular decompositions of NH3 on polycrystalline wires and foils of Pd and Ir are examined and compared with the corresponding ones on Pt and Rh. The reactions were carried out in a differential flow reactor, at pressures from 0.01 to 1 Torr and temperatures from 500 to 1900 K. It was found that the rates of product formation could be fit by Langmuir-Hinshelwood unimolecular rate expressions, with an accuracy of ±20% under all conditions. Ammonia decomposes to N2 and the rate of decomposition is fastest on Ir by several orders of magnitude when compared with that on the other metals, becoming flux limited above about 750 K. Ir appears to be the choice catalyst for dehydrogenating ammonia. The heats of adsorption of NH3 on Pt, Rh and Pd are similar and equal to 16.7, 16.8 and 17.4 kcal/mol, respectively. The apparent activation energy for this reaction is similar on Pt and Rh and equal to 21 kcal/mol, while for Pd and Ir it is 26.2 and 31.2 kcal/mol, respectively.

Solitary waves on inclined films: Flow structure and binary interactions

The downstream evolution of disturbances, introduced at the inlet of a liquid film flowing along an inclined plane wall, is studied numerically by solving the full, time-dependent Navier–Stokes equation. Computational results are validated against the predictions of spatial linear stability analysis and against detailed data of the entire evolution process. The structure of the flow field below the waves is analyzed, and the results are used to test assumptions frequently invoked in the theoretical study of film flow by long-wave equations. An interesting prediction is that solitary waves exhibit strongly nonparabolic velocity profiles in front of the main hump, including a slim region of backflow. The computational scheme is subsequently used to study solitary wave interactions. It is predicted that coalescence (the inelastic collision of two humps) is not inevitable but occurs only when the waves differ appreciably in height. Waves of similar size repel monotonically, whereas for intermediate difference...

Experimental study of inclined film flow along periodic corrugations: The effect of wall steepness

Liquid films on a flat wall at inclination are first destabilized at a criticalRe = 5=6cot . The instability mode is a long-wave disturbance that travels downstream fast enough to result in a convective instability. We presently investigate how the primary instability is modified when the flat wall is substituted by one with periodic corrugations. Film flow along a corrugated wall is studied experimentally in a 800 mm long by 250 mm wide channel, whose inclination can be adjusted from 0-50 . Two walls are considered, both with wavelength 12 mm and height 2 mm: in one case the corrugations are purely sinusoidal and in the other they consist of symmetric step-ups and step-downs (orthogonal shape). Multiple conductance probes are installed at different streamwise locations and record time-series of the local free surface elevation[1]. Consistent with previous studies[2], both corrugated walls are found to result in a strong delay of the primary instability of the liquid film, compared to the classical results for flat substrate. Moreover, a significant effect of inclination angle is observed, with the film becoming progressively more stable at higher inclinations. The stabilization also varies with the shape of corrugations: film flow along the orthogonally corrugated wall is more stable at intermediate inclinations. The above results are clarified and interpreted by the observation of a new instability mode, which manifests as a high-frequency oscillation ( 10 Hz) and corresponds to a traveling wave of small wavelength. The new mode appears to be the primary cause of instability at intermediate and high channel inclinations. This observation supports a computational prediction[3], that film flow along sinusoidal corrugations may be unstable not only to long-wave disturbances, but also to disturbances with the wavelength of the wall. It is conjectured that, through an interaction of inlet disturbances with the deformed steady flow, energy is transferred from long-wave modes to the newly observed short wave mode. Given that the latter is expected to remain stable at higher Re, this tentative mechanism may explain the observed enhanced stability of the film.

Observations of solitary wave dynamics of film flows

Experimental results are reported on non-stationary evolution and interactions of waves forming on water and water-glycerol solution flowing along an inclined plane. A nonlinear wave generation process leads to a large number of solitary humps with a wide variety of sizes. A fluorescence imaging method is applied to capture the evolution of film height in space and time with accuracy of a few microns. Coalescence-the inelastic interaction of solitary waves resulting in a single hump-is found to proceed at a timescale correlated to the difference in height between the interacting waves. The correlation indicates that waves of similar height do not merge. Transient phenomena accompanying coalescence are reported. The front-running ripples recede during coalescence, only to reappear when the new hump recovers its teardrop shape. The tail of the resulting solitary wave develops an elevated substrate relative to the front, which decays exponentially in time; both observations about the tail confirm theoretical predictions. In experiments with water, the elevated back substrate is unstable, yielding to a tail oscillation with wavelength similar to that of the front-running ripples

Laminar film flow down a wavy incline

Abstract Laminar flow of a liquid, down an inclined wall with sinusoidal corrugations, is considered. A linear analysis, valid for small-amplitude disturbances but arbitrary wavelength and Re number, leads to an Orr-Sommerfeld type equation with nonhomogeneus boundary conditions. The free-surface amplitude and phase relative to the wall are examined. In a range of Re numbers, a resonance phenomenon is calculated, leading to amplification of the wall corrugations. This behavior has not been encountered in previous analyses of thin film flow, based on the Stokes approximation.

Effect of channel width on the primary instability of inclined film flow

A procedure is developed to detect the onset of interfacial instability in inclined film flows (with estimated accuracy better than 5%) and is used to show that the finite width of experimental channels stabilizes the undisturbed liquid film. Deviation from the classical prediction scales inversely with the product of channel width and sine of inclination angle, and for small inclinations and/or narrow channels is of the order of 100%. The effect is tentatively attributed to the influence of sidewalls on the traveling disturbances, which results in curved crestlines and transverse variation of wave characteristics.

Nonlinear dynamics of inclined films under low-frequency forcing

The evolution of an inclined liquid film, subjected to regular inlet disturbances of small frequency, f, is studied experimentally and computationally. The fluorescence imaging method is used to quantitatively document film thickness at a downstream window of the flow channel, and a Galerkin finite-element solution of the Navier–Stokes equation of motion is invoked to predict the entire spatio-temporal dynamics of the free surface. Experiments confirm that, below a certain frequency, fp, the regular wave train is destroyed by the appearance of one or more parasitic crests behind each major wave. Experiments and simulations indicate that parasitic crests are not the result of spatially unlocalized instabilities in the substrate, but originate in a regular way from a depression developing at the tails of growing solitary waves. Their downstream fate is dictated by the proximity of the next major wave, and thus different scenarios are predicted and observed for f≈fp and f≪fp. A theoretical explanation of the...

Mass transfer in gas-liquid flow in small-diameter tubes

Data on the rate of liquid-phase controlled mass transfer in a small-diameter tube (ID 4 mm) are presented. The motivation is to model the performance of compact contact devices consisting of short, narrow passages. The mean volumetric mass transfer coefficient, KLa, is measured for a wide range of superficial gas and liquid velocities (UGS = 0.1−30 m s−1 and ULS = 0.01−1 m s−1). Results are related to flow regime transitions and are correlated in terms of Jepsens (1970, A.I.Ch.E. J.16, 705–711) fractional energy dissipation parameter.