S.V. Paras

Xanthan production by Xanthomonas campestris in batch cultures

The kinetics of growth and xanthan production by Xanthomonas campestris ATCC 1395 in batch culture were studied in a laboratory fermenter without pH control. Fermentations were carried out over a range of stirrer speeds (100–600 rpm) and the pyruvate content, as well as the molecular weight of the product were estimated. Increased agitation levels resulted in higher production rates and biomass levels, while product formation in this fermentation appeared to be partly growth associated. The chemical structure of xanthan was influenced by agitation, as the pyruvate content increased with increasing stirrer speeds. However, no significant effect was observed on xanthan molecular weight as the stirrer speed increased from 100 to 600 rpm.

STATISTICAL CHARACTERISTICS OF FREE FALLING FILMS AT HIGH REYNOLDS NUMBERS

Abstract The characteristics of films flowing inside a vertical pipe are studied experimentally. Using an accurate wire conductance technique the film thickness is measured over the Re range 509–13,090. The mean film thickness data are in good overall agreement with established relations. Extensive statistical analysis shows that film thickness fluctuations have a stochastic character. The description given by Dukler and co-workers, of a relatively well-defined substrate in between the large waves, is confirmed up to Re ≈ 7000. At higher Re the substrate tends to lose its identity, due to amplification of the small waves on its surface. Moreover, data on standard deviation, h max and h min indicate that at Re > 5000 the amplification of large waves essentially stops, and that the substrate thickness tends to increase. The calculated probability density functions of film thickness, the spectral density functions, the skewness and kurtosis provide new reliable information for the statistical description of film thickness fluctuations. Finally, by identifying the wave peaks it is found that the total number of waves is nearly constant, in the range of Re studied, and that the distribution of wave peaks becomes bimodal at Re ≳ 4000.

PROPERTIES OF THE LIQUID LAYER IN HORIZONTAL ANNULAR FLOW

Time records have been collected of local liquid film thickness, using parallel-wire conductance probes, in a new 50.8 mm i.d. pipe loop. Statistical analysis of these records allows the study of the circumferential variation of time-averaged thickness, of RMS values and of other quantities. The power spectra show that the dominant frequency associated with large disturbance waves is < 10 Hz at low gas velocities, displaying a tendency to increase with gas flow rate. By combining signal cross-correlation data with visual observations, it is shown that the disturbance waves tend to deform rapidly, move on a plane inclined with respect to the vertical and cover an increasingly larger portion of the circumference with increasing gas velocity. The mean large wave height at the pipe bottom is linearly related to the local RMS value. It is also a strong function of gas flow rate, with a moderate dependence on liquid flow rate. The wave steepness shows a similarly strong dependence on gas flow rate. However, the disturbance wave intermittency appears to be almost independent of gas as well as liquid flow rates.

Droplet entrainment and deposition in horizontal annular flow

Local measurements of droplet fluxes, in the core of horizontal annular flow, are employed in order to calculate the liquid concentration distribution as well as the circumferential variation of deposition rate. A relatively simple model is proposed for predicting the above quantities. Terms representing fluxes due to turbulent diffusion and gravitational settling are used in the model. An additional parameter a is introduced and is considered to represent a combination of fluxes; i.e. the net upward flux of droplets responding to turbulence and the flux of droplets that gain high inertia upon atomization. An interpretation is given of average deposition (or atomization) rates obtained from the data, by using detailed information on film thickness properties and by recognizing that only films with a local thickness greater than a critical value can contribute to atomization. The development of a physically realistic correlation for the mean deposition rate is explored.

Liquid layer characteristics in stratified—Atomization flow

Abstract Measurements of liquid film thickness, liquid-to-wall shear stress, pressure drop and visual observations have been carried out in a 50.8 mm i.d. horizontal pipe flow loop. Attention was paid to the lateral variation of the liquid properties by making measurements at locations Θ = 0° (pipe bottom) and Θ = 45° Statistical analysis of liquid film records led to the determination of local mean thickness, RMS values as well as of other wave characteristics (wave amplitude, intermittency, etc.) useful in computing gas-liquid interface friction. These data were complemented by similar statistical information from the shear stress measurements. The gas-liquid interface was found to deviate significantly from the usually assumed flat profile. A new result obtained here is that a small film thickness is associated with reduced local shear stress; i.e. that a lateral mean shear stress variation exists in this flow regime. In general, larger intensities of wall stress fluctuations are measured in thin films; e.g. at Θ = 45° as compared to Θ = 0° (thicker film). Power spectra of film thickness and of shear stress display similarities indicative of the effect of waves on wall stress. There is also evidence suggesting that damping of interfacial waves takes place in relatively thick films. Probability densities of local layer thickness and of wall stress exhibit striking similarities indicative of a substrate on which a moving wavy layer develops. Using the above new data, improved estimates of the interfacial friction factor f i are obtained. The data exhibit a linear dependence of f i on the liquid layer Reynolds number, as in previous studies. An expression relating an equivalent interface roughness to wave characteristics is also proposed.

Experimental investigation of the flow of a blood analogue fluid in a replica of a bifurcated small artery

The scope of this work is to study the pulsatile flow of a blood mimicking fluid in a micro channel that simulates a bifurcated small artery, in which the Fahraeus-Lindqvist effect is insignificant. An aqueous glycerol solution with small amounts of xanthan gum was used for simulating viscoelastic properties of blood and in vivo flow conditions were reproduced. Local flow velocities were measured using micro Particle Image Velocimetry (μ-PIV). From the measured velocity distributions, the wall shear stress (WSS) and its variation during a pulse were estimated. The Reynolds numbers employed are relatively low, i.e. similar to those prevailing during blood flow in small arteries. Experiments both with a Newtonian and a non-Newtonian fluid (having asymptotic viscosity equal to the viscosity of the Newtonian one) proved that the common assumption that blood behaves as a Newtonian fluid is not valid for blood flow in small arteries. It was also shown that the outer wall of the bifurcation, which is exposed to a lower WSS, is more predisposed to atherosclerotic plaque formation. Moreover, this region in small vessels is shorter than the one in large arteries, as the developed secondary flow decays faster. Finally, the WSS values in small arteries were found to be lower than those in large ones.

The influence of small tube diameter on falling film and flooding phenomena

Flooding experiments have been carried out in vertical small i.d. tubes (6, 7, 8 and 9 mm), using smooth inlet and outlet conditions, with air and two liquids (water and kerosene). Experimental data on free falling film characteristics have also been obtained, in the same test sections, which aid the interpretation of flooding phenomena. These new data suggest that the tube diameter strongly affects film flow development, possibly promoting wave interaction and damping. In turn, the wavy film evolution essentially determines flooding characteristics. At small liquid Reynolds numbers (ReL < 300) critical flooding velocities, UG, follow a trend already reported in the literature, i.e. decreasing with increasing liquid rate. However, at higher ReL the trend is reversed, i.e. increasing UG with increasing liquid rate. This has not been reported in the literature before and may be attributed to damping of waves. At still higher ReL, another region is evident in the flooding curves, characterized by nearly constant flooding velocity. The dominant mechanism in almost all cases is wave growth and upward dragging by the gas, initiated at the liquid exit. 2002 Elsevier Science Ltd. All rights reserved.

Incipient flooding in inclined tubes of small diameter

Abstract This is an experimental study on the effect of tube diameter, inclination angle and liquid properties on incipient flooding in inclined small diameter tubes, in gas–liquid counter-current flow. Flooding experiments are conducted with four relatively small i.d. tubes (6, 7, 8 and 9 mm) in the range of inclination angles 30–60° and smooth inlet/outlet, i.e. a porous tube segment for the liquid and tapered section for gas entry. The effect of liquid properties is examined by using two different liquids, namely water and kerosene. To facilitate the interpretation of flooding data, the free-flowing liquid layer characteristics are also investigated. The results indicate that under these conditions, the effect of tube i.d. and angle of inclination on incipient flooding is significant, but the influence of liquid physical properties is most pronounced.

Liquid-to-wall shear stress distribution in stratified/atomization flow

Abstract Experiments were conducted in a horizontal flow loop (24 mm i.d.), using an electrochemical technique, for measuring liquid-to-wall shear stress, at various positions around the pipe circumference. Measurements of liquid film thickness, pressure drop as well as visual observations were also made. These data complement similar information obtained by Paras et al. (1994), in a horizontal 50.8 mm i.d. flow system. Mean values and other statistical information are obtained from the analysis of the shear stress and film thickness data. Visual studies of the gas/liquid interface confirm that its profile is concave rather than flat. The data show that the time-averaged shear stress tends to decrease in the lateral direction (i.e. away from the pipe bottom, Θ = 0°) along which the liquid film gradually becomes thinner. Only for relatively low superficial gas velocities (e.g. UG

Visual observations of flooding in narrow rectangular channels

Abstract New flooding data in a vertical rectangular channel with 5 and 10 mm gap between its main parallel plates are reported. Visual observations and fast recordings are made to determine conditions associated with the onset of flooding in the channel. For the 10 mm gap channel a combination of mechanisms, i.e., levitation and upward transportation of laterally coherent or isolated waves, and local wave bridging, appear to be responsible for triggering flooding. The critical flooding velocity is curiously nearly independent of liquid flow rate above film Reynolds number approximately 500. In the 5 mm gap channel and for small Re L , flooding occurs at the liquid entrance, whereas “massive” wave bridging, occurring above the middle of the test section, is the dominant flooding mechanism for relatively higher Re L . Predictions based on small amplitude stability analysis are in good agreement with flooding data from the latter case.