Buddhika N. Hewakandamby

Parametric Study of Churn Flow in Large Diameter Pipes

Two phase flow in vertical risers are common place in oil and gas industry and many other process industries. Depending on the flow rates of the phases, there could be several flow patterns could exist inside the riser. These could vary from bubbly flow to annular flow with slug and churn flow in between. When the liquid phase flow rate is higher the bubbly flow exists while the annular flow is dominated by higher gas flow rate that forms a distinct gas core in the middle of the vertical riser. Of these flow regimes, churn flow is of particular interest as it is not well understood. The paper will report findings of an experimental campaign investigating the development of churn flow.Experiments were carried out in a closed loop flow facility with a 127 mm ID, 11 m long vertical test section. The maximum flow rates achievable in the system were 17 and 1.2 m/s for gas and liquid phases respectively. Compressed air was used as the gas phase while water and water/glycerol mixtures were used as the liquid phase. The mixtures of water and glycerol were used to investigate the influence of the viscosity on the flow regime investigated. The flow was investigated using a Wire Mesh Sensor (WMS), an intrusive measurement device that can map the cross sectional distribution of phases. Void fraction measurements were made at several axial locations for a number of flow rate combinations from onset of churn flow until it turns into annular flow.A region in flow rates where large liquid ligaments (wisps) suspended in the gas core was found and the breakup mechanism has been observed. Furthermore, huge waves were observed in this region. Analysis of results shows that the frequency of both huge waves and the wisps entrained in the gas core increase along the axial distance. The changes to the flow behaviour with the increase of viscosity and other findings will be presented in detail in the paper.© 2014 ASME

Characterisation of hydraulic jumps/falls with surface tension variations in thin film flows

Abstract The effects of surface tension gradients in flowing thin films are studied. Thermodynamic viability of inverted hydraulic jump-like waves is characterised through a dimensionless parameter β arising from the control volume analysis of hydraulic jumps. The factor β is the ratio of effective gravity forces upstream to downstream when the upstream and downstream surface tensions differ. Further, the linear stability theory is numerically solved to determine critical stability due to the surface tension gradients along the free surface. The surface tension is assumed to be changing on a longer scale than the depth. This assumption allows the treatment of the spatial dependency of surface tension parametrically. Approximate analytical solutions to the perturbation system of first order in wave number give a relationship between the parameters which determines the bifurcating value of Reynolds number, Re b . This bifurcation Reynolds number explicitly depends on the initial pressure gradient imposed along the flow direction. Surface tension gradients tend to destabilise the flow. For a surface tension gradient of 1%, Re b drops about 33% from its value in the absence of surface tension gradients. The numerical results are in accordance with analytical results for limiting cases of long waves and small inertia, respectively.

Multi-needle capacitance probe for non-conductive two-phase flows

Despite its variable degree of application, intrusive instrumentation is the most accurate way to obtain local information in a two-phase flow system, especially local interfacial velocity and local interfacial area parameters. In this way, multi-needle probes, based on conductivity or optical principles, have been extensively used in the past few decades by many researchers in two-phase flow investigations. Moreover, the signal processing methods used to obtain the time-averaged two-phase flow parameters in this type of sensor have been thoroughly discussed and validated by many experiments. The objective of the present study is to develop a miniaturized multi-needle probe, based on capacitance measurements applicable to a wide range of non-conductive two-phase flows and, thus, to extend the applicability of multi-needle sensor whilst also maintaining a signal processing methodology provided in the literature for conductivity probes. Results from the experiments performed assess the applicability of the proposed sensor measurement principle and signal processing method for the bubbly flow regime. These results also provide an insight into the sensor application for more complex two-phase flow regimes.

Separation of sulphuric acid from an acid suspension of cellulose nanocrystals by manual shaking

In this paper, the separation of sulphuric acid from a suspension of cellulose nanocrystal by manual shaking is described. Cellulose nanocrystals are prepared from acid hydrolysis of cotton using 64 wt% sulphuric acid at ca. 45 °C for 45 minutes. After the hydrolysis was complete, water was added to dilute the mixture to a resulting concentration of 30 wt% of the acid. This mixture was shaken rigorously in a closed container and after 48 hours, separation occurs such that cellulose nanocrystals float, with the bubbles introduced by the shaking, to give clear acid solution at the bottom. This shaking-floating process is repeatable for several cycles after the acid was removed from the bottom and more water was added. Using this simple process, the total acid recovery of > 90% has been achieved, and the concentration of all the acid recovered combined was 17.5 wt%. This work demonstrates a method that allows energy efficient and up-scalable separation of cellulose nanocrystals from the acidic suspension from which it was extracted.

Effect of Geometry on Droplet Generation in a Microfluidic T-Junction

The main aim of this study is to examine how the droplet formation in microfluidic T-junctions is influenced by the cross-section and aspect ratio of the microchannels. Several studies focusing on droplet formation in microfluidic devices have investigated the effect of geometry on droplet generation in terms of the ratio between the width of the main channel and the width of the side arm of the T-junction. However, the contribution of the aspect ratio and thus that of the cross-section on the mechanism of break up has not been examined thoroughly with most of the existing work performed in the squeezing regime. Two different microchannel geometries of varying aspect ratios are employed in an attempt to quantify the effect of the ratio between the width of the main channel and the height of the channel on droplet formation. As both height and width of microchannels affect the area on which shear stress acts deforming the dispersed phase fluid thread up to the limit of detaching a droplet, it is postulated that geometry and specifically cross-section of the main channel contribute on the droplet break-up mechanisms and should not be neglected. The above hypothesis is examined in detail, comparing the volume of generated microdroplets at constant flowrate ratios and superficial velocities of continuous phase in two microchannel systems of two different aspect ratios operating at dripping regime. High-speed imaging has been utilised to visualise and measure droplets formed at different flowrates corresponding to constant superficial velocities. Comparing volumes of generated droplets in the two geometries of area ratio near 1.5, a significant increase in volume is reported for the larger aspect ratio utilised, at all superficial velocities tested. As both superficial velocity of continuous phase and flowrate ratio are fixed, superficial velocity of dispersed phase varies. However this variation is not considered to be large enough to justify the significant increase in the droplet volume. Therefore it can be concluded that droplet generation is influenced by the aspect ratio and thus the cross-section of the main channel and its effect should not be depreciated. The paper will present supporting evidence in detail and a comparison of the findings with the existing theories which are mainly focused on the squeezing regime.Copyright

Experimental Investigation and Visualization of Helical Structures in a Novel Swirling Jet

This paper describes an experimental study carried out to investigate the dynamics of helical structures in an unforced swirling jet and in a forced swirling jet that is excited by acoustic methods. An experimental rig that consists of a novel swirling jet and acoustic generation chamber is utilized to study the characteristics and mutual interaction of helical structures. For the excited jet, the helical structures are acoustically triggered in the jet-mixing layer. The acoustic excitation is applied transversely by an array of eight loudspeakers using radial wave-guides. The loudspeakers are driven by harmonic acoustic signals mutually phase shifted by a quarter of the period between each two neighbors so as to act in effect to produce two rotating waves. To generate swirl in the flow, novel swirl generators are utilized. The approach is to substitute the usual vanes set at an angle relative to the incoming flow by fixed vanes at a zero angle of attack, with the flow deflection based on super-circulation produced by the Coanda effect. The particle image velocimetry method was employed to analyze, quantify and visualize the flow field.Copyright

Experimental Investigation of Air-Silicone Oil Slug Flow Developmentaround a Horizontal 90 Degree Bend

An experimental investigation of slug flow around 90 horizontal bend is presented. This paper demonstrates the visual observations during the onset of hydrodynamic slugs. It also presents the axial development of slug flow downstream a horizontal 90 bend. The available slug flow correlations are tested against the experimental results. Air-Silicone oil (viscosity 5 mPa.s) experiments were conducted in a horizontal test section of a 68 mm ID. The behaviour of slug characteristics was studied at 5D upstream and four locations (10D, 40D, 69D and 75D) downstream of the bend using ECT and WMS. The flow around the bend was observed using a high speed imaging system. This work demonstrates that at low gas flow rate hydrodynamic slug flow is mainly generated from the Kelvin-Helmholtz instability. While at high gas superficial velocities (>2.29m.s-1), slugs are developed from the coalescence of roll waves travelling at different velocities. Horizontal 90° bends have inconsiderable effects on the behaviour of slugs. This is due to that the phase separation and momentum transfer being insignificant. The horizontal bend has a minimum influence on the velocity and frequency of the flow structure.

Phase Behaviour of Cellulose Nanocrystal Dispersion in Aqueous Sulphuric Acid and Development of an Energy Efficient Separation Technique for the Acid-Cellulose Nanocrystal System

In this paper, the phase behaviour of a cellulose nanocrystal (CNCs) dispersion in sulphuric acid solutions was investigated, aimed at the development of an energy efficient separation method for this mixture. The system in consideration was a mixture of 30 wt% aqueous sulphuric acid (ρl = 1219 kg/m3) containing 12.6 mg/ml of cellulose nanocrystals (CNCs) (ρs = 1590 kg/m3, volume fraction of CNCs less than 1%). This volume filling mixture was obtained directly from a CNC extraction process, as obtained after the hydrolysis of cotton using 64 wt% sulphuric acid at ca. 45 ̊C for 45 minutes (this condition was required for the extraction of CNCs from cotton) followed by quenching the hydrolysis with water. The CNCs form the desired product and need to be separated from the acid that can then be recycled. Conventionally this separation has been difficult and requires a large input of energy. This work addresses this problem by investigating into the phase behaviour and physicochemical and hydrodynamic character of this mixture. This understanding led to the development of a very energy efficient separation mechanism for this mixture, which is 5 orders of magnitude more energy efficient than the most widely used centrifugation systems.

Numerical Simulation of the Effect of Rheological Parameters on Shear-Thinning Droplet Formation

Immiscible non-Newtonian-Newtonian fluid systems in microfluidics constitute an essential study as non-Newtonian fluids consistently met in medical and biological systems. Although a large number of experimental investigations have been reported in this area, attempts to develop predictive models appear to be limited. This paper is an attempt to incorporate a non-Newtonian stress model together with front-tracking scheme used in computational fluid dynamics. A conservative two-phase level set method (LSM) was applied for capturing the droplet breakup dynamics and relevant hydrodynamics of shear-thinning carboxymethylcellulose (CMC) droplets. Our droplets comprise of 0.02wt% to 1.2wt% CMC solutions in a Newtonian continuous fluids system (olive oil) employed in a T-shaped microfluidic cell. A Carreau-Yasuda viscosity model for shear-thinning CMC droplets has been implemented. This shear-dependent constitutive model fitted well to our steady state non-linear shear measurements for polymeric CMC solutions, with asymptotic viscosities at zero and infinite shear rates, and with different degrees of shear thinning (η0/η∞) in steady state. The particular focus of this study was to systematically undergo parametric studies on the influence of rheological parameters of the specified model such as zero (η0) and infinite shear viscosity (η∞), and relaxation time (λ) on the droplet formation processes. The level set simulation predicted that the droplet diameter increases with increasing η0/η∞. The effect of η0/η∞ has been found to have more prominent impact on droplet diameter for higher CMC concentrations. The variation in droplet diameter becomes less significant at the higher degrees of shear-thinning for all concentrations of CMC dispersed solutions. In the limit of zero shear-thinning effect, the droplet diameter increases when the dispersed phase viscosity decreases. Additionally, the effect of λ on the droplet diameter is also discussed. The reciprocal of the characteristic relaxation time (1/λ) corresponds to a critical shear rate that indicates the onset shear rate for shear-thinning. As λ increases, the numerical studies clearly reveal that the droplet diameter is increasing until it reaches a plateau for larger values of λ. The influence of λ leads to a more significant impact on droplet diameter for higher CMC concentration. These findings will ultimately help in understanding the sensitivity of rheological parameters to the microdroplet formation.Copyright

Swirl Induced Flow Through a Venturi-Ejector

A swirl body inserted into the entry of the primary fluid to a venturi-ejector influences the subsequent mixing of the two fluids and increases the suction rate of the ejector. An experimental facility has been developed to study the influence that a variation in the swirl angle has on the operational performance of a model venturi-ejector. Water and air were the primary and secondary fluids employed. Three swirl body configurations representing three different swirl angles were inserted upstream of the entry nozzle and the resultant downstream flow mixing characteristics were studied. The performance of the venturi-ejector with and without the use of the swirl body inserts was compared. The paper presents a description of the experiments performed and an analysis and discussion of the results obtained from this initial experimental study.© 2012 ASME