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Atomisation & Spray combustion

Liquid atomisation is the process wherein a liquid bulk is converted to a collection of droplets leading to the formation of a spray and has numerous applications in engineering and science. Upon atomisation, the bulk liquid is fragmented to a spray system where the droplets disperse interacting with each other and the gaseous ambient environment. Thus, atomisation can be considered as the result of the inertia and the external forces acting on the jet surface. Atomisation modelling supplies the initial conditions for spray computations i.e. the droplet size, velocity, temperature e.t.c. Understanding the fragmentation process is a challenging task since there are still many uncertainties about the fundamental mechanisms of the liquid disintegration. Atomisation depends on the complex interactions between the aerodynamic and capillary forces. Turbulence and shear layers deform the liquid-gas interface while the surface tension can amplify instabilities. The solver can capture both Rayleigh-Taylor and Kelvin-Helmholtz instabilities using advanced modelling ideas for interface tracking between the two phases. One of the features of MPflow is the natural transition from an Eulerian to Lagrangian framework as shown below using Large-Eddy-Simulations.

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Dispersed Phase and Separated Flows

In dispersed phase flows one phase consists of discrete elements, such as droplets in a gas medium or bubbles in a liquid. The discrete elements are not connected. In a separated flow, the two phases are separated by a line of contact. An annular flow is a separated flow in which there is a liquid layer on the pipe wall and a gaseous core. In other words, in a separated flow one can pass from one point to another in the same phase while remaining in the same medium.

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Mixers

Mixer-settlers typically contain an impeller mounted on a shaft, and optionally it can contain baffles. In this study, the effect of the number of baffles on the separation characteristics of the system, and the effect of blade width and inlet velocity on baffles number have been investigated. CFD models in MPflow are developed to predict the separation characteristics.

The development of solvent extraction mixer settlers aims at achieving clean phase separation, minimizing the loss of the reagents and decreasing the surface area of the settlers. The role of baffles in a mechanically agitated vessel is to ensure even distribution, reduce settler turbulence, promote the stability of power drawn by the impeller and to prevent swirling and vortexing of liquid, thus, greatly improving the mixing of liquid. Inserting the appropriate number of baffles clearly improves the extent of liquid mixing. However, excessive baffling would interrupt liquid mixing and lengthen the mixing time. CFD provides a tool for determining detailed information on hydrodynamics which is required for modelling sub-processes in mixers.

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Hazardous materials releases

Numerical modelling using Computational Fluid Dynamics (CFD) analysis and mathematical modelling.

Our CFD team has extended expertise in simulating damage hazards developing specialised models for highly specialised applications of CFD for supporting safety cases. The main processes safety engineers need to consider for LNG source terms are categorized broadly as:

Jets (liquid and two-phase)

Pool formation

Vaporisation from within the containment

Rapid Phase Transitions

Pool spread and vaporisation

CFD modelling and analysis can be used for a range of hazards such as fire, smoke, steam and the dispersion of dangerous substances such as hydrogen is crucial in process engineering. CFD modelling may include mitigating activities such as ventilation controls and the effect of fire-fighting systems.

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LNG Dispersion

When LNG accidentally escapes its containment, a pool is usually considered to form, which provides, by means of spread and vaporisation, a source of a flammable heavy gas cloud. MPflow can be used for detailed simulations of LNG releases in the atmosphere in case of an accident like a rupture in a pipeline system or a crack in a storage vessel, valve e.t.c. Both low or high-pressure releases, on/off-shore spills can be modelled using specialised numerical methods for the investigating the associated complex phenomena.

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Critical flows

Bubble formation and growth in two-phase mixtures within nozzles have a significant impact on the atomisation process and spray dynamics. Depending on the vaporisation rate and conditions the flow pattern might be bubbly, slug or annular. The regime of the flow has a significant impact on the flow characteristics and the mass flow rate. Calculation of the critical mass flow rate is extremely important in industry typical applications of which, require prior knowledge of the mass flow rate for safety and design purposes. Critical flow for a single-phase gas occurs when the flow is sonic (Mach number is one) at the smallest cross-section. The molecular relaxation phenomena are rapid enough so that the thermodynamic equilibrium is achieved. In cases of two-phase flows, the relaxation time-scales of heat and mass transfer are comparable to the residence time of the fluid at the choking region.