Advantages of using multi-hole injector over single hole injector when used in a CI engine

One of the reason for using multi-hole injectors is spray dispersion due to large spray cone angle achieved due to the increase in the number of injector holes. As the bore diameter of CI engines is quite large, the spray needs to be very well dispersed for uniform mixture formation.

Efficient mixing of fuel & air is critical in any engine in order to obtain optimum power and fuel consumption. When using multi-hole over single-hole injectors in a diesel engine it it possible to employ smaller holes and this results in superior atomisation of the fuel and more thorough fuel/air mixing thereby generating greater efficiency.

The larger spray cone is another aspect – distributing the droplets more evenly in the volume thus resulting in a more uniform combustion.

Dam break flows or floods have become a very important topic for engineers. As a part of a risk and safety analysis it is now current to produce inundation maps consistent with different dam failure scenarios and conditions. These maps are based on numerical simulations and on computational models of dam-break floods. The dam-break problem is now as a real problem in engineering practice that must be solved by computational models according to real experience and knowledge, based on historical events, physical models and sensitivity analysis. MPflow uses state-of-the-art numerical methods for interface capturing of the water-air interface providing highly accurate modeling.

Cryogens such as liquid nitrogen (), liquid oxygen (() and liquid methane (() are widley used in aerospace industry for rocket engines. The study of the jets is extremely important since the atomisation process of these liquids is directly related to the engine efficiency quality of combustion. The vapour produced from cryogenic liquids like or are also extremely cold and can be extremely harmful. The vapour might condense the moisture in the surrounding air, creating a highly visible fog. This fog can also be formed around cold equipment when no release of the cold liquid or vapours has occurred. A key thing to remember is that these fog clouds do not define the vapour cloud. They define the area where the vapours are still cold enough to condense the moisture in the air. An example of  the Large-Eddy-Simulations capabilities of the solver for simulating liquid methane jets atomisation is shown below:

Cavitation is the formation and collapse of small vapor cavities or bubbles in a liquid. It usually occurs when a liquid is subjected to changes of pressure such that the static pressure goes lower than the liquid’s vapor pressure which will cause the formation of bubbles in the liquid. When subjected to higher pressure, the bubbles implode. When these implosions happen near a solid material for e.g. pump impeller, the bubbles implode with a microjet hitting the material surface. Repeated bursts of microjets will cause further erosion which spreads and ultimately results in catastrophic equipment failure. Cavitation occurs in pipes and can cause various unpleasant effects such as:

* Material Erosion
* Noise
* High Vibration
* Decreased flow/pressure
* Equipment Failure
* Increased power consumption for machinery

Studies in cavitation within nozzles have shown that it has a key role in spray dynamics and atomisation efficiency. The geometry, initial conditions and the composition of the mixture are mainly responsible for preserving or damping the transient phenomena. In case of sharp nozzle inlets the fluid flows towards a smaller cross section and accelerates. The fluid reaches its maximum velocity at a short distance from the nozzle inlet. The result is flow separation in the vicinity of the nozzle walls with the stream diameter becoming minimum (vena contracta).

In cases where a liquid, stored in a high-pressure vessel, flows towards a low-pressure environment, some interesting phenomena might be triggered that can change the morphology of the flow. If the liquid’s local pressure drops below the local saturated vapour pressure, boiling process, generally termed flashing, occurs. This phenomenon has been exploited in clean combustion technologies to improve mixing and combustion processes, and multiple aerosol industrial applications where small droplets in the vicinity of the nozzle exit are desired. Flashing is known to produce fine sprays and like cavitation has an impact on the atomisation and spray dynamics. The phase change is manifested by bubbles forming within the liquid, changing progressively the regime of the flow. The exact time and position of the bubble generation is not a trivial task especially in the presence of turbulence. The flow is strongly dependent on the initial pressure and superheat or subcooling degree. The flow characteristics dominantly affect the subsequent atomisation upstream of the nozzle exit. The regime of the jet varies depending on the local topology including the length of the tubes, size and shape of the orifice. In flashing, extensive bubble nucleation occurs in the liquid. Thus, multiphase CFD codes dealing with flash boiling need to be able to account for the impact of the phase change throughout the liquid
dealing with flash boiling need to be able to account for the impact of the phase change.

MPflow incorporparates a novel numerical approach for simulating the atomisation of flashing liquids accounting for the distinct stages, from primary atomisation to secondary break-up to small droplets using the Eulerian-Lagrangian-Spray-Atomisation model coupled with the homogeneous relaxation model. The surface density equation (Σ-equation) is implemented in the code and solved in a fully Eulerian approach for tracking liquid structures of any shape, and computes the spray characteristics. A modified version for the transport equation of the surface density is used and new source terms accounting for the changes in Σ due to evaporation in both dense and dilute spray regions are added. This approach has the advantage of avoiding the unrealistic common assumption of pure liquid rather than a mixture at the nozzle exit. It models the change in the regime inside the nozzle treating flashing in a unified approach, simulating the metastable jet both inside and outside the nozzle. Important mechanisms such as thermal non-equilibrium, aerodynamic break-up, droplet collisions and evaporation are modelled in a novel atomisation model.