Lionel Ganippa

Coupled simulations of nozzle flow, primary fuel jet breakup, and spray formation

Presented are two approaches for coupled simulations of the injector flow with spray formation. In the first approach the two-fluid model is used within the injector for the cavitating flow. A primary breakup model is then applied at the nozzle orifice where it is coupled with the standard discrete droplet model. In the second approach the Eulerian multi-fluid model is applied for both the nozzle and spray regions. The developed primary breakup model, used in both approaches, is based on locally resolved properties of the cavitating nozzle flow across the orifice cross section. The model provides the initial droplet size and velocity distribution for the droplet parcels released from the surface of a coherent liquid core. The major feature of the predictions obtained with the model is a remarkable asymmetry of the spray. This asymmetry is in agreement with the recent observations at Chalmers University where they performed experiments using a transparent model scaled-up injector. The described model has been implemented into AVL FIRE computational fluid dynamics code which was used to obtain all the presented results. Copyright

Physics of puffing and microexplosion of emulsion fuel droplets

The physics of water-in-oil emulsion droplet microexplosion/puffing has been investigated using high-fidelity interface-capturing simulation. Varying the dispersed-phase (water) sub-droplet size/location and the initiation location of explosive boiling (bubble formation), the droplet breakup processes have been well revealed. The bubble growth leads to local and partial breakup of the parent oil droplet, i.e., puffing. The water sub-droplet size and location determine the after-puffing dynamics. The boiling surface of the water sub-droplet is unstable and evolves further. Finally, the sub-droplet is wrapped by boiled water vapor and detaches itself from the parent oil droplet. When the water sub-droplet is small, the detachment is quick, and the oil droplet breakup is limited. When it is large and initially located toward the parent droplet center, the droplet breakup is more extensive. For microexplosion triggered by the simultaneous growth of multiple separate bubbles, each explosion is local and independent initially, but their mutual interactions occur at a later stage. The degree of breakup can be larger due to interactions among multiple explosions. These findings suggest that controlling microexplosion/puffing is possible in a fuel spray, if the emulsion-fuel blend and the ambient flow conditions such as heating are properly designed. The current study also gives us an insight into modeling the puffing and microexplosion of emulsion droplets and sprays.

Combustion characteristics of diesel sprays from equivalent nozzles with sharp and rounded inlet geometries

The ignition delay, flame structure, temperature, and soot distribution in a diesel spray injected at 80 MPa in a high-temperature (830 K) and high-pressure (6 MPa) quiescent air was studied for two nozzles, one with 0% hydrogrinding (HG) and another with 20% HG. HG in diesel nozzles is the process of forcing an abrasive fluid through the nozzles with sharp inlets; the abrasive fluid wears the sharp inlet edge of the spray holes until a prescribed flow rate is achieved. The percentage of HG used in this article is a measure of an increase in the volume flow rate after the HG process in a low-pressure flow test. The difference is substantial. For convenience, at some instances 0% HG is referred to as the sharp inlet and 20% HG as the rounded inlet. Spray impingement studies were made to evaluate the time-resolved spray momentum, nozzle discharge coefficient, and turbulence kinetic energy to characterize the nozzle internal flow effects on spray combustion. Equivalent nozzles were selected such that the momentum rates of the spray from both nozzles, as determined by the spray impingement, were the same. This was obtained by increasing the orifice diameter of the nozzle with 0% HG to compensate for the higher friction losses and lower discharge coefficient of the nozzle. The differences in discharge coefficient indicate that the flows inside the nozzles have different turbulence and cavitation levels. Inspite of the strong differences in internal flow, the sprays, which had the same momentum rate, behaved identically. In particular, the spray dispersion, penetration, ignition delay, combustion temperatures, flame volumes, soot concentration, and liftoff distances were almost the same for both sprays. Also, the use of noncircular injection orifices was shown not to change the combustion and emission performance of a diesel engine when the momentum of the fuel jets is the same. The work thus shows that diesel spray combustion is fully controlled by the spray momentum and that for realistic injection and combustion conditions the internal nozzle flow structure does not matter as long as it does not change the momentum.

Gas phase thermometry of hot turbulent jets using laser induced phosphorescence

The temperature distributions of heated turbulent jets of air were determined using two dimensional (planar) laser induced phosphorescence. The jets were heated to specific temperature increments, ranging from 300 - 850 K and several Reynolds numbers were investigated at each temperature. The spectral ratio technique was used in conjunction with thermographic phosphors BAM and YAG:Dy, individually. Single shot and time averaged results are presented as two dimensional stacked images of turbulent jets. YAG:Dy did not produce a high enough signal for single shot measurements. The results allowed for a direct comparison between BAM and YAG:Dy, revealing that BAM is more suitable for relatively lower temperature, fast and turbulent regimes and that YAG:Dy is more suited to relatively higher temperature, steady flow situations.

Nonthermal Plasma System for Marine Diesel Engine Emission Control

A nonthermal plasma reactor (NTPR) using two 2.45-GHz microwave (MW) generators for the abatement of nitrogen oxides (NOx) and sulfur (SOx) contained in the exhaust gas of a 200-kW marine diesel engine was built and tested. Numerical analysis based on a nonthermal plasma kinetics model for the abatement of NOx and SOx from marine diesel engine exhaust gas was performed. A generic kinetic model that implements electron collisions and plasma chemistry has been developed for applications involving low-temperature (50-100 K) nonthermal plasma. Abatement efficiencies of NOx and SOx were investigated for a range of mean electron energies, which directly impact on the rate constants of electron collisions. The simulation was conducted using the expected composition of exhaust gas from a typical two-stroke, slow-speed marine diesel engine. The simulation results predict that mean electron energy of 0.25-3.2 eV gives abatement efficiency of 99% for NOx and SOx. The minimum residence time required was found to be 80 ns for the mean electron energy of 1 eV. Multimode cavity was designed using COMSOL multiphysics. The NTPR performance in terms of NOx and SOx removal was experimentally tested using the exhaust from a 2-kW lab scale, two-stroke diesel engine. The experimental results also show that the complete removal of NO is possible with the MW plasma (yellow color) generated. However, it was found that generating required MW plasma is a challenging task and requires further investigation.

Effect of diesel injector tip deposits on transient spray behavior

The structural characteristics of high pressure diesel sprays have been investigated using two different complementary techniques, and the impact of injector fouling on the spray development has been assessed. In the first novel technique, fuel was injected into a reservoir of water, a liquid that offered high density ambient conditions. In a second technique, fuel was injected into a pressurised constant volume chamber.