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Title: Numerical simulations of high temperature and pressure diesel spray and combustion
Author: Nicholson, Sean Louis Francis
ISNI:       0000 0004 9356 4509
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2020
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Compression Ignition (CI) engines are state of the art power generating machines, and as such have found widespread use in a variety of implementations due to their high thermal efficiency, fuel efficiency, durability, reliability, and low carbon dioxide (CO2) emissions. These include power generation, marine propulsion, light and heavy duty road vehicles, and off-road applications. However, due to the nature of a CI engine’s combustion process, emissions of other harmful pollutants is increased, such as nitrogen oxides (NOx) and particulate matter (PM). Multiple injection strategies have been used to combat this rise in emissions; along with low temperature combustion (LTC) methodologies these have been able to reduce both NOx and PM emissions, and as such this thesis will focus on the modelling of the transient period of the diesel spray and its impact on the combustion of the fuel. Initially, this thesis will consider the transient period of diesel injection by focusing on the prediction of the early stage of the spray formation at the Engine Combustion Network’s (ECN) "Spray A" condition, comprising of a single-hole injection of ndodecane (diesel surrogate) fuel. This is achieved by comparing two different commercially available Computational Fluid Dynamics (CFD) codes and their predictions of the liquid and vapour lengths, initially with different computational set-ups before these set-ups are converged to being identical with each other. All simulations are undertaken under a Reynolds Averaged Navier Stokes (RANS) framework, in a well characterised domain for both CFD codes. This convergence of set-ups shows that the transient region of the spray is highly dependent on the break-up model, however comparison with experimental data showed a deficiency in the implementation of the break-up model within Star-CD. This was corrected with the inclusion of a novel break-up length criterion, with the corrected model showing good agreement with experimental data, with particular strengths in decoupling the liquid and vapour length predictions. Following the implementation of the novel break-up length criterion within Star- CD, the performance of this model at a combusting condition is tested. This study was performed under the same framework as previously, however with the implementation of a commonly used chemical mechanism for n-dodecane combustion. When the novel break-up length criterion is compared to the original baseline case within Star-CD the results match very well to each other, with predicted ignition delays, lift-off-lengths and combustion fields being closely aligned. An over-prediction in lift-off-length to experimental data is noted, however this is commonly seen for the mechanism used. Finally, by utilising the decoupling of the liquid and vapour penetrations offered through the novel break-up length criterion, the impact of the vaporising match on the combustion criteria detailed previously is investigated. A variety of cases are considered, with high, low and matched variations on both the liquid and vapour lengths compared against each other. The results from these tests show a strong effect of certain model constants on the combusting criteria, with break-up model constants especially having a large impact on the mixture fraction and temperature predictions. In contrast, the turbulence model constants often used when matching simulated tests to experimental results have very minimal impact on either the mixture fraction or temperature fields, with only the position of the combustion field changing, as expected. The effect of the combustion field position on the combustion temperatures is also considered, further reinforcing the break-up model constant’s impact on combustion prediction.
Supervisor: Davy, Martin Sponsor: Engineering and Physical Sciences Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available