Numerical Simulations of the Effect of Cold Fuel Temperature on In-Nozzle Flow and Cavitation Using a Model Injector Geometry
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Abstract
In the present study, Large Eddy Simulations (LES) have been performed with a 3D model of a step nozzle injector, using n-pentane as the injected fluid, a representative of the high-volatility components in gasoline. The influence of fuel temperature and injection pressure were investigated in conditions that shed light on engine cold-start, a phenomenon prevalent in a number of combustion applications, albeit not extensively studied. The test cases provide an impression of the in-nozzle phase change and the near-nozzle spray structure across different cavitation regimes. Results for the 20oC fuel temperature case (supercavitating regime) depict the formation of a continuous cavitation region that extends to the nozzle outlet. Collapse-induced pressure wave dynamics near the outlet cause a transient entrainment of air from the discharge chamber towards the nozzle. The extent of air entrainment appears noticeably reduced on the simulations with colder fuel (−10oC, incipient cavitation) due to the shorter length of the continuous cavitation region, which impedes the backwards air motion. A comparison of the examined cases suggests that in addition to the in-nozzle flow, the injection temperature also has a noticeable effect on the macroscopic structure of the emanating fuel spray. On the colder injection, a symmetric jet is formed in the near-nozzle region, in contrast to the supercavitating cases that demonstrate a concave surface on the side of the step nozzle edge (a byproduct of intense air entrainment) and an increased spray opening angle. The above observations attest that the size and intensity of the cavitation features tend to become suppressed as the temperature of the injected fuel decreases. This has a binary effect: the injection of cold fuel may adversely influence the near-nozzle jet formation, while at the same time contributing to enhanced fuel delivery towards the combustion chamber due to the reduced flow obstruction from the entrained air and the diminished in-nozzle vapour structures.