Modelling of nozzle cavitation in newtonian and viscoelastic fluids

Cavitation is the abrupt process of vapour formation due to pressure drop in liquid flows, which may occur in various equipment and hydraulic machinery such as propellers, bearings or fuel injectors. In fuel injector nozzles, cavitation formation may be either beneficial or detrimental to the engine performance, depending on the location and type of the vaporous structures. Cavitation can enhance the turbulence levels inside the nozzle and therefore improve the spray atomisation, moreover large vortex cavities known as string cavitation, can increase the spray cone angle. However, cavitation bubbles collapsing near the internal surfaces result in erosion and ultimately failure of the injector parts. Furthermore, excessive vapour formation inside the nozzle significantly reduces the nozzle discharge coefficient and may eventually result in chocked flow conditions. Therefore, understanding the cavitation dynamics and cavitation control is vital for injector design and an active topic in fluid dynamics research. Newly developed deposit control fuel additives have the potential to reduce the in-nozzle cavitation and enhance the flowrate even in clean injectors due to their non-Newtonian properties. This research aims to provide an understanding about the association between viscoelastic detergent additives and turbulent cavitating nozzle flows using computational fluid dynamics. An accurate framework for modelling the in-nozzle cavitation is developed. Performance of several RANS and LES models is assessed in predicting incipient and developed cavitation regimes. The Reboud at al. eddy viscosity correction is utilized to compensate the effect of mixture compressibility on turbulent viscosity in k-ɛ RNG and k-ω SST models. WALE LES results are validated against the velocity and rms of turbulent velocity measurements in a step nozzle. A homogeneous equilibrium cavitation model based on Wallis speed of sound formula is utilised to study the effect of vapour-liquid mass transfer rate compared to Zwart Gerber-Belamri (ZGB) and Schnerr-Sauer (SS) cavitation models. Cavitation vapour fraction values are validated against X-ray CT measurements. The Phan-Thien-Tanner (PTT) fluid model is implemented to simulate the shear-induced viscoelastic behaviour of Quaternary Ammonium Salt (QAS) surfactants in the additised fuel. The model is validated against analytical solution for channel flow and experimental measurements of corner vortex in a square contraction geometry. The effect of liquid viscosity on cavitation development is presented using LES simulations. Finally, the effect of viscoelasticity on different cavitation regimes, namely cloud cavitation inside a step nozzle and string cavitation in a realistic injector geometry is investigated using LES. The physical flow characteristics are investigated and the effect of the additive on cavitation inside the nozzle is presented. The numerical findings are found to be in agreement with experimental studies of additised fuel

Performance of turbulence and cavitation models in prediction of incipient and developed cavitation

The aim of this article is to assess the impact of turbulence and cavitation models on the prediction of diesel injector nozzle flow. Two nozzles are examined, an enlarged one, operating at incipient cavitation, and an industrial injector tip, operating at developed cavitation. The turbulence model employed includes the re-normalization group k–ε, realizable k–ε and k–ω shear stress transport Reynolds-averaged Navier–Stokes models; linear pressure–strain Reynolds stress model and the wall adapting local eddy viscosity large eddy simulation model. The results indicate that all Reynolds-averaged Navier–Stokes and the Reynolds stress turbulence models have failed to predict cavitation inception due to their limitation to resolve adequately the low pressure existing inside vortex cores, which is responsible for cavitation development in this particular flow configuration. Moreover, Reynolds-averaged Navier–Stokes models failed to predict unsteady cavitation phenomena in the industrial injector. However, the wall adapting local eddy viscosity large eddy simulation model was able to predict incipient and developed cavitation, while also capturing the shear layer instability, vortex shedding and cavitating vortex formation. Furthermore, the performance of two cavitation methodologies is discussed within the large eddy simulation framework. In particular, a barotropic model and a mixture model based on the asymptotic Rayleigh–Plesset equation of bubble dynamics have been tested. The results indicate that although the solved equations and phase change formulation are different in these models, the predicted cavitation and flow field were very similar at incipient cavitation conditions. At developed cavitation conditions, standard cavitation models may predict unrealistically high liquid tension, so modifications may be essential. It is also concluded that accurate turbulence representation is crucial for cavitation in nozzle flows

Genetic Variation and Selection of Domain I of the Plasmodium vivax Apical Membrane Antigen-1(AMA-1) Gene in Clinical Isolates from Iran

Background: Apical Membrane antigen 1 (AMA-1) is positioned on the surface of merozoite and it may play a role in attack to red blood cells. The mainaim of present study was to determine the genetic variation, as well as, to detect of selection at domain I of AMA-1 gene Plasmodium vivax isolates in Iran. Methods: Blood samples were collected from 58 patients positive for P. vivax,mono infection and the domain I of AMA-1 gene was amplified by nested PCR and then sequenced. Results: A total 33 different haplotypes were identified among 58 Iranian sequences.The 23 new haplotypes were determined in this study that was not reported previously in other regions of the world. There were totally observed 36 point mutations at the nucleotide level in the analyzed sequences. Sequences analyses indicated 25 amino acid changes at 20 positions in which 5 sites demonstrated thrimorphic polymorphism and the others were dimorphic in the domain I of the Iranian PvAMA-1 isolates. Conclusion: Our findings indicated relatively high level of allelic diversity at the domain I of PvAMA-1 among P.vivax isolates of Iran. Since, PvAMA-1 is considering as vaccine candidate antigen, these data provide valuable information for the development of a PvAMA-1 based malaria vaccine