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- Liquid film dynamic on the spray impingement modelingPublication . Rodrigues, Christian; Barata, Jorge M M; Silva, AndréThe present paper addresses a liquid film submodel included into a computational model that aims at reproducing the spray impingement phenomena. This numerical extension incorporates the spread of the liquid film over the neighboring nodes due to the dynamic motion induced by the film inertia and also the exchange of mass between the liquid layer and the incident and splashing particles. Moreover, a dimensionless film thickness parameter is embedded in the submodel by mean of an experimentally deduced correlation that can be fitted and updated to specified conditions. In order to realize how the model behaves with different influencing parameters, a thorough investigation is performed: the results that are obtained with and without the inclusion of the liquid film submodel are compared against the experimental data for two crossflow velocities. The integration of the computational extension with the spread/splash transition criterion is also evaluated by considering two types of expressions: one that includes the effect of the film thickness and one that does not. The results show that the latter option combined with the submodel does not distinctly enhance the simulation results, contrary to what happens with the transition criterion that considers the film thickness as an influencing parameter. In this case, the model with the computational extension reveals better prediction results, which indicates the necessity of considering it for spray impingement simulations along with a splash threshold that depends on the liquid layer.
- On the Modeling of a Spray Impingement Onto a Hot SurfacePublication . Silva, A. R. R.; Rodrigues, Christian; Barata, Jorge M MThe present investigation addresses the impingement of a spray onto a heated solid surface under crossflow conditions at low pressure injection - typical of port-injection engines - during cold start. The characteristics of the initial spray are established by employing an empirical procedure, which relies on a comprehensive set of free spray measurements. This computational study considers the presence of a crossflow moving perpendicularly to an interposed surface and the formation of a dynamic liquid film over the impingement wall. Both previous conditions are often neglected in numerical simulations, despite their importance on the final outcome. Distinct wall and crossflow temperatures are analyzed systematically to evaluate the influence of droplets evaporation on the final outcome of spray impingement, and, particularly, on the distribution of the thin liquid film over the surface. The present computational model already proved to deliver accurate predictions of the spray/wall interactions under different conditions. In this work, the conditions are extrapolated to a heated environment, which reproduce more adequately what is found in in-cylinder situations. The computational model is adapted to meet the new requirements and perform within the range of conditions for which it is now formulated. The analysis shows that higher temperatures lead to smaller impinging droplets, an increase in the number of depositing droplets but a decrease in the fraction of mass of particles contributing to the liquid film; and a more uniform distribution of the liquid layer over the surface.
- Numerical study of the spray impingement onto a solid wallPublication . Rodrigues, Christian Michel Gomes; Silva, André Resende Rodrigues da; Barata, Jorge Manuel MartinsThe modelling of turbulent multiphase flows has been gathering high interest in the last decades in the scientific community due to its relevance in several applications, such as in industrial and environmental processes or for chemical and biomedical purposes. In fact, regarding the industrial applications, the impingement of liquid fuel sprays onto engine surfaces has become a subject of interest due to its influence on the mixture preparation prior to combustion and, consequently, engine performance and pollutants emission (Barata and Silva, 2005). However, there is still a lack of knowledge concerning the spray-wall interaction but also concerning the exact phenomenon occurring during the process. These gaps do not allow defining the most favourable conditions for the optimal engine performance. Hence, the main challenge for the investigators lies in attaining a much deeper understanding of the phenomena involved in the spray impingement process, through either theoretical analysis or experimental investigation. Meanwhile, the splash phenomenon has been the focus of many researchers due to its relevance in the combustion process of small-bore, direct-injected gasoline and diesel engines, as well as in a variety of other industrial devices in which sprays impinge on solid surfaces. Bai and Gosman (1995) developed a model to predict the outcomes of spray droplets impacting on a wall with temperatures below the fuel boiling point. This model, which has been formulated using a combination of simple theoretical analysis and experimental data from a wide variety of sources, was later improved (Bai et al., 2002) by refining the dissipation energy term and by enhancing the post-splashing characteristics. In fact, recently, significant attention has been given to this regime either through the definition of transition criteria that better fit specific conditions of the experimental configuration under study or by characterizing the behaviour of the drop during all stages of the regime (expansion of the lamella, crown formation and propagation, etc.) through both theoretical analyses and experimental data. Beyond the transition criteria, another aspect that controls the characteristics of the secondary droplets after the impacts is the energy dissipation term and thus, it is essential its proper definition for adequately modelling these multi-phase flows. However, contrary to spreading, there is little literature available related to this particular parameter and, more important than that is the fact that there is a certain ambiguity even for what it represents exactly. In addition, the majority of the dissipative energy loss relationships have been deduced for the spread regime, i.e., from the beginning of the expansion of the lamella until the drops reaches its maximum extent (without splashing). This situation can be overcome through some simplifying assumptions, which obviously carries inaccuracy. The present work is dedicated to the study of the sprays impingement onto a solid wall through a crossflow. The major purpose of the thesis is to improve the accuracy of the base model, which is the model of Bai et al. (2002), through the employment of both new correlations for the deposition/splash transition criteria and energy dissipation loss relationships available in the literature. The numerical predictions are then compared with the experimental data of Arcoumanis et al. (1997) for two crossflow rates ( and ). From the results, it can be concluded that the employment of different transition criteria can bring better results (see also Silva et al., 2011). On the other hand, no improvements were seen by the employment of the new energy dissipative loss relationships in the base models, which calls for further research in this particular matter.
- Development of a New Dynamic Liquid Film Extension and Integration into a Spray Impingement ModelPublication . Rodrigues, Christian; Barata, Jorge M M; Silva, AndréThis paper is concerned with the improvement of macroscopic spray impingement simulations, for which a sub-model has been proposed for the formation of the liquid film over the impingement wall due to the deposition of particles. This computational extension deals with the interaction and spreading of the liquid between adjacent nodes due to the dynamic motion induced by the film inertia and crossflow velocity, as well as, the exchange of mass between incident drops, secondary droplets and liquid film. Moreover, an empirical correlation deduced from experimental data is used to define the relative liquid film thickness. Experimental data are used to validate the numerical model. Good agreement is obtained between numerical and experimental results for the test cases performed. The distribution of the relative liquid film thickness for two cases of crossflow velocities are also illustrated, as well as, its temporal evolution from start of injection. The dynamic behavior of the layer proved to be in concordance with the observations reported in experimental investigations available in the literature.
- Spray impingement modelling: Evaluation of the dissipative energy loss and influence of an enhanced near-wall treatmentPublication . Rodrigues, Christian; Barata, Jorge M M; Silva, AndréThe goal of the present research is to contribute to improve the knowledge about the spray impingement topic through a numerical study that is aimed at investigating the impact of using the dissipative energy terms that are available in the literature when they are embedded into a specific dispersion model. Comparing all the numerical approaches, a non-negligible disagreement is observed between the relationship proposed in the original model and the other ones drawn from the literature. This fact evidences the influence of the energy dissipated term on the secondary atomization outcome. The present work also provides a comprehensive study on the estimation of the energy dissipated during the splash event. This is a major contribution to the permanent literature since the few works found only addressed the spread regime. In addition, this paper gives an in-depth analysis on the influence that an enhanced treatment of the boundary layer in the region close to the wall may have in the simulation of such flows. The work revealed that this near-wall droplets tracking method provides an alternative way to increase the accuracy of the dispersed phase and achieve more consistent results without the necessity of a direct mesh refinement.
- Modelling of Drop Deformation and BreakupPublication . Rodrigues, Christian; Barata, Jorge M M; Silva, AndréThe present paper addresses the macroscopic atomization characteristics of liquid-fuel droplets when subjected to the infuence of a high velocity air crossfow. A breakup model is conceived by using a set of correlations available in the literature with the purpose of replicating such phenomena. The computational results are compared against experimental data to validate the model. The results show a reasonable agreement between measurements and predictions in both qualitative and quantitative outcomes evaluated, which sustain the mathematical formulation adopted. However, further improvements may be aspired given the fact that there is a lack of experimental data available when shearing effects come into play in the mechanisms occurring during the atomization process. On the other hand, the use of two fuels (diesel and bio-diesel) allowed to perceive a relevant impact of the liquid properties (particularly surface tension and viscosity) in the characteristics of the fragments resulting from the breakup event.
- Influence of the Spread/Splash Transition Criteria in the Spray Impingement ModelingPublication . Rodrigues, Christian; Barata, Jorge M M; Silva, AndréThe present paper reports a numerical study of a spray impinging on a surface through a crossflow. This work is intended to study the influence of the spread/splash transition criteria in the modeling of the spray impingement phenomenon. Several experimental correlations available in the literature are inserted in the same base model and the results are tested against experimental data. It can be concluded that the employment of an accurate transition criteria can improve the quality of the results.
- Modeling of Droplet Deformation and BreakupPublication . Rodrigues, Christian; Barata, Jorge M M; Silva, A. R. R.The present paper addresses the macroscopic atomization characteristics of liquid-fuel droplets when subjected to the influence of a high velocity air crossflow. A breakup model is conceived by using a set of correlations available in the literature with the purpose of replicating such phenomena. The computational results are compared against experimental data to validate the model. The results show a reasonable agreement between measurements and predictions in both the qualitative and quantitative outcomes evaluated, which sustain the mathematical formulation adopted. However, further improvements may be aspired to, given the fact that there is a lack of experimental data available when shearing effects come into play in the mechanisms occurring during the atomization process. On the other hand, the use of two fuels (diesel and biodiesel) allowed to perception of a relevant impact of the liquid properties (particularly surface tension and viscosity) in the characteristics of the fragments resulting from the breakup event.
- Modelling of spray-wall impingementPublication . Rodrigues, Christian Michel Gomes; Silva, André Resende Rodrigues da; Barata, Jorge Manuel MartinsWhen a drop collides with an interposed surface, three phases are usually involved: liquid (the drop), solid (the substrate) and gas (the surrounding environment). Such an event involves a number of parameters associated with the physical characteristics of the incident particles, the properties of the target surface, and the natural features of the air flow. Each occurrence leads to a singular outcome, since each particle experiences a different reality throughout the injection cycle. Therefore, the development of appropriate modelling strategies of this complex multi-phase flow requires a thorough understanding of the mechanisms underlying the spray impingement process. Several computational models have been reported in the open literature, although not always successfully. From these, only a few have attempted to replicate the more intricate scenarios that include the formation and development of a liquid film over the surface due to the deposition of previously injected particles, the presence of a high velocity cross-flowing gas, and the thermal effects promoted by the existence of hot walls. Even though these elements are some of the more influential parameters affecting the final outcome of spray-wall impacts, most of the simulations still neglect some of them in their formulation. Therefore, in order to capture the majority of the physical phenomena observed in experimental studies, CFD codes must be equipped with superior mathematical formulations. During the present doctoral research, three independent computational extensions have been devised and integrated into the model used by our research group to simulate spray-wall interactions. The upgrades — that have been proposed over the course of the study — have been denominated as the liquid film, evaporation and breakup sub-models. They are intended to complement the basic mathematical formulation adopted in the original simulation procedure. This approach has contributed to enhance the prediction capabilities of the model, since it is now capable of capturing some phenomena that were not considered previously. On the other hand, it has also extended the range of applicability of the CFD code to a new set of impact conditions (i.e., in hot environments and with a high velocity crossflow). Furthermore, the present work provides a detailed analysis of the results obtained, with major emphasis given to the disintegration mechanisms and secondary droplet characteristics. Both quantitative and qualitative comparisons between computational and experimental results are presented. When pertinent, the impact of a particular sub-model onto the outcome predicted is also evaluated by comparing the versions of the model with and without the corresponding computational extension. Moreover, a systematic approach is adopted at each section to infer the influence of different parameters on the final outcome. This methodology has been decisive to better understand the factors affecting the phenomena occurring during impact.
- Modeling of Evaporating Sprays Impinging onto Solid SurfacesPublication . Rodrigues, Christian; Barata, Jorge M M; Silva, A. R. R.The present paper presents a numerical study on evaporating droplets impinging onto a solid surface through a crossflow. The characteristics of the initial spray are established by employing an empirical procedure that relies on a comprehensive set of free spray measurements. Distinct wall and crossflow temperatures are analyzed systematically to evaluate the influence of droplet evaporation on the final outcome of the simulation and, particularly, on the distribution of the thin liquid film that forms over the surface due to the deposition of incident droplets. The present computational model already has been proven to deliver accurate prediction results of the spray-wall interactions under different conditions. In this work, the conditions are extrapolated to a heated environment, which more adequately reproduces what is found in in-cylinder situations. The computational model is adapted to meet the new requirements and to perform within the range of conditions for which it is now being formulated. The analysis shows that higher temperatures lead to a decrease in the size of impinging droplets, an increase in the number of depositing droplets, a decrease in the fraction of mass of droplets contributing to the liquid film, and a more uniform distribution of the liquid layer over the surface.