Using a homogeneous equilibrium model for the study of the inner nozzle flow and cavitation pattern in convergent-divergent nozzles of diesel injectors

[EN] In this paper, the behaviour of the internal nozzle flow and cavitation phenomenon are numerically studied for non-conventional Diesel convergent-divergent nozzles in order to assess their potential in terms of flow characteristics. The used nozzles differs each other in the convergence-divergence level of the orifices but all of them keep the same diameter at the middle of the nozzle orifice. The calculations have been performed using a code previously validated and able to simulate cavitation phenomenon using a homogeneous equilibrium model for the biphasic fluid and using a RANS method (RNG k-ε) as a turbulence modelling approach. For the simulations, one injection pressure and different discharge pressures were used in order to assess the characteristics of nozzles for different Reynolds conditions involving cavitating and non-cavitating conditions. The comparison of the nozzles has been carried out in terms of flow characteristics such as mass flow, momentum flux, effective velocity and other important dimensionless parameters which help to describe the behaviour of the inner flow: discharge coefficient (Cd), area coefficient (Ca) and velocity coefficient (Cv). Additionally, the nozzles have been compared in terms of cavitation inception conditions and cavitation development. The study has shown a high influence on the results of the level of convergence-divergence used in the nozzles. In these nozzles, the vapour originated from cavitation phenomenon came from the throttle of the orifice at the midpoint, and it extended along the whole wall of the divergent nozzle part towards the outlet of the orifice. The main results of the investigation have shown how the different geometries modify the cavitation conditions as well as the discharge coefficient and effective velocity. In particular, the nozzle with highest convergence-divergence level showed cavitation for all the tested conditions while for the nozzle with lowest convergence-divergence level, the cavitation phenomenon could be avoided for high discharge pressures. Additionally, the nozzle with highest convergence-divergence level showed the lowest discharge coefficient values but similar effective injection velocity than the nozzle with lowest level of convergence-divergence level despite of its higher orifice outlet area.This work was partly sponsored by ‘‘Ministerio de Economía y Competitividad’’ of the Spanish Government, in the frame of the project ‘‘Estudio de la interacción chorro-pared em condiciones realistas de motor’’, Reference TRA2015-67679-c2-1- R. This support is gratefully acknowledged by the authors. Mr. Jaramillo’s thesis is supported by ‘‘Conselleria d’Educació, Cultura I Esports’’ of ‘‘Generalitat Valenciana’’ through the program ‘‘Programa VALI+D para investigadores en Formación’’, Reference ACIF/2015/040. The authors would like to express gratitude for the computer resources, technical expertise and assistance provided by the Universidad de Valencia relating to the use of the supercomputer ‘‘Tirant’’.Salvador, FJ.; Jaramillo-Císcar, D.; Romero, J.; Roselló, M. (2017). Using a homogeneous equilibrium model for the study of the inner nozzle flow and cavitation pattern in convergent-divergent nozzles of diesel injectors. Journal of Computational and Applied Mathematics. 309:630-641. https://doi.org/10.1016/j.cam.2016.04.010S63064130

Comparative study of the internal flow in diesel injection nozzles at cavitating conditions at different needle lifts with steady and transient simulations approaches

[EN] The motion of the needle during the injection process of a diesel injector has a marked influence on the internal flow, the fuel characteristics at the nozzle exit, the spray pattern and the fuel-air mixing process. The current paper is focused on the computational study of the internal flow and cavitation phenomena during the injection process, with inclusion of the opening where the needle is working at partial lifts. This study has been performed with a homogeneous equilibrium model (OpenFOAM) customized by the authors to simulate the real motion of the needle. The first part of the study covers the analysis of the whole injection process with a moving mesh using the boundary conditions provided by a one-dimensional (1D) model of the injector created in AMESim. This 1D model has offered the possibility of reproducing the movement of the needle with real lift law and real injection pressure evolution during the injection. Thus, it has been possible to compare the injection rate profiles provided by OpenFOAM against those obtained both in AMESim and experimentally. The second part compares the differences in mass flow, momentum flux, effective velocity and cavitation appearance between steady (fixed lifts) and transient (moving mesh) simulations. The aim of this comparison is to establish the differences between these two approaches. On the one hand is a more realistic approach in its use of transient simulations of the injection process and where the needle movement is taken into account. On the other hand, is the use of steady simulations at partial needle lifts. This analysis could be of interest to researchers devoted to the study of the diesel injection process since it could help to delimit the uncertainties involved in using the second approach which is more easily carried out, versus the first which is supposed to provide more realistic results.The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was partly sponsored by the 'Ministerio de Economa y Competitividad' of the Spanish Government, in the frame of the Project 'Estudio de la interaccion chorro-pared en condiciones realistas de motor', Reference TRA2015-67679-c2-1-R.Salvador, FJ.; De La Morena, J.; Crialesi Esposito, M.; Martínez López, J. (2018). Comparative study of the internal flow in diesel injection nozzles at cavitating conditions at different needle lifts with steady and transient simulations approaches. Proceedings of the Institution of Mechanical Engineers Part D Journal of Automobile Engineering. 232(8):1060-1078. https://doi.org/10.1177/0954407017725672S10601078232

Simulation of Venturi Tube Design for Column Flotation Using Computational Fluid Dynamics

Froth flotation is one of the most important methods in mineral processing. Previous studies have found that using cavitation bubbles reduces both energy consumption and operating costs. Hydrodynamic cavitation is the most economical method used for creating tiny bubbles for flotation. In this study, the efficiency of cavitation bubble generating devices and their geometry design is analyzed using computational fluid dynamics (CFD).;To optimize the geometric design of the venturi tube, a response surface method was conducted for evaluating the effect of six important parameters affecting the efficiency of the venturi tube. Critical velocities of cavitation of different designs were compared. The CFD model was validated by comparing it with previous experimental work.;Population Balance Module (PBM) was developed to analyze the bubble size distribution of particles for the secondary phase. This module includes the rates of nucleation, growth, dispersion, aggregation, and breakage. A mathematical model was created and written as User Defined Functions (UDF) code and complied with ANSYS-FLUENT, to calculate the nucleation rate for the PBM model. In order to understand the interactions of bubble size distribution with particle size distribution, a 200 microm and a 400 microm diameter ball were added to the venturi grid. The CFD results found were consistent with experimental measurement results.;Different orders of the packed column and venturi tube in series were tested. The first design placed the packed column first, then the venturi tube second. The second design placed the venturi tube first, then the packed column second. Vapor volume of fractions and bubble sizes generated with a scale up venturi and multiple lab-scale venturis were analyzed. The results and recommendations of this study will provide more optimum yield in industry

Analizar y comparar, el fenómeno de condensación por contacto directo, mediante simulación por CFD entre una tobera convergente - divergente de vapor y una flauta de flujo de vapor.

A simulation model of direct contact condensation DCC, of steam jets in stationary liquid water is presented. The analysis takes as a baseline the studies of phase change by nucleation given by Borishanski and the thermal behavior of the fluid that is supported by the thermal theory of Clausius Clapeyron. It is possible to determine the change of phase of vapor to liquid through the formation of annular flow between the surface of the steam and water, the dependence of the change of phase is fulfilled while the differential of temperature is maintained, according to the laws of transport Fick of dough. The numerical study was carried out with CFD tools, computational fluids dynamics, supported with the Navier-Stokes equations using the Lee model of evaporation and condensation, in this way, the thermal and mass relationship of a new biphasic flow was obtained. The result is validated with the evident dependence of the steam jet on the differentiation of the Reynolds number, which allows to relate the rate of variation of mass with the thermal diffusivity evaluated in Prandtl. The numerical study corroborates the existing advances of the interfacial friction factor in the phase change, thus allowing to know the thermodynamic parameters of the phenomenon under established initial conditions. The steam injectors used to check the phenomenon are converging-diverging nozzle and steam flute, models IN15 and IN40M manufactured by SpiraxSarco®. The stable working speeds are 2836,66 ms-1 and 500 ms-1, 8 comparative cases were made with different speeds outside the manufacturer's recommendations. The results are corroborated with contour analysis of vapor volume fraction, density, pressure and velocity, compared between all the assigned cases.Se presenta un modelo de simulación de condensación por contacto directo DCC, de chorros de vapor en agua líquida estacionaria. El análisis toma como línea base los estudios de cambio de fase por nucleación dados por Borishanski y el comportamiento térmico del fluido que se respalda con la teoría térmica de Clausius Clapeyron. Se logra determinar el cambio de fase de vapor a líquido mediante la formación de flujo anular entre la superficie del vapor y agua, la dependencia del cambio de fase se cumple mientras el diferencial de temperatura se mantenga, según se cumplan las leyes de Fick de transporte de masa. El estudio numérico se realizó con herramientas CFD, computational fluids dynamics, respaldadas con las ecuaciones de Navier-Stokes usando el modelo de Lee de evaporación y condensación, de esa manera, se obtuvo la relación térmica y másica de un nuevo flujo bifásico. El resultado se valida con la evidente dependencia del chorro de vapor a la diferenciación del número de Reynolds, la cual permite relacionar la tasa de variación de masa con la difusividad térmica evaluada en Prandtl. El estudio numérico corrobora los avances existentes del factor de fricción interfacial en el cambio de fase, permitiendo así conocer los parámetros termodinámicos del fenómeno bajo condiciones iniciales establecidas. Los inyectores de vapor usados para comprobar el fenómeno son tobera convergente – divergente y flauta de vapor, modelos IN15 e IN40M fabricados por SpiraxSarco®. Las velocidades estables de trabajo son 2836,66 ms-1 y 500 ms-1, se realizaron 8 casos comparativos con distintas velocidades fuera de las recomendaciones del fabricante. Los resultados se corroboran con análisis de contornos de fracción de volumen de vapor, densidad, presión y velocidad, comparados entre todos los casos asignados

An Investigation of Cavitation Initiation and Length of the Two-Phase Region in a Converging-Diverging Nozzle

Doctor of PhilosophyDepartment of Mechanical and Nuclear EngineeringMohammad H HosniA traditional refrigeration cycle has four main system components which are an evaporator, a compressor, a condenser, and an expansion valve. Different types of refrigerants are used in most cooling cycles. The main objective of this project was to develop a water-based cooling system by investigating the cavitation/flash phenomena for the flow through converging-diverging nozzles. Although, cavitation can be harmful in some engineering applications and causes damage to pumps, refrigeration expansion valves, and capillary tubes, on the other hand, it can be managed and used in a beneficial way. Cavitation in a flowing fluid can cause a reduction in temperature, which can result in energy being absorbed and hence, demonstrating a cooling potential. Cavitation/flash can occur when the static pressure of the fluid falls below the vapor pressure, into a metastable liquid state. This phenomenon has been shown in previous experimental work to occur in water flowing through a converging-diverging nozzle, where the cross-sectional area is constricted at the throat causing the velocity to increase and the pressure to decrease below the saturation pressure. The research presented in this dissertation is focused on developing a complete theoretical model and evaluation techniques to predict the results of the cavitation phenomena in a converging-diverging nozzle. The conservation equations and the laws of thermodynamics are presented to understand the fundamental thermodynamics phenomena and to develop predictive models relevant to the cavitation process. The developed models were used to predict pressure distributions, the onset of flash/cavitation, the condensation shock location, and the length of the two-phase region for water within a converging-diverging nozzle. The predicted results were shown to compare well with the previous experimental work; in particular, with the length of the two-phase region. The length of the two-phase region is defined as the distance from the flash inception point to the location where the condensation shock formed in the diverging section of the nozzle. The length of the two-phase region is also used as a measure for the heat absorption area in the nozzle. The larger the area of heat absorption, the higher the cooling potential is likely to be for the system. Experimental results using water have shown only small temperature drops due to a cavitation process in converging-diverging nozzles, mainly due to the physical properties of water. However, the models developed should also apply to cavitation with other fluids. Hence, this analysis can form the basis for future evaluation and potential optimization of the nozzle geometry, and the identification of alternative fluids (properties), necessary to achieve maximum cooling potential for fluids flowing through converging-diverging nozzles