25 research outputs found
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Improved droplet breakup models for spray applications
The current study examines the performance of two zero-dimensional (0D) aerodynamically-induced breakup models, utilized for the prediction of droplet deformation during the breakup process in the bag, multi-mode and sheet-thinning regimes. The first model investigated is an improved version of the widely used Taylor analogy breakup (TAB) model, which compared to other models has the advantage of having an analytic solution. Following, a model based on the modified Navier–Stokes (M-NS) is examined. The parameters of both models are estimated based upon published experimental data for the bag breakup regime and CFD simulations with Diesel droplets performed as part of this work for the multi-mode and sheet-thinning regimes, for which there is a scarcity of experimental data. Both models show good accuracy in the prediction of the temporal evolution of droplet deformation in the three breakup regimes, compared to the experimental data and the CFD simulations. It is found that the best performance of the two is achieved with the M-NS model. Finally, a unified secondary breakup model is presented, which incorporates various models found in the literature, i.e. TAB, non-linear TAB (NLTAB), droplet deformation and breakup (DDB) and M-NS, into one equation using adjustable coefficients, allowing to switch among the different models
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Numerical investigation on the evaporation of droplets depositing on heated surfaces at low Weber numbers
The evaporation of water droplets, impinging with low Weber number and gently depositing on heated surfaces of stainless steel is studied numerically using a combination of fluid flow and heat transfer models. The coupled problem of heat transfer between the surrounding air, the droplet and the wall together with the liquid vaporisation from the droplet’s free surface is predicted using a modified VOF methodology accounting for phase-change and variable liquid properties. The surface cooling during droplet’s evaporation is predicted by solving simultaneously with the fluid flow and heat transfer equations, the heat conduction equation within the solid wall. The droplet’s evaporation rate is predicted using a model from the kinetic theory of gases coupled with the Spalding mass transfer model, for different initial contact angles and substrate’s temperatures, which have been varied between 20–90° and 60–100 °C, respectively. Additionally, results from a simplified and computationally less demanding simulation methodology, accounting only for the heat transfer and vaporisation processes using a time-dependent but pre-described droplet shape while neglecting fluid flow are compared with those from the full solution. The numerical results are compared against experiments for the droplet volume regression, life time and droplet shape change, showing a good agreement
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Numerical investigation of the evaporation of two-component droplets
A numerical model for the complete thermo-fluid-dynamic and phase-change transport processes of two-component hydrocarbon liquid droplets consisting of n-heptane, n-decane and mixture of the two in various compositions is presented and validated against experimental data. The Navier–Stokes equations are solved numerically together with the VOF methodology for tracking the droplet interface, using an adaptive local grid refinement technique. The energy and concentration equations inside the liquid and the gaseous phases for both liquid species and their vapor components are additionally solved, coupled together with a model predicting the local vaporization rate at the cells forming the interface between the liquid and the surrounding gas. The model is validated against experimental data available for droplets suspended on a small diameter pipe in a hot air environment under convective flow conditions; these refer to droplet’s surface temperature and size regression with time. An extended investigation of the flow field is presented along with the temperature and concentration fields. The equilibrium position of droplets is estimated together with the deformation process of the droplet. Finally, extensive parametric studies are presented revealing the nature of multi-component droplet evaporation on the details of the flow, the temperature and concentration fields
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Aerodynamic breakup of an n-decane droplet in a high temperature gas environment
The aerodynamic droplet breakup under the influence of heating and evaporation is studied numerically by solving the Navier-Stokes, energy and transport of species conservation equations; the VOF methodology is utilized in order to capture the liquid-air interphase. The conditions examined refer to an n-decane droplet with Weber numbers in the range 15–90 and gas phase temperatures in the range 600–1000 K at atmospheric pressure. To assess the effect of heating, the same cases are also examined under isothermal conditions and assuming constant physical properties of the liquid and surrounding air. Under non-isothermal conditions, the surface tension coefficient decreases due to the droplet heat-up and promotes breakup. This is more evident for the cases of lower Weber number and higher gas phase temperature. The present results are also compared against previously published ones for a more volatile n-heptane droplet and reveal that fuels with a lower volatility are more prone to breakup. A 0-D model accounting for the temporal variation of the heat/mass transfer numbers is proposed, able to predict with sufficient accuracy the thermal behavior of the deformed droplet
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Transient heating effects in high pressure Diesel injector nozzles
The tendency of today’s fuel injection systems to reach injection pressures up to 3000 bar in order to meet forthcoming emission regulations may significantly increase liquid temperatures due to friction heating; this paper identifies numerically the importance of fuel pressurization, phase-change due to cavitation, wall heat transfer and needle valve motion on the fluid heating induced in high pressure Diesel fuel injectors. These parameters affect the nozzle discharge coefficient (Cd), fuel exit temperature, cavitation volume fraction and temperature distribution within the nozzle. Variable fuel properties, being a function of the local pressure and temperature are found necessary in order to simulate accurately the effects of depressurization and heating induced by friction forces. Comparison of CFD predictions against a 0-D thermodynamic model, indicates that although the mean exit temperature increase relative to the initial fuel temperature is proportional to (1 − Cd2) at fixed needle positions, it can significantly deviate from this value when the motion of the needle valve, controlling the opening and closing of the injection process, is taken into consideration. Increasing the inlet pressure from 2000 bar, which is the pressure utilized in today’s fuel systems to 3000 bar, results to significantly increased fluid temperatures above the boiling point of the Diesel fuel components and therefore regions of potential heterogeneous fuel boiling are identified
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Non-dimensionalisation parameters for predicting the cooling effectiveness of droplets impinging on moderate temperature solid surfaces
The conjugate problem of fluid flow and heat transfer during the impact of water droplets onto a heated surface is studied numerically using the Volume of Fluid (VOF) methodology; adaptive grid refinement is used for increased resolution at the droplet moving interface. The phenomenon is assumed to be 2D-axisymmetric and the wall temperature is moderated to prevent the onset of nucleate boiling. Parametric studies examine the effect of Weber number, droplet size, wall initial temperature and liquid thermal properties on the cooling process of the heated plate during the impaction period. The main variables describing the evolution of the phenomenon are non-dimensionalised with expressions arising from the transient conduction theory. It is proved that for all cases examined, these non-dimensional expressions can be grouped together for describing the hydrodynamic and thermal behavior in a similar manner. Additionally, semi-analytic expressions are derived, which, for a given range of variation, describe the spatial distribution and the temporal evolution of the temperature of the wall as well also the heat flux absorbed from the droplet, cooling effectiveness and mean droplet temperature
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Numerical investigation of the aerodynamic breakup of Diesel and heavy fuel oil droplets
The present work examines numerically the aerodynamic breakup of Diesel and heavy fuel oil (HFO) droplets in ambient pressures ranging from atmospheric up to those encountered in Diesel engines. The numerical model solves the Navier-Stokes equations coupled with the Volume of Fluid (VOF) methodology along with an adaptive local grid refinement technique to enhance the resolution near the high deformable interface. Simulations are performed both in 2D axisymmetric and 3D computational domains. The capabilities of the model are evaluated by comparing its results against published experimental data for Diesel fuel droplets at small Ohnesorge numbers (Oh<0.04), Weber (We) numbers ranging from 14 up to 264, and liquid to air density ratios (ε) from 79 up to 695. These conditions correspond to the bag, multimode and sheet-thinning breakup regimes. Following model validation for Diesel droplets, the breakup mechanism of HFO droplets is investigated for the same range of We numbers, two relatively large Oh numbers (0.96 and 1.53) and two density ratios (30 and 72); these conditions are representative for Diesel engines operating with HFO. The simulations reveal the effect of Oh number and density ratio on the breakup mode, drop deformation, liquid surface area and drag coefficient. Finally, a correlation is proposed for the prediction of the breakup initiation time as function of the non-dimensional numbers We, Re, ε and Oh
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Numerical investigation of heavy fuel oil droplet breakup enhancement with water emulsions
The heating and explosive boiling leading to fragmentation of immiscible heavy fuel oil-water droplets, termed as W/HFO emulsions, is predicted numerically by solving the incompressible Navier-Stokes and energy equations alongside with a set of three VoF transport equations separating the interface of co-existing HFO, water liquid and water vapour fluid phases. Model predictions suggest that explosive boiling of the water inside the surrounding HFO, ought to their different boiling points, accelerates droplet breakup; this process is termed as either puffing or micro-explosion. In contrast to past studies which predefine the presence of vapor bubbles inside the water droplet, this is predicted here with a phenomenological model based on local temperature and superheat degree. Following their formation, the growth rate of the bubbles is computed with OCASIMAT phase-change algorithm. Moreover, the fuel droplet is simultaneously subjected to convective air flow which further contributes to its deformation. As a result, the performed simulations quantify the relative time scales of the aerodynamic-induced and the emulsion-induced breakup mechanisms. The conditions examined refer to a highly viscous emulsified heavy fuel oil droplet in a gas phase having fixed temperature and pressure equal to 1000 K and 30 bar, respectively. Initially, a benchmark case demonstrates the detailed mechanisms taking place, concluding that droplet fragmentation occurs only at a part of the fuel-air interface, resembling characteristics similar to puffing. Next, a parametric study with Weber number (Oh=0.9,We<200) shows that puffing process can speed up to 10 times the breakup of the droplet relative to aerodynamic breakup
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Predicting the evaporation rate of stationary droplets with the VOF methodology for a wide range of ambient temperature conditions
This paper presents CFD predictions for the evaporation of nearly spherical suspended droplets for ambient temperatures in the range 0.56 up to 1.62 of the critical fuel temperature, under atmospheric pressures. The model solves the Navier-Stokes equations along with the energy conservation equation and the species transport equations; the Volume of Fluid (VOF) methodology has been utilized to capture the liquid-gas interface using an adaptive local grid refinement technique aiming to minimize the computational cost and achieve high resolution at the liquid-gas interface region. A local evaporation rate model independent of the interface shape is further utilized by using the local vapor concentration gradient on the droplet-gas interface and assuming saturation thermodynamic conditions. The model results are compared against experimental data for suspended droplet evaporation at ambient air cross flow including single- and multi-component droplets as well as experiments for non-convective conditions. It is proved that the detailed evaporation process under atmospheric pressure conditions can be accurately predicted for the wide range of ambient temperature conditions investigated
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Parametric Investigations of the Induced Shear Stress by a Laser-Generated Bubble
The present paper focuses on the simulation of the growth and collapse of a bubble in the vicinity of a wall. Both liquid and gas phases are assumed compressible, and their interaction is handled with the volume-of-fluid method. The main interest is to quantify the influence of the induced shear stress and pressure pulse in the vicinity of the wall for a variety of bubble sizes and bubble–wall distances. The results are validated against prior experimental results, such as the measurements of the bubble size, induced pressure field, and shear stress on the wall. The simulation predictions indicate that the wall in the vicinity of the bubble is subjected both to high shear stresses and large pressure pulses because of the growth and collapse of the bubble. In fact, pressure levels of 100 bar or more and shear stresses up to 25 kPa have been found at localized spots on the wall surface, at the region around the bubble. Moreover, the simulations are capable of providing additional insight to the experimental investigation, as the inherent limitations of the latter are avoided. The present work may be considered as a preliminary investigation in optimizing bubble energy and wall generation distance for ultrasound cleaning applications