Droplet Momentum Research Articles

Slip effects on the exergy loss due to irreversible heat transfer in a condensing flow

This paper studies the effect of slip between vapour and liquid phases on entropy generation owing to irreversible heat transfer between these phases. The paper offers governing equations considering slip between two phases and resolves droplet momentum equation. The governing equations are solved using fourth-order Runge-Kutta scheme. Also, a valid model is used to work out the entropy generation owing to nucleation. Afterwards, slip effect on entropy generation is investigated for variables affecting the entropy generation by comparing slip and no-slip results. Results verify that slip consideration increases the entropy generation and the exergy losses.

Characterization of Ni–YSZ anodes for solid oxide fuel cells fabricated by suspension plasma spraying with axial feedstock injection

Composite Ni–Y0.15Zr0.85O1.925 anodes were fabricated by axial-injection suspension plasma spraying in open atmosphere conditions. The composition of the anode is controllable by adjustment of the plasma gas composition, stand-off distance, and suspension feed rate. The total porosity is controllable through the addition of carbon black to the suspension as a sacrificial pore-forming material as well as by adjustment of the suspension feed rate. The size of the NiO particles in suspension affects both the composition and total porosity, with larger NiO particles leading to increased Ni content and porosity in the deposited coatings. The surface roughness increases with a decrease of the in-flight droplet momentum, which results from both smaller NiO particles in suspension and the addition of low density pore-forming materials. A solid oxide fuel cell was fabricated with both electrodes and electrolyte fabricated by axial-injection plasma spraying. Peak power densities of 0.718 W cm−2 and 1.13 W cm−2 at 750 °C and 850 °C, respectively, were achieved.

On the role of physiochemical properties on evaporation behavior of DISI biofuel sprays

Biofuels and alternative fuels are increasingly being blended to conventional gasoline fuel to reduce the overall CO2 emissions. The effect on NOx and soot formation is still unclear as the atomization and evaporation of gasoline with biocomponents differ depending on fuel specific physiochemical properties. This work focuses on describing the biofuel evaporation behavior of gasoline sprays at homogeneous charge (early injection timing) and stratified-charge conditions (late injection timing mode) used in modern direct injection spark ignition engines (DISI). A spray plume of a 6-hole solenoid injector is analyzed in terms of liquid spray propagation, and local droplet sizes studied in an injection chamber. Depending on the operating conditions, different physiochemical properties are found to dominate the atomization and evaporation processes: For low and moderate ambient temperature and pressure, high-boiling point components show a strong influence on the spray droplet size distribution. However, at elevated temperature and pressure, the evaporation behavior changes completely. Due to a high degree of evaporation, the evaporation cooling effect dominates the local droplet sizes. Fuel mixtures owing a larger heat of vaporization show larger droplet sizes—even if these fuels have a lower boiling point. Depending on the local evaporation behavior, the different remaining droplet momentum in the spray controls the air entrainment and the subsequent progress of evaporation and mixing. Overall, it can be stated that the heat of vaporization is a dominating physiochemical property for the droplet evaporation rate at high-level supercharged conditions.

Research into Configuration and Flow of Wall Oil Film in Bearing Chamber Based on Droplet Size Distribution

The lubrication design and heat transfer determination of bearing chambers in aeroengine require a sufficient understanding of the oil droplet-film interaction and physical characteristic in an oil/air two-phase flow state. The analyses of oil droplet movement, mass and momentum transfer during the impingement of droplet/wall, as well as wall oil film thickness and flow velocity are very important for the bearing chamber lubrication and heat transfer calculation. An integrated model in combination with droplet movement, droplet/wall impact and film flow analysis is put forward initially based on the consideration of droplet size distribution. The model makes a contribution to provide more practical and feasible technical approach, which is not only for the study of droplet-film interaction and physical behavior in bearing chambers with oil/air two-phase flow phenomena, but also useful for an insight into the essence of physical course through droplet movement and deposition, film formation and flow. The influences of chamber geometries and operating conditions on droplet deposition mass and momentum transfer, and wall film thickness and velocity distribution are discussed. The feasibility of the method by theoretical analysis is also verified by the existing experimental data. The current work is conducive to expose the physical behavior of wall oil film configuration and flow in bearing chamber, and also significant for bearing chamber lubrication and heat transfer study under oil/air two-phase flow conditions.

Deterministic model of open-space dust removal system using water spray nozzle: Effects of polydispersity of water droplet and dust particle

A deterministic model to predict the inertia-interceptional dust removal efficiency of open-space water spraying is developed based on momentum and mass balances of water droplets and dust particles. The effect of polydisperse dust particles and water droplets generated by different nozzles on the total removal efficiency ( η total ) is examined. The small water droplets with narrower distribution could provide the acceptable removal of dust particles while the relatively large dust particles also provide the large η total value. The η total predicted by the developed model is in the range of 19.6–23.8% with a relative error of 2% when compared with the measuring data. The η total with the use of polydispersity of water droplets and dust particles is more accurate than that with the use of average size of water droplets and dust particles.

Structure of evaporating single- and multicomponent fuel sprays for 2nd generation gasoline direct injection

In the present paper the effect of fuel properties on spray formation and evaporation was investigated for a hollow-cone spray of a piezoelectric injector for Direct Injection Spark Ignition (DISI) engines. Late injection timing in a high-pressure atmosphere (1.5 MPa, 200 °C) was simulated in an injection chamber. Liquid and vapor phase structure of the hollow-cone spray were studied with 2D-Mie scattering, laser-induced fluorescence (LIF) as well as phase-Doppler anemometry (PDA). The spray structure was investigated for several alkanes with high and low volatility ( n-hexane, n-heptane, iso-octane, n-decane) and a three-component mixture of the n-alkanes with similar fuel properties like a multicomponent gasoline fuel. It is found that the rapid evaporation of high volatility fuels can lead to spray destabilization, whereas low volatility single-component fuels overestimate radial spray propagation and vortex formation. For iso-octane the droplet size distribution is shifted to smaller droplets and the spray appeared to be less dense compared to n-heptane despite almost identical boiling behavior. However, the much higher viscosity of iso-octane determines the internal nozzle flow which results in a reduced injected fuel mass and changed atomization. A well defined three-component fuel models the global spray characteristics as well as the droplet size, droplet momentum distribution and evaporation behavior of the used multicomponent gasoline fuel very precisely. Small amounts of low volatility fractions delay the droplet evaporation and support the overall spray stability also for multicomponent mixtures. This leads to an increased spray width as well as larger droplet sizes and momenta. The evaporation characteristic of multicomponent fuels at increased ambient pressure is complex. At the studied injection conditions it is situated between the two limiting cases of distillation-like behavior and coevaporation of the components. Moreover, the results in comparison with theoretical estimations indicate a demixing of light and heavy boiling fractions in the three-component and multicomponent fuel under conditions which are typical for DISI strategies with late injection.

Determination of the momentum of droplets impinging on the pool during aluminium GMAW

The purpose of this work was to raise the kinematics and dynamics characteristics of the droplets under transference during aluminium GMA welding and quantify the partial effect of the momentum on the weld bead formation. It is initially claimed that a better way of quantifying this effect would be through the mass flow rate of the droplets reaching the weld pool; calculated parameter denominated here as Effective Momentum. Bead-on-plate GMAW, shielded with Ar, was carried out using an aluminium alloy wire. Three levels of current and two levels of arc length were employed in the experimental design. High-speed filming shadowgraphy, synchronized with the electrical signals, was used as measuring tool. Dedicated computational programs were employed to quantify arc lengths and size, frequency, acceleration and speed of the droplets. It was confirmed that in spite of the low values of the Effective Momentum for aluminium GMAW, the momentum is one of the governing factor in developing bead penetration. The interaction between Effective Momentum and welding current and its effect on the aluminium GMAW bead geometry is discussed.

Stationary GMAW-P weld metal deposit spreading

Non-isothermal spreading of non-travelling pulsed gas metal arc weld (GMAW-P) deposits was studied by experiments and numerical simulation. After an initial transient period, metal from the melting welding wire was deposited as a regular stream of droplets into a molten weld pool whose spreading was enhanced by direct arc heating of the solid substrate and weld pool convection. Accurate predictions of the final weld cross-section and time histories of base metal temperature and weld pool radius were produced by this simulation, which included mass, energy and momentum of transferring filler metal droplets. Dimensional analysis showed that speed of weld pool spreading was initially dominated by the momentum of liquid droplets while thermal convection by surface tension-driven flow became important at intermediate times. Eventually, the rate of spreading was matched to the thermal diffusion rate in the liquid. Capillary forces were never important in weld pool spreading. The weld metal deposit shape was well-approximated as a spherical cap with increasing volume and constant contact angle.

High Speed Droplet-based Delivery System for Passive Pumping in Microfluidic Devices

A novel microfluidic system has been developed that uses the phenomenon of passive pumping along with a user controlled droplet based fluid delivery system. Passive pumping is the phenomenon by which surface tension induced pressure differences drive fluid movement in closed channels. The automated fluid delivery system consists of a set of voltage controlled valves with micro-nozzles connected to a fluid reservoir and a control system. These voltage controlled valves offer a volumetrically precise way to deliver fluid droplets to the inlet of a microfluidic device in a high frequency manner. Based on the dimensions demonstrated in the current study example, the system is capable of flowing 4 milliliters per minute (through a 2.2 mm by 260 microm cross-sectional channel). Based on these same channel dimensions, fluid exchange of a point inside the channel can be achieved in as little as eight milliseconds. It is observed that there is interplay between momentum of the system (imparted by a combination of the droplets created by the valves and the fluid velocity in the channel), and the surface tension of the liquid. Where momentum provides velocity to the fluid flow (or vice-versa), equilibration of surface tension at the inlet provides a sudden stop to any flow. This sudden stop allows the user to control the flow characteristics of the channel and opens the door for a variety of biological applications, ranging anywhere from reagent delivery to drug-cell studies. It is also observed that when nozzles are aimed at the inlet at shallow angles, the droplet momentum can cause additional interesting fluid phenomena, such as mixing of multiple droplets in the inlet.

High Speed Droplet-based Delivery System for Passive Pumping in Microfluidic Devices

A novel microfluidic system has been developed that uses the phenomenon of passive pumping along with a user controlled droplet based fluid delivery system. Passive pumping is the phenomenon by which surface tension induced pressure differences drive fluid movement in closed channels. The automated fluid delivery system consists of a set of voltage controlled valves with micro-nozzles connected to a fluid reservoir and a control system. These voltage controlled valves offer a volumetrically precise way to deliver fluid droplets to the inlet of a microfluidic device in a high frequency manner. Based on the dimensions demonstrated in the current study example, the system is capable of flowing 4 milliliters per minute (through a 2.2mm by 260um cross-sectional channel). Based on these same channel dimensions, fluid exchange of a point inside the channel can be achieved in as little as eight milliseconds. It is observed that there is interplay between momentum of the system (imparted by a combination of the droplets created by the valves and the fluid velocity in the channel), and the surface tension of the liquid. Where momentum provides velocity to the fluid flow (or vice-versa), equilibration of surface tension at the inlet provides a sudden stop to any flow. This sudden stop allows the user to control the flow characteristics of the channel and opens the door for a variety of biological applications, ranging anywhere from reagent delivery to drug-cell studies. It is also observed that when nozzles are aimed at the inlet at shallow angles, the droplet momentum can cause additional interesting fluid phenomena, such as mixing of multiple droplets in the inlet.

Influence of welding parameters on distribution of wire feeding elements in CO2 laser GMA hybrid welding

The effect of welding parameters on the distribution of wire feeding elements has been investigated during CO2 laser and pulsed gas metal arc hybrid welding process. The molten metal flow on the pool surface and inside of the samples was observed by a high speed video camera and an in situ X-ray transmission imaging system respectively. The results indicate that the fluid flow towards the inside of keyhole, namely inward flow, improves the homogeneity of weld metal. The distribution of alloying elements is more homogeneous in leading laser compared with leading arc, since both of the drag force of the plasma jet and momentum of droplet promote the inward flow in leading laser. Almost homogeneous distribution of alloying elements can be attained if the oxygen content in the shielding gas is more than 2%, since the Marangoni flow direction changes from outward to inward with increasing the oxygen content.

Simulation of Droplet-gas Flow in the Effervescent Atomization Spray with an Impinging Plate

A comprehensive three-dimensional model of droplet-gas flow was presented to study the evolution of spray in the effervescent atomization spray with an impinging plate. For gas phase, the N-S equation with the k- ɛ turbulence model was solved, considering two-way coupling interaction between droplets and gas phase. Dispersed droplet phase is modeled as Lagrangian entities, accounting for the physics of droplet generation from primary andsecondary breakup, droplet collision and coalescence, droplet momentum and heat transfer. The mean size and statistical distribution of atomized droplets at various nozzle-to-plate distances were calculated. Some simulation results were compared well with experimental data. The results show that the existence of the impinging plate has a pronounced influence on the droplet mean size, size distribution and the droplet spatial distribution. The air-to-liquid ratio has obvious effects on the droplet size and distribution.

Numerical Investigation of the Time Scales of Single Droplet Burning

The effect of gas phase velocity fluctuations on single droplet burning is investigated numerically. The main objective of this study is to understand the effect of gas phase turbulence on nitric oxide formation in single droplet flames. Since the interaction of gas phase velocity fluctuations with droplet burning is of sequential character, a separate investigation of droplet momentum coupling and droplet burning is performed. Momentum coupling controls droplet relaxation against changes of the gas phase velocity along the droplet trajectory and, thereby, determines to what extend gas phase velocity fluctuations translate into droplet slip velocity fluctuations. This coupling effect acts as a high pass filter with a cutoff frequency determined by the droplet Reynolds number and diameter. In the simulation of single droplet burning detailed models for chemical reaction, diffusive species transport and evaporation are used. A significant effect of slip velocity fluctuations on the mean values of NO formation rate is observed. The effect of slip velocity fluctuations on the mean NO formation rate is frequency dependent. The frequency response of the droplet flame is similar to that of a low pass filter. The droplet flame time scale characterizing the response to slip velocity fluctuations is found to correlate with chemical time scales. This time scale is not affected by droplet diameter.

Application of Electrohydrodynamic Atomization to Two-Phase Impingement Heat Transfer

The effect of electric fields applied to two-phase impingement heat transfer is explored for the first time. The electric field applied between a capillary tube and heated surface enhances the heat transfer by controlling the free boundary flow modes from discreet drops to jets, to sprays. Through an experimental study, the impingement heat transfer was evaluated over a range of operating conditions and geometrical parameters with subcooled ethanol used as the working fluid. The ability to change the mode of impinging mass did change the surface heat transfer. The characteristics of the impinging mass on heat transfer were dependent on flow rate, applied voltage, capillary tube to heated surface spacing, capillary tube geometry, heat flux, heater surface geometry, and capillary tube array configuration. Enhancement occurred primarily at low heat fluxes (below 30W∕cm2) under ramified spray conditions where the droplet momentum promoted thin films on the heated surface resulting in 1.7 times enhancement under certain conditions. Higher heat fluxes resulted in greater vapor momentum from the surface, minimizing the effect of different impingement modes. The use of capillary tube array allowed for electrohydrodynamics atomization enhancement and higher liquid flow rates, but electrostatic repulsive forces diverted the spray from the heater surface. This reduced the mass flux to the surface, leading to premature dryout under certain conditions.

Large-Eddy Simulation of Interactions Between a Reacting Jet and Evaporating Droplets

Large-eddy simulation of a turbulent reactive jet with and without evaporating droplets is performed to investigate the interactions among turbulence, combustion, heat transfer and evaporation. A hybrid Eulerian–Lagrangian approach is used for the gas–liquid flow system. Arrhenius-type finite-rate chemistry is employed for the chemical reaction. To capture the highly local interactions, dynamic procedures are used for all the subgrid-scale models, except that the filtered reaction rate is modelled by a scale similarity model. Various representative cases with different initial droplet sizes (St0) and mass loading ratios (MLR) have been simulated, along with a case without droplets. It is found that compared with the bigger, slow responding droplets (St0 = 16), smaller droplets (St0 = 1) are more efficient in suppressing combustion due to their preferential concentration in the reaction zones. The peak temperature and intensity of temperature fluctuations are found to be reduced in all the droplet cases, to a varying extent depending on the droplet properties. Detailed analysis on the contributions of respective terms in a transport equation for grid-scale kinetic energy (GSKE) shows that the droplet evaporation effect on GSKE is small, while the droplet momentum effect depends on St0. When the MLR is sufficiently high, the bigger (St0 = 16) droplets can have profound influence on GSKE, and consequently on the formation and evolution of large-scale flow structures. On the other hand, the turbulence level is found to be lower in the droplet cases than in the pure flame case, due to the dissipative droplet dynamic effect.

Simulation and Analysis of High-Speed Droplet Spray Dynamics

Abstract An experimentally validated computer simulation model has been developed for the analysis of gas-phase and droplet characteristics of isothermal sprays generated by pressure jet atomizers. Employing a coupled Euler-Lagrange approach for the gas-droplet flow, secondary droplet breakup (based on the ETAB model), was assumed to be dominant and the k-ε model was selected for simulating the gas flow. Specifically, transient spray formation in terms of turbulent gas flow as well as droplet velocities and size distributions are provided for different back pressures. Clearly, two-way coupling of the phases is important because of the impact of significant gas entrainment, droplet momentum transfer, and turbulent dispersion. Several spray phenomena are discussed in light of low back-pressure (1atm) and high back-pressure (30atm) environments. At low back-pressure, sprays have long thin geometric features and penetrate faster and deeper than at high back-pressures because of the measurable change in air density and hence drag force. Away from the nozzle exit under relatively high back pressures, there is no distinct droplet size difference between peripheral and core regions because of the high droplet Weber numbers, leading to very small droplets which move randomly. In contrast to transient spray developments, under steady-state conditions droplets are subject to smaller drag forces due to the fully-developed gas entrainment velocities which reduce gas-liquid slip. Turbulent dispersion influences droplet trajectories significantly because of the impact of random gas-phase fluctuations.

Direct numerical simulations of a planar jet laden with evaporating droplets

A direct numerical simulation (DNS) study is conducted on the various aspects of phase interactions in a planar turbulent gas-jet laden with non-evaporative and evaporative liquid droplets. A compressible computational model utilizing a finite difference scheme for the carrier gas and a Lagrangian solver for the droplet phase is used to conduct the numerical experiments. The effects of droplet time constant, mass-loading and mass/momentum/energy coupling between phases on droplet and gas-jet fields are investigated. Significant changes in velocity, temperature, density and turbulence production on account of the coupling between the liquid and gas phases are observed in non-isothermal jets with evaporating droplets. Most of these changes are attributed to the density stratification in the carrier gas that is caused by droplet momentum and heat transfer.

High Pressure Spray Cooling of a Moving Surface

A novel technique is described for investigating spray cooling of moving hot surfaces. An experimental investigation is described for vertically downwards water sprays impinging on a horizontal steel annulus of 250mm diameter with a surface temperature up to 600°C, and rotating at up to 120rpm, giving a tangential velocity of 1.35ms−1. The central homogeneous zones of sprays from full-cone atomizers are used at pressures up to 2.07MPa and the ranges of impacting spray parameters are 0.98to12.5kgm−2s−1 for mass flux, 49-230μm for volume median drop diameter, and 9.8-32.3ms−1 for impinging velocity (Yule, A. J., Sharief, R. A., and Nasr, G. G., 2000, “The Performance Characteristics of Solid Cone Spray Pressure Swirl Atomizers,” Ann. Tokyo Astron. Obs., 10(6), pp. 627–646). Time histories of the steel temperature, at positions within the annulus, are presented and analyzed to deduce the transient cooling as the instrumented section of the annulus was swept repeatedly under the spray. Discussion is provided on the physical processes occurring on the basis of the observations. Correlation equations derived to find relationships of surface heat flux with the spray and surface parameters provide further insight into these processes. The results confirm results for static surfaces, that droplet size is a relatively weak parameter, while droplet momentum flux and surface velocity are important. As the surface velocity is increased, peak heat transfer rate at the surface reduces, and its position moves downstream with respect to the spray centerline.

Modeling of droplet collision-induced breakup process

The present article proposes a new droplet collision model considering droplet collision-induced breakup process with the formation of satellite droplets. The new model consists of several equations to investigate the post-collision characteristics of colliding droplets and satellite droplets. These equations are derived from the conservations of droplet mass, momentum, and energy between before and after collision, and make it possible to predict the number of satellite droplets, and the droplet size and velocity in the analytical way. To validate the new collision model, numerical calculations are performed and their results are compared with experimental data published earlier for binary collision of water droplets. It is found from the results that the new model shows good agreement with experimental data for the number of satellite droplets. It can be also shown that the predicted mean diameter by the new model decrease with increasing the Weber number because of the collision-induced breakup, whereas the O’Rourke model fails to predict the size reduction via the binary droplet collision.

Self Similarity of Cross-Stream Droplet Momentum Displacement in Dispersed Multiphase Flow

Analytical methods are developed and applied to droplet motion, as it relates to aircraft icing. Impinging droplets largely affect the heat balance at an iced aircraft surface, as well as the final ice shape. In this study, a similarity solution of the Eulerian droplet momentum equation is developed. Droplet motion near a flat plate is investigated with a similarity solution. By using scaling, sensitivity, order of magnitude and similarity methods, a momentum displacement of droplets (or particles) due to the presence of the solid surface is predicted. Self similarity of the droplet profiles is established, such that downstream propagation can be expressed in terms of a single independent coordinate. Limiting trends of momentum/drag induced and Blasius-diffusion profiles are found to identify the spatial range encompassing the droplet motion. The predicted results are successfully compared against the scaling requirements.