Multijet Sprays Research Articles

General understanding on spray collapse process of an asymmetrical multi-hole direct injection gasoline injector under wide flash-boiling conditions

How the under-expansion impacts the flash-boiling spray collapse process over wide superheat levels ( Rp, defined as the ratio of saturation pressure to ambient pressure) is not well understood. In the present study, n-hexane flash-boiling sprays issued from a five-hole asymmetrical injector were experimentally and numerically studied to obtain a more general understanding of the spray collapse with wide Rp variation. The experimental results proved that the collapse in the transitional region occurs in the far field, unlike the fully collapse that occurred in the near-nozzle region. The numerical results demonstrated the complexity of individual jet evolutions and their interactions over wide Rp. For individual flash-boiling jets, there were different behaviors in the near-nozzle region. In the case with Rp slightly larger than 1, no shock waves can be observed, but a set of compression-expansion chains. The further increase in Rp caused the generation of shock waves, and resultantly the primary cells were established. For the multi-jet sprays, the further increase in Rp enlarged the primary cells, leading to their interactions and the generation of secondary cells. When Rp was sufficiently higher, the further interactions among primary and secondary cells could cause the generation of tertiary cell. Orderly interactions of shock cells were observed with increasing Rp, that is, the interactions initially occur between the adjacent jets with smaller distances, and then other jets were involved. Based on the results: It was found that the compression-expansion chains caused the low- Rp flash-boiling spray collapse in the far field; With the increase in Rp, the shock waves and shock-to-shock interaction become the main contributor to spray collapse, leading collapse appearing in the near-nozzle region.

Investigation into shock-to-shock interactions induced by flash boiling and the impact on spray behaviors

Through recent numerical studies, the shock-to-shock interactions between flash-boiling jets are believed to be the reason for multi-jet spray collapse in gasoline direct injection (GDI) engines. But this sort of interaction was not well understood through the experimental investigation. In the present study, a two-hole GDI injector was used to examine this process via optical diagnostics and CFD simulations. With the increase in superheat level (Rp), the shock waves (primary cells) in individual jets became observable. Further increase in Rp could cause shock-to-shock interactions, and a secondary cell could be formulated between two primary cells. The different intensity of shock-to-shock interactions would result in different jet-to-jet interaction behaviors. In the moderate interaction region, the secondary jet appeared inside and downstream of the secondary cell, but the sprays did not collapse. In the highly interaction region, both the secondary jet and spray collapse could be observed, where the secondary jet would laterally develop perpendicular with the plane of the two jets. The detailed numerical results showed that the secondary cell was small in the moderate interaction region, but became much larger in the highly interaction region. The occurrence of secondary cell directed the fuel from primary cells into the secondary cell, contributing to the generation of secondary jet. After flowing out of the secondary cell, the incipient secondary jet could subsequently entrain the nearby low-velocity fuel. Further, the strong shock-to-shock interactions would alter the shape of primary cells, reducing the transverse velocity component of jets and promoting collapse. Based on the experimental and numerical results, the existence of shock-to-shock interactions and its important role in jet-to-jet interactions and spray collapse were fully illustrated, and this understanding would be of help in nozzle optimization.

Simulation of high pressure, direct injection processes of gaseous fuels by a density-based OpenFOAM solver

The direct injection of a gaseous fuel in internal combustion engines involves under-expanded supersonic jets and complex air/fuel fluid-dynamics. Furthermore, with the high pressure ratios between the injector and the cylinder, the gaseous flow usually becomes choked even inside the injector. Knowledge of all these phenomena is essential to achieve a deeper understanding of the air–fuel mixing process that follows, influencing combustion and pollutant formation. In this framework, this study deals with the development and validation of a fully explicit, density-based solver for supersonic compressible flows, using the OpenFOAM library and featuring Runge–kutta fourth order time discretization and the Kurganov central flux splitting scheme. This methodology was applied to analyze the inner and the external flow of an innovative, multi-hole, high pressure injector for heavy-duty vehicle applications. The adoption of multi-hole patterned injectors in gaseous fuel combustion systems is believed to be an efficient way of achieving a better air/fuel mixture and, therefore, improving the combustion reaction. The present work aims to evaluate the reliability of the aforementioned mathematical approach for such kinds of complex flows and, especially, provide a comprehensive characterization of the multi-jet spray. It was found that shock waves in the internal-nozzle deeply modify the flow development and the external Mach disk as shock cells move the mixing activity on a lateral shear layer. It was also observed that a methane cloud grows downstream and, although flammable conditions are present, it later inhibits air recirculation toward the near nozzle zone.

Morphology analysis on the effect of injection pressure on jet trajectory deviation of multi-jet sprays

Fuel spray behaviors play an important role in mixture formation, efficiency and emission of internal combustion engines. In this study, the effect of injection pressure on spray jet trajectory was investigated by using an asymmetric five-hole GDI injector. With schlieren imaging method, the ethanol spray development was captured under superheat degrees from 0.46 to 9.48, and the ambient pressure was fixed at 101.3 kPa. For the non-flash-boiling cases, the jet trajectory gradually deviated inward in the far field with the increase of injection pressure. For the flash boiling cases, the jet trajectory near the nozzle exit deviated outward under the relatively low injection pressures and deviated inwards with the increase of injection pressure. The outward deviation under relatively low injection pressure was attributed to the non-uniform vapor formation inside the nozzle orifice. The disappearance of near-field deviation under flash boiling conditions under high injection pressures could be attributed to the reduced time for nucleation and bubble growth inside the nozzle. Additionally, under both non-flash-boiling and flash-boiling conditions, the far-field deviation under high injection pressures could be attributed to the increased jet speed and the stronger jet-to-jet interaction. Compared with non-flash-boiling cases, the far field deviation under flash boiling conditions were more intense because of the enhanced atomization and it is believed that the smaller droplets were more inclined to move inwards. The remarkable effect of injection pressure on jet interaction of flash boiling sprays shows the potential of flexible jet targeting control at different engine loads.

Radial expansion of flash boiling jet and its relationship with spray collapse in gasoline direct injection engine

In order to mitigate spray collapse and fully utilize the advantage of flash boiling injection in gasoline direct injection (GDI) engines, it is important to understand the flashing jet behavior and its relationship with spray collapse. In this study, n-hexane sprays discharged from a modified single-hole injector and the original five-hole injector were studied. The tests were carried out in a constant volume vessel with injection temperatures from 30 to 130 °C and ambient pressures (Pamb) from 20 to 101 kPa. By analyzing the relationship between the jet width and different parameters including superheat level, nucleation rate, and chemical potential of phase change (Δμ), strong correlation was found between the jet width and Δμ·Pamb−0.5, indicating the radial expansion of flashing jets was determined by chemical potential of phase change and ambient resistance. Beyond 20d0, the correlation was gradually weakened along the axis, as flash boiling process was finished. Besides, it was found that the extent of single-jet radial expansion was positively related to that of multi-jet spray collapse in the transitional collapse region. Therefore, to mitigate spray collapse, it is necessary to restrict the radial jet expansion, and generating external flashing jets is proposed as a practical approach to realize it.

A compact refrigeration system based on multijet sprays for electronics thermal management

The present study integrates a spray impingement heat sink and a compact vapor compression cooling system driven by a R-134a small-scale oil-free linear compressor. The cooling strategy is based on the impact of multiple cylindrical jets at a single point to create a low-pressure spray that impinges on a heated surface. An experimental performance evaluation is presented focusing on the influence of the applied thermal load, orifice-to-surface distance (jet chamber height), number of colliding jets and hot reservoir temperature. For a hot reservoir temperature of 25 °C, the heat sink configuration with three colliding jets and a jet chamber height of 23.8 mm sustained imposed thermal loads as high as 225 W (heat flux of 354.7 kW/m2), maintaining the impingement surface temperature below 40 °C with a refrigerant mass flow rate measured at approximately 5 kg/h. The resulting average heat transfer coefficients ranged from 7.5 to 18.1 kW/(m2 K), and calculated vapor mass qualities as high as 0.93 have been achieved at the outlet of the cooling unit. High-speed video sequences showing the interaction between the impinging spray and the heated surface were collected to help illustrate the phase change heat transfer mechanisms and the influence of the independent variables.

Large Eddy Simulation of GDI Single-hole and Multi-hole Injector Sprays with Comparison of Numerical Break-up Models and Coefficients

In the present study the fuel spray of a gasoline direct injected engine with multi-hole injector is simulated. Simulation inputs data, injection flow rate and spray cone angle are obtained from previous experimental studies. Log-normal distribution with different standard deviation is used for initial droplet size as the primary break-up model in order to reach the agreement between experimental and calculated spray tip penetration. As the first step, only one plume of spray injected into a quiescent air environment is simulated and validated by varying break-up model and standard deviation. Then, with coefficient obtained from the single jet simulation all six spray jets are simulated based on the injector nozzles geometry. The comparison between single-jet simulation and multi-jet simulation shows that validated model coefficients for the single-jet spray cannot be used for multi-jet spray simulation without significant modifications due to adjacent jet interaction and pressure drag. A set of new coefficients for the multi-jet spray is presented

Intermittent multijet sprays for improving mixture preparation with low-pressure injection systems

In this work, the characteristics of droplets produced by a multijet impingement atomization process are measured with a Phase-Doppler Interferometer and statistically described using finite mixtures of weighted probability density functions. Through this statistical tool, drop size and axial velocity distributions are involved in the physical interpretation of the flow, instead of limiting it to first- and second-order distribution moments. Each group of droplets with similar size characteristics has been modeled by lognormal distributions and normal distributions relatively to drop axial velocity. The analysis based on finite mixtures identified three groups of droplets with similar size characteristics, although the group with smaller sizes is negligibly represented in the statistical finite mixture. Also, the lognormal standard deviation in all groups is well correlated with the corresponding geometric mean diameter allowing to simplify the description of the spray. In terms of axial velocity, mainly one distribution has been identified with a relatively constant standard deviation, and a characteristic velocity slightly dependent on the duty cycle associated with the spray intermittent condition. Furthermore, droplets characteristics are correlated with the heat transfer rate obtained for several operating conditions that maintain the surface temperature in steady-state at 125 °C. The effect of the time between consecutive injections is analyzed. Concerning the potential use of multijet impingement sprays for fuel injection systems, results evidence the importance of an interaction between thin liquid film heat transfer and droplets axial velocity for enhancing heat transfer and promote evaporation. This would decrease the amount of fuel deposited on interposed surfaces, thus, improving mixture preparation in low-pressure injection systems for internal combustion engines.

Microprocessor cooling based on an intermittent multijet spray system

The power dissipation of electronic devices is reaching the limit of the cooling capacity of current heatsinks, demanding new developments in cooling technologies. Moreover, the dissipated power varies with time, reducing the efficiency of the cooling system, unless an Intelligent Thermal Management is applied. In this paper, a control strategy is devised based on the Intermittent Spray Cooling concept. A multijet atomization defined by the simultaneous impact of two jets, together with an electromechanical valve, produce an intermittent spray made of small and slow droplets, which are likely to deposit on the surface for cooling purposes. The multijet spray cooling is characterized in the transition region between non-boiling and boiling regimes. The experiments consider a maximum imposed heat dissipation of 100W in a 6.25cm2 copper plate. While the Duty Cycle (the percentage of liquid injection within the cycle time) is known as the key parameter for controlling heat transfer in Intermittent Spray Cooling, optimal cooling conditions are obtained for high injection frequencies, up to 20Hz in the present work. Also, the boiling curves presented evidence how an Intermittent Spray Cooling is able to benefit from phase-change in the non-boiling regime enhancing the evaporation in the non-injection time between consecutive cycles. The transient behaviour of the cooling process is studied through the assessment of the response time to an overshoot situation. Finally, the Intermittent Spray Cooling control strategy devised is applied to a typical processor power profile with constant and variable injection conditions, for a maximum and controllable project temperature. The results presented demonstrate how the control of the cooling process through the spray intermittency leads to an increase of the efficiency and liquid savings, demonstrating the feasibility and advantage of implementing Intermittent Spray Cooling for developing Intelligent Thermal Management.

Transient analysis of intermittent multijet sprays

This paper analyzes the transient characteristics of intermittent sprays produced by the single-point impact of multiple cylindrical jets. The aim is to perform a transient analysis of the intermittent atomization process to study the effect of varying the number of impinging jets in the hydrodynamic mechanisms of droplet formation. The results evidence that hydrodynamic mechanisms underlying the physics of ligament fragmentation in 2-impinging jets sprays also apply to sprays produced with more than 2 jets during the main period of injection. Ligaments detaching from the liquid sheet, as well as from its bounding rim, have been identified and associated with distinct droplet clusters, which become more evident as the number of impinging jets increases. Droplets produced by detached ligaments constitute the main spray, and their axial velocity becomes more uniformly distributed with 4-impinging jets because of a delayed ligament fragmentation. Multijet spray dispersion patterns are geometric depending on the number of impinging jets. Finally, an analysis on the Weber number of droplets suggests that multijet sprays are more likely to deposit on interposed surfaces, thus becoming a promising and competitive atomization solution for improving spray cooling.

High-power electronics thermal management with intermittent multijet sprays

Abstract Thermal management plays a crucial role in the development of high-power electronics devices, e.g. in electric vehicles. The greatest energy demands occur during power peaks, implying dynamic thermal losses within the vehicle’s driving cycle. Therefore, the need for devising intelligent thermal management systems able to efficiently respond to these power peaks has become a technological challenge. Experiments have been performed with methanol in order to quantify the maximum heat flux removed by a multijet spray to keep the 4 cm2 surface temperature stabilized and below the threshold of 125 °C. A multijet atomization strategy consists in producing a spray through the multiple and simultaneous impact of Nj cylindrical jets. Moreover, the spray intermittency is expressed through the duty cycle (DC), which depends on the frequency and duration of injection. Results evidence that: i) a shorter time between consecutive injection cycles enables a better distribution of the mass flow rate, resulting in larger heat transfer coefficient values, as well as higher cooling efficiencies; ii) compared with continuous sprays, the analysis evidences that an intermittent spray allows benefiting more from phase-change convection. Moreover, the mass flux is mainly affecting heat transfer rather than differences induced in the spray structure by using different multijet configurations.