Compared with port fuel injection engines, direct injection(DI) gasoline engine is becoming the mainstream of gasoline engines because of its higher fuel economy and excellent transient response. It has been proven th...Compared with port fuel injection engines, direct injection(DI) gasoline engine is becoming the mainstream of gasoline engines because of its higher fuel economy and excellent transient response. It has been proven that fuel spray characteristics in DI engines are crucial to the performance and emission quality of the engine. Flash boiling spray has great potential to achieve high fuel economy and low emission by dramatically improving the fuel atomization and vaporization and it has different spray-air interaction behavior as compared with non-flash boiling one, while its mechanism is more complex as compared with subcooled spray. We investigate the time-resolved spatial velocity field of the spray using 2-camera high-speed 3 D3 C(3-dimension 3-component)tomographic particle image velocimetry(PIV) diagnostic technique. A 10 mm thick laser sheet is used to illuminate the fuel spray. Characteristics of both non-flash and flash boiling sprays are studied. A single-hole injector is mounted within a heat exchanger so that different fuel temperature can be accessed. In the experiment, n-pentane is used as the fuel. For the non-flash boiling spray, the velocity field of the liquid spray is mostly consistent to the injection direction. With the increase of the degree of superheat(Do S), the overall velocity scale decreases especially at the spray tip. Meanwhile, larger swirls occur at the lower part of the flash boiling spray, which means stronger spray-air interaction occurs at a higher Do S.展开更多
Flash boiling atomization(FBA)is a promising approach for enhancing spray atomization,which can generate a fine and more evenly distributed spray by increasing the fuel injection temperature or reducing the ambient pr...Flash boiling atomization(FBA)is a promising approach for enhancing spray atomization,which can generate a fine and more evenly distributed spray by increasing the fuel injection temperature or reducing the ambient pressure.However,when the outlet speed of the nozzle exceeds 400 m/s,investigating high-speed flash boiling atomization(HFBA)becomes quite challenging.This difficulty arises fromthe involvement ofmany complex physical processes and the requirement for a very fine mesh in numerical simulations.In this study,an HFBA model for gasoline direct injection(GDI)is established.This model incorporates primary and secondary atomization,as well as vaporization and boilingmodels,to describe the development process of the flash boiling spray.Compared to lowspeed FBA,these physical processes significantly impact HFBA.In this model,the Eulerian description is utilized for modeling the gas,and the Lagrangian description is applied to model the droplets,which effectively captures the movement of the droplets and avoids excessive mesh in the Eulerian coordinates.Under various conditions,numerical solutions of the Sauter mean diameter(SMD)for GDI show good agreement with experimental data,validating the proposed model’s performance.Simulations based on this HFBA model investigate the influences of fuel injection temperature and ambient pressure on the atomization process.Numerical analyses of the velocity field,temperature field,vapor mass fraction distribution,particle size distribution,and spray penetration length under different superheat degrees reveal that high injection temperature or low ambient pressure significantly affects the formation of small and dispersed droplet distribution.This effect is conducive to the refinement of spray particles and enhances atomization.展开更多
The high-speed digital imaging technique is applied to observe the developing process of flash boiling spray of dimethyl ether at low ambient pressure, and the effects of nozzle opening pressure and nozzle hole diamet...The high-speed digital imaging technique is applied to observe the developing process of flash boiling spray of dimethyl ether at low ambient pressure, and the effects of nozzle opening pressure and nozzle hole diameter on the spray shape, spray tip penetration and spray angle during the injection are investigated. The experimental results show that the time when the vortex ring structure of flash boiling spray forms and its developing process are determined by the combined action of the bubble growth and breakup in the spray and the air drag on the leading end of spray; with the enhancement of nozzle opening pressure, the spray tip penetration increases and the spray angle decreases. The influence of nozzle hole diameter on the spray tip penetration is relatively complicated, the spray tip penetration is longer with a smaller nozzle hole diameter at the early stage of injection, while the situation is just opposite at the later stage of injection. This paper establishes that the variation of spray angle is consistent with that of nozzle hole diameter.展开更多
Spray atomization of liquid fuel plays an important role in droplet evaporation,combustible mixture formation and subsequent combustion process.Well-atomized liquid spray contributes to high fuel efficiency and low po...Spray atomization of liquid fuel plays an important role in droplet evaporation,combustible mixture formation and subsequent combustion process.Well-atomized liquid spray contributes to high fuel efficiency and low pollutant emissions.Gasoline direct injection(GDI)has been recognized as one of the most effective ways to improve fuel atomization.As a special direct injection method,the air-assisted direct injection utilizes high-speed flow of high-pressure air at the injector exit to assist liquid fuel injection and promote spray atomization at a low injection pressure.This injection method has excellent application potential and advantages for high performance and lightweight engines.In this study,the hollow cone spray emerging from an air-assisted injector was studied in a constant volume chamber with the ambient pressures ranging from 5 kPa to 300 kPa.External macro characteristics of spray were obtained using high speed backlit imaging.Phase Doppler particle analyzer(PDPA)was utilized to study the microcosmic spray characteristics.The results show that under the flash boiling condition,the spray will generate a strong flash boiling point which causes the cone shape spray to expand both inwards and outwards.The axisymmetric inward expansion would converge together and form a lathy aggregation area below the nozzle and the axisymmetric outward expansion greatly increases the spray width.The sauter mean diameter(SMD)of flash boiling condition can be reduced to 5μm compared to the level close to 10μm in the non-flash boiling condition.展开更多
Ammonia is a suitable carbon-free alternative fuel for power equipment.Direct combustion of liquid ammonia has the potential to reduce system costs and heat loss of gas turbine(GT).However,its tendency to flash and th...Ammonia is a suitable carbon-free alternative fuel for power equipment.Direct combustion of liquid ammonia has the potential to reduce system costs and heat loss of gas turbine(GT).However,its tendency to flash and the high latent heat of vaporization can lead to combustion deterioration.Previous research suggests that stabilizing a liquid ammonia flame requires swirling and preheated air.So far,the influencemechanism of preheated air on liquid ammonia swirl spray remains inadequately explored.To fill this research gap,this study conducted a large eddy simulation(LES)to investigate the effect of preheated air temperature(T_(a))on a liquid ammonia flash spray in a swirl combustor.The influence of T_(a) on the spray morphology and the axial velocity,diameter,and temperature distributions of the droplets were investigated to understand the spray characteristics.Besides,the effects of T_(a) on the evaporation characteristics,the properties,and the possible ignition performance of themixture were studied.The results show that with the increase of T_(a),the heating capacity of air is enhanced,leading to a greater proportion of droplets reaching flash boiling conditions.This greatly optimizes the evaporation process,resulting in more complete evaporation and significantly smaller volume.The bulk air flow velocity is increased,causing the expansion of the inner recirculation zone(IRZ),and the gaseous temperature and mixture concentration distribution are optimized.In addition,the low gaseous ammonia concentration makes ignition difficulty at T_(a)=300 K.The high|τ|value(τis the shear stress)and large inner recirculation zone area lead to a larger RegionM and a smaller RegionL at T_(a)=300 K compared to the case of T_(a)=500 K.展开更多
基金the National Natural Science Foundation of China(No.51376119)
文摘Compared with port fuel injection engines, direct injection(DI) gasoline engine is becoming the mainstream of gasoline engines because of its higher fuel economy and excellent transient response. It has been proven that fuel spray characteristics in DI engines are crucial to the performance and emission quality of the engine. Flash boiling spray has great potential to achieve high fuel economy and low emission by dramatically improving the fuel atomization and vaporization and it has different spray-air interaction behavior as compared with non-flash boiling one, while its mechanism is more complex as compared with subcooled spray. We investigate the time-resolved spatial velocity field of the spray using 2-camera high-speed 3 D3 C(3-dimension 3-component)tomographic particle image velocimetry(PIV) diagnostic technique. A 10 mm thick laser sheet is used to illuminate the fuel spray. Characteristics of both non-flash and flash boiling sprays are studied. A single-hole injector is mounted within a heat exchanger so that different fuel temperature can be accessed. In the experiment, n-pentane is used as the fuel. For the non-flash boiling spray, the velocity field of the liquid spray is mostly consistent to the injection direction. With the increase of the degree of superheat(Do S), the overall velocity scale decreases especially at the spray tip. Meanwhile, larger swirls occur at the lower part of the flash boiling spray, which means stronger spray-air interaction occurs at a higher Do S.
基金supported by the National Natural Science Foundation of China(Project Nos.12272270,11972261).
文摘Flash boiling atomization(FBA)is a promising approach for enhancing spray atomization,which can generate a fine and more evenly distributed spray by increasing the fuel injection temperature or reducing the ambient pressure.However,when the outlet speed of the nozzle exceeds 400 m/s,investigating high-speed flash boiling atomization(HFBA)becomes quite challenging.This difficulty arises fromthe involvement ofmany complex physical processes and the requirement for a very fine mesh in numerical simulations.In this study,an HFBA model for gasoline direct injection(GDI)is established.This model incorporates primary and secondary atomization,as well as vaporization and boilingmodels,to describe the development process of the flash boiling spray.Compared to lowspeed FBA,these physical processes significantly impact HFBA.In this model,the Eulerian description is utilized for modeling the gas,and the Lagrangian description is applied to model the droplets,which effectively captures the movement of the droplets and avoids excessive mesh in the Eulerian coordinates.Under various conditions,numerical solutions of the Sauter mean diameter(SMD)for GDI show good agreement with experimental data,validating the proposed model’s performance.Simulations based on this HFBA model investigate the influences of fuel injection temperature and ambient pressure on the atomization process.Numerical analyses of the velocity field,temperature field,vapor mass fraction distribution,particle size distribution,and spray penetration length under different superheat degrees reveal that high injection temperature or low ambient pressure significantly affects the formation of small and dispersed droplet distribution.This effect is conducive to the refinement of spray particles and enhances atomization.
文摘The high-speed digital imaging technique is applied to observe the developing process of flash boiling spray of dimethyl ether at low ambient pressure, and the effects of nozzle opening pressure and nozzle hole diameter on the spray shape, spray tip penetration and spray angle during the injection are investigated. The experimental results show that the time when the vortex ring structure of flash boiling spray forms and its developing process are determined by the combined action of the bubble growth and breakup in the spray and the air drag on the leading end of spray; with the enhancement of nozzle opening pressure, the spray tip penetration increases and the spray angle decreases. The influence of nozzle hole diameter on the spray tip penetration is relatively complicated, the spray tip penetration is longer with a smaller nozzle hole diameter at the early stage of injection, while the situation is just opposite at the later stage of injection. This paper establishes that the variation of spray angle is consistent with that of nozzle hole diameter.
基金Supported by Beijing Institute of Technology Research Fund Program for Young Scholars(2019CX04-031)Foundation Research Funds of Ministry of Industry and Information Technology(JCKY2019602D018)。
文摘Spray atomization of liquid fuel plays an important role in droplet evaporation,combustible mixture formation and subsequent combustion process.Well-atomized liquid spray contributes to high fuel efficiency and low pollutant emissions.Gasoline direct injection(GDI)has been recognized as one of the most effective ways to improve fuel atomization.As a special direct injection method,the air-assisted direct injection utilizes high-speed flow of high-pressure air at the injector exit to assist liquid fuel injection and promote spray atomization at a low injection pressure.This injection method has excellent application potential and advantages for high performance and lightweight engines.In this study,the hollow cone spray emerging from an air-assisted injector was studied in a constant volume chamber with the ambient pressures ranging from 5 kPa to 300 kPa.External macro characteristics of spray were obtained using high speed backlit imaging.Phase Doppler particle analyzer(PDPA)was utilized to study the microcosmic spray characteristics.The results show that under the flash boiling condition,the spray will generate a strong flash boiling point which causes the cone shape spray to expand both inwards and outwards.The axisymmetric inward expansion would converge together and form a lathy aggregation area below the nozzle and the axisymmetric outward expansion greatly increases the spray width.The sauter mean diameter(SMD)of flash boiling condition can be reduced to 5μm compared to the level close to 10μm in the non-flash boiling condition.
基金National Natural Science Foundation of China,Grant/Award Number:52176130Open Research Fund of Beijing Key Laboratory of Powertrain for New Energy Vehicle,Beijing Jiaotong University。
文摘Ammonia is a suitable carbon-free alternative fuel for power equipment.Direct combustion of liquid ammonia has the potential to reduce system costs and heat loss of gas turbine(GT).However,its tendency to flash and the high latent heat of vaporization can lead to combustion deterioration.Previous research suggests that stabilizing a liquid ammonia flame requires swirling and preheated air.So far,the influencemechanism of preheated air on liquid ammonia swirl spray remains inadequately explored.To fill this research gap,this study conducted a large eddy simulation(LES)to investigate the effect of preheated air temperature(T_(a))on a liquid ammonia flash spray in a swirl combustor.The influence of T_(a) on the spray morphology and the axial velocity,diameter,and temperature distributions of the droplets were investigated to understand the spray characteristics.Besides,the effects of T_(a) on the evaporation characteristics,the properties,and the possible ignition performance of themixture were studied.The results show that with the increase of T_(a),the heating capacity of air is enhanced,leading to a greater proportion of droplets reaching flash boiling conditions.This greatly optimizes the evaporation process,resulting in more complete evaporation and significantly smaller volume.The bulk air flow velocity is increased,causing the expansion of the inner recirculation zone(IRZ),and the gaseous temperature and mixture concentration distribution are optimized.In addition,the low gaseous ammonia concentration makes ignition difficulty at T_(a)=300 K.The high|τ|value(τis the shear stress)and large inner recirculation zone area lead to a larger RegionM and a smaller RegionL at T_(a)=300 K compared to the case of T_(a)=500 K.
