Spray Evaporation Research Articles

Numerical study of spray cooling with flash evaporation

This work aims to apply Computational Fluid Dynamic (CFD) method to establish a flash evaporation spray cooling (FESC) model to simulate the heating process and find the optimum cooling performance. The heat transfer process during FESC is studied through numerical simulation using commercial code ANSYS FLUENT. The species transport model and the discrete phase model are applied to simulate the multiphase flow and heat transfer process. The turbulence effect is included. The effects of flow rate, nozzle pressure, nozzle angle, and the nozzle orifice size on spray cooling are investigated through analyzing the final surface temperature distributions. This work revealed the mechanism of the heat transfer process in FESC by means of particle tracks and velocity magnitude distribution. The simulated results for the effect of flow rate were compared with other researchers’ previous published experimental results. The comparison shows same trend, which verified the model and the simulation result. The optimum cooling performance is found by analyzing various working conditions. The results show that high flow rate, high nozzle pressure, small nozzle angle and small nozzle orifice can improve the FESC characteristics. The detailed mechanisms of these effects under various working conditions are also discussed. Under giving working conditions, the optimum cooling performance is obtained for the condition where mass flow rate of working fluid is 2L/min, the nozzle pressure is 100MPa, the nozzle angle is 15 degrees and the orifice size of the nozzle is 1mm.

A novel approach of water injection optimization applied in gasoline engines: An investigation on heated water injection

Water injection technique is a promising solution for accurate combustion control, but oil emulsion obstacle its commercial application. The present study proposes a practical approach for optimizing water spray atomization and evaporation, in which water is heated through exhaust gas recovery and directly injected into combustion chamber with a flash boiling. Firstly, a heated water injection (HWI) system was designed and established. Then, experiments were conducted to explore the effects of HWI parameters on engine combustion and emissions characteristics in terms of water injection strategies. The results present that HWI increase water evaporation rate and improves combustion efficiency. The HWI lowers flame propagation speed and improves specific heat ratio of air–fuel mixture. Elevated injection temperature lowers duration of flame kernel formation, which shortens ignition delay prolonged under conventional water injection. CA10 is advanced 4.5°CA under same spark advance while compared to conventional water injection. Moreover, HWI can not only reduce production of NOX and CO emissions but also effectively lower HC emissions by 26.2%, as water injection temperature increased to 205℃. However, excessive water injection mass and inappropriate water injection timing may worsen ignition and flame propagation, increasing unburned loss and misfire probability. The present results provide considerable evidence for the application of HWI within modern gasoline engines.

Schlieren and mie-scattering visualization of an evaporating spray under marine diesel engine conditions

Due to the critical influence on combustion, extensive research has been carried out to deepen the understanding of diesel spray evolution. However, for the greater share of land-based diesel engines, most of the studies have focused on the injectors with small nozzle diameter and rare studies can be found on marine diesel engines, which has a larger mass flow rate and longer injection duration with large nozzle diameter. In this study, the influence of injection pressure, ambient temperature, ambient density and nozzle diameter on spray morphology for medium speed marine diesel engine injectors (with nozzle diameter up to 0.53 mm) were investigated under various non-reacting but evaporative conditions. The experiments were performed via a highly enhanced constant volume chamber (highest temperature of 900 K and highest pressure of 8 MPa) with an optical window diameter of 205 mm. Liquid and vapor penetration of sprays were recorded by Mie-scattering and schlieren optical configurations respectively. Results show that the vapor penetration velocity is mainly affected by injection pressure. Ambient temperature has the most significant effect on liquid penetration length, but less effect can be found on the vapor penetration velocity. Given the obvious deviation from heavy duty engine sprays, the empirical correlations for liquid penetration length and vapor spray evolution are updated to fit the spray characteristics of medium speed marine diesel engine injectors, which is helpful to understand the spray behavior for large nozzle diameter injector under marine diesel engine conditions.

Inorganic geochemistry of saline substrates associated with coastal turf, Otago Peninsula, New Zealand

ABSTRACT Halophytic plant communities have developed on exposed coastal landforms to form low-stature turf in several locations around New Zealand. We quantify the inorganic geochemistry, mineralogy and simulated ephemeral surface water composition of saline soil-free bare ground at a well-preserved site at Maori Head on Otago Peninsula, where halophytes have colonised from adjacent coastal turf. The community at this site is dominated by Leptinella dioica, Samolus repens, Disphyma australe and Selliera radicans. Maori Head substrates are dominated by clay-altered basalt colluvium and show a general trend from high electrical conductivity (EC > 10 mS/cm) and weakly acid pH (< 6) on bare ground to low EC (< 1 mS cm−1) and higher pH (> 7) in turf and adjacent pasture soils. High salinities on dry substrates and in simulated ephemeral surface waters result from surface evaporation of airborne sea spray and contain halite (NaCl) with some gypsum (CaSO4.2H2O). Chemical interactions with the substrate are minor. The high surface salinities were maintained despite a major winter rain event with strong onshore winds. However, surface salinity is frequently leached by rain and shallow groundwater, whereas inland saline sites (pH 8–11) have substantial salt accumulations driven by lower rainfall, high evaporation and chemical interaction with Otago Schist-derived substrates.

Investigation on diesel spray flame evolution and its conceptual model for large nozzle and high-density of ambient gas

To reveal the theory behind the ignition phenomenon of large nozzle diameter spray and better organize the combustion of two-stroke marine diesel engine, in this article, the numerical simulation has been made by CONVERGE to study the developing process of large nozzle spray and flame in constant volume combustion chamber. Spray and flame characteristics for different nozzle diameters 0.5 mm, 0.625 mm and 0.875 mm are compared to investigate the effect of nozzle diameter on spray and flame characteristics. Then the ambient pressure is increased and a swirl is added to discuss the spray and flame characteristics under high-density of ambient gas and swirl. It was shown that for large nozzle led spray heads, there would be several stripping processes of the spray head periphery under air-dragging work. The first stripped fuel remains on the periphery of the spray upstream, evaporating and igniting. The flame propagates through the stripped fuel evaporation path from the ignition position to the spray tip. Flame propagation stops after nearly reaching the spray tip due to the lack of the combustible mixture but the presence of unevaporated liquid fuel, which was caused by the large nozzle diameter slowing down the spray tip evaporation. The liquid fuel at the spray tip then evaporates and mixes with the air. Then flame begins to propagate through the vapor fuel path, enveloping the entire spray. Increasing ambient pressure accelerates spray break-up and evaporation. The flame can directly envelop the spray through the combustible mixture in the spray tip. Adding swirl pushes the separated swirl-facing side-periphery fuel to the spray tip. Ignition occurs in the spray tip and mid-part of spray leeside then flame propagates in the direction of each other direction.

Analysis of Spray Evaporation in a Model Evaporating Chamber: Effect of Air Swirl

Analysis of Spray Evaporation in a Model Evaporating Chamber: Effect of Air Swirl

STUDY ON THE EFFECT OF THE PRESENCE OF FUEL VAPOR IN THE SURROUNDING GAS ON THE MIXING CONTROLLED VAPORIZATION OF FUEL SPRAY

In different combustion devices, liquid fuel is injected inside the combustion chamber. The liquid fuel spray atomizes, vaporizes, and mixes with the surrounding gas before combustion. The vaporization rate of the injected fuel is very important because it controls the air-fuel mixing and finally the combustion rate. Similar to droplet vaporization, the concentration of fuel vapor in the surrounding gas influences the spray vaporization process. It changes the maximum liquid phase penetration or liquid length (LL) of a vaporizing spray. Hence the liquid length model should consider the ambient fuel concentration effect, but has been ignored up until now. The present work studies the liquid lengths of vaporizing sprays at different fuel vapor concentrations in the surrounding gas. The effects of ambient fuel concentration on heating and vaporization of the spray are analyzed in the context of a mixing controlled vaporization model. The results shows that the fuel concentration in the surrounding gas changes the maximum liquid phase fuel temperature and the specific energy required for vaporization. It is observed that as the ambient fuel concentration effect is ignored in the existing mixing controlled model, it fails to predict liquid lengths when the surrounding gas is mixed with fuel vapor. A modified model is presented where the mass fraction of fuel vapor in the surrounding gas is included in the model. The modified model is able to predict the LL values more accurately when the surrounding gas is mixed with fuel vapor.

Conditional Generative Adversarial Networks for modelling fuel sprays

In this study, the probabilistic, data driven nature of the generative adversarial neural networks (GANs) was utilized to conduct virtual spray simulations for conditions relevant to aero engine combustors. The model consists of two sub-modules: (i) an autoencoder converting the variable length droplet trajectories into fixed length, lower dimensional representations and (ii) a Wasserstein GAN that learns to mimic the latent representations of the evaporating droplets along their lifetime. The GAN module was also conditioned with the injection location and the diameters of the droplets to increase the generalizability of the whole framework. The training data was provided from highly resolved 3D, transient Eulerian–Lagrangian, large eddy simulations conducted with OpenFOAM. Neural network models were created and trained within the open source machine learning framework of PyTorch. Predictive capabilities of the proposed method was discussed with respect to spray statistics and evaporation dynamics. Results show that conditioned GAN models offer a great potential as low order model approximations with high computational efficiency. Nonetheless, the capabilities of the autoencoder module to preserve local dependencies should be improved to realize this potential. For the current case study, the custom model architecture was capable of conducting the simulation in the order of seconds after a day of training, which had taken one week on HPC with the conventional CFD approach for the same number of droplets (200,000 trajectories).

End-to-end modeling of fuel injection via static coupling of internal flow and ensuing spray

AbstractAccurate knowledge of fuel spray behavior is of utmost importance for liquid-fuel-based combustion systems. Fuel properties, injector geometry, operating conditions, and thermal state of the combustion chamber determine the fuel’s ability to mix and burn efficiently. Three-dimensional computational-fluid-dynamics models can reveal the complex dynamics of the injector’s internal flow, as well as the spray breakup, evaporation, mixing, and combustion. However, time and length scales of in-nozzle flow and ensuing spray can differ by several orders of magnitude, limiting the feasibility of a simultaneous solution of the entire chain of physics. This work explores an end-to-end approach to decouple the problem at the injector outlet via a static-coupling framework. Flowfields are sampled at the injector exit, stored into spatiotemporally resolved maps, and used to initialize a Lagrangian spray whose properties reflect the flow’s instantaneous state as predicted by the in-nozzle flow simulations. Comparisons against typical rate-of-injection results and qualitative validation against optical spray data highlighted the ability of static coupling to unveil spray physics that would otherwise be missed.

Statistical modeling of spray formation, combustion, and evaporation of liquid fuel droplets

This article is devoted to computer simulation of the atomization, combustion, and evaporation processes of liquid fuel drops (heptane) at high turbulence. The paper describes the main characteristics and methods of atomization of liquid fuels and shows the types of special devices used in fuel supply injection systems. As proposed in the paper mathematical model of the combustion of liquid fuel injections at high turbulence consists of the laws of conservation of mass, momentum, transfer of kinetic energy, and conservation concentration of the mixture components. Also, when setting up computational experiments, the chemical model of reacting systems, the turbulence model, and the equations describing evaporation, heat transfer, and interaction with the gaseous medium were taken into account. As a result of the computational experiments performed to study the liquid fuel droplets’ injection rate influence on the processes of its spraying and combustion, the optimal combustion mode was established. The obtained data that determine the optimal mode are confirmed by a graphical interpretation of the distribution of temperature fields, the number of combustion products, and particles along the height of the model combustion chamber. These results will contribute to a deep study and construction of the liquid fuels’ combustion theory, and an understanding of complex thermal and physical phenomena in combustion chambers

A preliminary computational analysis towards the use of Electrically Heated Mixing Catalyst for innovative SCR after-treatment systems

This work aims at investigating the possible advantages of substituting the mixer in traditional SCR systems with an Electrically Heated Catalytic structure (EHC). First of all, EHC technology is being widely investigated in literature because it offers a concrete solution for catalyst thermal activation and film formation reduction in engine cold start conditions; however, its adoption as a mixer can also guarantee other important improvements in reducing pollutant emissions. In this work, a low-pressure injection of Ad-Blue impacts an electrically heated structure and a complete analysis of liquid droplets and film evaporation is carried out. A hybrid Eulerian-Lagrangian model has been adopted on a multi-region configuration, accounting for fluid-solid conjugate heat transfer (CHT), which plays a key role in the conversion strategy: it has been demonstrated that the heated structure can be exploited to significantly increase the exhaust gas enthalpy in the cold start, which represents an important improvement for pollutant conversion. Different heating strategies are analysed, with the objective of maximising the spray evaporation and the uniformity of the ammonia distribution downstream of the mixer. The objectives of this work are emphasizing the improvements that an electrically heated mixing catalyst can bring to the traditional SCR configuration, laying the foundation for other following studies.

Effects of nozzle diameter on the characteristic time scales of diesel spray two-stage ignition under cold-start conditions

Diesel engine cold-start performances are closely related to ignition delay time which in turn is strongly influenced by the nozzle diameter (Dnoz). In this study, ignition delay time and ignition location of No. −50 diesel spray at different Dnoz are investigated by high-speed direct photography in a constant volume combustion chamber (CVCC) under cold-start conditions. The experimental results clarify the non-monotonic variation of the high-temperature ignition delay time (τHTI) with decreasing nozzle diameter. The minimum experimental high-temperature ignition delay time of about 1.85 ms is obtained when Dnoz = 160 μm. To investigate the mechanism that the influence of nozzle diameter on high-temperature ignition delay time, numerical simulations of the n-heptane spray ignition process are performed under the same conditions as optical diagnostics. Five characteristic time scales for the two-stage ignition process are defined as: physical preparation time (τphy), low-temperature pre-reaction time (Δτ1), low-temperature ignition delay time (τLTI), high-temperature pre-reaction time (Δτ2), and high-temperature ignition delay time. The physical preparation time decreases linearly with nozzle diameter caused by better spray atomization, evaporation, and air-fuel mixing. Changes in nozzle diameter barely affect low-temperature pre-reaction time, indicating that the linear shortening of low-temperature ignition delay time is attributed to the acceleration of the physical preparation process. The high-temperature pre-reaction time initially decreases and then increases with decreasing Dnoz, which is the result of a smaller scalar dissipation rate promoting the decomposition of H2O2 and inhibiting flame propagation. The combined effect of decreasing Dnoz at each stage leads to a non-monotonic trend in high-temperature ignition delay time.

Performance analysis of an open absorption heat pump and thermal desalination system for heat and water recovery of coal-fired flue gas based on zero liquid discharge

An innovative combined system based on an open absorption heat pump and low-temperature multi-effect desalination system is proposed to provide fresh water and domestic hot water in order to achieve zero liquid discharge from coal-fired power plants and recover the waste heat of flue gas. This system takes into consideration the evaporation of desulfurization wastewater by flue gas. By using a 600 MW unit as an example, energy and exergy analysis is used to examine the impacts of flue gas humidity, temperature, mass, generator pressure, solution concentration, and boiler load on system performance. The findings showed that because coal-fired power plants produce a significant amount of dry flue gas, an increase in gas humidity has little effect on product supply. As a result, little variation in the combined system is caused by the desulfurization wastewater spray evaporation, which allows power plants to achieve zero liquid discharge with only a small loss in exergy efficiency of about 6%. The recovery of sensible heat rather than latent heat benefits from an increase in the bulk of dry flue gas. The recovery of heat and water benefits more from an increase in operating load or flue gas temperature than the open absorption heat pump system itself. The generator pressure and rich solution concentration have negligible effects on heat and water recovery. The open absorption heat pump system's increasing exergy efficiency trend and the boiler operating loads are also related. The exergy efficiency of the generator is the highest by about 75%, that of the desalination system is the lowest by about 29%, and the exergy destruction of the absorber is the highest by about 43%. The findings are likewise applicable to coal-fired power stations with various levels of electrical output.

Spatial evolution of multi-scale droplet clusters in an evaporating spray

Evaporative sprays are encountered in a wide range of engineering applications. Since clustering of droplets in sprays leads to strong inhomogeneity in the spatial distribution of droplet concentration that impacts mass, momentum, and energy exchange between the spray and the surrounding flow, a detailed investigation of droplet clustering in evaporating sprays is important. In the current research work, we experimentally investigate the spatial evolution of droplet cluster characteristics in an evaporating acetone spray injected from an air-assist atomizer. The droplet size and velocity are measured using Interferometric Laser Imaging for Droplet Sizing technique. In detail, characterization of the droplet clusters is achieved by the application of Voronoi analysis to particle image velocimetry images of the spray droplets. This approach not only identifies the droplet clusters but also provides area, length scale, and local droplet number density within the clusters. The identified droplet clusters are multi-scale and could be classified into either large- or small-scale clusters, which scale with spray half-width and Kolmogorov length scale, respectively. Experiments are also conducted in water spray under the same operating conditions. Despite the similarity in the droplet clustering process between the two sprays at small scales of air turbulence, some distinct trends are observed for the large-scale clusters in the acetone spray. This is attributed to the higher evaporation rate of acetone droplets, which promotes preferential accumulation of droplets.

Evaporation of single moving liquid nitrogen droplet: Experimental study and numerical simulation

Liquid nitrogen (LN2) spray is composed of numerous LN2 droplets. Understanding the spray evaporation process requires firstly studying the evaporation mechanism of single moving LN2 droplet. This study focuses on experimental study and numerical simulation for evaporation characteristics of single moving LN2 droplet. A visualized experimental setup was established to investigate the dynamic evaporation characteristics of single moving LN2 droplet. The temporal evolution of LN2 droplet diameter and traveling distance was measured. The mass, energy and momentum equations between the droplet and the surrounding gases, as well as their coupled solving algorithm, are also developed to predict the temporal diameter, temperature and velocity of single moving droplet. Six typical gas-phase evaporation models are integrated into an in-house program so as to evaluate their accuracy and applicability for the droplet with a high evaporation rate. The results show that all of the six models underestimate the evaporation rate of single moving LN2 droplet. It is suggested that the Stefan blowing effect should not be taken into consideration for single moving LN2 droplet. A novel gas-phase model is then proposed for the evaporation of single moving LN2 droplet. The model can accurately predict the evaporation rate of single moving LN2 droplet when the relative velocity of LN2 is greater than 0.5 m/s, and the prediction error is within 1.1%. Furthermore, the model is also used to investigate the effects of key droplet parameters in real LN2 spray, including the initial diameter, the initial velocity, the ambient temperature and the mass fraction of nitrogen in the ambient gases, on the evaporation of single moving droplet.

LES study of turbulent ethanol spray flames using CSE coupled with non-adiabatic chemistry tables

Conditional Source-term Estimation (CSE) is applied to three turbulent ethanol spray flames (EtF3, EtF6, and EtF8) in Large Eddy Simulation (LES). The objectives of this paper are to include the heat losses due to spray evaporation and gas radiation in the chemistry tabulation, assess the impact of these changes on the temperature and droplet statistics, and evaluate the performance of LES-CSE for the selected flames. The profiles of gas temperature, spray velocity, velocity root mean square (rms) and droplet size distribution are well reproduced in the simulations compared to available experimental data. Temperature underpredictions near the centreline are observed, in particular, at locations closer to the jet exit for flames with lower jet velocity. A wider flame is predicted in EtF8 compared to the experiment and regions of local extinction are visible. The use of non-adiabatic chemistry library results in a noticeable improvement in the temperature predictions near the peak locations, especially for flames with higher velocity and closer to the jet exit. The heat losses due to evaporation are larger than those from radiation, confirming the importance of including the evaporation effects in the chemistry tables. The droplet velocity is well predicted, except for EtF8 where an underprediction is observed far downstream. The velocity rms is slightly underpredicted at some locations, probably due to the simple stochastic model used. Overall, LES-CSE with non-adiabatic chemistry tables successfully captures the gas-spray quantities in the selected flames.

Large Eddy Simulations of complex multicomponent swirling spray flames in a realistic gas turbine combustor

Large Eddy Simulations of the realistic liquid fueled gas turbine combustor LOTAR operated at ONERA are performed for two fuels; a conventional JetA-1 and an alternative alcohol to jet fuel At-J, each modeled by a 3-component formulation. JetA-1 is composed of n-dodecane, methyl-cyclohexane and xylene each corresponding to the major hydrocarbon families found in real fuel. At-J is a synthetic drop in fuel composed of only branched chain alkanes, iso-octane, iso-dodecane and iso-hexadecane. Analytically Reduced Chemistry and multicomponent spray evaporation model coupled to the dynamic thickened flame turbulent combustion model are employed to understand the processes involved in turbulent spray flames in the LOTAR configuration. The objectives are to predict and understand the potential effects of staged vaporisation and consumption of the fuel components, and their impact on the spray flame structures. Simulations confirm the role of preferential evaporation in establishing and stabilising the reaction zone. JetA-1 evaporation zones extend deep into the rich burnt gasses resulting in a combustion regime with the possibility of droplet clusters burning individually. At-J which is more volatile, leads to complete combustion with the majority occurring due to the premixed lean reactions of the smaller pyrolysed components. The need to further include models capable of identifying and handling combustion regimes encountered in such spray flames is hence highlighted. This work is intended as a starting point for improving multicomponent spray modelling and requires additional experimental data for validation.

A compacted non-pump self-circulation spray cooling system based on dual synthetic jet referring to the principle of two-phase loop thermosyphon

A traditional spray cooling system usually requires a pump to circulate the fluid, and the kinetic energy of high-temperature steam is wasted. A two-phase loop thermosyphon (TPLT) which can circulate its working fluid is thermally driven. However, heat leakage may occur for low heating power. This paper proposes a compacted non-pump self-circulation spray cooling system named active two-phase loop thermosyphon (ATPLT). Dual synthetic jet integrated with spray cooling (DSJS) is used to enhance the performance of evaporator of ATPLT. Without an external pump, the waste heat and evaporation of liquid spray drive the system internal pressure to increase, which pumps the water to the reservoir as well as avoid heat leakage. It only needs a little working fluid and a little energy consumption for actuator, but can maintain hundreds of Watts of heat dissipation capability for a long time, which can facilitate the development of the energy systems. The performance of ATPLT is studied through temperature, pressure, laser particle size and particle image velocimetry experimental researches. The experimental results show that the cooling capability of ATPLT is mainly influenced by Re, We and Ja, and finally a correlation for ATPLT cooling is established with relative errors within ±18%.

Topological transitions of Jet A-1 lean azimuthal flames (LEAF)

Prior studies about liquid fuel combustion in a vitiated air environment have shown increased combustion efficiency with reduced NOx, CO, and soot emissions. The concept of lean azimuthal flame (LEAF), which can be associated to the latter combustion mode, is based on opposed injections of air and liquid fuel sprays in an axisymmetric chamber with a central outlet, which can result in a highly turbulent toroidal reaction zone. The mixture of fresh air and hot combustion products of each spray provides a vitiated cross-flow configuration to the next spray distributed along the chamber circumference, leading to ignition and sequential combustion of the sprays by the others. The present paper deals with a LEAF combustor with air-assisted spray atomization, which has not been investigated so far. The combustor is fueled with Jet A-1 and operated from 15 to 25 kW with variations in the atomization-air to liquid mass flow ratio (ALR). This study focuses on the flame topology transitions as a function of atomizer ALR. Experimental results based on flame chemiluminescence and OH planar laser-induced fluorescence show two flame topologies: tubular and LEAF topology for ratio of 2 and 4, respectively (denoted ALR2 and ALR4). The spray Mie scattering indicates a significant presence of unburnt droplets for ALR2, whereas quick evaporation is observed for ALR4 cases. In this paper, we propose and validate a basic model based on the spray droplet size distribution, and the evaporation and convection timescales, which are the prominent factors governing the flame topology. Indeed, for ALR2, the evaporation timescale is longer than the convective timescale, which causes incomplete spray evaporation and insufficient vitiated environment, leading to a tubular flame topology and preventing a LEAF to develop. In contrast, for ALR4, the spray evaporation timescales are smaller than the convective timescales, which aids the LEAF topology.

Effects of injection pressure on the flame front growth in an optical direct injection spark ignition engine

• Optical diagnostics performed at wide open throttle and 1200 rpm conditions. • Elevated injection pressure does not monotonically increase engine output. • Higher spatial variation in turbulence measured for very high injection pressure. • Local overmixing and increased mixture inhomogeneity are likely causes. • Despite decreased power, soot is still lower at very high injection pressure. The fuel injection pressure of direct injection spark ignition (DISI) petrol engines has increased significantly to improve engine efficiency and reduce air-polluting emissions. The present study performs a detailed analysis of spray penetration, burn duration, flame front vectors and turbulence intensity to provide scientific explanations about the optimal injection pressure observed for the maximum power output/efficiency in a single-cylinder optically accessible DISI engine. Five injection pressures of 5, 10, 15, 20 and 25 MPa were tested based on an equal-split double injection strategy using a side-mounted six-hole direct injector. A high-speed camera with a 20 kHz frame rate was used to record spray development, propagating petrol flame and soot luminosity. The Mie-scattering technique was used to image the fuel dispersion under varied injection pressure and to calculate spray penetration. For time-resolved, two-dimensional flame front vector extraction, a newly developed flame image velocimetry (FIV) was applied to high-speed natural combustion luminosity movies. A spatial filtering method was also used for the flow decomposition to acquire flame front turbulence intensity. A total of 100 combustion cycles were FIV processed for each test pressure to tackle the inherent cyclic variations. The spray images showed that higher fuel injection pressure leads to faster spray tip penetration and enhanced spray evaporation for better air/fuel mixing. However, the in-cylinder pressure and power output do not continue to increase with an increase in injection pressure; they peak at 15 MPa and decrease at higher injection pressure. The spatially averaged flame front vector magnitude and turbulence intensity also peak at 15 MPa, indicating the optimal injection pressure was due to the maximum turbulent flame front growth. The distribution of turbulence intensity along the flame boundary shows increased variations at injection pressure higher than 15 MPa, because local over-mixing caused low turbulent flame growth regions. Although the highest turbulent flame front growth was not measured at 25 MPa, the soot luminosity was much lower than that of 15 MPa, which explains the development trend towards higher injection pressure.