Supercritical Spray Research Articles

Numerical simulation study of the injection characteristics affecting trans-/supercritical sprays based on orthogonal experiment design

A mixture of numerical models and orthogonal experiments was used to better understand the mixing behavior and heat/mass transfer in jets operating under supercritical conditions. The goal is to investigate the effects of injector diameter (Dinj), ambient pressure (Pamb), injection temperature (Tinj), injection velocity (Vinj), and ambient temperature (Tamb) on jet evolution. The findings highlight that, among these parameters, Tinj and Dinj exert the most substantial impact on jet mixing characteristics. Higher Tinj and Dinj enhance the jet spreading angle (θhsa) and mixing layer thickness (δ) for improved mixing efficiency. Increasing Tinj and reducing Dinj effectively shorten the potential core length (Lp). Moreover, higher Tinj enhances the temperature rise rate (Tr), effectively raising fuel temperature over a shorter distance. The velocity decay rate (Vd) increases with Vinj and Tinj. Furthermore, this study explores the effects of injection density ratio on evaluation indexes (θhsa, Lp, Vd, Tr and δ) for jet mixing efficiency and trends of jet structural development (Lp, turbulent chaos length (CL) and self-similarity length (SL)). These findings are important for understanding supercritical spray development, guiding engine chamber design and setting operational conditions based on spray behavior.

Numerical Investigations of Pseudo-Boiling and Multi-Component Mixing Under Trans-/supercritical Conditions for Engine Applications

ABSTRACT The fuel injection and mixing process are often carried out under trans-/supercritical conditions for engine applications; however, the process is still unclear. In this study, based on OpenFOAM, a multi-component trans-/supercritical spray model based on the KT/KNP scheme is developed, which covers the special real fluid equation of state, various mixing rules and modified thermodynamic properties. In addition, the PIMPLE algorithm is extended to deal with the high nonlinearity. First, a one-dimensional advection case is applied to evaluate the performance of the model. Then, the effect of various mixing rules and pseudo-boiling phenomenon on n-heptane/nitrogen jet are thoroughly analyzed under trans-/supercritical conditions. The results show that the jet is extremely sensitive to the changes of the initial jet temperature and chamber pressure, and the “heat shield” effect of pseudo-boiling delays the breakup and mixing in the transcritical jet. With the mixing of different components, unlike the single-component condition, the pseudo-boiling intensity will increase. The increase of pressure and occurrence of mixing will lead to the decrease of pseudo-boiling temperature, and the pseudo-boiling will end earlier, which is obviously different from the single-component fluid case. For the n-heptane/nitrogen mixture, when the mole fraction of n-heptane in the mixture changes from 1.0 to 0.7, and the pseudo-boiling temperature decreases from 571 K to 465 K. With increasing injection temperature or chamber pressure, the pseudo-boiling intensity gradually decreases, and the potential core shortens, the difference among various mixing rules decreases. When the injection temperature is higher than 571 K, the pseudo-boiling phenomenon no longer exists. When the chamber pressure reaches 9 MPa, the pseudo-boiling strength is very low. Therefore, the spray mixing under supercritical conditions will be more sufficient. With the occurrence of mixing, the critical parameter gradually transitions from the critical value of n-heptane to the critical value of nitrogen, which strongly depends on the mixing rule.

Experimental study on the phase inhomogeneity and separability in supercritical state of acetone fuel

Sufficient understanding of the supercritical state is important for exploring supercritical jet spray in thermodynamic cycle power machinery. However, in conventional studies, the phase division of supercritical state and supercritical jet spray is simple and rough, and whether the pseudo-boiling (or widom) line exists widely in various fluids, especially fossil fuels, is still not proved by experiments. In this paper, the phase inhomogeneity and separability in the supercritical state of fossil fuels are studied by using high-speed Mie scattering and UV-LAS (ultraviolet–visible laser absorption/scattering) optical diagnostic techniques. Acetone is selected as the single representative component of fossil fuels. The results show that acetone is inhomogeneous in the supercritical state at fuel temperature Tf<1.13Tc and fuel pressure Pf<1.74Pc, where Tc and Pc are the critical temperature and pressure, and can be further divided into liquid-like phase and gas-like phase, which is similar to the properties of inert fluids and simple fluids reported in previous literatures. The transition region (measured at interval of pressure 0.2 MPa) between the liquid-like phase and gas-like phase contains thermodynamic extremum lines such as constant pressure specific heat, volume expansion coefficient, and isothermal compression rate, which supports the view that the widom (or pseudo-boiling) line is used as the crossover in the supercritical state. When the state of acetone spray passes through the transition region, the liquid-like and gas-like phases will have dramatic changes in the optical and macroscopic properties of spray (For example, when the injection temperature is equal to the critical temperature of 235 °C, the stable liquid-like areas crossing the pseudo-boiling line decreases by 9 times as much as that without crossing the pseudo-boiling line). The results of this paper have important reference values for the study of the supercritical spray atomization regime and the establishment of the simulation model.

Rate of heat release characteristics of supercritical sprays of dieseline blend in constant volume combustion chamber

In present experimental study, combustion characteristics of homogeneous supercritical (SC) fuel spray injection system are analyzed in cylindrical constant volume combustion chamber (CVCC) at experimental conditions similar to diesel engine-like operating conditions. A comparative study is also performed between SC fuel spray and normal diesel spray combustion under similar conditions. Combustion characteristics of SC fuel sprays includes rate of heat release (ROHR) and peak pressure (PP). Operating parameters include hot surface temperature range (673 ​K–723 ​K), pre-firing cylinder air pressure range (20 ​bar–40 ​bar) and injection pressure (IP) range of 100 ​bar–300 ​bar. Piezoresistive pressure transducer is employed for indicating and recording combustion processes. Diesel fuel and dieseline blend (50% gasoline and 50% diesel by v/v) are used for normal diesel spray and SC spray combustion respectively. It is found that ROHR in SC fuel spray combustion is smaller than in normal diesel spray combustion at all operating conditions. In SC fuel spray combustion, ROHR increases with increase in IP but decreases with increase in HST and CP due to homogeneous and hot air combustion nature of SC fuel sprays. Percentage reduction in ROHR of SC fuel spray combustion as compared to ROHR in normal diesel spray combustion is found to be nearly 34% at normal operating conditions of diesel engine. PP reached during SC spray combustion is noted to be less than PP reached during normal diesel spray combustion in all cases and is responsible for reduced NOx formation in SC spray combustion.

Real-Fluid Modeling for Turbulent Mixing Processes of N-Dodecane Spray Jet Under Superciritical Pressure

The multi-environment probability density function (MEPDF) approach using the real-fluid Equation of State (EoS) has been developed to numerically investigate the physical processes of the non-cryogenic n-dodecane fuel spray jet under the supercritical pressure. The MEPDF approach together with the conserved scalar formulation and the real-fluid library is utilized to account for the scalar fluctuation effects on the turbulent mixing processes of real fluids over the transcritical and supercritical spray conditions. To correctly predict the highly non-linear thermodynamic properties over a wide range of pressures and temperatures, this approach has adopted the RK-PR equations of state and dense-fluid correction models. In order to realistically and efficiently predict the supercritical spray jet with the strong anisotropic turbulence, the present study has adopted the RANS-based v2-f model and the LES-based turbulence model. Simulations are made for the “Spray-A” benchmark case in the Engine Combustion Network. Based on numerical results, the detailed discussions are also made for the prediction capability of RANS/LES based real-fluid approach as well as the essential features of the real-fluid n-dodecane jet under the supercritical pressure.

Combustion and soot formation characteristics of homogeneous supercritical sprays of dieseline blend in constant volume chamber

In present work, combustion and soot formation characteristics of supercritical (SC) sprays are studied in a cylindrical constant volume combustion chamber under diesel engine-like experimental conditions. Hot surface temperature range (673 K to 723 K), initial cylinder pressure range (2 MPa to 4 MPa) and injection pressure range of 10 MPa to 30 MPa are the experimental conditions. Photo sensor and piezoelectric sensors are employed for indicating combustion and injection events respectively. Commercial diesel fuel and dieseline blend are employed for analyzing combustion of normal diesel and SC sprays respectively. It is investigated that duration of combustion of SC sprays are substantially lower than normal diesel sprays at all experimental conditions. Percentage reduction in DOC of SC sprays as compared to diesel sprays is above 35% for all cases. Percentage reduction in DOC of SC sprays is observed highest at higher IP of 30 MPa. Percentage reduction in soot formation (SF) in SC spray combustion is more than 90% at all operating conditions. At normal diesel engine operating conditions, nearly 82% reduction in DOC and about 96% reduction in SF are found. Combustion of SC sprays seems hot air and single-phase homogeneous combustion unlike that of normal diesel sprays having hot surface ignition/combustion. SC spray injection system proves out to be a faster as well as cleaner combustion technology for future high speed direct-injection engines.

Aircraft engines’ needs from combustion science and engineering

Turbulent multiphase combustion plays an important role in both nature (e.g., volcanos and pool fires) and industry (e.g., industrial furnaces, aeroengines, and internal combustion engines). It is a highly complex multiscale and multi-physicochemical process in which interactions between the dispersed and continuous phases, phase change, droplet collisions, evaporation, mixing, heat transfer, and chemical reactions occur simultaneously. In recent years, significant progress has been made in understanding the mechanisms of spray flames and their behaviors in combustion engines. This paper covers key and representative developments in the area of turbulent spray combustion with a focus on spray–chemistry–turbulence interactions. The effects of turbulence–chemistry, spray–turbulence, and spray–chemistry interactions on the spray process, ignition, flame stabilization and emission are comprehensively discussed at elevated pressures and temperatures. Furthermore, spray–radiation and spray flame–wall interactions, which are important to engine performance and emission characteristics, are scrutinized. Supercritical spray flames and turbulent spray flames in dual-fuel engines are also discussed. Finally, outlooks and further challenges for the research field are outlined.

Supercritical Fluid Spray Dyeing: A New Method of Dyeing and Simulations of its Flow Field

The supercritical fluid flow jetted out from a nozzle was divided into two stages, free jetting and passing through the porous media, in a supercritical spray dyeing process. According to the nature of the flow, the free jetting model and passing through porous media model were established separately. Based on the established models, the characteristics of supercritical fluid flow field in the supercritical fluid spray dyeing process were investigated firstly in this paper. Distributions of the temperature and pressure and their influencing factors in the flow field, which are the determinants of the spray dyeing process, were obtained and analyzed. Besides, the mass flow and covering diameter sprayed by supercritical fluid were studied. The simulating analysis may provide a theoretical basis and guidance for the actual process of supercritical fluid spray dyeing.

Real Fluid Modeling of Multiphase Flows in Liquid Rocket Engine Combustors

The characteristics of propellant mixing near the injector have a profound effect on liquid rocket engine performance. Injector element geometry and propellant momentum are the only parameters available to control such mixing. A multiphase flow combustion model utilizing propellant real-fluid properties was developed to predict the mixing and thermal environment created by element configurations. The multiphase model was incorporated into a mature computational fluid dynamics code that can be used to simulate the flowfield near the injector and to analyze the combustion chamber and nozzle wall heating. The validity of the present model and code was investigated by comparing predictions to test data for two liquid oxygen/gaseous hydrogen unielement shear coaxial injector and two cryogenic nitrogen jet injection experiments. These simulations and those of several other investigators were critically compared at the 2nd International Workshop on Rocket Combustion Modeling. Based on these comparisons, it appears that the present model is a good, computationally efficient approximation of liquid rocket injector flows for both subcritical and supercritical spray combustion.