Investigation on the Interaction of an Impinging Jet with Cylinder Wakes

Jet impingement cooling is a widely used cooling method due to the high heat transfer rates associated with it. Research for improving heat transfer rates for this cooling is still being carried out due to its broad application in various fields like gas turbine blade cooling, electronic component cooling, and paper drying. The unsteady jet oscillation effectively enhances the stagnation region and the time-averaged heat transfer rates. It is shown that a novel passive jet oscillation technique can be achieved using the vortices periodically shed from a cylinder placed upstream in a channel with an initial crossflow. Preliminary CFD results prove the hypothesis of jet oscillation induced by the cylinder vortices and that the lateral jet oscillation is an efficient method for uniform distribution of heat transfer. The statistical analysis concluded jet oscillation is most sensitive to cylinder vortex strength. A frequency spectral analysis is performed to classify oscillating and non-oscillating cases. Finally, unsteady numerical and experimental research is carried out to determine the effect of cylinder-jet distance, cylinder diameter, and velocity ratio on jet oscillation and heat transfer rate. The range of cylinder-jet distance and velocity ratio tested are S/d = 2 – 4 and VR = 4 – 12, respectively. The flow interaction mechanism leading the jet oscillation is analyzed using TKE, vorticity, and velocity contours in time. The flow feature analysis concluded the cylinder wakes deformed the jet core inducing lateral and angular oscillations. The heat transfer results showed the Nusselt number is proportional to the velocity ratio for oscillating jet cases. The non-oscillating jet enhances the heat transfer rate by 94% in the wall jet region due to crossflow interaction. And the optimum oscillating jet case improved the stagnation region Nusselt number by 19%

Effects of motion on jet exhaust noise from aircraft

The various problems involved in the evaluation of the jet noise field prevailing between an observer on the ground and an aircraft in flight in a typical takeoff or landing approach pattern were studied. Areas examined include: (1) literature survey and preliminary investigation, (2) propagation effects, (3) source alteration effects, and (4) investigation of verification techniques. Sixteen problem areas were identified and studied. Six follow-up programs were recommended for further work. The results and the proposed follow-on programs provide a practical general technique for predicting flyover jet noise for conventional jet nozzles

Numerical and Experimental Analysis of Injection and Mixture Formation in High-Performance CNG Engines

L'abstract è presente nell'allegato / the abstract is in the attachmen

Large eddy simulation of highly turbulent under-expanded hydrogen and methane jets for gaseous-fuelled internal combustion engines

Burning hydrogen in conventional internal combustion (IC) engines is associated with zero carbon-based tailpipe exhaust emissions. In order to obtain high volumetric efficiency and eliminate abnormal combustion modes such as preignition and backfire, in-cylinder direct injection (DI) of hydrogen is considered preferable for a future generation of hydrogen IC engines. However, hydrogen's low density requires high injection pressures for fast hydrogen penetration and sufficient in-cylinder mixing. Such pressures lead to chocked flow conditions during the injection process which result in the formation of turbulent under-expanded hydrogen jets. In this context, fundamental understanding of the under-expansion process and turbulent mixing just after the nozzle exit is necessary for the successful design of an efficient hydrogen injection system and associated injection strategies. The current study used large eddy simulation (LES) to investigate the characteristics of hydrogen under-expanded jets with different nozzle pressure ratios (NPR), namely 8.5, 10, 30 and 70. A test case of methane injection with NPR = 8.5 was also simulated for direct comparison with the hydrogen jetting under the same NPR. The near-nozzle shock structure, the geometry of the Mach disk and reflected shock angle, as well as the turbulent shear layer were all captured in very good agreement with data available in the literature. Direct comparison between hydrogen and methane fuelling showed that the ratio of the specific heats had a noticeable effect on the near-nozzle shock structure and dimensions of the Mach disk. It was observed that with methane, mixing did not occur before the Mach disk, whereas with hydrogen high levels of momentum exchange and mixing appeared at the boundary of the intercepting shock. This was believed to be the effect of the high turbulence fluctuations at the nozzle exit of the hydrogen jet which triggered Gortler vortices. Generally, the primary mixing was observed to occur after the location of the Mach disk and particularly close to the jet boundaries where large-scale turbulence played a dominant role. It was also found that NPR had significant effect on the mixture's local fuel richness. Finally, it was noted that applying higher injection pressure did not essentially increase the penetration length of the hydrogen jets and that there could be an optimum NPR that would introduce more enhanced mixing whilst delivering sufficient fuel in less time. Such an optimum NPR could be in the region of 100 based on the geometry and observations of the current study

Simulations And Experiments Of Fuel Injection, Mixing And Combustion In Di Gasoline Engines

Direct Injection (DI) has been known for its improved performance and efficiency in gasoline spark-ignition engines. In order to take all the advantages of the GDI technology, it is important to investigate in detail the interactions of fuel spray and combustion system, such as air-fuel mixing, in-cylinder flow development, surface wetting, and turbulence intensity. The characterizations of the internal nozzle flow of DI injector are first studied using the multidimensional computational fluid dynamic (CFD) simulation. In the meanwhile the numerical and experimental studies are carried out to observe the external spray and wall impingements in an optical constant volume vessel. The fuel film deposit characteristics were derived using the Refractive Index Matching (RIM) technique. Finally, the interactions of sprays with the charge motion are investigated in an optical accessible engine using CFD simulation and high-speed imaging of sprays inside engines. The numerical results DI injector nozzle show that the complicated unsteady flow features dominate the near-nozzle breakup mechanisms which are quite unlike those of diesel. The spray impingement, wetted area, fuel film thickness, and the resultant footprint mass were investigated experimentally. The CFD simulation with selected models of spray validated first for its transport in the air is used to compare the impingement models with the experimental measurements. The spray cone, tip penetration and fuel film shapes were in very good agreement. The effects of spray patterns, injection timing and flexible valve-train on the bulk flow motion and fuel-air mixing in an optical accessible engine, in terms of tumble and swirl ratios, turbulence level, and fuel wall film behaviors are discussed. Using integral analyses of the simulation results, the mechanisms in reducing fuel consumption and emissions in a variable valve-actuation engine, fueled by side-mounted multi-hole DI injectors are illustrated. The implications to the engine mixing and the resultant combustion in a metal engine are also demonstrated

Aeronautical Engineering. A continuing bibliography, supplement 115

This bibliography lists 273 reports, articles, and other documents introduced into the NASA scientific and technical information system in October 1979

Analysis of injection, mixture formation and combustion processes for innovative CNG Engines

Natural gas is a promising alternative fuel for internal combustion engines application due to its low carbon content and high knock resistance. The work presented in this thesis deals with the fluid dynamics, experimental study and optimization of different technologies aimed at exploiting the potentials of such fuel at best. The first section of the work is aimed at the combustion chamber optimization with the focus on the combustion stability. The engine considered in the study is a prototype specifically dedicated to CNG. It features a variable valve actuation system and has been released with different and very high compression ratios ranging from 12 to 14. An innovative experimental methodology based on hot wire anemometry (HWA) purposely developed by Centro Ricerche Fiat (CRF) has been adopted for the characterization of the steady-state tumble. The HWA method has been validated against the well-known Ricardo method and is used as a basis for the development and validation of a numerical “virtual flow bench”. The numerical model has been used to gain a deeper insight into the fluid dynamic phenomena and to replace the experimental campaign considering a head variant and quantifying its tumbling and volumetric performances. A transient 3D CFD analysis for the complete engine cycle has been performed in order to evaluate the effect on the combustion process of different compression ratios and head designs.The results showed that the HWA technique represents a factual alternative to the integral technique for the tumble characterization. The “Virtual flow box” model turned out to be accurate enough to evaluate the main flow motions induced by the head design and to be a valid tool complementary to the experimental method. Finally, the transient model used in combination with the ECFM-3z combustion model is fairly accurate for the comparative analysis between different engine designs and/or valve actuations. Despite the main findings of the flow model activity, importance should also be placed onto advanced technologies for natural gas engines such as direct injection. Thus, the second section is aimed at the numerical study of a natural gas direct injection engine. The numerical complexity caused by the high pressure ratio at nozzle exit has been faced using an accurate mesh procedure able to correctly capture the formation of shocks structures. Moreover, the actual needle geometry and the realistic needle movement has been taken into account in order to correctly simulate the opening and closing transient. The final mix and turbulence level has been evaluated comparing two engine prototypes and considering several injection strategies. Finally, a qualitative validation of the computational model has been performed comparing the simulation results with the available experimental data obtained through the PLIF procedure on an equivalent optical engine. The CFD model resulted to be accurate in the prediction of the mixing quality and it shows to be a reliable tool for the analysis of the main mixing mechanism and so for the assessment of the best injection strategy

Aeronautical engineering: A continuing bibliography, supplement 122

This bibliography lists 303 reports, articles, and other documents introduced into the NASA scientific and technical information system in April 1980

HIGH INJECTION PRESSURE IMPINGING DIESEL SPRAY CHARACTERISTICS AND SUBSEQUENT SOOT FORMATION IN REACTING CONDITIONS

The spray impingement in diesel engines attracts the attention of engine researchers in recent decades as the physical size of the engine is reduced. Due to the spray impingement, the atomization, vaporizing and air-fuel mixing quality is altered compared to a free spray. For emission control, soot is one of the major particulate emissions from diesel combustion and its formation in an impinged spray is worthy to be investigated. Firstly, to understand the impinged spray characteristics, the experiments for both non-vaporizing and reacting conditions were conducted in a constant volume combustion vessel. The impinged spray was captured by a high-speed camera and the instantaneous spray propagation distance and rate were obtained. For a better understanding, the microscopic behavior of the spray propagation, the curvature of the impinged spray was calculated and a relationship between local fuel distribution and soot formation was found. After that, the apparent heat release rate from an impinge spray combustion and the heat flux through the impingement were analyzed. The apparent heat release rate was obtained by the internal chamber pressure and the heat flux was measured by heat flux probes embedded in the impinging plate. Then, the soot formation of an impinged spray was both studied from experiments and simulations. In the experiments, the natural luminosity mainly due to the incandescence of soot particles was captured by the high-speed camera. A computational fluid dynamics (CFD) approach was adopted to quantitatively study the soot formation in terms of absolute soot mass and soot mass fractions in the vicinity of the wall. In the last, the film formation under different ambient temperatures, impinging distances, and oxygen concentration was investigated in terms of film area and thickness. The impact of film formation on the soot outcomes was then investigated by comparing the rate of film vaporization and soot formation. To summarize, the main goal of this dissertation is going to benefit the understanding of the impinged spray in reacting diesel-relevant engine conditions. From experiments, a global view of soot formation in an impinged spray was analyzed and the mechanism of soot formation was further revealed by the CFD simulations