Premixed charge compression ignition (PCCI) is a promising low-emission combustion concept. By partially mixing the fuel, air and exhaust gas before auto-ignition, the soot and NOx emissions are lower than for conventional diesel combustion. However, the fundamental aspects of the mixing process of the fuel spray with the ambient air are still not well understood, especially not in terms of the temperature distribution of the fuel/air mixture. This thesis focuses on the 2D temperature distribution measurement of fuel sprays under conditions relevant to PCCI-mode combustion in heavy-duty Diesel engines. Experiments are performed in the High Pressure Cell (HPC), which simulates engine conditions while providing much better optical accessibility than a real engine. The temperature field which is produced by the pre-combustion technique is also measured, to characterize the ambient condition which the sprays are injected in. Advanced diagnostics are applied to provide detailed information on fuel sprays and the ambient condition, to improve the understanding of the mixing mechanism and its consequences for the combustion process. Chapter 2 describes the pre-combustion technology of the HPC and the modeling of the cooling process. The analytical modeling is based on experimental observations, assuming that turbulent convection is dominating the cooling process. A natural and a forced convection model are used to estimate the thermal boundary layer thickness and the core temperature in the scenarios without and with fan-mixing, respectively. Without fan-mixing the heat transfer rate decays together with the turbulent kinetic energy. If fan mixing is added, a constant turbulent flow is maintained, that is, the turbulent kinetic energy is constant during the cooling process. The modeling results match experimental results very well. In the first scenario, the core temperature is about 5 – 7% higher than the bulk temperature. In the second scenario the difference is 2 – 6%. These results are reasonable when compared with experiments. The model provides a good estimation for the ambient condition during fuel spray measurements. In Chapter 5, Laser Induced Phosphorescence (LIP) is chosen to measure the temperature field of the ambient gas prior to fuel injection. The BAM (BaMgAl10O17:Eu) is chosen as a tracer phosphor for its high signal-to-noise ratio and its capability to survive the pre-combustion. A seeding device to seed the 3 µm solid particles into gaseous flow was designed and implemented, which performed beyond expectation. Particle agglomeration was not observed, probably due to the high shear forces induced. Particle sticking is not a major concern as long as stainless steel tubing is used and Teflon material is avoided. BAM-LIP is excited by a 355 nm YAG laser. Results show that BAM-LIP can be used to measure the temperature field of the residual gas in the HPC below 650 K. The precision of the experiments is better than 30 K at 400 K and 60 K at 650 K. The spatial resolution was estimated to be 3 mm in the plane of the laser sheet and 10 mm along the line of sight, primarily determined by multiple scattering present in the experiments. Temperature field results show that there is a significant temperature gradient in the vertical direction present during the cooling phase in the HPC when the mixing fan is not used. This finding supports the interpretation of the analytical model, which overpredicts the temperature for neglecting the buoyancy effect. However the BAM-LIP method is currently not able to provide 2D temperature distribution prior to fuel injection, due to the lack of signal due to particle falling. Possible improvements have been recommended. In Chapter 3, the physical processes in a fuel spray, as it is injected into stagnant ambient gas, are explained and two phenomenological spray models are compared in their prediction of temperature distribution in a fuel spray. The major difference between the Versaevel and the Valencia model exists in their assumptions for radial profiles of fuel concentration and velocity. The Versaevel model assumes a top-hat profile while the Valencia model assumes a Gaussian profile, which is observed by averaging multiple injections. Both models predict spray penetration very well, however, they differ considerably in their prediction of the temperature distribution. The Valencia model predicts lower central line temperatures than the Versaevel model. Laser Induced Fluorescence (LIF) using 10% toluene as a tracer, as described in Chapter 5, is used as a tool to measure 2D temperatures during and after injection of fuel inside the HPC and an optical engine. The error analysis and evaluation of the toluene LIF method was performed on the HPC, while the calibration was performed in the optical engine. The toluene LIF method is capable of measuring temperatures up to 700 K; above that the signal becomes too weak. The precision of the spray temperature measurements is 4% and the spatial resolution is 1.3 mm. Experimental results from the HPC reveal a hot zone in the fully developed spray. Two camera configurations are compared. An opposite side camera setup seems to be beneficial over a one-side setup because it avoids the dichroic beam splitter requirement, but the precision is lower because of different light paths. The toluene LIF method offers a relatively simple and precise way to measure the 2D temperature distribution in fuel sprays. However, several improvements can be done to improve the absolute accuracy, For example using more sensitive camera and applying flat field correction with a light source of the relevant wavelength. In general, the toluene LIF method is capable of providing 2D temperature information in a fuel spray with 4% precision, which makes it possible to detect the temperature gradients in sprays. The BAM-LIP method could be used in measuring the temperature distribution in a gas-phase environment, where combustible tracers, such as toluene, are not applicable. Both methods might be applied in more applications such as in burners and internal combustion engines.
|Qualification||Doctor of Philosophy|
|Award date||19 Nov 2012|
|Place of Publication||Eindhoven|
|Publication status||Published - 2012|