Abstract
Piston temperature prediction is very important for a recent high power, high speed diesel engine because the piston temperature affects its durability very much. It is well conducted to decrease piston temperature with an oil jet and cooling channel. However, the oil movement and heat absorption characteristics in the cooling channel have not been investigated in detail. Therefore, we have tried to make clear the heat absorption characteristics in the piston with experimental and numerical approach.
As an experimental approach, the visualization of the oil movement in the cooling channel, the measurement of the oil flow rate and the heat transfer rate around the cooling channel were conducted. The visualization was done with two dimensional fluorescence method. A fluorescent dye Coumarin-6 was mixed to the oil and the images were taken with a video camera with synchronizing the lightening of a xenon flash lamp. The air-mixed oil was observed around the upper and lower side in the channel at the piston top and bottom dead center by piston shaking effect, respectively. The passing oil flow rate* through the channel was also measured by weighing the oil discharged from the channel outlet. The average value of the passing oil flow rate at 3000 rpm was about 75 %. This tendency was supposed to be based on the reverse flow and the relative speed between the piston and the oil jet. The heat absorption rate of the oil in the channel for the passing oil flow rate was measured. The heat absorption rate increased with the passing oil flow rate for all engine speeds. From this result, increasing the passing oil flow rate seemed to be one measure for improving the piston cooling performance.
Numerical simulations were performed with a multiphase 3D-CFD to predict the oil movement, the oil flow rate and the heat transfer rate around the cooling channel. The results were compared with measured data. The calculation results of oil location showed good agreement with measured images at each crank angle. The passing oil flow rate also showed good agreement with measured ones at each engine speed. From these results, it is clear that the passing oil flow rate is reduced by interference between the supplied oil and the oil in the channel. It is found that the interference mainly affects the maximum passing oil flow rate. The heat flow distribution derived from measurement was compared with calculated heat transfer coefficient. Qualitatively, the amount of heat transfer at the inlet side was very large for both measurement and calculation because of the collision of oil jet. From these comparisons, the developed simulation technique showed enough accuracy to predict the oil movement and the heat transfer in the cooling channel and this approach could help us to understand its detail. (* passing oil flow rate = passing oil flow through channel / supply oil flow by oil jet)
Keywords:Piston, Cooling Channel, Oil Jet, Heat Transfer Rate, Prediction