Numerical investigation on the effects of spatial angle on oscillating flow and heat transfer characteristics of piston cooling galleries

摘要

The oscillating cooling method is currently one of the most efficient cooling solutions for highly reinforced pistons. Previous studies only focused on the piston galleries with the reciprocating motion along a specified direction relative to the direction of the gravity force, e.g., the vertical direction in inline multi-cylinder engines. There are strong needs to study the cooling performance of the pistons that are designed to have reciprocating motions with different directions relative to gravity. In this paper, a concept of spatial angle is introduced to describe the design angle between the direction of the piston reciprocating motion and the direction of the gravity force in a coordinate system. When the spatial angle is changed, the gravity components of the piston are changed along the direction of the piston axial motion and its perpendicular direction. Moreover, with a given spatial angle, different piston designs can be achieved to obtain different relative positions of the cooling oil inlet and outlet passages, being higher or lower relative to each other. Because both the spatial angle and the relative positions of the oil passages affect flow and cooling performance, several simulation models are established to cover five spatial angles including -90 degrees, -45 degrees, 0 degrees, 45 degrees, and 90 degrees as well as two groups of relative positions, i.e., groups A and B. The CFD simulation is validated by experimental testing. The results show that the flow and heat transfer performance of group B is more stable than that of group A because the inlet and outlet of the cooling passages of group B are at the same height. It is found that the steady-state oil charge ratio and steady-state heat transfer coefficient of group A gradually decrease with the increase of the spatial angle. The fluctuation patterns of the instantaneous oil charge ratios and heat transfer coefficients of groups A and B are very similar, but their spatial distribution values are different at the same crank angle (CA). The maximum difference in the instantaneous heat transfer coefficients between A1 and A3 reaches 23.27% at 303 degrees CA. Moreover, it is found that when the piston moves between 150 degrees CA and 300 degrees CA, a phenomenon of liquid plug attenuates with the increase of the spatial angle. These findings can support the cooling performance analysis and optimization design of the piston galleries of the V-type, W-type, and horizontal opposed-piston diesel engines.