| The oscillating cooling is currently one of the most efficient cooling technologies for highly reinforced pistons.Previous studies only focused on the piston galleries with the reciprocating motion along the direction of the gravity force.However,for different types of engines,there are also the inline horizontal,V-type and Radial engines according to cylinder arrangement,and the reciprocating motion direction of piston is not necessarily the gravity direction.The cooling performance of piston gallery under different cylinder arrangement is also different,but the research in related fields is relatively deficient.Therefore,a concept of spatial angle is introduced to describe the angle between the direction of the piston reciprocating motion and the direction of the gravity force in a coordinate system,for the piston galleries of highly reinforced pistons with different cylinder arrangements.Considering the characteristics of the relative positions between the inlet passages and outlet passages,the numerical simulation models of the cooling galleries with different spatial angles are established,and the oscillation flow visualization test is carried out.The oscillating flow and heat transfer characteristics of piston galleries in different spatial angles are investigated by simulation analysis and visualization experiment.The main conclusions are as follows:(1)The visualization test of oscillation flow in two-phase flow with different spatial angles were carried out.The effects of oil charge ratio and simulation test speed on the formation and transformation of two-phase flow pattern were explored,and the flow law of fluid in the square cavity with different spatial angles was revealed.The results show that the simulation test speed is the key factor affecting the flow pattern conversion of two-phase flow in the oscillation process.When the test speed is 129r/min,the flow patterns of two-phase flow are mainly elastic flow and wavy flow,and they are converted to each other.When the test speed is 180 r/min,the flow patterns of two-phase flow are mainly elastic flow and bubble flow,and they are converted to each other.When the speed increases to 232 r/min,the further enhancement of turbulence makes the flow pattern of two-phase flow are mainly dispersed bubble flow.In addition,in the oscillating flow process,the flow laws around the wall of the fluid at different spatial angles are different,resulting in a large difference in the fluid distribution on the wall of the square cavity under the same crankshaft angle.(2)The numerical simulation models of the cooling galleries in different spatial angles were established.The steady-state flow characteristics,instantaneous flow characteristics and oil distribution characteristics of the galleries in different spatial angles were analyzed,combined with the characteristics of piston motion,oil inlet mass flow,oil outlet mass flow,oil flow velocity.The results show that the oil only flows out of the galleries between 86°CA and 334°CA.Due to the increase of engine speed and the decrease of oil inlet mass flow,there is a significant reflux phenomenon in the inlet passages between 233° CA and 310 ° CA,and the maximum reflux mass flow is not less than 0.0055 kg/s.The instantaneous flow patterns of different spatial angle models are basically the same,but the relative position of the inlet and outlet oil passages of the cooling galleries will affect the oil charge rate and oil distribution.When the engine speed is 1000 r/min,the oil inlet passage and outlet passage are not in the same vertical height of the piston galleries.With the increase of the spatial angle,the circumferential gravity component decreases,and the oil circumferential flow velocity increases.The inlet passage is gradually higher than the outlet passage,the oil outlet mass flow increases,and the steady-state oil charge rate of the galleries decreases.When the spatial angle increases from-90° to 90°,the steady-state oil charge rate of the cooling gallery decreases from 63% to 34%,the oil coverage area of the wall is reduced.When the inlet passage and outlet passage of the piston galleries are at the same vertical height,the closer the spatial angle is to 0°,the smaller the oil charge rate of the cooling galleries,and the smaller the oil coverage area of the cooling gallery walls.(3)The steady-state heat transfer characteristics,instantaneous heat transfer characteristics and uneven heat transfer performance of circumferential and axial of piston galleries with different spatial angles were systematically studied.The results show that the steady-state heat transfer coefficient decreases with the increase of spatial angle when the inlet and outlet oil passages of piston galleries are not in the same vertical height,and the heat transfer of the gallery walls are more uneven.When the engine speed exceeds 2700 r/min,the instantaneous heat transfer characteristics of cooling galleries are no longer affected by the spatial angle.For the piston galleries with the same vertical height,the closer the spatial angle is to 0°,the greater the variation amplitude of instantaneous heat transfer coefficient is,and the more uneven the heat transfer of the gallary is.When the engine speed exceeds 1600 r/min,the instantaneous heat transfer characteristics of cooling galleries with different spatial angles is basically the same.In addition,the spatial angle has the greatest influence on the heat transfer of the axial outer wall,the axial inner wall and the circumferential inlet and outlet middle region of the cooling galleries.(4)The research on the flow characteristics and heat transfer characteristics of piston galleries with different spatial angles,it is found that for the galleries of non-vertical engine pistons such as in-line horizontal and V-shaped engines,the heat transfer performance of piston galleries can be enhanced by adopting the design method of the inlet passage below and the outlet passage above.By optimizing the axial outer wall and inner wall of the piston galleries,and the circumferential inlet and outlet middle region of the piston galleries,the heat transfer performance of piston gallery at different spatial angles can be improved. |
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