Numerical Simulation Of Two-phase Oscillating Flow And Heat Transfer In Piston Cooling Galleries For Disel Engines

Increasing strengthening degrees of diesel engines have caused significant concerns regarding the thermal loading to the pistons.The most common and effective measure for decreasing piston temperatures is to place cooling oil galleries inside the pistons.The mixed air and oil oscillates inside the piston galleries,during which the energy is exchanged with inner surface of the galleries.The heat transfer performance in different operating conditions should be fully estimated during the design process to design cooling galleries that meet the technical requirements.Establishing numerical codes for two-phase oscillating flow and heat transfer inside piston galleries,studying on oscillating flow and heat transfer characteristics systematically,providing references for optimal design of piston galleries,are of great engineering significance for reducing thermal loads and building stronger pistons.In this study,the two-phase interface flow simulating module is constructed under the framework of the finite volume method for unstructured meshes,and the numerical model of two-phase interface flow and heat transfer is established,based on the in-house software GTEA(General Transport Equation Analyzer).The two-phase oscillating flow and heat transfer mechanism in annular galleries and complex galleries coupled with a telescopic pipe has been studied based on the established model.The main research work includes the following aspects:(1)The module for solving volume fraction equations is added to the original GTEA solver.Systematic comparisons have been made in terms of construction idea,implementation strategy,calculation accuracy and computational time-consuming between the CICSAM(Compressive Interface Capturing Scheme for Arbitrary Meshes)scheme and the THINC(Tangent of Hyperbola INterface Capturing)scheme.The CICSAM scheme is easy to implement,and the time step is highly limited.Great deficiencies have been existed for solving interface deformation problems on sparse grids and unstructured grids.The implementation steps for THINC scheme are relatively complex,but the computational accuracy is higher,and the calculation time is about 2-3 times that of the CICSAM scheme.(2)The direct time-integral THINC scheme is proposed and developed.The classical THINC method uses third-order total variation diminishing Runge–Kutta schemes to update the volume fraction,requiring three reconstruction steps during one time step.The new TI/THINC avoids such computational consuming.The improved TI/THINC scheme is much more efficient than the original THINC approach using RK3 with no apparent reduction in computational accuracy.The CPU time is similar with the CICSAM scheme.Moreover the TI/THINC scheme is better able to retain the boundedness of the volume fraction field for a wide range of β values.As with the non-physical values near the interface,the explicit adaptive bounding procedure is introduced and proved to be useful in minimizing non-physical values.(3)Numerical solver for two-phase oscillating flow and heat transfer is tested and verified.The two-phase oscillating flow and heat transfer problem in piston galleries is divided into two sub problems: oscillating flow in closed chamber and contact heat transfer between two-phase fluid and walls of oil chamber.The applicability of the current software to oscillating flow problem is verified by simulating the sloshing problem under external forces and comparing with the corresponding experimental and numerical results.Through the simulation of droplet impacting on the high temperature wall covered with horizontal liquid film,the change rules of flow and heat flux distribution are analyzed,and the applicability of the current program to the flow and heat transfer near the wall is verified.Both provide a basis for studying on two-phase oscillating flow and heat transfer in real oil galleries.(4)Research on two-phase oscillating flow and heat transfer for annular galleries has been carried.The two-dimensional profile partially filled with oil and the three-dimensional oil chamber with oil injected from the nozzles have been studied.The flow and heat transfer between the cooling oil and chamber walls during axial reciprocating motion and circumferential flow is discussed,by studying the influence of engine speed,oil injection speed,inclination angle and oil filling ratio on the oil coverage ratio,oil-gas conversion rate and heat transfer coefficient.It is founded that the extreme value of oil-gas conversion rate corresponds to the position where the oil coverage ratio changes fastest,and the heat transfer coefficient is related to both the oil coverage ratio and the flow state near the wall.Increasing the injection speed and reducing the crankshaft speed will lead to an increase in the oil filling ratio of the cavity.With the increase of oil filling ratio,the heat transfer coefficient first increases and then decreases,reaching the maximum value near 70%.For high-speed engines,the reciprocating motion direction of piston has little effect on the heat transfer coefficient of the cavity.(5)Research on two-phase oscillating flow and heat transfer for complex galleries coupled with a telescopic pipe has been carried.A complete cooling system is first established,and the influence of inlet pressure and crank shaft speed on the two-phase distribution,pressure and temperature inside the oil chamber is analyzed.It is founded that the telescopic pipe directly impacts the two-phase flow and pressure distribution of the gallery.The negative pressure effect caused by the tension of the telescopic pipe causes a large amount of air to be sucked into the gallery system,forming the gas phase medium for oscillating cooling.Increasing the inlet pressure and reducing the crankshaft speed will lead to an increase in the oil filling ratio of the gallery and an increase in the heat taken by the cooling oil from the piston head per unit time.Then a simplified model of flow and heat transfer only including the piston head gallery is constructed by overfitting the boundary conditions at the inlet of the piston head,combined with the method of loading momentum source term.The numerical results verify the practicability of the simplified method.