| The piston heat load problems are becoming obvious with the continuous improvement of the intensity of internal combustion engine. Solving of the piston heat load has become a bottleneck of enhancing internal combustion engine. The enhanced piston cooling technology is one of the most effective means to solve this problem. At present, the oil cooling gallery structure is commonly used in internal combustion engine piston and the two-phase flow formed by cooling oil and air oscillates in the oil cooling gallery and can enhance the effect of strengthening heat exchange.Therefore, the flow and heat transfer of two-phase flow in the oil cooling gallery have become a hot spot in recent years,but the mechanism is still lacking.This topic is devoted to the study of piston heat transfer system,using the computational fluid foundation and the piston temperature field finite element analysis, basing on much of experiment data, several assumptions are proposed which is about the re-circulation characteristics of two-phase flow,and a injection model of the nozzle is established; a special transient oil target visualization test platform is created, according to the experiment results, combined with finite element simulation,CFD simulation and derivation calculation method, the initial and boundary conditions of fluid heat transfer are determined, then the main influencing factors of oscillatory flow characteristics of two-phase flow are proposed, and the functional relationship of convective heat transfer with the correction term is established, the mechanism of flow and heat transfer of two-phase flow in oil cooling gallery is revealed, providing a direct and reliable basis for the design optimization of piston cooling system,and laying the theoretical foundation of improvement of engine’s performance.The main research work of this paper is as follows:1 Study on jet characteristics of cooling nozzle in oil cooling gallery cooled systemA nozzle jet test bench is set up to study the nozzle jet at different oil injection pressure and oil injection temperature, and the relationship between the capture rate of the oil inlet and the distance from the nozzle outlet to the injection cross section is obtained. It is found that the higher the oil injection pressure or the higher the oil injection temperature, the larger the divergence angle of fuel injection oil, influencing the return flow of the inlet of oil cooling gallery. In addition, according to the experimental study, the nozzle jet is laminar flow.Based on the simulation and analysis of nozzle jet, the relationships between nozzle oil injection pressure, oil injection velocity and oil injection distance, jet radius and so on are studied. The simulation results show that, the maximum velocity of the core area near the ejection outlet is 0.2-0.6mm in the center of the offset, with the pressure increases, the velocity gradient of the maximum velocity of the core region increases; with the increase of distance between the jet flow and the nozzle, the effect of axial velocity of the section is getting smaller, and the attenuation of the radial velocity of the jet is more gentle, the velocity of the jet flow varies with the position of the radial section of the jet, and its distribution is similar; When the fluid is injected into the air, the jet velocity decreases gradually with the distance ranging from 0 to 28.6 mm. With the development of jet, the mass and energy of the oil and air are drastically exchanged at 28.6 mm from the nozzle, axial velocity attenuation increases,resulting in a sharp decline in jet velocity. In addition, the study shows that the injection volume of the nozzle has a great influence on the static filling in the oil cooling gallery.Finally, a non-submerged jet model of the laminar flow of the coolant from the nozzle to inlet is established. The relationships of velocity,flow,oil injection pressure,nozzle radius and expansion angle at each section are obtained, and the variation of the axial velocity and the jet distance.2 Experimental study on dynamic characteristics of two-phase flow in oil cooling galleryIn order to reveal the influence of each factor on the flow pattern of the coolant and the corresponding flow .pattern conversion mechanism, the prediction model of the flow form is studied. Regarding the influence of the piston motion on the flow pattern, the formation and transformation of two-phase flow patterns in the oil cooling gallery are analyzed theoretically, a concept of area coverage is proposed,a prediction model of the flow pattern of two-phase flow is established,the heat transfer intensity of the two-phase flow in the oil cooling gallery is determined.A dynamic visual shooting test bench is created, which can monitor the flow in the oil cooling gallery real-time. The two-phase flow patterns in oil cooling gallery could be observed directly through the camera, it is compared under different conditions of speed, injection pressure and temperature, static oil filling rate, gallery size and sectional shape. It shows that, the wave flow dominates when the engine on low speed,with the engine speed increases,the "liquid plug" becomes more obvious.The higher the engine speed, the greater the oil oscillation, the more complex the two-phase flow patterns. The difference of oil injection temperature directly results in the change of oil viscosity. The flow pattern’s change accelerates as the increasing of oil viscosity,leading to the emergence of "liquid plug" requires a lower speed. The cross-sectional shape of the oil cooling gallery influence its motion greatly, the two-phase flow distribution in the kidney-shaped gallery is more regular, when it moves up, the right side oil hit the top earlier, when it moves down, the left side oil hit the bottom earlier, the positions of trend of forming "liquid plug" are more in oval-shaped gallery, and drop-shaped gallery moves down, most of the cycles are close to the oil inlet and the oil is coming down to form a "liquid plug", forming a relatively large air area in the middle. In contrast, the effect of oil injection pressure on the flow is negligible.On the whole, the main influencing factors of the two-phase flow in the oil cooling gallery are the oil filling rate, the oil cavity shape and the engine speed.3 Numerical study on dynamic characteristics of two-phase flow in internal coolingThe flow of two-phase flow in the oil cooling gallery is simulated calculation by the CFD model. The simulation results of the area coverage rate and heat transfer characteristics of the two-phase flow in the oil cooling gallery are in good agreement with the experimental results, which is compared under different conditions of engine speed,injection pressure,injection temperature,oil cooling gallery size and section shape of annular cavity.The results show that the variation of the heat transfer coefficient of the top is contrary to the bottom of the oil cooling gallery with the crank angle, but the inner and outer wall’s variation is consistent; The injection temperature changes the viscosity-temperature characteristic of oil,thus affects the area coverage rate and heat transfer characteristics of the oil cooling gallery; The engine speed varies the oscillation intensity of the oil, thus changes the turbulence intensity and its heat transfer characteristics in the oil cooling gallery; The difference of oil cooling gallery structure directly affects the oil filling rate and impact level of the oil cooling gallery on its reciprocating motions, thus affects the heat transfer effect.4 Steady heat transfer prediction model of oil cooling gallery of internal combustion engine pistonFrom the view of engineering application, combined with the experimental research and numerical simulation results, using the coercive convective basis correlation formula in the tube,based on the Nusselt,Prandtl and Renault criterion,the correlation formula attached correction is fit out through the least squares method,a prediction mathematical correlations formula of convective heat transfer coefficient is established.During the progress of establishment of the prediction mathematical correlations formula, the thickness of the viscous sublayer decreases with the increasing of engine speed. No matter how much the engine speed or different pistons, the thickness of the viscous sublayer is greater than the roughness of the inner wall. The thickness of the viscous sublayer decreases as the piston bore increasing when the engine speeds are same; The natural logarithm of the Reynolds number and the Prandtl product is only related to the engine speed, and it increases as engine speed increasing.The influences of the assumptions, neglect factors, instability and so on are analyzed through the numerical method, the finite element analysis and the hardness test are used in the error analysis of heat transfer coefficient, calculated by the heat transfer prediction model. Therefore, the accuracy and applicability of the calculation method and results can be ensured. The results show that, the prediction model can effectively predict the convective heat transfer coefficient of the two-phase flow in the oil cooling gallery under the different conditions of engine speed, bore and oil temperature, and provide the theoretical basis for the designing of oil cooling gallery.Combined with the experimental study and the numerical simulation results, it shows that the heat transfer coefficient at different crank angle can be corrected by the area coverage rate of the wall based on the correlation formula.5 Influence of heat transfer characteristics of oil cooling gallery on piston reliabilityCombined with templug test, this paper studies the influence of the oil cooling gallery position on the heat load of the piston, the effect of the oil cooling gallery on the second-order movement of the piston, and the influence of the new piston structure on the piston strength, through simulating and analyzing by finite element analysis software,fatigue analysis software and dynamic analysis software. The cooling effect proves that the using of the oil cooling gallery greatly reduces the piston head temperature, and the higher the position of piston head, the better the cooling effect, the lower the heat load. The study also proves that the using of oil cooling gallery reduces the piston’s deformation and the force between the piston and cylinder,thereby significantly improves the piston friction and wear, lateral force and skirt pressure and so on. However, the stability of the piston second-order movement will be reduced, also the piston knocking noise will increase, then the piston skirt line needs to be optimized accordingly. The studies of the structure of the new type of oil cooling gallery show that, the structure of the oil cooling gallery in the ring carrier is better to avoid the stress concentration phenomenon, it can reduce the piston heat load through the oil cooling gallery,and also improve the strength of the first ring groove and the combustion chamber through the ring carrier effectively.Above all the studies, it can be found that the heat transfer efficiency of the oil cooling gallery affects the heat load of the piston, and the flow pattern of the two-phase flow in the oil cooling gallery is closely related to its heat transfer efficiency. The influential factors such as engine speed, oil injection pressure, oil injection temperature and cross section of oil cooling gallery affect the flow pattern of two-phase flow directly or indirectly. The flow pattern of two-phase flow directly reflects its distribution, which determines the effective scouring area of the liquid relative to the oil cooling gallery,and characterizes the intensity of heat transfer. In addition, the numerical simulation results are lower compared with the results obtained from the calculated value of correlation in this paper, because the change of gas-liquid interface in the axial plane is not taken into account, and the heat dissipation of the gas-liquid two-phase flow is neglected in the simulation calculation. |
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