Research On Fluid-solid Coupling Numerical Simulation Of Boiling Heat Transfer And Reliability In Cylinder Head

For water-cooled internal combustion engine, sub-cooled boiling heat transfer is an important heat transfer mode of cooling water-jacket in cylinder head. However the existing boiling heat transfer model does not consider the influence both of local flow and air bubbles on flow boiling heat transfer and the velocity on boiling heat transfer flux, the critical heat flux point and the void fraction. Therefore, in order to simulate, predict and evaluate the boiling heat transfer phenomena of cooling water-jacket in cylinder head more accurately and design cylinder head more rationality, the fluid-solid coupling numerical simulation of boiling heat transfer and reliability in cylinder head is studied in this paper including the following aspects:1,Based on the existing single-phase flow boiling heat transfer model, considered the influence of velocity for boiling heat transfer flux and the critical heat flux point, a conservative void mass equation was established through theoretical analysis and mathematical derivation. The void fraction distribution of the flow was estimated indirectly by the relationship between void mass and void fraction to reflect the existence of two-phase flow. Calculated the boiling heat transfer flux by the linear superposition theory and described the boiling heat transfer curve completely, then a new homogenous flow boiling heat transfer model was established. Embedded the new boiling heat transfer model into CFD software by UDF and UDS subroutines to solve the mixed flow with single-phase flow equations and void mass equation. Comparation between the simulation results of the new boiling heat transfer model and the experimental results shows that they were achieved a good agreement.2,For the convenience of numerical simulation of boiling heat transfer, a corrected algorithm that simulated boiling heat transfer was put forward based on the single-phase flow boiling heat transfer model. The impact of boiling heat transfer was reflected by correcting single-phase convective heat transfer. Aiming at the application of the boiling heat transfer model which is applied to the design of cylinder head, it was proposed that the critical value of mean void fraction which is located from the wall to the height of 5 mm was used as the criterion for judging the limit state of nucleate boiling of cooling water-jacket in cylinder head and to evaluate whether the boiling heat transfer in cooling water-jacket of cylinder head exceed the limit state of nucleate boiling. The reasonable mean void fraction was between 0.20 and 0.94. Then the boiling heat transfer system was established completely.3,Based on the basic theory of fluid-solid coupled algorithm, combined with the characteristics of flow and heat transfer in cylinder head, a new automated fluid-solid numerical simulation software was developed adopting C and python language with CFD and FEA software. The point cloud quick registration algorithm is used to obtain rapid match of mass dataon coupling face. Via MPCI to control and manage the transformation of different process automation, used python language to convert and transfer data between the different software. Take 226B diesel engine for example, compared simulation results of automated fluid-solid coupled algorithm with results of traditional methods showed that this automated fluid-solid coupled algorithm had more enough precision and important industrial application value.4,The automated numerical simulation of fluid-solid coupling on flow and boiling heat transfer of cooling water-jacket in cylinder head of diesel engine was employed using the software of CFD and FEA, and obtained the distribution of the stress field and flow field in the cooling water-jacket. According to the simulation results, the shape of cooling water-jacket was modified, the modified results showed that the flow uniformity improved obviously and the velocity around the exhaust valve and injector improved. The valve of mean void fraction which is located from the wall to the height of 5 m m in the cooling water-jacket was less than its'critical value, indicating that the nucleate boiling heat transfer in cooling water-jacket of cylinder head was safe and the criterion of mean void fraction had more extensive significance. Considered the impact of boiling heat transfer, boiling plays an evident role in enhancing heat transfer of cooling water-jacket in cylinder head, which should not be ignored in numerical calculation.5,The numerical simulation of heat transfer in cylinder head was performed using three kinds of methods by boiling heat transfer correction algorithm, the simulation results indicated that the result of three kinds of methods have a good agreement. According to the simulation iteration convergence time and the degree of difficulty, the second method was the most reasonable one. Comparation the simulation results of the new homogenous flow boiling heat transfer model with boiling heat transfer correction algorithm, indicating that had a good agreement. In order to verify the accuracy of numerical simulation, the temperature on interior of cylinder head in diesel engine was measured using copper-constantan thermocouple. Compared the simulation results with the experimental results, the two results were in good agreement with each other, the new homogenous flow boiling heat transfer model and boiling heat transfer correction algorithm have enough simulation precision.6,The finite element analysis for structure stress and thermal-structure stress of cylinder head were calculated and the fatigue analysis for cylinder head was calculated. According to the simulation results the shape of the cylinder head was modified, the modified results showed that the fatigue strength was improved. Meanwhile the influence of boiling heat transfer on both stress and fatigue were analyzed, it showed that cylinder head could work in a lower temperature condition because of boiling heat transfer which could reduce the thermal stress value, increase safety coefficient and intensifie their capability of preventing thermal fatigue.