Study On Solid-Fluid Conjugate And Boiling Heat Transfer For Gasoline Engine

The heat generated by the gasoline engine which is running should be emitted into theenvironment through cooling system in time, otherwise it will increase heat load of theengine components which are heated and there maybe hot fault, then affects the enginereliability, durability and longevity. The strengthening degree of modern engines is higherand higher, and then the working environment of engine components is worse and worse,so the demand of research on engine cooling system is increased more and more. Coolingwater jacket is an important component of engine cooling system in water-cooled vehicleengine, which has complex structure, and it is difficult to make sure the flow status of itsinternal coolant. It is difficult to analyze and measure the flow status by traditional fluiddynamics theory and experimental methods.Previously, the study on calculation of heat transfer was carried out by the cylindersolid and the cooling water separately. This method needs to adjust the boundary conditionsof the cooling water side during calculation continuously, until the calculation is coincidentwith the experimental results. This process is very fussy. Taking into account the heattransfer condition on the solid-liquid boundary is not uniformed, this method of calculationdoesn't accord with the actual. In addition, the calculation of engine heat transfer does notinclude the effect of boiling heat transfer to the temperature distribution. In recent years,with computational fluid dynamics and computer technology development, new simulationcalculation methods and models are applied more and more in many engineering designs.For study on the problems of the flow and heat transfer in engine cooling systemdeeply, and further reveal the microcosmic mechanism, a four-cylinder gasoline engine ischosen as the research object, cooling fluid flow of which the coolant flow was supposed assingle-phase flow, solid-liquid conjugate heat transfer of which was supposede as the coolant flow was single-phase flow, and the boiling heat transfer in which the coolant flowwas supposed as two-phase flow were simulated, and a comparison was made between thecalculated results and experimental data in this thesis.The main work includes:1. CFD analysis was made for the gasoline engine water jacket and the structurewas improved: it is necessary to re-design the engine body structure for the need of thetechnology innovation. And the new water jacket was designed to fit the new model. Firstly,an elementary water jacket structure for the new engine was designed, then the coolantflow was calculated by applying commercial CFD software—FLUENT. The analysis wasmade on the results of velocity field, pressure field, flow distribution and heat transfercoefficient. The velocity field problems were found, and then three kinds of projects werebrought forward. The improved projects was calculated and compared. Finally, thepreferred structure of the water jacket was determined. In order to analyze the designmethod for the separated cooling type water jacket, many kinds of improved models wereset up and calculated. The influence of the flow guide structure on the distribution of flowfield was analyzed and summarized.2. Research on solid-liquid conjugate heat transfer of gasoline engine: In order toreduce human impacts and intervention in the calculation, and make the calculation resultsshow the temperature distribution of heated engine parts directly, the heat transfer researchwas made by looking solid and fluid region of the engine as a whole, which is calledsolid-liquid conjugate heat transfer. Using the integer -division-assembly method, thehigh-quality, suitable grids for two different region characteristics were generated, it ismore convenient to set up the grids control parameters by this method, and the grids at theinterface of two different regions are assembled. The heat transfer boundary conditions ofgas in cylinder were made by the GT_POWER simulation model, which had been verifiedby experiment. A UDF program was wrote and linked into the FLUENT.3. The test platform of engine temperature field was established: in order tounderstand the temperature distribution in the designed engine when the engine is running normally, verify the simulation results through experimental, determine whether appearsthe boiling phenomenon in the local region of cooling water jacket, test platform of enginetemperature field was established. The measuring point was arranged, and then differentthermocouple seal methods were discussed and compared. The experiment implementationwas monitored visually by the existing data acquisition system and test control interface.4. Research on coolant boiling heat transfer: near the nose of the cylinder headexhaust side and the cooling bridge of the cylinder block, there is a little big differencebetween the calculation results of engine solid-liquid conjugate heat transfer andexperimental data, which shows that it is very likely that boiling heat transfer phenomenonoccurred. In order to verify this judgment, research on coolant boiling heat transfer wascarried out. Previous studies have shown that the boiling heat transfer in water jacketbelongs to the nucleate region, the Chen model and Franz model were used to research theboiling heat transfer. General to consider, it can be concluded: the calculation results gainedby Franz model are closer with the experimental results, so it is more suitable for researchon the boiling heat transfer for engine coolant. The results of the two-phase flow in theliquid region by CFD simulation are consistent with the calculation of heat transfer. Itexplains the causes of temperature distribution from a microcosmic point of view.The main innovations of the thesis include:1. Analyzing the influence and effect of the flow guide structure on the flow fieldin the separated type water jacketIn former studies, during structural optimization of the engine water jacket, it wasoften used to modify the number and size of the gasket holes, but this method is onlysuitable for the coolant mixed flow structure of water jacket. In this thesis, the water jacketstructure of the gasoline engine is separated type. The method of modifying the gasketholes to improve the model does not work. In order to analyze the design method for thesepareated structure of the water jacket, summarize the design rules and experience, manykinds of improved models were set up and calculated. The influence of the flow guidestructure on the distribution of flow field was analyzed. 2. Proposing numerical simulation method model of solid-liquid conjugate heattransfer for the engine.Using integer -division-assembly methods, high-quality, suitable grids for twodifferent region characteristics were generated. Considering the characteristics of cylinderheat transfer, the simulation software--GT-POWER was used to make partition of thecylinder, the results of the simulation was written as a UDF program which was linked tothe FLUENT software. The calculated results gotten from that method were compared withthe experimental data, except for the data of the nose of the cylinder head and the coolingbridge of the cylinder block, there are smaller differences between the calculation resultsand the experiment data, which shows that such a numerical simulation method is suitablefor analysis and calculation of the engine temperature field.3. Proposing that Franz model is more suitable for the calculation of enginecoolant boiling heat transfer.The mechanism of boiling heat transfer is complex, and it is affected by many factors,so the application of some models must meet the applicable conditions, and there are stillmore differences between the calculated and experimental results. In this thesis, enginecoolant boiling heat transfer was studied through the Chen model and the Franz model. Theresults of the Franz model are closer to the experimental results, so it is more suitable forengine coolant boiling heat transfer research.