| Recently,the requirements on the internal combustion engine power,economy and emissions gradually increased,and IC engine aims at high power and high strengthening coefficient,the problem of heat load has become one of the major problems that IC engines need to face.Boiling heat transfer is an important method in cooling water jacket in the cylinder head of the IC engine,which results in an increased heat transfer coefficient and a smaller volume of cooling system.It can effectively solve the heat load of the IC engine by utilizing the high heat exchange capacity of the boiling bubbles in the water jacket of the cylinder head to achieve high temperature cooling.However,the realization of high temperature cooling must accurately comprehend the state of boiling heat transfer in the water jacket of the cylinder head,otherwise it will cause more serious heat load problems,so that it poses a security threat to IC engine.Due to the complexity of IC engine cylinder head,in order to improve the internal heat transfer condition,in this paper,a simplified horizontal rectangular channel is used as the research object,and make an in-depth study of boiling heat transfer mechanism in the water jacket of the cylinder head by theoretical analysis and experimental research,the main contents are as follows:(1)The design and construction of the visualization experiment platform have completed for the characteristics of boiling heat transfer in cylinder head.In order to study the effect of flow parameters on characteristics on boiling heat transfer,the effects of different flow parameters at the entrance such as flow rate,temperature and pressure on the wall temperature,heat flux and heat transfer coefficient were analyzed respectively,the study shows: When the heat wall temperature is high,low flow rate of the same conditions can achieve a higher flow rate of heat transfer effect,that reduce the flow rate appropriately can optimize the structure of the cooling system in the case of not affecting the heat transfer,while improve the cooling temperature and pressure of medium will reduce the boiling heat transfer efficiency.(2)The evolutionary behavior of bubble formation,slip,detachment anddisappearance is monitored by visualization platform,and the internal causes of heat transfer mechanism and boiling heat transfer characteristics in different boiling stages are analyzed and explained.The number and size of the bubbles can reflect different boiling heat transfer conditions,and the evolution of the bubbles can form a disturbance in the cooling medium,thus promoting the occurrence of heat transfer.The bubbles are generated on the wall,when the flow rate is 0.8m/s,cooling water temperature is 90 ?,pressure is 0.1MPa,respectively,and the wall superheat reaches17?.When the superheat gradually increased to around 65?,the wall was completely covered by the bubble,the boiling heat transfer efficiency can be increased by nearly 40% compared to the pure convection heat transfer.The primary cause of the change of heat transfer curve is explained from the perspective of bubble behavior,which provides a theoretical reference for the design of boiling heat transfer for cylinder head of IC engine.(3)This paper studies the boiling pressure in the cooling channel of cylinder head,and monitoring signals of pressure fluctuations caused by boiling are mainly collected and analyzed.And compared the difference between the time-domain pressure fluctuation signal in the three boiling states which is part of the development of boiling stage,the full development of boiling stage and dangerous boiling stage respectively is compared,the pressure fluctuation signal corresponding to the first two stages of boiling is more regular,while time-domain pressure signal in the state of dangerous boiling is extremely unstable The experiment shows that boiling pressure monitoring can be used as a new effective monitoring method for boiling heat transfer of cylinder head of IC engine. |
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