Experimental Study On Bubble Behavior Of Subcooled Flow Boiling Heat Transfer

With the continuous advancement of human society and the importance of environmental protection issues,the emission regulations for internal combustion engines in various countries have become increasingly strict.The wide application of many new internal combustion engine technologies,including high-pressure common rail and turbo charging technology,is not only its power.The improvement of the performance also makes the engine more and more developed in the direction of high explosion and high power density,and the subsequent increase in thermal load poses a new challenge to the heat exchange capacity of the engine cooling system.Many new concepts such as "precise cooling" and "moderate cooling" to improve the cooling efficiency of the cooling system have emerged and are widely used in internal combustion engines.The area near the nose bridge area of the cylinder head cooling water chamber is the most intense part of the engine subjected to the alternating heat load during normal operation.Understanding and recognizing the sub-cooling flows boiling heat transfer mechanism of the cooling medium inside the water chamber is to improve the cooling efficiency of the internal combustion engine.The key is when boiling occur,heating the wall to a certain temperature will generate bubbles.The nucleation,development,detachment and extinction of the bubbles directly characterize the complex two-phase flow heat transfer mechanism of boiling heat transfer,and the law of bubble growth and detachment.Conditional research,also known as bubble dynamics,is important for understanding the sub-cooled flow boiling that occurs inside the cooling water chamber of an internal combustion engine.In this paper.the boiling heat transfer test bench is built to simulate the internal boiling heat transfer form of the engine,and the high-speed camera is used to record the complete life process of the large number of bubbles generated during boiling at the bottom of the simulated flow channel,mainly the equivalent of the bubble at different times.Collection of diameters,the multi-case experiment was designed to study the effects of different factors on bubble behavior,including flow rate,pressure,superheat and heat flux.By counting the frequency distribution of the diameter of the heated wall bubble,it can be found that the diameter distribution obeys the Gaussian distribution,and as the heat flux density increases,the bubble density of the heating surface also increases.The influence of the three parameters of the transverse contrast on the bubble distribution on the heated surface shows that the change of the superheat degree has a greater influence on the distribution of the bubble diameter when the heat flux density is higher.As the superheat degree increases,the bubble diameter distribution value increases overall.The trend of flow velocity on the bubble distribution is mainly due to its influence on the flow field in the local region.When the flow velocity is high,the heat exchange is rapid,and the bubble is difficult to reach a relatively high value,and the phenomenon of detachment occurs,which also causes the bubble on the heating surface.The number is reduced;an increase in pressure will cause the bubble diameter to increase as a whole.Through statistical analysis of the complete life cycle data of a large number of bubbles from nucleation,detachment to extinction,it can be found that the bubble starts from nucleation and the diameter increases continuously until the wall is heated,and the bubble reaches the maximum volume of the whole life cycle before and after the detachment.Then gradually decrease until it dies.There are many nucleation points on the heating wall.It is found that the overall law of the bubble is consistent under different test conditions.Under the same working condition,due to the different positions of the nucleation sites.,the difference of the local microstructure makes the bubble life cycle scale and the maximum bubble diameter respectively.There is a deviation of 0?1ms and 0?0.8mm.By longitudinally comparing the behavior characteristics of the detachment bubbles under three different gradients of heat flux density;it is found that with the increase of heat flux density,the life cycle duration of a large number of bubbles shows a decreasing trend.The influence of the three parameters on the behavior of the detached bubble is found.Under different heat flux densities,the flow velocity has a significant effect on the behavior of the bubble life cycle and its regularity is consistent.The overall size of the bubble includes the detachment diameter and the achievable The maximum diameter decreases with increasing fluid flow rate;as the pressure increases,the bubble life cycle duration tends to decrease,and this trend is particularly pronounced at lower pressures;the main fluid flow rate and heat flux density are not In the case of a change,the bubble size decreases as the degree of superheat increases,and the lower temperature gradient of the boundary layer around the bubble reduces the rate of bubble fusion,so that the bubble can reach a higher equivalent diameter throughout the life cycle.Finally,the data of the detached bubble collected in this experiment is brought into the prediction correlation of the predecessors for calculation.The comparative analysis shows that most of the prediction relations are not well applied to the experimental situation in this subject,and the error is large.Three common methods for predicting bubble detachment diameter are summarized and summarized.According to their advantages and disadvantages,based on the force balance method of the subject,the function correlation between bubble diameter and four dimensionless parameters is established.The results show that the prediction effect is good.The average relative error of the predicted bubble detachment diameter is 10.25%.