Experimental Study Of The Effect Of Bubble Movement On Boiling Heat Transfer On Bundle With A Big Length To Diameter Ratio

With the advantages of remarkable heat transfer performance in response to the small temperature difference between heating surface and working fluids,nucleate boiling is widely used in many fields,such as energy,dynamics and chemical industrial.The pressurizer of the primary loop in nuclear power plant is the key equipment to maintain the pressure of the loop.Inside there are some electrical heating tubes,arranged vertical and upside down.The heating tubes need to be operating stably under very high pressure with the boiling flow pattern developing along the tube.The ability of mastering the law of nucleate boiling phenomenon of big length-diameter ratio vertical heating tubes and predicting the heat transfer performance can provide theoretical references for design and safe operation of the pressurizer.Nuclear safety accidents can be avoided,consequently.Furthermore,influence mechanism of the bubble behavior and boiling flow pattern on the heat transfer performance was explained within this paper.The visual experimental investigations on boiling heat transfer characteristics of electrical heating tubes of AP1000 pressurizer were performed.The heat transfer performance along the tube,boiling flow patterns and nucleating characteristics on the wall were studied and analyzed quantitatively.The whole research contents can be divided into three parts.The first part,boiling heat transfer visualization experiment system was built.The thesis made single tube and a bundle of three tubes arranged in different ways the targets of comparative research.The effects of heat flux and tube pitch on the heat transfer performance were analyzed in an experimental way.The second part,the bubble dynamics and development of boiling flow pattern under different working conditions were observed by means of a high-speed camera.The third part,a quantitative analysis of statistics on the nucleating parameters,such as nucleation site density,bubble departure diameter and bubble departure frequency,were conducted to gain the nucleating trend on the heating surface under different working conditions.The visualization results and high-speed photography showed that the nucleate boiling flow pattern on the big length-diameter ratio heating tube had changed dramatically.There is also a correspondence relationship between heat transfer performance and boiling phenomenon.It is can be perceived as an entire liquid convection with a relatively low HTC(Heat transfer coefficient)when the boiling phenomenon is within the isolated bubble region.Violent two-phase flow can be seen when the boiling flow has developed to be a continuous column regime corresponding to a higher HTC.The HTC of the mist flow is in the middle.The correlation of the local heat transfer coefficient was fitted with the experimental data.The correlation was compared with the former empirical correlations and the results were acceptable under low heat flux.A lot of high-speed photography videos were analyzed and nucleation site density,bubble departure diameter and bubble departure frequency were counted.The results show that the dominant effecting factor on the nucleation sites is the wall superheat.At a given heat flux,the bubble departure diameters for the different heights of the tube were nearly the same.At a given height of the tube,the bubble departure diameter was influenced by heat flux obviously.The bubble departure frequency increased with the heat flux.The vertical coalescence is more likely to happen under high bubble departure frequency.The optimum heat transfer performance was got when the frequency was high enough.