| In a periodically changing force field generated by ocean or in some accident conditions,the flow rate,pressure and void fraction of nuclear power equipment will also correspondingly periodically change.For certain two-phase flows,attention should be paid especially on the fluctuation ranges,frequency of pressure and void fraction,as it would appears flow-loss or dry-outphenomenon at some extreme situation,which will cause serious accident,for example,in boiling heat transfer equipment,if the pressure is oscillating too severe,somewhere of the channel's wall can't be wetted timely,which will be melted through by the accumulated heat,then cause the serious LOCA.Besides,plate components(thin gap),for their advantage in smaller size than circular pipes with same heat transfer ability,are used commonly in compact-layout-need nuclear power equipment.While the disadvantage of the plate components than the circular pipes is that their bearing capacity is weaker.Therefore,the purpose of this study is to understand the tin gap's flow resistance characteristics and its calculating method of both single-phase and two-phase flow,and its fundamental influencing factor,void fraction,of two-phase flow,in periodically changing force field.The study methods are looking for the corresponding relation among the pressure drop,flow rate and void fraction experimental data,and numerical simulating those parameters evolution with time.This study enriches the contents of nuclear thermal hydraulic research,and provides references for thermal hydraulic analysis and safety analysis of nuclear power plant under moving conditions.In the study,the periodically changing field is idealized as a harmonic pulsatile(sinusoidal)force field.According to Fourier series,a complicated periodically changing force field can be got by superimposing some simple harmonic force fields,additionally the simple harmonic force field is the first order approximation for other periodically changing force field.In the experiments,thesingle harmonic pulsatile flow was gotten by controlling the rotational speed of the pump.The test section is a horizontally placed transparent rectangular channel with cross-section of 40mm?3mm.A high-definition camera and high accuracy measuring devices with Lab VIEW software were set to record the pressure drop,flow rate and the phase distribution in the channel so as to visualize studying the parameters changing.By studying flow rate and pressure drop data of the single phase harmonic pulsatile flow,we found that the average pressure drop and its amplitude influence each other,and at the same time they are infected by the pulsatile frequency.With Laminar model the formulas of velocity radial distribution and phase difference between pressure drop and flow rate were derived,which shows that the velocity radial distribution varied with time and the deviation from parabolic are influenced by the pulsatile frequency and amplitude;the phase difference between flow rate and pressure drop positively correlates with pulsatile frequency and negatively correlates with the diameter of the pipe,but irrelevant with the pipe's shape.Based on similarity theory,the calculation equations of average pressure drop and pressure drop amplitude were got by fitting method.The Dynamic-Respond method was proposed to be made in use for calculating the pressure drop relative amplitude and the phase difference between pressure drop and flow rate.The method can simplify the harmonic pulsatile problem from studying the pulsatile frequency,amplitude and phase difference of each flow condition to studying the constants in Dynamic-Respond transfer function.And the transfer function's constants are independent of the pulsatile frequency,amplitude and the phase difference.By studying the pressure drop,flow rate and void fraction in harmonic pulsatile flow,it was found that the average pressure drop is not sensitive to the pulsatile frequency/period and amplitude.There is a strong linear relationship between the two-phase multiplier and the reciprocal of the liquid content,not square relationship,when calculating the average friction pressure drop with Lockhart-Martinelli method.Calculating the friction pressure drop with this linear relationship,the calculation errors are tiny,within 5%.In order to find out the accuracy of calculating the friction pressure drop with the instantaneous void fraction,an image processing function was programed to identify the bubble boundary,and the identification performance was good.Based on the bubble boundary identification results,an iterative method is used to restore the instantaneous value.It was found that the instantaneous void fraction changes relatively randomly,which cannot accurately calculate the two-phase instantaneous pressure drop.While the pressuredrop amplitude can be calculated with the single-phase pressure drop amplitude equation in Chapter 3,by which the instantaneous pressure drop can be got relatively accurately.The Dynamic Respond method can also perform well in calculating the phase difference in two-phase harmonic pulsatile flow.The homogeneous model empirical function for calculating the average friction pressure drop was corrected for harmonic pulsatile flow.The calculation formula of instantaneous friction pressure drop was also given,and at the relative pressure drop less than 1 condition,the errors are small,within 5%.In order to find out the reason that the two-phase harmonic pulsatile flow pressure drop change predictably but the void fraction is relatively random,two-dimensional direct numerical simulation on the two-phase harmonic pulsatile bubbly flow was carried out by using the front tracking method.It is found that there is a strong uniform variation between flow and pressure gradient in the calculation area.The phenomenon of bubble cluster occurs when the pulsation frequency is small,but the bubble cluster size is relatively random,which can confirm the experimental phenomenon to a certain extent.In addition,the bubble cluster phenomenon can also be seen in the 3D direct numerical simulation. |
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