Study On The Flow Distribution In The Compact Parallel Heat Exchanger

In order to ensure the integrity of the containment under accident conditions,The passive containment cooling system(PCCS)has been generally introduced to the new generation of PWR nuclear power plants;a natural circulation loop is applied to export the heat in the containment to the environment to achieve the contorl of the temperature and the pressure.In the passive containment cooling system,the internal heat exchanger is the key equipment,and its heat exchange capacity has a decisive influence on the natural circulation of the system.The uniformity of flow distribution in tubes of the internal heat exchanger is one of the core issues in the design of the internal heat exchanger,which has an important influence on the flow resistance and heat transfer characteristics of the cooling water in the tube.Aiming at the core issue of the flow characteristics in the heat exchanger tube,this project intends to use CFD software to establish a calculation model of the internal heat exchanger,and analyze the structure,geometric dimensions,arrangement of heat transfer tube bundles,and convection flow such as phase change.The influence law of distribution uniformity is to determine the influence of these parameters on flow uniformity.Through the research of this subject,it is not only conducive to optimizing the structure of the internal heat exchanger of the containment vessel,strengthening the heat transfer of the heat exchanger,but also improving the design method of the internal heat exchanger.At the same time,it also has a certain degree of design for other types of heat exchanger.Reference significance.main tasks as follows:For the first time,the three-wave classification method is proposed for the flow distribution in the heat exchanger with a T-junction,and the classification methods and characteristics of these three types of waveforms are given.And based on the three-wave classification method,under the background of the influence of eddy current at the header tee of the central heat exchanger,the mutual influence of the header diameter and the center pipe spacing on the flow distribution characteristics in the pipe is determined.In addition,the influence of the inlet flow rate,the number of heat transfer tubes and the diameter of the heat transfer tubes on the flow distribution characteristics in the tubes is also analyzed.On the basis of the completion of the above-mentioned research work,the three waveforms are classified and discussed according to their different characteristics,and the influence of header diameter matching on the flow characteristics in the pipe is determined in three cases.On the basis of the previous research on the single row and single row tube bundle arrangement,eight tube bundle arrangements have been expanded.The calculation of these nine tube bundle layouts found that in addition to the recognized geometric structure size that will have a greater impact on the flow distribution characteristics in the tube,the tube arrangement will also have a significant impact on the pressure distribution in the header and the flow distribution in tubes.For example,in a symmetrical arrangement,the flow of the fluid in the header to both sides will introduce additional pressure loss.In the in-line arrangement of more than two rows,the flow of the fluid to one side up and down will also introduce additional pressure loss.When the pressure loss caused by the shunting effect is introduced into the system,the static pressure distribution in the header is changed from the static pressure recovery effect and friction loss dominated distribution law of the traditional structure(single row and single row)to static pressure recovery.The friction loss and the pressure loss introduced by the diversion on both sides are dominantly distributed.Three geometric structure improvement schemes are proposed,(1)double entrance design;(2)curved tee;(3)pipe extension method.The first two schemes can significantly improve the uniformity of flow distribution while reducing the overall flow loss of the heat exchanger.The advantage of the latter scheme is that the structure is very simple and the uniformity of flow distribution in the tube can be greatly improved.The additional resistance loss is small.Considering for the first time that under low pressure conditions,when the heat flux density is high,the supercooled boiling may occur in the tube.Using the low pressure supercooled boiling model,the influence of the supercooled boiling on the flow characteristics in the heat exchanger is obtained.And through the discrete analysis of the pressure distribution in the heat exchanger,the calculation results are explained.