| Marine diesel engines had been used as the main propulsion power of ships since the inception.From the 1950 s,marine diesel engines had almost completely replaced steam engines.Marine diesel engines had high thermal efficiency,good startability,and good economy.They were the main power of civil ships,small and medium-sized ships and conventional submarines.As an important heat exchange equipment on diesel engines,heat exchangers were used to cool down the fresh water and lubricating oil.A heat exchanger with excellent performance played an irreplaceable role in improving the thermal efficiency and maneuverability of diesel engines.Shell and tube heat exchangers and tube-fin heat exchangers were used as the heat exchange equipment on diesel engines.In this kind of heat exchanger,the tube side had a simple structure and a fixed flow pattern,while the shell side had very complicated flow and heat exchange.Heat transfer performance and resistance performance were a pair of contradictions.Recently,in order to improve the heat exchangers performance,some researches were focused on enhancing the heat transfer performance while ignoring the resistance performance,which resulted in poor overall performance.Recently,as the concept of energy saving and emission reduction had became more and more deeply rooted in the hearts of the people,it was necessary to seek one method that can not only enhance heat transfer performance,but also improve overall performance.Improving heat transfer performance on the basis of saving energy had been an urgent problem to be solved.In order to enhance both heat transfer and overall performance by improving the shell side structure scientifically and reasonably,it was particularly important to study the influence of the structure outside tube on the flow and resistance in the heat transfer process,and it was also important to explain the relationship between the heat transfer and flow characteristic.This thesis was aimed to explore the influence of the structure outside the tube on the heat transfer and flow performance in the process of fluid crossing the heat exchange tube.The research on the heat transfer and flow characteristic in the surrounding area of the heat exchange tube provided guidance for improving the shell side flow and heat transfer performance and comprehensive performance scientifically and reasonably.This thesis had great engineering practical significance for improving the shell side flow characteristic and heat transfer performance of tube-fin heat exchanger or shell-and-tube heat exchanger.Firstly,the different effects between the louver structure and the heat exchange tube on the local flow characteristic and heat transfer were researched.The results showed that the louver structure had certain limitations on heat transfer enhancement.The gap between heat transfer tubes was the main flow area,and the enhancement of heat transfer around the heat exchange tube was mainly considered to enhance heat transfer;Then,by comparing the flow characteristic and heat transfer effects of the vortex generators at different positions,the different effects of flow characteristic and heat transfer performance were researched.The results showed that the front side of the heat exchange tube was the main heat exchange area,where the fluid flow direction was perpendicular to the temperature drop direction,which resulted in good heat exchange performance and high resistance performance.The area besides the heat exchange tube was the main fluid flow areas,where the fluid flow direction and the temperature drop direction were parallel.The heat exchange performance was poor and the resistance was small.In addition,the area besides the tube would greatly increase the resistance performance;The area behind the tube side was the wake area,and improving the wake area can simultaneously improve the heat exchange performance and resistance performance.Then,the flow chaaracteristic and heat transfer of the elliptical tube and the V-shaped streamlined tube had been studied,the results showed that the windward area directly determined the strength of the heat transfer performance,and the slender streamlined heat transfer tube was great helpful to reduce the flow resistance and improve the overall performance.Obviously improving the wake area had positive effects on three aspects: heat transfer performance,resistance performance and comprehensive performance.Finally,according to the relationship between local heat exchange and flow characteristic,and combined with the integral shell-and-tube heat exchanger experiment,an improvement was proposed for this type of heat exchanger.The heat transfer performance,resistance performance and comprehensive performance of the diamond-shaped shell-and-tube heat exchanger were significantly improved.The main content of the full text was as follows:1)In the case of low-power heat supply,a test bench was designed to meet the test requirements of high-power energy consumption heat exchanger.At the same time,the old experimental equipment and the newly designed experimental equipment were connected through LAN to improve the utilization rate of experimental equipment.The shell side of LQ10(shell and tube heat exchanger)was tested to study its heat transfer performance and resistance performance.Besides,it provided basic experimental data for the establishment of the simulation2)The louver finned tube heat exchanger was investigated to compare the comprehensive performance of the louver structure and heat exchanger tube,and analyze the heat transfer enhancement mechanism of louver finned heat exchanger.When the total inlet flow rate remained constant,the effect of the structure of the louver fin and the fluid velocity of the heat exchange tube on the overall thermal performance was compared.The influence of louver fin and heat exchange tube on flow performance and thermal performance was investigated.3)By studying three different positions of the same type of vortex generator,the effect of three areas of the front,besides and back of the heat exchange tube on the heat transfer performance and flow resistance performance were explored.In this section,by comparing the heat transfer performance and resistance performance of the three regions around the heat exchange tube,the vortex generator placed in the wake area of the heat exchange tube was proposed to enhance both the heat transfer performance and comprehensive performance.4)The influence of the wake region on the heat transfer performance and resistance performance was investigated by studying two groups of novel heat exchanger tubes.Different cross-section of heat exchange tubes was used to optimize the fluid flow channel and improve the wake area of the heat exchange tube.The influence of different cross-section heat exchange tubes on the thermal hydraulic performance was studied.5)Combined with the experiment of shell and tube heat exchanger,the numerical simulation of LQ10(shell and tube heat exchanger)was carried out.The improvement scheme of heat exchange tube was put forward,and the numerical simulation was carried out to improve the shell side channel structure of shell and tube heat exchanger.Compared with the circular tube shell heat exchanger,the pressure drop between the inlet and outlet of the rhombic tube shell heat exchanger was decreased by 16.68%-56.3%,the temperature difference is increased by 4.42%-9.29% and the comprehensive performancewan increased by 24.27%-107.8%... |
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