Research On The Heat Transfer Characteristics Of Spiral Tubes Enhanced By Square Wave Pulsating Flow And Its Composite Spherical Corrugated Structur

Heat transfer enhancement is a main way to improve the heat transfer capacity of equipment and achieve energy saving.As an efficient heat transfer device,helical tube has been widely used in various industrial fields.The study of fluid flow and compound heat transfer enhancement in helical tube has been one of the research hotspots at home and abroad.The application of pulsating flow and spherical corrugated walls are both effective compound heat transfer enhancement methods for helical tubes.Base on the literature analyzing and summarizing of the two enhanced heat transfer technologies of the helical tube,this thesis firstly reveals the enhanced heat transfer mechanism of the square wave pulsating flow in a single helical tube.And then a new heat transfer enhancement technique for square wave pulsating flow composite spherical corrugated wall is proposed.The new technology is applied to the helical tube to further enhance the heat transfer performance of the fluid in it.Experimental and numerical simulation methods were used to analyze the flow and heat transfer characteristics of fluid in the helical tube under square wave pulsating flow satation.The effects of pulsating parameters(including pulsating frequency and pulsating amplitude)and fluid properties on heat transfer enhancement performance are investigated.Based on the―field synergy principle‖,the heat transfer mechanism of a single helical tube strengthened by the square wave pulsating flow is revealed.The results showed that with the increase of dimensionless amplitude((?))and dimensionless frequency(Wo),the Nusselt number(Nu)and fluid flow friction coefficient(f _D)of the helical tube increased.The square wave pulsating flow of Wo=9 and(?)=0.25 performed best in comprehensive heat transfer enhancement within the studied range.When the working medium was water,engineering oil and ethylene glycol,the comprehensive enhanced heat transfer coefficient(TP)was within 1.06～1.08,1.02～1.06 and 1.0～1.03,respectively.It showed that square wave pulsating flow had better heat transfer enhancement effect on the fluid with smaller Prandt number.The analysis results showed that the disturbance of square wave pulsating flow enhanced the average secondary flow intensity and average turbulence intensity of the mainstream in the helical tube.In addition,the backflow emerged near the inner wall due to the flow rate sudden change.All of these were favorable to improving the synergy between the velocity field and the temperature field and achieving the heat transfer enhancement.The heat transfer enhancing performance of the square wave pulsating flow combined with spherical corrugated wall in the helical tube was investigated numerically.The composite flow field structure and the local heat transfer enhancement characteristics of the fluid in a pulsating period were analyzed.The results showed that the square wave pulsating flow further improved the local flow field characteristics in spherical corrugation.At the same time,the intensity and range of the average secondary flow were further enhanced and the turbulent ability of the mainstream fluid was increased.Thus the efficient heat transfer enhancement of the helical tube was further realized.During more than 3/5 moments of one pulsating period,the circumferential average Nusselt number(Nu_m)was noticeably greater than that in steady flow with the same average velocity.At the end of the first half period,the value of Nu_m was the largest,which increased by 38.5%compared with the corresponding value in the steady flow state.Within the scope of Dean number(De)De=2936-4894,as(?) increased,the overall average Nusselt number and friction resistance coefficient increased,but the comprehensive heat transfer enhancement performance decreased.The optimum dimensionless pulsation frequency was Wo=35 and TP was in the range of 1.06～1.13.The thesis proposed a new type of compound heat transfer enhancement technology of the helical tube.The research content provided a theoretical basis for the technical innovation and design of relevant heat transfer equipment.