| Increasing the performance of heat exchanger can effectively improve the energy utilization efficiency of energy conversion device,and reduce the size of heat exchanger.At present,the heat transfer enhancement under steady unidirectional flow has been widely studied.However,the flow of working gas in various energy conversion devices like Stirling engine and refrigerator was in reciprocating oscillatory flow.The flow direction of working fluid reversed regularly with time.Moreover,the flow parameters,such as velocity,pressure,temperature,density and so on changed every moment.Thus,the conclusions of heat transfer enhancement under steady unidirectional flow could not be directly applied to the design of heat exchanger under reciprocating oscillatory flow.Up to date,there was still lack of researches on heat transfer enhancement under reciprocating oscillatory flow.In this dissertation,the physical and mathematical models of reciprocating oscillatory flow were established by three-dimensional numerical simulation.The changes of fluid velocity,pressure and flow direction with time were realized by setting sine or cosine function in boundary conditions.Twelve transient moments at intervals of 30° phase angle for a reciprocating oscillatory flow cycle were selected to carry out steady numerical simulation.These results based on steady unidirectional flow were compared with the calculation results in a cycle by transient simulation based on reciprocating oscillatory flow.The results show that the heat transfer of working gas in heating tube at a certain phase angle calculated by transient simulation was greatly affected by the working gas state in the previous period.The acceleration and deceleration of flow also affected the heat transfer of working gas.Compared with the steady simulation,the heat transfer performance of heating tube calculated by the transient simulation was slightly weaker,and a larger pressure was consumed.Moreover,the average velocity was similar,but the spatial distribution of velocity was quite different.Because the change of pressure difference between the inlet and outlet of tube caused the velocity change of working gas,and the velocity change of working gas caused the change of heat transfer in heating tube.Thus,for the transient simulation,the change time of pressure difference,velocity and heat transfer were earlier than,substantially equal to,and later than the change time of steady simulation.The heat transfer characteristics of reciprocating oscillatory flow in a two-start spirally corrugated tube were investigated.The results indicate that the longitudinal swirl vortexes were generated under the guidance of a spirally corrugated channel and the mixture between fluid in boundary region and in core region was significantly enhanced,which promoted the heat transfer of two-start spirally corrugated tube.The heat absorption of a two-start spirally corrugated tube exceeded that of a smooth tube.Under the geometric parameters and working conditions in this dissertation,their difference of the average heat absorption in a cycle was 561 W and their ratio was 1.36.The outlet temperature of fluid in the enhanced tube was 160 K higher than that in smooth tube,and the pressure consumption of enhanced tube was 430 Pa more than that of smooth tube.When compared with the pressure in heating tube(1.5?2.5MPa),the additional pressure consumption by enhanced tube was negligible.However,the outlet temperature of heating tube could be increased significantly.Thus,the benefits were apparent for the heat transfer enhancement by using spirally corrugated tubes in the heater.Two types of performance evaluation criterion(PEC)calculation methods were used to evaluate the heat transfer enhancement of a spirally corrugated tube.The average transient PEC value in a cycle was 1.69,and the PEC value calculated by average parameters in a cycle was 1.38.Thus,the heat transfer enhancement of two-start spirally corrugated tube was superior.Thermal expansion of working gas is one of the driving forces for reciprocating oscillatory flow in a Stirling engine.Therefore,the performance of heater is critical to a Stirling engine.A tube inserted with spiral spring was propsed to enhace the heat transfer of a Stirling engine heater with an attention to avoide the disadvantages of special section tube,such as the difficult manufacture,the subdued endurance for high temperature and high pressure,and so on.It is found that longitudinal vortices were formed in the tube inserted with spiral spring.The size,number and distribution of vortices were various with time and location.Under the geometric parameters and working conditions in this dissertation,the Nu,f and PEC value increased with h and decreased with p for enhanced tube.The influence of h on flow and heat transfer was more remarkable than that of p.With hydrogen,helium,nitrogen and carbon dioxide as working gas,compared with smooth tube,the PEC value of enhanced tube(p=24mm,h=1.8mm)were 1.33,1.22,1.21 and 1.25,respectively;the fluid outlet temperature of enhanced tube increased by 68,64,57,and 63 K in the entry process,respectively;and the temperature increased by 32,34,40,and 51 K in the return process,respectively;the average pressure consumption in a cycle increased by 446,1256,7079,and 13320 Pa,respectively.Considering the safety and heat transfer enhancement,helium was recommended as the working gas for the enhanced tube.Cold compression of the working gas is another driving force for reciprocating oscillatory flow in a Stirling engine.A tube inserted with helical wire of constant or variable pitch was proposed to enhace the heat transfer for a Stirling engine cooler.Under the geometric parameters and working conditions in this dissertation,the results show that the average temperature of outlet fluid decreased with the decrease of helical pitch for enhanced tube.The average temperature of outlet fluid decreased with the decrease of diameter and the increase of length for smooth tube.Under the condition of same average pitch,the heat transfer enhancement for enhanced tube with constant or variable pitch was similar.With the same size,the temperature drop of enhanced tube was 1.08?1.54 times as much as that of smooth tube for different flow stages.The extra pressure consumption by inserting helical wire was no more than 3956 Pa.The advantages of enhanced tube under reciprocating oscillatory flow were evaluated by the decrease of outlet temperature or the reduction of heat exchanger size according to the actual situation of Stirling engine in this study.Under the working conditions in this dissertation,when obtaining the same outlet temperature of fluid,the length of smooth tube should be increased to above 2 times as much as that of enhanced tube(p=12.5mm)with same tube diameter(d=5mm).In another case,the diameter of smooth tube was nearly half of that of enhanced tube with same tube length(l=180mm).This implies that the amount of smooth tube needed to increase to 3.7 times as much as that of enhanced tube.Therefore,the tube with a helical wire could decrease the size of cooler and improve the compactness of Stirling engine. |
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