Research On Gas Heat Transfer Enhancement In High Power Fast Axial Flow CO₂ Laser

Heat exchanger is one of the important parts of high power fast axial flow CO₂ laser. So improving the efficiency of heat transfer can make the laser more compact and stable. In order to achieve maximum output power, the structure of heat exchanger has been optimized and three different structure vortex generators were introduced into the channel of heat exchanger. Main contends of present work are shown as follow:A numerical model has been built for the heat exchanger of fast axial flow CO₂ laser. The influence of the arrangement of tubes on efficiency of heat transfer and flow resistance was analyzed with boundary layer theory, turbulent theory and induced velocity distribution theory. A staggered arrangement heat exchanger has been designed to take the place of online arrangement heat exchanger that make the volume of heat exchanger decrease by 14.2%. When the range of Re was from 400 to 1400, compared with the heat exchanger with online arrangement, the heat transfer efficiency of staggered arrangement increased by 9%²4% with the same pressure loss.The vortex and wake area of heat exchanger of fast axial flow CO₂ laser which has rectangular vortex generator in flow channel were analyzed with a low Re numerical model. It was found that the vortex generator could disturb the temperature boundary layer, promote the gas mixing and compress the wake area behind the tube. So it could enhance the performance of heat exchanger greatly. In order to make an all-around consideration of each factor, ten kinds of models have been made and the characteristics of flow and heat transfer of each model were analyzed. Over the range of Re mentioned, It was found that the rectangle vortex generator could provide a better integrated performance to the heat exchanger of fast axial flow CO₂ laser. The Nusselt number and friction factor of the optimal configuration increased by 5%¹7% and 23%³0% respectively.A novel longitudinal vortex generator,which is comprised of a modified rectangular wing and an accessory wing, was proposed. The influences of a single row of novel combined winglet pair?NCWP?, mounted in the first row of tubes, on heat transfer enhancement and flow field structure in the finned-tube heat exchanger were analyzed. The Re?based on hydraulic diameter? range was from 400 to 2400. In comparison of rectangular winglet pair?RWP?, the NCWP generates main vortex with larger centric intensity. Furthermore, the accessory vortex has a lower core and flows into the back of tubes compressing wake regions' size behind tubes due to the influence of accessory wing. It could affect the developing of boundary layers on the bottom fin and enhance effectively the heat transfer of the wake region behind the tube. Compared with RWP case, the heat transfer for finned-tube heat exchanger with NCWPs is improved by 5.5%, while the pressure penalty decrease by 3.5%. Therefore, the performance of heat transfer for finned-tube heat exchanger is significantly enhanced by NCWP with a moderate pressure drop penalty. So the fan load greatly reduced.In this work, a new type of elliptical curved vortex generator?ECVG? was reported. A numerical simulation has been presented for three-dimensional trajectory model of particles flowing through the ECVG. Three different kinds of vortices generated in the channel of heat exchanger. They are horizontal vortex, longitudinal vortex and horseshoe vortex respectively. Backward-facing step flow is reason for horizontal vortex. The horseshoe vortex is located in the bottom of the flow channel. So it could block the boundary layer separation on the surface of tube. The effects of shape, size, and installation form of vortex generator on the performance of the ECVG have been conducted by experimental method. It was found that attack angle of 0 degree can enhance the heat transfer and decrease the friction factor of heat exchanger at the same time that cannot be achieved by the vast majority heat enhancement technologies. So the fast axial CO₂ laser with this kind of heat exchanger could achieve a higher output power.