摘要
轴快流CO2激光器是目前大功率激光工业加工应用的主力光源,随着加工工业的发展,不仅要求激光器具有更高功率,而且要求具有体积小、重量轻的紧凑结构。实现这一目标的关键在于气体循环系统的优化设计,其核心在于热交换以及流场的优化。本文针对高功率轴快流C02激光器的热交换以及流场特性对激光器实现高效、稳定的激光输出以及长时间运行稳定性的影响进行了研究,将气体循环系统分成了四个主要区域,就决定热交换过程以及流场分布的三个关键区域：放电区域、热交换区域、风机入口流道区域进行了分析研究。论文的主要研究工作如下：
(1)研究了轴快流CO2激光器气体循环系统的第一个关键区域—放电区域内部流场的优化方法。比较了不同入流喷嘴偏转角度的放电管数值模拟结果,主要将分析重点放在沿管长方向上的速度和湍流强度分布,以及横截面上的速度和湍流强度分布上。可以看出工作气体的涡流在阳极区是有利的,但在直管段要尽量抑制；入流喷嘴偏转45角,可以实现管内最利于稳定辉光放电的气体流动状态,既在阳极区产生强湍流流动,又保证了直管段均匀、稳定的流动。为了提高放电管的输出功率,将管径由19 mm增加到23 mm,需要通过合理的设计来消除流道变大带来的流动不稳定性影响。通过比较不同入流喷嘴尺寸设计的23 mm放电管的内部流场分布,得到了优化设计的原则,即放电管管径和入流喷嘴直径存在一个最佳的比值R最优=1.5,增加放电管直径时需要按照此比值来确定入流喷嘴尺寸。并且进一步研究了入流喷嘴尺寸对于气体流场分布均匀性和稳定性的影响。
(2)针对激光器热交换过程的特点,比较了适用于轴快流CO2激光器的各种换热器的主要特点及优劣,并最终选择了矩形翅片管式换热器作为所研制激光器气体循环系统中的热交换部件。并以7 kW轴快流CO2激光器为例,给出了此类换热器结构参数设计、换热能力以及流阻计算的传统经验公式方法,通过这种方法设计的热交换设备已经运用于所研制的7 kW轴快流CO2激光器,效果良好,在输出功率达到8 kW以上时也能很好的满足换热需求。
(3)研究了轴快流C02激光器气体循环系统的第二个关键区域,热量交换区域的数值模拟方法。通过分析轴快流CO2激光器内部热交换过程的特点,得到了几何模型、物理模型以及数学模型简化和近似的方法,进行了气体流场以及热交换过程的数值模拟的仿真。比较数值模拟的结果与经验公式的计算结果,得到了较好的一致性：换热量相差3.5%、压力损失相差仅2.9%,验证了本课题采用的数值模拟方法是合理且符合实际情况的。通过分析、比较不同管排布置的换热器数值模拟结果,得到了换热器内部流场的分布特点：压强损失与沿气体流动方向上管排数的增加而增大；管子迎风侧换热效果最好,背风侧换热效果最差,为了提高换热器换热效果,要尽量抑制背风侧的死区。根据以上特点,为4 kW轴快流CO2激光器选择了最为合理的管排布置,保证了所需的换热量,也满足了压阻尽量小的设计原则。解决了激光器的热交换设备设计计算繁琐且灵活性不高的问题,为其提供了准确、高效的新方法。
(4)研究了轴快流CO2激光器气体循环系统研究的第三个关键区域,风机入口流道区域。分析了热交换过程以及流场分布的数值模拟方法,计算得到了区域内部气体流场的详细分布。首先针对传统结构进行数值模拟研究,计算结果验证了数值模拟的合理性,根据传统结构的内部流场分析可以得到优化设计原则：进口气体的速度和温度都较高,因此入口端需要保证足够大的空间,以利于高速高温气体充分扩散到换热器的整个翅片区域；风机出口的涡流区域,导致了出口端流动阻力增加,压强降低也比较明显,因此,出口端尽量不要有障碍物遮挡,以免增加压阻。根据此原则,设计了紧凑型的风机入口流道结构。通过分析比较两个典型的紧凑型流道结构的数值模拟结果,为所研制3 kW轴快流CO2激光器确定了最优的风机入口流道结构,优化后的紧凑型结构不仅比传统结构节省了34%的空间,流阻减小了6%,并且可以实现3.6 kW的激光输出,最高电光转换效率达到23.4%。
论文的研究工作,改进了传统只能采取经验和试错的轴快流CO2激光器气体循环系统设计方法,为热交换以及流场的优化设计提供了理论指导,缩减了设计周期、节省了成本,使优化设计过程更加高效和准确。为轴快流CO2激光器朝更高功率和更紧凑结构发展,提供了有效的模拟仿真与设计工具。
The high power fast axial flow (FAF) CO2 laser is a very well established processing tool in the manufacturing industry. With the development of processing industry, the FAF CO2 laser needs not only a higher power, but also a smaller, lighter and more compact structure. The key factors in achieving all these characteristics are the optimization of heat exchanging and flow field of the gas circulation system. According to the analysis of the influence of the heat exchanging and flow field on the efficient laser output and the long-time stable work, the gas circulation system is divided into four parts. The three key parts which affect the heat exchanging and flow field are studied. The main work of this paper is:
(1) The optimization method of the internal flow field distribution of the discharge region which is the first critical part of the gas circulation system is studied. The influence of the nozzle structure and tube diameter on the internal flow field stability is analyzed, and an efficient and accurate optimization method for the discharge tube design is provided. According to the comparison of different numerical simulation results, the following conclusions can be obtained:the vortex of the working gas is benefit in the anode region, but disadvantaged in the straight length. Moreover, the situation that the tube diameter increase from 19 mm to 23 mm is considered, the internal flow field variation tendency is studied. The optimization design principle is obtained:there is an optimum ration between the tube diameter and the inflow nozzle size. The inflow nozzle size should be determined by this ratio after increasing the tube diameter. The influence of the inflow nozzle size on the gas flow filed is also further studied.
(2) The heat balance equation of the FAF CO2 laser is established. Meanwhile, in view of the heat exchanging characteristics of the high power laser, various types of heat exchangers are compared; the rectangle fin circle tube heat exchanger is chose as the laser radiator because of its prominent heat transfer capacity and compact structure. Further more, the traditional empirical formula method is used to design the heat exchanger for the 7 kW FAF CO2 laser. The results indicate that the rectangle fin circle tube heat exchanger is suitable to the FAF CO2 laser we designed.
(3) The numerical simulation method of the heat exchanging region is studied. According to the analysis of the inner heat exchanging process characteristics of the FAF CO2 laser, the geometric model, physical model and mathematical model of the heat exchanger are established, moreover, the inner gas distribution is obtained. The results between the numerical simulation and the empirical formula calculation are compared. The results are in good agreements, so the reasonableness of the numerical simulation method is proved to be an efficient method to the heat exchanger design for the gas circulation system. Moreover, the inner gas distribution in different structures of the heat exchangers is compared, and then the most reasonable structure for the 4 kW FAF CO2 laser is selected. The results can not only meet the requirements of the heat transfer capacity, but also achieve a compact structure.
(4) According to the analysis of the characteristics of the blower given gas circulation system, the importance of the inlet pressure of the turbo blower is studied, and the key part to the optimization design of the gas circulation system is identified. According to the working characteristics and structure of this region, the geometrical model and the physical model is established. Moreover, the flow field distribution is calculated by using the computational fluid dynamics method. The following optimization design principles are obtained:the inlet of the flow region should be ensured enough space, in order to facilitate the high-speed and hot gas fully spread to the entire fin area of the heat exchanger; the obstacles block should be avoided in the outlet region, in order to suppress the influence of the swirl. According to those principles, an optimized structure is designed. Compare to the traditional structure, the 34% length and 6% pressure loss is saved.
The research of this thesis improve the traditional design method of the heat exchanging process and flow field of the FAF CO2 laser, and make the design process more efficient and more accurate. The research provides an efficient tool for simulation and optimal design to promote the FAF CO2 towards higher power and more compact structure development.
引文
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