Study On The Cooling Of High-power Solid-State Lasers

The high-power solid-state lasers have a good prospect of applications in many fields such as the industry, the agriculture, the national defense and military, and the modern high technology. With the development of the laser output and the packages being more compact in size, the thermal effects become the major barriers obstructing the technological development in high-power solid-state laser and the traditional cooling methods are reaching their limits. In this dissertation, the high-effective spray cooling method outside the laser medium and the non-classical heat transfer in the laser medium were studied systematically and deeply. Both experiments and numerical simulations were carried out, which provides important experimental and theoretical instructions for resolving the problem of thermal energy removal of the high-power solid-state lasers.In order to understand the non-boiling heat transfer behavior of the spray cooling, an experimental setup with two full cone spray nozzles was established using pure water as the working fluid. A microscopic lens system in conjunction with an advanced high-speed camera was used to observe the spray process. The influences of the liquid flux, the spacing between the nozzles and the heated surface and the inlet temperature of the liquid on the cooling effect were investigated. It is found that the non-boiling spray cooling method can remove high heat flux from surfaces while maintaining low surface temperature, which satisfies the cooling requirements of the high-power solid-state lasers and the electronic components. The flow and heat transfer mechanism is believed to consist of convective heat transfer and direct evaporation from the surface of the liquid film. The droplet diameter is about 500μm. It is concluded that increasing the liquid flux increases the overall heat transfer coefficient and enhances the cooling effect distinctly. When increasing the space between the nozzles and the heated surface, the heat transfer coefficient increases first and then reduces. So there exists an optimal spacing in the experiment. With the reducing of the inlet temperature of the liquid, the surface temperature decreases and the heat transfer coefficient increases. The thermal performance can be improved further by adding proper surfactant. The concentration of surfactant plays an important role in spray cooling. A heat transfer correlation about Nu was developed on the basis of large amounts of experiments. The effective method to promote heat transfer and reducing the thermal stress is proposed.A three-dimensional geometric model was developed and the numerical simulations have been carried out to investigate the flow of the liquid pressure atomization and spray process using the Discrete Phase Model which follows the Euler-Lagrange approach combined with the Wall-Film boundary conditions. The complicated flow and atomization process was discovered. It shows that the spray pressure is the main factor to realize the atomization. After atomization, the droplet is basically uniform in the diameter and was also distributed on the heated surface uniformly. The droplet collision velocity reduces from the center to the edge. Increasing the liquid mass flux can increase the average droplet collision velocity, but the corresponding maximum film thickness on the heated surface declines. The film thickness changed irregularly with the increase of the spray distance, however, the temperature distribution tends to be uniform and the surface temperature happened to be the lowest where the corresponding film thickness was the least. Hence, both the surface temperature and its uniformity should be considered when confirming the best spray distance.A mathematical model was developed to investigate the details of two-phase flow and heat transfer in a thin liquid film. The model considers the effects of phase change between vapor and liquid, gravity, surface tension and viscosity. The dynamics of bubble growth in the film, the movement of the interface between two fluids and the surface heat transfer characteristics were numerically simulated for two cases:(1) when a liquid droplet impacts a thin liquid film with vapor bubble growing and (2) when the vapor bubble grows and merges with the vapor layer above the liquid film without droplet impacting. The influences of droplet falling velocity, droplet diameter and initial position, the multi-droplets impact on the flow and heat transfer were discussed. The interaction mechanism between the droplet and the thin film, the complicated multiphase flow and heat transfer characteristics were revealed. It is found that the droplet impact can improve the surface heat transfer notably because it can quicken the distortion speed of bubble, and leads to the secondary nuclei in the film. The peak value of surface heat transfer coefficient increases with the increase of the droplet velocity and meanwhile the position of the peak value moves left gradually. The heat transfer coefficient does not increase linearly with the increase of the diameter. The initial position of the droplet plays an important role in the heat transfer. When the droplet impacts the right side of the bubble where the collision leads more acute disturbing to the thin liquid film and promotes the bubble distortion speed, the surface heat transfer coefficient is largest. The heat transfer due to multi-droplets impact exceeds the case of single droplet impact significantly when the diameter and the falling speed are same. The distinct peak value of the surface heat transfer coefficient would appear according to the difference of the droplet number and the impact center. The surface heat transfer coefficient will tend to be uniform if the number of the droplets is enough. The above conclusions provide the theoretical basis on designing the spray cooling system by controlling the droplet parameters, which can realize the best cooling effect under low surface superheat.Considering the non-classical heat transfer phenomena in the process of series or impulse solid-state lasers, a one-dimensional unsteady non-Fourier heat conduction model with non-uniform heat source was developed to simulate the transient heat transfer in the laser medium under extreme conditions for the first time. The temperature fields were numerically analyzed using the finite difference method combined with the TDMA algorithm for different pump power densities, pulse durations, thermal relaxation time and cooling intensities, respectively. The calculated results are compared with those predicted by the classical Fourier heat conduction law. The results indicate that the non-classical heat transfer phenomenon of laser medium should be considered at the moment of pumping when the pump power density is more than 104W/m2. The larger the pump power density, the more apparent the non-Fourier effects. The longer the pulse duration, the less significant the non-Fourier effects in the laser medium. At the same time, increasing the size of the laser medium can reduce the pump power density, which also is helpful to debase the maximum temperature. The thermal relaxation time is a crucial factor to determine whether the non-classical heat transfer behavior is significant or not. The non-Fourier effects will be improved as the increase of the thermal relaxation time. So it is necessary to further confirm this value. This study provides an important theoretical basis and a new research method. Also, it expands the applications of the Non-Fourier heat transfer law.