Study On The Heat Transfer And The Thermal Stress In High-power Solid-State Lasers

Thermal effects are among the major barriers obstructing the technological development in high-power solid-state laser. In this dissertation, the problems of conductive and convective heat exchange in laser medium and pump DLAs, as well as the associated problems of thermal stress and thermal strain are studied to seek the methods for enhancing the heat transfer, lowering the device temperature, improving the temperature uniformity and reducing the thermal stress, in order to promote the laser output and the beam quality.Mathematical models of the convective heat exchange in the cooling channel, the heat conduction and the thermal stress in the laser medium were developed for a side-pumped, side-cooled cylindrical laser rod. The spatially varying fluid temperature and the convective heat transfer coefficient were determined from the theoretical solutions or experimental correlations of convective heat transfer in an annular passage with prescribed heat fluxes. An analytical solution was derived from a two-dimensional axisymmetric heat conduction model with non-uniform boundary conditions. The first term of the analytical solution coincides with the result of the one-dimensional model. The other terms indicate that the axial temperature-rise in a laser rod has positive correlations with the axial coolant temperature-rise, the radial Biot number and the length-to-radius ratio of the laser rod. Subsequently, a conjugate numerical simulation that couples up the fluid convection and the solid conduction was performed. Compared with the analytical solution, the conjugate numerical simulation better exhibits the entrance effects of flow and heat transfer, therefore may provide more accurate solution in specific cases. It is suggested that the laser rod portion with large temperature gradient caused by serious thermal entrance effect be excluded from the working portion during the design of solid laser. Analytical solutions were further derived for the thermal stress and thermal strain in the laser rod. The analysis shows that under the traction free boundary conditions, the longitudinal rise of fluid temperature has little effect on the thermal stress profile in the laser rod. However, the thermal strain field caused by both the temperature and the thermal stress fields has an evident variation in the longitudinal direction, which will affect the laser transmission characteristics and the beam quality.Mathematical models were presented for the heat conduction and the thermal stress problems in an orthotropic laser medium under three pumping schemes. Analytical expressions for the three-dimensional temperature profiles were derived via an integral transform method. Analytical solutions for the thermal stress fields were derived through the Airy's thermal stress function method, according to the obtained 3-D temperature expressions, based on the plain-strain assumption. Comparisons of temperature,thermal stress and thermal strain components were made among different pumping schemes, and also between orthotropic and isotropic media. The results indicate that, the inner regions of the laser medium are subject to compressive stress, whereas the outer regions bear tensile stress; the maximum temperature and thermal stress under a top-hat pump beam are lower than those under a Gaussian pump beam, whereas the thermal effects under uniform pumping are the least significant of all; in the absence of external forces, the effects of orthotropic thermal properties on the thermal strain are more significant than on the thermal stress. These analytical solutions are applicable to the temperature and thermal stress problems in various orthotropic media under laser irradiation.Three-dimensional conjugate numerical simulations using the inlet, average and variable thermal properties, respectively, were performed for the laminar water flow and heat transfer in rectangular microchannels with Dh of 0.333mm at Re of 101-1775. The average and variable properties were adopted in the data reduction stage, respectively. The calculated local and average characteristics of flow and heat transfer were compared among different methods, and also with the experiments, correlations and simplified theoretical solution data from published literatures. Compared with the inlet property method, both the average and the variable property methods have significantly lower f_app, but higher convective heat transfer coefficient h_z and N_uz. Compared with the average property method, the variable property method has higher f_appRe_ave and lower h_z at the beginning, but lower f_appRe_ave and higher h_z at the later section of the channel. The calculated Nu_ave agree well with the Sieder-Tate correlation and the recently reported experiment, validating the traditional macroscale theory in predicting the flow and heat transfer characteristics in the dimension and Re ranges of the present work. A liquid-flow microchannel heat sink was designed and fabricated for a pump DLA by adopting dense, thin and short channels instead of the originally designed sparse, wide and long passages, in order to increase the cooling capacity, improve the temperature uniformity over the heating surface, while achieving a very low flow resistance, thus overcoming the problems involved in the old heat sink design. Under the maximum flow rate of 70ml/s in this experiment, the pressure drop across the channels is only 10.3kPa; when the average temperature rise of the heating surface is 25.7℃, the cooling capacity of the heat sink reaches 730W, corresponding to an interfacial heat flux of 146.9W/cm², while the overall thermal resistance is as low as 0.035K/W, or 0.199 K·cm²/W. The cooling capacity is expected to be further increased if the Reynolds number is increased by using a pump with higher power. It is also found that the Shah and London correlation of apparent flow friction coefficient and the Sieder-Tate correlation of Nusselt Number agree well with the present experimental data.