Yujia Tao

Analysis of thermal effects in an orthotropic laser medium

The temperature, thermal stress, thermal strain, and optical path difference (OPD) in an orthotropic laser medium under Gaussian, top-hat, and uniform pumping schemes are solved both analytically and numerically. The results indicate that, provided the same total heat loading, the thermal effects under the top-hat pumping scheme are lower than under the Gaussian pumping scheme, whereas the thermal effects under uniform pumping are the least significant of all; in the absence of external forces, the orthotropic thermal properties have more significant effects on the thermal strain than on the thermal stress. The theoretical OPD agrees well with published experimental data and shows evident orthotropy.

Influence of longitudinal rise of coolant temperature on the thermal strain in a cylindrical laser rod

The thermal strain in a laser rod with a longitudinal temperature increase is modeled and analytically derived through the method of thermoelastic displacement potential and the method of Love displacement function. The analytical results show that in the absence of external forces, the longitudinal rise of fluid temperature has an unnoticeable effect on the thermal stress profile in the laser rod. However, the thermal strain field caused by the temperature distribution under the traction free boundary condition has an evident variation in the longitudinal direction, which will considerably affect the laser transmission characteristics and the beam quality.

Numerical Simulation of Droplet Impact on an Isothermal Micro-Grooved Solid Surface

The process of a micro droplet of distilled water impact on an isothermal micro-grooved solid surface is numerical simulated in this paper. To accurately represent the droplet dynamics, special attention is given to the variation of the droplet pressure and velocity, the movement of the free surface between two fluids and the deforming of the droplet after impact. The Volume Of Fluid method is used to track the position and the shape of the liquid region. The PISO algorithm is selected to solve the pressure-velocity coupling. The influences of the droplet initial velocity, the contact angle for water on the surface perpendicular to the groove direction and the surface tension coefficient on the impact process are discussed in detail. The results show that the droplet spreading factor improves notably with the increase of the initial velocity, and reduces with the increase of the contact angle. When the surface tension coefficient increases, the spreading factor reduces greatly. The spreading factor is the largest and the time elapsing is the longest in the case of σ = 0.038 N/m.Copyright

An Analytical Solution to the Thermal Stress and Thermal Strain in a Cylindrical Laser Rod With Longitudinal Temperature Rise

In this paper, a mathematical model of the thermal stress and thermal strain in the laser medium was presented. An analytical solution was further derived for the thermal stress and thermal strain in the laser rod through the method of Thermoelastic Displacement Potential and the method of Love Displacement Function. The analytical solution results show 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.Copyright

Microscopic Boiling of Liquid Nitrogen Induced by a Pulsed Laser Irradiation

An experimental investigation was conducted to explore the characteristics of microscopic boiling induced by firing a microsecond pulsed laser beam on a thin platinum (Pt) film that immerged in the liquid nitrogen (LN2) cryostat. High-speed photography aided by a high-voltage lighting system was employed to visually observe the bubble formation and the dynamical boiling process of LN2. A rapid transient temperature-measuring system was designed to record the temperature evolution of the heating surface. Explosive boiling, characterized by bubble cluster, was observed within LN2 at the early stage of laser heating, and conventional boiling followed after a certain time. The transition time, therefore, was introduced for separating these two different boiling modes. The temperature of Pt film rose sharply to its maximum during laser pulse, with a very high rising rate of about 107 K/s, and then dropped rapidly after laser irradiation. A model of bubble cluster was proposed to describe the explosive boiling heat transfer, and the latent heat released by bubble collapse in explosive boiling was explored as an important mechanism considerably influencing the boiling heat transfer.