Unsteady heat transfer problems related to a high-power laser flow loop

Abstract

Introduction A PULSED closed-cycle electric discharge laser (EDL) is a very important device for achieving high energy intensity laser beams. Figure 1 shows a simplified sketch of the EDL. A working gas flows through a closed loop as in a closed circuit wind tunnel used in aerodynamic applications. The flow is circulated by the fan. In the cavity section, pulsed electron beam energy is deposited into the flowing gas to achieve population inversion leading to lasing action. Pulse frequencies of 100 Hz are typical, and each pulse typically lasts for 5 //s. As far as the fluid dynamics of the cavity flow is concerned, the energy deposition is instantaneous, and, therefore, the process can be described as an instantaneous constant volume heating process. Thus, the gas pressure and temperature are instantaneously increased by the energy deposition in the cavity. Because of this, shock waves traveling away from the cavity in both upstream and downstream directions are created. Acoustic mufflers for attenuating these shocks are located upstream and downstream of the cavity. The pressure in the cavity region relaxes back to near its original value in a short period of time. However, the temperature discontinuities created by the energy deposition do not relax in a short period of time. A hot region of gas remains long after the pressure waves have traveled away. This hot slug of gas flows from the cavity at the speed of the flow and encounters the heat exchanger located downstream. The heat exchanger not only removes the heat from the hot slug of gas but also causes longitudinal mixing of this hot slug with the previous cold slug which passed through the heat exchanger. The fan also causes mixing and, thus, the working gas enters the cavity at a lower temperature. For the cavity medium to produce a high-quality output laser beam, there are stringent requirements on density homogeneity. Typically (Ap/p) rms ~ 10~. Variations in density are due to several sources. One source is the unsteady temperature profile at the inlet of the heat exchanger, shown in Fig. 2. The pulse width in Fig. 2 is small compared to the interpulse time, and, therefore, the pulse width is considered infinitesimal. A second source of density variation is the temperature balance between the gas and the solid surfaces in the loop the gas temperature changes with time. Estimating the relaxation times and density changes requires a knowledge of unsteady heat transfer. One unique feature of these unsteady heat transfer problems is that the fluid bulk temperature fluctuates as opposed to problems where the body surface temperature fluctuates. Available literature shows that even though a large number of papers are published concerning unsteady heat transfer, little work is available which considers fluid bulk temBOUNDARY LAYER SUCTION