CFD ANALYSIS AND EXPERIMENTAL INVESTIGATION OF THE HEAT DISSIPATION CHARACTERISTICS OF FLUORESCENT LIGHT FIXTURES IN THE NATIONAL IGNITION FACILITY

摘要

The National Ignition Facility (NIF) at Lawrence Livermore National Laboratory is being constructed as the latest in a series of high-power laser facilities to study inertial confinement fusion. In particular, NIF will generate and amplify 192 laser beams and focus them onto a fusion fuel capsule the size of a BB. The energy deposited by the laser beams will raise the core temperature of the target to 100,000,000°C, which will ignite the fusion fuel and produce a fusion energy output that is several times greater than the energy input. NIF is comparable in size to a large sports arena. Its massive bays contain laser beam amplification, conditioning, and diagnostic equipment, large laser beam transport tubes, and a variety of optical shaping and alignment devices. The ability to generate, condition, and focus 192 laser beams onto a target the size of a BB, requires not only precision optical hardware and instrumentation, it also demands extreme environmental control measures. For example, small spatial and temporal excursions in the NIF environmental air temperature may cause minute distortions in beam alignment hardware as well as index of refraction gradients in the beam transport tubes, thereby degrading the operating performance and net energy yield of the NIF. To minimize the impact of thermal gradients, the NIF HVAC system has been designed to maintain a mean air temperature field of 20.00 ± 0.28°C throughout the facility. Unfortunately, heat sources within the facility will create local hot spots in which air temperatures will exceed the upper limit of this range. Consequently, the magnitude and extent of the local hot spots must be characterized and engineering control measures must be developed to minimize their impact on the NIF operating performance. The purpose of this study was to assess the extent of spatial temperature excursions created by fluorescent lights in the NIF laser bays, estimate their detrimental influence on the beam transport tubes, and offer design recommendations to minimize their impact on operating performance. To achieve this, experiments were performed to characterize the heat dissipation of a fluorescent light fixture and establish meaningful boundary conditions for a computational fluid dynamics (CFD) model. Next, both CFD and analytical models were developed to investigate the magnitude and extent of thermal plumes and radiation heat transfer from fluorescent: light fixtures. By incorporating the results of the CFD and analytical models with facility layout drawings, several design modifications were recommended.