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High heat flux phase change thermal management of laser diode arrays




Bevis, Taylor A., author
Bandhauer, Todd M., advisor
Williams, John D., committee member
De Miranda, Michael A., committee member

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Laser diodes are semiconductor devices than convert electrical work into light emitted at a specific wavelength over a small spectral bandwidth at a high intensity. A small array of laser diodes can be fabricated on an internally reflective bar that emits light through one edge. If a large number of edge-emitting bars are packed closely together and arrayed to emit light towards the same target, a very high brightness (i.e., light power per unit area) can be achieved, which is useful for a wide range of applications, including advanced manufacturing, inertial confinement fusion energy, and pumping laser gain media. The principle limit for achieving higher brightness is thermal management. State of the art laser diodes generate heat at fluxes in excess of 1 kW cm-2 on a plane parallel to the light emitting edge. As the laser diode bars are packed closer together, it becomes increasingly difficult to remove the heat generated by the diodes in the diminishing space between neighboring diode bars. In addition, the wavelength of the laser diode changes with temperature, and minimizing the variation in wavelength among diodes in very large arrays is very challenging. Thermal management of these diode arrays using conduction and natural convection is practically impossible, and therefore, some form of forced convective cooling must be utilized. Cooling large arrays of laser diodes using single-phase convection heat transfer has been investigated for more than two decades by multiple investigators. Unfortunately, either large temperature increases or very high flow velocities must be utilized to reject heat to a single phase fluid, and the practical threshold for single phase convective cooling of laser diodes appears to have been reached. In contrast, liquid-vapor phase change heat transport can occur with a negligible increase in temperature and, due to a high enthalpy of vaporization, at comparatively low mass flow rates. However, there have been no prior investigations at the conditions required for high brightness edge emitting laser diode arrays: heat fluxes >1 kW cm-2 and a volumetric heat generation rate >10 kW cm-3. In the current investigation, flow boiling heat transfer at heat fluxes up to 1.1 kW cm-2 was studied in a microchannel heat sink with plurality of very small channels (45 × 200 mm) for a phase change fluid (R134a). The high aspect ratio channels (5:1) were manufactured using MEMS fabrication techniques, which yielded a large heat transfer surface area to volume ratio in the vicinity of the laser diode. To characterize the heat transfer performance, a test facility was constructed that enabled testing over a wide range of fluid properties and operating conditions. Due to the very small geometric features, significant heat spreading was observed, necessitating numerical methods to determine the average heat transfer coefficient from test data. The heat transfer correlations were predicted well (mean absolute error, MAE, of ±38.7%) by the correlation of Bertsch et al. This correlation was modified to account for the effect of fin conduction, in the calculation of average heat flux, which yielded an improved MAE of ±8.1%. The new correlation was then used to investigate a range of potential phase change fluids and an alternative microchannel geometry for the laser diode phase change heat exchanger. Finally, a next generation test section design and operating conditions are proposed which are expected to improve diode array brightness up to 5.3× over the state of the art with R134a. If ammonia is used at the working fluid instead of R134a, the brightness could potentially increase by more than 17× over the state of the art.


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