| 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. If a large number of edge-emitting diode 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. 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, and are cooled by liquid convection heat transport. It is proposed that liquid-vapor phase change heat transport can dissipate more energy 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: >1 kW cm-2 and >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 evaporator with plurality of very small channels (45 x 200 microns) for a phase change fluid (R134a). 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. An experimental increase in brightness of 1.87x compared to the state of the art was demonstrated. From this data a heat transfer correlation was generated with a mean absolute error, MAE, of +/-8.1%. The new correlation was used propose a range of potential phase change fluids and an alternative microchannel geometry for the laser diode phase change heat exchanger to reach a target increase in brightness of ten-fold over the current state of the art. |
