The thermal-hydraulic characterisation of a microchannel heat exchanger for use in next generation photonic integrated circuits

The context of this thesis is a microfluidic thermal management solution for a transceiver photonics integrated circuit (PIC), a device that forms the backbone of optical telecommunication networks. The PIC features laser bar arrays which, due to their small footprint, generate heat fluxes of order ~103 W/cm2. Moreover, the lasers must be maintained to within ± 0.1 K of their operating temperature to minimise wavelength changes. To meet increased bandwidth demands, higher laser densities are necessary within transceiver PICs. However, today’s air-cooling technology is limiting development. Macro-thermoelectric coolers and resistive heaters, used to control laser temperature, add to the inefficiency of contemporary technology. In addition, an uneven spreading of heat exists within a laser array due to thermal cross talk. A combination of microthermoelectrics and integrated microfluidics has been proposed, within the EU-funded TIPS consortium, to thermally control laser arrays, enabling higher laser densities and, therefore, higher data throughput within transceiver PIC packages. The overall objective of this thesis is to thermally and hydraulically characterise a rectangular microchannel heat exchanger in order to assess its feasibility as a thermal control solution for next generation transceiver PICs. To examine the thermal conditions within a transceiver PIC in order to obtain a thermal baseline for microfluidic testing, an active III/V laser device was thermally characterised across a range of power dissipations (0 – 375 mW). Silicon microfluidic test chips, with integrated microchannel and heater structures, were thermally and hydraulically characterised to demonstrate the effectiveness of microfluidics as a thermal management solution. The rectangular microchannel heat exchanger, 500 µm x 250 µm in section, was characterised across flow rates ranging from 2 – 20 ml/min (Re = 93 – 931) and under a range of heating conditions. The effect of flow rate on the heater surface temperature was recorded using IR thermography, with flow parameters measured using a flow meter, manometry and thermal sensors. The cooling effect of the fluid on the complete array of heater structures while powered simultaneously was examined. To improve the resolution of the IR thermography, heater power dissipation was increased, and single heaters were thermally characterised at higher heat fluxes. This research found that with an increase in fluid flow rate from 2 – 20 ml/min, heater surface temperature decreased for all heaters and heating conditions, and the change in heater surface temperature due to a variation in flow rate was determined to be largely independent of heater streamwise position and power dissipation. Above a flow rate of 12 ml/min, enhanced convective cooling effects were found to diminish while pumping power requirements continued to increase, indicating towards an optimal flow rate for thermal control and minimised operating costs. Above a flow rate of 12 ml/min, heater surface temperatures were found to be within ± 3.5°C for all heaters within the array. Thermal cross talk between neighbouring heater structures was found to decrease with increasing flow rate due to the decrease in thermal resistance to one-dimensional heat transfer into the cooling fluid. Experimental thermal and hydraulic characterisation of a rectangular microchannel heat exchanger determined the suitability of the technology for use as a thermal management solution in next generation transceiver PICs. Thermal measurements showed that at flow rates > 12 ml/min, a reasonable degree of thermal control of the heater array was achieved while also ensuring that minimal operating costs would be incurred.