K. Nolan

A Vision for Thermally Integrated Photonics Systems

Thermal management has traditionally been relegated to the last step in the design process. However, with the exponential growth in data traffic leading to ever-greater levels of component integration and ever-higher levels of energy consumption, thermal management is rapidly becoming one of the most critical areas of research within the ICT industry. Given the vast use of optics for efficient transmission of high-speed data, this paper focuses on a new thermal solution for cooling the components within pluggable optical modules. Thermally Integrated Photonics Systems (TIPS) represents a new vision for the thermal building blocks required to enable exponential traffic growth in the global telecommunications network. In the TIPS program, existing thermal solutions cannot scale to meet the needs of exponential growth in data traffic. The main barriers to enabling further growth were identified and a research roadmap was developed around a scalable and efficient integrated thermal solution. In particular, the effects of replacing inefficient materials and large macroTECs with better thermal spreaders and μTECs are investidated. In addition, new forms of μChannel cooling into the package to more efficiently remove the heat generated by the lasers and the TECs are being studied which can lead to future photonic devices that can be deployed in a vastly more dense and integrated manner to address the requirements of future telecommunication networks.

Towards AlN optical cladding layers for thermal management in hybrid lasers

Aluminium Nitride (AlN) is proposed as a dual function optical cladding and thermal spreading layer for hybrid ridge lasers, replacing current benzocyclobutene (BCB) encapsulation. A high thermal conductivity material placed in intimate contact with the Multi-Quantum Well active region of the laser allows rapid heat removal at source but places a number of constraints on material selection. AlN is considered the most suitable due to its high thermal conductivity when deposited at low deposition temperatures, similar co-efficient of thermal expansion to InP, its suitable refractive index and its dielectric nature. We have previously simulated the possible reduction in the thermal resistance of a hybrid ridge laser by replacing the BCB cladding material with a material of higher thermal conductivity of up to 319 W/mK. Towards this goal, we demonstrate AlN thin-films deposited by reactive DC magnetron sputtering on InP.

Forced convection in the wakes of sliding bubbles

Both vapour and gas bubbles are known to significantly increase heat transfer rates between a heated surface and the surrounding fluid, even with no phase change. However, the complex wake structures means that the surface cooling is not fully understood. The current study uses high speed infra-red thermography to measure the surface temperature and convective heat flux enhancement associated with an air bubble sliding under an inclined surface, with a particular focus on the wake. Enhancement levels of 6 times natural convection levels are observed, along with cooling patterns consistent with a possible hairpin vortex structure interacting with the thermal boundary layer. Local regions of suppressed convective heat transfer highlight the complexity of the bubble wake in two-phase applications.

2D Lattice Boltzmann Simulation of Droplet Jumping in a Viscous Fluid

In order to investigate the coalescence and out-of-plane jumping of two incompressible droplets on a non-wetting surface surrounded by an incompressible fluid with matched viscosity in the low Ohnesorge number regime, a two-dimensional lattice Boltzmann phase-field model is implemented. An interfacial force of potential form is used to model the internal surface tension force and capture the fluid-surface interaction, viz. the contact-line dynamics. We evaluate the simulated velocity fields and interface shape evolution during coalescence and the subsequent jumping event. We confirm that the coalescence dynamics of the binary droplet system is similar to the case where the outer fluid viscosity is small compared to that of the droplet fluid, as is the case of condensed water droplet jumping on superhydrophobic surfaces in a gaseous ambient. An argument is also developed to demonstrate that the dynamics in 2D, when appropriately scaled, should be approximately equivalent to the corresponding 3D case. A simple drag model is used to capture the rapid velocity decay of the jumping droplet as it moves away from the surface into the viscous fluid. The results suggest the possibility of experimentally observing coalescence-induced droplet jumping in liquid-liquid systems that may be potentially exploited for microfluidic applications.Copyright

Mixing enhancement due to viscoelastic instability in serpentine microchannels at very large Weissenberg numbers

The flow of shear-thinning viscoelastic fluids is investigated experimentally in a serpentine microchannel at very large Weissenberg numbers (Wi > 104) undergoing elastic instability. The effects of geometric curvature on local flow instability and the consequent heat transfer enhancement are reported. Unlike previous studies where fluids with large zero-shear viscosities (up to 300 mPa.s) were used, we employ a working fluid with a lower viscosity (η0 = 9 mPa.s) more suited to microfluidic heat transfer applications while exhibiting viscoelastic characteristics. This results in Elasticity number (EI = Wi/Re) flows an order of magnitude larger than previously reported in the literature with apparent viscosities close to the solvent viscosity under flow conditions. Detailed Micro Particle Image Velocimetry (μPIV) measurements reveal the local enhancements due to instantaneous flow structures which result in vigorous local mixing at sub-critical Reynolds numbers. In addition the pressure drop increase is moderate as mixing occurs locally and the flow is maintained undisturbed elsewhere throughout the flow path.

Numerical simulations of viscoelastic flow over a confined cylinder in microchannels

In this study, several 2D numerical simulations on a non-Newtonian flow over a confined cylinder placed in a rectangular microchannel are carried out at different Weissenberg (Wi) numbers. In particular, the Oldroyd-B model implemented in open source code OpenFOAM is employed to capture the three basic ingredients of polymer rheology, viz., anisotropy, elasticity and relaxation. Numerical calculations indicate that the flow structure particularly in the downstream is influenced by the presence of the cylinder. As Wi or the channel height increases, the velocity-recovery length required increases. It is observed that both the pressure drop across the channel and the elastic stress magnitude in the downstream grow exponentially with Wi. However it is interesting to observe that recirculation zones appear at Wi = 1.2 with a modest increase in pressure drop compared to Newtonian flow.