Adaptive Liquid Cooling Methods in Microchannel Heat Sinks

Abstract

This work investigates the effect of implementing three varying cooling techniques for high heat flux electronics cooling. High heat fluxes are being experienced in data centres as the number of transistors per inch increases according to Moore?s law in the central processing units (CPUs) of servers with the thermal design powers (TDPs) increasing year after year. Heat fluxes comparable to the surface of the sun are being encountered in CPU hot spots resulting in large thermal gradients, diminishing the operational lifetime of the components. The cooling challenge introduced by this complex spatially and temporally varying load is just one side of the problem, the other side is the ever rising power consumption of the data centres across the globe. Globally the information and communications technology (ICT) sector consumes approximately 2.5% of the total electricity consumed, and expected to rise to 8% by the end of the decade. This power consumption is especially a concern in Ireland where a large number of large global data providers have set up large scale facilities. EirGrid, the Irish national power transmission operator who deals with all current and future large scale grid connections predicts that the data centre sector will consume 29% of the total Irish electricity demand by 2028. The European Union has set out clear target for greenhouse gas emissions, which Ireland will not meet on time as they currently are the third worst polluters in the EU per capita. Ireland faces fines (> 250 million euro) for their first offence for missing the agreed to targets. Combining this with the predicted cost to refit the energy grid to support the growing data centre sector, with a monetary cost of 9 billion euro, and an environmental cost of an additional 1.5 million tonnes of greenhouse gases to Irelands carbon emissions by 2030. It is clear that large scale changes are required in the data centre sector and especially in cooling which can comprise of up to 33% of the facility power consumption if Ireland wants to curb its growing consumption trend.

The cooling techniques investigated in this work pertain only to liquid cooling solutions and disregard the outdated and inefficient cooling medium of air. This is due to the large power consumption related to high speed server fans, the inability of air cooling to deal with rising heat fluxes, and the poor thermal carrying capacity of air, which excludes it from waste heat re-use schemes. Re-use of waste heat streams from servers is one effective way of reducing national power consumption as well as carbon emissions. Heated water exiting server racks can be utilised for district heating in nearby building/greenhouses/farms or used in low temperature organic Rankine cycles to generate onsite electricity. To achieve high enough outlet temperatures while maintaining the CPU junction temperatures below critical limits, heat transfer enhancement methods must be utilised. These take the form of passive, active and adaptive methods. The passive enhancement method of staggered wall mounted micromixer baffles is investigated using experimental and numerical methods. A full micro-PIVsystem is designed and built by the author for the investigation of micro fluidic phenomena. For the passive enhancement case significantly higher outlet temperatures and Nusselt numbers were found over a traditional empty channel configuration. Micro-PIV delivers high spatial and temporal resolution of the flow velocity data. It was found that at certain flow rates the steady flow bifurcates into a time periodic flow with large scale velocity fluctuations, vortex shedding and scalar transport downstream which is linked to enhanced cooling.

Using micro-PIV the active enhancement technique of flow pulsation is investigated in microchannels. Traditionally excitation waveforms have been limited to sinusoidal waveforms. This study investigated novel asymmetric excitation waveforms for Womersley numbers below 5 and pulsation amplitudes below 0.055. The novel asymmetric waveforms exhibited notably lower channel temperatures and higher Nusselt numbers, linking them to enhanced heat transfer. This is attributed to large and rapid switches in inertial and viscous dominated regions within the flow and the presence of higher order fluctuations in the flow. Modifying the waveform shape adds an additional parameter to tune to enhance heat transfer and was shown to outperform the symmetric waveforms. This knowledge was then combined into a fast control algorithm to instantaneously adapt the excitation waveform, amplitude and frequency depending on a monitored temperature value. Using this control loop the channel temperatures can be maintained at notably lower temperatures compared to a plain no control utilisation scenario.

An adaptive cooling technique in the form of a shape memory alloy (SMA) valve/fin is experimentally investigated. By programming the SMA structure to have a two way shape memory effect the structure can independently respond to thermally varying loads without the need for external monitoring and control, reducing the number of points of failure. The fabricated SMA structures were demonstrated to rapidly and reliably respond to temporally fluctuating thermal loads and alter the pressure drop accordingly to reduce or increase flow rate and forced convection. The amplitude of thermal actuation can accurately be captured by a representation of the system, allowing for the prediction of pressure relief with temperature to be modelled. The technique shows promise if more complex microstructures are fabricated and used to more comprehensively control the system.