Two-Phase Flow Boiling in a Single Layer of Future High-Performance 3D Stacked Computer Chips

The present study focuses on an experimental investigation of two-phase flow boiling in a silicon multi-microchannel evaporator, which emulates a single layer of a 3D stacked computer chip. The micro-evaporator is comprised of 67 parallel channels, each having a 100 x 100 μm2 cross-section area, and separated by 50 μm-wide fins. Two aluminium micro-heaters were sputtered onto the backside of the test section to provide two 0.5 cm2 heated areas in order to simulate the power dissipated by active component in 3D CMOS chips. The experiments were performed with a second identical test section having 50 μm-wide, 100 μm-deep, and 100 μm-long restrictions (micro-orifices) at the inlet of each channel to stabilize the two-phase flow. The goal of this experimental campaign was to perform simultaneous high-speed flow visualization and infra-red measurements of the two-phase flow and heat transfer dynamics across the entire micro-evaporator area. Refrigerants R245fa, R236fa and R1234ze(E) were chosen as the working fluids. The micro-orifices successfully suppressed back flow, eliminated flow instabilities, provided a good flow distribution, and started the boiling process with some flashed vapor. Thermal performance was found to be uniform widthwise using these orifices

Transient Simulation of Two-Phase On-chip Liquid Pump Cycle for Processor's Cooling

Transient modeling and regulation of a two-phase on-chip liquid pump cooling cycle is studied. The purpose is to cool down multiple micro-processors in parallel and a set of memories, DC/DC converters in series. The cycle will be incorporated in a blade/drawer of a rack as shown in Figure 1. The cooling system is composed of multiple on-chip micro-evaporators, a condenser, a liquid accumulator, a pump and all piping joining components. In order to take advantage of enhanced heat transfer and more uniform chip temperature, two-phase refrigerant R134a is used as working fluid. The dynamic of the system is relevant as the heat load of microprocessors is changing continuously with time. Transient simulations allow firstly verifying that the critical heat flux (CHF) is not reached during heat loads disturbances. Secondly, for energy recovery purpose, it allows tracking with time the available heat at the condenser. The thermal accumulation is thus taken into account in the energy balance. However, since the pressure equilibrium is a much faster phenomenon than thermal inertia, the momentum accumulation is neglected in the momentum balance. Transient two-dimensional conduction in the body of the micro-evaporators is also considered. Regulators, the roles of which are to control the mixing outlet vapor quality and the pressure drops equilibrium in the parallel branches, are also implemented in the code. Preliminary simulations with four microprocessors in parallel considering different levels of heat load (36, 30, 25 and 10 W/cm2) show the robustness of the predictive-corrective solver used. In 14 minutes of computing time, 46 seconds of real time were simulated. For a desired mixing vapor quality of 30%, at a pressure of 16 bars (saturation temperature of 58°C) and a subcooling of 2K, the mass flow rates in the micro-evaporators were respectively 3.6, 4.0, 4.5 and 7.4 kg/h (largest flow rate for lowest heat load) and the total pressure drop over the section was 0.6 kPa

Efficiency Improvements of a Thermal Power Plant by Making Use of the Waste Heat of Large Datacenters using Two-Phase On-Chip Cooling

Cooling of datacenters is estimated to have an annual electricity cost of 1.4 billion dollars in the United States and 3.6 billion dollars worldwide. Currently, refrigerated air is the most widely used means of cooling datacenter’s servers. Modern datacenters require a heat dissipation rate in the order of 5 to 15 MW and current air cooling technology represents around 45% of the total energy consumed. Based on the above issues, thermal designers of datacenters and server manufacturers now seem to agree that there is an immediate need to improve the server cooling process. On-chip cooling research is being developed in this context to propose a new, more efficient cooling technology. This also allows the recoveryof the heat rejected by the servers in a proper way, making it possible to reuse elsewhere, in another application. The present investigation develops a case study considering two different cooling systems applied on a datacenter and exploring the application of energy recovered in the condenser on a feedwater heater of a coal power plant. The effects of the evaporating and condensing temperatures on the cooling cycle performance and the potential to recover energy, and consequently the effect on the power plant efficiency, are evaluated. The analyses consider the main objective function to be the minimization of energy consumption, the corresponding CO2 footprint and operating costs. From the datacenter’s point of view, when compared with traditional air-cooling systems, energy consumption, without considering energy recovery, can be reduced by as much as 45% when using a liquid pumping cycle and 35% when using a vapour compression cycle. From the power plant point of view, the results showed that, when the pressure of the feedwater heater is optimized, an increase of up to 6.5% of the overall power plant efficiency can be obtained when using a vapour compression cycle to cool the datacenter. Considering the vapour compression cycle and a datacenter of 100000 blades, overall savings (considering the power plant and the datacenter as a whole system) of 2170 tons of CO2 and $0.34 million per MW of electricity production were obtained. Additional investigation was developed considering the effects of partial operation of the datacenter and/or the power utility on the parameters mentioned beforehand. For such an investigation the start-up was the ideal match between a datacenter of 100000 blades and a power plant, both with 100% of operational uptime. It has been shown that some cases could lead to impossible thermodynamic operations, meaning that special attention must be given when the design of such integrated utilities (datacenter and power plant) is made

On-chip two-phase cooling of datacenters: Cooling system and energy recovery evaluation

Cooling of datacenters is estimated to have an annual electricity cost of 1.4 billion dollars in the United States and 3.6 billion dollars worldwide. Currently, refrigerated air is the most widely used means of cooling datacenter’s servers, which typically represents 40-45% of the total energy consumed in a datacenter. Based on the above issues, thermal designers of datacenters and server manufacturers now seem to agree that there is an immediate need to improve the server cooling process. The goal of the present study is to propose and simulate the performance of a novel hybrid two-phase cooling cycle with micro-evaporator elements (multi-microchannel evaporators) for direct cooling of the chips and auxiliary electronics on blade server boards (savings in energy consumption of over 60% are expected). Different working fluids were considered, namely water, HFC134a and a new, more environmentally friendly, refrigerant HFO1234ze. The results so far demonstrated that the pumping power consumption is on the order of 5 times higher for the water-cooled cycle. Additionally, a case study considering the hybrid cooling cycle applied on a datacenter and exploring the application of energy recovered in the condenser on a feedwater heater of a coal power plant was also investigated (modern datacenters require the dissipation of 5-15 MW of heat). Aspects such as minimization of energy consumption and CO2 footprint and maximization of energy recovery (exergetic efficiency) and power plant efficiency are investigated

A micro particle shadow velocimetry (mu PSV) technique to measure flows in microchannels

A micro particle shadow velocimetry (mu PSV) system based on back-lit illumination and forward scatter observation of light from non-fluorescent particles has been developed. Relatively high luminous efficiencies and particle image contrasts were achieved by using the condenser stage of a standard transmitted light microscope and a continuous incoherent collimated light emitting diode (LED). This paper includes a critical review of the operating principles, benefits and practical problems associated with the predominant epifluorescent micro particle image velocimetry (mu PIV) technique, and the less common light scattering mu PIV methods of which mu PSV is a development. This mu PSV system was then successfully used to measure axial velocity profiles in a 280-mu m-diameter circular channel up to a Reynolds number of 50 which corresponds to peak velocities of around 0.4 m/s. These velocity profiles were then integrated to provide instantaneous flow rates on the order of 100 mu l/min to an accuracy of +/- 5 % relative to average flow rates determined using a digital balance. Due to the incoherent nature of the LED light source, the back-lit forward scatter observation mode and the applied refractive index matching system, the location of the test section walls and thus the local velocity fields were also accurately obtained. As a result of this, mu PSV provides a low cost and safe way to investigate microfluidics, especially in lab-on-a-chip applications where the necessary optical access through transparent test sections is often available

Green Cooling of High Performance Micro Processors: Parametric Study between Flow Boiling and Water Cooling

Due to the increase in energy prices and spiralling consumption, there is a need to greatly reduce the cost of electricity within data centers, where it makes up 50% of the total cost of the IT infrastructure. A technological solution to this is using on-chip cooling with a single-phase or evaporating liquid to replace energy intensive air-cooling. The energy carried away by the liquid or vapour can also potentially be used in district heating, as an example. Thus, the important issue here is “what is the most energy efficient heat removal process?” As an answer, this paper presents a direct comparison of single-phase water, a 50% water ethylene glycol mixture and several two-phase refrigerants, including the new fourth generation refrigerants HFO1234yf and HFO1234ze. Two-phase cooling using HFC134a had an average junction temperature 9 to 15˚C lower than for single-phase cooling, while the required pumping power for the CPU cooling element for single-phase cooling was on the order of 20-130 times higher to achieve the same junction temperature uniformity. Hot-spot simulations also showed that two-phase refrigerant cooling was able to adjust to local hot-spots because of flow boiling's dependency on the local heat flux, with junction temperatures being 20 to 30˚C lower when compared to water and the 50% water-ethylene glycol mixture, respectively. An exergy analysis was developed considering a cooling cycle composed by a pump, a condenser and a multi-microchannel cooler. The focus was to show the exergetic efficiency of each component and of the entire cycle when the subject energy recovery is considered. Water and HFC134a were the working fluids evaluated in such analysis. The overall exergetic efficiency was higher when using HFC134a (about 2%) and the exergy destroyed, i.e. irreversibilities, showed that the cooling cycle proposed still have a huge potential to increase the thermodynamic performance

A Review of On-Chip Micro-Evaporation: Experimental Evaluation of Liquid Pumping and Vapor Compression Driven Cooling Systems and Control

Thermal designers of data centers and server manufacturers are showing a greater concern regarding the cooling of the new generation data centers, which consume considerably more electricity and dissipate much more waste heat, a situation that is creating a re-thinking about the most effective cooling systems for the future beyond conventional air cooling of the chips/servers. A potential significantly better solution is to make use of on-chip two-phase cooling, which, besides improving the cooling performance at the chip level, also adds the capability to reuse the waste heat in a convenient manner, since higher evaporating and condensing temperatures of the two-phase cooling system (from 60-95°C) are possible with such a new “green” cooling technology. In the present project, two such two-phase cooling cycles using micro-evaporation technology were experimentally evaluated with specific attention being paid to (i) controllability of the two-phase cooling system, (ii) energy consumption and (iii) overall exergetic efficiency. The controllers were evaluated by tracking and disturbance rejection tests, which were shown to be efficient and effective. The average temperatures of the chips were maintained below the limit of 85°C for all tests evaluated in steady state and transient conditions. In general, simple SISO strategies were sufficient to attain the requirements of control. Regarding energy and exergy analyses, the experimental results showed that both systems can be thermodynamically improved since only about 10% of the exergy supplied is in fact recovered in the condenser in the present setup