Pritish R. Parida

Extreme energy efficiency using water cooled servers inside a chiller-less data center

The paper summarizes part of a project that was undertaken to develop highly energy efficient warm liquid cooled servers for use in chiller-less data centers that could save significant data center energy use and reduce data center refrigerant and make up water usage. One of the key concepts developed as part of this project is the Dual-Enclosure-Liquid-Cooling (DELC), which comprises of a 100% liquid cooled server cabinet and an outdoor dry cooler unit for heat rejection to the ambient and this configuration is used to reject the Information Technology (IT) equipment heat load directly to the outside ambient air without the use of a chiller. Demonstration hardware for server liquid cooling and a chiller-less data center was built and is operational for a 15 kW rack fully populated with liquid cooled servers which has been designed for use for up to 45°C liquid coolant to the rack. The anticipated benefits of such energy-centric configurations are significant energy savings of as much as 25% at the data center level. This paper builds on recent work that focused on the server liquid cooling, the rack enclosure with heat exchanger cooling and liquid distribution, and the data center level cooling infrastructure and which also presented sample data from experiments in support of the DELC concept. This paper presents experimental data related to the novel data center loop in a new manner, which is used to create a simplified thermodynamic model using curve-fit of surfaces of heat exchanger approach temperatures and power use of cooling devices. The model is validated using experimental data for a 22 hour test that was conducted in August of 2011. Subsequent to model validation, the simplified model is then used to make projections for DELC prototype performance (thermal and energy) under different conditions including different simple control schemes and weather conditions in the US. Weather data from nine different US cities is analyzed for a single day in August and realizable energy and energy cost savings over traditional chiller based data center cooling designs are presented. The results show that the new innovative data center cooling configuration presented could reduce cooling energy use to be less than 3.5% of the IT power for most US locations even in warm summer times of the year.

Experimental Investigation of Cooling Performance of Metal-Based Microchannels

Metal-based microchannel heat exchangers (MHEs) are of current interest due to the combination of high heat transfer performance and improved mechanical integrity. Efficient methods for fabrication and assembly of functional metal-based MHEs are essential to ensure the economic viability of such devices. Al- and Cu-based high-aspect-ratio microscale structures (HARMS) have been fabricated through molding replication using metallic mold inserts. Such metallic HARMS were assembled through eutectic bonding to form Al- and Cu-based MHEs, on which heat transfer tests were conducted to determine the overall cooling rate and time constants. Electrically heated Cu blocks were placed outside the MHEs and provided a constant flux, and water flowing within the microchannels acted as the coolant. Experimental results show a great influence of the type of metal, flow rate, and the surrounding conditions on the overall cooling performance of the MHEs.

Innovative liquid cooling configurations for high heat flux applications

Breakthroughs in the recent cutting-edge technologies have become increasingly dependent on the ability to safely dissipate large amount of heat from small areas. Improvements in cooling techniques are therefore required to avoid unacceptable temperature rise and at the same time maintain a high efficiency. Jet impingement is one such cooling scheme which has been widely used to dissipate transient and steady concentrated heat loads. But with constantly increasing cooling needs, conventional jet impingement cooling is no longer a viable option. Considerable improvements are therefore required to meets such stringent requirements. A combination of swirl-impingement-fin generating geometry is one such alternative. Even without a fin, an overall enhancement of 150% – 200% in the maximum heat transfer coefficient has been recorded both experimentally and computationally due to impingement and associated swirl. Moreover, the presence of fins further increases the cooling area. The present scheme is therefore expected to overcome the existing heat distribution and cooling problems in high heat flux dissipating devices.

High Efficiency Minichannel and Mini-Impingement Cooling Systems for Hybrid Electric Vehicle Electronics

Over the years, electronic equipment, especially semiconductor based devices, have found their applications in almost all fields of research. The demand for more power and performance from such electronic equipment has constantly been growing resulting in an increased amount of heat dissipation from these devices. While conventional cooling solutions have performed the task of heat removal, no straightforward extension has been possible for significantly high heat fluxes dissipated by smaller and more efficient electronic devices. Thermal management of high-density power control unit for hybrid electric vehicle is one such challenging application. Over the last few years, the performance of this power control unit has been improved and size has been reduced to attain higher efficiency and performance causing the heat dissipation as well as heat density to increase significantly. Efforts are constantly being made to reduce the PCU size even further and also to reduce the manufacturing costs. As a consequence, heat density will go up (∼200–250 W/cm2) and thus, a better high performance cooler/heat exchanger is required which can operate under the existing cooling system design (pressure drop limitation) and at the same time, maintain active devices temperature within optimum range (<120–125°C) for higher reliability.The focus of this paper is to discuss the development of various cooling options for high heat flux dissipating devices with severe size constraints. A parametric and optimization study on the selected designs was performed. Finally, the optimized cooler/heat exchanger was tested under actual running conditions. The methodology was to explore various high performance cooling options such as impinging jets, pin fins, and ribbed mini-channels and to arrive at new promising, conceptual designs. These new designs were then compared against similar conventional designs both numerically and experimentally. Additionally, conjugate heat transfer simulations were performed on partial packaging model to compare the various designs. Experiments were also performed to validate the simulation models and characterize the meshing parameters to perform cost and time effective calculations/simulations.Copyright

Novel PCM and Jet Impingement Based Cooling Scheme for High Density Transient Heat Loads

Breakthroughs in the recent cutting-edge technologies have become increasingly dependent on the ability to safely dissipate large amount of heat from small areas. Improvements in cooling techniques are therefore required to avoid unacceptable temperature rise and at the same time maintain high efficiency. Jet impingement is one such cooling scheme which has been widely used to dissipate transient and steady concentrated heat loads. With constantly increasing transient cooling needs, conventional pin-fin cooling and conventional jet impingement cooling are not meeting the requirements. Considerable improvements are therefore required to meet such stringent requirements without any significant changes in the cooling system. A combined cooling scheme based on jet impingement and phase change materials (PCMs) is presented as one such alternative to existing cooling systems. A high heat storage capability of PCMs in combination with a high heat transfer rates from impingement cooling can help overcome the existing heat distribution and transient cooling problems in high heat flux dissipating devices. Preliminary conjugate CFD simulations show promising results. Additionally, experimental validation of the simulation predictions has also been performed. A reasonably good agreement was found between the predictions and experiments.Copyright

Numerical Prediction of Flow and Heat Transfer Rates in Metal Based Microchannels Using Lattice Boltzmann Method

Metal-based Microchannel Heat Exchangers (MHEs) are of current interest due to the combination of high heat transfer performance and improved mechanical integrity. In the present work, a simple two-dimensional thermal lattice Boltzmann model without viscous heat dissipation and pressure compressible work has been developed to simulate the heat transfer phenomenon in Cu- and Al-based micro-channels. A 2D fluid-solid conjugate heat transfer problem is solved using LBM and Fluent. For the Cu specimen, the height of the channel considered was 204 μm and the top and bottom wall thickness was taken to be same as the channel height. The LBM results were compared with 3D and 2D fluent models. The study also compares the numerically computed velocity profile with the analytical results and compares the Nusselt number values predicted by LBM and Fluent with the experimental data. Owing to the simplicity of the thermal LB model, promising results were obtained from the LBM predictions.© 2008 ASME