Sukhvinder S. Kang

Heat and Mass Transport in Heat Pipe Wick Structures

temperature field,areobtainedfortheporouswicksundertheactionofadiscreteheatsource(evaporator)mounted on one end. The working fluid, supplied from a condenser pool, evaporates from the wick surface primarily in the evaporator region and is condensed and collected into a container separate from the pool, to yield mass flow rates. Thus the liquid-pumping capability of the wick, coupled with flow impedance, is measured as a function of applied heat flux.Repeatableresultswithlowuncertaintyareobtained.Acarefulanalysisofthetransportpathsforheatand masstransferin thewickstructure confirmsthatmasstransfer duetovaporization oftheworking fluidisthelargest contributor to heat dissipation from the wick. The expected and measured values of evaporation rate are in good agreement. Results are also presented in terms of overall effective conductance based on measured temperatures.

A methodology for the design of perforated tiles in raised floor data centers using computational flow analysis

Data centers are used to house multiple servers, mainframes, supercomputer systems, and storage systems used in business data processing and scientific analysis. Typically, data processing (DP) equipment is cooled using forced flow of air. Modular chillers are commonly used to cool the hot air exhausted from the DP equipment and a raised floor to recirculate the conditioned air back into the room. Therefore, data centers need well-designed ventilation systems, appropriate placement of the DP equipment, and modular chillers to ensure that the air used for cooling the processing equipment is within the desired temperature range. An important aspect of the design of data centers involves sizing of the perforated floor tiles for return of cold air, the size of the space under the raised floor, and placement of the DP equipment and modular chillers. The flow through individual perforated tiles needs to fulfil the cooling requirements of the computer equipment placed adjacent to them. The novelty of the paper lies in the treatment of the volume under the raised floor as a uniformly pressurized plenum. The accuracy of the Pressurized Plenum model is demonstrated with reference to a Computational Fluid Dynamics (CFD) analysis of the recirculating flow under the raised floor and the limits of its validity are also identified. The simple model of the volume under the raised floor enables use of the technique of Flow Network Modeling (FNM) for the prediction of the distribution of flow rates exiting from the various tiles. An inverse design method is proposed for one-step design of the perforated tiles and flow balancing plates for individual chillers. Subsequent use of the FNM technique enables assessment of the performance of the actual system. Further, modifications to an existing system design needed to accommodate the changes in the cooling requirements can also be evaluated using the FNM analysis in a simple, quick, and accurate manner. The resulting design approach is very simple and efficient, and is well suited for the design of modern data centers.

Loop heat pipe technology for cooling computer servers

An air cooled loop heat pipe (LHP) system was developed for cooling a dual CPU 1U server. The LHP uses newly developed flat plate evaporators and air cooled condensers with millimeter scale condensation channels and plain parallel fins. Bench tests were conducted using 20 x 20 mm uniform heat sources dissipating 100 watts and air preheating to simulate air-cooled operation within a 1U server chassis. Thermal resistance was decreased as the air temperature increased from 25degC to 50degC. At 50degC air temperature, the LHP evaporator surface temperature reached 65degC showing an effective evaporator surface to air thermal resistance of 0.15degC/W. A commercial 1U server with dual 100 W rated Intel Xeon processors was used to compare the effectiveness of the LHP cooling system to the standard processor fan heat sinks. Multiple instances of CPU Burn were used to exercise the CPUs for maximum power dissipation. Under similar test conditions with room air temperature in the range 26-30degC, the interface temperature at the processors stabilized at ~75degC with the fan heat sinks and ~55degC with the LHP cooling system.

A novel hybrid heat sink using phase change materials for transient thermal management of electronics

A hybrid heat sink concept which combines passive and active cooling approaches is proposed. The hybrid heat sink is essentially a plate fin heat sink with the tip immersed in a phase change material (PCM). The exposed area of the fins dissipates heat during periods when high convective cooling is available. When the air cooling is reduced, the heat is absorbed by the PCM. The governing conservation equations are solved using a finite-volume method on orthogonal, rectangular grids. An enthalpy method is used for modeling the melting/resolidification phenomena. Results from the analysis elucidate the thermal performance of these hybrid heat sinks. The improved performance of the hybrid heat sink compared to a finned heat sink (without a PCM) under identical conditions, is quantified. In order to reduce the computational time and aid in preliminary design, a one-dimensional fin equation is formulated which accounts for the simultaneous convective heat transfer from the finned surface and melting of the phase change material at the tip. The influence of the location, amount, and type of PCM, as well as the fin thickness on the thermal performance of the hybrid heat sink is investigated. Simple guidelines are developed for preliminary design of these heat sinks.

CLOSED LOOP LIQUID COOLING FOR HIGH PERFORMANCE COMPUTER SYSTEMS

The power dissipation levels in high performance personal computers continue to increase rapidly while the silicon die temperature requirements remain unchanged or have been lowered. Advanced air cooling solutions for the major heat sources such as CPU and GPU modules use heat pipes and high flow rate fans to manage the heat load at the expense of significant increases in the sound power emitted by the computer system. Closed loop liquid cooling systems offer an excellent means to efficiently meet the combined challenges of high heat loads, low thermal resistance, and low noise while easily managing die level heat fluxes in excess of 500 W/cm 2 . This paper describes the design and attributes of an advanced liquid cooling system that can cool single or multiple heat sources within the computer system. The cooling system described use copper cold plates with meso scale channels to pick up heat from CPU and GPU type heat sources and highly efficient liquid-to-air heat exchangers with flat copper tubes and plain fins to transfer the heat to air by forced convection. A water based coolant is used for high thermal performance and additives are used to provide burst protection to the cooling system at temperatures down to -40 ο C and corrosion protection to critical components. A highly reliable compact pump is used to circulate the fluid in a closed loop. The overall system is integrated using assembly methods and materials that enable very low fluid permeation for long life.

Application of flow network modeling and CFD to computer system design

This paper describes the air flow design of a computer system using commercially available flow network modeling (FNM) and computational fluid dynamics (CFD) software and proposes a new Hybrid approach that combines the best features of both. The basis of the proposed approach lies in the recognition that air flow within different regions of a computer system can be divided into two categories. One category, consisting of regions or subsystems through which the flow direction is well defined (e.g. Channels formed between card arrays, power supplies, an array of disk drives etc.) is well modeled using a flow impedance component in a FNM representation whereas the second type of region where the flow pattern is poorly defined (e.g. Air flow plenums) and highly dependent on the characteristics of adjoining subsystems requires CFD to model adequately. The FNM model of the sample design problem provides quick results and allows many design alternatives to be assessed but at the inevitable cost of oversimplifying the second type of region. The full system CFD model is large in size, requires a large computational time and significant post processing effort to understand the results. These issues are addressed by the Hybrid method whose key attributes and implementation within FNM and CFD codes is described. A CFD model is used to illustrate the proposed approach and demonstrate that, for the specific design problem used here, the method yields good accuracy while achieving 14/spl times/ reduction in model size and 30/spl times/ reduction in simulation time compared to the full CFD model.

Computational method for generalized analysis of pumped two-phase cooling systems and its application to a system used in data-center environments

Two-phase pumped-loop systems are being actively considered for cooling of high heat load electronics. In the present study, a computational method based on a two-level approach is developed for generalized system-level analysis of two-phase pumped-loop cooling systems containing multiple branches under steady-state conditions. Detailed one-dimensional analysis of components with distributed two-phase flow is performed to determine their flow and thermal characteristics. System-level analysis utilizes these compact representations for analyzing the component interaction in a generalized manner to predict the system performance. Component models have been developed for the microchannel-heat-sink, finned-tube evaporator and condenser, reservoir, and positive displacement pump. In order to illustrate the utility of the computational method in the design of practical two-phase cooling systems, it has been applied for the analysis of a pumped-loop system being explored for the cooling of hot-air exhausts from server racks in data centers. The system consists of multiple finned-tube evaporators in parallel branches, a water-cooled condenser, a reservoir, and a pump. Results of the analysis show the occurrence of flow maldistribution among the evaporators due to absorption of varying heat loads.

Computational method for system-level analysis of two-phase pumped loops for cooling of electronics

Two-phase pumped-loop systems are being actively explored for the cooling of high-heat-flux electronics cooling because of their compactness and low thermal resistance. A typical two-phase pumped loop consists of a microchannel heat sink based evaporator, a finned-tube condenser, a reservoir, and a positive displacement pump. In the present study, the physics of operation of a two-phase pumped-loop system is presented. A two-level computational method, involving coupled component-level and system-level analyses, is then presented for predicting the performance of this system. Component models for the finned-tube condenser and the microchannel-heat-sink evaporator incorporate analysis of one-dimensional two-phase flow within the flow passages combined with correlations for friction factor and heat transfer coefficient under two-phase conditions. Further, the component model for the microchannel-heat-sink evaporator considers the interaction between conduction in the solid region and two-phase flow in individual channels. The system-level solution exploits the simplicity of the two-phase loop to analyze system-level interactions among the components. The computational method has been applied for the analysis of the performance of a practical two-phase pumped loop. Results of analysis illustrate the utility of the computational model in the design of two-phase pumped-loop systems.

Computational method for characterization of a microchannel heat sink with multiple channels involving two-phase flow

Two-phase microchannel heat sinks are of interest in electronics cooling because of their compactness and low thermal resistance. In this study, a computational method is presented for the analysis of conjugate heat transfer and two-phase flow in a heat sink containing multiple microchannels that have prescribed flow rates. It involves coupled analysis of conduction within the solid and simultaneous two-phase flow and heat transfer in the microchannels. The flow and thermal behavior in each microchannel is determined by solving the one-dimensional momentum and energy conservation equations. Coupling between the heat transfer within the solid region and the two-phase flow in the microchannels is handled through iterations involving the transfer of heat flux distributions on the channel bounding surfaces. The method has been applied for the prediction of the flow and thermal performance of a heat sink with equal mass flow through individual microchannels. The results of our analysis show the important physical effects in the two-phase flow regime, namely the acceleration pressure drop and the boiling enhancement of heat transfer, that strongly affect the relative behavior of the individual microchannels. The study demonstrates quantitatively that the disparity in the heat loads is the cause of flow redistribution among the microchannels. Finally, the computational method presented in this study enables determination of the flow resistances of individual microchannels for a prescribed flow distribution. When combined with a network-based approach, this method will enable the prediction of both the two-phase flow distribution and thermal performance of practical heat sinks containing multiple microchannels

Computational Method for Characterization of a Microchannel Heat Sink Involving Two-Phase Flow

Microchannel heat sinks are being increasingly considered for the cooling of electronic equipment because of their ability to absorb high heat fluxes directly from the heat-dissipating components in a compact manner with a low thermal resistance. In this study, a computational method is presented for the analysis of conjugate heat transfer and two-phase flow in a heat sink containing a single microchannel. It involves a two-domain solution of the three-dimensional conduction within the solid region and the one-dimensional two-phase momentum and energy transfer within a microchannel. The nonlinear coupling between the two domains that occurs through the heat exchange at the walls of the microchannels is handled using an iterative calculation. Analysis of the flow and heat transfer in the microchannel is based on the homogenous flow assumption that is deemed to be accurate for the flow of low surface tension coolants such as methanol, isobutane, and HFC’s. Representative single and two-phase correlations are used for the calculation of the friction factor and the heat transfer coefficient. The computational model is applied for the prediction of the performance of a microchannel heat sink over a range of mass flow rates. The results of the analysis show the important physical effects that govern the performance of the microchannel heat sink involving two-phase flow. These include the acceleration of the flow in the microchannel in the two-phase region that influences the pressure drop through it and the two-phase enhancement of heat transfer that determines the temperature field within the solid region. This paper was also originally published as part of the Proceedings of the ASME 2005 Heat Transfer Summer Conference.Copyright