Molecular Dynamics and Monte Carlo Simulations for Heat Transfer in Micro and Nano-channels

Abstract. There is a tendency to cool mechanical and electrical components by microchannels. When the channel size decreases, the continuum approach starts to fail and particle based methods should be used. In this paper, a dense gas in micro and nano-channels is modelled by molecular dynamics and Monte Carlo simulations. It is shown that in the limit situation both methods yield the same solution. Molecular dynamics is an accurate but computational expensive method. The Monte Carlo method is more efficient, but is less accurate near the boundaries. Therefore a new coupling algorithm for molecular dynamics and Monte Carlo is introduced in which the advantages of both methods are used.

Is Hot IT a False Economy? An Analysis of Server and Data Center Energy Efficiency as Temperatures Rise

As demand for digital services grows, there is need to improve efficiency and reduce the environmental impact of data centers. The largest energy consumer in any data center is the IT, followed by the systems dedicated to cooling. Aiming to improve efficiency, and driven by metrics like PUE, there is a trend towards running data centers hotter to reduce the cooling energy. There is little research investigating the effect this will have on the IT beyond failure rates. To ensure overall efficiency is improving, we must view the data center as a system of systems, taking a holistic view rather than focusing on individual sub-systems. In this paper we use industry standard benchmarks and a wind-tunnel to profile typical enterprise IT. We analyze the effect of environmental conditions on IT efficiency, showing minor increases in temperature or pressure impact the efficiency of servers. Using an idealized, simulated data center case study we show that the interaction between cooling systems, server behaviour and local climate are non-trivial and increasing temperatures has potential to worsen efficiency

A review of linear compressors for refrigeration

Linear compressor has no crank mechanism compared with conventional reciprocating compressor. This allows higher efficiency, oil-free operation, lower cost and smaller size when linear compressors are used for vapour compression refrigeration (VCR) system. Typically, a linear compressor consists of a linear motor (connected to a piston) and suspension springs, operated at resonant frequency. This paper presents a review of linear compressors for refrigeration system. Different designs and modelling of linear compressors for both domestic refrigeration and electronics cooling (miniature VCR system) are discussed. Key characteristics of linear compressor are also described, including motor type, compressor loss, piston sensing and control, piston drift and resonance. The challenges associated with the linear compressors are also discussed to provide a comprehensive review of the technology for research and development in future

A molecular dynamics boundary condition for heat exchange between walls and a fluid

In molecular dynamics simulations of heat transfer in micro channels, a lot of computation time is used when the wall molecules are explicitly simulated. To save computation time, implicit boundary conditions, such as the Maxwell conditions, can be used. With these boundary conditions, heat transfer is still a problem. In this work, we derive a new boundary condition based on a vibrating potential wall. The heat-transfer properties of the new boundary condition are shown to be comparable with those of the explicit wall. The computation time needed for the implicit boundary condition is very small compared with that needed for the explicit simulation

Visualization study on the instabilities of phase-change heat transfer in a flat two-phase closed thermosyphon

This paper presents systematic experiments and visualization on the instabilities of phase-change heat transfer for water, ethanol and acetone in a flat evaporator of a two phase closed system, respectively. The effects of the heat flux, filling ratio, coolant temperature and working fluid type on the instabilities and their mechanisms have been systematically investigated. The experimental results show that the instabilities of phase-change heat transfer are strongly related to the corresponding heat transfer modes. The instabilities of temperature and heat transfer coefficient (HTC) of the evaporator are mainly caused by the bubble behaviours, the physical properties and the operation pressures. Natural convection, intermittent boiling and fully developed nucleate boiling are the main heat transfer modes in the present study. The condensate droplets may affect the instabilities due to inducing periodic boiling at lower heat fluxes. The maximum standard deviations of the evaporator temperature and vapor pressure fluctuations can reach 3.1 °C and 0.8 kPa respectively during the intermittent boiling. There is no intermittent boiling regime for ethanol and acetone in the present study. Therefore, no instability phenomena of nucleate boiling with ethanol and acetone are observed in the present study

Fundamental issues, mechanisms and models of flow boiling heat transfer in microscale channels

This paper presents state-of-the-art review on the fundamental and frontier research of flow boiling heat transfer, mechanisms and prediction methods including models and correlations for heat transfer in microscale channels. First, fundamental issues of current research on flow boiling in microscale channels are addressed. These mainly include the criteria for macroscale and microscale channels. Then, studies on flow boiling heat transfer behaviours and mechanisms in microscale channels are presented. Next, the available correlations and models of flow boiling heat transfer in microscale channels are reviewed and analysed. Comparisons of 12 correlations with a database covering a wide range of test parameters and 8 fluids are presented. It shows that all correlations poorly agree to the database. No generalized model or correlation is able to predict all flow boiling heat transfer data. Furthermore, comparisons of the mechanistic flow boiling heat transfer models based on flow patterns including the Thome et al. three-zone heat transfer model for evaporation in microchannel and the flow pattern based model combining the Thome et al. three zone heat transfer models with the Cioncolini-Thome annular flow model for both macro- and microchannel to the database are presented. It shows that the flow pattern based model combining the three zone model with the annular flow model gives better prediction than the three zone heat transfer model alone. The flow pattern based heat transfer model favourably agrees with the experimental database collected from the literature. According to the comparison and analysis, suggestions have been given for improving the prediction methods in the future. Next, flow patterned based phenomenological models and their applications to microscale channels are presented. Finally, as an important topic, unstable and transient flow boiling phenomena in microscale channels are briefed and recommendations for future research are given. According to this comprehensive review and analysis of the current research on the fundamental issues of flow boiling, mechanisms and prediction methods in microscale channels, the future research needs have been identified and recommended. In general, systematic and accurate experimental data of flow boiling heat transfer in microscale channels are still needed although a large amount of work has been done over the past decades. The channel size effect on the flow boiling behaviours should be systematically investigated. Heat transfer mechanisms in microscale channels should be further understood and related to the corresponding flow patterns. Furthermore, effort should be made to develop and improve generalized mechanistic prediction methods and theoretical models for flow boiling heat transfer in microscale channels according to the physical phenomena/mechanisms and the corresponding flow structures. The effects of the channel size and a wide range of test conditions and fluid types should be considered in develop new methods. Furthermore, systematic experimental, analytical and modeling studies on unstable and transient flow boiling heat transfer in microscale channels should be conducted to understand the physical mechanisms and theoretical models

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

Particle-based evaporation models and wall interaction for microchannel cooling

Microchannel cooling, in which a coolant flows through a microchannel, is a efficient met-hod for heat removal. The heat removal can be improved by having the coolant evaporate inside the microchannel. Although it is clear that this method can achieve large local cooling, the process is not fully understood yet. CFD descriptions fail at the small length scales, but particle-based method still work. Two of the most important particle-based techniques are Direct Simulation Monte Carlo (DSMC) and Molecular Dynamics (MD). DSMC is computationally faster, but is less accurate and only works for gases. MD is more time-consuming, but also works in the liquid and solid phase. In the simulation of microchannels, the channel walls can also be simulated explicitly with MD, which in-creases the accuracy. The methods (CFD, DSMC and MD) are analyzed on their weak-nesses and strengths, and compared in three standard problems: a Poiseuille flow, a gas with a temperature gradient between two walls, and lubrication flow around a heated obstacle. The results show that the modelling of the fluid-wall interaction crucial for the overall behavior. In the simulation of evaporation inside microchannels, the treatment of fluid-wall heat transfer is the most important. In gases close to walls, density fluctuations occur, which have a large effect on the heat transfer between wall and gas; these fluctuations have been analyzed in more detail. For heat transfer between a micro channel wall and the coolant, the explicit wall model can be used in MD with great accuracy, but also with great computational costs. Other wall models that are computationally cheaper exist, but they are less accurate, or have parameters that are unknown a priori. To overcome this problem, a new model was introduced, based on a vibrating wall model, that cuts back on computation time but has an accuracy comparable to the explicit wall model. Evaporation can be modeled in MD, but to be sure that the simulations give the right result, it has to be validated with experimental results. First, the intermolecular interaction was validated, by looking at vaporation of Argon. Then, the bonds were validated, by analyzing Oxygen evaporation. In this way, more complex molecules consisting of atoms with internal bonds have also been checked. The two problems of fluid-wall interaction and evaporation come together at the microregion. This microregion is important in the analysis of a microchannel, as the most heat is transferred here. To validate the MD simulation of this microregion, simulation results are compared to the results of a model of this region based on a continuous description. With the models now available, a complete microchannel with evaporative cooling can be simulated. However, it is computationally too expensive to useMDfor the full domain, so MD is only used for the most relevant parts where CFD methods are inaccurate (close to the walls, at the evaporation interface and in the microregion), and CFD methods are used for the bulk