Data Center Energy Conservation by Heat Pipe Cold Energy Storage System

In this paper, design and economic analysis for applying a novel type of heat pipe into cold energy storage systems have been proposed and discussed. The heat pipe cold energy storage systems can be designed into several types that are ice storage, cold water storage and pre-cool heat exchanger. Those systems can be used for co-operating with conventional chiller system for cooling data centers. The heat load used for discussing in this paper is 8800 kW which represents a large scale data center. The methodology addressed in this paper can be also converted into the middle and small sizes of the data centers. This type of storage system will help to downsize the chiller and decrease its running time that would be able to save significant electricity cost and decrease green house gas emissions from the electricity generation. The proposed systems can be easily connected into the existing conventional systems without major design changes. The analysis in this paper is using Air Freezing Index AFI >= 400 °C-days/year for sizing the heat pipe modules. For the locations where AFI has different value the storage size will be varied accordingly. The paper also addressed a result that an optimum size of cold energy storage system that should be designed at a level to handle 60% of total yearly heat load of a data center.

Thermal Performance Improvement of a Heat Sink With Piezoelectric Vibrating Fins

A heat sink with piezoelectric vibrating fins is developed through attaching piezoelectric actuators to the fins of a heat sink, and the heat transfer characteristics of the heat sink are experimentally investigated. Thermal performance improvement of the heat sink by the vibration of the fins is observed compared to the thermal performance of a natural convection heat sink with static fins under a fixed heat sink geometry condition. The thermal performance of the heat sink changes as the vibration amplitude of the fins or the vibration frequency of the fins changes. Particularly, if the vibration frequency of the fins matches up to the natural frequency of the fins, the vibration amplitude is significantly increased by resonance and the thermal performance also increases. The natural frequency of the fins changes with the heat sink temperature because the geometry of the fins changes and the properties of the fins change due to the temperature change.

Capillary Flow Separation in 2- and 3-Dimensional Laminar Falling Liquid Films

Due to the selective use of liquid films in specialized technical equipment (e.g. new generation nuclear reactors), a fundamental understanding of underlying momentum and heat transport processes inside these thin liquid layers (with a thickness of approximately 0.5 mm) is required. In particular, the influence of surface waves (which develop due to the film’s natural instability) on these transport processes is of interest. For a number of years, experimental and numerical observations in wavy falling liquid films have suggested that momentum and heat transfer in the capillary wave region, preceding large wave humps, undergo drastic modulations. Indeed, some results have indicated that upward flow, i.e. counter to the gravitational acceleration, takes place in this region. Further, evidence of a substantial increase in wall-side and interfacial transfer coefficients has also been noted. Recently, Dietze et al. [1,2] have established that flow separation takes place in the capillary wave region of 2-dimensional laminar falling liquid films, partially explaining the above mentioned observations. Thereby, it was shown that the strong third order deformation (i.e. change in curvature) of the liquid-gas interface in the capillary wave region causes an adverse pressure gradient sufficiently large to induce flow detachment from the wall. In the present paper, a detailed experimental and numerical account of the capillary flow separation’s kinematics and governing dynamics as well as its effect on heat transfer for two different 2-dimensional flow conditions is presented. Experimentally, velocity measurements (using Laser Doppler Velocimetry (LDV) and Particle Image Velocimetry (PIV)) and film thickness measurements (using a Confocal Chromatic Imaging technique) were performed in a specifically designed optical test setup. On the numerical side, simulations of the full Navier-Stokes equations as well as the energy equation using the Volume of Fluid (VOF) method were performed. In addition to the 2-dimensional investigations, the characteristics of capillary flow separation under 3-dimensional wave dynamics were studied based on the 3-dimensional numerical simulation of a water film, which was previously investigated experimentally by Park and Nosoko [3]. Results show that flow separation persists over a wide area of the 3-dimensional capillary wave region, with multiple capillary separation eddies occurring in the shape of vortex tubes. In addition, strong spanwise flow induced by the same governing mechanism is shown to occur in this region, which could explain the drastic intensification of transfer to 3-dimensional liquid films.

Viscosity Effect on Thermocapillary Rupture of Falling Liquid Films

Rupture of a subcooled liquid film flowing over an inclined plate with a 150×150 mm heater is studied for a wide range of liquid viscosity (dynamic viscosity μ = (0.91–17.2)x10−3 Pa·s) and plate inclination angle with respect to the horizon (Θ = 3–90 deg). The main governing parameters of the experiment and their respective values are: Reynolds number Re = 0.15–54, heat flux q = 0–224 W/cm2. The effect of the heat flux on the film flow leads to the formation of periodically flowing rivulets and thin film between them. As the heat flux grows the film thickness between rivulets gradually decreases, and, upon reaching a certain threshold heat flux, qidp, the film ruptures in the area between the rivulets. The threshold heat flux increases with the flow rate of liquid and with the liquid viscosity, while the plate inclination angle has little effect on qidp. Criterion Kp, which is traditionally used in the literature to predict thermocapillary film rupture, was found to poorly generalize data for high viscous liquids (ethylene glycol, and aqueous solutions of glycerol) and also data for Θ≤45 deg. The criterion Kp was modified by taking into account characteristic critical film thickness for film rupture under isothermal conditions (no heating), deduced from existing theoretical models. The modified criterion has allowed to successfully generalize data for whole ranges of μ, Re, Θ and q, studied.

Numerical Simulation of Heat Transfer and Fluid Dynamics in Supersonic Chemically Reacting Flows

An implicit fully coupled numerical method for modeling of chemically reacting flows is presented. Favre averaged Navier-Stokes equations of multi-component gas mixture with nonequilibrium chemical reactions using Arrhenius chemistry are applied. A special method of splitting convective fluxes is introduced. This method allows for using spatially second-order approximation in the main flow region and of first-order approximation in regions with discontinuities. To consider the effects of high-speed compressibility on turbulence the author suggests a correction for the model, which is linearly dependent on Mach turbulent number. For the validation of the code the described numerical procedures are applied to a series of flow and heat and mass transfer problems. These include supersonic combustion of hydrogen in a vitiated air, chemically reacting flow through fluid rocket nozzle, afterburning of fluid and solid rocket plumes, fluid dynamics and convective heat transfer in convergent-divergent nozzle. Comparison of the simulation with available experimental data showed a good agreement for the above problems.

Effect of Reactant Preheating on the Stability of Non-Premixed Methane/Air Flames

This study details the influence of reactant temperature on the stability of non-premixed CH4/air co-flow jet flames. Flame characteristics have been studied for five temperature levels (from 295 K to 600 K). The hysteresis zone formed by the limits between attached and lifted flame translates towards higher methane jet velocities with an increase of initial temperature, independently of the air velocity range. Moreover, critical velocities vary linearly with initial temperature. In addition, attachment and lift-off heights have been obtained from CH* chemiluminescence visualization. Results point out that the attachment height decreases significantly with temperature. Observations also indicate that the intrinsic process of lifting is modified. Pre-lifting anchored flame local extinctions, not observed at room-temperature, appear at higher initial temperatures; their occurrence increases with temperature. The lift-off height of turbulent lifted flames is also reduced with temperature. Overall, results show that increasing local temperature in the stabilization zone enhances flame stability.

Analysis of Radiation Reabsorption Effects on Flame Characteristics and NOx Emission in Laminar Flames

The radiation reabsorption effects on NOx formation and flame characteristics in CH4/Air laminar flames were numerically investigated by using full chemistry mechanism and detailed transport properties. The radiative gases were treated as non-gray gas and their spectral radiative properties were evaluated by means of the statistical narrow-band model. The radiative heat transfer equation was solved by the discrete ordinate method. It was found that the reabsorption of emitting radiation leads to substantially wider flame thickness and higher flame temperature than those calculated by using the optically thin model, and the radiation reabsorption effect on the “radiation extinction limit” becomes more important. The results show that the level of NOx is predicted to be highest in the adiabatic flames, that is, flames without radiation heat loss, and that the level of NOx is predicted to be lowest in the flames by the optically thin model. In the flames by the SNB model, the predicted amount of NOx lies between these two levels. The calculated results also show that the radiation reabsorption effect on NOx formation grows stronger as the stretch rate decreases, particularly when CO2, a strong absorber, is added to the unburned gas mixture. In this study, the effectiveness and validity of the optically thin radiation model for calculating NOx formation in laminar flames was also investigated in comparison with the SNB model.

Hybrid Superlattice Coolers for Dynamic Thermal Management of Microprocessor Hotspots

A hybrid cooling scheme for thermal management of hotspots (300–500 W/cm2) in the presence of low background heat flux (100 W/cm2 over 1 cm2) is being investigated. It uses superlattice coolers (SLCs) to remove ultra high power density hotspot and microchannel heat sink for lower background heat flux. In this paper, transient response of the SLC for hotspot removal is studied. The effect of contact resistance, chip thickness, and hotspot size on the performance of the hybrid cooling scheme is also investigated.

Numerical and Experimental Study on Soot Accumulation on the Wall of Falling Fuel Film Micro-Combustor

Fluid flow contributes much to fuel-air mixture formation in a micro-combustor, the RNG k-ε turbulence model was used to simulate the cold flow field of a falling fuel film microcombustor, and comparison was made between numerical result and experimental results. It is shown that the RNG k-ε turbulence model translated the flow field of a complex structure micro-combustor and the soot accumulation on the wall of combustion chamber. The experimental results showed that soot accumulation occurs in vortex backflow area near the wall of combustion chamber and the numerical methods is helpful for understanding the way of soot accumulation in the wall of combustion chamber. Therefore, modifications on the flow field with different diameters and entrance direction of the air flow into the primary combustion chamber were made. The numerical simulation of flow distribution showed that the flow field of micro-combustor could be ideal for eliminated soot accumulation.

Mass Nature of Heat and Its Applications I: Motion of Thermomass Fluid and General Heat Conduction Law

The concept of thermomass is defined as the equivalent mass of thermal energy according to the Einstein’s mass-energy relation. Hence, the phonon gas in dielectrics can be regarded a weighty, compressible fluid. Heat conduction in the medium, where the rest mass lattices or molecules acts the porous framework, resembles the gas flow through the porous medium. Newton mechanics has been applied to establish the equation of state and the equation of motion for the phonon gas as in fluid mechanics, since the drift velocity of a phonon gas is normally much less than the speed of light. The momentum equation of the thermomass gas, including the driving, inertial and resistant forces, is a damped wave equation, which is in fact the general conduction law. This is because it reduces to the CV (Cattaneo-Vernotte) model or the single phase-lag model as the heat flux related inertial terms are neglected, and reduces to Fourier’s heat conduction law as all inertial terms are neglected. Therefore, the underlying physics of Fourier’s heat conduction law is the balance between the driving force and the resistant force of the heat motion, and Fourier’s law will break down when the inertial force is comparable to the resistant force, for instance, in the case of ultra-short pulse laser heating or heat conduction in carbon nanotubes at ultra-high heat flux.