Ali, Adya Alisha
13 July 2004
Technological advancement, as well as consumer demands, has motivated the miniaturization of electronic/mechanical systems and increase of device power and performance. The notebook computer is not an exception, and innovative thermal management solutions must be employed to compensate for the increased heat dissipation in the space-constrained enclosures. The majority of current cooling systems in laptop computers rely on heat pipes attached to a remote heat exchanger with micro-fans providing forced convection to reject heat to the ambient, however this technique can not accommodate the increasing heat fluxes in the confined laptop enclosure. In this study, a two-phase closed loop cooling system is designed and tested for a laptop computer. The cooling system consists of an evaporator structure containing boiling structures connected to a compact condenser with mini fans providing external forced convection. A pump is also incorporated to assist the return of the condensate back to the evaporator. The cooling system is characterized by a parametric study which determines the effects of volume fill ratio of coolant, initial system pressure, and pump flow rate on the thermal performance of the closed loop. Experimental data shows the optimum parametric values which can dissipate 25 W of chip power with a chip temperature maintained at 95C. Numerical analysis provides additional data to further enhance the heat dissipation from the external air-cooled side of the condenser by studying the effects of ventilation and air flow rate across the system. Thermal management of mobile systems must be considered during the early design phases, and this research shows the feasibility of implementing of a two-phase cooling system to dissipate 25 W in a laptop computer.
Operation and Heuristic Design of Closed Loop Two-Phase Wicked Thermosyphons (CLTPWT) for Cooling Light Emitting Diodes (LEDs)Remella Siva Rama, Karthik 15 May 2018 (has links)
No description available.
Material and Processing Development Contributions Toward the Development of a MEMS Based Micro Loop Heat PipeShuja, Ahmed A. 03 July 2007 (has links)
No description available.
Two-phase cooling solutions employing subcooled nucleate boiling flows e.g. thermosyphons, have gained a special interest during the last few decades. This interest stems from their enhanced ability to remove extremely high heat fluxes, while keeping a uniform surface temperature. Consequently, modelling and predicting boiling flows is very important, in order to optimise the two-phase cooling operation and to increase the involved heat transfer coefficients. In this work, a subcooled boiling model is implemented in the open-source code OpenFOAM to improve and extend its existing solver reactingTwoPhaseEulerFoam dedicated to model boiling flows. These flows are modelled using Computational Fluid Dynamics (CFD) following the Eulerian two-fluid approach. The simulations are used to evaluate and analyse the existing Active Nucleation Site Density (ANSD) models in the literature. Based on this evaluation, the accuracy of the CFD simulations using existing boiling sub-models is determined, and features leading to improve this accuracy are highlighted. In addition, the CFD simulations are used to perform a sensitivity analysis of the interfacial forces acting on bubbles during boiling flows. Finally, CFD simulation data is employed to study the Onset of Nucleate Boiling (ONB) and to propose a new model for this boiling sub-model, with an improved prediction accuracy and extended validity range. It is shown in this work that predictions associated with existing boiling sub-models are not accurate, and such sub-models need to take into account several convective boiling quantities to improve their accuracy. These quantities are the thermophysical properties of the involved materials, liquid and vapour thermodynamic properties and the heated surface micro-structure properties. Regarding the interfacial momentum transfer, it is shown that all the interfacial forces have considerable effects on boiling, except the lift force, which can be neglected without influencing the simulations' output. The new proposed ONB model takes into account convective boiling features, and it able to predict the ONB with a very good accuracy with a standard deviation of 2.7% or 0.1 K. This new ONB model is valid for a wide range of inlet Reynolds numbers, covering both regimes, laminar and turbulent, and a wide range of inlet subcoolings and applied heat fluxes.
DATA CENTER CONDENSER OPTIMIZATION: A DISCRETIZED MODELLING APPROACH TO IMPROVE PUMPED TWO-PHASE COOLING CYCLESTyler John Schostek (16613160) 19 July 2023 (has links)
<p>Rising interest in high-performance servers in data centers to support the increasing demands for cloud-computing and storage have challenged thermal management systems. To prevent these increased power density servers from overheating due to the high heat fluxes dissipated, new cooling methods have continued to be investigated in recent years. One such solution is pumped two-phase cooling which shows promise over traditional air cooling due to the reduced power consumption it requires to operate, while also being able to dissipate large amounts of heat from the small components in servers.</p> <p> </p> <p>Although pumped two-phase systems as a cooling strategy have existed for multiple decades, sub-optimal component design have hindered the potential efficiencies achievable. This is especially prevalent in the condenser where, in order to meet required metrics, these heat exchangers are commonly oversized due to maldistribution at low vapor qualities and a lack of understanding about the condensation behavior within certain geometries.</p> <p><br></p> <p>Through the work presented in this thesis, the capabilities of an air-cooled microchannel condenser model are explored for future use in optimization studies for data center applications. To perform this research, an investigation into the boundary conditions of these systems and common condenser modeling strategies were carried out. Using this knowledge, a flexible discretized condenser model was developed to capture the behavior of pumped two-phase cooling in data centers under a wide range of operating conditions. In conjunction, an experimental test setup was sized, designed, and constructed to provide validation for the model. Then, using the model, some initial parametric studies were conducted to identify the sensitivity effects of various parameters on overall condenser performance. In this initial study, some favored boundary conditions and geometries were found that both minimize refrigerant pressure drops and maximize heat transfer. For an air-cooled condenser operating with R1234ze(E), these include: refrigerant entering the condenser around 40% quality, operating at moderate refrigerant mass fluxes through the channels (130 - 460 kg/m^2-s), and designing microchannel condenser tubes with many tightly packed square ports. Continued investigation into the contributing parameters of weight in the future using the tools developed in this thesis will lead to further optimized condenser designs and operating conditions.</p>
Jeongmin Lee (13133907)
21 July 2022
<p>The present study investigates the capability of computational fluid dynamics (CFD) extensively to predict hydrodynamics and heat transfer characteristics of FC-72 flow boiling in a 2.5-mm ´ 5.0-mm rectangular channel and experimentally explores system instabilities: <em>density wave oscillation</em> (DWO), <em>pressure drop oscillation</em> (PDO) and <em>parallel channel instability</em> (PCI) in a micro-channel heat sink containing 38 parallel channels having a hydraulic diameter of 316-μm. </p> <p>The computational method performs transient analysis to model the entire flow field and bubble behavior for subcooled flow boiling in a rectangular channel heated on two opposite walls at high heat flux conditions of about 40% – 80% of <em>critical heat flux</em> (CHF). The 3D CFD solver is constructed in ANSYS Fluent in which the <em>volume of fluid</em> (VOF) model is combined with a <em>shear stress transport</em> (SST) <em>k</em>-<em>ω</em> turbulent model, a surface tension model, and interfacial phase change model, along with a model for effects of shear-lift and bubble collision dispersion to overcome a fundamental weakness in modeling multiphase flows. Detailed information about bubble distribution in the vicinity of the heated surface, thermal conduction inside the heating wall, local heat fluxes passing through the solid-fluid interface, and velocity and temperature profiles, which are not easily observed or measured by experiments, is carefully evaluated. The simulation results are compared to experimental data to validate the solver’s ability to predict the flow configuration with single/double-side heating. The added momentum by shear-lift is shown to govern primarily the dynamic behavior of tiny bubbles stuck on the heated bottom wall and therefore has a more significant impact on both heat transfer and heated wall temperature. By including bubble collision dispersion force, coalescence of densely packed bubbles in the bulk region is significantly inhibited, with more giant bubbles even incurring additional breakup into smaller bubbles and culminating in far less vapor accumulation along the top wall. Including these momentums is shown to yield better agreement with local interfacial behavior along the channel, overall flow pattern, and heat transfer parameters (wall temperature and heat transfer coefficient) observed and measured in experiments. The computational approach is also shown to be highly effective at predicting local phenomena (velocity and temperature profiles) not easily determined through experiments. Different flow regimes predicted along the heated length exhibit a number of dominant mechanisms, including bubble nucleation, bubble growth, coalescence, vapor blankets, interfacial waviness, and residual liquid sub-layer, all of which agree well with the experiment. Vapor velocity is shown to increase appreciably along the heated length because of increased void fraction, while liquid velocity experiences large fluctuations. Non-equilibrium effects are accentuated with increasing mass velocity, contributing minor deviations of fluid temperature from simulations compared to those predicted by the analytical method. Predicted wall temperature is reasonably uniform in the middle of the heated length but increases in the entrance region due to sensible heat transfer in the subcooled liquid and decreases toward the exit, primarily because of flow acceleration resulting from increased void fraction. When it comes to analyzing heat transfer mechanisms at extremely high heat flux via CFD, predicted flow pattern, bubble behavior, and heat transfer parameters (such as wall temperature excursion and thermal energy concentration) clearly represent phenomena of premature CHF, which take place slightly earlier than actual operating conditions. But, despite these slight differences, the present computational work does demonstrate the ability to effectively predict the severe degradation in heat transfer performance commonly encountered at heat fluxes nearing CHF. </p> <p>Much of the published literature addressing flow instabilities in thermal management systems employing micro-channel modules are focused on instability characteristics of the module alone, and far fewer studies have aimed at understanding the relationship between these characteristics and compressive volume in the flow loop external to the module. From a practical point of view, developers of micro-channel thermal management systems for many modern applications are in pursuit of practical remedies that would significantly mitigate instabilities and their impact on cooling performance. Experiments are executed using FC-72 as a working fluid with a wide range of mass velocities and a reasonably constant inlet subcooling of ~15°C. The flow instabilities are reflected in pressure fluctuations detected mainly in the heat sink’s upstream plenum. Both inlet pressure and pressure drop signals are analyzed in pursuit of amplitude and frequency characteristics for different mass velocities and over a range of heat fluxes. The current experimental study also examines the effects of compressible volume location in a closed pump-driven flow loop designed to deliver FC-72 to a micro-channel test module having 38 channels with 315-μm hydraulic diameter. Three accumulator locations are investigated: upstream of the test module, downstream of the test module, and between the condenser and pump. Both high-frequency temporal parameter data and high-speed video records are analyzed for ranges of mass velocity and heat flux, with inlet subcooling held constant at ~15°C. PDO is shown to dominate when the accumulator is situated upstream, whereas PCI is dominant for the other two locations. Appreciable confinement of bubbles in individual channels is shown to promote rapid axial bubble growth. The study shows significant variations in the amount of vapor generated and dominant flow patterns among channels, a clear manifestation of PCI, especially for low mass velocities and high heat fluxes. It is also shown effects of the heat sink’s instabilities are felt in other components of the flow loop. The parametric trends for PCI are investigated with the aid of three different types of stability maps which show different abilities at demarcating stable and unstable operations. PDO shows severe pressure oscillations across the micro-channel heat sink, with rapid bubble growth and confinement, elongated bubble expansion in both directions, flow stagnation, and flow reversal (including vapor backflow to the inlet plenum) constituting the principal sequence of events characterizing the instability. Spectral analysis of pressure signals is performed using Fast Fourier Transform, which shows PDO extending the inlet pressure fluctuations with the same dominant frequency to other upstream flow loop components, with higher amplitudes closer to the pump exit. From a practical system operation point of view, throttling the flow upstream of the heat sink eliminates PDO but renders PCI dominant, and placing the accumulator in the liquid flow segment of the loop between the condenser and pump ensures the most stable operation.</p>
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