2,850 research outputs found
Thermocapillary actuation of liquid flow on chemically patterned surfaces
We have investigated the thermocapillary flow of a Newtonian liquid on hydrophilic microstripes which are lithographically defined on a hydrophobic surface. The speed of the microstreams is studied as a function of the stripe width w, the applied thermal gradient |dT/dx| and the liquid volume V deposited on a connecting reservoir pad. Numerical solutions of the flow speed as a function of downstream position show excellent agreement with experiment. The only adjustable parameter is the inlet film height, which is controlled by the ratio of the reservoir pressure to the shear stress applied to the liquid stream. In the limiting cases where this ratio is either much smaller or much larger than unity, the rivulet speed shows a power law dependency on w, |dT/dx| and V. In this study we demonstrate that thermocapillary driven flow on chemically patterned surfaces can provide an elegant and tunable method for the transport of ultrasmall liquid volumes in emerging microfluidic technologies
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An analysis of the thermal interaction between components in power converter applications
Accurately predicting the temperature of semiconductor devices is very important in the initial design of power electronics converter. RC thermal models derived from well-known methods have some ability to predict the temperature. However, the accuracy is boundary condition specific, hence, these methods cannot be used in the reliability analysis. To make the thermal model more accurate and robust the factors contributing to discrepancies need to be analyzed carefully. These are power-module-materials’ non-linear properties, thermal grease layer and the cooling system (i.e., liquid-cooled cold plate). In this work, estimation of accurate RC parameters from FEA thermal model is demonstrated in COMSOL. The electrical model having temperature dependent power loss model is coupled to refined thermal model and solved in a circuit simulator, PLECS. The proposed method is applied in two applications: assessing thermal interaction between IGBTs and anti-parallel diodes in a half-bridge power module, and assessing thermal interaction among the discrete switches in an interleaved bidirectional DC-DC converter. Results show that the impact of material non-linearity, thermal grease layer and cooling boundary conditions are significant for accurate prediction of IGBT and diode temperatures. The proposed model is consistent to FEA results and differs by 2-6.5% comparing to the experimental results
The benefit of high-conductivity materials in film cooled turbine nozzles
This study presents an experimental and numerical investigation of the beneficial effect of higher conductivity materials in HP turbine nozzles. Most of the literature studies focus on the maximum temperature that a nozzle can withstand, whereas the effect of thermal gradients is often neglected. However thermal gradients have higher influence on the life of the components and they have to be given careful consideration. In this work it is shown that thermal gradients are reduced by using high conductivity materials and, as a consequence, the nozzles life is appreciably increased. A representative film cooled leading edge with an internal impingement plate was studied experimentally at Texas AM University. Two materials were used, namely polycarbonate and stainless steel, in order to highlight the impact of conduction on coolant effectiveness. Numerically conjugate heat transfer simulations have been carried out with an in house solver to analyse in detail the impact of conduction and internal convection. Both experimental and numerical results show that by increasing the conductivity in the solid region, the thermal gradients are strongly reduced. Numerically it is shown that using inserts of nickel-aluminide alloys in nozzles may reduce the thermal gradients from 3 to 4 times if compared to nowadays design. © 2012 Elsevier Inc
Experimental study of transient boiling
The clad to coolant heat transfer is a key issue in the safety evaluation of the clad failure risk during a reactivity initiated accident (RIA). It has been demonstrated that under very fast transient conditions such as those of RIA, the steady state correlations are not valid. Dedicated experiments simulating RIA conditions are required to determine the transient transfers and deeper understanding of the transient boiling phenomena is desirable. This paper presents an experimental study of the onset and development of boiling of a refrigerant flowing over a half-cylindrical wall whose heating rate is mastered. Thanks to the design of the facility, synchronized wall temperature measurement and flow visualization allow to characterize and analyse the heat transfer for the different boiling regimes. From the main results of the first tests campaigns, the impact of the wall heating rate on the boiling curve is discussed. We then draw some perspectives for further campaigns and analysis
Direct Pore Level Simulation of Heat Transfer in Open Cell Reticulated Porous Ceramics
The project involved in studying the fluid transport, heat and mass transport inside various ceramic porous inserts by Direct Pore Level Simulations (DPLS). The geometric grid data required for the simulations are reconstructed from the computer tomographic scan images of the real porous media. The simulation results are used to study the influence of the structural properties of porous media on the fluid flow, heat transfer and mass transfer
Analytical and numerical studies on macro and micro scale heat sinks for electronics applications
Thesis (M.S.) University of Alaska Fairbanks, 2003From the practice in computer industry the standard approach for electronics cooling is fan-cooled heat sinks. We developed thermal models for forced convection heat sinks. An Intel Pentium ill chip has been adopted as a preliminary design case to develop necessary equations. We found the heat dissipated from the aluminum heat sink, based upon different modes of airflow over the fins. We also considered radiation heat transfer. We performed transient heat transfer analysis to determine the time to attain the steady state temperature for the whole system for macro and micro scale also. Next, we refined our one-dimensional analytical convection analysis using the numerical analysis. This was done using the computational fluid dynamics code Fluent to obtain accurate velocity fields over the fins. Using these improved velocities, convective heat transfer coefficients were computed. Next, we have miniaturized the processor chip size to the micrometer scale and have designed a heat sink based upon the models we have developed. Calculations of mean free path and Knudsen number shows the continuum theory for air still holds for our designed micro-channels. Equations for natural convection heat sinks are also explored as a part of this study. In the microscale study, we did forced and natural convection analysis.1. Introduction -- 2. Theory -- 3. Analytical solutions for macro scale chip -- 4. Application to microscale chips -- 5. Conclusions -- References
On the Interaction of Chemically Conditioned Water with Steel Heating Surfaces during Saturated Pool Boiling: An Experimental Thermotechnical Approach
The heat transfer from steel surfaces to boiling water is subject to irreversible negative temporal changes. In this work, the influence of the responsible physicochemical processes on the heating surface is investigated by thermotechnical measurements. Therefore, the influence of exemplary inorganic and organic feed water agents on the heat transfer, critical operating conditions and the surface properties of horizontal heating surfaces made from carbon-steel is examined thermotechnically in a special apparatus under pool boiling conditions and further characterised by analytical methods.Der Wärmeübergang von Stahlheizflächen an siedendes Wasser unterliegt irreversiblen negativen zeitlichen Änderungen. In dieser Arbeit wird die Wirkung der verantwortlichen physikochemischen Prozesse auf der Heizfläche wärmetechnisch untersucht. Dazu wird der Einfluss exemplarischer anorganischer und organischer Speisewasseradditive auf den Wärmeübergang, die kritischen Betriebszustände und die Eigenschaften horizontaler Kohlenstoffstahlheizflächen beim Behältersieden in einer speziellen Anordnung wärmetechnisch beschrieben und darüber hinaus mithilfe analytischer Methoden charakterisiert
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