178 research outputs found
Jayathi Y. Murthy, head and shoulders portrait
No full textPurdue University Faculty. Jayathi Y. Murthy, appointed the Robert V. Adams Professor of Mechanical Engineerin
Jayathi Y. Murthy and student working on computer simulation
No full textJayathi Y. Murthy (standing), a professor in Purdue's School of Mechanical Engineering, works with graduate student Dipali Pradhan on a computer simulation to analyze how heat is transferred through a carbon nanotube. Murthy will lead a new center based at Purdue's Discovery Park to develop advanced simulations for commercial and defense.applications.College of Engineering
Design of Integrated Nanostructured Wicks for High-Performance Vapor Chambers
No full textThe performance of passive phase-change cooling devices, such as vapor chambers or heat pipes, may be significantly enhanced by exploiting the superior thermal properties of carbon nanotube (CNT) arrays. The potential for large reductions in overall package resistance with the use of high-conductivity wick materials enhanced with CNT nanostructures is investigated. While such nanostructured wicks feature very small pore sizes that support high capillary pressures, it is shown that the high fluid flow resistance through these dense arrays prevents their use as the lone fluid transport mechanism. It is proposed that evaporator surfaces comprised of nanostructured wicks fed by interspersed conventional wick materials (such as sintered powders) can provide the required permeability for fluid flow while simultaneously decreasing the effective evaporator thermal resistance. Optimization of wicks with integrated sintered and nanostructured areas requires a study of the trade-offs between the greater permeability of the sintered materials and the greater capillary pressure and thin-film evaporation area offered by the nanostructures. A numerical model is developed to estimate the thermal resistance of the evaporator region compared to that of a homogeneous sintered powder wick. The inputs needed for this model include the permeability and the capillary pressure in the two regions. A parametric study is conducted as a function of the ratio of conduction and evaporative resistances for the nanostructured and sintered regions. For a given heat input, the optimal liquid-feeding geometry that minimizes thermal resistance is obtained. In the best cases, the thermal resistance is reduced by a factor of thirteen through the use of the integrated nanostructured wicks compared to the resistance of a homogeneous sintered powder wick
Transient three-dimensional modeling of flat heat pipes with discrete heat sources
No full textA stable numerical procedure is developed to analyze the transient performance of flat heat pipes for large input heat fluxes and high wick conductivity in the incompressible flow limit. Computation of flow and heat transfer in a heat pipe is complicated by the strong coupling among the velocity, pressure and temperature fields with phase change at the interface between the vapor and wick. A structured collocated finite volume scheme is used in conjunction with the SIMPLE algorithm to solve the continuity, energy and momentum equations. A numerical scheme is devised to compute system pressurization in the incompressible limit using overall mass balance. The stability of the standard sequential procedure is improved by accounting for the coupling between the evaporator/condenser mass flow rate and the interface temperature and pressure as well as the system pressure. One-dimensional, two-dimensional and three-dimensional models have been developed using the improved and standard numerical scheme to analyze the transient and steady-state performance of flat heat pipes. The model predictions are validated by comparing the heat pipe wall temperatures against experimental measurements. Numerical wall and vapor temperature predictions in a cylindrical heat pipe are also benchmarked with experimental and model results in the literature. Two-dimensional simulations of vapor chambers are performed to analyze the heat spreading characteristics of typical flat heat pipes. Comparisons are made with experimental measurements of the cold and hot wall temperature profiles, as well as with simulations of solid copper spreaders. In the three-dimensional analysis with discrete heat sources, predictions are made of the magnitude of heat flux at which dry-out would occur in a flat heat pipe. The input heat flux and the spacing between the discrete heat sources are studied as parameters. The location in the heat pipe at which dry-out is initiated is found to be different from that of the maximum temperature. The algorithm that has been developed to analyze the transient and steady-state performance of flat heat pipes is implemented into a commercial software package through user defined functions, for ease of use in practical applications
Direct simulation of transport through stochastic porous media
No full textPorous sintered microstructures are critical to the functioning of heat transport devices such as heat pipes and vapor chambers that are employed in a variety of thermal management applications. Accurate understanding of the pore-scale transport phenomena is important for enhancing the thermal performance of such devices. In the first part of the thesis, a direct simulation methodology based on the actual, detailed microstructure of the porous media obtained via X-ray microtomography is developed for predicting fundamental transport characteristics. Open-celled aluminum foams are first considered for validating the procedure. The approach is then employed for predicting single- and two-phase transport properties of commercial sintered copper wicks. In the later part of the thesis, two different approaches useful for designing optimized wick structures are presented. X-ray microtomography is a novel, non-destructive 3D imaging technique suitable for analyzing intricate porous media. Three commercial metal foam samples of similar volumetric porosity (in the range ∼ 91–93%), but with different pore sizes (10, 20 and 40 pores per inch), are first considered for validating the direct simulation approach developed here. Effective transport properties such as thermal conductivity and interfacial heat transfer coefficient are computed and successfully compared against data and models from the literature. A network model for the estimation of effective thermal conductivity of open-celled metal foams, constructed out of 3D image skeletons is then presented, and significant computational cost savings relative to detailed numerical analysis are demonstrated. A thorough microstructural characterization of foam features—pore size, ligament thickness, ligament length and pore shapes—is also performed. All the three foam samples are observed to have similar pore shapes and volumetric porosity, while the other features scale with pore size. The validated direct simulation approach is then employed for a detailed characterization of single-phase transport properties of commercially available sintered copper wick microstructures, in the second part of the thesis. A scan resolution of 5.5 µm is employed, and the current computations are compared with correlations and other experimental data available in the literature. Based on the computational results, new correlations for predicting convective heat transfer through porous sintered beds are also proposed. Pore-scale analysis of thin-film evaporation through sintered copper wicks is subsequently performed, again employing real microstructures. For improving convergence, modifications are introduced into the Volume of Fluid (VOF) model available in a commercial software package. Important two-phase characteristics, such as capillary pressure, effective pore radius, and evaporative mass and heat fluxes, are estimated. Based on the analysis, the best performing sample (particle size range) is identified along with the optimum contact angle. The final part of the thesis focuses on reverse-engineering and design of sintered wick structures with two approaches. In the first approach, a cellular automaton model is developed for predicting microstructural evolution during sintering. After thorough validation, the developed model is employed to predict the sintering dynamics of randomly packed multi-particle configurations in two and three dimensions. The effect of sintering parameters, particle size, and porosity on fundamental transport properties, viz., effective thermal conductivity and permeability, is quantified. The second approach employs 2D image data which are readily obtainable via techniques such as SEM. Firstly, based on the two-point autocorrelation function, a detailed microstructural characterization of sintered beds is performed. Further, a reconstruction technique is implemented for reconstructing a three-dimensional stochastic equivalent structure of the considered thin-sections. These reconstructed domains are employed for predicting single-phase fluid-thermal characteristics, and a detailed comparison with the actual 3D XMT data is also performed. Finally, based on the nature of two-point autocorrelation functions, a new parametrized model is proposed for the design of porous materials. The utility of this model in reconstructing three-dimensional porous microstructures with controllable fluid-thermal properties of interest is demonstrated. With advances in additive manufacturing techniques, such an approach may eventually be employed to design intricate porous structures with properties tailored to specific applications
A meshless finite difference method for fluid flow and heat transfer
No full textMesh generation consumes a substantial portion of human time in computational fluid dynamics (CFD) simulations of complex industrial geometries. Despite the progress in developing solvers and mesh generation techniques for unstructured meshes, the task remains onerous. Therefore, there has been a great deal of interest in recent years to develop computational techniques that eliminate the mesh generation task altogether, through the use of meshless methods. A number of successful schemes have been published, most commonly for structural analysis, but also for fluid flow. Nevertheless, it is fair to say that the field is still in its infancy, and many needs exist for improving discretization accuracy and solution speed. In this thesis, a meshless finite difference scheme is developed for steady incompressible flows of Newtonian fluids using a weighted least-squares method. The weighted least-squares method is used to fit a polynomial which is then compared to the Taylor series in order to compute approximations to the derivatives appearing in the governing equations. The method is applied in sequence to the heat diffusion equation, the convection-diffusion equation, and finally to the incompressible steady Navier-Stokes equations, and its accuracy and convergence properties are evaluated. Heat conduction in a constant conductivity domain is first computed using structured and unstructured distributions of points in order to establish the order of accuracy of the method. Conjugate heat conduction problems are addressed subsequently, with conductivity ratios of up to 1000. The solutions obtained using the meshless finite difference method are compared to those obtained using the commercial software, FLUENT. Good comparisons with published analytical and numerical solutions are obtained. The scheme is shown to be free of spurious spatial oscillations that plague many published meshless schemes for conjugate heat transfer, especially at high conductivity ratios. Scalar transport in the presence of a given velocity field is simulated next. The focus here is to develop analogues to convection schemes used in traditional finite differences. These include the first-order upwind scheme, the second-order central difference scheme and a new technique called the minimum gradient method, inspired by the essentially non-oscillatory (ENO) scheme. The stability of the solution procedure is demonstrated for a range of Peclet numbers. The order of accuracy is established by comparing computed solutions to available exact solutions. Finally, the most important element of CFD, namely the fluid flow solution method, is tackled. A non-staggered velocity-pressure formulation is the most convenient option for a meshless method. Therefore, the scheme stores pressure and velocity at all computational points. Our focus is on the solution of incompressible flows using sequential and iterative schemes. Hence, a modified explicit fractional-step technique is developed for the meshless method. At each time step, the momentum equations are first solved without a pressure gradient term using an explicit time step, and yield an auxiliary velocity field. This auxiliary velocity is not continuity satisfying, and therefore must be corrected; the pressure is computed in such as way as to ensure that the resulting velocity field is divergence-free. In this work, the auxiliary velocity field is decomposed into curl-free and divergence-free components. The curl-free component is cast as the gradient of a scalar field, and this field is solved for, using boundary conditions derived from those imposed on pressure. The pressure is then computed posteriori from the scalar field through a simple algebraic relationship. The explicit fractional time-stepping algorithm for fluid flow is tested on three fluid flow problems: 2D channel flow, the driven cavity problem, and a vortical flow problem based on the method of manufactured solutions. In all three cases, stable and accurate solutions free of pressure and velocity checkerboarding are obtained. Two different convective schemes, the first order upwind scheme and the central difference scheme, are tested for each of the three problems. The order of accuracy of the solution is established, and is found to be limited by that of the convective operator. Furthermore, the computational effort and CPU time for the computations are also found. The thesis establishes that a viable meshless finite difference method may be developed for incompressible flows. Future work includes the extension of the work to implicit fractional time-stepping schemes, alternative u-v-p coupling algorithms, application to complex geometries in porous media, particle beds and foams, and in fluid-structure interaction problems combining meshless methods for both fluid and structure
Modeling and Design Optimization of Ultra-Thin Vapor Chambers for High Heat Flux Applications
No full textPassive phase-change thermal spreaders, such as vapor chambers have been widely employed to spread the heat from small-scale high-flux heat sources to larger areas. In this paper, a numerical model for ultrathin vapor chambers has been developed, which is suitable for reliable prediction of the operation at high heat fluxes and small scales. The effects of boiling in the wick structure on the thermal performance are modeled, and the model predictions are compared with experiments on custom-fabricated vapor chamber devices. The working fluid for the vapor chamber is water and a condenser side temperature range of 293 K–333 K is considered. The model predictions agree reasonably well with experimental measurements and reveal the input parameters to which thermal resistance and vapor chamber capillary limit are most sensitive. The vapor space in the ultrathin devices offers significant thermal and flow resistances when the vapor core thickness is in the range of 0.2 mm–0.4 mm. The performance of a 1-mm-thick vapor chamber is optimized by studying the variation of thermal resistance and total flow pressure drop as functions of the wick and vapor core thicknesses. The wick thickness is varied from 0.05 to 0.25 mm. Based on the minimization of a performance cost function comprising the device thermal resistance and flow pressure drop, it is concluded that the thinnest wick structures (0.05 mm) are optimal for applications with heat fluxes below 50 W/cm2, while a moderate wick thickness of 0.1 mm performs best at higher heat flux inputs (\u3e50 W/cm2)
Electrical and thermal transport in nanotube based thin film transistors
No full textThin-film transistors (TFTs) based on networks of carbon nanotubes (CNTs) or silicon nanowires (NW) promise improved performance and novel applications in microelectronics and macroelectronics. Network transistors suggest the possibility of low voltage, highly reliable, high-speed (\u3eGHz) flexible plastic electronics with potential applications in displays, e-paper, e-clothing, biological and chemical sensing, conformal radar, and others. Despite many promising experimental demonstrations, a number of puzzling technical difficulties have stymied the systematic development (and eventual commercial adoption) of the technology. The statistical, electrical, thermal and mechanical properties of such a network transistor are not presently understood; this requires detailed theoretical and numerical analyses in order to interpret experimental observations. The goal of the present work is develop a computational electro-thermal model to analyze the transport properties of nanocomposites composed of isotropic 2D ensembles of nanotubes in a substrate for use as the channel region of TFTs. The visually complex nanotube-network transistor is studied by representing it as a simple, two-dimensional, interpenetrating percolating network of metallic and semiconducting nanosticks. A model based on percolation theory, drift-diffusion and Fourier-conduction equations is developed to predict electrical/thermal characteristics for different parameters such as channel length, tube length, network density, tube-tube contact-conductance, tube-substrate contact-conductance and substrate-tube conductivity ratio (kS/kt). The developed computational model is used to compute the conductance properties of a CNT network. The numerical results are in good agreement with analytical results for short channel transistors with low tube densities, and with experimental measurements for longer channels at higher densities, providing broad validation of the model. Thermal transport in finite nanocomposites provides insight into the dominant transport mechanisms. For low values of kS/kt, percolating conduction in the network is seen to dominate over a wide range of tube-tube and tube-substrate contact parameters; as kS/kt increases, thermal transport through the substrate begins to dominate. An effective medium theory based analytical model is presented to compute the effective thermal conductivity of 2D composite. Theoretical results depart significantly from numerical predictions for higher volume fractions because of tube-tube interaction. Finally, the contact resistance between tubes and tube-tube coupling is characterized through classical molecular dynamics (MD) and wavelet methods
Investigation of actuated droplet motion on smooth and superhydrophobic surfaces
No full textEfficient droplet transport is critical in microassays, microfluidic devices, and a range of heat transfer applications. The main advantages of miniaturizing assays and bioanalytical tools include improved performance and speed, reduced cost, and the ability to perform parallel and integrated analysis. A careful study of the electrically (and gravitationally) actuated droplet motion on hydrophobic surfaces is essential to understanding and improving performance in these applications. The first part of the thesis focuses on understanding the physics of droplet motion under gravitational actuation. Investigation of droplet actuation and motion under the action of electrical forces is conducted in the remaining part of the thesis. The physics of droplet motion on a smooth surface before rolling off and at terminal velocity are studied under gravitational actuation. An experimentally validated model based on the Volume of Fluid - Continuous Surface Force (VOF-CSF) framework with varying contact angles along the triple contact line is developed to predict droplet statics and dynamics on an incline. The model is successfully used to predict critical inclination angle and the terminal velocity of the droplet beyond the critical inclination angle. The effect of contact angle models on the terminal velocity prediction is investigated. The physics of droplet motion, including the internal fluid motion, is explained in detail. The effect of electrowetting is incorporated into the VOF-CSF framework for droplets on smooth surfaces. The droplet motion is shown to originate from the contact line. Contact line friction is shown to be the dominant damping force. An approximate mathematical model is successfully developed to predict the overall contact line motion of the droplet. The numerical model is extended to include a treatment of superhydrophobic surfaces through geometrical modeling of the microstructured surface with full fidelity. The model accurately predicts the droplet shapes, apparent contact angle and the voltage required to induce Cassie-Wenzel transition on two different surface morphologies. The transient features of the Cassie-Wenzel transition are explained through the analysis of the transient surface energy and contact line lengths. The effective contact line friction coefficient on surfaces is predicted using the approximated mathematical model developed for smooth surfaces
Phonon transport in confined structures and at interfaces
No full textThe objective of this thesis is to develop a fundamental understanding of the role of the interface in thermal transport in nanostructures. This task is accomplished using molecular dynamics (MD) simulations and the atomistic Green’s function (AGF) method as the primary tools. A molecular dynamics simulation tool for large-scale MD simulation is developed and validated through a variety of tests. First, the thermal conductivity of bulk silicon is computed using the Green-Kubo method and its dependence on temperature and system size explored. The dispersion relation along the [100] direction of silicon is predicted and compared to experimental data. Finally, non-equilibrium MD is applied to predict the out-of-plane thermal conductivity of silicon thin films as a function of film thickness. A wave-packet MD technique is developed to evaluate interface transmissivities. It is firstly applied to study thermal transport across interfaces with controlled roughness between silicon and an artificially heavy silicon system. The transmissivity is found to be strongly dependent on phonon frequency, polarization and surface asperity. In the low-frequency limit, the roughness structure at interfaces is transparent to acoustic phonons and the transmission coefficients nearly equal the ideal-interface results. In the mid-frequency range, phonon transmission is significantly decreased when the roughness characteristic length is comparable to the phonon wave length. Complex phonon mode conversions are observed and wave interference at this range is conjectured to be the reason for decreased transmission. At frequencies close to the cutoff frequency, the transmissivity drops rapidly to zero and the roughness influence is not evident. The method is also applied to the study of an Si/Ge rough interface. Strong phonon wave interference effects are found to restore the transmissivity as the number of roughness layers goes larger. The AGF method is employed and verified by application to a 1-D atomic chain. The AGF is then implemented to simulate phonon transport between two semi-infinite semiconductors to obtain the phonon transmission function with respect to interface atomic configuration, roughness layer thickness and phonon frequency. It is found that the in-plane structure of the roughness strongly modify the transmission while the roughness thickness has only a small effect to the extent that it modifies the device DOS. Finally, the development of a massively-parallel molecular dynamics program that has been used through the thesis work is presented. The parallelization scheme and message passing treatment are described in detail. Code performance is tested on a wide range of computer platforms and its efficiency and scalability on different architectures are discussed
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