Keynote Lectures

This talk is intended to provide a perspective and review of a journey toward a flexible thermal superconductor, which can be bent or twisted, to control heat transfer in heat generating devices of various shapes. The thermal superconductor exploits recent advances in micro pulsating heat pipes, which consists of liquid-vapor slug-train units oscillating within a microchannel. It has the following characteristics: an effective thermal conductivity of 1,000 W/mK, 2.5 times higher than copper, and 5,000 times higher than current flexible materials, and a thickness of 0.6 mm. Compared to conventional pulsating heat pipes, micro pulsating heat pipes with hydraulic diameters of less than 1 mm have interesting features like orientation independent performance. In addition, several new ideas for thermal performance enhancement in the micro pulsating heat pipes will be presented. This talk will conclude with an overview of ongoing research activities associated with a prestigious 9 year grant by Korea’s Creative Research Initiative to develop Flexible and Thin Thermal Superconductors.

In this presentation, the impact of sustainable energy research and the development of new technologies on the lives of people are discussed. The research carried out at CEEE during the last eight years as related to buildings, industry, and cities are outlined. The presentation emphasizes the importance of human comfort and its control in operation of buildings, and how they can be part of the design principles starting from the integrated engineering and architecture concepts. To achieve thermal and visual comfort and energy efficiency in buildings, sensor and automation networks can be employed. Yet, these can be most effective if the building-user interactions are streamlined, which may require the modeling of human behavior based on a transdisciplinary approach. This approach allows buildings to be responsive to human needs and to be designed considering the details of light, air flow and temperature distributions, and with the help of interactive ventilations and solar shading systems. All these ideas need to be coupled not only for buildings, but also for regions and cities of the future. The presentation will also address the development of sustainable and inexpensive materials to be used at larger scales for buildings and industrial systems.

Batteries with increasing energy and power densities are developed rapidly to enable massive electrification of the transport sector. Many vehicle manufacturers are gradually releasing private cars, trucks and buses with an electrified powertrain. Currently batteries based on the Li-ion concept are dominating. For batteries there exist several thermal issues to handle and these present multi-physics and multiscale problems. For instance, the cells have to be maintained at a desired temperature level (Too hot: decreased battery life, decreased performance, risk of fire explosion. Too cold: sluggish chemistry, lithium plating and dendrite formation). The cell-to-cell temperature variations should be minimized and a small amount of energy should be consumed for the operation. Accordingly, it has been necessary to develop various thermal management principles. In the thermal analysis, many processes need consideration, i.e., heat and mass transport including heat generation, charge transport, electrode kinetics as well as electrode-electrolyte interfacial processes. Thus detailed modeling approaches require solution of the so-called Butler-Volmer equation for the electron current, conservation of charge (ions and electrons), conservation of species (ions) and energy conservation.

This lecture aims to present an overview of thermal issues in batteries, methods of analysis and some solutions of thermal management.

The microfluidic systems developed for focusing and sorting cells and biological objects are based on hydrodynamic interactions responsible for the cross-flow migration of small objects conveyed by a low Reynolds number flow of liquids. Recent studies bring us plethora of examples how nature cope with creeping flow limitations. For example it appeared that bacteria can swim at extremely low Reynolds numbers (Re~10-5) by rotating one or more helical filaments called flagella. At slightly higher flow rates, but still in the regime of the laminar flow, local velocity gradients generated by the interface corrugation induce cross-flow temporal and spatial fluctuations. Their effect efficiently increases mass and heat fluxes desired in several practical applications of microfluidic devices. Several numerical and experimental studies have been undertaken to elucidate existence of irreversible hydrodynamics, responsible for inertial diffusion regime and redistribution of suspended particles, droplets and filaments across the channel flow. It becomes obvious that presence of residual inertial forces and breakup of the flow symmetry in case of suspended deformable objects or complex channel configurations leads to irreversible hydrodynamics, imposing limited use of so called Stokesian approximation in the theoretical models. Careful look at the flow patterns indicated serious discrepancies with numerical models based on Stokesian hydrodynamics. Tumbling, coiling, and cross-flow migration of long filaments, resembling DNA behavior, gave us motivation to look closer at conditions literally forbidding application of simple, linear modelling of hydrodynamic problems. In this talk we will try to extract formulations defining sources of nonlinearities based on the local flow parameters, like spatial and temporal gradients of velocity, replacing commonly used in hydrodynamics global Reynolds number.

The Laplace transformation is commonly used to get analytical solutions for transient conduction problems where the heat equation and its associated conditions are Linear with Time Invariant coefficients (LTI) and where the geometry is simple [1]. In that case, the temperature solution at a given point in the material system is a convolution product between the intensity of the heat or temperature source and a corresponding impulse response, which is the inverse Laplace transform of a transfer function whose expression is analytical in the Laplace domain. Inversion of such an analytical transfer function can now be achieved using numerical inversion algorithms. Once the impulse function retrieved, the temperature solution of the heat transfer problem can be found by implementation of the convolution product for any time shape of the source. It short-circuits the need for finding more than once the solution of the Partial Differential Equation (PDE) problem, that is the “detailed model”: the convolution product is its corresponding “reduced model” and its structure is not biased.

The Laplace transformation can also be applied to LTI systems where heat transfer occurs in a domain characterized by any non simple 3D geometry. It is the case for a heterogeneous physical system (including solids and flowing fluids, even with linearized radiation in a cavity configuration) if the coefficients of the heat equation system do not vary with time, without being necessarily uniform. In particular, the velocity field can be 3D but in a steady state regime. This also requires each single transient source (the input) to be separable, which means it can be written as a product of a time part, its intensity, by a space part, its geometrical support [2]. The temperature response at any point (output) is still a convolution product in time between its cause (source or input), and the impulse response.

In practice, the impulse response has to be found through solving an inverse problem, here a deconvolution. One can either use a numerical temperature solution of a Finite Element simulation code for a given source (model reduction), or the experimental noisy temperature signal delivered by a local sensor for a measured source in a calibration experiment (model identification, which requires some kind of regularization in the inversion). Examples of experimentally identified impulse responses, for characterizing heat exchangers [3, 4] are given in this presentation: impedances, between a temperature and a thermal power, or transmittances, linking temperatures at two different points.

The use of heat in medicine dates back from remote centuries. Recently, for example, nanoparticles have been concentrated in solid tumors to locally increase the absorption of electromagnetic waves imposed by external energy sources. Thus, thermal damage is mostly caused to the tumorous cells, with minimum heating of surrounding healthy cells. Similarly, bioheat transfer problems have been advanced for diagnosis and treatment of other diseases. Promising techniques for the nonintrusive measurement of the temperature of internal tissues include magnetic resonance and photo-acoustics. Uncertainties in these techniques can be reduced by solving inverse problems related to the temperature measurements. In addition, for the practical application of bioheat transfer problems, uncertainties must be considered not only in available measurements, but also in mathematical models used for computational simulations. The coupling of measurements and mathematical models for the better understanding of physical phenomena falls within the inverse problem paradigm. This presentation will summarize the application of inverse problems in bioheat transfer, by our group and close collaborators, aimed at the hyperthermia treatment of cancer, temperature measurements of internal tissues, diagnosis of inflammatory bowel diseases (IBD) and model selection for the thermal damage of tissues. The synthesis and characterization of nanostructured Pd and PdCeO2 hydrides, for possible hyperthermia treatment application, will also be presented.