Keynotes 2014
Bubble Deformation in Nucleate Boiling
July 14, 2014 08:40
Abstract: Nucleate boiling is one of the most efficient modes of heat transfer, yet many aspects of nucleate boiling are not well understood even today. Important aspects of nucleate flow boiling currently under research are the interactions of • an isotropic growing bubble with approaching flow • shape deformation and bubble detachment with flow and angle of inclination • neighboring bubble nucleation sites The onset of nucleate boiling and the net heat transfer coefficient are heavily influenced by these interactions. At the same time, the predictive capacity of mechanistic heat transfer and pressure drop models suffers from a lack of knowledge of these interactions. This paper intends to survey the present state of knowledge of these interactions, including some interesting recent findings.
There are two main force balances acting on the center of mass of a growing boiling bubble. One is parallel to the presumably plane wall which heats the flow. The other is normal to this wall. This normal component is often used for a criterion to predict bubble size at detachment. Bubble is then said to have departed from the wall if its gas-liquid interface is closed. The force components parallel to the wall may cause the bubble to "slide" over the wall before actually departing. Whether the contact line is actually moving in this case is questionable. However, next to these two force components other forces govern growth and detachment of a bubble in forced convection. They are related to bubble deformation and the most well-known and significant dynamic relationship involved is often named after Rayleigh and Plesset. The more complex the shape deformation, the more force balances must be taken into account. A coupled set of second order differential equations governs bubble growth dynamics in the general case. In each of these governing equations drag occurs. Drag is partly related to vorticity. Two areas or layers with significant vorticity built up can be distinguished in the liquid in nucleate boiling. The layer adjacent to the wall which exists in single phase flow and which is heated by the wall is one. The downstream wake of a bubble is the other. Quite often bubble growth rate is that fast and viscosity is that small that vorticity built-up in the wake is negligible and hardly contributing to the drag on the bubble. The main force components are then given by capillary action and by the overpressure inside the bubble. These two forces are enormous and nearly balancing one another. The difference is finely tuned by shape deformation and compensates gravity and -if the bubble volume is significant or bubble deformation is large- inertia forces. The process conditions in which the above relative force estimations are valid will be specified in the paper. Various independent ways to assess the force components from measured bubble growth histories will be discussed. Several validation examples will be shown, some for pool boiling, others for forced convection in micro- and in terrestrial gravity.
The bubble size at detachment depends, amongst others, on orientation of the heated plate with respect to gravity, on the velocity profile of the flow approaching the bubble and on the system pressure. Physical properties in the Jakob number cause this number to increase with decreasing system pressure, which decreases the overpressure inside the bubble and, hence, decreases the corresponding detaching force component as well. At the same time, a slight increase in the adhering capillary force is caused by the decrease in system pressure and the corresponding decrease in saturation temperature. Both effects increase the bubble size at detachment with decreasing system pressure, a trend often observed. The bigger the bubble, the more important inertia forces are and the more sensitive the growing boiling bubble is for acoustic triggering or for disturbances of a different nature. Force analysis of pool boiling bubbles at low system pressure reveal the importance of inertia forces of strongly deforming bubbles. The number of eigenmodes of a bubble attached to a plane wall exceeds that of a free bubble by one, because rectilinear motion of the center of mass loses symmetry by introduction of the wall. With a dedicated experiment, the relation between the frequency of this additional eigenmode and geometrical properties of an attached bubble has been measured. Theoretical analysis predicts the radian frequency found, as well as other frequencies such as the high one connected to isotropic deformation, f_RP, and shape deformation frequencies, f_n. Boiling bubbles have been triggered acoustically in the audible regime at various frequencies around bubble eigenmodes. Precisely at predicted frequencies of f_RP and f_n a decrease in bubble size at detachment and an increase in bubble release frequency have been found. Theoretical analysis shows that in principle another type of resonance might occur if 4f_n^2 is forced to become close to f_RP^2. However, sustained large amplitude oscillation in the near-resonance case always involves more than one fundamental mode. The nearby presence of a wall merely enhances this effect of cross-over of fundamental modes. The frequency f_n is found to decrease with increasing amplitude and with decreasing distance to a plane wall. Nucleation site interactions were extensively studied in pool boiling, which provided better insight in the phenomenon. Only recently these interactions have been studied in a flow boiling setup (Coen Baltis). Several bubble generators have been equally spaced and operated independently. High speed camera recordings at 5,000 fps have been synchronized with instantaneous voltage and power measurements of the bubble generators at 10 kHz data acquisition frequency. The added convective heat of an upstream generator is found to promote bubble nucleation at a downstream site up to the point when the upstream generator starts to create boiling bubbles of its own. From this moment on, the bubbles originating from the upstream generator have an inhibitive effect on bubble frequency of the downstream generator and can even cause deactivation of bubble nucleation. Another very interesting discovery was made during these experiments, which will be discussed in the paper.

- President of the Assembly of World Conferences on Experimental Heat Transfer, Fluid Mechanics and Thermodynamics (AWC). Chairman of the Nusselt-Reynolds prize committee in 2009-2013
- Editor of Experimental Thermal and Fluid Science. Frequent reviewer for high-ranked journals
- Member or co-chair of organizational committees of various conferences, e.g. in Poland, Jamaica, China, Italy. Chairman of the contact group Two-Phase Flow in the Netherlands in 1987-2003
- Member of various international scientific committees, such as the standard committee ISO/TC 185/WG 1 Working group Two-phase flow.
- Representative of Eindhoven University for the CAPES-funded BRANETEC program for exchange of students with Brazil.
- Coordinator of over 100 MSc thesis, co-advisor of 15 PhD students, member of 6 Ph.D. committees abroad
Cees was in the period 2001-2009 (co-) author of 27 refereed journal papers, in the period 2010-2012 of 17 refereed journal papers. Recent cooperation with the Amsterdam Medical Centre and Erasmus Medical Centre (Rotterdam) resulted in 14 articles, so far. In 2007-2013 was Cees involved in 12 subsidized projects, 9 times as principal investigator and mostly funded by the Dutch Technology Foundation STW. XXX
Multiphase Flow and Heat Transfer on Micro/Nanostructured Surfaces
July 14, 2014 09:20
Abstract: Multiphase flow and heat transfer in mini/microspaces are of significant interest for thermal management applications, where the latent heat of phase change offers an efficient method to dissipate large heat fluxes in a compact device, such as a heat spreader or a heat pipe. However, a significant challenge for the implementation of microscale phase change heat spreader is associated with flow instabilities due to insufficient bubble removal, leading to liquid dry-out which severely limits the heat removal efficiency. Specifically, in the micrometric scale, the relative importance of surface tension and viscous forces become predominant compared to inertia and buoyancy forces. Because the instability of micro/nanobubble formation and breakup is sensitive to the interfacial surface tension, and the temperature distribution, due to the wettability of the surface, the capillary forces are particularly sensitive to the geometry of the surface wettability patterns. Vapor bubbles generated on the micro/nanostructure wettability patterned surfaces may create instability of bubble dynamics that may have the potential to enhance heat transfer and critical heat flux from the surfaces. To address this challenge, several techniques have been proposed to enhance two-phase flow and heat transfer in mini/microspaces.
This work will conduct a review on the challenges and opportunities that surfaces with micro/nanostructure patterned wettabilities. Bubble dynamics, fluid flow and heat transfer caused by the micro/nanostructure patterned surfaces will be discussed. The effects of micro/nanostructure patterned surfaces on flow generation under a vapor bubble in a microchannel under very low Reynolds number will be demonstrated. The effects of wettability patterned surfaces on nucleation pool boiling and flow boiling heat transfer processes will be described. Bubble formation, breakup and departure will be visualized and measured. It is found that bubble dynamics and pool boiling performance are enhanced significantly on smooth and flat surfaces combining hydrophilic and hydrophobic patterns in comparison with a hydrophilic surface. Wettability patterned micro/nanostructure surfaces are manufactured on glass wafers and copper surfaces, respectively. Different surface hexagonal pattern size will be used. Indium Tin Oxide (ITO), Fluoroalkylsilanes (FAS) and Copper Oxide (CuO) will be used for glass and copper surfaces, respectively. It was found that a micro/nanostructure patterned surface can significantly improve the nucleation process which has great potential for enhancing heat transfer. Fluid flow and boiling heat transfer in microspaces with patterned surfaces will be discussed. An asymmetrical heat spreader with Micro/nanostructured evaporator will be demonstrated. Bubble dynamics and heat transfer in a mini/microspace under acoustic excitation will also be presented, which can further enhance heat transfer in mini/microspaces.

On Conservation of Scattered Energy and Angle in Radiative Transfer Computations
July 14, 2014 09:20
Abstract: To compute accurately radiative transfer in anisotropically-scattering media, conservation of both scattered energy and angle after discretization is required. Alteration of asymmetry factor (i.e., average cosine of scattering angles) due to angular discretization leads to a third type of numerical error, named as angular false scattering. The error that was known as "false scattering" is actually caused by spatial discretization and has nothing to do with scattering; and thus, it is more appropriate to be called "numerical smearing". Five phase-function normalization techniques, designed to attempt to conserve scattered energy, angle, or both, are analyzed here using DOM and FVM for both diffuse and ballistic radiation, to determine their capability to mitigate errors and produce accurate radiative transfer. Comparisons with Monte Carlo benchmark predictions are used to gauge accuracy. The two normalization techniques that conserve both scattered energy and asymmetry factor simultaneously are found to result in substantial improvements in radiative transfer accuracy with respect to MC predictions and comparison to each other. Normalization for ballistic radiation situations is shown to be crucial. Normalization impacts FVM and DOM in similar manners, as the accuracy of both is equal. In terms of computational efficiency, it is found that the DOM is more efficient than the FVM when both have the same number of angular directions.

Distributed Heat Conversion Technologies Based on Organic-Fluid Cycles for a High-Efficiency and Sustainable Energy Future
July 15, 2014 08:00
Abstract: The recent heightened involvement in the energy debate, in the scientific community, government and policy circles, and the public domain, has given rise to an intensified interest and rapid developments in a variety of fuel-to-power and heat-to-power conversion technologies. These technological developments have come from all of the areas of energy supply, conversion, storage and provision, spanning a range of scales and diverse applications. In particular, two important aspects of the energy challenge concern: (i) the improved utilisation of the vast amount of rejected energy to the environment, in particular in the form of waste heat from domestic and commercial settings and from a wide range of industrial processes, as well as (ii) the harnessing of renewable and sustainable energy sources, such as the solar resource, for the provision of heat and power, and also, depending on the application, cooling. The widespread deployment of any successful solution to this challenge must be associated with, beyond a high technical performance, either a cost benefit or at the very least a cost that is affordable and economically justifiable to the end-user.
A series of technologies are being proposed that aim to respond to the energy challenge. This paper presents and discusses the emergence of two distinct classes of energy conversion systems based on thermodynamic vapour-phase heat engine cycles undergone by organic working fluids, namely organic Rankine cycles (ORCs) and two-phase thermofluidic oscillators (TFOs). Each type of system has its own distinctive characteristics, advantages and limitations. ORCs are a more well-established and mature technology, are more efficient, especially with higher temperature heat sources and at larger scales, whereas TFOs have the potential to be more cost-competitive, in particular at lower temperatures and at smaller scales. Specifically, ORC systems are particularly well-suited to the conversion of low- to medium-grade heat (i.e. hot temperatures up to about 300 - 400 °C) to mechanical or electrical work, and at an output power scale from a few kW up to 10s of MW. Thermal efficiencies in excess of 25% are achievable at the higher temperatures, and efforts are currently in progress to develop improved ORC systems by focussing on advanced architectures, working fluid selection, heat exchangers and expansion machines. Correspondingly, TFO systems are a more recent development aimed at the affordable conversion of low-grade heat (i.e. hot temperatures from 20 - 30 °C above ambient, up to about 100 - 200 °C) to hydraulic work for fluid pumping and/or pressurisation. Ultimately, TFOs could emerge at scales of up to a few hundred W and with a thermal efficiency of the order of a few % points.
The two energy conversion systems are complementary, and together have a great potential to be used for distributed power generation and improved energy efficiency, leading to primary energy (i.e. fuel) use and emission minimisation. Relevant applications and fields of use include the recovery of waste heat and conversion to useful work including mechanical, hydraulic or electrical energy, or the effective utilisation of renewable energy sources such as geothermal, biomass/biogas and solar energy. In both cases, models capable of accurate and reliable predictions of system performance are used to provide insight on operational characteristics and performance. In particular, the minimisation of thermodynamic losses inflicted by finite heat transfer effects is considered. It is shown that these losses can arise from inherently unsteady, conjugate and nonlinear thermal processes between the working fluids within the systems of interest and the solid walls of key system components. This forms an interesting and important avenue for further work.

Thermal and Fluid Dynamic Processes Occurring At Liquid Boiling Over Micro-And-Nano Enhanced Surfaces
July 15, 2014 08:00
Abstract: Several strategies for surface modification have been addressed within the last decades to enhance heat transfer between liquids and solid surfaces, particularly involving phase transitions. The fast advance of micro-and-nano technologies provided the researchers with a wide range of solutions to modify the topology and even the chemistry of the surfaces (e.g. micro-patterning, nano-structuring, coating, nanowires and microporous). Alternative solutions comprise the modification of the liquid properties by the addition of surfactants or nanoparticles. The main goal is usually to increase the heat transfer coefficient and/or, in the particular case of pool boiling systems, increase the upper limit imposed by the Critical Heat Flux. However, many researchers focused mainly on the application of the surface modification technologies to achieve their goal, in a trial and error process, leaving the understanding of the physics governing the energy and mass transport phenomena to a secondary role. This is particularly evident when one tries to describe the effect of surface topology and wettability. Despite there is a known, although complex relation between these two factors, many studies do not account for such relation in the analysis of the results. Also, these effects are often mixed up and even misinterpreted. Hence, although the theory dealing with heat and mass transfer at liquid-solid interfaces, has been extensively explored, particularly in pool boiling systems, there are still many issues to solve related to the accurate description of the effect of surface properties. In this context, this talk will revise the effect of the surface properties on the fluid dynamics and heat transfer processes occurring at liquid-solid interfaces, with transition of the liquid phase. Emphasis is given to the heat transfer and bubble dynamics phenomena occurring in pool boiling, although other systems such as droplet/surface interfaces will be considered, particularly for the introduction of wettability concepts. To overcome the historical aforementioned empiricism related with the evaluation of the pool boiling heat transfer on modified surfaces, an alternative approach is suggested to the definition of a theoretical model. This model takes into account the different parcels of heat flux associated with the boiling process and their relation with bubble dynamics and interaction mechanisms. The independent data required for bubble nucleation and heat transfer characterization are collected coupling high speed camera visualization, PIV (Particle Image Velocimetry) and heat flux/surface temperature measurements. Some case study applications are then exploited, mainly related to electronics cooling, thus using dielectric fluids.

Fundamental Issues in Nanoscale Heat Transfer: From Phonon Coherence in Heat Conduction to Acoustic Based Near-Field Radiation
July 15, 2014 09:20

Jet in Crossflow: Experiments on the Interaction of Flowstructure and Cooling Efficiency
July 15, 2014 09:20

Study of Unsteady Combustion Processes Controlled by Detailed Chemical Kinetics
July 16, 2014 08:00
Abstract: Our understanding of the fundamentals of combustion processes to a large extent was heavily based on the use of a fairly simplified one-step Arrhenius kinetics model. However, the chemical mechanisms are an important factor significantly influencing the processes. The range of validity of simplified chemical schemes is necessarily very limited. Furthermore, it became clear that the use of a one-step Arrhenius model may lead to only a very basic picture describing qualitatively a few major properties of the combustion phenomena with some poor accuracy if any, often rendering misinterpretation of a variety of combustion phenomena. Moreover, many important features of combustion can not be explained without account of the reactions chain nature. An accurate description of unsteady, transient combustion processes controlled by chemical kinetics requires knowledge of the detailed reaction mechanisms for correctly reproducing combustion parameters in a wide range of pressures and temperatures. The availability of such models is essential for gaining scientific insight into the most fundamental combustion phenomena and it is an essential factor for design of efficient and reliable engines and for controlling emissions. In this lecture we consider the option of a reliable reduced chemical kinetic model for the proper understanding and interpretation of the unsteady combustion processes using hydrogen-oxygen combustion as a quintessential example of chain mechanisms in chemical kinetics. Specific topics cover several of the most fundamental combustion phenomena including: the regimes of combustion wave initiated by initial temperature non-uniformity; ignition of combustion regimes by the localized transient energy deposition; the spontaneous flame acceleration in tubes with no-slip walls; and the transition from slow combustion to detonation.

Role of Energy and Exergy Analyses in Performance Improvement and Development of Advanced and Sustainable Energy Systems
July 16, 2014 08:00
Abstract: The energy demand is growing and this has to be met with reduced pollutants and greenhouse gas emissions. Apart from natural gas and coal energy systems, alternative energy sources are receiving a great deal of attention in recent years for power generation. The presentation will discuss natural gas, coal, biomass, solar power generation and hybrid power generation systems. The role of thermodynamics (energy and exergy analyses) in the analysis and performance improvement of natural gas, coal and biomass advanced and integrated energy systems will be discussed. Biomass power systems, biomass co-firing hybrid power generation systems are also receiving attention to reduce greenhouse emissions. The biomass power generation systems and the recent developments in biomass hybrid power generation systems will also be presented. The presentation will also focus on integrated and multi-generation systems. The exergy analysis for power generation systems is also receiving attention due to the ability to analyze a power generation system on a component basis and also as a whole system from quality point of view. The exergy analysis for natural gas, coal and biomass power generation systems and its role on plant performance will be discussed.

Can We Obtain First Principle Results for Turbulence Statistics?
July 16, 2014 09:20
Abstract: Text-book knowledge proclaims that Lie symmetries such as Galilean transformation lie at the heart of fluid dynamics. These important properties also carry over to the statistical description of turbulence, i.e. to the Reynolds stress transport equations and its generalization, the multi-point correlation equations (MPCE). Interesting enough, the MPCE admit a much larger set of symmetries, in fact infinite dimensional, subsequently named statistical symmetries.
Most important, theses new symmetries have important consequences for our understanding of turbulent scaling laws. The symmetries form the essential foundation to construct exact solutions to the infinite set of MPCE, which in turn are identified as classical and new turbulent scaling laws. Examples on various classical and new shear flow scaling laws including higher order moments will be presented. Even new scaling have been forecasted from these symmetries and in turn validated by DNS.
Turbulence modellers have implicitly recognized at least one of the statistical symmetries as this is the basis for the usual log-law which has been employed for calibrating essentially all engineering turbulence models. An obvious conclusion is to generally make turbulence models consistent with the new statistical symmetries.

MAAT System Design - Weight Model of Very Large Lighter-Than-Air Vehicle
July 16, 2014 09:20
Abstract: The main objective of this paper is to provide a realistic weight model, based on the physical-mathematical foundations, for the design of the new very large lighter-than-air vehicle, called Multibody Advanced Airship for Transport (MAAT), the ongoing European FP7 project under intensive research and development activities. The Modeling and Simulation (M&S) principles, aided with simulations and visualization tools, have been extensively used, as the key enablers to combine, manage and structure such highly complex engineering process, which emerged as a natural integration mechanism and evidence provider of the encountered complexity, successfully encompassing the MAAT multidisciplinary design requirements. The authors experience, in solving the M&S problems, gained within the European R&D projects, was efficiently reused, where the use of such software technologies have been successfully demonstrated, and today, further applied for the new generation transportations solutions, as envisaged by MAAT, especially addressing the best practices in taking advantage of the variety of multi-physics software and their related analysis tools. The MAAT system is envisaged to be composed of two airships: the Cruiser, which stays at a constant altitude of 16 km, travelling horizontally; and the Feeder, which acts like an elevator system connecting the Cruiser to the ground. In this paper, the proposed weight model is similar to the typical one applied in the aircraft design process. The main difference is primarily the airship teardrop shape, which is commonly applied for the currently produced airships. The main challenge is that MAAT has a very large shape, which has required the introduction of new elements and references, as being presented in this work. The achieved results show that MAAT can be realized, by taking into account the significant weight estimated for such aircrafts, to be for the Cruiser about 533 tons, while the Feeder weight is about 12 tons. As highlighted before, the MAAT design is still under intensive developments, and thus, it is expected that in the coming years, by taking into account the new emerging technological solutions, the lightening of such aircrafts structure is inevitable. In addition, the authors plans are to further investigate new materials and their related applications, in order to improve the structural part of the MAAT system, as one of the essential parts in such new transportation system, expected to become the reality in the forthcoming future.
