Heat-transfer enhancement by adaptive reorientation of flow fields

Scope is enhancement of scalar transport (heat, chemical species) in engineered flow systems by reorientations of a laminar base flow. Practical applications include mixing in inline heat exchangers by downstream reorientation of baffles, stirring in bio-reactors by cyclic repositioning of impellers, and subsurface chemicals distribution for in situ minerals mining by unsteady pumping schemes. Conventional reorientation schemes consist of a periodic reorientation (in space or time) of the flow designed to accomplish efficient fluid mixing. However, whether this approach indeed yields optimal scalar transport for significant diffusion and/or chemical reactions is unclear. The present study explores an alternative approach: adaptive reorientation of the flow by interval-wise selection of the reorientation that is predicted to yield optimal scalar transport for a future time horizon. Key enabler for fast predictions is a compact model based on the spectral decomposition of the scalar evolution in the base flow. The adaptive reorientation scheme is investigated for a representative problem: enhanced heating of a cold fluid in a 2D circular domain by an unsteady flow driven by step-wise activation of moving boundary segments. This reveals that the adaptive reorientation scheme can substantially accelerate the heating compared to conventional time-periodic reorientation designed for efficient mixing and thus demonstrates its potential for attaining optimal scalar transport in reoriented flows

Rapid thermalization by adaptive flow reorientation

Aim of this paper is the enhancement of scalar transport (heat, chemical species) in flow systems with reorientations of a laminar base flow. Conventional heating/mixing protocols comprise of temporal or spatial periodic reorientations of these base flows to promote fluid mixing. However, thermal homogenisation rates of scalar fields are not necessarily accelerated with these approaches due to the substantial effect of diffusion and/or chemical reactions on heat/chemical transport. In the present study we numerically study heat transport with an adaptive approach for an entire parameter space of fluid and flow properties. Key to the approach is real-time control of the fluid flow based on the scalar field due to an efficient numerical model. Results show that the adaptive approach can significantly enhance heat transport over the conventional periodic heating/mixing approach designed for efficient mixing

Lyapunov-based temperature regulation by flow reorientation

Transport of scalars, in the form of heat or chemicals, by fluid flow is a key feature for the effective operation of applications that range from chemicalspecies mixing to subsurface resource extraction. Therefore, enhancing transport of these scalars by improving their dispersion will prove beneficial toa large variety of industries. Systems that involve scalar transfer from the boundary and have a substantial influence of diffusion and/or chemical reactionson heat/chemical transport are of particular interest. The system considered in this work intends to rapidly homogenize a scalar field by reorienting a laminar base flow. In conventional heating/mixing approaches a periodic reorientationscheme is designed towards effective fluid mixing and thus lacks robustness to perturbations required for widespread application. In this work we present two novel methods that accomplishes transport acceleration by adjusting the fluid flow reorientation. Rationale behind these methods is that influencingtransport rates by fluid flow is analogous to influencing the decay rate of a Lyapunov function. This reasoning leads to the design of a bang-bang regulatorand a general nonlinear regulator. We numerically investigate the performance of these regulators on a representative thermal flow problem: boundary heating of an initially cold fluid by reorientation of a 2D flow fields. Results show that the proposed regulators improve heating rates by up to 80 % compared to mere diffusive heating

Fast fluid heating by adaptive flow reorientation

Transport of scalar quantities such as e.g. heat or chemical species in laminar flows is key to many industrial activities and stirring of the fluid by flow reorientation is a common way to enhance this process. However, "How best to stir?" remains a major challenge. The present study aims to contribute to existing solutions by the development of a dedicated flow-control strategy for the fast heating of a cold fluid via a hot boundary in a representative case study. In-depth analysis of the dynamics of heating in fluid flows serves as foundation for the control strategy and exposes fluid deformation as the "thermal actuator" via which the flow affects the heat transfer. This link is non-trivial, though, in that fluid deformation may both enhance and diminish local heat exchange between fluid parcels and a fundamental "conflict" between local heat transfer and thermal homogenisation tends to restrict the beneficial impact of flow to short-lived episodes. Moreover, the impact of fluid deformation on the global fluid heating is primarily confined to the direct proximity of the moving boundary that drives the flow. These insights imply that incorporation of the thermal behaviour is essential for effective flow-based enhancement strategies and efficient fluid mixing, the conventional approach adopted in industry for this purpose, is potentially sub-optimal. The notion that global heating encompasses two concurrent processes, i.e. increasing energy content ("energising") and thermal homogenisation, yields the relevant metrics for the global dynamics and thus enables formulation of the control problem as the minimisation of a dedicated cost function. This facilitates step-wise determination of the "best" flow reorientation from predicted future evolutions of actual intermediate states and thereby paves the way to (real-time) regulation of scalar transport by flow control in practical applications. Performance analyses reveal that this "adaptive flow reorientation" significantly accelerates the fluid heating throughout the considered parameter space and thus is superior over conventional periodic schemes (designed for efficient fluid mixing) both in terms of consistency and effectiveness. The controller in fact breaks with conventions by, first, never selecting these periodic schemes and, second, achieving the same superior performance for all flow conditions irrespective of whether said mixing occurs. The controller typically achieves this superiority by thermal plumes that extend from the hot wall into the cold(er) interior and are driven by two alternating and counter-rotating circulations

Design and numerical analysis of an electrostatic energy harvester with impact for frequency up-conversion

Integration of vibration energy harvesters (VEHs) with small-scale electronic devices may form an attractive alternative for relatively large batteries and can, potentially, increase their lifespan. However, the inherent mismatch between a harvester’s high-frequency resonance, typically in the range 100 - 1000 Hz, relative to the available low-frequency ambient vibrations, typically in the range 10–100 Hz, means that low-frequency power generation in microscale VEHs remains a persistent challenge. In this work, we model a novel electret-based, electrostatic energy harvester (EEH) design. In this design, we combine an out-of-plane gap-closing comb (OPGC) configuration for the low-frequency oscillator with an in-plane overlap comb configuration for the high-frequency oscillator and employ impact for frequency up-conversion. An important design feature is the tunability of the resonance frequency through the electrostatic nonlinearity of the low-frequency oscillator. Impulsive normal forces due to impact are included in numerical simulation of the EEH through Moreau’s time-stepping scheme which has, to the best of our knowledge, not been used before in VEH design and analysis. The original scheme is extended with time-step adjustments around impact events to reduce computational time. Using frequency sweeps, we numerically investigate power generation under harmonic, ambient vibrations. Results show improved low-frequency power generation in this EEH compared to a reference EEH. The EEH design shows peak power generation improvement of up to a relative factor 3.2 at low frequencies due to the occurrence of superharmonic resonances