Results are reported from a three-dimensional computational stability analysis of flow over a backward-facing step with an expansion ratio (outlet to inlet height) of 2 at Reynolds numbers between 450 and 1050. The analysis shows that the first absolute linear instability of the steady two-dimensional flow is a steady three-dimensional bifurcation at a critical Reynolds number of 748. The critical eigenmode is localized to the primary separation bubble and has a flat roll structure with a spanwise wavelength of 6.9 step heights. The system is further shown to be absolutely stable to two-dimensional perturbations up to a Reynolds number of 1500. Stability spectra and visualizations of the global modes of the system are presented for representative Reynolds numbers.

Three-dimensional instability in flow over a backward-facing step

Air-Side Heat Transfer Enhancement Utilizing Design Optimization and an Additive Manufacturing Technique

In this work, we discuss the fundamental aspects of Electrohydrodynamic (EHD) conduction pumping of dielectric liquids. We build a mathematical model of conduction pumping that can be applied to all sizes, down to microsized pumps. In order to do this, we discuss the relevance of the Electrical Double Layer (EDL) that appears naturally on nonmetallic substrates. In the process, we identify a new dimensionless parameter related to the value of the zeta potential of the substrate-liquid pair, which quantifies the influence of these EDLs on the performance of the pump. This parameter also describes the transition from EHD conduction pumping to electro-osmosis. We also discuss in detail the two limiting working regimes in EHD conduction pumping: ohmic and saturation. We introduce a new dimensionless parameter, accounting for the electric field enhanced dissociation that, along with the conduction number, allows us to identify in which regime the pump operates.

/pdf/in-depth-description-of-electrohydrodynamic-conduction-f7onnoj5c6.pdf

In-depth description of electrohydrodynamic conduction pumping of dielectric liquids: Physical model and regime analysis

Amorphous graphene – Transformer oil nanofluids with superior thermal and insulating properties

Enhancement of convection heat transfer using EHD conduction method

In the fabrication of paper, a slurry with cellulose fibers and other matter is drained, pressed, and dried. The latter step requires considerable energy consumption. In the structure of wet paper, there are two differ-ent types of water: free water and bound water. Free water can be removed most effectively. However, removing bound water consumes a large portion of energy during the process. 
The focus of this paper is on the intermediate stage of the drying process, from free water toward bound water where the remaining free water is present on the surfaces of the fibers in the form of a liquid film. For simplicity, the drying process considered in this study corresponds to pure convective drying through the paper sheet. The physics of removing a thin liquid film trapped between fibers in the paper drying process is explored. The film is assumed to be incompressible, viscous, and subject to evaporation, thermocapillarity, and surface tension. By using a volume of fluid (VOF) model, the effect of the previously mentioned parameters on drying behavior of the thin film is investigated.

Fundamental understanding of removal of liquid thin film trapped between fibers in the paper drying process: A microscopic approach

Nowadays, heat transfer enhancement devices are becoming very essential in many applications. Most of electronic devices, from the simplest to the most sophisticated, contain processors. Advanced processors require efficient cooling for an optimal performance. Due to the growing interest in faster and lighter devices, researchers always seek to innovate and optimize cooling strategies. In spatial applications, the use of typical cooling systems can cause complications due to the vibrations during takeoff and due to zero gravity effects. Electrohydrodynamic (EHD) pumping, which is based on the interaction of a dielectric liquid with an electric field, could present a solution to all these challenges. EHD pumps were considered a breakthrough in the field of cooling since they demonstrate many advantages over other types of pumps. EHD devices are less power consuming, lighter and cheaper. They don’t have moving parts and they are suitable for microgravity applications. The hydrofluoroethers (HFE) are dielectric liquids that could be ideally utilized in these pumps for many applications. Being dielectric fluids with eco-friendly properties, HFEs could replace CFCs, HFCs, HCFCs, and PFCs. Due to their promising EHD applications, studies must be done to investigate the variation of their dielectric behavior with electric field and with temperature. This work presents an experimental investigation of the dielectric characteristics of HFE-7000 with temperature variations. Understanding this aspect can help enhance and optimize the performance of EHD systems.

Study of the Dielectric Behavior of HFE-7000 in function of Electric Field and Temperature Variations

Electrohydrodynamics (EHD) pumps enable to manipulate fluids by means of electrical forces. Such electrical forces are driven by the ions transport that arises as a natural response of the fluid subjected to an electric field. EHD pumps, therefore, make possible to act on the fluid at practically our discretion via an appropriate electrodes arrangement. Because of this ability to alter the flow, pumps arouse a wide variety of potential applications from biotechnology to microelectronics, to name but a few. However, although the EHD pump effect is well-known since 60’s, most new generations of applications coming nowadays require a more in-depth insight. Particularly, most of those new applications such as bioreactors, microcooling systems, etc., require being adjustable to the working environment. Flexible pumps are devised to cover this requirement in terms of geometrical versatility, although elucidating the optimal setting in such conditions involve investigating and exploring a wider variety of scenarios. This work is aimed at tackling how the curvature on the substrate modifies the net pumping effect induced by the EHD force. Our results suggest that for a moderate curvature the EHD pumping effect remains unaltered. Nonetheless, more complex curvature and setups, including the possibility of a moving substrate, will need to be considered in future studies.

The Effect of Substrate Curvature on Flexible EHD Conduction Pumping Performance: A Numerical Study

We are pleased to announce that the Center for Advanced Research in Drying (CARD) was officially approved as a U.S. National Science Foundation (NSF) Industry/University Cooperative Research Center...

Center for Advanced Research in Drying (CARD) approved as a U.S. National Science Foundation (NSF) Industry/University Cooperative Research Center (I/UCRC)

A novel approach for paper drying using airborne ultrasound technology is presented. A unique experimental setup is developed, and a systematic study is conducted using 23 factorial design of experiments and Analysis of Variance. Three controlling factors are considered in the experiments including the initial moisture content, basis weight and refining condition. The outcome of the experiments is compared to a previous work on direct-contact ultrasonic drying of paper. The results confirm that similar to direct-contact, for airborne ultrasonic drying, the basis weight/thickness of the sample is the most important factor in ultrasonic drying and it is followed by the effect of initial moisture content. Using linear regression model, a correlation for predicting the total time of ultrasonic drying is provided. Quality of the dried samples is evaluated, and the permeability measurements confirmed the effect of pore characteristics on ultrasonic drying. The analysis for energy consumption reveals that airborne ultrasonic drying is more efficient at higher moisture contents. 

Airborne power ultrasound for paper drying: an experimental study

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