Passive Enhancement Research Articles

Passive Control and Enhancement of Low Reynolds Number Slot Jets Through the Use of Tabs and Chevrons

Liquid microjets are emerging as candidate primary or secondary heat exchangers for the thermal management of next generation photonic integrated circuits (PICs). However, the thermal and hydrodynamic behavior of confined, low Reynolds number liquid slot jets is not yet comprehensively understood. This investigation experimentally examined jet outlet modifications—in the form of tabs and chevrons—as techniques for passive control and enhancement of single-phase convective heat transfer. The investigation was carried out for slot jets in the laminar flow regime, with a Reynolds number range, based on the slot jet hydraulic diameter, of 100–500. A slot jet with an aspect ratio of 4 and a fixed confinement height to hydraulic diameter ratio (H/Dh) of 1 was considered. The local surface heat transfer and velocity field characteristics were measured using infrared (IR) thermography and particle image velocimetry (PIV) techniques. It was found that increases in area-averaged Nusselt number of up to 29% compared to the baseline case could be achieved without incurring additional hydrodynamic losses. It was also determined that the location and magnitude of Nusselt number and velocity peaks within the slot jet stagnation region could be passively controlled and enhanced through the application of outlet tabs of varying geometries and locations.

Experimental correlations on critical Reynolds numbers and friction factor in tubes with wire-coil inserts in laminar, transitional and low turbulent flow regimes

This paper analyses 23 circular helicoidal wire-coils with different geometric characteristics ranging from: dimensionless pitch p/d=[0.25–3.37], dimensionless thickness e/d=[0.071–0.286] and a Reynolds number interval from 50 to 8000. This interval widely includes the Reynolds number range in which rigid wire-coil inserts present better performance as passive enhancement technique for tubular heat exchanger applications Re=[200–2000]. Based on their hydraulic performance, the wire-coil inserts are categorized according to a new dimensionless parameter: the Transition Shape Parameter (TSP). A new set of correlations are obtained to predict the Fanning friction factor coefficient as a function of Reynolds number and geometrical characteristics of the insert within the three flow regimes: laminar, transitional and low turbulent. Additional correlations are proposed to estimate the critical Reynolds number at the beginning and ending of the transition region, which allows to select the most adequate friction factor correlation as a function of the operational Reynolds number for a heat exchanger design application. Finally, a comparative between the proposed and the published correlations in the open literature for laminar and turbulent regimes is presented. This brings to light the need and interest of having the suitable and reliable set of correlations presented in this paper to compute the friction coefficient covering all the wire-coil applicability range as an enhancement technique.

Development of passive film and enhancement of corrosion protection of mild steel in hydrochloric acid through the synergistic interaction of 2-amino-4-methyl benzothiazole (AMBT) and (E)-2-methylbenzo[d]thiazol-2-yl) imino-4-methyl) phenol (MBTP)

2-amino-4-methylbenzothiazole (AMBT) and one of its derivatives (E)-2-methylbenzo[d]thiazol-2-yl)imino-4-methyl) phenol (MBTP) were used as synergistic pair for the protection of mild steel in 0.5M HCl. The combination of inhibitors, AMBT-MBTP, adsorbs on electrode surface through chemical and physical means. The antagonistic interaction was more pronounced on higher temperature, and an inhibition efficiency of 92% was recorded at 313K and 93% at 303K. The formation of protective film of AMBT-MBTP combination is further evidenced by SEM and AFM images. The process of adsorption followed Langmuir isotherm model and corresponding thermodynamic activation parameters were also calculated and discussed. Corrosion inhibition efficiency and the reaction kinetics of synergistic pair were evaluated by electrochemical impedance spectroscopy (EIS), potentiodynamic polarization and weight loss measurements. H-bonding interaction and resulting enthalpy change and reactivity descriptors were computed for correlating the corrosion inhibition efficiency and molecular structure by B3LYP/6-31G∗ level of Density Functional Theory (DFT).

HEAT TRANSFER ENHANCEMENT USING CIRCUMFERENTIAL FINNED TWISTED TAPE HEAT EXCHANGER

The most desirable feature in any thermal equipment is the enhancement of heat transfer. Heat transfer is basically a slow process and is enhanced by adopting passive or active methods of enhancement. In passive enhancement methods, heat transfer is increased without demanding any external power source; while in active method, enhancement in heat transfer demand external power. In this work, a passive enhancement method is proposed and tested to check the extent of heat transfer enhancement noticed. A tube in shell heat exchanger is designed with circumferential fins attached along the length of tube and a spiral insert running inside the tube. One fluid is made to flow inside the tube under the influence of twisted tape and the shell side fluid is running around the tube continuously provoked by fins. Therefore, the hot and cold fluids were estimated to exchange more heat because of thorough mixing initiated in their flow paths. In this work, analysis was made in CFD package by creating a model that simulates experimentations observed in the literature. The results of experiments and results of CFD analysis were compared. Noticing the agreement between the results, the CFD model is given enhancements like circumferential fins and twisted tape to check the enhancement in heat transfer. The velocity and temperature contours were observed at various flow conditions (Reynolds numbers). Based on results of analysis, thermal performance factor is also estimated to check the increment in heat transfer with reference to hydraulic (or flow) parameters.

The effect of broadened linewidth induced by dispersion on the performance of resonant optical gyroscope

Anomalous dispersion enhancement physical mechanism for Sagnac effect is described by special relativity derivation, and three kinds of definitions of minimum detectable angular rate of resonance optical gyroscope (ROG) are compared and the relations among them are investigated. The effect of linewidth broadening induced by anomalous dispersion on the sensitivity of ROG is discussed in this paper. Material dispersion-broadened resonance linewidth deteriorates the performance of a passive ROG and dispersion enhancement effect, while the sensitivity of a structural dispersion ROG is enhanced by two orders of magnitude even considering the dispersion-broadened resonance linewidth.

Orientation effect in helical coils with smooth and rib-roughened wall: Toward improved gas heaters for supercritical carbon dioxide Rankine cycles

Helical coils have gained increasingly interest in the field of supercritical carbon dioxide (SCCO2) Rankine cycles during the last decade due to the compact structure and high heat transfer rate. Past studies mainly focused on effects of operating conditions and the gravitational buoyancy, and are not sufficient to fully understand the behavior of SCCO2 helically coiled gas heaters. Influence of some other key factors, such as the coil orientations and internal wall roughness, on overall performance has been seldom reported and is still unclear to date. In this work we filled this gap with a solid-to-fluid conjugate heat transfer model where supercritical flow turbulence is solved by the Shear-Stress Transport k-ω equations. The orientation effect on performance of helical coils was revealed with the coil axis arranged in horizontal, vertically upward and downward directions, respectively. Results demonstrate that the criterion of Bo < 10−5 for negligible buoyancy effect was applicable to coils with dimensionless curvature δ = 0.01–0.1. The Orientation effect only emerged when the buoyancy effect became significant with Bo > 10−5. Deterioration and enhancement of heat transfer occurred alternatively in horizontal coils. The overall performance of horizontal coils was much worse than vertical coils, among which the downward orientation was slightly better than the upward one. The increase of δ barely improved the performance of horizontal coils. Instead, at larger δ the intensive fluctuation in heat transfer coefficients was more frequent due to the more drastic change of the flow direction. The location and extent of local impairment also varied with the inlet arrangement. Finally, the passive compound enhancement technique of helical coils in conjunction with internal helical-rib roughness was found to be very effective in mixed convection of SCCO2, especially in the horizontal coils.

Numerical study on convective mass transfer enhancement by lateral sweep vortex generators

Passive mass transfer enhancement from the surface of a liquefying substance using Lateral Sweep Vortex Generators (LSVG) placed in high-speed flow is presented. Numerical simulation of three-dimensional turbulent compressible flowfield involving species transport has been carried out using Advection Upstream Splitting Method (AUSM) scheme based Finite Volume solver. A temperature dependent mass efflux boundary condition has been developed and implemented in the computational procedure for updating the boundary mass flux. Computational procedure has been validated using the experimental wall pressure profile reported for a similar vortex generator placed in high speed flow and could capture the complex flow features resulted by the vortex-boundary layer interactions. Convective mass transfer of species is found to get improved in wake region of the LSVG by the horseshoe vortex and further get transported downstream by the counter-rotating vortex pair. Extensive numerical simulations have been carried out with several lateral sweep angles of LSVG to explore the role of vortices in promotion of the convective mass transfer. It has been observed that the mass efflux enhances with increase in sweep angle of the vortex generator due to the vorticity augmentation. Analysis of the vortex trajectories could figure out the influence of vortex interactions leading to the peak mass efflux. Further, an exponential-power law based correlation between relative Sherwood number and relative streamwise vorticity has been developed.

Blind image forensics using reciprocal singular value curve based local statistical features

In this article, passive contrast enhancement detection technique is presented using block based reciprocal singular value curve features. Contrast enhancement operation changes the natural statistics of the image and variation in singular value curve is exploited for constructing the feature vector for forgery detection. Various statistical features using reciprocal singular value curve are extracted after multilevel 2-Dimensional wavelet decomposition. Fisher criterion is employed to choose the most discriminating and to discard the redundant features. Experimental results are presented using gray scale, G component and C b image database and support vector machine classifier. Robustness against anti-forensic algorithm and JPEG compression is also presented. The algorithm outperforms all the existing feature based blind contrast enhancement detection methods in terms of detection accuracy.

Passivity Enhancement in Renewable Energy Source Based Power Plant With Paralleled Grid-Connected VSIs

Harmonic instability is threatening the operation of renewable energy based power plants where multiple grid connected voltage source inverters (VSIs) are connected in parallel. To analyze and improve the stability of the grid-connected VSIs, the real part of the output admittance of the VSIs is first investigated in this paper. It has been shown that the negative real part of the output admittance will make the system unstable under certain grid condition. Based on the stability analysis, this paper proposes a method to cancel the negative real parts by using VSIs with different LCL filters, sampling frequencies, and control strategies. The proposed method does not require the VSIs to have any active damping function. Three specific cases are studied where the LCL parameters, sampling frequencies, and current control strategies of the VSIs are different. The results show that the combination of different types of grid-connected VSIs can improve the stability of the renewable power plant.

Computational Investigation of the Thermal-Hydraulic Performance for Twisted Tape Enabled High Heat Flux Components

High heat flux thermal management is an important challenge for upcoming nuclear fusion and plasma physics experiments. The plasma facing components (PFCs) in devices such as ITER or Wendelstein 7-X (W7-X) will be subjected to extreme heat loads on the order of 10–20 MW/m2 in the divertor region. The heat dissipation issue will become critical in this next generation of experiments, and active cooling will be necessary. The current state-of-the-art water cooled technologies can accommodate extreme heat fluxes and often utilize passive heat transfer enhancement techniques, such as swirl flow, to decrease the thermal loading on PFCs. Swirling flow is commonly induced with a twisted tape that is inserted into a circular tube. Twisted tape devices are planned for use in both W7-X and ITER. Computational modeling was performed to investigate the thermal-hydraulic performance for single-phase, turbulent flow of water through a twisted tape device. This study exploited the advantage of computational simulations by analyzing local flow information. It was shown that points of low wall shear stress corresponded to locations of low heat transfer coefficient and high surface temperatures. Thus, decreased wall shear stress could be an indicator for early burnout in twisted tape geometries. This analysis was the first step towards informing the design of twisted tape devices utilized in PFCs.

Study on the Law of Ultrasonic Vibration-Assisted Adhesive Bonding of CFRP-to-Aluminum Joints

A novel ultrasonic vibration-assisted adhesion method for CFRP-to-aluminum joints is proposed. Extra force caused by ultrasonic vibration is introduced to reinforce the process of CFRP adhesion, which can overcome shortcomings in passive adhesion enhancement methods. The relationship between each ultrasonic vibration factor is explored, and the optimal ultrasonic vibration frequency, 15 KHz, is found. Through orthogonal experiment optimizing adhesive bonding process, it is found that the order of the influence of the factors on the adhesion strength is: Vibration time > Vibration amplitude > Vibration pressure > Vibration position, and the optimal ultrasonic vibration-assisted adhesive bonding of CFRP-to-aluminum joints (vibration time is 8s, vibration pressure is 0.40MPa, vibration position is 30mm and vibration amplitude is 56 μm) can improve adhesion strength by 40.14% and improve consistency of adhesion strength by 58.29%. This study provides a feasible and effective method for CFRP component adhesion.

Thermodynamic and geometrical characteristics of mixed convection heat transfer in the shell and coil tube heat exchanger with baffles

The article presents passive heat transfer enhancement method in the form of baffles to increase the energy efficiency of the shell coil heat exchanger. Conducted literature review shows that, despite numerous studies, there is little work on the intensification of heat transfer at the shell side. Most of the work focuses on the impact of geometrical parameters of the coil itself. This article successfully proves that it is possible to increase the efficiency of heat exchange in the heat exchanger shell type coil with baffle inserts. This work shows that, due to the presence of mixed convection, natural convection has a significant effect for small values of Reynolds numbers and large heat flux. Also, the baffle and inlet configuration have a high impact on results. Presented simple experimental correlation gives a satisfactory compliance with experimental results. It should be noted that the correlation can be applied for all of the presented heat exchanger configurations.

Numerical study on passive convective mass transfer enhancement

Passive mixing mechanisms are widely used for heat and mass transfer enhancement. Vortices generated in flowfield lead to gradients that favour convective mass transfer. Computations on enhancement of convective mass transfer of sublimating solid fuel by baroclinic torque generated vortices in the wake of a swept ramp placed in high speed flow is presented here. Advection Upstream Splitting Method (AUSM) based computational scheme employed in the present study, to solve compressible turbulent flow field involving species transport, could capture the complex flow features resulted by vortex boundary layer and shock boundary layer interactions. Convective mass transfer is found to get improved in regions near boundary layer by horseshoe vortex and further transported to other regions by counter rotating vortex pair. Vortices resulted by flow expansion near aft wall of wedge and recompression wave-boundary layer interactions also promotes convective mass transport. Extensive computations have been carried out to reveal the role of vortices dominance at various lateral sweep angles in promotion of convective mass transfer in turbulent boundary layer.

Development of Passive Film and Enhancement of Corrosion Protection of Mild Steel Exposed in Hydrochloric Acid due to the Adsorption of Water Dispersed 4-[(E)-(3,4-Dihydroxybenzylidene)amino]-6-Methyl-3-Mercapto-1,2,4-Triazin-5(4H)-one(DHMMT)

Corrosion inhibition activity of 4-[(E)-(3,4-Dihydroxybenzylidene)amino]-6-methyl-3-mercapto-1,2,4-triazin-5(4H)-one (DHMMT), a triazine-based Schiff base, for the corrosion of mild steel in HCl has been investigated by weight loss and electrochemical methods. Effect of inhibitor concentration, immersion time, acid concentration and temperature on inhibition efficiency has been studied. It is shown that inhibition efficiency increased with increase in inhibitor concentration and decreased with increase in acid concentration, immersion time and temperature. Polarization studies show that DHMMT act as mixed type inhibitor and adsorption of DHMMT on mild steel obeys Langmuir adsorption isotherm. The presence of protective film on mild steel surface is further confirmed by surface morphological studies.

Second principle approach to the analysis of unsteady flow and heat transfer in a tube with arc-shaped corrugation

Passive convective heat transfer enhancement techniques are well known and widespread tool for increasing the efficiency of heat transfer equipment. In spite of the ability of the first principle approach to forecast the macroscopic effects of the passive techniques for heat transfer enhancement, namely the increase of both the overall heat exchanged and the head losses, a first principle analysis based on energy, momentum and mass local conservation equations is hardly able to give a comprehensive explanation of how local modifications in the boundary layers contribute to the overall effect. A deeper insight on the heat transfer enhancement mechanisms can be instead obtained within a second principle approach, through the analysis of the local exergy dissipation phenomena which are related to heat transfer and fluid flow. To this aim, the analysis based on the second principle approach implemented through a careful consideration of the local entropy generation rate seems the most suitable, since it allows to identify more precisely the cause of the loss of efficiency in the heat transfer process, thus providing a useful guide in the choice of the most suitable heat transfer enhancement techniques.

Expanding the domain of drug delivery for HIV prevention: exploration of the transdermal route.

Constant efforts for HIV prevention using antiretroviral drugs, pre- and postexposure prophylactic agents, and microbicides are being made by researchers. Drug-delivery systems such as oral tablets and coitally dependent vaginal gels are short acting, require daily application, and are associated with user adherence issues, whereas the coitally independent systems such as injectables and biodegradable implants are long acting, lasting several months, during which time the termination of prophylaxis is impractical in case of adverse effects. An effective drug-delivery system to be used for an intermediate duration, if available, would be an attractive alternative option for users in terms of adherence. Transdermal delivery systems, overcoming most of the limitations of the other routes of administration and aiming to provide sustained delivery of drugs through skin, may be explored for HIV prevention. Passive and physical enhancement techniques may be designed strategically to improve the transdermal delivery of HIV preventive agents.

Passivity Enhancement of Grid-Tied Converters by Series LC-Filtered Active Damper

The series LC -filtered active damper can be used for stabilizing a grid converter tied to the nonideal grid. Its operation principle is to mimic a damping resistance at the resonance frequencies appearing in the grid. However, the selection of the damping resistance has not been fully analyzed in the literature. Its effect with parasitic capacitance present in the grid has also usually been ignored, even though it may bring new challenges to the active damper. To address these issues, passivity is applied to study the grid converter stability before the understanding gained is used for formulating a damping resistance selection method. The method formulated can further be improved by admittance shaping so that system stability can always be ensured even when considering grid parasitic capacitance and control imperfection. Experimental results obtained have verified the expectations, and, hence, the suitability of the presented methods for the series LC -filtered active damper.

Analysis of the passive heat removal enhancement for AP1000 containment due to the partially wetted coverage

AP1000 containment uses the water film evaporation, coupled with containment inner condensation, to remove the core decay heat. However, water film cannot fully cover heat transfer surface and dry-wetted strips appear. As a result, heat transfer within the containment shell is a two-dimension thermal conduction. Current work numerically studied the AP1000 heat removal enhancement due to the partially wetted coverage phenomenon. It used the evaporation and condensation boundary conditions and Fluent software to calculate the local heat fluxes and their distributions. Results show that the maximum heat transfer enhancement can reach 63% and this enhancement peak appears when the dry strip fraction approximately equals 90%. The influences of dry coverage and dry-wet strip width were carefully discussed. It indicates that the heat transfer enhancement for small dry-wet strip is linear with dry strip fraction (0<α<80%). Finally, according to the wetted coverage ranges for AP1000 passive containment cooling system, an empirical correlation for heat transfer enhancement is given and it can be used to improve PCCS analysis.

Extraction of enhanced, ultrashort laser pulses from a passive 10-MHz stack-and-dump cavity.

Periodic dumping of ultrashort laser pulses from a passive multi-MHz repetition-rate enhancement cavity is a promising route towards multi-kHz repetition-rate pulses with Joule-level energies at an unparalleled average power. Here, we demonstrate this so-called stack-and-dump scheme with a 30-m-long cavity. Using an acousto-optic modulator, we extract pulses of 0.16 mJ at 30-kHz repetition rate, corresponding to 65 stacked input pulses, representing an improvement in three orders of magnitude over previously extracted pulse energies. The ten times longer cavity affords three essential benefits over former approaches. First, the time between subsequent pulses is increased to 100 ns, relaxing the requirements on the switch. Second, it allows for the stacking of strongly stretched pulses (here from 800 fs to 1.5 ns), thus mitigating nonlinear effects in the cavity optics. Third, the choice of a long cavity offers increased design flexibility with regard to thermal robustness, which will be crucial for future power scaling. The herein presented results constitute a necessary step towards stack-and-dump systems providing access to unprecedented laser parameter regimes.

Metal-Free Carbon-Based Nanomaterial Coatings Protect Silicon Photoanodes in Solar Water-Splitting.

The decreasing cost of silicon-based photovoltaics has enabled significant increases in solar electricity generation worldwide. Silicon photoanodes could also play an important role in the cost-effective generation of solar fuels, but the most successful methods of photoelectrode passivation and performance enhancement rely on a combination of precious metals and sophisticated processing methods that offset the economic arguments for silicon. Here we show that metal-free carbon-based nanomaterial coatings deposited from solution can protect silicon photoanodes carrying out the oxygen evolution reaction in a range of working environments. Purified semiconducting carbon nanotubes (CNTs) act as a hole extraction layer, and a graphene (Gr) capping layer both protects the CNT film and acts as a hole exchange layer with the electrolyte. The performance of semiconducting CNTs is found to be superior to that of metallic or unsorted CNTs in this context. Furthermore, the insertion of graphene oxide (GO) between the n-Si and CNTs reduces the overpotential relative to photoanodes with CNTs deposited on hydrogen-passivated silicon. The composite photoanode structure of n-Si/GO/CNT/Gr shows promising performance for oxygen evolution and excellent potential for improvement by optimizing the catalytic properties and stability of the graphene protective layer.