Low Flow Resistance Research Articles

Performance enhancement of fin and tube heat exchanger employing curved delta winglet vortex generator with circular punched holes

The generation of vortices has recently gained attention as a potential passive technique for improving air-side heat transfer in a fin and tube heat exchanger. This study proposes novel configurations of a curved delta winglet vortex generator (CDWVG) with and without circular holes to improve heat transfer in FTHEs. The present study utilizes a three-dimensional numerical analysis to investigate the thermal-hydraulic performance analysis of fin and tube heat exchanger (FTHE) with curved delta winglet vortex generators (CDWVG) with or without circular holes, operating across a wide Reynolds number range (i.e., from 400 to 2000). In addition to being arranged in vertical and horizontal configurations, the CDWVGs are oriented in the same way as the flow direction. Therefore, the pressure distribution, temperature distribution, and flow structure distribution of an FTHE with a four-in-line circular tube configuration are analyzed and compared between FTHE without vortex generator (VG) and five different configurations of CDWVG (i.e., CDWVG without hole, CDWVG with 1 hole, CDWVG with 2 holes, CDWVG with 3 holes and CDWVG with 6 holes). The pressure drop (ΔP), London area goodness factor (LAGF) (j/f), Nusselt number (Nu), and Colburn factor (j) are also used to evaluate the thermo-hydraulic performance of FTHE. The FHTE performance with CDWVGs can be affected by the number of punched holes, which is evaluated using a dimensionless number including Performance Evaluation Criteria (PEC), Colburn factor, etc. The thermo-hydraulic efficiency of the FTHE is improved significantly by using CDWVGs with circular punched holes. Nusselt number decreases across all the VG configurations (i.e., CDWVG without hole, CDWVG with 1 hole, CDWVG with 2 holes, CDWVG with 3 holes, and CDWVG with 6 holes) due to the lower flow resistance. Compared to other CDWVG configurations, the 6-hole configuration of CDWVG is the most effective. The Nusselt number of CDWVG with 6 holes increases by 77.25% and 42.51% at Reynolds numbers of 400 and 2000, with respect to fin and tube heat exchangers without vortex generator, respectively. On the other hand, friction is decreased by 5.11%. Therefore, when considering the London area goodness factor, CDWVG with six holes is found to be superior to other CDWVG configurations.

Comprehensive study on parabolic trough solar receiver-reactors via a novel efficient optical-thermal-chemical model of catalyst packed bed characteristics

The catalyst packed bed structure plays an important role in the promising solar-driven catalyst packed bed based thermochemical hydrogen production systems. However, corresponding accurate yet efficient numerical methods for characterizing and optimizing the catalyst packed bed structure as well as the whole complex solar-driven thermochemical process is still urgently needed. In this paper, a novel three-dimensional comprehensive model is proposed for the hydrogen production of methanol steam reforming reaction (MSRR) in parabolic trough solar receiver-reactors (PTSRR). A relatively realistic porosity distribution obtained by the discrete element method was combined with the optical-thermal-chemical coupling model. The proposed model was validated, proved to improve the accuracy of numerical predictions in a more efficient way. Moreover, it was effectively applied to tune the overall energy flow of the complex optical-thermal-chemical process of PTSRR-MSRRs for better performance, by optimizing the catalyst packed bed structure in aspects of catalyst sizes and packing methods. The results revealed that single packed beds of larger catalyst particles show better performance within the selected catalyst size range. The radial layered packing method of larger particle-to-particle diameter ratios can bring higher energy conversion efficiency and lower flow resistance, as compared to the single packing method and fully mixed packing method. This study could provide an efficient numerical modelling method and significant guidance for other similar solar-driven thermochemical applications of catalyst packed beds.

Correlation between permeation performance and structural evolution of Al-doped lamellar porous Ti3SiC2 through freeze casting

In order to meet the requirements for heat and mass transport and filtration applications, a lamellar porous Ti3Si(Al)C2 has been prepared. The resulting scaffold phase and porous structure have been thoroughly investigated to fully understand the thermal and fluid transport properties. It has been observed that the porous structure evolves in response to changes in solid content, which subsequently leads to changes in permeability. Specifically, as the solid content of the material increases from 15 % to 25 %, the pressure drop increases from 0.22 to 2.81 MPa/m. The unique lamellar structure, low tortuosity, small roughness, and small surface area of the Ti3Si(Al)C2 contribute to lower flow resistance and induces a decrease in pressure drop. Additionally, high temperatures have been found to have little effect on the porous Ti3Si(Al)C2 material, in terms of both its structural integrity and permeability. Overall, the results of this study suggest that the lamellar porous Ti3Si(Al)C2 has significant potential for application in mass transport and filtration applications due to excellent permeability.

Effect of Dynamic Pressure Feedback Orifice on Stability of Cartridge-Type Hydraulic Pilot-Operated Relief Valve

Current research on pilot-operated relief valve stability is primarily conducted from the perspective of system dynamics or stability criteria, and most of the existing conclusions focus on the spool shape, damping hole size, and pulsation frequency of the pump. However, the essential factors pertaining to the unstable vibration of relief valves remain ambiguous. In this study, the dynamic behavior of a pilot-operated relief valve is investigated using the frequency-domain method. The result suggests that the dynamic pressure feedback orifice is vital to the dynamic characteristics of the valve. A large orifice has a low flow resistance. In this case, the fluid in the main spring chamber flows freely, which is not conducive to the stability of the relief valve. However, a small orifice may create significant flow resistance, thus restricting fluid flow. In this case, the oil inside the main valve spring chamber is equivalent to a high-stiffness liquid spring. The main mass–spring vibration system has a natural frequency that differs significantly from the operating frequency of the relief valve, which is conducive to the stability of the relief valve. Good agreement is obtained between the theoretical analysis and experiments. The results indicate that designing a dynamic pressure feedback orifice of an appropriate size is beneficial to improving the stability of hydraulic pilot-operated relief valves. In addition, the dynamic pressure feedback orifice reduces the response speed of the relief valve. This study comprehensively considers the stability, rapidity, and immunity of relief valves and expands current investigations into the dynamic characteristics of relief valves from the perspective of classical control theory, thus revealing the importance of different parameters.

Support-free multi-walled carbon nanotubes-based membrane for forward osmosis applications

This study aims to fabricate a thin-film composite membrane with high water flux and low reverse solute flux for the application in a forward osmosis (FO) system. Instead of using traditional non-woven fabric or polymer materials, multi-walled carbon nanotubes (MWCNTs) layers prepared via the vacuum filtration method to reinforce the membrane are fabricated. In this work MWCNTs are modified by the mixture of polydopamine (PDA) and polyvinyl alcohol (PVA) to enhance the mechanical strength and hydrophilicity of the membrane, which is denoted as PVAm pCNTn. Finally, a polyamide (PA) active layer, used as molecular selection, is synthesized on the PVAmpCNTn film using the interfacial polymerization method, which is assigned as PVAmpCNTn-PA. Experimental results show that PDA and PVA improve adherence among the MWCNTs, strengthening the tolerance and increasing hydrophilicity of the membrane. Membrane performance is also affected by the amount of PVA(m) and PVA(n), and thickness can be controlled by adopting various amounts of MWCNTs suspension during the vacuum filtration process; the thinner membrane results in higher water flux because of lower flow resistance. In addition, the PVA concentration influences the pore size distribution and configuration within the membrane, as a result, affects the FO performance. The best FO performance are achieved by PVA0.25pCNT3-PA, with a water flux of 30.16 L m−2 h−1 and reverse solute flux of 9.34 g−2 h−1. The excellent FO performance and the simple process used in this study indicate the great potential of this work in the FO application.

Feedback Control of Electrospray with and without an External Liquid Pump Using the Spray Current and the Apex Angle of a Taylor Cone for ESI-MS.

An electrospray operated in the steady cone-jet mode is highly stable but the operating state can shift to pulsation or multijet modes owing to changes in flow rate, surface tension, and electrostatic variables. Here, a simple feedback control system was developed using the spray current and the apex angle of a Taylor cone to determine the error signal for correcting the emitter voltage. The system was applied to lock the cone-jet mode operation against external perturbations. For a pump-driven electrospray at a regulated flow rate, the apex angle of the Taylor cone decreased with increasing voltage. In contrast, for a voltage-driven electrospray with low flow resistance, the angle was found to increase with the emitter voltage. A simple algorithm based on iterative learning control was formulated and implemented using a personal computer to automatically correct the emitter voltage in response to the error signal. For voltage-driven electrospray ionization (ESI), the feedback control of the spray current can also be used to regulate the flow rate to an arbitrary value or pattern. Electrospray ionization-mass spectrometry (ESI-MS) with feedback control was demonstrated to produce ion signal acquisition with long-term stability that was insusceptible to the emulated external disturbances.

Multi-Objective Design Automation for Microfluidic Capture Chips.

Microfluidic capture chips are useful for preparing or analyzing a wide range of different chemical, biological, and medical samples. A typical microfluidic capture chip contains features that capture certain targets (i.e. molecules, particles, cells) as they flow through the chip. However, creating optimal capture chip designs is difficult because of the inherent relationship between capture efficiency and flow resistance: as more capture features are added to the chip, the capture efficiency increases, but the additional features slow the flow of fluid through the chip. This paper introduces the use of multi-objective optimization to generate capture chip designs that balance the trade-off between maximizing target capture efficiency and minimizing resistance to fluid flow. Design automation for this important class of microfluidic chips has not been attempted previously. Our approach automatically produces a Pareto front of non-dominated chip designs in a reasonable amount of time, and most of these designs have comparable capture efficiency to hand-designed chips with far lower flow resistance. By choosing from the chip designs on the Pareto front, a user can obtain high capture efficiency without exceeding the flow resistance constraints of their application.

Innovative Implantable Left Ventricular Assist Device—Performance under Various Resistances and Operating Frequency Conditions

This paper presents the operation of an innovative left ventricular assist device under various resistances and operating frequencies. The operating principle of the device is based on pulsatile blood flow, which is forced by a suction–discharge device pumping helium into a set of intra-cardiac balloons. In this way, the ejection fraction of the left ventricle is increased, and the mitral valve is additionally occluded. What is more, the suction–discharge device is part of a portable pumping system that is synchronized with the heart cycle by monitoring the ECG signal. The device is implanted in a minimally invasive manner and is suitable for patients with stage D heart failure accompanied with residual mitral regurgitation. A model of the heart was built on the basis of a realistically reconstructed heart geometry and is part of an overall test stand that allows for realistic conditions in the heart of patients with end-stage heart failure to be reproduced. In the following sections, example measurements of the pressures in the heart chambers and balloons are shown, demonstrating that the device works correctly at least on a laboratory scale. The entire device, including the pumping system, is portable and powered by a set of lithium-ion batteries. From the measurements, it was observed, for example, that the flow rate varies with the frequency of the portable external balloon pumping system, up to 2.5 kg/min for 100 cycles/min at low flow resistance. As the flow resistance of the hydraulic system increases, the pressure in the heart chamber and aorta increases while the flow rate decreases.

Numerical study of time and number cycle to failure for the new bipolar plate in the PEM fuel cell used in hybrid energy storage

The operation of a fuel cell poses significant problems related to local current density and the failure rate of components. The issue results from high flow resistance and stresses that guarantee the tightness of the stack. Low flow resistance allows the stack to be used in Hybrid Energy Storage. This study shows that the selection of the best variant of the channel shape in a fuel cell should be based on more than just observing electrochemical performance. Increasing the current density flux can lead to faster component degradation. Older material degradation models based on the strain tensor will only accurately identify some risks. Thanks to the use of Perzyna's viscoplastic model, it was possible to protect the fuel cell from premature failure. Earlier studies did not indicate how the changes in the shape introduced, improving the electrochemical properties, impacted the component's service life during cyclic loads.

Factors Associated with Complications after Total Pharyngo-Laryngo-Esophagectomy and Free Jejunal Flap Reconstruction

Introduction: Free jejunal flap (FJF) reconstruction is a standard procedure for pharyngeal and cervical esophageal defects resulting from head and neck cancer resection. However, improvements in patients’ quality of life after surgery require a further statistical approach. Methods: An observational, retrospective, multivariate analysis was designed to report the incidence of postoperative complications and their association with clinical factors in 101 patients who underwent total pharyngo-laryngo-esophagectomy and FJF reconstruction for head and neck cancer at a university hospital between January 2007 and December 2020. Results: Postoperative complications were observed in 69% of patients. In the reconstructive site, anastomotic leak, observed in 8% of patients was associated with vascular anastomosis in the external jugular vein system (age-adjusted odds ratio [OR]: 9.05, p = 0.044) and anastomotic stricture, observed in 11% of patients was associated with postoperative radiotherapy (age-adjusted OR: 12.60, p = 0.02). Cervical skin flap necrosis was the most common complication (34%) and was associated with vascular anastomosis on the right cervical side (age- and sex-adjusted OR: 4.00, p = 0.005). Conclusion: Although FJF reconstruction is a useful procedure, 69% of patients suffer a postoperative complication. We suppose that anastomotic leak is related to the low blood flow resistance of the FJF and inadequate drainage of the external jugular venous system, and anastomotic stricture is related to the vulnerability of the intestinal tissue to radiation. Furthermore, we hypothesized that the location of the vascular anastomosis may affect the mesenteric location of the FJF and the dead space in the neck, leading to the development of cervical skin flap necrosis. These data contribute to increasing our knowledge about postoperative complications related to FJF reconstruction.

BIO11: Ambulatory Rabbit ECLS Model for Long-term Biocompatibility Testing

Background: Thrombosis causes rapid failure of oxygenators within extracorporeal life support (ECLS) circuits. To combat this problem, various groups are developing new surface coatings and anticoagulants, but their testing typically relies on either (a) inexpensive, but overly-brief (< 8 hour) in vitro or small animal in vivo testing or (b) expensive, long-term (days to weeks) testing in large animals. A more inexpensive model is thus needed to assess the long-term biocompatibility of new anticoagulant technologies. In this study, the feasibility of a long-term, ambulatory rabbit ECLS model was assessed. Methods: A miniaturized ECLS circuit was attached to rabbits (2.5-4 kg) under anesthesia using a pumpless arteriovenous (AV) configuration. The circuit contained a miniature artificial lung (surface area of 400 cm2) using the same materials as full-scale artificial lungs. The circuit was tunneled behind the animal’s back and secured just below the scruff of the neck. IV heparin was provided via a battery-powered infusion pump (SAI 3D Mini Infusion Pump) within a small jacket. Twelve non-recovery experiments were used to optimize the model, and two subsequent recovery experiments were run until oxygenator failure (blood flow rate below 5 mL/min for over 2 hours and/or resistance > 5x baseline). Non-recovery experiments were used to optimize the cannulation strategy. This included cannula type and circuit placement on the rabbit. Once optimized, two survival studies were performed. The average circuit blood flow during the first experiment was 50-60 mL/min. The circuit blood flow was then limited to 35-45 mL/min with a Hoffman clamp for the second experiment. Blood flow resistance was measured to assess anticoagulation effectiveness, and physiology was monitored through hepatic and renal panels. Results: Bilateral cannulation was ideal for circuit placement. A flexible 16G single-lumen central line was chosen because of its low blood flow resistance and resistance to kinking. The circuit was tunneled 40 cm, emerging just below the scruff. This enabled safe handling of the animal without impacting its mobility and ability to eat. The recovery experiments lasted 47 and 21 hours prior to oxygenator failure. Animals were completely ambulatory with free access to food and water. Heparin delivery could be controlled simply using the infusion pump, and coagulation could be assessed by drawing blood samples and measuring oxygenator resistance. The first survival rabbit developed hyperlipidemia on Day 1. The cause is unclear at this time but could be attributed to high blood flow through AV circuit. Conclusion: This study demonstrates the feasibility of using a lower-cost rabbit model for long-term anticoagulation testing that requires less staffing. Future studies will use this model to compare the anticoagulation effectiveness of heparin, a selective Factor XIIa inhibitor, and surface coatings for a period of five days.

Slurry Based Lithium-Ion Flow Battery with a Flow Field Design

Slurry based lithium-ion flow battery has been regarded as an emerging electrochemical system to obtain a high energy density and design flexibility for energy storage. The coupling nature of electrode thickness and flow resistance in previous slurry flow cell designs, demands a nuanced balance between power output and auxiliary pumping. To address this issue, a slurry based lithium-ion flow battery featuring a serpentine flow field and a stationary porous carbon felt current collector is proposed in this work. The carbon felt serves to provide a stable and efficient pathway for electron transport, while the flow field helps distribute active slurry onto the felt for electrochemical reactions. With such a design, the LiFePO4 (LFP) slurry based flow battery shows a low flow resistance and good flow stability without forming severe filter cakes on the felt surface, similar to cross-flow filtration. A maximum power density of 84.5 mW cm−2 and a stable coulombic efficiency of ∼98% under intermittent flow, and a specific capacity of 164.87 mAh g−1 (based on the total LFP in the tank) in continuous flow are successfully demonstrated. These preliminary yet encouraging results may put forward new avenues for future structural design and optimization of slurry based flow batteries.

Appealing sheath‐core spun high‐performance composite carbon molecular sieve membranes

AbstractCarbon molecular sieve (CMS) membranes are attractive candidates to meet requirements for challenging gas separations. The added ability to maintain such intrinsic properties in an asymmetric morphology with a structure that we term a “Pseudo Wheel+Hub &amp; Spoke” asymmetric form offers new opportunities. For CMS membrane, specifically, the structure provides both selective layer support and low flow resistance even for high feed pressures and fluxes in CO2 removal from natural gas. This capability is unavailable to even rigid glassy polymers due to the much higher modulus of CMS materials. Combining precursor asymmetric hollow fiber formation and optimized pyrolysis creates a defect free CMS proof‐of‐concept membrane for this application. Facile formation of the sheath‐core spun precursor with a 6FDA‐DAM sheath and Matrimid® core also avoids the need to seal defects before or after the carbonization of the precursors. The composite CMS membrane shows CO2/CH4 (50 : 50) mixed gas feed with an attractive CO2/CH4 selectivity of 64.3 and CO2 permeance of 232 GPU at 35 °C. A key additional benefit of the approach is reduction in use of the more costly high performance 6FDA‐DAM in a composite sheath‐core CMS membrane with the “Pseudo Wheel+Hub &amp; Spoke” structure.

Appealing sheath-core spun high-performance composite carbon molecular sieve membranes.

Carbon molecular sieve (CMS) membranes are attractive candidates to meet requirements for challenging gas separations. The added ability to maintain such intrinsic properties in an asymmetric morphology with a structure that we term a "Pseudo Wheel+Hub & Spoke" asymmetric form offers new opportunities. For CMS membrane, specifically, the structure provides both selective layer support and low flow resistance even for high feed pressures and fluxes in CO2 removal from natural gas. This capability is unavailable to even rigid glassy polymers due to the much higher modulus of CMS materials. Combining precursor asymmetric hollow fiber formation and optimized pyrolysis creates a defect free CMS proof-of-concept membrane for this application. Facile formation of the sheath-core spun precursor with a 6FDA-DAM sheath and Matrimid® core also avoids the need to seal defects before or after the carbonization of the precursors. The composite CMS membrane shows CO2 /CH4 (50 : 50) mixed gas feed with an attractive CO2 /CH4 selectivity of 64.3 and CO2 permeance of 232 GPU at 35 °C. A key additional benefit of the approach is reduction in use of the more costly high performance 6FDA-DAM in a composite sheath-core CMS membrane with the "Pseudo Wheel+Hub & Spoke" structure.

Potential of the KOBO extrusion process for nonferrous metals in the form of solids and chips

The research undertaken for this paper was inspired by the results of theoretical analyzes and strong experimental evidence confirming the possibility to induce the phenomenon of viscous flow (typical for liquids) in the process of low-temperature KOBO extrusion of metals. Thus, despite the constant solid state of the metal, the KOBO plastic deformation process requires much lower stress (low flow resistance) in comparison with conventional extrusion. On the other hand, the KOBO method is particularly attractive due to the generation of high strength properties in the product/extrudate favorable for low (room) temperature deformation as well as superplastic features at higher temperature ranges.This paper presents the results of the studies on the efficiency of low-temperature KOBO extrusion of hardly-deformable materials in massive form (Mg4Li, AK11, Cu6.5P) and fragmented fractions (2014, 7075, AZ91, titanium grade 2). Particular attention was paid to the structure and mechanical properties of the obtained extrudates, which were rated as exceedingly favorable. As such, the paper provides new technological arguments for the application of the KOBO method in plastic deformation of materials as it guarantees their functional properties.

Dead-end forward osmosis as an alternative for deep sludge dewatering: Evaluation method and characteristics analysis

Sludge deep dewatering technologies that are effective and energy-saving are urgently required. Forward osmosis (FO) technology was recently used for dewatering sludge. However, the final moisture content was too high—above 95.0%—to meet the treatment and disposal requirements. In this study, a novel dead-end forward osmosis (DEFO) technique was proposed and verified for deep dewatering of excess sludge. In DEFO, the feed side is in dead-end mode, while the draw side is in cross-flow mode. Further, when 30.8 g/L excess sludge, 2.0 M NaCl draw solute, 3.0 cm/s cross velocity on the draw side, and 3‰ cationic polyacrylamide dosage was used, the moisture content of the filter cake was as low as 69.2%, equating to 90.0% sludge reduction. A proposed evaluation model for DEFO sludge dewatering behavior includes critical characteristics, such as water flow osmosis resistance and water permeability coefficient of the FO membrane. The sludge dewatering process continued regardless of whether the filter cake broke. In contrast to the conclusions endorsed in traditional dead-end pressure filtration, the DEFO filter cake exhibits an abnormal structure with a dense upper cake and a loose structure for the lower cake, resulting in a permeable filter cake. The photographs, moisture content distribution, and filter cake frozen scan verified this. The solid compressive pressure and hydraulic pressure in the DEFO filter cake were negligible, resulting in low water flow resistance of the filter cake. These findings provide further insights into the use of DEFO technology for deep sludge dewatering.

Numerical investigation of enhanced mass transfer flow field on performance improvement of high‐temperature proton exchange membrane fuel cell

AbstractThe enhanced mass transfer flow fields have been proven to be an effective measure to improve the cell performance of low‐temperature proton exchange membrane fuel cells, yet little research has been done for high‐temperature proton exchange membrane fuel cells (HT‐PEMFC). In this work, three types of cathode‐enhanced mass transfer flow fields (tapered, staggered‐blocked, and blocked) are designed. The effects of various flow fields on the reactant delivery, current density distribution uniformity, and net power output of HT‐PEMFC are quantitatively investigated and compared. It is found that the three enhanced mass transfer flow fields can effectively increase the performance of HT‐PEMFC by transforming the traditional diffusion into a combination of diffusion and forced convection. In the sight of the superior performance and lower flow resistance, the tapered flow field is thought to be the optimal candidate for HT‐PEMFC among the four flow fields, with a 12.21% net power increment and 5.32% current density distribution uniformity improvement at 1.4 A/cm2 compared to the conventional flow field. These results support further performance enhancements and applications of HT‐PEMFC.

Evaluation and Application of Fractal-Based Hydraulic Constitutive Model for Unsaturated Flow in Heterogeneous Soils

The physical-based hydraulic constitutive model developed by fractal theory has received much attention, however, its performance in two-phase flows has not been fully evaluated. This study evaluates the ability of fractal-based hydraulic constitutive models to describe the two-phase flow combining mass and momentum conservation equations. After that, we investigate the injection and displacement characteristics of the amendment at NAPL (non-aqueous phase liquids)-contaminated low-permeability sites based on this fractal model. Points of interest are the effect of the location of hydraulic fracturing in layered soils on amendment injection and distribution and the corresponding key parameters. Comparison with experimental results reveals that the fractal model describes the two-phase flow behavior very well. Fractal dimension and intrinsic permeability of soils have significant effects on the two-phase flow. For layered soils, the lower part of the low-permeability layer is preferably recommended as the main fracturing and injection zone to obtain the optimal injection efficiency. Besides, due to the lower flow resistance, hydraulic fractures across layers can be treated as fractures in highly permeable layers. This work provides new insights into the application of fractal theory in describing two-phase flows.

Investigation of flow characteristics in a twin-surface dielectric barrier discharge reactor by Schlieren imaging

Dielectric barrier discharges are an emerging technology for the plasma-catalytic removal of volatile organic compounds and other gas purification challenges such as the removal of O2 traces from H2. Packed-bed reactors are mainly used for these applications, but surface dielectric barrier discharges (SDBDs) typically printed on thin dielectric plates are promising alternatives for the treatment of large volumetric flow rates due to their low flow resistance causing a low pressure drop. Especially for SDBDs the flow conditions are crucial, because the active plasma filled volume covering the mentioned plates with a typical thickness of 0.1 mm is small in comparison to the overall reactor volume with a typical distance of some tens of millimeters to the reactor wall. In this study, the flow conditions of a twin-SDBD were investigated by Schlieren imaging applied in converting O2 traces in H2 containing gas mixtures to H2O and compared to fluid dynamics simulations. Schlieren imaging was used to visualize local gradients of the refractive index inside the SDBD reaction chamber, while gas composition, dissipated power, or flow rate were varied. Without a plasma discharge, laminar flow dominates, resulting in a conversion below 10% over a Pt-coated electrode configuration in the reaction of O2 traces with H2. With the plasma discharge, full conversion was achieved for the same reaction without catalyst, although the plasma is also confined to the surface of the electrode configuration. Schlieren structures covering the complete cross section of the reaction chamber were observed, showing that strong radial mass transport is induced by the plasma. The shape and extent of the Schlieren structures is ascribed to a superimposition of gas flow, thermal expansion from the plasma volume, thermal buoyancy as well as an electrohydrodynamic force between the electrodes and the grounded reactor walls. Fluid dynamics simulations show vortex formation above and below the electrode, created by the electrohydrodynamic force further implying extensive mass transport by the plasma, which is visualized in addition by carbonaceous deposits on the reactor lid. This emerging deposition pattern during toluene decomposition closely corresponds to the electrode geometry. It is proposed that the reaction proceeds only in the active plasma volume and that reactive species transported to the bulk gas phase only have a minor contribution. Thus, the degree of conversion of the SDBD reactor is not only determined by the chemical reactivity in the plasma volume, but also by its plasma-induced mass transport resulting in efficient gas mixing. These findings reveal new possibilities to improve SDBD reactors for gas purification applications based on their favorable flow conditions.

Study on heat transfer characteristics of annular pulsating heat pipe with temperature oscillation characteristic

A new annular structure of pulsating heat pipe (APHP) was designed for evacuated solar collector. To reduce the contact thermal resistance and alleviate the overheating problem, each tube of the pulsating heat pipe can be installed to fit snugly against the inner wall of evacuated tube. Experiments on filling ratio, heat transfer distance, inclination angle, and working fluid were carried out. The flow circumstances and heat transfer performance were analyzed based on the difference in frequency and amplitude of temperature oscillation curve and thermal resistance data. The results showed that high-frequency and low-amplitude temperature curve appeared when the APHPs were in the situation of low flow resistance, normally accompanied with better heat transfer effect. The optimum filling ratio and inclination angle are 60% and 60 °, respectively. The heat transfer performance of APHP decreased with the increase of heat transfer distance. The APHPs charged with self-rewetting fluid demonstrated a significant effect on heat transfer enhancement. Over different heat transfer distance, the application of self-rewetting fluid reduced the thermal resistance on average by 32.46%, 34.15% and 37.40%, respectively.