Tandem Design Research Articles

Tandem nozzle supersonic wind tunnel design

A new tandem nozzle supersonic wind tunnel was designed for Technische Universitat Braunschweig. Based on the infrastructure of the existing M = 6 Ludwieg tube in Braunschweig (HLB), two nozzles in tandem configuration are designed to get supersonic flow with a similar test section size. Preliminary design trades are presented based on one-dimensional flow analysis. Using alternate methods, contours of the second nozzle are designed. Numerical optimisations of the second nozzle are accomplished using the DLR TAU-Code, by which reliable estimates of flow uniformity in the test section are obtained. A settling chamber with suitable flow straighteners is also employed. The flow analysis determines a reasonable configuration of the overall tandem nozzle wind tunnel and yields realistic estimates of flow uniformity in the test section. In conclusion the present work provides quantitative design trades, detailed flow quality and performance data for cost-efficient extensions of hypersonic Ludwieg tubes into the supersonic flow range.

Tandem cathode for proton exchange membrane fuel cells

The efficiency of proton exchange membrane fuel cells is limited mainly by the oxygen reduction reaction at the cathode. The large cathodic overpotential is caused by correlations between binding energies of reaction intermediates in the reduction of oxygen to water. This work introduces a novel tandem cathode design where the full oxygen reduction, involving four electron-transfer steps, is divided into formation (equilibrium potential 0.70 V) followed by reduction (equilibrium potential 1.76 V) of hydrogen peroxide. The two part reactions contain only two electron-transfer steps and one reaction intermediate each, and they occur on different catalyst surfaces. As a result they can be optimized independently and the fundamental problem associated with the four-electron catalysis is avoided. A combination of density functional theory calculations and published experimental data is used to identify potentially active and selective materials for both catalysts. Co-porphyrin is recommended for the first step, formation of hydrogen peroxide, and three different metal oxides - SrTiO3(100), CaTiO3(100) and WO3(100) - are suggested for the subsequent reduction step.

Recent progress on tandem structured dye-sensitized solar cells

Tandem structured dye-sensitized solar cells (DSSCs) can take full advantage of sunlight, effectively broadening the absorption spectrum of the cell, resulting in a higher open circuit voltage or short circuit current than that of the conventional DSSC with single light absorber. The theoretical maximum efficiency is therefore suggested to be over the Schottky-Queisser limit of 33%. Accordingly, tandem design of DSSC is thought to be a promising way to break the performance bottleneck of DSSC. Besides, the tandem designs also broaden the application diversity of DSSC technology, which will accelerate its scale-up industrial application. In this paper, we have reviewed the recent progress on photo-electrochemical applications associated with kinds of tandem designs of DSSCs, in general, which are divided into three kinds: “n-type DSSC + n-type DSSC,” “n-type DSSC + p-type DSSC” and “n-type DSSC + other solar conversion devices.” The working principles, advantages and challenges of these tandem structured DSSCs have been discussed. Some possible solutions for further studies have been also pointed out together.

On the capacity of highway checkpoints: Models for unconventional configurations

Widening a highway toll plaza, border crossing or other checkpoint by adding travel lanes and vehicle-processing stations in parallel fashion can be expensive, and sometimes even infeasible. Rather than expanding laterally, checkpoint capacities can be increased by branching or staggering the vehicle-processing stations, or by placing them in tandem. Analytical models are formulated to estimate the vehicle-processing capacities achievable via these layouts. The models indicate that tandem designs tend to produce the highest capacities among the three alternatives. Other insights are unveiled as well.

Numerical Investigation of Low Solidity Vaned Diffuser Performance in a High-Pressure Centrifugal Compressor—Part III: Tandem Vanes

As Part III, following the authors’ previous studies, the aerodynamic performance of two different tandem LSDs (low solidity diffusers), Tandem (A) and (B), in a high-pressure centrifugal compressor was numerically investigated over flow rates from impeller choke to minimal flows available in computation. Tandem (A) was of conventional design where the first row came directly from the authors’ previous studies (Part I Oh and Agrawal, 2007, “Numerical Investigation of Low Solidity Vaned Diffuser Performance in a High-Pressure Centrifugal Compressor - Part I : Influence of Vane Solidity,” ASME Paper No. GT2007-27260, and Part II: Oh et al., 2008, “Numerical Investigation of Low Solidity Vaned Diffuser Performance in a High-Pressure Centrifugal Compressor - Part II : Influence of Vane Stagger,” ASME Paper No. GT2008-50178) selected as the highest efficiency vane at design flow, and the second row was designed to be added considering flow conditions at the exit of the first row vane. Tandem (B) followed a creative patent-pending concept where the number of the first row vanes was doubled with much smaller vane chord keeping a low solidity. A position parameter of RCP (relative circumferential position) was introduced to see the effect of the relative location of the second row vane. Using an in-house Navier-Stokes solver with finite volume time marching methods, overall performance was predicted to be compared with each other. Detailed investigation on the behavior of the static pressure recovery and the total pressure loss coefficient in both diffuser designs helps determine why Tandem (A) design is better and the case of RCP = 0.3 gives the best performance.

Bifacial transparent solid-state dye-sensitized solar cell with sputtered indium-tin-oxide counter electrode

In this study, we report a solid state dye-sensitized solar cell (SSDSC) made with a transparent ITO film as a counter electrode using the sputtering technology. For the first time, a bifacial transparent SSDSC is realized and irradiated from FTO and ITO side. The SSDSCs give short circuit photocurrent density (Jsc) of 4.16mAcm−2, open circuit voltage (Voc) of 0.74V, and fill factor (FF) of 0.64, corresponding to the photoelectric conversion efficiency of 1.96% from FTO side illumination (AM 1.5G, 100mWcm−2). Moreover, it is found that Jsc of SSDSCs (2.85mAcm−2) when irradiated from ITO side is less than that from FTO side. This result is because of the cut-off of incident photons in the blue region by the ITO film and the light screening effect by the hole transport material (HTM) absorption. Our results demonstrate the possibility of production scalable sputtering process for SSDSCs electrodes fabrication and pave the avenue for tandem design application which requires a transparent intermediate layer for interconnection.

Influence of absorber doping in a-SiC:H/a-Si:H/a-SiGe:H solar cells

This work deals with the design evaluation and influence of absorber doping for a-Si:H/a-SiC:H/a-SiGe:H based thin-film solar cells using a two-dimensional computer aided design (TCAD) tool. Various physical parameters of the layered structure, such as doping and thickness of the absorber layer, have been studied. For reliable device simulation with realistic predictability, the device performance is evaluated by implementing necessary models (e.g., surface recombinations, thermionic field emission tunneling model for carrier transport at the heterojunction, Schokley—Read Hall recombination model, Auger recombination model, bandgap narrowing effects, doping and temperature dependent mobility model and using Fermi—Dirac statistics). A single absorber with a graded design gives an efficiency of 10.1% for 800 nm thick multiband absorption. Similarly, a tandem design shows an efficiency of 10.4% with a total absorber of thickness of 800 nm at a bandgap of 1.75 eV and 1.0 eV for the top a-Si and bottom a-SiGe component cells. A moderate n-doping in the absorber helps to improve the efficiency while p doping in the absorber degrades efficiency due to a decrease in the VOC (and fill factor) of the device.

A TCAD-based modeling of GaN/InGaN/Si solar cells

Theoretical efficiency potential of GaN/InGaN/cSi tandem solar cells is investigated using two-dimensional numerical computer simulation (i.e. technology-based computer aided design tool: TCAD). With double-junction GaN/InGaN/cSi tandem design, a conversion efficiency of 27% is achieved using a 1.0 µm In0.5Ga0.5N absorber of top cell over crystalline silicon (cSi) bottom cell. This efficiency is further improved to 29.0% with grading of the InxGa1−xN absorber layer close to the top heterointerface (p+-GaN/n−-InxGa1−xN) of the solar cell. A maximum conversion efficiency is obtained when the band discontinuity ratio (i.e. ΔEC:ΔEV) is set to 0.65:0.35. While efficiency remains approximately constant with moderate n-doping (up to 5 × 1016 cm−3) in the top InGaN absorber layer, sensitivity of the efficiency to the interface trap density and trap cross-section (when traps are located only at the heterointerfaces) shows degraded behavior with increasing trap density and trap cross-section. A temperature coefficient for open-circuit voltage (efficiency) of −0.15 (−1.72 × 10−3 °C−1), −0.09 (−0.95 × 10−3 °C−1) and −0.2 (−2.38 × 10−3 °C−1)%/°C for single heterojunction (SHJ), double-heterojunction (DHJ) and tandem-graded design is predicted from the numerical simulations.

Automating the TANDEM Design Method in End-User Programming Environments

Mashups are newly envisaged applications, made up from local information sources and processes, Web services and other distributed resources, bound together technically in some way. Interactive Development Environments (IDE) used to build mashups are becoming more accessible to end-user programmers. Design methods that end-users may apply to a given problem addressed by a mashup, are much less prevalent. This paper describes an end-user-friendly design method called TANDEM and demonstrates the use of it in detail, by way of an example: the design of a mashup of services that solves the so-called movie-cinema problem. An implementation of the newly designed movie-cinema app is then built within the DigitalFriend, an end-user programmer IDE. Furthermore, a significant part of the TANDEM design method, is then automated within the development tool itself. This automation removes the most skilled task required by TANDEM of the end-user: the automation of the process of Data Normalization. The automation applies data normalization to the initial model of components and data sources that feed into the mashup. The presentation here relies on some understanding of Data Normalization, so a simple example is presented. After this demonstrated example of the method and the implementation, the paper discusses the applicability of a model achievable by end-users using TANDEM coupled with the automated normalization process built into the IDE, versus, using a top-down model by an experienced information analyst. In conclusion, the TANDEM method combined with the automation as demonstrated, does empower an end-user to a significant degree in achieving a workable mashup or distributed application. And furthermore, the TANDEM method does have broader applicability to designing a broad class of logic programs, complementing the use of collected patterns in logic programs.

Air stable hybrid inverted tandem solar cell design

In order to get an air stable solar cell with high open circuit voltage (Voc), we fabricated an inverted tandem solar cell based on hybrid wet chemistry and vacuum thermal deposition techniques. A thin metallic interfacial layer was applied to improve charge recombination and maximize both the fill factor and Voc of the tandem solar cell. A cationic dye doped electron transport layer was used to minimize space charge induced Voc loss. The tandem cell Voc reached 1.02 V, which equals the sum of the two subcells’ Voc. Increase of the metal nanoparticles’ layer thickness reduces the short circuit current density of the tandem owing to increasing light extinction. Our tandem cell design offers superior air stability due to additional encapsulation effect from top metal oxide layers. It retains about 80% of its original efficiency after storage in air for three months.

Tandem Ion Trap Design with Enhanced Mass Analysis Capabilities for Large Populations of Ions

A new arrangement consisting of two separate radio frequency (rf) quadrupole ion traps is used to analyze large populations of ions over a wide mass-to-charge (m/z) range. The setup consists of an "accumulation" trap that is maintained at a higher pressure than the second high-performance "analyzer" trap. The two traps are scanned simultaneously, with a mass difference between that determines the residence time and mass range of ions in the analytical trap. Initially, all ions are trapped in the accumulation trap and then mass-selectively ejected into the analyzer trap. As ions arrive in the analyzer trap, they cool through collisions with the buffer gas and then are mass selectively ejected toward the detector. This concurrent linked mass scanning reduces the total number of ions present in the analyzer trap during mass analysis, thereby reducing space charge effects and leading to improved resolution and mass accuracy of analytical spectra.

Computer analysis of thin-film amorphous silicon heterojunction solar cells

A two-dimensional numerical computer analysis for thin-film-based hydrogenated amorphous silicon (i.e. a-Si : H) solar cells is presented. A comparative performance assessment for various absorbing layers such as a-Si, a-SiGe, a-SiC, combined a-Si+a-SiGe, a-Si/a-SiGe-graded layers and tandem (tdm) design is shown. The device performance is evaluated by implementing special surface recombination model, thermionic field emission model for transport at the heterojunctions and interface traps model at top and bottom side of the heterojunction interfaces. Single absorber with a graded design gives an efficiency of 10.1% for 800 nm thick multiband absorption. Similarly, tandem design shows an efficiency of 10.4% with a total absorber of thickness of 800 nm at bandgap of 1.75 eV and 1.0 eV for the top a-Si and bottom a-SiGe component cells. Numerical simulations predict improved efficiency for tandem design over large range of band discontinuity and interface trap density. Although comparable, the efficiency performance for graded design (i.e. absorbing layer of a-Si and a-SiGe graded layers) approach is very sensitive to the limited range of band discontinuity and interface trap density available at the heterojunctions.

An Interview with Koreen Olbrish and Kristen Cromer

Pennsylvania-based Tandem Learning designs training and education programs, with an emphasis on games-based learning and alternate reality games, for a wide variety of organizations and corporations. Founded by Koreen Olbrish, CEO, and Kristen Cromer, vice president of learning design, the company has been creating and rolling out e-learning content since 2008. Here, the two co-founders speak about what kinds of things they see and hear being repeated from one client to the next, as well as how they know a client is truly ready to take on a games-based learning approach.

Drift-Wave Eigenmodes and Spectral Gaps in Tandem Mirrors

In recent years, breakthroughs in the GAMMA-10 tandem mirror device and new designs such as the Kinetically Stabilized Tandem Mirror have renewed enthusiasm for investigation of mirror plasmas. With a discrete eigenmode solver and shooting code we calculate kinetic Alfvén waves in a model GAMMA-10 experiment, and then examine the nature of the spectral gaps. We validate the results of these calculations by comparing with magnetic probe signals recorded in the LAPD as a function of frequency of the antenna and the axial position.

Tandem design of mechanisms for function generation and output speed variation

A new approach for generation of functions based on application of a multi-gear drive is proposed. The approach provides solution to the functional ψ ( α ) = g n ( g n - 1 ( ⋯ ( g 1 ( α ) ) ) ) wherein ψ ( α ) is the function assigned for generation and g i ( α ) , i = 1 , … , n , is the transmission function of pair i of conjugated non-circular gears. The specifics characteristics of the proposed approach are: (a) integrated impact by application of n synchronized gear drives, (b) favourable shape of centrodes, and (c) observation of limits of pressure angle. In addition to the approach for function generation, tandem design for the following types of planar linkages has been developed: double-crank linkage, crank-slider linkage and Scotch–Yoke mechanism. Tandem design for mechanism formed by a rack and non-circular gear has been developed as well. The developed theory is illustrated with seven detailed numerical examples.

The Detection of Flat and Depressed Colorectal Lesions: Which Endoscopic Imaging Approach?

The secondary prevention of colorectal cancer (CRC) assumes that early detection and resection of precursor lesions disrupts the adenoma– carcinoma sequence and halts progression to invasive neoplastic disease. The adenoma– carcinoma sequence has until now formed the rationale for endoscopic therapies directed at reducing the incidence of CRC. The fact that snare polypectomy of exophytic lesions fails to prevent progression to carcinoma in up to 24% of cases has prompted reevaluation of the prevalence and clinicopathologic significance of flat and depressed colorectal lesions in Western population cohorts. Such lesions, although well-described in the Japanese literature, have only recently been reported in Western population cohorts. Debate has continued regarding their prevalence, anatomic localization, histopathologic characteristics, and proposed imaging modality of choice. Within this perspective, we summarize the key data pertinent to recently described endoscopic diagnostic modalities and detail their clinical applicability both for the detection of flat neoplastic lesions occurring in the “sporadic” patient cohort, familial polyposis syndromes, and chronic ulcerative colitis.

Energy Confinement Scaling Predictions for the Stabilized Tandem Mirror

The absence of toroidal curvature and the rel- atively weak internal parallel currents in a tandem mirror gives the system favorable stability and transport proper- ties. GAMMA-10 experiments demonstrate that sheared plasma rotation suppresses turbulent radial losses through control of the radial potential profiles. Recent achieve- ments of the GAMMA-10 include 3 keV ion confinement potentials and Te ‡ 800 eV. Total energy confinement times for the GAMMA-10 experiment exceed by an order of magnitude the corresponding empirical confinement times in toroidal devices. At the temperatures achieved in the GAMMA-10, the end loss rate sp . 100 ms so that radial losses determine sE, as intended in tandem mirror reactor designs. Drift-wave results on radial confinement times developed using Bohm, gyro-Bohm, and electron temperature gradient (ETG) scalings imply that the tandem mirror has a qualitatively different form of drift-wave ra- dial transport from that in toroidal devices. Drift-wave ei- genmodes for the GAMMA-10 are analyzed for the fluctuating electrostatic potential and magnetic perturba- tions.

How irritant is alcohol?

Alcohol-based hand rubs are used worldwide to prevent transmission of nosocomial pathogens. To investigate skin irritation caused by alcohols alone and in combination with detergent washing. Single and repetitive patch testing with 60-100% alcohols [ethanol, 1-propanol, 2-propanol (synonyms: isopropyl alcohol, isopropanol)], a positive control [0.5% sodium lauryl sulphate (SLS)] and negative controls (empty chamber and water) were performed. Wash tests were performed with 80% ethanol and 0.5% SLS on the forearms with each agent alone and with both agents in a tandem design. Skin hydration, erythema and barrier disruption [measured as transepidermal water loss (TEWL)] were evaluated (always 15 volunteers). We found no significant change in skin barrier or erythema induced by the alcohols in the patch tests, whereas skin hydration decreased significantly. Application of alcohols to previously irritated skin did not show a stronger skin barrier disruption than application of SLS alone. Wash tests demonstrated that alcohol application caused significantly less skin irritation than washing with a detergent (TEWL, P < 0.001; skin hydration, P < 0.05; erythema, P < 0.05). Even on previously irritated skin, ethanol did not enhance irritation. By contrast, a protective effect of ethanol used after skin washing was observed (TEWL, P < 0.05; skin hydration, P < 0.05; erythema, P < 0.05). Alcohol-based hand rubs cause less skin irritation than hand washing and are therefore preferred for hand hygiene from the dermatological point of view. An alcohol-based hand rub may even decrease rather than increase skin irritation after a hand wash due to a mechanical partial elimination of the detergent.

A study of commercial vehicle brake judder transmission using multi-body dynamic analysis

Braking-induced forced vibration, known as brake judder in road vehicles, causes dissatisfaction to drivers and passengers and also damage and possible early failure in components and systems. In this paper, the transmission of judder vibration from the point of generation (the brake friction pair) through the vehicle structure to the driver is investigated for the particular case of a heavy commercial vehicle. The investigation uses a computer simulation multi-body dynamic model based on the automatic dynamic analysis of mechanical systems software to identify any characteristics of the vehicle suspension design that might influence the vibration transmission from the wheel to the driver. The model uses a simplified rigid chassis and cab to lump the chassis parameters, so that the investigation can focus on the front axle/suspension design, which is a beam axle leaf spring arrangement, and the rear axle/suspension assembly, which is a tandem axle bogie design. Results from the modelling indicate that brake judder vibration is transmitted to the chassis of the vehicle through a leaf spring ‘wind-up’ mode and a ‘walking’ mode associated with the rear tandem axle. Of particular interest is the longitudinal vibration transmitted through the chassis, since this creates a direct vibration transmission path to the cab and driver. The simulation results were compared with the previously published experimental work on the same design of commercial vehicle, and agreement between the predicted and the measured vibration characteristics and frequencies was found. It is concluded that the rear suspension design parameters could affect the transmission of brake judder vibration to the cab and driver and that a tandem rear axle offers some design opportunity to control the transmission of brake judder vibrations from the wheel to the cab and driver. Given that brake judder has so far defied all attempts to eliminate completely from vehicle brake systems, this is potentially an important opportunity.

Designing efficient methods for the tandem AGV network design problem using tabu search and genetic algorithm

A tandem AGV configuration connects all cells of a manufacturing facility/plant by means of non-overlapping, single-vehicle closed loops. Each loop has at least one additional P/D station, provided as an interface between adjacent loops. This study describes the development of tabu search and genetic algorithm procedures for designing tandem AGV systems. The objective is to minimize the maximum workload of the system. Both algorithms have mechanisms to prevent solutions with intersecting loops. The new algorithms and the partitioning algorithm presented by Bozer and Srinivasan are compared using randomly generated test problems. Results show that for large-scale problems, the partitioning algorithm often leads to infeasible configurations with crossed loops in spite of its shorter running time. However the newly developed algorithm avoids infeasible configurations and often yields better objective function values.