bluuebunny/crossref_metadata_2025 huggingface dataset:
Cooling molecular electronic junctions by AC current
Authors: Preston Riley J. , Honeychurch Thomas D. , Kosov Daniel S.
Published: September 2020

Electronic current flowing in a molecular electronic junction dissipates significant amounts of energy to vibrational degrees of freedom, straining and rupturing chemical bonds and often quickly destroying the integrity of the molecular device. The infamous mechanical instability of molecular electronic junctions critically limits performance and lifespan and raises questions as to the technological viability of single-molecule electronics. Here, we propose a practical scheme for cooling the molecular vibrational temperature via application of an AC voltage over a large, static operational DC voltage bias. Using nonequilibrium Green’s functions, we computed the viscosity and diffusion coefficient experienced by nuclei surrounded by a nonequilibrium ”sea” of periodically driven, current-carrying electrons. The effective molecular junction temperature is deduced by balancing the viscosity and diffusion coefficients. Our calculations show the opportunity of achieving in excess of 40% cooling of the molecular junction temperature while maintaining the same average current.
Sensitivity of pair statistics on pair potentials in many-body systems
Authors: Wang Haina , Stillinger Frank H. , Torquato Salvatore
Published: September 2020

We study the sensitivity and practicality of Henderson’s theorem in classical statistical mechanics, which states that the pair potential v(r) that gives rise to a given pair correlation function g2(r) [or equivalently, the structure factor S(k)] in a classical many-body system at number density ρ and temperature T is unique up to an additive constant. While widely invoked in inverse-problem studies, the utility of the theorem has not been quantitatively scrutinized to any large degree. We show that Henderson’s theorem has practical shortcomings for disordered and ordered phases for certain densities and temperatures. Using proposed sensitivity metrics, we identify illustrative cases in which distinctly different potential functions give very similar pair correlation functions and/or structure factors up to their corresponding correlation lengths. Our results reveal that due to a limited range and precision of pair information in either direct or reciprocal space, there is effective ambiguity of solutions to inverse problems that utilize pair information only, and more caution must be exercised when one claims the uniqueness of any resulting effective pair potential found in practice. We have also identified systems that possess virtually identical pair statistics but have distinctly different higher-order correlations. Such differences should be reflected in their individually distinct dynamics (e.g., glassy behaviors). Finally, we prove a more general version of Henderson’s theorem that extends the uniqueness statement to include potentials that involve two- and higher-body interactions.
Atypical non-Basset particle dynamics due to hydrodynamic slip
Authors: Premlata A. R. , Wei Hsien-Hung
Published: September 2020

Surface slip does not simply reduce drag but strongly influences the behavior of unsteady particle motion. In this work, we revise the Maxey–Riley type equations in conjunction with the modified Faxen laws, showing that slip particles in unsteady motion, even if the amounts of slip are minuscule, can behave markedly different than no-slip particles due to the non-Basset history force and torque. The non-Basset memory kernels here are identified to be of Mittag–Leffler type but featured with the unique slip–stick transition that exists only for partial slip particles but not for full slip bubbles. The impacts especially manifest in the short time regime, illustrated with transient sedimentation, translational response to a suddenly applied stream, and angular response to a torque impulse. In these examples, the translational and angular velocities of a slip sphere are found to vary with time in different powers compared to those of single no-slip spheres. Dynamic distinctions to a spherical bubble can be best revealed by the asynchronous spinning of a slip sphere in an oscillatory vortical flow, showing that an additional inertia torque can arise from slip to give rise to a non-monotonic spinning response when the sphere is lighter than the surrounding fluid. As these non-Basset particle dynamics are rather atypically sensitive to the slip length, the impacts could be crucial to aerosol suspensions and inertial swimming of active hydrophobic particles where slip effects can no longer be negligible. The features might also have potential uses for achieving efficient hydrodynamic sorting of slip particles.
Neural network model for structure factor of polymer systems
Authors: Huang Jie , Li Shiben , Zhang Xinghua , Huang Gang
Published: September 2020

As an important physical quantity to understand the internal structure of polymer chains, the structure factor is being studied both in theory and experiment. Theoretically, the structure factor of Gaussian chains has been solved analytically, but for wormlike chains, numerical approaches are often used, such as Monte Carlo simulations, solving the modified diffusion equation. In these works, the structure factor needs to be calculated differently for different regions of the wave vector and chain rigidity, and some calculation processes are resource consuming. In this work, by training a deep neural network, we obtained an efficient model to calculate the structure factor of polymer chains, without considering different regions of wavenumber and chain rigidity. Furthermore, based on the trained neural network model, we predicted the contour and Kuhn lengths of some polymer chains by using scattering experimental data, and we found that our model can get pretty reasonable predictions. This work provides a method to obtain the structure factor for polymer chains, which is as good as previous and more computationally efficient. It also provides a potential way for the experimental researchers to measure the contour and Kuhn lengths of polymer chains.
Recent advances in the catalytic cyclopropanation of unsaturated compounds with diazomethane
Authors: Menchikov Leonid G. , Shulishov Evgeny V. , Tomilov Yury V.
Published: September 2020

The main achievements and development trends of the past 10–15 years related to the catalytic cyclopropanation of unsaturated compounds with diazomethane are integrated and analyzed. The attention is focused on the most efficient catalysts based on palladium compounds. Data on the effects of substrate structure and nature of catalyst components on the regio- and stereoselectivity of these reactions are systematized. Characteristic features of safe methods for diazomethane generation are considered, including the use of membrane technologies and continuous-flow andin situpreparation methods, which have prospects for industrial application.

The bibliography includes 281 references.
The finger flexors occlusion threshold in sport‐climbers: an exploratory study on its indirect approximation
Authors: Bergua Pedro , Montero‐Marin Jesús , Gomez‐Bruton Alejandro , Casajús José A.
Published: September 2020

Blood flow partially determines specific climbing endurance (SCE) as it mediates oxygen bio‐availability in the finger flexors. Blood flow is related to occlusion threshold (OT), which is defined as the contraction intensity at which intramuscular pressure exceeds perfusion blood pressure resulting in the cessation of local blood flow. The OT is represented as an inflection point on a force‐time graph when isometric force is registered and applied through maximal and continuous tests. Endurance time (ET) to exhaustion is influenced by the relative isometric applied force and is different for each climber. The aim of this study was to explore whether an approximation of the finger flexoŕs OT in sport climbers through records of ET to exhaustion at different isometric relative intensities was possible. We measured maximum finger hang ETs at 6 intensities ranging from 85% to 35% maximal force in 34 sport climbers of advanced and elite level. The values obtained were analysed by two different methods in an attempt to determine a change in the shape of the curve in the intensity‐ET relationship graphs that approximated the OT for each climber. The results suggest that the finger flexoŕs OT could be different among climbers, regardless of their strength and ability level. The presented methods do not accurately reflect the OT, but could indicate the intensity at which blood flow is restored in the active muscles. This is the first study to indirectly approximate the finger flexors OT in sport‐climbers, a parameter that could be essential to assess SCE.
Nesprin-2G tension fine-tunes Wnt/β-catenin signaling
Authors: Gottardi Cara J. , Luxton G.W. Gant
Published: September 2020

How LINC complexes mediate nuclear mechanotransduction remains unclear. In this issue, Déjardin, Carollo, et al. (2020. J. Cell Biol.https://doi.org/10.1083/jcb.201908036) show that the LINC complex protein nesprin-2G is a mechanosensor of epithelial–mesenchymal transitions (EMTs), recruiting α-catenin to the nucleus to attenuate Wnt/β-catenin signaling.
Immunotherapy during the acute SHIV infection of macaques confers long-term suppression of viremia
Authors: Nishimura Yoshiaki , Donau Olivia K. , Dias Joana , Ferrando-Martinez Sara , Jesteadt Eric , Sadjadpour Reza , Gautam Rajeev , Buckler-White Alicia , Geleziunas Romas , Koup Richard A. , Nussenzweig Michel C. , Martin Malcolm A.
Published: September 2020

We report that combination bNAb immunotherapy initiated on day 3 post-infection (PI) maintained durable CD8+ T cell–mediated suppression of SHIVAD8 viremia and preinoculation levels of CD4+ T cells in 9 of 13 treated monkeys during nearly 6 yr of observation, as assessed by successive CD8+ T cell–depletion experiments. In an extension of that study, two treatment interventions (bNAbs alone or cART plus bNAbs) beginning on week 2 PI were conducted and conferred controller status to 7 of 12 monkeys that was also dependent on control mediated by CD8+ cells. However, the median time to suppression of plasma viremia following intervention on week 2 was markedly delayed (85 wk) compared with combination bNAb immunotherapy initiated on day 3 (39 wk). In both cases, the principal correlate of virus control was the induction of CD8+ T cellular immunity.
Enabling Compact Thermal Systems through Insights on Microscale Coupled Heat and Mass Transfer Phenomena
Authors: Garimella S
Published: September 2020

Heat transfer in microscale geometries offers the advantage of high heat transfer coefficients and large surface area-to-volume ratios, and therefore, the potential to develop compact heat and mass transfer devices and systems. This paper provides an overview of a variety of applications ranging from tens of Watts to Megawatts of heat transfer rates that can benefit from these insights into coupled heat and mass transfer. Practical challenges and innovations in implementing microscale heat exchangers are briefly discussed. This is followed by examples that include wearable cooling systems, thermally driven heat pumps, cooling of aircraft carriers, carbon capture and methane gas cleaning using microscale hollow fibers, and the incorporation of microchannel heat exchangers into electrochemical storage systems and nuclear reactor cores.
Thermal Performance Investigation by Infrared Analysis of Mini Pulsating Heat Pipe
Authors: Bonfiglio C , Bozzoli F , Cattani L , Iwata N , Rainieri S. , Vocale P
Published: September 2020

A promising solution in the field of passive two-phase heat transfer devices is represented by Pulsating Heat Pipes (PHPs). They are undoubtedly appealing due to the high heat transfer capability, efficient thermal control, adaptability and low cost. In the last years they are raising concern for space applications that are characterised by extreme environmental conditions, strictly constrains in terms of compactness, reliability and the need to dissipate efficiently heat in microgravity conditions. In this study, the thermal performance of oscillating heat pipes that consists of extra-thin metallic pipes are investigated: the adoption of metallic pipes with an inner diameter less than 0.4 mm permits to couple flexibility and compactness with high heat transfer performance. HFC-134a is used as working fluid. Many authors have investigated the pulsating behaviour of this type of heat transfer devices only considering the average temperature of the evaporator and condenser. In this work, to deeply investigate the oscillating behaviour of the proposed PHP, it is adopted an approach based on the study of the local temperature distributions on the wall of the PHP, acquired with a high-speed and high-resolution infrared camera. The local analysis of the temperature trends is of fundamental importance in the understanding of the complex phenomena that govern the pulsating field.
Experimental investigation of two-side heat transfer in spacer-filled channels
Authors: Ciofalo M , Cacciatore F , Di Liberto M , Tamburini A
Published: September 2020

In Membrane Distillation (MD), spacers support the membranes and promote mixing, thus reducing temperature polarization. Their efficient design requires a knowledge of the distribution of the local heat transfer coefficient h and of its dependence on Reynolds number, spacer geometry and flow-spacer relative orientation. In previous work, we applied Thermochromic Liquid Crystals (TLC) and digital image processing to the measurement of h distributions for different spacer configurations; data were used to validate CFD simulations and select turbulence models. For constructive reasons, the test section allowed only one-side heat transfer, while in most MD configurations (e.g. spiral-wound modules) heat transfer occurs from both sides of the feed water channels. Analytical and numerical solutions show that changing from one-side to two-side heat transfer deeply affects h values. This motivated the design and construction of an improved test section in which a hot channel is sandwiched between two cold channels, and twin cameras and lighting equipment allow the simultaneous acquisition of TLC images on both walls. This paper describes this new test section and the experimental technique, discusses measurement uncertainty, and presents preliminary results.
Numerical prediction of dropwise condensation performances on hybrid hydrophobic-hydrophilic surfaces
Authors: Croce Giulio , Suzzi Nicola , D’Agaro Paola
Published: September 2020

The use of hybrid surfaces, alternating hydrophobic and hydrophilic (or less-hydrophobic) areas, has been recently proposed as a valuable tool for dropwise condensation performances enhancement. The hydrophilic region, in particular, is designed in order to promote the removal of the droplet from the hydrophobic region, reducing their average size and thus increasing the heat transfer performance. Here, dropwise condensation on heterogeneous surfaces composed by a texture of alternating hydrophobic and hydrophilic (or less-hydrophobic) vertical strips is numerically investigated through a Lagrangian-based phenomenological model, originally developed to study droplet evolution in the framework of in-flight icing phenomenon. The effects of droplet nucleation, growth, coalescence and motion are implemented. The model is extended to the case of curved surface. In order to properly assess the hybrid surface, however, a better knowledge of the interaction between the droplet and the film is required. Thus, additional numerical simulations are carried out using an in-house Eulerian full solver of the liquid film layer, in order to understand the migration mechanism of a drop between two different wettability regions. This will give useful information for a proper implementation of the effect of hydrophilic strips in the phenomenological Lagrangian model. A parametric analysis allows to find the optimal geometric configuration of the hybrid pattern, leading to maximum heat transfer performance, for a given set of wettability properties, nucleation density and condensation rate per unit area. Comparison with experimental data from the available literature ensures the capability of the current Lagrangian model to properly catch the physics behind dropwise condensation process on both planar and curved hybrid surfaces. The uncertainty of the estimate of some model input parameter and their influence on the computed solution are be also discussed.
Flow boiling heat transfer characteristics of R245fa refrigerant in a plate heat exchanger
Authors: Ghorbani Tari Z
Published: September 2020

Plate heat exchangers (PHEs) are used extensively in industrial applications and, owing to their compactness, and higher thermal efficiency, they are keys component of organic Rankine cycle (ORC) application. This study presents the experimental heat transfer characteristics during flow boiling of R245fa refrigerant with a commercial working fluid used in ORCs; inside brazed plate heat exchanger with chevron angle of 65 degree. The flow boiling heat transfer characteristics were measured with varying saturation temperatures, mass flux and heat flux, which range from 55.5 °C-61.8 °C, 15.5-17.4 kg m−2 s−1, and 6400-10120 W m−2, respectively. The experimental results showed that flow boiling heat transfer coefficient is dependent upon the heat flux and mass flux. The results showed that the heat transfer coefficient increases with the increase of heat flux, and then starts to decrease due to local dry out. However, at low mass flux, locally triggering dry out was more prominent than that high mass flux. The heat transfer coefficient showed to be sensitive to the change in the saturation temperature. Moreover, flow boiling heat transfer coefficient showed a linear relationship with mass flux of the refrigerant.
Simulations of paraffine melting inside metal foams at different gravity levels with preliminary experimental validation
Authors: Iasiello M , Mameli M , Filippeschi S , Bianco N
Published: September 2020

In this work, the results of a numerical code based on the porous media Local Thermal Non-Equilibrium (LTNE) and the apparent heat capacity methods, are compared with experiments aiming at a preliminary validation. The test cell consists in a 50 mm aluminum foam cube filled with a paraffin wax, heated and cooled on the same face. The heat flux is measured by two miniaturized sensors, while the temperature is measured in three different locations along the cube edge. Finally, one side is equipped with a Zinc Selenide window which is transparent to the long wave InfraRed. This system allows to track the paraffin melting front evolution together with the temporal trend of the whole temperature distribution simplifying the comparison with the numerical outputs at different time steps.

The numerical model is then set with the same boundary conditions (heat flux) to predict the experimental temperature fields, considering both conduction in the solid domain and natural convection in the liquid domain. The preliminary validation shows that the numerical results match the experimental data with good agreement. Results are also presented for different gravity levels. This study can be a starting point for all those applications where gravity has a major role.
Influencing factors on flow boiling of carbon dioxide in enhanced tubes and comparison with correlations
Authors: Mastrullo R , Mauro A W , Thome J R , Vanoli G P , Viscito L
Published: September 2020

Carbon dioxide two-phase flow characteristics are different from those of conventional refrigerants, due to the CO2particular thermodynamic and transport properties obtained by working at high reduced pressures. Moreover, the use of peculiar heat transfer surfaces such as grooves and internal fins are often preferred to enhance the boiling heat transfer performance. This paper collects CO2flow boiling heat transfer coefficient data from different independent databases available in scientific literature, regarding both smooth and enhanced geometries and a wide range of operative conditions, that are typical of refrigeration systems and heat pumps. The database for enhanced tubes covers internal diameters from 0.8 to 8.92 mm, saturation temperatures from -30 to +20 °C, imposed heat fluxes from 1.67 to 60 kW/m2and mass velocities from 75 to 800 kg/m2s, collecting more than 800 points. Heat transfer data for smooth and enhanced surfaces under the same conditions are collected, in order to measure the enhancement and to correlate it to the geometry augmentation. The assessment of quoted prediction methods explicitly developed for carbon dioxide is finally carried out, with a proposal for a correction factor.
Average Friction Factors of Choked Gas Flow in Microtubes
Authors: Kang D , Hong C , Rehman D , Morini G L , Asako Y , Faghri M
Published: September 2020

A micro-tube passage is a basic and important element in the design of micro heat exchangers and for this reason during the last decade a series of investigations have been made with the aim to clarify the main scaling effects playing an important role in microtubes. In this paper, a combined analysis of numerically and experimentally obtained average friction factors in microtubes under the situation of under-expanded (choked) gas flow is presented. The working fluid (nitrogen) passes through the microtube and discharges into the atmosphere under an increasing inlet pressure. Experiments and numerical computations are performed for microtubes with 249 and 528.9 mm in diameter, by varying the aspect ratio (i.e. length/diameter) from 100 to 200. The numerical methodology to solve the governing equations is based on the Arbitrary-Lagrangian-Eulerian (ALE) method. In order to capture the under-expansion characteristics of the flow during choking, the computational domain is extended in the downstream region beyond the microtube outlet. Both experimental and numerical results were obtained for a wide range of Mach number and Reynolds number. In the previous study, it was demonstrated how the outlet Mach number can be expressed as a function of the tube diameter under choked conditions. In this paper, a data reduction procedure for the estimation of the average friction factor between the inlet and the outlet of the microtube is proposed for choked flows in which the outlet gas temperature and pressure are obtained by using the outlet Mach number calculated numerically as a function of the microtube diameter. It is demonstrated how this data reduction method allows an accurate calculation of the average friction factors in microtubes by using a limited number of parameters which are easy to measure. The results obtained in this way are in good agreement with the numerical predictions as well as with the most common empirical correlations.
Influence of moisture content, temperature and absorbed solar radiation on the thermal performance of a spruce XLAM wall in the Italian climates
Authors: Danovska M , Pernigotto G , Baratieri M , Baggio P , Gasparella A
Published: September 2020

In building simulation codes, conduction transfer functions or finite difference methods are generally implemented to model the heat transmitted through the opaque components. Thermal conductivity is conventionally assumed constantly equal to its nominal value, even if it depends on both temperature and moisture content, especially for porous building materials, like timber. Therefore, the adoption of constant nominal values can bring inaccuracies on the calculated heat flux, affecting the predicted building energy performance. In this research, the magnitude of such inaccuracies is quantified for a spruce cross-lam (i.e. XLAM) wall, by comparing hourly specific heat fluxes calculated with variable thermal conductivity and those with nominal one. To achieve such a goal, a MATLAB® 1D heat and mass transfer model was developed according to the finite difference method. Material’s thermal conductivity was characterised as a function of moisture content and temperature. Finally, the developed model was calibrated and validated against experimental data collected in the Building Physics Laboratory of the Free University of Bozen-Bolzano. Annual simulations were run considering 110 Italian climates and different orientations and inclinations of the wall. Constant internal boundary conditions and hourly variable external conditions were adopted; in particular, external solicitations were expressed through the sol-air temperature definition.
Thermal Efficiency of a Concentrating Solar Collector Under High-Vacuum
Authors: De Maio D , Alessandro C D’ , De Luca D , Musto M , Di Gennaro E , Rotondo G , Russo R
Published: September 2020

A new frontier in solar thermal panel technology can be a high vacuum collector, thick enough to be equipped with solar concentrators based on non-imaging optics, such as the Compound Parabolic Concentrators (CPC). The high vacuum technology guarantees higher operating temperatures thanks to the enhanced thermal insulation, which leads to pay particular attention to the absorber radiative emission. In this paper, by means of numerical simulations, we compare the efficiency of a flat selective solar absorber under high vacuum to the efficiency of a CPC under high-vacuum collector.
Modelling of Natural Convective Heating of a Standard Wet Brick for Oven Energy Consumption Tests
Authors: Lucchi M. , Suzzi N. , Lorenzini M.
Published: September 2020

The environmental impact of buildings is directly influenced by low-efficiency appliances like electric ovens. Their energy class is estimated through a test prescribed by the EN 60350 European standard, where a wet clay brick is heated under set conditions and the energy consumption is measured; throughout the test, particular attention is devoted to the control of air temperature. In this work, a transient model of the oven suitable for control design was realized including the heat and mass transfer within the brick. A lumped-parameter approach was used to ensure good predictive properties and low computational cost. The dynamics of the cavity air and the Pt500, which is used in normal operation to maintain the desired set point, was also included in the model. Model parameters were determined through an optimization procedure based on the experimental data collected during energy consumption tests with the oven in natural convective heating mode. The model was then validated. Good results were obtained in the prediction of both temperature and heating time.
Experimental and modelling evaluation of possible solutions for compact design of producer-gas heat exchangers
Authors: Morselli N , Puglia M , Mason J , Parenti M , Ottani F , Tartarini P
Published: September 2020

An experimental and numerical campaign was carried out to investigate the performances of tubes in shell heat exchanger applied to cool down the producer gas of a small-scale commercial wood biomass gasifier built by All Power Labs. Producer gas is contaminated with soot and tars that condensate under 300 °C, for this reason the heat exchanger was designed in order to have an in-situ cleaning mechanism. The heat exchanger was tested both with standard plain tubes and with a set of metal twisted tapes (TT) with the aim of enhancing the heat transfer between producer gas and water. Temperatures and mass flows analyses shown a maximum increase of thermal power output and overall heat transfer coefficient respectively of 19% and 76% when TT are applied to the standard plain tubes in shell heat exchanger. Numerical simulations shown a consistency in the trends giving an average discrepancy with experimental results of the 12%.
Urban Building Energy Modeling: an hourly energy balance model of residential buildings at a district scale
Authors: Mutani G , Todeschi V
Published: September 2020

The energy consumption of buildings is related to several factors, such as the construction and geometric characteristics, occupancy, climate and microclimate conditions, solar exposure, and urban morphology. However, the interaction between buildings and the surrounding urban context should also be taken into consideration in energy consumption models. The aim of this work has been to create a bottom-up model in order to evaluate the energy balance of residential buildings at an urban scale, starting from the hourly energy consumption data. This modeling approach considers the building characteristics together with urban variables to describe the energy balance of the built environment; it can therefore be used to manage heterogeneous types of data at different scales and it can offer accurate spatial-temporal information on the energy performance of buildings. Detailed heat balance methods can be used at a building scale to estimate heating loads, but this urban-scale simplified model can also be used as a decision tool to support urban design explorations and for policy purposes. This urban energy consumption model was verified for a case study of a district in Turin, Italy, with the support of a GIS tool, considering hourly energy consumption data of about 50 residential users for two or three consecutive heating seasons. The results show that a simplified model, based on low quality and quantity data, which are typical of an urban scale, can be a powerful tool for the evaluation and spatial representation of the energy needs of buildings at an urban scale.
Load-shifting strategies in district heating networks with constant supply temperature: the case study of Verona
Authors: Vivian J , Quaggiotto D , Ploner E , D’Isep F , Zarrella A
Published: September 2020

District heating and cooling networks are a key infrastructure to decarbonise the heating and cooling sector. Besides the design of new networks according to the principles of the 4th and 5th generation district heating, operational aspects may significantly contribute to improve the efficiency of existing networks from both economic and environmental standpoints. In the present work, computer simulations using a district heating network model (NeMo) were carried out to investigate the load shifting potential offered by the thermal capacitance of the water volume enclosed in the network pipelines. The average return temperature is increased before the occurrence of the peak load by adjusting the flow rate according to the forecasted heat demand for the upcoming hours. The effectiveness of this control strategy was demonstrated for the district heating network of Verona, proving that the peak load can be significantly reduced, especially during middle season. Finally, the load shifting strategy was used to allow a change in the heat production mix, thereby reducing the primary energy consumption of the main heat supply station during the considered period.
A multiscale fluid structure interaction model derived from Koiter shell equations
Authors: Chierici A , Chirco L , Giovacchini V , Manservisi S , Santesarti G
Published: September 2020

In this work we propose the numerical simulation of fluid structure interaction (FSI) problem by using a membrane model, derived from the Koiter shell equations. With this approach the thickness of the solid wall can be neglected, with a meaningful reduction of the computational cost of the numerical problem. The fluid structure problem is then reduced to the fluid equations on a moving mesh together with a particular Robin boundary condition imposed on the surface corresponding to the solid moving wall. Furthermore an artificial absorbing outflow boundary condition has been implemented in order to reduce the damping and reflections of the pressure waves at the domain’s outlet. This model is implemented and solved with an in-house finite elements code, and tested through axisymmetric cases that show the robustness of the developed algorithm. Finally, we report a comparison of the implemented model with results of a FSI monolithic model, based on non-linear incompressible structure.
An adjoint-based temperature boundary optimal control approach for turbulent buoyancy-driven flows
Authors: Chirco L , Giovacchini V , Manservisi S
Published: September 2020

This paper deals with the adjoint optimal control for turbulent buoyancy-driven flows. The aim of this optimal control problem is to obtain a desired velocity profile and enhance the turbulence intensity in a well defined region by controlling the fluid temperature on domain boundaries and consequently the buoyancy forces. The fluid is assumed to be incompressible within the Boussinesq approximation, while turbulence is considered by coupling the Wilcox k-ω model with the Reynolds Averaged energy and Navier Stokes equations. The state, adjoint and control equations are derived by employing the Lagrangian multipliers method. The optimality system is solved with a finite elements code where a steepest descent algorithm has been implemented in order to find the optimal boundary control parameter. Numerical results are reported to show the robustness of the method in solving strongly-coupled optimality systems with a large number of unknowns.
Development and calibration of a 1D thermo-fluid dynamic model of ventilation in tunnels
Authors: Cingi Pietro , Angeli Diego , Cavazzuti Marco , Levoni Paolo , Cipollone Maurizio
Published: September 2020

In complex, large civil infrastructures where ventilation has a crucial role for the safety of users in both normal operation and hazardous scenarios, the correct prediction of flow and heat transfer parameters is of fundamental importance. While full 3D simulation is applicable only to a limited extent, and the resort to 1D modeling is a common practice in both design and evaluation phases, the limitation of such models lies in the choice of transfer parameters, such as friction loss coefficients and heat transfer coefficients. In this work, an original approach based on the Finite Volume integration of the 1D flow and energy equations is presented. Such equations are to be solved on a network of ducts, representing the ventilation system in the 11.6 km long Mont Blanc Tunnel with a spatial resolution of 10 m. A preliminary calibration of a set of friction loss coefficients against a rich experimental dataset collected throughout a dedicated set of in situ tests is of particular concern here, as it is carried out by means of genetic optimization algorithms. Predictions of the flow field are in remarkable agreement with the experimental data, with an overall RMS error of ± 0.42 m/s. Further refinements and possible parameter choices are also discussed.
A simple test method for measurement of the interface thermal resistance of coated and uncoated metal surfaces
Authors: Pedrazzi S , Vergnano A , Allesina G , Veronesi P , Leali F , Tartarini P , Muscio A
Published: September 2020

Molds for metal casting are often internally coated in order to minimize the interaction between the steel surface of the mold and the melted metal during its pouring and solidification. However, the added coating increases the thermal interface resistance during the cooling process and it can thus affect the process itself. For example, numerical predictions and experimental results are matched only if the interface resistance is even quintupled in the presence of the usual interface coating, compared to the uncoated mold. In order to have a reasonable estimate of the interface thermal resistance, an easy to use measurement approach has been tested. This consists of contacting a cold and a hot samples of metals with known thermal properties, one at ambient temperature and the other one much hotter. The temperatures of the two samples, assumed to be uniform in each sample, are measured by thermocouples placed inside the samples through a hole, and the interface resistance is calculated from the time evolution pattern of those temperatures during the thermal transient that follows the instant in which the samples have been contacted.
Condensation and vaporization heat transfer of low-GWP mixtures
Authors: Bortolin S , Azzolin M , Berto A , Guzzardi C , Del Col D
Published: September 2020

The present work investigates the heat transfer performance of non-azeotropic ternary low-GWP (Global Warming Potential) mixtures during condensation and vaporization inside a 8.0 mm inner diameter channel. The tested mixtures are R455A, which is a ternary blend of R32, R1234yf and R744 (21.5/75.5/3.0% by mass composition), and R452B, which is a ternary blend of R32, R1234yf and R125 (67.0/26.0/7.0% by mass composition). R455A presents a GWP100-years value equal to 146 and a temperature glide of 10 K at pressure p=17.6 bar, while R452B is characterized by higher GWP, equal to 676, but lower temperature glide, about 1 K at p=22.9 bar. Condensation tests have been run with the two mixtures R455A and R452B at a mean saturation temperature (average value between dew point and bubble point temperatures at constant pressure and given composition) equal to 40 °C and mass velocity ranging between 100 and 600 kg m−2 s−1. Flow boiling tests have been performed at around 18 °C mean saturation temperature and mass velocity ranging between 200 and 400 kg m−2 s−1. The effects of vapor quality, mass velocity and heat flux on the heat transfer coefficient are investigated. Moreover, the adoption of zeotropic mixtures poses the problem of how to extend the correlations developed for pure fluids. In the present paper, the contribution of the mass transfer resistance is assessed and the corrections needed in the models are discussed.
Effect of Refrigerant Properties Estimation on the Prediction Capabilities of Well-Established Two-Phase Heat Transfer and Pressure Drop Models for New Refrigerants
Authors: Righetti G , Akasaka R , Bobbo S , Brown J S , Fedele L , Higashi Y , Longo G A , Mancin S , Zilio C
Published: September 2020

R1233zd(E) and R1224yd(Z) are two HCFO (Hydro Chloro Fluoro Olefin) low-GWP refrigerants recently proposed as substitutes of the HFC (Hydro Fluoro Carbon) fluids commonly used in air conditioning, high temperature heat pumps and ORC applications. A few experimental works regarding the two phase heat transfer in small diameter smooth and microfin tubes have already been published in the open literature. The estimations of thermophysical properties may have a remarkable impact on the prediction of the heat transfer coefficients and pressure drops, especially during two-phase heat transfer. The thermophysical properties databases and predictive tools for these refrigerants are continuously being updated and improved as additional, accurate measurements are published in the open literature. The present paper aims to highlight the impacts of thermophysical properties prediction models on the consistency of estimations of heat transfer coefficient and pressure drop using correlations published in the open literature. As a benchmark, some experimental heat transfer and pressure drop data collected for two new HCFOs, R1233zd(E) and R1224yd(Z) are used. The experimental database has been collected at the Department of Management and Engineering of the University of Padova. For the thermophysical properties estimations, different Equations of States contained in REFPROP as well as a more “simplified” approach based on group contribution methods coupled with a Peng-Robinson Equation of State, are considered.
An integrated CO2 unit for heating, cooling and DHW installed in a hotel. Data from the field.
Authors: Tosato Giacomo , Girotto Sergio , Minetto Silvia , Rossetti Antonio , Marinetti Sergio
Published: September 2020

This paper presents a new heat pump unit intended to provide heating, cooling and hot water to a hotel and using CO2 as the working fluid. The heat pump was installed in mid-2018 in a hotel located in a touristic area in North Italy and open nearly all over the year. The unit can benefit from ground water as heat source or heat sink. The heat pump features a two-phase multi-ejector as expansion device. An original two-evaporator lay-out is implemented, where the first one is gravity driven and the second one is ejector driven. It is fully equipped with measurement instruments, data acquisition system and cloud data storage, developed and tailored for the unit. The unit was developed, instrumented, installed and monitored within the H2020 MultiPACK project, which aims at building confidence in integrated heating, ventilation, air conditioning and refrigeration packages based on CO2 technology in high energy-demanding buildings. In this paper, operational parameters and performance data will be presented, related to winter season, thus to heating and domestic hot water service.
Experimental analysis of a CO2 heat pump for instantaneous domestic hot water production
Authors: Trinchieri R , Pieve M , Boccardi G , Calabrese N , Rovella P , Saraceno L
Published: September 2020

The performance of a CO2 (R744) heat pump (HP) depends strongly on the application and on the environmental operating conditions. In particular, the CO2 HP becomes competitive compared to traditional heat pumps using halogenated refrigerants in applications characterized by a high hot water demand (hospitals, sport centres, etc.). This paper deals with the experimental analysis of an air-to-water commercial R744 HP tested in ENEA laboratories for instantaneous production of domestic hot water (DHW). The results show that HP efficiency is strongly affected by the inlet water temperature at the gas cooler. A test equipment capable of maintaining the ambient temperature at a desired value (between -10 °C and +20 °C), and managing water flow and water temperature (between 15 °C and 45 °C) is used to evaluate the machine COP as a function of the boundary conditions (inlet gas cooler water temperature, water flow rate, external ambient temperature). The analysis shows the considerable potential of R744 heat pumps for the instantaneous production of DHW, combining a high performance and relevant energy saving. As well, it points out that, to maximize the COP it is necessary to combine the HP with a high stratification storage system, which ensures a suited inlet water temperature, and with a correct value of water flow.
The impact of the fiber orientation angle on the dispersion characteristic of guided waves for multilayered GLARE composites
Authors: Barski M , Muc A
Published: September 2020

The present paper is devoted to the parametric analysis of the impact of the fiber orientation angle on the dispersion relations of elastic waves in the case of GLARE composite material. The studied material consists of 13 layers. The layers are made of aluminum alloy AW-6060 and glass fiber/epoxy resin TVR380M12/R-glass. It is assumed that general configuration of the GLARE composite is as follows: [Al, +θ, −θ, +θ, −θ, +θ, Al, +θ, −θ, +θ, −θ, +θ, Al] and the angle θ is the parameter, which values are taken from the following range [0°, 90°]. In order to determine the dispersion curves the stiffness matrix method is applied. Moreover, the special computer application is created with the use of C++, where the multithreading mechanism is adopted to significantly reduce the time of calculations. As a result of the variations of the fiber orientation angle θ the change of the phase velocity of the fundamental symmetric and shear horizontal mode is mainly observed. The sensitivity of the other modes on the angle θ is not significant.
Buckling Dynamics in Crystalline Membranes
Authors: Pezzutti Aldo D. , Hernández Hugo
Published: September 2020

We study the dynamics of a flexible crystalline membrane with hexagonal pattern. The relaxational dynamics towards equilibrium is described through a Brazovskii-Helfrich-Canham Hamiltonian model. The coupled dynamics between the hexagonal structure and the deformation modifies the elastic properties of the membrane. In this paper, we explore the properties of this coupling by analyzing the structure factor of the deformation out of the plane of the considered membrane system.
Divergence and Flutter of Multilayered Laminated Structures
Authors: Flis J , Augustyn M , Muc A
Published: September 2020

The aim of the present paper is to describe and discuss on possible methods used in the divergence and flutter analysis of composite multilayered laminated structures. Dynamic instability is a very dangerous phenomenon and aeroelastic optimization has become an important element of the composite flight vehicle design process. A special attention is focused on the analysis of cylindrical panels and flat plates using numerical (finite element) methods. The structures with different flow angles, fibre orientations, stacking sequences and boundary condition are considered.
Damage detection in composite structures by multipoint measuring system using guided wave propagation method.
Authors: Stawiarski A , Muc A
Published: September 2020

In this paper, the elastic wave propagation method was used in damage detection in composite cylindrical panels. The multipoint measuring system based on surface-mounted piezoelectric transducers was applied in the multistage measurement of the elastic wave propagation. The damage index definition based on the correlation coefficient was used to determine the state of the structure including localization and estimation of the damage size. Application of the PZT elements as actuators and sensors allows building a low-cost damage detection system that can be used in structural health monitoring (SHM) of composite parts. The resolution of the damage detection results depends on the number and localization of the sensors. However, the application of the multistage wave propagation analysis makes possible to improve the damage detection accuracy without raising the SHM system costs.
Identification of the oculo-motor system based on the Volterra model using eye tracking technology
Authors: Pavlenko Vitaliy D. , Milosz Marek , Dzienkowski Mariusz
Published: September 2020

Instrumental algorithmic and software tools for constructing a nonparametric dynamic model of the human oculo-motor system (OMS) based on its inertial and nonlinear properties are developed in the paper on the basis of the experimental studies data of «input-output» in the form of the Volterra model. Taking into account the specificity of the object investigated, test multistage signals (visual stimulus) were used for identification. Based on the experimental data obtained using the developed computational algorithms and data processing software, a nonparametric dynamic model of OMS in the form of a transition function and transition functions of the 2nd and 3rd orders is constructed. Verification of the constructed model showed its adequacy to the object investigated – a practical coincidence (within the acceptable error) of the object and model feedback for the same test effect.
Piezoelectric Glove Design and Test for Future Wearable Devices
Authors: Ulisse I. , Colaiuda D. , Ricci S. , Ferri G.
Published: September 2020

We present the first results of experimental measurements performed to estimate the available energy derived from the movement of the fingers of one hand. The proposed system consists of a mechanical hand made with a 3D printer, on which 5 piezoelectric strips are applied, electrically connected in series or in parallel. Measurements have shown that the series configuration produces the best result. In this configuration, in fact, the open circuit peak-to-peak voltage reaches 39.6V.
A review of simplified servovalve models for digital twins of electrohydraulic actuators
Authors: Berri P C , Dalla Vedova M D L
Published: September 2020

The development and detail design of complex electrohydraulic actuators for aircraft flight controls require the use of accurate, high fidelity fluid-dynamic simulations in order to predict the behaviour of the system within its whole operating envelope. However, those simulations are usually computationally expensive, and simplified models are useful for the preliminary design phases and real-time health monitoring. Within this context, this work presents a review of low fidelity models for the fluid-dynamic behaviour of an electrohydraulic servovalve. Those are intended to run in real time as digital twins of the physical system, in order to enable the execution of diagnostic and prognostic algorithms. The accuracy of the simulations is assessed by comparing their results against a detailed, physics-based high fidelity model, which computes the response of the equipment accounting for the pressure-flow characteristics across all the internal passageways of the valve.