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Based on a prototype presented in a prior publication, this research investigates the operational characteristics of a methane-fueled solid oxide fuel cell (SOFC) system with anode off-gas recirculation (AOGR) for electrical energy supply on sea-going vessels. The proposed first-principle system model utilizes a spatially segmented SOFC stack and lumped balance of plant components validated on the component level to accurately depict the steady-state and transient operating behavior. Five operational limitations are chosen to highlight permissible operating conditions with regard to stack and pre-reformer degradation. Steady-state operating maps are presented, emphasizing efficient operating conditions at maximum stack fuel utilization and minimal permissible oxygen-to-carbon ratio. Exemplary transient load changes illustrate increasing system control complexity caused by gas flow delays due to the spatially distributed plant layout. Actuation strategies are presented and underline the need for a top-level model predictive system controller to assure a dynamic and efficient operation within the defined constraints. [ABSTRACT FROM AUTHOR]

Two different additively manufacturable metal structures are evaluated with regard to their entropy production number. The structures are investigated at two temperature levels: 200/80 °C and 700/580 °C. At the low temperature level, the heat conduction within the structure is the main driver for entropy production. If the temperatures are raised, the total entropy production number increases sharply, where the dissipation due to fluid friction is the main cause for the losses occurring in both structures. [ABSTRACT FROM AUTHOR]

A computer‐aided process design methodology is used to determine the limits of NH3/H2O mixtures in different heat pump cycles from 20 °C to 150 °C. The evaluation is based on a defined parameter set consisting of the coefficient of performance, total heat transfer area and volumetric heat capacity. Simple cycles with two heat flows were chosen to meet good process integration capabilities. The results are shown in a sink outlet/temperature lift matrix considering current technical limits. R1366mzz(Z) in a standard compression cycle with internal heat exchanger serves as a benchmark. To provide temperatures up to 150 °C the lift of the heat pump has to increase to about 80 K of which only the wet compressions cycle with NH3/H2O is capable of. The working domains of NH3/H2O and R1366mzz(Z) are similar, when a two‐stage compression for NH3/H2O is applied. [ABSTRACT FROM AUTHOR]

The freedom of additive manufacturing allows for the production of heat-transferring structures that are optimized in terms of heat transfer and pressure loss using various optimization methods. One question is whether the structural optimizations made can be reproduced by additive manufacturing and whether the adaptations can also be verified experimentally. In this article, adjoint optimization is used to optimize a reference structure and then examine the optimization results experimentally. For this purpose, optimizations are carried out on a 2D model as well as a 3D model. The material chosen for the 3D optimization is stainless steel. Depending on the weighting pairing of heat transfer and pressure loss, the optimizations in 2D result in an increase in heat transfer of 15% compared to the initial reference structure with an almost constant pressure loss or a reduction in pressure loss of 13% with an almost constant heat transfer. The optimizations in 3D result in improvements in the heat transfer of a maximum of 3.5% at constant pressure loss or 9% lower pressure losses at constant heat transfer compared to the initial reference structure. The subsequent experimental investigation shows that the theoretical improvements in heat transfer can only be demonstrated to a limited extent, as the fine contour changes cannot yet be reproduced by additive manufacturing. However, the improvements in pressure loss can be demonstrated experimentally following a cross-section correction. It can therefore be stated that with increasing accuracy of the manufacturing process, the improvements in heat transfer can also be utilized. [ABSTRACT FROM AUTHOR]

The calculation of thermodynamic state variables, particularly derivatives of the pressure with respect to density and temperature, in conventional molecular-dynamics simulations is considered in the frame of the comprehensive treatment of the molecular-dynamics ensemble by Lustig [J. Chem. Phys. 100, 3048 (1994)]. This paper improves the work of Lustig in two aspects. In the first place, a general expression for the basic phase-space functions in the molecular-dynamics ensemble is derived, which takes into account that a mechanical quantity G is, in addition to the number of particles, the volume, the energy, and the total momentum of the system, a constant of motion. G is related to the initial position of the center of mass of the system. Secondly, the correct general expression for volume derivatives of the potential energy is derived. This latter result solves a problem reported by Lustig [J. Chem. Phys. 109, 8816 (1998)] and Meier [Computer Simulation and Interpretation of the Transport Coefficients of the Lennard-Jones Model Fluid (Shaker, Aachen, 2002)] and enables the correct calculation of the isentropic and isothermal compressibilities, the speed of sound, and, in principle, all higher pressure derivatives. The derived equations are verified by calculations of several state variables and pressure derivatives up to second order by molecular-dynamics simulations with 256 particles at two state points of the Lennard-Jones fluid in the gas and liquid regions. It is also found that it is impossible for systems of this size to calculate third- and higher-order pressure derivatives due to the limited accuracy of the algorithm employed to integrate the equations of motion. [ABSTRACT FROM AUTHOR]

This work presents an easy applicable method for estimating thermal conductivity of oxide ceramic samples in a high-temperature SOFC/SOEC system. An experimental investigation establishes a thermal conductivity map for various oxide ceramics in the temperature range of 700 - 900 °C. Material specific properties are determined using laser flash analysis, displacement measurement for density and differential scanning calorimetry for heat capacity. The total thermal conductivity, incorporating conduction and radiation, is assumed based on a linear relationship with temperature gradient. Deviations in measurements are attributed to increasing radiation effects. The method provides a rough estimation of thermal conductivity, with 8YSZ exhibiting good agreement with literature values and 10Sc1CeSZ showing a range of 2.3 (+0.32/-0.48) - 2.8 (+0.96/-1) W/mK. [ABSTRACT FROM AUTHOR]

Geothermal heat pipes are an effective heat source for heat pumps used for space heating. Because the area for the installation of borehole heat exchangers is limited in urban areas (one site per borehole), the maximum heat extractable from one borehole shall rise. In cooperation with the FKW Hannover, the Institute for Thermodynamics of the Leibniz University of Hannover is investigating the thermodynamic behavior of CO driven geothermal heat pipes of higher thermal power. Therefore two different types of geothermal heat pipes with a length of 400 m each have been installed. Furthermore a numerical simulation of the heat and mass transfer within the pipes is under development. The experimental setup and first results of the experiments are presented as well as the current status of the numerical simulation. A comparison of the two different types of heat pipes and a comparison of the experimental data with the numerical simulation is given. [ABSTRACT FROM AUTHOR]

This paper reports accurate measurements of the speed of sound in compressed pure nitrogen and a synthetic ternary mixture of nitrogen, oxygen, and argon with a composition close to that of natural dry air. The data have been measured by a double-path-length pulse-echo technique and cover the temperature ranges between 275 and 400 K for nitrogen and between 240 and 420 K for the dry air mixture with pressures up to 100 MPa. The expanded uncertainties (at the 0.95 confidence level) amount to 2.1 mK for temperature, 50 ppm for pressure, 80 ppm for speed of sound in nitrogen, and 110 ppm for speed of sound in the dry air mixture. [ABSTRACT FROM AUTHOR]

This paper presents experimental results on boiling heat transfer of R134a in a compact plate fin heat exchanger. The exchanger is made of aluminum and has high density offset fins (30 fins/in.). Such heat exchangers are widely used in air separation industry and aerospace applications because of their high compactness and low weight. The test heat exchanger is attached to a vapor cycle refrigeration basic module to study the effects of boiling phenomena and its influence on performance as there is limited information available for this type of fins. This in turn allows for discussion on boiling mechanism of RI34a inside the fins using the water circuit on the other side of the test heat exchanger. The water side single phase heat transfer coefficient (Colburn j factor) is calculated using the CFD tool FLUENT and validated with available open literature. The results are presented for heat fluxes up to 5500 W/m² and mass fluxes up to 20 kg/(m²'s) with water side flow rate varying from 0.033 to 0.17 kg/s for water temperatures of 10,15, 20, 25, and 30°C. [ABSTRACT FROM AUTHOR]

The use of plasmonic nanofluids in photothermal applications, such as solar thermal receivers, is a strong subject in current research. Additionally, other fields show interests in basefluids, of which the optical properties are tuned by adding nanoparticles. Exemplary research activities are plasmonic hyperthermia or nanoparticle-based sunscreen products. However, chosing the appropriate nanoparticle material is of great importance for the efficiency of such systems. The 'classical' approach is to measure the absorption or scattering behavior of known nanofluids, followed by an estimation whether or not this fluid is suitable for the designated application. This paper shows up a different approach: a method is presented to be used as a guided search for a global optimal nanoparticle material for a certain task. [ABSTRACT FROM AUTHOR]

Thermoelectric energy converters based on galvanic cells (TGC) offer the possibility of direct conversion of low-temperature waste heat into electrical energy and could therefore be a promising approach for an increase in the overall efficiency of energy conversion. Due to an externally applied heat source, a temperature gradient across the electrolyte is induced, leading to a gradient in the chemical potential of the species and an electrical potential difference between the electrodes. The aim of approaching an internal equilibrium state leads to various coupled molecular transport mechanisms taking place in the electrolyte, impacting the open circuit voltage (OCV) and the performance of the TGC. By applying the theory of non-equilibrium thermodynamics (NET) to describe these coupled processes, the interactions that occur can be characterized in more detail. In this work, a polymer electrolyte membrane (PEM)-based TGC with two H2/H2O electrodes of different temperatures and gas compositions is experimentally investigated. By controlling the gradients in temperature and concentration, different impacts on the resulting OCV can be identified. In addition, we present the measured coupling coefficient, representing the singular relation between the transport of the hydrogen ions inside the membrane and the electrical potential difference between the electrodes for a wide variety of working conditions. [ABSTRACT FROM AUTHOR]

Plate heat exchangers are well established due to their high thermal efficiency, compactness, and cost competitiveness. Unfortunately, there are only few published data and correlations for a reliable calculation of heat transfer and pressure drop in multiphase flow and phase transition situations. The objective of this work is the experimental investigation of two-phase heat transfer and pressure drop during condensation in a vertical corrugated plate gap. Experimental results for the condensation of refrigerant R134a and water steam are presented and validated by comparison with calculation models from the literature. [ABSTRACT FROM AUTHOR]

In this manuscript, off-set strip fin structures are presented which are adapted to the possibilities of additive manufacturing. For this purpose, the geometric parameters, including fin height, fin spacing, fin length, and fin longitudinal displacement, are varied, and the Colburn j-factor and the Fanning friction factor are numerically calculated in the Reynolds number range of 80–920. The structures are classified with respect to their entropy production number according to Bejan. This method is compared with the results from partial differential equations for the calculation of the irreversible entropy production rate due to shear stresses and heat conduction. This study reveals that the chosen temperature difference leads to deviation in terms of entropy production due to heat conduction, whereas the dissipation by shear stresses shows only small deviations of less than 2%. It is further shown that the variation in fin height and fin spacing has only a small influence on heat transfer and pressure drop, while a variation in fin length and fin longitudinal displacement shows a larger influence. With respect to the entropy production number, short and long fins, as well as large fin spacing and fin longitudinal displacement, are shown to be beneficial. A detailed examination of a single structure shows that the entropy production rate due to heat conduction is dominated by the entropy production rate in the wall, while the fluid has only a minor influence. [ABSTRACT FROM AUTHOR]

The evaporation heat transfer coefficient of the refrigerant R-134a in a vertical plate heat exchanger was investigated experimentally. The area of the plate was divided into several segments along the vertical axis. For each of the segments, the local value of the heat transfer coefficient was calculated and presented as a function of the mean vapor quality in the segment. Owing to the thermocouples installed along the plate surface, it was possible to determine the temperature distribution and vapor quality profile inside the plate. The influences of the mass flux, heat flux, pressure of system and the flow configuration on the heat transfer coefficient were also taken into account and a comparison with literature data was performed. [ABSTRACT FROM AUTHOR]

Experimental results for the pressure drop during the evaporation of the refrigerant 1,1,1,2-tetrafluoroethane (R-134a) in a vertical plate heat exchanger are presented in this paper. The influences of mass flux, heat flux and vapor quality on the two-phase pressure drop are specially analyzed and compared with previously published experimental data and literature correlations. All results are given in graphical form as the dependency of the frictional pressure drop on the mean vapor quality. [ABSTRACT FROM AUTHOR]

In this study the transfer coefficient of evaporation heat of the refrigerant 1,1,1,2-tetrafluoroethane (R-134a) in a vertical plate heat exchanger was experimentally investigated. The results are presented as the dependancy of the mean heat transfer coefficient for the whole heat exchanger on the mean vapor quality. The influences of mass flux, heat flux and flow configuration on the heat transfer coefficient were also taken into account and a comparison with previously published experimental data and literature correlations was made. [ABSTRACT FROM AUTHOR]

An instrument for highly accurate measurements of the speed of sound in fluids in the temperature range between 240 and 420 K with pressures up to 100 MPa is described. The measurement principle of the speed of sound sensor is based on a double path length pulse-echo technique. The achieved measurement uncertainties are 3 mK for the temperature, 0.01% for the pressure below 10 MPa and 0.005% for the pressure between 10 and 100 MPa, and 0.014% for the speed of sound. The high accuracy of the instrument is demonstrated by measurements in liquid water and compressed argon. The results for argon prove that our pulse-echo technique agrees with the highly accurate spherical resonator technique, which is commonly employed for speed of sound measurements in gases, in the pressure range where both methods overlap within our measurement uncertainty. [ABSTRACT FROM AUTHOR]

• Experimental investigation of additively manufactured high-temperature heat exchangers. • Different types of fins with special adaptations. • Algorithm for determination of Nusselt number from experiment presented. • Experiments up to 700 °C. • Very good agreement between experimental and numerical data. This article examines various additively manufactured heat exchangers for high-temperature applications. The heat exchangers differ in terms of their internal fin structure as well as optimizations in terms of manufacturing quality. Starting from a reference fin type, optimizations with focus on low pressure drop and high heat transfer are performed. The heat exchangers are investigated experimentally, focusing on the laminar flow regime between 60

In an extensive computer simulation study, the transport coefficients of the Lennard-Jones model fluid were determined with high accuracy from equilibrium molecular-dynamics simulations. In the frame of time-correlation function theory, the generalized Einstein relations were employed to evaluate the transport coefficients. This third of a series of four papers presents the results for the bulk viscosity. With comprehensive simulation data at over 350 state points, the temperature and density dependences of the bulk viscosity are characterized in this work over a wide range of fluid states. The bulk viscosity exhibits a large critical enhancement similar to that known for the thermal conductivity, but it extends much farther into the supercritical region and can be observed even at 4.5 times the critical temperature. An investigation of the pressure-fluctuation autocorrelation functions shows that the enhancement is caused by extremely slowly decaying pressure fluctuations.© 2005 American Institute of Physics. [ABSTRACT FROM AUTHOR]

In an extensive computer simulation study, the transport coefficients of the Lennard-Jones model fluid were determined with high accuracy from equilibrium molecular-dynamics simulations. In the frame of time-correlation function theory, the generalized Einstein relations were employed to evaluate the transport coefficients. This second of a series of four papers presents the results for the self-diffusion coefficient, and discusses and interprets the behavior of this transport coefficient in the fluid region of the phase diagram. The uncertainty of the self-diffusion data is estimated to be 1% in the gas region and 0.5% at high-density liquid states. With the very accurate data, even fine details in the shape of the self-diffusion isotherms are resolved, and the previously little-investigated behavior of the self-diffusion coefficient at low-density gaseous states is analyzed in detail. Finally, aspects of the mass transport mechanisms on the molecular scale are explored by an analysis of the velocity autocorrelation functions. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

In an extensive computer simulation study, the transport coefficients of the Lennard-Jones model fluid were determined with high accuracy from equilibrium molecular-dynamics simulations. In the frame of time-correlation function theory, the generalized Einstein relations were employed to evaluate the transport coefficients. This first of a series of four papers presents the results for the viscosity, and discusses and interprets the behavior of this transport coefficient in the fluid region of the phase diagram. Moreover, the kinetic-kinetic, kinetic-potential, and potential-potential viscosity contributions are resolved over the whole range of fluid states, and their characteristic dependence on temperature and density is described. Finally, an additional analysis of the shear-stress correlation functions reveals aspects of the momentum-transport mechanisms on the molecular scale. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

To make an all-electric aircraft possible, both high power densities and efficiencies are needed. However, particularly high demands are also placed on the thermal management system. Often, the electric motor and cooling system are considered without co-optimization. Particularly in the case of electric motors with conductors directly cooled by a liquid, there is great potential for optimization, since the temperature-dependent Joule losses determine the largest part of the losses. This publication shows the main influencing parameters for the electric motor and cooling system: coolant speed and winding temperature. In addition, the influence of the cooling system control during a flight mission is demonstrated and its potential in mass reduction is quantified. It could be shown that with a low utilized electric motor the maximum winding temperature of 130 ° C is beneficial, the cooling system should work in almost all operation points in its sized operation and the mass of the heat exchanger and pump is negligible compared to the mass of the electric motor and energy storage. [ABSTRACT FROM AUTHOR]

In addition to taste and melting behavior, the gloss is an important quality criterion of chocolate products. Differences considering the gloss on products can be attributed to different causes in the production process. The chocolate that has been pre‐crystallized by shearing and supercooling is further cooled, crystallized and transported as a solid chocolate out of the cooling process. The cooling of the chocolate is largely unexplored in a paternoster refrigerator and therefore the resulting streaming profile and temperature profile are numerically studied in this work. [ABSTRACT FROM AUTHOR]

Eco‐friendly mixtures may substitute pure working fluids in thermodynamic cycle processes, yet the existing calculation approaches of mixture condensation are complex due to iterative procedures and multiple input data. To simplify condenser design, this project aims for a new calculation method of mixtures. On this behalf, experimental data for the condensation of ethanol/water and ethanol/octamethyltrisiloxane is provided within a wide composition range to identify the most influential parameters. Finally, a practical prediction method for heat transfer coefficients of mixtures is suggested. [ABSTRACT FROM AUTHOR]

Zirconium dioxide (ZrO2) codoped with 10 mol % scandium(III) oxide Sc2O3 and 1 mol % cerium dioxide CeO2 (10Sc1CeSZ) is a relevant electrolyte material for high temperature fuel or electrolysis cells, due to its high ionic conductivity and its stability in the cubic phase in comparison to other metal oxides. Despite the many experimental studies for 10Sc1CeSZ, there are to our knowledge no computational studies of this material. In this paper, we calculate the diffusivities and ionic conductivity of 10Sc1CeSZ using classical molecular dynamics for different temperatures and the density at zero pressure in the canonical ensemble. Our results for the ionic conductivity are comparable with experimental data for temperatures equal to or above 1200 K. This may hopefully also be the case for the Brownian diffusion coefficient and the tracer diffusion coefficient calculated in this work, for which no literature values are available. The values given in this paper can be used in future studies describing transport phenomena in fuel or electrolysis cells with 10Sc1CeSZ electrolytes. [ABSTRACT FROM AUTHOR]

The two-phase flow in the corrugated gap created by two adjacent plates of a plate heat exchanger was investigated experimentally. One setup consisting of a transparent corrugated gap was used to visualize the two-phase flow pattern and study the local phenomena of phase distribution, pressure drop and void fraction. Saturated two-phase R365mfc and an air-water mixture were used as working fluids. In a second experimental setup, the heat transfer coefficients and the pressure drop inside an industrial plate heat exchanger during the condensation process of R134a are determined. Both experimental setups use the same type of plates, so the experimental results can be connected and a flow pattern model for the condensation in plate heat exchangers can be derived. In this work the results of the flow pattern visualization, the two-phase pressure drop in the corrugated gap and the void fraction analysis by measurement of the electrical capacity are presented. A new pressure drop correlation is derived, which takes into account different flow patterns, that appear during condensation. The mean deviation of the presented pressure drop model compared to the experimental data and data from other experimental works is 18.9%. 81.7% of the calculated pressure drop lies within ±30% compared to the experimental data. [ABSTRACT FROM AUTHOR]

A novel mathematical programming method was applied to acquire the optimal heat exchanger network (HEN) retrofit in the crude oil distillation unit. The mathematical model was based on the stagewise superstructure model with splits of streams from the work of Yee et al. [Chin. J. Chem. Eng. 2007, 15 (2), 296-301]. The objective function was developed into the minimum of the sum of the cost of modifications and the cost of utilities in one year, focusing on the changing heat transfer coefficients arising in the reused existing heat exchangers from stream splits. The HEN modifications included buying new heat exchangers, adding or reducing areas of the existing heat exchangers, moving the existing heat exchangers, and laying pipes. It was a mixed integer nonlinear program (MINLP) problem. A hybrid genetic algorithm was used to optimize the MINLP problem. In order to reduce the total cost of modifications, the heat exchangers and the corresponding matching structures between the hot and cold streams in the existing HENs were designed as elites introduced into the hybrid genetic algorithm. The Dittus-Boelter and Kern correlations were used to estimate the convective heat transfer coefficients of the corresponding streams. One example was given to show the effect of the novel retrofit strategy. [ABSTRACT FROM AUTHOR]

PEM fuel cell systems face highly dynamic load profiles in automotive application. This work showcases the impact of media supply adaption, system architecture and test rig restrictions on the transient voltage response of an automotive fuel cell stack. Current step and load profile experiments were conducted on a system test rig, featuring automotive balance of plant components, and a short stack test bench. A time scale analysis allowed us to identify the predominant effect for the voltage response in each test case. The voltage response measured in the test cases was dominated either by air supply, membrane humidification or coolant temperature dynamics. This systematic comparison of different types of test setups highlights the importance of application-like system level testing as, in contrast to common experiments, different phenomena shape the electrical stack behavior. [ABSTRACT FROM AUTHOR]

In addition to taste and melting behavior, the gloss is an important quality criterion of chocolate products. Differences considering the gloss on products can be attributed to different causes in the production process. The chocolate which has been precrystallized by shearing and supercooling is further cooled, crystallized, and transported as a solid chocolate out of the cooling process. The cooling of the chocolate is largely unexplored in a paternoster refrigerator. Therefore, the resulting flow profile and temperature profile are numerically studied in this work. [ABSTRACT FROM AUTHOR]

Pressure drop during flow boiling of R410A was experimentally investigated at a saturation temperature of 279 K over a mass flux range of 50–80 kg/(m2s) and a vapor quality range of 0.2–0.8. The inner tubes with the same outside diameter of 19 mm contain a smooth tube and three enhanced tubes: a micro-porous (No. 1) tube, a ring-shaped finned (No. 2) tube, and an independent small-bosses surface (No. 3) tube. It was observed that the pressure drop in single-phase and flow boiling conditions increased with the increase of the mass flux, and the ring-shaped finned tube showed the highest pressure drop (more than 1.35 times and 1.31 times that of the smooth tube, respectively) due to the effect of the ring-shaped fins. The pressure drop in the micro-porous tube and the smooth tube increased with the increase of outlet vapor quality in the range of the mass fluxes tested. Results from correlation analysis showed that two existing correlations can be implemented to predict the frictional pressure drop in a smooth tube within a deviation of ± 10%. Improved correlations to predict pressure drop in the enhanced tubes were presented, and 94% of the data points of the micro-porous tube were predicted within a ± 10% error band. [ABSTRACT FROM AUTHOR]

With its outstanding performance characteristics, the SOFC represents a promising technology for integration into the current energy supply system. For cell development and optimization, a reliable quantitative description of the transport mechanisms and the resulting losses are relevant. The local transport processes are calculated by a 1D model based on the non-equilibrium thermodynamics (NET). The focus of this study is the mass transport in the gas diffusion layers (GDL), which was described as simplified by Fick's law in a previously developed model. This is first replaced by the Dusty-Gas model (DGM) and then by the thermal diffusion (Soret effect) approach. The validation of the model was performed by measuring U , j -characteristics resulting in a maximum deviation of experimental to simulated cell voltage to up to 0.93%. It is shown that, under the prevailing temperature, gradients the Soret effect can be neglected, but the extension to the DGM has to be considered. The temperature and heat flow curves illustrate the relevance of the Peltier effects. At T = 1123.15 K and j = 8000 A/m2, 64.44% of the total losses occur in the electrolyte. The exergetic efficiency for this operating point is 0.42. Since lower entropy production rates can be assumed in the GDL, the primary need is to investigate alternative electrolyte materials. [ABSTRACT FROM AUTHOR]

Falling film evaporation is widely used in solar desalination systems. Fouling is an important problem to be addressed in many applications involving heat transfer including processes involving the utilization of solar energy in desalination applications. In the research upon which this paper partly reports, an experimental investigation was carried out on a vertical tube in falling film evaporation to determine the effects of temperature, velocity, the use of a porous-sintered tube, and the use of Teflon coating on calcium carbonate deposition characteristics. During the fouling experiments, the pressure inside the test tubes was maintained constant at 101.3 kPa, and the inlet temperature was maintained at 373 K, while allowing the water mass velocity to vary from 0.42 to 1.05 kg m-1 s-1. Results show that the fouling in the test tube becomes more serious as the temperature increases and the flowrate decreases. Compared with stainless-steel tubes, porous-sintered tubes can significantly reduce fouling resistance, but at the same time they bring about a decrease in the heat transfer coefficient. The Teflon coating also has anti-fouling performance but does not affect the heat transfer coefficient in stainless-steel tubes. Through the weighing of local fouling deposits, it has been found that the mass of the fouling deposits in the lower part of the tested tubes is greater than that in the upper part. [ABSTRACT FROM AUTHOR]

Plate heat exchangers are widely used for two-phase heat transfer in the industrial applications, and recently more attention has been paid to the plate heat exchangers with enhanced surface due to their better heat transfer performance. In this paper, the local condensation heat transfer coefficients are studied using R134a in a micro-structured plate heat exchanger. In order to obtain a more accurate prediction model, a series of measurements are conducted under various operating conditions. The mass flux of R134a varied from 47 kg/m2s to 77 kg/m2s, the saturation pressure in the condenser ranged from 6.32 bar to 8.95 bar, and the value of the heat flux was between 13 kW/m2 and 22 kW/m2. The local two-phase Nusselt number increases with the increase of the mass flux. As the saturation pressure increases, the local two-phase Nusselt number increase at the beginning of the condensation and decrease at the end of the condensation. However, the effect of heat flux on local heat transfer is irregular, due to the interaction of these parameters in the experiment. Comparing with the unstructured plate heat exchanger, R134a condenses faster at the beginning of the process in the micro-sturctured plate heat exchanger, and the local heat transfer performs better when the vapor quality is lower. Combing with the phenomenon that the overall heat flux in micro-structured plate is larger under the same working conditions, it shows that the overall heat transfer of the micro-structured plate is improved, but the local heat transfer uprades only at lower vapor qualities. A new correlation is developed, it predicts all the experimental data within the root mean square error 10%, and a new correlation for the waterside is suggested as well. [ABSTRACT FROM AUTHOR]

This paper presents a novel tool for interactive 3D visualization and computational steering of molecular simulations and other computer simulation techniques such as computational fluid dynamics in parallel computing environments. The visualization system consists of three major components--data source, streaming server and viewer--which are distributed in intra/internet networks. A parallelized data extraction and visualization library, which generates 3D scenes, is integrated in the simulation software. The 3D scenes can be stored locally or sent to the streaming server parallel to the simulation, where they are stored on a fast RAID hard disk system. The streaming server transfers the 3D scenes to viewer clients on demand for display. A multi-platform 3D viewer software is provided as a plug-in embeddable into different WWW browsers. Many models of broad interest in molecular simulations can be visualized, from simple spherical particles to moderately complex molecules, and several volume visualization methods are implemented efficiently. Examples from thermophysical property research applications demonstrate the utility of the visualization system. One example shows that the transport coefficients of the Lennard-Jones fluid at subcritical temperatures in the gas region are influenced by the formation of small clusters of particles. [ABSTRACT FROM AUTHOR]

Pool boiling characteristics on horizontal tubes with diameter lying between wires and industrial tubes have been investigated experimentally. Boiling experiments are carried out at near atmospheric pressures with water and R-123 as boiling liquids. The experimental results show quite a different boiling behavior compared to larger tubes or plates. The commonly used correlations are found to be ineffective over this range of diameters. A developing sliding bubble mechanism can be attributed to such behavior which shows a strong diameter effect. The heat transfer is found to increase with diameter which is contrary to that on industrial size tubes. Useful boiling data are presented in this range of diameters in which experimental data are scarce. The need for more investigations is stressed in view of emerging applications in this area. [ABSTRACT FROM AUTHOR]

The prediction of mixture condensation is still complex due to the coupled heat and mass transfer and insufficient data of thermophysical mixture properties. This article analyzes the impact of various heat and mass transfer correlations on the non-equilibrium approach for mixture condensation in a vertical plain tube. Furthermore, the influence of thermophysical properties from different databases is investigated. The results are shown for ethanol-water, but allow conclusions to other fluid mixtures. They indicate that the liquid heat transfer coefficient in the non-equilibrium approach dominates the qualitative behavior of the condensation process, but the vapor mass transfer coefficient can only decrease or increase the quantitative level of the effective heat transfer with minor impact. More importantly, the logarithm in the vapor mass transfer term is central for the prediction of the condensation heat transfer. As this logarithm contains vapor-liquid equilibrium (VLE) data, it proves that there is a strong connection between VLE and overall prediction of mixture condensation. A demonstration of available data for thermophysical mixture properties of ethanol-water shows significant deviations, which affect the calculations as well. Besides, data from our own experiments are presented for mixture viscosity of ethanol-water. It is recommended to focus not only on improved heat and mass transfer correlations but also on thermophysical properties and VLE data for a precise prediction of mixture condensation. [ABSTRACT FROM AUTHOR]

• Apparatus for ultrasonic velocity is proposed and tested. • Speed of sound of four nanofluids were investigated. • Molecular interactions between the nanofluid components were analyzed. • Comparison with the literature data was performed. The speed of sound of four different nanofluids consisting of Ag or Al 2 O 3 nanoparticles and water or ethylene glycol base fluid is measured in the temperature range (298.15–343.15) K and at atmospheric pressure. One aim of this analysis is to learn whether the change from pure base fluid speed of sound to the nanofluid situation is a smooth or disruptive transition when adding nanoparticles. The measurements were conducted on the newly proposed apparatus consisting mainly of function generator, ultrasonic sensor, switch, voltage source, oscilloscope, ultrasonic container and the necessary sensors and equipment for data acquisition. The values for speed of sound were calculated based on the Time of Flight method. Deionised water and ethanol were used to calibrate the apparatus which is additionally tested with pure base fluids water and ethylene glycol and showed excellent agreement with average absolute percentage deviations between experimental and literature data for speed of sound of 0.12% and 0.36%, respectively,. Beside the temperature influence, the influence of the nanoparticle concentration on the speed of sound of nanofluid were investigated and followed. The analysis of the sound velocity allows insight into the molecular level interactions taking place between the nanoparticles and the base fluid molecules of the nanofluid. The decrease in velocity values with the increase of concentration, noticeable in the tested nanofluids, is a result of decrease in nanoparticle-fluid interaction and dominance of particle–particle interaction. [ABSTRACT FROM AUTHOR]

Fluid flow and heat transfer in annuli appear in various technical applications. One of these are actively cooled underground cables, which are a solution for high-voltage transmission grids that face overloads due to a growing capacity of renewable sources. The cooling of these systems depends on precise prediction of heat transfer of a highly eccentric annulus in a way that has not been subject to any known literature or former research. This article presents Nusselt numbers obtained from CFD calculations for turbulent flow (20,000 < Re < 150,000) in highly eccentric annuli (dimensionless eccentricity e up to 0.95). The mesh and turbulence models are validated with well-known correlations from literature, which show very good accordance. Nusselt numbers show a high dependency on the eccentricity. They strongly decrease for e > 0.5 and drop to a third and less for e = 0.95 in comparison to concentric annuli. This has significant impact on the assurance of safe operation in actively cooled cable systems. Results also indicate a change of flow characteristics from laminar to turbulent depending on the eccentricity that should be considered in order to improve heat transfer. [ABSTRACT FROM AUTHOR]

Compact heat exchanger is a kind of advanced heat transfer equipment with small size and high efficiency. It has wide application prospects in industry. However, due to the highly compact structure, it is difficult to measure the wall temperature of heat transfer surface by traditional test methods. For measuring the thermal performance of compact heat transfer surfaces more accurately, a single-blow transient test technique using pulse change inlet condition with optimized pulse width and matching time is developed. By turning on and off the electric air heater, the pulse change in the inlet temperature can be realized and is fitted as the superposition of a positive and a negative exponential function with a time shift. In order to reduce the effect of the uncertainty of temperature measurement, the optimum pulse width and optimum matching time is obtained by numerical calculations. By means of the newly extended test method, the heat transfer performance of a parallel-plate test core is measured and compared with the results from the literature. The analysis shows that the present pulse change technique considering the optimal pulse width and matching time have to be considered for the heat transfer surfaces with NTU > 4.5 to reduce the uncertainty in temperature measurement. The experimental results are in good agreement with those given in the literature. [ABSTRACT FROM AUTHOR]

A good water management is very important for the operation of PEM fuel cell systems as the proton conductivity is dependent on the membrane water content. In contrast to state of the art approaches, this study focuses on an anode based water management approach of fuel cell systems with an anode recirculation loop. The aim of the anode based water management is to reach a high and homogeneously distributed anode humidity without condensation in all relevant operating conditions. A criterion is defined to evaluate the anode humidity distribution. A macroscopic discrete 2D+1D model was developed that can simulate humidity distributions and the cell voltage for various flow directions of the fluids and operating conditions. The model considers the system behavior including the anode recirculation loop. This study shows that flow directions that support an internal water circulation are beneficial for fuel cell systems without external humidification. Furthermore, the study shows a correlation between the anode humidity distribution at the membrane and the cell voltage. The higher the temperature is, the more important is a flow field that supports a high and homogeneously distributed anode humidity. • Homogeneity of anode humidity is a good criterion for water management evaluation. • The system behavior has to be considered for the optimization of flow directions. • High and homogeneously distributed anode humidity results in high cell voltage. • Internal water circulation is important for fuel cell systems without humidifier. • High temperature operation requires supportive flow directions of the fluids. [ABSTRACT FROM AUTHOR]

Recently, several publications gave attention to nanofluid based solar absorber systems in which the solar radiation energy is directly absorbed in the volume of the fluid. This idea could provide advantages over conventionally used surface absorbers regarding the optical and thermal efficiency. For the evaluation of this concept, a numerical approach is introduced and validated in this contribution. The results show that the optical efficiency of a volumetric absorber strongly depends on the scattering behavior of the nanofluid and can reach competitive values only if the particle size distribution is narrow and small. If this is achieved, the surface temperature and therefore the heat loss can be lowered significantly. Furthermore, the surface absorber requires very high Reynolds numbers to transfer the absorbed energy into the working fluid and avoid overheating of the absorber tube. This demand of pumping power can be reduced significantly using the concept of volumetric absorption. [ABSTRACT FROM AUTHOR]

In the first part of the study, we present a model of the solid oxide fuel cell (SOFC) system with anode off-gas recirculation (AGR) in the SchIBZ-project. In this last second paper, we conduct an exergy analysis of this real SOFC plant, using validated models for the reformer and the SOFC module from our first study. We find that an optimum recirculation and low O / C -ratios yield a high exergetic efficiency, due to the small exergy losses in the conversion of chemical energy to electrical power in the SOFC-module under these operation conditions. We conduct our simulations based on a real concept and use validated models, so that the exergy analysis given in this work may be a reliable estimate to the real thermodynamic behavior of this SOFC plant. • The efficiency of the fuel cell system of the well-known SchIBZ project is analysed. • An exergy analysis depending on the recirculation rate and O/C-ratio is conducted. • The SOFC model is successful validated within the system in part load. • The fuel cell characteristics are calculated with the validated model. [ABSTRACT FROM AUTHOR]

Solid oxide fuel cell (SOFC) systems with anode off-gas recirculation (AGR) and diesel pre-reforming are advantageous because they can operate with the current fuel infrastructure. In the SchIBZ-project, the prototype of such a SOFC system for maritime applications has already been commissioned. In this first paper, we model the system devices to conduct an exergy analysis of this real SOFC plant and validate them with experimental values from experiments in laboratory scale. The results of our simulation agree well with the experimental values. The calculations with the validated results may be closer to the real thermodynamic behavior of such system components than previous literature. [ABSTRACT FROM AUTHOR]

Micro-fin tube, 3-D enhanced tube and smooth tube with an inner diameter of 9.52 mm were used as test tubes to study the condensation heat transfer performance with R410A and R32 as the working fluids at different mass flow rates (150–400 kg/m2s) and vapor qualities (0.2–0.8). For R410A and R32, the heat transfer coefficient of the micro-fin tube is 2.0–2.2 times and 1.5–2.0 times that of the smooth tube, and the heat transfer coefficient of the 3-D enhanced tube is 1.4–1.5 times and 1.5–1.6 times that of the smooth tube, respectively. The micro-fin tube is effective in thinning the condensate thickness and reducing the thermal resistance. The 3-D enhanced tube promotes the generation of turbulence and droplet entrainment, which improves heat transfer of enhanced tubes. The heat transfer coefficient of R32 is greater than that of R410A due to its higher thermal conductivity, latent heat and specific heat capacity. The frictional pressure drop increases monotonically with the mass flow rate. Considering the increment in surface area and the additional pressure drop penalty, the performance evaluation factor of the enhanced tubes ranges from 0.9 to 1.4. The study presents flow pattern maps for smooth and enhanced tubes. Enhanced tubes promote the appearance of intermittent and annular flow. [ABSTRACT FROM AUTHOR]

The synthesis of heat exchanger networks (HENs) is a complex problem because of the nonlinearity that results from the integer and continuous variables. Here, a bi-level algorithm for the optimal design of a HEN is proposed that attempts to optimize separately the integer and continuous variables on two levels. The master level is a problem-oriented evolution method generating new candidate HEN structures. The slave level is a memetic particle swarm optimization, an improved particle swarm optimization combined with a local search component, improvement of neighbourhood topologies and control parameter preference. The slave level minimizes the total annual cost (TAC) of a given structure received from the master level, and then sends this value back to the master level for structure evolution. The proposed bi-level method is applied to several cases taken from the literature, which demonstrate its reliable search ability in both structure space and continuous variable space and its ability to optimize the system, producing generally lower TACs than previously used methods. [ABSTRACT FROM AUTHOR]

A modeling study on a polymer electrolyte membrane fuel cell by means of non-equilibrium thermodynamics is presented. The developed model considers a one-dimensional cell in steady-state operation. The temperature, concentration and electric potential profiles are calculated for every domain of the cell. While the gas diffusion and the catalyst layers are calculated with established classical modeling approaches, the transport processes in the membrane are calculated with the phenomenological equations as dictated by the non-equilibrium thermodynamics. This approach is especially instructive for the membrane as the coupled transport mechanisms are dominant. The needed phenomenological coefficients are approximated on the base of conventional transport coefficients. Knowing the fluxes and their intrinsic corresponding forces, the local entropy production rate is calculated. Accordingly, the different loss mechanisms can be detected and quantified, which is important for cell and stack optimization. [ABSTRACT FROM AUTHOR]

A laboratory-scale solid oxide fuel cell (SOFC) system using liquefied natural gas (LNG) as a fuel is designed to be used as an energy converter on seagoing vessels (MultiSchIBZ project). The presented system design phase is supported by thermodynamic system simulation. As heat integration plays a crucial role with regard to fuel recirculation and endothermic pre-reforming, the heat exchanger and pre-reforming component models need to exhibit a high degree of accuracy throughout the entire operating range. Compact additively manufactured tube-bundle and plate-fin heat exchangers are designed to achieve high heat exchange efficiencies at low pressure losses. Their heat transfer correlations are derived from experimental component tests under operating conditions. A simulation study utilizing these heat exchanger characteristics is carried out for four configuration variants of pre-reforming and heat integration. Their system behaviour is analyzed with regard to the degree of pre-reforming and the outlet temperature of the fuel processing module. The combination of allothermal pre-reforming with additively manufactured plate-fin heat exchangers exhibits the best heat integration performance at nominal full load and yields a partial load capability to up to 60% electrical load at net electrical efficiencies of 58 to 60% (LHV). [ABSTRACT FROM AUTHOR]

• An experimental investigation on the tube side condensation heat transfer inside helix micro fin tube and three dimensional dimple tube is conducted. • Effect of mass flux, vapor quality, saturation temperature, tube thermal conductivity, tube diameter, and tube surface structure on the condensation heat transfer are studied. • An evaluation factor to compare the sensitivity of the enhanced tube heat transfer coefficient for a variety of operational conditions and tube parameters (material, properties, and structure) is proposed. Condensation heat transfer characteristics were experimentally investigated inside two dimensional helix micro fin (HX) tubes, three-dimensional dimple (1EHT) tubes and smooth (ST) tubes; while using R410A, for a variety of operating conditions and tube parameters. Tubes evaluated had a length of 2 m, with outer diameters of 9.52 and 12.7 mm; tube materials included copper and stainless steel. Experimental procedure was verified using a single-phase heat balance; with a comparison of results (enhanced tubes to smooth tube) being performed. The condensation experiments were performed at a saturated temperature of 35–45 °C; for the vapor quality range from 0.1 to 0.9; with mass fluxes that ranged from 70 to 450 kg m−2 s−1. Additionally, the effect of various parameters (mass flux, mean vapor quality, saturation temperature, tube thermal conductivity, tube diameter, and surface structure) on the tube side condensation heat transfer was evaluated. For the test conditions considered here the HTC enhancement ratio is in the range from 1.15 to 2.05 for the 1EHT tube and for the HX tube it ranged from 1.18 to 1.69; while the performance factor (PF takes into account the pressure drop) is 1.02–1.57 (1EHT) and 1.23–1.78 (HX). Heat transfer performance of the smooth tube was determined to be slightly affected by the thermal conductivity of the tube; however, for enhanced tubes there is a more dramatic increase in the HTC for an increase of thermal conductivity. Additionally, better heat transfer performance is demonstrated for smaller diameter tubes. Finally, the enhancement effect of the experimental parameters on the enhanced tubes was quantified through the use of a newly introduced evaluation factor. [ABSTRACT FROM AUTHOR]