Thermal performance measurement of additive manufactured high-temperature compact heat exchangers

Due to increased distribution of high-temperature processes in energy and process plants, more efficient and compact high-temperature heat exchangers are being developed. The additive manufacturing allows the construction of compact sizes and application-specific requirements. To evaluate the thermal performance of these heat exchangers, experimental investigations are evident. This study presents a test rig for testing compact high-temperature heat exchangers as well as a first set of thermal performance data of an additively manufactured plate-fin heat exchanger. The test rig can provide a maximum fluid temperature of 900°C and a maximum mass flow rate of 0.8 kg/min. A steam unit can add steam to the fluid stream to evaluate the influence of gas radiation on the thermal performance. The capabilities of this test rig are being tested with the plate-fin heat exchanger, varying the mass flow rate between 0.2 - 0.52 kg/min at a hot and cold inlet temperature of 750°C and 250°C. The overall effectiveness of the heat exchanger is approx. 0.9

Fundamental material properties of the 2LiBH4-MgH2 reactive hydride composite for hydrogen storage: (I) Thermodynamic and heat transfer properties

Thermodynamic and heat transfer properties of the 2LiBH4-MgH2 composite (Li-RHC) system are experimentally determined and studied as a basis for the design and development of hydrogen storage tanks. Besides the determination and discussion of the properties, different measurement methods are applied and compared to each other. Regarding thermodynamics, reaction enthalpy and entropy are determined by pressure-concentration-isotherms and coupled manometric-calorimetric measurements. For thermal diffusivity calculation, the specific heat capacity is measured by high-pressure differential scanning calorimetry and the effective thermal conductivity is determined by the transient plane source technique and in situ thermocell. Based on the results obtained from the thermodynamics and the assessment of the heat transfer properties, the reaction mechanism of the Li-RHC and the issues related to the scale-up for larger hydrogen storage systems are discussed in detail.Fil: Jepsen, Julian. Helmholtz-Zentrum Geesthacht; AlemaniaFil: Milanese, Chiara. University of Pavia; ItaliaFil: Puszkiel, Julián Atilio. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina. Helmholtz-Zentrum Geesthacht; AlemaniaFil: Girella, Alessandro. University of Pavia; ItaliaFil: Schiavo, Benedetto. Universidad de Palermo; Argentina. Istituto per le Tecnologie Avanzate; ItaliaFil: Lozano, Gustavo A.. Helmholtz-Zentrum Geesthacht; Alemania. BASF; AlemaniaFil: Capurso, Giovanni. Helmholtz-Zentrum Geesthacht; AlemaniaFil: Von Colbe, José M. Bellosta. Helmholtz-Zentrum Geesthacht; AlemaniaFil: Marini, Amedeo. University of Pavia; ItaliaFil: Kabelac, Stephan. Leibniz Universität Hannover; AlemaniaFil: Dornheim, Martin. Helmholtz-Zentrum Geesthacht; AlemaniaFil: Klassen, Thomas. Helmholtz-Zentrum Geesthacht; Alemani

Fundamental material properties of the 2LiBH4-MgH2 reactive hydride composite for hydrogen storage: (II) Kinetic properties

Reaction kinetic behaviour and cycling stability of the 2LiBH4-MgH2 reactive hydride composite (Li-RHC) are experimentally determined and analysed as a basis for the design and development of hydrogen storage tanks. In addition to the determination and discussion about the properties; different measurement methods are applied and compared. The activation energies for both hydrogenation and dehydrogenation are determined by the Kissinger method and via the fitting of solid-state reaction kinetic models to isothermal volumetric measurements. Furthermore, the hydrogen absorption-desorption cycling stability is assessed by titration measurements. Finally, the kinetic behaviour and the reversible hydrogen storage capacity of the Li-RHC are discussed.Fil: Jepsen, Julian. Helmholtz-Zentrum Geesthacht Zentrum für Material- und Küstenforschung; AlemaniaFil: Milanese, Chiara. Università degli Studi di Pavia; ItaliaFil: Puszkiel, Julián Atilio. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina. Helmholtz-Zentrum Geesthacht Zentrum für Material- und Küstenforschung; Alemania. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; ArgentinaFil: Girella, Alessandro. Università degli Studi di Pavia; ItaliaFil: Schiavo, Benedetto. Università degli Studi di Palermo; ItaliaFil: Lozano, Gustavo A.. Helmholtz-Zentrum Geesthacht Zentrum für Material- und Küstenforschung; AlemaniaFil: Capurso, Giovanni. Helmholtz-Zentrum Geesthacht Zentrum für Material- und Küstenforschung; AlemaniaFil: Von Colbe, José M. Bellosta. Helmholtz-Zentrum Geesthacht Zentrum für Material- und Küstenforschung; AlemaniaFil: Marini, Amedeo. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; ArgentinaFil: Kabelac, Stephan. Leibniz Universität Hannover; AlemaniaFil: Dornheim, Martin. Helmholtz-Zentrum Geesthacht Zentrum für Material- und Küstenforschung; AlemaniaFil: Klassen, Thomas. Helmholtz-Zentrum Geesthacht Zentrum für Material- und Küstenforschung; Alemani

Analytical modeling for the heat transfer in sheared flows of nanofluids

We developed a model for the enhancement of the heat flux by spherical and elongated nano- particles in sheared laminar flows of nano-fluids. Besides the heat flux carried by the nanoparticles the model accounts for the contribution of their rotation to the heat flux inside and outside the particles. The rotation of the nanoparticles has a twofold effect, it induces a fluid advection around the particle and it strongly influences the statistical distribution of particle orientations. These dynamical effects, which were not included in existing thermal models, are responsible for changing the thermal properties of flowing fluids as compared to quiescent fluids. The proposed model is strongly supported by extensive numerical simulations, demonstrating a potential increase of the heat flux far beyond the Maxwell-Garnet limit for the spherical nanoparticles. The road ahead which should lead towards robust predictive models of heat flux enhancement is discussed.Comment: 14 pages, 10 figures, submitted to PR

THERMODYNAMIC VIEW ON THE LOSS MECHANISMS IN PEM-FUEL CELLS

ABSTRACT The fuel cell, which is a highly promising candidate for high efficiency energy conversion, is not reaching expected conversion efficiencies of η > 0,5 yet. Parallel to standard explanations of loss mechanisms by means of overvoltages, a thermodynamic view of addressing irreversibilities by calculating local entropy production rates is helpful. Entropy production rates are calculated by multiplying local transport fluxes with appropriate driving forces, i.e., gradients of temperature, chemical potentials and electric potentials. These gradients have to be calculated by solving the set of constitutive balance equations. Before this tedious task is done, simplified model equations have to be used. The reversible fuel cell is the starting point of analysis. Results for a one-dimensional PEM-FC are shown. INTRODUCTION Although a fuel cell will not solve all the energy problems of mankind on one strike, it is still a very challenging energy conversion device from a thermodynamic point of view. The availability (or exergy) of the part of the internal energy of fuels like methane or hydrogen which are called chemical energy is very high. In the case of hydrogen and methane we have availabilities of 95 % and 102 % of their lower heating values, respectively. By simply burning this fuel to convert the chemical energy into thermal energy, the availability is reduced to around 60 %, depending on the temperature T of the resulting intermediate heat flux as characterized by the Carnot efficienc

A new setup for speed of sound measurements and experimental data for nanofluids Al2O3 or Ag with water or ethylene glycol

The speed of sound of four different nanofluids consisting of Ag or Al2O3 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

The mid-ir absorption spectrum of gas-phase clusters of the nucleobases guanine and cytosine

Contains fulltext : 98896.pdf (publisher's version ) (Open Access

Heat transfer coefficient and pressure drop during refrigerant R-134a condensation in a plate heat exchanger

The condensation heat transfer coefficient and the two-phase pressure drop of refrigerant R-134a in a vertical plate heat exchanger were investigated experimentally. The area of the plate was divided into several segments along the vertical axis. For each of the segments, local values of the heat transfer coefficient and frictional pressure drop were 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 and the heat flux on the heat transfer coefficient and the pressure drop were also taken into account and a comparison with previously published experimental data and literature correlations was carried out