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

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

HT2005-72855 MEASUREMENT OF LOCAL CONVECTIVE HEAT TRANSFER COEFFICIENTS WITH TEMPERATURE OSCILLATION IR THERMOGRAPHY AND RADIANT HEATING

ABSTRACT A method using temperature oscillations to measure local convection coefficients from the outside of a heat-transferring wall has been developed. This method is contact-free, employing radiant heating with a laser and an IR camera for surface temperature measurements. The numerical model extends previous research to three dimensions and allows for rapid evaluation of the convection coefficients distribution of sizable heat exchanger areas. The technique relies first on experimental data of the phase-lag of the surface temperature response to periodic heating, and second on a numerical model of the heattransferring wall that computes the local convection coefficients from the processed data. The temperature data processing includes an algorithm for temperature drift compensation and Single Frequency Discrete Fourier Transformations. The inverse heat conduction problem of deriving a surface map of convection coefficients from the phase-lag data is solved with a new numerical approach based on a complex 3-D finite-difference method. To validate the experimental approach, measurements of the temperature response of a semi-infinite specimen were analyzed. The results obtained were within 1.6% agreement with the analytical solution. The numerical model was verified by comparison with data generated by the FEM program ANSYS. The results of preliminary experiments investigating the local Nusselt number of water entering a tube are in agreement with established correlations. Future applications of this method will involve an aerodynamic vortex generator in a wind tunnel and plate heat exchangers. Another possible application of the experimental method is non-destructive testing of materials known as Lock-In Thermography

Flow boiling of R134a and ammonia in a plate heat exchanger

This paper presents experimental results on evaporation heat transfer for flow boiling of ammonia and of R134a in a chevron-pattern corrugated plate heat exchanger (PHE). The measurements enable the evaluation of a quasi-local heat transfer coefficient along the plate, which in turn allows discussing the two-phase distribution and the heat transfer mechanism during evaporation in a plate channel. The saturation temperature varied between 268 K < TNH 3s < 283 K (3.55 bar < pRs < 5.73 bar) for ammonia and 265 K < TR 134 as < 283 K (2.157 bar < pRs < 4.14 bar) for R134a. The heat transfer coefficient is discussed in relation to the vapor quality, mass flux, heat flux and the type of refrigerant. The secondary fluid is a water/ethylene glycol mixture flowing in counter flow or parallel flow arrangement within the PHE. It is shown that the parallel flow case yields better overall performance than the counterflow case, and that plates with low chevron angle corrugations increase the evaporation heat transfer. Comparison with the limited data available from the literature shows good agreement. The Danilova equation and the Steiner boiling correlation are adapted to PHEs and show the need for further theoretical development

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 &gt; 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