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03119 Modelling results for the thermal managementsub-system of a combined heat and power (CHP) fuel cell system (FCS)
16 Fuel science and technology (fundamental science, analysis, instrumentation) 04/03114 Measurement and modeling of surface tensions of asymmetric systems: heptane, eicosane, docosane, tetracosane and their mixtures Queimada, A. J. et al. Fluid Phase Equilibria, 2003, 214, (2), 211 221. To extend the surface tension database for heavy or asymmetric nalkane mixtures, measurements were performed using the Wilhelmy plate method. Measured systems included the binary mixtures heptane + eicosane, heptane + docosane and heptane + tetracosane and the ternary mixture heptane+eicosane+tetracosane at temperatures from 313.15 K (or above the melting point of the mixture) up to 343.15 K. All the measurements were performed at atmospheric pressure. Using these data, along with data previously measured by us and collected from the literature, a recently proposed corresponding states model was assessed. It is shown that using a new generalized combining rule for the critical temperature, the data can be described with deviations of about 1% that is within the experimental uncertainty of the measurements.
04•03115 Measurement and modelling of bubble and dew points in the binary systems carbon dioxide + cyclobutanone and propane + cyclobutanone Cruz Duarte, A. R. et al. Fluid Phase Equilibria, 2003, 214, (2), 121136. The fluid phase behaviour for the binary systems carbon dioxide + cyclobutanone and propane+cyclobutanone has been determined experimentally, using Cailletet equipment. For both the systems bubble points have been determined for a number of isopleths covering the whole mole fraction range. Additionally, for the binary system carbon dioxide+cyclobutanone dew points and critical points could be observed for a number of overall-compositions rich in carbon dioxide. The temperature and pressure range were, respectively, from 278 to 369 K and from 0.1 to 14.0 MPa. Correlation of the experimental data of both systems has been performed using the Soave-Redlich-Kwong (SRK) equation of state. Satisfactory results have been achieved using only one binary interaction parameter.
04/03116 Micro-thermoelectric cooler: interfacial effects on thermal and electrical transport da Silva, L. W. and Kaviany, M. International Journal of Heat and Mass' Transfer, 2004, 47, (10 1I), 2417 2435. The flows of heat and electricity in a column-type micro-thermoelectric cooler are analysed by modelling the various interfacial resistances. Electron (barrier tunnelling) and phonon (diffuse mismatch) boundary resistances at the thermoelectric/metal interface, and thermal nonequilibrium between electrons and phonons adjacent to this interface (cooling length), increase the thermal conduction resistance and decrease the Seebeck coefficient of the thermoelectric elements. These in turn reduce the device cooling performance, which is also affected by the thermal and electrical contact resistances at the thermoelectric/ metal and metal/electrical-insulator interfaces. To produce a temperature drop of 10 K with a cooling load of 10 roW, the optimum number of thermo-element pairs, operating current, and coefficient of performance, based on vapour deposited 4 ~tm thick films of Bi2Te3 and Sb2Tes, are predicted for a micro-thermoelectric cooler operating with a 3 V battery.
04/03117 Modeling the coalescence of heterogenous amorphous particles Yadha, V. and Helble, J. J. J. J. Journal of Aerosol Scienee, 2004, 35, (6), 665-681. A model is developed for the coalescence of unequal-sized amorphous primary particles commonly found in aerosol aggregates. Computational efficiency is achieved by deriving an analytical expression for the surface geometry of the coalescing particles and subsequently decoupling surface evolution from the fluid dynamic equations. Calculated shrinkage lengths and surface areas are in good agreement with results reported from finite element calculations for all particle size ratios considered. Results indicate that above a size ratio of 4, the normalized coalescence rate is independent of size ratio. Applying these results to the coalescence of SiO2 particles indicates that rates are primarily dependent on the diameter of the smaller particle, although coalescence times do not scale linearly with either particle size ratio or total particle mass. Comparison with the widely used exponential model of Koch and Friedlander suggests that rates of surface area decrease are under-predicted by the Koch and Friedlander model. Modified values of the decay constant for both equal sized and heterogeneously sized particles are therefore proposed, with the value decreasing with increasing size ratio of the coalescing pair.
04/03118 Modeling turbulent flow in stirred tanks with CFD: the influence of the modeling approach, turbulence model and numerical scheme Aubin, J. et al. Experimental Thermal and Fluid Science, 2004, 28, (5), 431~445.
438
Fuel and Energy Abstracts
November 2004
Single phase turbulent flow in a tank stirred by a down- and an uppumping pitched blade turbine has been simulated using computational fluid dynamics. The effect of the modelling approach, discretization scheme and turbulence model on mean velocities, turbulent kinetic energy and global quantities, such as the power and circulation numbers, has been investigated. The results have been validated by laser doppler velocimetry (LDV) data. The stationary and timedependent modelling approaches were found to have little effect on the turbulent flow, however the choice of the numerical scheme was found to be important, especially for the predicted turbulent kinetic energy. A first-order method was found to highly underestimate LDV data compared with higher order methods. The type of the turbulence model was limited to the k-e and R N G models due to convergence difficulties encountered with a Reynolds stress model and there was found to be little effect of these models on the mean flow and turbulent kinetic energy. This latter quantity was found to be largely underpredicted in the discharge region of the down-pumping impeller in comparison with LDV data. Better agreement was found for the uppumping pitched blade turbine. Estimated power numbers were found generally to be in good agreement for the down- and up-pumping data. However, the circulation number tended to be over-predicted by about 30% and 40% for the down- and up-pumping agitators, respectively.
04/03119 Modelling results for the thermal management sub-system of a combined heat and power (CHP) fuel cell system (FCS) Colella, W. G. Journal of Power Sources, 2003, 118, (1-2), 129-149. Although the fuel cells research and development community has traditionally focused the majority of its efforts on improving the fuel cell stack's voltage (electrical efficiency), combined heat and power (CHP) fuel cell system (FCSs) may achieve a competitive advantage over conventional generators only if the research and development community refocuses its efforts on cultivating other inherent technical qualities of such systems. Based on an analysis of their use within energy markets, these inherent qualities include (1) an ability to vary their electrical load rapidly, (2) an ability to vary their heat to power ratio during operation, and (3) an ability to deliver their waste heat to a useful thermal sink. This article focuses on the last of three design objectives: effectively capturing heat from a CHP FCS. This article (1) delineates the design specifications for a 6 kWe CHP FCS, (2) analyses four possible cooling loop configurations for this system, and (3) concludes which one of these provides the optimal heat recovery performance.
04/03120 Modelling the passive thermal response of a building using sparse BMS data Lowry, G. and Lee, M.-W. Applied Energy, 2004, 78, (l), 53 62. This paper describes the application of system identification to the thermal response of a conventional office space using data collected from an existing building management system. Autoregressive models are estimated and it is found that the output-error model provides the best fit between simulated and measured internal-air temperature data. The inclusion of electricity consumption data improves the fit for all model structures used, though this is evidently not modelled as providing a simple internal heat-gain. The model error is comparable with that reported for models validated in laboratory test rooms, but without the use of full instrumentation.
04•03121 Monte-Carlo simulation of unipolar diffusion charging for spherical and non-spherical particles Biskos, G. et al. Journal of Aerosol Science, 2004, 35, (6), 707-730. This paper presents a 3D Monte-Carlo model that simulates diffusion charging of aerosol particles in positive unipolar environments. Calculations are performed for Nit products up to 5× 1012 ions m -3 s (with Ni being the ion concentration and t the charging time), and particles with diameter 5-1000 nm, covering a wide range of Knudsen numbers at atmospheric pressure. Apart from the average charge, the code allows for the calculation of the charge distribution, which is shown to be well described by Gaussian statistics for monodisperse particles. Standard deviations of the charge distribution calculated with the source-and-sink approach show good agreement with the MonteCarlo results. Comparison of the Monte-Carlo calculations with Fuchs' limiting-sphere theory shows good agreement for the whole size range and highlights the importance of the image force effect for smaller particles. The diffusion-mobility theory of the continuum regime matches the simulation results for the larger particle sizes while differences with Fuchs' limiting-sphere theory in this regime are relatively small. Simulations of non-spherical particles show the power of the code to easily handle more complicated situations. Results of rectangular-shaped and elongated chain-aggregate particles show different charging behaviour compared to theoretical predictions, and indicate the importance of the assumptions for the surface distribution of charges on the particle. In contrast, calculations of 3D cross-shaped aggregate particles, despite having a very irregular geometry, indicate that the spherical shape assumption is reasonable.
04•03115 Measurement and modelling of bubble and dew points in the binary systems carbon dioxide + cyclobutanone and propane + cyclobutanone Cruz Duarte, A. R. et al. Fluid Phase Equilibria, 2003, 214, (2), 121136. The fluid phase behaviour for the binary systems carbon dioxide + cyclobutanone and propane+cyclobutanone has been determined experimentally, using Cailletet equipment. For both the systems bubble points have been determined for a number of isopleths covering the whole mole fraction range. Additionally, for the binary system carbon dioxide+cyclobutanone dew points and critical points could be observed for a number of overall-compositions rich in carbon dioxide. The temperature and pressure range were, respectively, from 278 to 369 K and from 0.1 to 14.0 MPa. Correlation of the experimental data of both systems has been performed using the Soave-Redlich-Kwong (SRK) equation of state. Satisfactory results have been achieved using only one binary interaction parameter.
04/03116 Micro-thermoelectric cooler: interfacial effects on thermal and electrical transport da Silva, L. W. and Kaviany, M. International Journal of Heat and Mass' Transfer, 2004, 47, (10 1I), 2417 2435. The flows of heat and electricity in a column-type micro-thermoelectric cooler are analysed by modelling the various interfacial resistances. Electron (barrier tunnelling) and phonon (diffuse mismatch) boundary resistances at the thermoelectric/metal interface, and thermal nonequilibrium between electrons and phonons adjacent to this interface (cooling length), increase the thermal conduction resistance and decrease the Seebeck coefficient of the thermoelectric elements. These in turn reduce the device cooling performance, which is also affected by the thermal and electrical contact resistances at the thermoelectric/ metal and metal/electrical-insulator interfaces. To produce a temperature drop of 10 K with a cooling load of 10 roW, the optimum number of thermo-element pairs, operating current, and coefficient of performance, based on vapour deposited 4 ~tm thick films of Bi2Te3 and Sb2Tes, are predicted for a micro-thermoelectric cooler operating with a 3 V battery.
04/03117 Modeling the coalescence of heterogenous amorphous particles Yadha, V. and Helble, J. J. J. J. Journal of Aerosol Scienee, 2004, 35, (6), 665-681. A model is developed for the coalescence of unequal-sized amorphous primary particles commonly found in aerosol aggregates. Computational efficiency is achieved by deriving an analytical expression for the surface geometry of the coalescing particles and subsequently decoupling surface evolution from the fluid dynamic equations. Calculated shrinkage lengths and surface areas are in good agreement with results reported from finite element calculations for all particle size ratios considered. Results indicate that above a size ratio of 4, the normalized coalescence rate is independent of size ratio. Applying these results to the coalescence of SiO2 particles indicates that rates are primarily dependent on the diameter of the smaller particle, although coalescence times do not scale linearly with either particle size ratio or total particle mass. Comparison with the widely used exponential model of Koch and Friedlander suggests that rates of surface area decrease are under-predicted by the Koch and Friedlander model. Modified values of the decay constant for both equal sized and heterogeneously sized particles are therefore proposed, with the value decreasing with increasing size ratio of the coalescing pair.
04/03118 Modeling turbulent flow in stirred tanks with CFD: the influence of the modeling approach, turbulence model and numerical scheme Aubin, J. et al. Experimental Thermal and Fluid Science, 2004, 28, (5), 431~445.
438
Fuel and Energy Abstracts
November 2004
Single phase turbulent flow in a tank stirred by a down- and an uppumping pitched blade turbine has been simulated using computational fluid dynamics. The effect of the modelling approach, discretization scheme and turbulence model on mean velocities, turbulent kinetic energy and global quantities, such as the power and circulation numbers, has been investigated. The results have been validated by laser doppler velocimetry (LDV) data. The stationary and timedependent modelling approaches were found to have little effect on the turbulent flow, however the choice of the numerical scheme was found to be important, especially for the predicted turbulent kinetic energy. A first-order method was found to highly underestimate LDV data compared with higher order methods. The type of the turbulence model was limited to the k-e and R N G models due to convergence difficulties encountered with a Reynolds stress model and there was found to be little effect of these models on the mean flow and turbulent kinetic energy. This latter quantity was found to be largely underpredicted in the discharge region of the down-pumping impeller in comparison with LDV data. Better agreement was found for the uppumping pitched blade turbine. Estimated power numbers were found generally to be in good agreement for the down- and up-pumping data. However, the circulation number tended to be over-predicted by about 30% and 40% for the down- and up-pumping agitators, respectively.
04/03119 Modelling results for the thermal management sub-system of a combined heat and power (CHP) fuel cell system (FCS) Colella, W. G. Journal of Power Sources, 2003, 118, (1-2), 129-149. Although the fuel cells research and development community has traditionally focused the majority of its efforts on improving the fuel cell stack's voltage (electrical efficiency), combined heat and power (CHP) fuel cell system (FCSs) may achieve a competitive advantage over conventional generators only if the research and development community refocuses its efforts on cultivating other inherent technical qualities of such systems. Based on an analysis of their use within energy markets, these inherent qualities include (1) an ability to vary their electrical load rapidly, (2) an ability to vary their heat to power ratio during operation, and (3) an ability to deliver their waste heat to a useful thermal sink. This article focuses on the last of three design objectives: effectively capturing heat from a CHP FCS. This article (1) delineates the design specifications for a 6 kWe CHP FCS, (2) analyses four possible cooling loop configurations for this system, and (3) concludes which one of these provides the optimal heat recovery performance.
04/03120 Modelling the passive thermal response of a building using sparse BMS data Lowry, G. and Lee, M.-W. Applied Energy, 2004, 78, (l), 53 62. This paper describes the application of system identification to the thermal response of a conventional office space using data collected from an existing building management system. Autoregressive models are estimated and it is found that the output-error model provides the best fit between simulated and measured internal-air temperature data. The inclusion of electricity consumption data improves the fit for all model structures used, though this is evidently not modelled as providing a simple internal heat-gain. The model error is comparable with that reported for models validated in laboratory test rooms, but without the use of full instrumentation.
04•03121 Monte-Carlo simulation of unipolar diffusion charging for spherical and non-spherical particles Biskos, G. et al. Journal of Aerosol Science, 2004, 35, (6), 707-730. This paper presents a 3D Monte-Carlo model that simulates diffusion charging of aerosol particles in positive unipolar environments. Calculations are performed for Nit products up to 5× 1012 ions m -3 s (with Ni being the ion concentration and t the charging time), and particles with diameter 5-1000 nm, covering a wide range of Knudsen numbers at atmospheric pressure. Apart from the average charge, the code allows for the calculation of the charge distribution, which is shown to be well described by Gaussian statistics for monodisperse particles. Standard deviations of the charge distribution calculated with the source-and-sink approach show good agreement with the MonteCarlo results. Comparison of the Monte-Carlo calculations with Fuchs' limiting-sphere theory shows good agreement for the whole size range and highlights the importance of the image force effect for smaller particles. The diffusion-mobility theory of the continuum regime matches the simulation results for the larger particle sizes while differences with Fuchs' limiting-sphere theory in this regime are relatively small. Simulations of non-spherical particles show the power of the code to easily handle more complicated situations. Results of rectangular-shaped and elongated chain-aggregate particles show different charging behaviour compared to theoretical predictions, and indicate the importance of the assumptions for the surface distribution of charges on the particle. In contrast, calculations of 3D cross-shaped aggregate particles, despite having a very irregular geometry, indicate that the spherical shape assumption is reasonable.