Using liquid air as a way to store energy

Paper presented at the 8th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Mauritius, 11-13 July, 2011.Energy storage is becoming an issue in a world were renewable energy will probably be the major way of energy production. Liquefaction of gasses in order to store the liquid is being looked at. This liquid can then be expanded in a Rankine cycle in order to recover the energy. If waste heat is used as input for the boiler, this can be an interesting technology to raise energy efficiency in industry. In this paper an analysis is made of the efficiency of storing liquefied air. Starting from the thermodynamics of the basic cycle, more complex combined cycles are studied. It is clear the ideal cycles have a good efficiency. Taking real expander efficiencies into account reduces the output a lot. Using combined cycles for liquefaction and energy production do not raise efficiency in a significant way. Conversion efficiencies are in the order of 20 to 50% , making these cycles comparable to hydrogen storage and compressed air storage.mp201

Short-term health effects in the general population following a major train accident with acrylonitrile in Belgium

Background: Following a train derailment, several tons of acrylonitrile (ACN) exploded, inflamed and part of the ACN ended up in the sewage system of the village of Wetteren. More than 2000 residents living in the close vicinity of the accident and along the sewage system were evacuated. A human biomonitoring study of the adduct N-2-cyanoethylvaline (CEV) was carried out days 14-21 after the accident. Objectives: (1) To describe the short-term health effects that were reported by the evacuated residents following the train accident, and (2) to explore the association between the CEV concentrations, extrapolated at the time of the accident, and the self-reported short-term health effects. Methods: Short-term health effects were reported in a questionnaire (n=191). An omnibus test of independence was used to investigate the association between the CEV concentrations and the symptoms. Dose-response relationships were quantified by Generalized Additive Models (GAMs). Results: The most frequently reported symptoms were local symptoms of irritation. In non-smokers, dose-dependency was observed between the CEV levels and the self-reporting of irritation (p=0.007) and nausea (p=0.007). Almost all non-smokers with CEV concentrations above 100 pmol/g globin reported irritation symptoms. Both absence and presence of symptoms was reported by non-smokers with CEV concentrations below the reference value and up to 10 times the reference value. Residents who visited the emergency services reported more symptoms. This trend was seen for the whole range of CEV concentrations, and thus independently of the dose. Discussion and conclusion: The present study is one of the first to relate exposure levels to a chemical released during a chemical incident to short-term (self-reported) health effects. A dose-response relation was observed between the CEV concentrations and the reporting of short-term health effects in the non-smokers. Overall, the value of self-reported symptoms to assess exposure showed to be limited. The results of this study confirm that a critical view should be taken when considering self-reported health complaints and that ideally biomarkers are monitored to allow an objective assessment of exposure

Influence of porosity and pore density on the thermal and hydraulic performance of metal foam heat exchangers

Paper presented to the 10th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Florida, 14-16 July 2014.Metal foam shows a great potential for heat transfer applications. In this work the influence of the volumetric porosity, the pore density and the foam material (aluminum or copper) on the heat transfer and pressure drop characteristics are investigated. Two-dimensional simulations are performed using a porous medium approach: the Darcy-Forchheimer-Brinkman flow model is combined with the two-equation energy model. Round tube heat exchanger with a staggered tube layout are considered. Simulations are performed for inlet velocities between 1.2 m/s and 3.2 m/s. The validation experiment shows a good match between the simulations and the measurements, proving the quality of the simulations. It is found that the friction factor is mainly determined by the porosity, while the Colburn j-factor is mainly determined by the pore density. For a given pumping power the heat exchanger volume increases with decreasing PPI value to perform the same heat duty. For the same PPI value and a fixed pumping power, the heat exchanger volume increases with increasing porosity. For the same fan power, the heat transfer rate for copper foam is up to 20% higher compared to foam made of the aluminum alloy AlSi7Mg0.3. Comparison to a bare tube bundle shows that the heat transfer rate of a foamed heat exchanger is up to 6 times larger for the same fan power. This article illustrates that optimization is required to design heat exchangers which fully benefit from the unique advantages of open-cell metal foam.dc201

Thermal influence of non-uniform inlet flow conditions for a commercial plate fin heat exchanger

Paper presented to the 10th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Florida, 14-16 July 2014.In this study, a commercially available plate heat exchanger with wavy fins is experimentally characterized under uniform and three non-uniform flow conditions. The heat exchanger is 275 mm wide and 295 mm high and has a distance in-between the plates of 3 mm and the wavy fins are 11 mm high. Internally, the heat exchanger is finned with offset strip fins. Hot water at 55°C is sent through the plates and the waterside mass flow rate is kept high in order to minimize the internal thermal resistance. For the internal convection coefficient, a correlation from literature is used. The airside mass flow rate is varied by adjusting the fan speed. In case of uniform flow conditions, the frontal air velocity varies between 1.7 and 7.6 m/s. The airside velocity is measured over a nozzle and the uniformity of the wind tunnel is verified by hot wire measurements. Three non-uniformities are placed 10 cm upstream of the heat exchanger: the first one covers the right-hand side of the heat exchanger, the second one covers the top half of the heat exchanger and the last (and most severe) non-uniformity consists of a circular hole of 150 mm diameter in the middle of a plate. Only the third non-uniformity has a significant influence on the heat transfer rate: up to 25% higher for the external convective resistance in comparison with the uniform case.dc201

Numerical study of the thermal and hydraulic characteristics of a round tube heat exchanger with louvered fins and delta winglets

Paper presented at the 8th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Mauritius, 11-13 July, 2011.Louvered fin and round tube heat exchangers are widely used in air conditioning devices and heat pumps. In this study the effect of punching delta winglet vortex generators into the louvered fin surface in the near wake region of each tube was investigated. Numerical simulations were performed on the compound design and the thermal and hydraulic characteristics were evaluated. It was found that the delta winglets can significantly reduce the size of the wake regions. This results in an enhanced heat transfer. Further, it was shown that the vortices do not propagate far downstream. Due to the flow deflection they are destroyed in the downstream louver bank. For the configuration studied, the pressure drop penalty of adding vortex generators was also significant, indicating that an optimization is necessary to select a compound design with improved overall performance.mp201

Modelling heat and moisture transport in porous materials with CFD for building applications

Paper presented at the 8th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Mauritius, 11-13 July, 2011.Heat and moisture transport in buildings have a large impact on the building envelope durability, the energy consumption in buildings and the indoor climate. Nowadays HAM (Heat, Air and Moisture) models are widely used to simulate and predict the effect of these transport phenomena in detail. Recently these HAM models are being coupled to CFD (Computational Fluid Dynamics) to study the moisture exchange between air and porous materials on a local scale (microclimates). A direct coupling approach between CFD and HAM is applied. The transport equations for heat and moisture in a porous material are directly implemented into an existing CFD package and the transport equations in the air and in the porous material are solved in one iteration by only one solver. In this paper a model for moisture transport in the hygroscopic range and over-hygroscopic range is developed. This way a broad range of problems can be tackled such as drying phenomena and interstitial condensation in building components. The model is verified and validated with data from literature.mp201

Increasing the efficiency and generated electricity of organic rankine cycles by using zeotropic mixtures as working fluids

Paper presented at the 9th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Malta, 16-18 July, 2012.We present a systematic and comparative study of the potential of zeotropic mixtures as working fluids in ORCs, considering most of the commonly used hydrocarbon and siloxane substances as components in various concentrations. We investigate the impact on the operation, the efficiency and power output of an ORC. The ORC cycle is realistically simulated in steady state conditions, taking into account all elements of an actual cycle (including an internal heat exchanger). By performing a pinch analysis, the power output is maximized, given the heat profiles of both the heat source and heat sink. The use of suitable zeotropic mixtures as working fluids has a positive effect on the ORC performance in all investigated cases. The potential increase in cycle efficiency and generated electricity is larger for lower temperature heat sources and when the temperature drop over the heat source exchanger is larger. When the ORC is optimized for operation with a low temperature heat source, the potential for increase in electricity production (maximum reported value approximately 23%) by using mixtures is particularly remarkable.dc201

Applying the volume averaging theory to open-cell metal foam in natural convection/radiation

Papers presented to the 11th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, South Africa, 20-23 July 2015.Heat sinks made out of open-cell aluminium foam are investigated numerically in natural convection. Results derived from a 2D numerical model are compared to results for in-house experiments. Different foam heights are studied. The numerical model is based on the volume averaging theory. The aluminium foam that is used has 10 pores per linear inch and a porosity of 93%. The temperature of the substrate was varied between 55°C and 95°C. The geometry used in the numerical model replicates the experimental test rig as well as possible. A discussion of the determination of the closure terms is given. If only convective heat transfer is taken into account in the numerical model, the relative differences between the numerical and experimental results are smaller than 29% for all foam heights studied. However, when the influence of radiation is included in the numerical model, it is shown that the numerical results differ less than 9% with the experimental ones. This validates the choice of closure terms used in the model and this shows that it is necessary to properly model radiative heat transfer in numerical models of open-cell aluminium foam in natural convection.am201