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

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

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

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

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

Feasibility study of a plastic helical coil heat exchanger for a domestic water storage tank

The main goal of this study is to investigate whether it is possible to use a polymeric helical coil heat exchanger as an alternative to conventional metallic helical coil. More specifically this work focuses on a helical coil design for a domestic water storage tank application. Corrosion and fouling resistance, scarcity of the materials, low weight and cost are the driving forces to consider designing polymeric heat exchangers rather than metallic heat exchangers. However, simply replacing the metallic material by the polymer material and applying the traditional design methods used for metallic heat exchangers do not lead to an acceptable design. If one wants to design a good polymer heat exchanger, heat transfer and structural problems have to be solved first. In addition, the pressure drop limitation should not be neglected. In order to reach this goal, a model is developed to predict the optimal design of a helical coil heat exchanger immersed in the water storage tank for a certain water mass flow rate and temperature. This design compensates for the low thermal conductivity and strength of the polymer.Papers presented to the 12th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Costa de Sol, Spain on 11-13 July 2016

Thermo-hydraulic comparison of 10 PPi metal foam and louvered fins for low velocity applications

Paper presented at the 9th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Malta, 16-18 July, 2012.To maximize the effectiveness and thus minimize the airside resistance, a relatively new heat enhancing material, aluminium foam, is compared with the current state-of-the-art, louvered fins, in a wind tunnel experiment. The comparison between both heat exchangers is done based on a well-defined Performance Evaluation Criterion (PEC), taking heat transfer and pressure drop in account. Furthermore, as the studied heat exchangers are so-called ‘low-capacity’ units, a non-uniform temperature field downstream the heat exchanger is induced. Therefore an area-mean temperature reading has been developed, with the aid of an infra red camera. Finally, the contribution of the contact resistance to the overall thermal resistance of these pressed-fit heat exchangers is investigated.dc201

Influence of the geometry on the thermohydraulics of a compound heat exchanger consisting of louvered fins and delta winglets

Paper presented at the 9th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Malta, 16-18 July, 2012.Louvered fin heat exchangers with round tubes are frequently used in heating, ventilation, air conditioning and refrigeration applications. In this paper delta winglet vortex generators are punched out of the louver surface, resulting in a so called compound design. Three-dimensional numerical simulations were performed. The delta winglets serve to reduce the size of the tube wakes, which are zones of poor heat transfer. A screening analysis of the most important geometrical parameters showed that the delta winglet geometry highly contributes to the thermal and hydraulic performance at low Reynolds numbers, while at higher Reynolds numbers the performance is mainly determined by the louver geometry. The compound heat exchanger has a better thermal hydraulic performance than when the louvers or the delta winglets are applied separately. The performance of the compound design is also compared to louvered, slit and plain fin heat exchangers. This clearly shows its potential. Especially for low Reynolds applications, the compound heat exchanger can be made smaller in size and thus more economical in cost

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