Linear stabitlity analysis of a supercritical loop

Paper presented at the 9th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Malta, 16-18 July, 2012.Because of their unique properties, supercritical fluids are becoming increasingly popular for industrial applications. These fluids behave liquid like at low temperatures and gas like at higher temperatures, with a smooth transition in between. This makes them very suited as a solvent for chemical extraction and separation processes. Another important use is as a power fluid. Modern fossil fuel fired power plants all operate using supercritical water, and on a smaller power scale they are considered for organic rankine cycles and refrigeration. As they heat up, the density of a supercritical fluid changes shows a very sharp drop for temperatures close to the critical point. This large density difference can be used as the driving force to circulate the fluid in a loop, rather than using a pump. This idea is similar to natural circulation boiling loops, but the density difference is larger. It adds a layer of inherent safety to a design, as active components such as pumps are no longer required; but also adds an additional complexity: flow instabilities. It is well known from natural circulation boiling systems, that these loops can become unstable under certain conditions (e.g. high power and low flow rate). In this study, a simple supercritical loop is studied to determine the neutral stability boundary. This is done through linear stability analysis: the set of one-dimensional governing equations is first linearised and then the eigenvalues are determined. These describe the response, indicating if it is stable or not. The results indicate that there is a clear unstable area, which can be linked to different types of instabilities.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

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

Thermal Conductivity of Carbon Nanotubes and their Polymer Nanocomposites: A Review

Thermally conductive polymer composites offer new possibilities for replacing metal parts in several applications, including power electronics, electric motors and generators, heat exchangers, etc., thanks to the polymer advantages such as light weight, corrosion resistance and ease of processing. Current interest to improve the thermal conductivity of polymers is focused on the selective addition of nanofillers with high thermal conductivity. Unusually high thermal conductivity makes carbon nanotube (CNT) the best promising candidate material for thermally conductive composites. However, the thermal conductivities of polymer/CNT nanocomposites are relatively low compared with expectations from the intrinsic thermal conductivity of CNTs. The challenge primarily comes from the large interfacial thermal resistance between the CNT and the surrounding polymer matrix, which hinders the transfer of phonon dominating heat conduction in polymer and CNT. This article reviews the status of worldwide research in the thermal conductivity of CNTs and their polymer nanocomposites. The dependence of thermal conductivity of nanotubes on the atomic structure, the tube size, the morphology, the defect and the purification is reviewed. The roles of particle/polymer and particle/particle interfaces on the thermal conductivity of polymer/CNT nanocomposites are discussed in detail, as well as the relationship between the thermal conductivity and the micro- and nano-structure of the composite

Long-term safety, tolerability, and efficacy of efgartigimod (ADAPT+): interim results from a phase 3 open-label extension study in participants with generalized myasthenia gravis

ObjectiveADAPT+ assessed the long-term safety, tolerability, and efficacy of efgartigimod in adult participants with generalized myasthenia gravis (gMG).MethodsADAPT+ was an open-label, single-arm, multicenter, up to 3-year extension of the pivotal phase 3 ADAPT study. Efgartigimod was administered in treatment cycles of 4 intravenous infusions (one 10 mg/kg infusion per week). Initiation of subsequent treatment cycles was individualized based on clinical evaluation. Safety endpoints included incidence and severity of adverse events. Efficacy endpoints assessed disease severity using Myasthenia Gravis-Activities of Daily Living (MG-ADL) and Quantitative Myasthenia Gravis (QMG) scores.ResultsAs of January 2022, 151 participants had rolled over to ADAPT+ and 145 had received ≥1 dose of efgartigimod, of whom, 111 (76.6%) were AChR-Ab+ and 34 (23.4%) were AChR-Ab−. Mean study duration (treatment plus follow-up) was 548 days, and participants received up to 17 treatment cycles, corresponding to 217.6 participant-years of exposure. In the overall population, 123 (84.8%) participants reported ≥1 treatment-emergent adverse event; most frequent were headache (36 [24.8%]), COVID-19 (22 [15.2%]), and nasopharyngitis (20 [13.8%]). Clinically meaningful improvement (CMI) in mean MG-ADL and QMG scores was seen as early as 1 week following the first infusion across multiple cycles in AChR-Ab+ and AChR-Ab− participants. Maximal MG-ADL and QMG improvements aligned with onset and magnitude of total IgG and AChR-Ab reductions. For AChR-Ab+ participants at any time point in each of the first 10 treatment cycles, more than 90% had a maximum reduction of ≥2 points (CMI) in MG-ADL total score; across the 7 cycles in which QMG was measured, 69.4% to 91.3% of participants demonstrated a maximum reduction of ≥3 points (CMI) in QMG total score. Many participants demonstrated improvements well beyond CMI thresholds. In AChR-Ab+ participants with ≥1 year of combined follow-up between ADAPT and ADAPT+, mean number of annualized cycles was 4.7 per year (median [range] 5.0 [0.5–7.6]).ConclusionResults of ADAPT+ corroborate the substantial clinical improvements seen with efgartigimod in ADAPT and support its long-term safety, tolerability, and efficacy, as well as an individualized dosing regimen for treatment of gMG.Clinical trial registrationhttps://classic.clinicaltrials.gov/ct2/show/NCT03770403, NCT03770403

Comparison of Earth-Air and Earth-Water Ground Tube Heat Exchangers for Residentialal Application

Earth-air systems are already commonly applied as a passive technique to reduce the overall energy use of buildings by reducing the required cooling or heating demand. However, they often require large surface area for their installation, and make use of large diameter tubes to reduce the pressure drop. As an alternative, waterearth systems are being considered. To explore these two options, one dimensional analytical models were derived. The impact of different design parameters, including tube length, tube diameter, fluid flow rate, etc., have been investigated. For the earth-air system, it is shown that for high Reynolds numbers, the soil resistance is in fact dominating, and as such this should be carefully considered in the design. For the water-earth systems, the addition of a compact heat exchanger to transfer the heat to the air, has a strong impact on the overall performance compared to an earth-air system. To allow for comparable performance, a compact heat exchanger with a high effectiveness is required (0.8 or higher). A strong interaction between the effectiveness of these two heat exchangers was found as the water flow rate varies. For the earth-water heat exchanger, the soil resistance is even more dominant than for the earth-air system