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In the frame of EU-project sCO2-flex the design of a 25 MWe supercritical CO2 (sCO2) Brayton cycle will be designed. The system will be optimized to meet flexibility requirements, while reducing environmental impact and focusing on cost efficiency. In the context of a sCO2 Brayton cycle, the gas cooler is a key component to achieve a high overall efficiency. Close to the critical point, due to varying properties, heat transfer and pressure drop of carbon dioxide (CO2) are difficult to predict. In case of vertical flow, acceleration and buoyancy effects induced by strong density gradients can cause a significant deterioration of the heat transfer. In this publication, the cooling heat transfer coefficient (htc) is investigated in a 3 mm diameter tube with vertical flow orientation. Commonly used calculation methods of the heat transfer coefficient are presented. Although developed for heating of sCO2, the mixed convection criterion of Jackson and Hall [7] is used to evaluate the heat transfer deterioration. The effects of the CO2 mass flux of 141 − 354 kg/m2s and bulk fluid temperatures of 20 − 50 °C with a constant pressure of 80 bar on the heat transfer were examined. The transition between forced and mixed convection can be explained by the htc -values. The upwards flow shows a steady decrease in the htc with the reduction of the mass flux. However, the downwards flow shows significant effects of buoyancy. At low mass flux the distinct peak in the htc at the pseudocritical temperature (Tpc) disappears., Conference Proceedings of the European sCO2 Conference4th European sCO2 Conference for Energy Systems: March 23-24, 2021, Online Conference, p. 146

In the frame of the sCO2-QA (qualification of analysis tools for the evaluation of a residual heat-driven, self-sufficient system for decay heat removal) project, a residual heat-driven self-sufficient sCO2-operated decay heat removal system based on a Brayton cycle is simulated with the German thermal-hydraulics system code ATHLET (analysis of thermal-hydraulics of leaks and transients). The heat removal system consists of a compact heat exchanger in the containment, a turbo-compressor system located in the reactor building and a gas cooler in the outdoor area. The validation of ATHLET and other numerical codes as well as understanding the heat transfer characteristics of sCO2 near the critical point requires experimental data. At IKE (Institute of Nuclear Technology and Energy Systems), the SCARLETT (supercritical carbon dioxide loop at IKE Stuttgart) test facility is available for various experiments with sCO2. This publication includes an experimental investigation of the thermal stratification in heated horizontal sCO2 pipe flows. For this investigation, eight test series with overall 48 experiments were carried out in two pipes with inner diameters of 4 mm and 8 mm. The experiments were carried out at a pressure of approximately 7.75 MPa. The target values of mass flux were set at 400 kg/m2s and 800 kg/m2s and those of heat flux at 50, 90 and 130 kW/m2, resulting in a heat to mass flux ratio of 62.5-225 J/kg. The inlet Reynolds numbers are between 16000 and 120000. The measured parameters are the flow rate, the pressure, the inlet and outlet fluid temperature as well as the outer surface temperature along the test pipe in three different radial angles. The results show the influence of the pipe diameter, Reynolds number, mass and heat flux on the temperature stratification. Also, inflow lengths were determined for a fully developed temperature stratification. This data set can be used for the validation of computer codes., Conference Proceedings of the European sCO2 Conference4th European sCO2 Conference for Energy Systems: March 23-24, 2021, Online Conference, p. 38

In the frame of EU-project sCO2-flex the design of a 25 MWe supercritical CO2 (sCO2) Brayton cycle will be developed. The system will be optimized to meet flexibility requirements, while reducing environmental impact and focusing on cost efficiency. In the context of a sCO2 Brayton cycle, the gas cooler is a key component to achieve a high overall efficiency. Close to the critical point, due to varying properties, heat transfer and pressure drop of carbon dioxide (CO2) are difficult to predict. By performing experiments with the “SCARLETT” facility of the University of Stuttgart, expertise will be gained using CO2 as working fluid in the pseudocritical region. The results of the fundamental experiments will be used for validation and improvement of correlations to develop heat exchangers working with high efficiency. The heat transfer and pressure drop of carbon dioxide near the critical point cooled in a 2 mm diameter was investigated. The outer wall surface temperature is determined by soldering calibrated T-type thermocouples into the copper tube wall. Thermocouples are evenly distributed along the 1.2 m cooled length. The effects of the CO2 mass flux of 400-1400 kg/m²s, inlet pressures of 7.7-8.5 MPa, bulk fluid temperatures of 10-85 °C and the flow orientation (upward, downward and horizontal) on the heat transfer and pressure drop were examined., Conference Proceedings of the European sCO2 Conference3rd European Conference on Supercritical CO2 (sCO2) Power Systems 2019: 19th-20th September 2019, p. 255

Conference Proceedings of the European sCO2 Conference3rd European Conference on Supercritical CO2 (sCO2) Power Systems 2019: 19th-20th September 2019, p. 39

Die Aufgabenstellung dieser Arbeit umfaßt die theoretische und experimentelle Untersuchung der Wärmeübertragungsphänomene beim Behältersieden von Kohlenwasserstoffen an rohrförmigen Wärmeübertragungsoberflächen. Die Untersuchung solcher Wärmeübertragungsoberflächen, speziell Oberflächen mit hinterschnittenen Strukturen, soll der Weiterentwicklung von kompakten Wärmeübertragern dienen. Es wurden Experimente mit Rohren aus verschiedenen Materialien (Cu, CuNi, Edelstahl, St35.8) mit unterschiedlich strukturierten Oberflächen durchgeführt. Als Wärmeträger wurden der Kohlenwasserstoff Propan und in Einzelfällen auch der Kohlenwasserstoff n-Pentan bei Sättigungstemperaturen im Bereich von 253 K bis 293 K verwendet. Zur Beurteilung der Verbesserung der Wärmeübertragung durch den Einsatz strukturierter Oberflächen wurden Experimente mit glatten Referenzoberflächen herangezogen. Zur Visualisierung der Wärmeübertragungsphänomene und der Zweiphasenströmung von den Verdampferoberflächen wurde ein Hochgeschwindigkeitsvideosystem verwendet. Die visualisierten Vorgänge bei der Wärmeübertragung mit Phasenwechsel wurden mit einer am IKE entwickelten Software zur digitalen Bilddatenverarbeitung bezüglich wichtiger Verdampfungsparameter wie zum Beispiel Blasenfrequenz und Blasenabreissdurchmesser ausgewertet., The topic of this thesis comprises of the theoretical and experimental investigation of the heat transfer phenomena of hydrocarbons from tubular heat transfer surfaces in the pool boiling mode. The results of the investigations of these heat transfer surfaces, especially of surfaces with re-entrant cavities, will be used to improve the efficiency of compact heat exchangers. The tested tubes, made of different wall materials (Cu, CuNi, Stainless Steel, St35.8), were provided with various surface structures. The hydrocarbon propane, and for some test also n-pentane, were used as working fluids at saturation temperatures in the range from 253 K to 293 K. The improvement factors of the structured surfaces are determined by comparison of their heat transfer coefficients with those of smooth reference surfaces. A high-speed videosystem was used to visualize the boiling phenomena and the two-phase flow from the heat transfer surfaces. A software for digital image processing, developed at IKE, is used for the evaluation of the video data to provide information about important boiling parameters, e.g. bubble departure diameter and bubble frequency.