Heimo Walter

Flow reversal in natural circulation systems

Boiling flow in natural circulation systems can be susceptible to thermohydrodynamic instabilities which can be the cause of mechanical damage. This paper presents an analysis of a circulation system where two unequal heated riser circuits are connected to a common downcomer. It can be shown that for such systems a critical heat absorption ratio exists which is dependent on the geometry and the overall heating conditions. Based on computer simulations, the static and dynamic behaviour of the riser systems with emphasis on the lower heat absorption circuit is discussed. Criteria to avoid flow reversal are presented.

Simulation von Kraftwerken und Feuerungen

Dieses Lehrbuch beinhaltet neben den Grundlagen der Stromungssimulation auch die Modellbildung von Komponenten und Gesamtanlagen der Kraftwerkstechnik. Die Grundgleichungen fur den Warme- und Stoffaustausch werden fur die Anwendung in der numerischen Simulation aufbereitet. Ausgewahlte numerische Methoden werden ausfuhrlich diskutiert. Inhalt des Buches ist die Simulation von Feuerungen und Gasstromungen als auch die der Wasser/Dampfstromung. Regelung und Steuerung, vereinfachte Modelle und Hybridmodelle sowie die Validierung von Messwerten werden ebenfalls behandelt. Zahlreiche ausgearbeitet Beispiele und Illustrationen sollen dem Leser das Verstandnis der Zusammenhange von Theorie und Praxis erleichtern. Einfache Plausibilitatskontrollen sollen dem Studierenden zeigen, wie er seine numerischen Ergebnisse einer ersten Uberprufung unterziehen kann. In der 2., korrigierten und erweiterten Auflage wird die noch relativ neue Discrete Element Method in Theorie und Praxis aufgenommen. Einzelne Kapitel wurden erganzt, wie zum Beispiel die Simulation und das Monitoring von Speisewasserpumpen. Das Lehrbuch wendet sich an Studierende und Forscher sowie an in der Industrie tatige Praktiker, die hier Anregungen fur ihre berufliche Tatigkeit finden konnen.

Dynamic simulation of natural circulation steam generators with the use of finite-volume-algorithms – A comparison of four algorithms

Abstract The paper reports the results of a comparison of the four finite-volume-algorithms SIMPLE, SIMPLEC, SIMPLER and PISO. The analysis has been done to explore and compare the suitability of the algorithms for the dynamic simulation of natural circulation steam generators based on a so called tube-header-model. The suitability of the algorithms was tested with the results of two simulation examples – a heat recovery steam generator with a horizontal tube bank ( Fig. 3 a) and a two-drum steam generator with a vertical tube bank ( Fig. 3 b). The calculations are done at low system pressure and under hot start-up conditions for the boilers. The results of the computations demonstrate that the SIMPLER algorithm should be preferred for dynamic simulations which are based on the tube-header-model. The algorithm shows a high stability, an extended field of variability of the under-relaxation factors as well as small required under-relaxation. The strong dependence of the calculation time on the number of control volumes is the only unfavourable characteristic of the SIMPLER algorithm.

Experimental and Numerical Investigation of the Gas Side Heat Transfer and Pressure Drop of Finned Tubes—Part I: Experimental Analysis

In this study, a heat transfer and pressure drop correlation are determined for helically I- and U-shaped finned tubes as well as for solid I-finned tubes at constant transverse and longitudinal spacing. In the heat transfer correlation, the influence of the number of tube rows arranged in flow direction is taken into consideration. A detailed description of the test rig and the data reduction procedure is presented. A thorough uncertainty analysis was performed to validate the results. The investigation has shown that the influence of the fin geometry on the heat transfer of the helically segmented I- and U-shaped tubes can be disregarded. The heat transfer correlation, which is valid for the helically segmented I- and U-shaped tubes in a staggered arrangement, can describe 90% of all measurement data within ±15%. All measurements are performed for constant transverse and longitudinal spacing. For the pressure drop coefficient, two new correlations, which are only valid for helically segmented U shaped finned tubes in a staggered arrangement, show an average deviation of approximately ±13% for 90% of all measurement results. All new correlations are compared with correlations from open and established literature for industrial boiler applications. The new heat transfer and pressure drop correlations show a relative deviation of ±20% in comparison with correlations in open literature. The new pressure drop correlations show the same characteristic as most correlations in the open literature.

Flow Stability of Heat Recovery Steam Generators

This paper presents the results of theoretical flow stability analyses of two different types of natural circulation heat recovery steam generators (HRSG)-a two-drum steam generator-and a HRSG with a horizontal tube bank. The investigation shows the influence of the boiler geometry on the flow stability of the steam generators. For the two-drum boiler, the steady-state instability, namely, a reversed flow, is analyzed. Initial results of the investigation for the HRSG with a horizontal tube bank are also presented. In this case, the dynamic flow instability of density wave oscillations is analyzed.

Transient Analysis of an Improved Finned Tube Heat Exchanger for Thermal Energy Storage System

The melting and solidification process of sodium nitrate, which is used as energy storage material, is studied in a vertical arranged energy storage device with two different bimetal finned tube designs (with and without transversal fins) for enhancing the heat transfer. The finned tube design consists of a plain steel tube while the material for the longitudinal (axial) fins is aluminum. The investigation analyses the influence of the transversal fins on the charging and discharging process. 3-dimensional transient numerical simulations are performed using the ANSYS Fluent 14.5 software. The results show that every obstruction given by transversal fins reduces the melting and solidification velocity in direction to the outer shell. In the present study also a comparison of the simulation results between 2D and 3D simulation of the melting and solidification behavior of the sodium nitrate is presented.Copyright

Numerical Simulation of Power Plants and Firing Systems

The book comprises the fundamentals of the numerical simulation of fluid flows as well as the modelling of a power plant and plant components. The fundamental equations for heat and mass transfer will be prepared for the application in the numerical simulation. Selected numerical methods will be discussed in detail. The book will deal with the gas as well as with the water/steam flow. Regulation and controller, simplified models and hybrid models as well as the validation of measurement data are also included in the book.

New Design of a Bimetallic Finned Tube for the Use in Latent Heat Thermal Energy Storage Units

The use of finned tubes as enhancement method to increase the heat flow rate into a phase change material, which has in many cases a low thermal conductivity, is a common method. A highly efficient and easy-to-assemble solution for finned heat exchanger tubes is a key component for innovative thermal energy storage systems which play a key-role in electricity production and industrial heat management.In the present article the results of the investigation for different designs of bimetallic heat exchanger tubes is presented. These tube designs are developed for the use in latent heat thermal energy storage systems (LHTES) at a medium temperature range. For the use in latent heat thermal energy storage systems, the probably high pressure of the heat transfer medium and the high temperature differences between the operating temperature and the ambient temperature are challenging. Therefore, the bimetallic finned heat exchanger tube consists of a steel tube, where the heat transfer fluid flows, and an aluminum tube with longitudinal fins, which should improve the heat transfer to the phase change material. Due to different thermal expansion coefficients, displacements of the tubes are given. To guarantee a high heat transfer rate between the two connected tubes the contact between aluminum and steel plays an important role.In the present study 4 prototypes (including the new design) were designed, analyzed and compared on the connection strength. Long-term tests for simulating the application in a LHTES were done to determine the creep rupture properties of the compositions. All prototypes were tested successfully; the new design is convinced in many aspects of that challenge and is submitted to the Austrian patent office. Main advantages of the new design are the simple production and assembling compared to other analyzed prototypes. Furthermore, the new design shows the best results under the analyzed operation conditions and the layout of the geometry has a high optimization potential in terms of stresses.Copyright

Flow Reversal in a Horizontal Type Natural Circulation Heat Recovery Steam Generator

Natural circulation heat recovery steam generators (HRSG) are used in many applications for the thermal recycling of the waste flue gas. Many of these boilers are designed as a horizontal type HRSG (see fig. 1). The evaporator of such a steam generator is characterized by an array of parallel tubes with different heat input. The paper presents the results of a theoretical stability analysis for a HRSG with a vertical tube bank. For the horizontal type HRSG the static instability, namely the reverse flow was analysed. The study was done at low system pressures and under hot start-up conditions for the boiler. The investigations show the influence of the geometry, the system pressure and the heat absorption of the individual tubes in the evaporator on the stability of the boiler. The aim of the study was to find design criteria to avoid reverse flow in the tubes of the evaporator. Addition of flow resistance at certain locations of the evaporator can improve the stability. A higher stability will be also achieved by the homogenization of the heat absorption in the individual layers of the bundle heating surface.Copyright

Durability of a fin-tube latent heat storage using high density polyethylene as PCM

Polymers have rarely been used as storage materials in latent heat storages up to now. Thus, we systematically screened all polymers available on a large-scale, selected promising ones based on their theoretical properties and experimentally tested more than 50 candidates. We found that polyethylene, polyoxymethylene and polyamides are promising even as recycled material. Especially high density polyethylene (HDPE) turned out to be suitable as was shown by detailed thermophysical characterization including more than 1000 heating and cooling cycles for INEOS Rigidex HD6070EA. We built a storage with 170 kg HDPE and a total mass of 600 kg based on a fin-tube heat exchanger and characterized its energy capacity, power characteristics and temperature profiles using a thermal oil test rig. In total we performed 30 melting and crystallization cycles where the whole storage was above 100 °C for more than 140 hours. After usage we examined the interior of the storage by cutting it into various pieces. A thin layer of degradation was observed on the surfaces of the PCM which is most likely related to thermo-oxidative degeneration of HDPE. However, the bulk of the PCM is still intact as well as the heat exchanger itself.