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Session: Experiments
Session starts: Tuesday 08 October, 15:40
Presentation starts: 16:00
Room: Willem Burger Zaal

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Florian Heberle (Universität Bayreuth, Zentrum für Energietechnik (ZET), Lehrstuhl für Technische Thermodynamik und Transportprozesse (LTTT))
Markus Preißinger (Universität Bayreuth, Zentrum für Energietechnik (ZET), Lehrstuhl für Technische Thermodynamik und Transportprozesse (LTTT))
Theresa Weith (Universität Bayreuth, Zentrum für Energietechnik (ZET), Lehrstuhl für Technische Thermodynamik und Transportprozesse (LTTT))
Dieter Brüggemann (Universität Bayreuth, Zentrum für Energietechnik (ZET), Lehrstuhl für Technische Thermodynamik und Transportprozesse (LTTT))

Abstract:
EXTENDED ABSTRACT INTRODUCTION Organic Rankine Cycle (ORC) for high-temperature industrial waste heat recovery or biomass combined heat and power plants has a high growth potential (Tchanche et al., 2011). Due to high efficiency, low toxicity and zero Global Warming Potential, siloxanes are often used as working fluids in ORC systems with heat source temperatures higher than 200 °C. For this class of substance numerous investigations concerning fluid properties and cycle efficiency have been performed (Colonna et al., 2006; Lai et al., 2009; Angelino and Colonna di Paliano, 2000; Drescher and Brüggemann, 2007; Fernández et al., 2011). Regarding experimental studies, Spinelli et al. (2011) present a construction to investigate flows of typical ORC turbine passages for a siloxane. Other interesting fields represent heat transfer characteristics and thermal stability. In this context experimental data are necessary for identification of appropriate predictive correlations and limitations in context of fluid decomposition. In addition, obtained data for pure fluids can serve as database for evaluation of ORC systems with fluid mixtures as working fluids regarding the additional heat transfer area (Angelino and Colonna di Paliano, 1998; Heberle et al., 2012). In this paper convective flow boiling of hexamethyldisiloxane (MM), octamethyltrisiloxane (MDM) and decamethyltetrasiloxane (MD2M) is examined. Local heat transfer coefficients are measured depending on pressure, heat flux density and mass flux density. In the context of thermal stability decomposition products of MM are analyzed by gas chromatography for increasing temperature. EXPERIMENTAL SETUPS Test rig for heat transfer coefficients at evaporation The realized test rig enables experimental data for local heat transfer coefficients at evaporation with temperatures up to 250 °C. A scheme is shown in Fig. 1. The circuit of the examined fluid includes a pump, a preheater, a test section for convective flow boiling, a throttle valve, a condenser and a reservoir tank. Pressure and temperature sensors at inlet and outlet of each component allow energy balancing. The outlet temperature of the electrically heated preheater is controlled to 0.5 K below saturation temperature. The test section consists of a plain stainless tube with a length of 5 m, an inner diameter of 10 mm and a wall thickness of 1 mm. A DC power supply provides a maximum heat input of 3.3 kW. By applying electric current directly to the tube a constant heat flux can be assumed. This is verified by heating the evacuated tube, which shows a uniform temperature distribution with a relative standard deviation below 0.6 %. The outer wall temperature of the test section is measured at 10 evenly distributed monitoring points. At each cross-section 3 thermocouples are fixed, 90° spaced, on the tube. The saturated vapor is expanded in the throttle valve and then condensed by transferring heat into the cooling circuit in a counter current double-pipe heat exchanger. Finally the working fluid is forced to a higher pressure level by the pump. The local heat transfer coefficient can be calculated from the ratio of heat flux density and temperature difference between saturation temperature and inner wall temperature. To determine the inner wall temperature thermal conduction is considered based on the averaged measured temperature of the outer wall. In addition, the corresponding vapour quality at each cross-section is calculated by an energy balance. Test rig for thermal stability To ensure that the results of heat transfer characteristics up to saturation temperatures of 250 °C are not influenced by decomposition products, thermal stability of siloxanes is investigated at 240 °C, 300 °C and 360 °C. Therefore a pressurized, tubular reactor which consists of a cylinder (stainless steel) covered with 5 m of heating wire and an insulation layer is used. The cylinder can be heated up to 360 °C, the temperature is controlled via a digital controller. Temperatures at the inside and the outside of the cylinder are measured by thermocouples of type K (absolute error: ± 1.5 K). Pressure is measured with a piezo resistant pressure transmitter which gives absolute values up to 50 bar (relative error: ± 0.5 %). Decomposition products within the gas phase are analyzed using gas chromatography. RESULTS Heat transfer characteristics The local heat transfer coefficient is examined for the considered working fluids depending on process pressure p, mass flux density G and heat flux density q. The uncertainties for the experimental heat transfer coefficient associated with the sensors and calculated fluid properties are below 14 %. The reproducibility was evaluated for MM and different process conditions. The standard deviation is 33 W/m2K in case of a mass flux density of 50 kg/(m2s) and 572 W/m2K for 300 kg/(m2s). As an example the results for MM are described in the following. In general, the heat transfer coefficient decreases with increasing vapor quality. In this context the reduction of thermal conductivity in the vapor phase predominates the effect of increasing flow velocity. For an increase in pressure a small reduction of the heat transfer coefficient can be observed. In the examined range of 8 kW/m2 to 17 kW/m2 for heat flux density, the local heat transfer coefficient decreases by up to 50 %. The influence of mass flux density on heat transfer characteristics is shown in Fig. 2A. The results show local heat transfer coefficients between 0.5 kW/m2K and 14.3 kW/m2K. In general, the heat transfer coefficient increases with rising mass flux density due to the higher flow velocity and turbulence. A comparison of the examined working fluids at constant parameters is displayed in Fig. 2B. On average MM and MDM lead to quite similar heat transfer coefficients. The lowest local heat transfer coefficients are obtained for MD2M. The unsteadiness in the curve of MDM is due to a change of flow characteristics from stratified flow to annular flow. Regarding existing correlations for flow boiling in plain tubes the best agreements of the experimental data are obtained for the model of Liu and Winterton (1991). Thermal stability The decomposition products of MM (Wacker® AK 0.65) at mentioned temperatures for a test period of 72 h are investigated. The purity of MM is given with more than 99 mass-%. At a temperature of 240 °C the gas phase consists of about 95 mass-% MM. In addition, a mass concentration of 3-methylpentane (2 mass-%) is detected. The analyses by gas chromatograph show a further peak of a volatile component with 1 mass-%. According to gas chromatography and an additional mass spectroscopy, the component is identified as methylsilane at the moment. The equilibrium shifts by increasing the temperature to 300 °C. Concentration of MM accounts for 89 mass-% and of 3-methylpentane for 3 mass-%. Even more significant is the increase of methylsilane to 5 mass-%. For a temperature of 360 °C the concentration of MM decreases to 83 mass-% and the olefin 2-methylpentene-1 (2 mass-%) is formed due to the high temperature. Furthermore, volatile products (methane: 7 mass-%; and ethane: 1 mass-%) are detected and methylsilane concentration drops down to almost zero. The results show, that for ORC using MM as working fluid below 240 °C, degradation does not play a role. Therefore, the measured heat transfer coefficients in this study are not influenced by decomposition products. However, at higher temperatures (>300 °C) decomposition has to be taken into account. CONCLUSIONS New experimental data for flow boiling and thermal stability of siloxanes were presented: For the considered fluids heat transfer coefficients show a low dependence on pressure and increase significantly with increasing mass flux density. Among the examined fluids MD2M shows the lowest heat transfer coefficients. The predictive method of Liu and Winterton (1991) works best compared to our experimental data. This study serves as a basis for further evaluation of the use of zeotropic mixtures in ORC systems under consideration of heat transfer characteristics. Additionally, decomposition products may influence the heat transfer. ACKNOWLEDGEMENTS The authors gratefully acknowledge financial support by Deutsche Forschungsgemeinschaft. LITERATURE Angelino, G., Colonna di Paliano, P., 1998. Multicomponent Working Fluids For Organic Rankine Cycles (ORCs). Energy 23, 449–463. Angelino, G., Colonna di Paliano, P., 2000. Air cooled siloxane bottoming cycle for molten carbonate fuel cells. Portland (USA). Colonna, P., et al., 2006. Multiparameter equations of state for selected siloxanes. Fluid Phase Equilibria 244, 193–211. Drescher, U., Brüggemann, D., 2007. Fluid selection for the Organic Rankine Cycle (ORC) in biomass power and heat plants. Applied Thermal Engineering 27, 223–228. Fernández, F.J., et al., 2011. Thermodynamic analysis of high-temperature regenerative organic Rankine cycles using siloxanes as working fluids. Energy 36, 5239–5249. Heberle, F., et al., 2012. Zeotropic mixtures as working fluids in Organic Rankine Cycles for low-enthalpy geothermal resources. Renewable Energy 37, 364–370. Kandlinkar, S.G., 1990. A general correlation for saturated two-phase flow boiling heat ransfer inside horizontal and vertical tubes. Journal of Heat Transfer 112, 219–228. Lai, N.A., et al., 2009. Description of linear siloxanes with PC-SAFT equation. Fluid Phase Equilibria 283, 22–30. Liu, Z., Winterton, R.H.S., 1991. A general correlation for saturated and subcooled flow boiling in tubes and annuli, based on a nucleate pool boiling equation. International Journal of Heat and Mass Transfer 34, 2759–2766. Spinelli, A., et al., 2011. Design, Simulation and Construction of a Test Rig for Organic Vapours. Presented at the First International Seminar on ORC Power Systems, Delft (NL). Tchanche, B.F.,et al., 2011. Low-grade heat conversion into power using organic Rankine cycles – A review of various applications. Renewable and Sustainable Energy Reviews 15, 3963–3979.