Chair: Dr. Andrea Spinelli
START-UP OF A TEST RIG FOR ORGANIC VAPORS
Andrea Spinelli, Vincenzo Dossena, Paolo Gaetani
Abstract: The Test Rig for Organic Vapors (TROVA) represents a novel facility built with the purpose of providing experimental data on the typical expansion flows taking place within organic Rankine cycle (ORC) turbines.
The facility has been built at the Fluid-dynamics of Turbomachines Laboratory of Politecnico di Milano (Italy), in collaboration with Turboden s.r.l..
It consists in a blow down facility in which an organic vapour is expanded from a high-pressure reservoir, kept at controlled supeheated or supercritical conditions, into a low-pressure reservoir, where the vapour is condensed and pumped back to the high pressure reservoir. Expansion from subsonic to supersonic speeds occurs through a converging-diverging Laval nozzle, which has been chosen as the test section for initial tests. The test rig can also accommodate liner blade cascade, as it is required by later experiments, and can operate with different working fluids of interest for ORC applications.
Within the test section the flow field can be characterized by independent measurements of pressure, temperature, and velocity, allowing also to verify the consistency of thermodynamic models currently employed to predict the typical real-gas behaviour of ORC turbine flows.
The present paper describes the commissioning of the TROVA, illustrates the test section setup and the adopted measurement techniques, and finally presents the early tests.\\
EXPERIMENTAL INVESTIGATIONS OF HEAT TRANSFER CHARACTERISTICS AND THERMAL STABILITY OF SILOXANES
Florian Heberle, Markus Preißinger, Theresa Weith, Dieter Brüggemann
Abstract: EXTENDED ABSTRACT
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.
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.
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).
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.
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.
The authors gratefully acknowledge financial support by Deutsche Forschungsgemeinschaft.
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FLEXIBLE ASYMMETRIC SCHOCK TUBE (FAST) SET-UP: STATUS AND FIRST EXPERIENCES
Piero Colonna, Mauro Gallo, Emiliano Casati, Tiemo Mathijssen
Abstract: The Flexible Asymmetric Shock Tube setup has been designed and built at the Process and Energy Laboratory of the Delft University of Technology in order to study non-classical gasdynamic phenomena in flows of dense organic fluid vapors . It operates according to the Ludwieg tube principle. One of the main objectives is the detection of rarefaction shock waves, which are theoretically predicted to occur at operating conditions close to the vapour-liquid critical point in the superheated vapor thermodynamic region of so called BZT fluids . Fluids of the siloxanes family qualify as BZT fluids therefore they are employed in the FAST setup . Siloxanes are also working fluids for organic Rankine cycle power plants . Gasdynamic measurements performed with the FAST setup are therefore also relevant especially for the aerodynamic design of ORC expanders. The FAST can be operated at temperatures up to 400 °C and pressures up to 30 bar.
Technical and experimental activities have been carried out to assess the performance of the main set-up components: the Fast Opening Valve (FOV), the vapor generator system and the thermal control system. Such activities have brought to light several problems which have hampered their correct functioning at the challenging test conditions. Therefore a partial redesign of the main components has been necessary and the current status with respect to the commissioning of the setup is illustrated.
The FOV plays a key role for assuring the success of the experiment and, at the same time, its repeatability. Due to its mechanical complexity, time-consuming activities are necessary to make reliable its functioning. Therefore, a different fast opening system has been conceived and tested: the Induced Breaking Diaphragm (IBD). Even though the IBD shows limitations related to the experiment repeatability, it represents a temporary solution, which allows for performing gasdynamic measurements both with air and siloxanes. Through these tests it will be possible to perform the fine-tuning of the PID of the thermal control system at conditions very close to those necessary for the RSW generation. Moreover it will possible to optimize both the acquisition procedure and the post-processing of the digital signals provided by the dynamic pressure transducers used to detect the RSW travelling along the tube. The results of the mentioned tests will be illustrated at the conference.