Design of a test set-up for experimental investigation of forced convective heat transfer to fluids operating at supercritical conditions. Stijn Daelman Supervisor: Prof. dr. ir. Michel De Paepe Counsellor: Marija Lazova Master's dissertation submitted in order to obtain the academic degree of Master of Science in Electromechanical Engineering Department of Flow, Heat and Combustion Mechanics Chairman: Prof. dr. ir. Jan Vierendeels Faculty of Engineering and Architecture Academic year 2013-2014 Design of a test set-up for experimental investigation of forced convective heat transfer to fluids operating at supercritical conditions. Stijn Daelman Supervisor: Prof. dr. ir. Michel De Paepe Counsellor: Marija Lazova Master's dissertation submitted in order to obtain the academic degree of Master of Science in Electromechanical Engineering Department of Flow, Heat and Combustion Mechanics Chairman: Prof. dr. ir. Jan Vierendeels Faculty of Engineering and Architecture Academic year 2013-2014 The author gives permission to make this master dissertation available for consultation and to copy parts of this master dissertation for personal use. In the case of any other use, the limitations of the copyright have to be respected, in particular with regard to the obligation to state expressly the source when quoting results form this master dissertation. Ghent, June 2014 The author S. Daelman "Wisdom is not a product of schooling but of the lifelong attempt to acquire it." - Albert Einstein The reason a lot of people do not recognize opportunity is because it usually goes around wearing overalls looking like hard work. - Thomas A. Edison Preface This master dissertation is a final conclusion of my career as an engineering student at Ghent University. Even through the relative short timespan of my studies, technology and science have changed a lot. The energy market and the technologies related to it not the least. Putting the knowledge gained during the past years to a practical use within this context was something I was looking forward to for my dissertation. The design and building of a new test set-up was engineering in all its possible aspects. The combination of the theoretical, mathematical, technical, economic and especially social aspects of engineering were an enrichment of my education. I would like to thank a lot of people and friends that have been important during my studies and especially during this final year. First of all, I would like to thank all the other people of the Department of Flow, Heat and Combustion Mechanics. Especially my supervisor Prof. dr. ir. M. De Paepe and my counsellor Marija Lazova for the trust they had to let me work on a project this big and important. Special thanks also go out to Hugo Bellinck, Patrick De Pue, Adriaan Lebbe and Yves Maenhout for the technical support and to Annie Harri and Griet Blond´efor the administration. I know it has been a lot of work! Furthermore I would like to thank the other students in the lab. Marijn Billiet, Thomas Deruyter, Piet Scheerlinck, Ben D'Haeger, your company and support are well appreciated, especially when things didn't go as expected. Finally, I would like to thank my parents for giving me the opportunity to complete my studies and for all their support! Stijn Daelman Ghent, June 2014 i Design of a test set-up for experimental investigation of forced convective heat transfer to fluids operating at supercritical conditions. Stijn Daelman Master's dissertation submitted in order to obtain the academic degree of Master of Science in Electromechanical Engineering Supervisor: Prof. dr. ir. Michel De Paepe Counsellor: Marija Lazova Department of Flow, Heat and Combustion Mechanics Chairman: Prof. dr. ir. Jan Vierendeels Faculty of Engineering and Architecture Ghent University Academic year 2013-2014 Summary The goal of this master's dissertation is to develop a test set-up that is able to perform measure- ments on the heat transfer process to refrigerants in supercritical conditions. In chapter 1, an introduction will be given on the current energy challenges and how Organic Rankine Cycles (ORCs) can provide a solution to these challenges. In chapter 2, a general overview is given of the different types of ORCs and why the use of a heat transfer process to fluids at supercritical conditions can be important to improve their performance. Furthermore, the general selection procedure of working fluids is clarified. A review of past work and existing correlations for forced convective heat transfer and existing set-ups concludes this chapter. Chapter 3 gives a description of the actual developed set-up and its general working principles. A more detailed version of how the set-up was designed and why certain choices have been made can be found in chapter 4, which discusses the design procedures. The importance of the data-acquisition and control hardware is discussed in chapter 5. The general ideas for the control software are discussed in chapter 6, whereas chapter 7 gives more information on the operational procedures. In chapter 8, the processing of the acquired data and the modified Wilson plot technique for determination of the heat transfer coefficient are discussed. Finally an error analysis is performed on the calibration of the temperature sensors and the calculations that are used in the processing of the data. Keywords Design, Test set-up, Supercritical, Forced convective heat transfer, Organic Rankine Cycle iii Extended abstract v EINDWERKEN 2014 Onderzoeksgroep Technische Thermodynamica en Warmteoverdracht Vakgroep Mechanica van Stroming, Warmte en Verbranding - UGent Paper number: SD EXPERIMENTAL FACILITY TO STUDY FORCED CONVECTIVE HEAT TRANSFER TO WORKING FLUIDS FOR ORGANIC RANKINE CYCLES AT SUPERCRITICAL CONDITIONS Daelman Stijn, Lazova Marija, De Paepe Michel Department of Flow, Heat and Combustion Mechanics Ghent University Sint-Pietersnieuwstraat 41 – B9000 Gent – Belgium E-mail: [email protected] ABSTRACT NOMENCLATURE Organic Rankine Cycles (ORC) have proven their use to Thermodynamic symbols convert ‘waste’ heat (e.g. from process industry) and other low- Cp [kJ/kgK] Heat capacity temperature heat sources (e.g. solar thermal) to usable electrical h [W/m²K] Convection coefficient energy. The efficiency of these cycles however, has still room k [W/mK] Conduction coefficient for improvement. Transcritical ORCs, in which the heat Nu [-] Nusselt number transfer from the heat source to the cycle’s working fluid P [bar] Pressure happens at supercritical conditions, can provide a higher energy Pr [-] Prandtl number conversion efficiency. q [W/m3] Heat flux In order to design these transcritical cycles, the heat transfer R [m2K/W] Thermal resistance process to the working fluids at supercritical conditions has to Re [-] Reynolds number be studied in order to provide useful correlations for design T [K] Temperature calculations of such a heat exchanger. In order to get to these v [m³/kg] Specific volume correlations, a test set-up that provides data on this heat transfer process has been designed. Abbreviations This paper reports on the design study in terms of GWP Glowal Warming Potential operational parameters, technical limitations, technical ODP Ozone Depletion Potential implementations and analytical methods used in such a set-up. ORC Organic Rankine Cycle The results obtained during this study will be used in the ORCNext project to design a supercritical heat exchanger to be Subscripts applied in a transcritical ORC. b Bulk c Critical INTRODUCTION min Minimum A shift towards the use of low-temperature energy sources, w Wall such as flue gasses, have led to a wider use of Organic Rankine 0 Ambient or reference Cycle. A common problem in ORCs however, is the exergy destruction in the heat transfer process in the evaporator within the ORC. The temperature mismatch between working fluid and heat source, such as visualised in Figure 1 is the mean economic feasibility of the transcritical ORCs, which can be a reason for this exergy destruction. threat to this technology within the current energy markets. In order to provide a solution, a transcritical cycle can be Consequently, further application of ORCs necessitates the used. In this cycle architecture, the heat transfer during the development of a heat transfer correlation for the applied evaporation phase takes place at supercritical pressures, working fluids under the supercritical conditions. Thanks to providing a better thermal match between heat source and these correlations, heat exchangers for heat input in transcritical working fluid. This is indicated in Figure 1. ORCs can be dimensioned in an appropriate way, thus lowering The lack of knowledge of the heat transfer to the applied the costs of such installations. This can result in more efficient working fluids within these transcritical ORCs leads to an over- energy cycles, using low-temperature energy sources, whilst dimensioning of heat exchangers, thus resulting in a lower being economic sustainable as well. Figure 1 Temperature profiles. Figure 3: Ts-diagram of R125. PROPERTIES OF SUPERCRITICAL HEAT TRANSFER Supercritical heat transfer to fluids has already been studied In the reviewed research findings, the effects of buoyancy by other researchers. However, the fluids in question were forces and acceleration effects come forward. These two mainly water, helium and CO2. The working fluids that would aspects tend to be able to have a significant impact on the heat be used within transcritical ORCs have not been tested yet. This transfer process in certain conditions. However, before doesn’t mean that the results and findings of these former discussing how these effects influence the heat transfer process, studies are of no use for these working fluids. It can be assumed a brief discussion of their existence is at hand. that the general properties that have been found for supercritical heat transfer to water, helium and CO2 are also applicable to the When neglecting the pressure losses over the heat working fluids of interest used in transcritical ORC exchanger, the heat transfer process can be modelled as a applications.