Experimental and numerical investigation of micro-scale radial turbocompressors for miniaturized sustainable energy systems
- Autores: Andrés Sebastián Herrera
- Directores de la Tesis: Rubén Abbas Cámara (dir. tes.)
- Lectura: En la Universidad Politécnica de Madrid ( España ) en 2022
- Idioma: español
- Programa de doctorado: Programa de Doctorado en Energía Sostenible Nuclear y Renovable por la Universidad Politécnica de Madrid
- Materias:
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- Resumen
The need to strengthen efforts to tackle the great energy and environmental challenges that humankind must face demands the development of sustainable and high-efficient novel technological solutions. Miniaturized turbomachinery arises as one promising technology which can be part of different novel environmentally friendly systems, not only in the power sector but also in heating and cooling sector. Particularly, micro-turbocompressors offer a wide range of applicability in these fields, from hydrogen fuel cells turbocharging to renewable thermal based micro-trigeneration systems. Nevertheless, turbocompressor downsizing entails numerous challenges that prevent them from being high-efficient turbomachines. Therefore, the present Thesis is focused on the characterization of these key adverse aerothermal effects derived from miniaturization in order to shed light on their application to renewable thermal based micro-trigeneration systems.
The assessment of these aerothermal effects has been undertaken in the present dissertation by means of developing three different methodological approaches. The first one consists in a similarity-based analytical prediction method to characterize the off-design behavior of one of the most characteristic miniaturization effect: the low Reynolds-number phenomenon. The proposed pseudo-homologous mapping methodology allows to easily predict the off-design response obtained when the Reynolds-number varies. This method, therefore, provides a straightforward calculation of the effect of considering a different working fluids at any other inlet pressurization rate, which may result in a beneficial efficiency increase if moving away from the laminar-to-turbulent flow regime.
The second methodological approach is based on the development of a three-dimensional numerical model for the characterization of these adverse effects on a reference micro-scale machine. This model has been used to validate the proposed analytical model and also to further explore the viscous effects in the transitional flow regime. Moreover, the assessment of a second miniaturization effect is carried out: the tip clearance losses. The quantification of this tip clearance flow and its exploration varying the working fluid and inlet pressurization have been conducted. Finally, a numerical study of the effects derived from the lack of adiabaticity due to the reduced scale of the machine is carried out, being this the third adverse aerothermal effect studied. The resultant diabatic compression has been characterized by means of considering diabatic walls across the different sections of the micro-turbocompressors.
The experimental investigation is the last methodological approach to study the identified miniaturization effects, especially the low Reynolds-number phenomenon. A specifically designed test rig is developed and commissioned for the reference micro-scale turbocompressor used through this Thesis. This test rig allows to vary the working fluid and its inlet pressurization through a closed-loop cycle. Hence, the Reynolds-number effects can be identified at different boundary conditions. Moreover, this investigation has required the characterization of the external heat losses effects due to proven non-adiabatic behavior of the machine.
This Thesis has led to several important findings about the influence of the key adverse aerothermal effects derived from the miniaturization of turbocompressors. Some of them are briefly summarized here after. First, efficiency variations in the range of 2-6 percentage points using air and carbon dioxide at different inlet pressures have been analytically, numerically and experimentally proven due to the low Reynolds-number phenomenon. This characterization on the whole experimentally feasible off-design map is in line with the most recent Reynolds-number correction methods. Second, tip leakage flow structure has been shown to be preserved when moving to other working fluids and inlet pressurization rates keeping pseudo-homologous conditions, which is fundamental for the similarity-based concept raised. Finally, the diffuser and the volute have been proven to be the regions with the largest impact when dealing with diabatic walls while the inlet and the impeller domain provide very low impact. This allows the proposal of heat transfer correction methods for experimentation, starting from the impeller outlet flow thermodynamic properties.
The application of micro-turbocompressors on promising sustainable miniaturization energy systems is finally assessed in this dissertation. Hence, a novel modular and flexible micro-trigeneration layout, based on micro-turbomachinery, is put forward for the use of low-to-moderate temperature renewable heat sources. The restricted space of design is analyzed for four different configurations proposed to achieve the smartest heat management of the system. The use of isobutane and propane for these cycles has been proven to provide reasonable exergy efficiency values. The implementation of this system has been examined for a 15-kW prototype of an innovative conception of small scale rotatory solar Fresnel collector, intended to reduce costs by means of a simple design. The proposed integration has been proven to be an efficient way to harness renewable thermal energy within a miniaturized power scale.
