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Modelling framework for a low-grade heat thermofluidic oscillator : the Evaporative Reciprocating Piston Engine

Timmer, M.A.G. (2014) Modelling framework for a low-grade heat thermofluidic oscillator : the Evaporative Reciprocating Piston Engine.

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Abstract:This work presents the construction of a modelling framework for the Evaporative Reciprocating Piston Engine (ERPE). This engine belongs to the group of thermofluidic oscillators, in which steady boundary conditions cause oscillatory, thermodynamic behaviour by evaporation and condensation of a working fluid. Due to the constant temperature phase change, these engines are able to operate across small temperature differences between the heat source and heat sink, thus they are able to utilize low-grade heat and convert it into power. The modelling framework for describing the ERPE makes use of the electrical analogy between the fluid and thermal domain with the electrical domain. In this analogy, voltage represents a pressure/temperature difference and a current represents a volumetric/entropy flowrate. The engine is divided into independent, linearised and spatially lumped components, for which the governing equations are represented in the electrical domain. This results in an electrical circuit representation of the ERPE, where the passive electrical components of the circuit are resistors, inductors and capacitors. For the heat exchanger of the engine, two linear models are investigated. The first model imposes a linear temperature profile along the heat exchanger wall (the LTP model) and the second model imposes a linear power input gradient along the heat exchanger wall and the dynamic ability to store and release energy (the DHX model). For validation purposes, both models are solved to acquire their performance indicators, such as operating frequency and exergy efficiency, which are then compared with experimental results from an early stage ERPE prototype. These comparisons show that the LTP model is not able to capture the correct behaviour of the ERPE prototype, especially in the cases were the load component of the prototype was set to high resistances. The DHX model was able to predict the behaviour of the ERPE prototype up to the correct order of magnitude, especially for the cases were the load was set to high resistances, which are the most realistic cases to represent the actual engine with. It can therefore be concluded that the DHX model can be used for predicting the behaviour of the ERPE correctly within an order of magnitude, whilst the LTP model can not. The DHX model was then used to perform a parametric study, which gave insight into the trends in the performance indicators with changing operating conditions or design variables of the engine. These results can subsequently be used to optimize future designs of the ERPE, which should eventually lead to a full-scale design of an ERPE for power production using low-grade heat. For the latter purpose, it is suggested to not only use the linear model of the current work, but also develop a nonlinear model. This model would be able to better describe the nonlinear behaviour that certain components of the engine exhibit, and therewith predict the behaviour of the entire ERPE more accurately. As a prequel for a nonlinear model, experiments have been done to identify the temperature profile along the heat exchanger wall, which is needed to develop a nonlinear model. These experiments showed that the temperature profile of the heat exchanger wall can be described with a tanh function. The analysis of 18 sets of experimental data, with changing operating conditions for every experiment, showed that the slope of the tanh function at the zero point is almost constant for all cases. Therefore, the nonlinear model can use a tanh function, with the identified slope from the experiments, as a temperature profile for the heat exchanger wall.
Item Type:Internship Report (Master)
Imperial College London, United Kingdom
Faculty:ET: Engineering Technology
Subject:52 mechanical engineering
Programme:Mechanical Engineering MSc (60439)
Keywords:thermofluidic, engine, piston, electrical analogy, heat exchanger
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