The gas circuit and the drive mechanism are the main subsystems of a Stirling engine. Models can be used to study these subsystems. The numerical computing power currently available allows the development of models with fewer simplifying assumptions than was possible decades ago. Because all models must be validated experimentally, the benefit of the advanced models is not greater accuracy but the ability to analyse physical phenomena dependent on variables whose experimental measurement is practically impossible. For now, simple models are needed at the preliminary design stage, while the advanced models can be appropriate for optimisation tasks. Beale and West numbers were probably the most commonly used criteria for sizing of engines at the preliminary design stage. Without prejudice to its historical importance, it should be recognized that both concepts are experimental correlations where great simplifications have been made. So far it has hardly been highlighted that these dimensionless numbers have two important limitations, namely: (a) do not provide criteria of independent design for the gas circuit and the drive mechanism, because they refer to brake power; and (b) do not allow to accurately estimate the maximum brake power, since there is no correlation or design criteria generally accepted to estimate the speed corresponding to the maximum brake power. In previous publications, more complete preliminary design criteria have been proposed by means of dimensional analysis, considering separately the gas circuit and the drive mechanism. In this method, the concept of quasi-static simulation and characteristic Mach number are essential for dimensioning the gas circuit, while the characteristic Stirling number is important from the standpoint of mechanical efficiency. In this article we update a database that contains the main geometric parameters, operating variables and experimental results of power and efficiency corresponding to engines of different size and characteristics. The database also includes simulation results. Since recent investigations claims that a Stirling machine operating at very high pressure and moderate maximum temperature allows the realization of small engines with significant useful power, with advantages in structural problems and the seals realization, the simulations also consider operating conditions for which the working fluid may evidence real gas effects. The concept of dimensionless quasi-static indicated power introduced in previous work using the ideal gas model, is now substituted by the analogous concept that results from an equation of state for real gases. Both values are compared with the approximation derived from the model of Schmidt, in order to evaluate separately how the mechanism simplification and the equation of state affect. We analyse the ratio between the dimensionless value of the maximum indicated power (a sort of ‘indicated’ Beale number) and the procedure to estimate the maximum brake power can be completed by analysing the ratio between the dimensionless rotation frequencies at the operating points of maximum brake power and maximum indicated power, as well as the values of the mechanical efficiency at the point of maximum brake power.

`http://hdl.handle.net/11379/453125`

Titolo: | Preliminary design criteria of Stirling engines taking into account real gas effects |

Autori interni: | SALA, Fernando INVERNIZZI, Costante Mario |

Data di pubblicazione: | 2014 |

Abstract: | The gas circuit and the drive mechanism are the main subsystems of a Stirling engine. Models can be used to study these subsystems. The numerical computing power currently available allows the development of models with fewer simplifying assumptions than was possible decades ago. Because all models must be validated experimentally, the benefit of the advanced models is not greater accuracy but the ability to analyse physical phenomena dependent on variables whose experimental measurement is practically impossible. For now, simple models are needed at the preliminary design stage, while the advanced models can be appropriate for optimisation tasks. Beale and West numbers were probably the most commonly used criteria for sizing of engines at the preliminary design stage. Without prejudice to its historical importance, it should be recognized that both concepts are experimental correlations where great simplifications have been made. So far it has hardly been highlighted that these dimensionless numbers have two important limitations, namely: (a) do not provide criteria of independent design for the gas circuit and the drive mechanism, because they refer to brake power; and (b) do not allow to accurately estimate the maximum brake power, since there is no correlation or design criteria generally accepted to estimate the speed corresponding to the maximum brake power. In previous publications, more complete preliminary design criteria have been proposed by means of dimensional analysis, considering separately the gas circuit and the drive mechanism. In this method, the concept of quasi-static simulation and characteristic Mach number are essential for dimensioning the gas circuit, while the characteristic Stirling number is important from the standpoint of mechanical efficiency. In this article we update a database that contains the main geometric parameters, operating variables and experimental results of power and efficiency corresponding to engines of different size and characteristics. The database also includes simulation results. Since recent investigations claims that a Stirling machine operating at very high pressure and moderate maximum temperature allows the realization of small engines with significant useful power, with advantages in structural problems and the seals realization, the simulations also consider operating conditions for which the working fluid may evidence real gas effects. The concept of dimensionless quasi-static indicated power introduced in previous work using the ideal gas model, is now substituted by the analogous concept that results from an equation of state for real gases. Both values are compared with the approximation derived from the model of Schmidt, in order to evaluate separately how the mechanism simplification and the equation of state affect. We analyse the ratio between the dimensionless value of the maximum indicated power (a sort of ‘indicated’ Beale number) and the procedure to estimate the maximum brake power can be completed by analysing the ratio between the dimensionless rotation frequencies at the operating points of maximum brake power and maximum indicated power, as well as the values of the mechanical efficiency at the point of maximum brake power. |

Handle: | http://hdl.handle.net/11379/453125 |

Appare nelle tipologie: | 4.1 Contributo in Atti di convegno |