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[en] Highlights: • Stacked-woven metal screens have been used as regenerator matrix materials. • Copper has been found as a superior regenerator matrix material than stainless steel. • Working gas flow direction has to be normal to screen surface to produce good engine performance. • Pressure drop through the regenerator plays a very important role on performance. • There exists an optimal fill factor. - Abstract: In this paper, a helium charge γ-type twin power piston Stirling engine has been studied experimentally to understand the effects of several regenerator parameters on the overall performance of the engine. The regenerator incorporated in this engine is a moving regenerator which is housed inside the displacer of the engine, and the parameters investigated include regenerator matrix material, matrices arrangement, matrix wire diameter, and fill factor. Stacked-woven metal screens have been used as regenerator matrix materials. The results include engine shaft torque, power, and efficiency versus engine speed at several engine’s hot-end temperatures. It is found that all parameters pose significant impact on engine performance. Copper is a superior regenerator material than stainless steel for the current engine; regenerator matrix screens have to be installed in a manner that the working-gas-flow direction is normal to the surface of matrix screens; very small wire diameter results in large pressure drop and reduce regenerator effectiveness; and there exists an optimal fill factor. The study offers some important information for the design of moving regenerator in a γ-type Stirling engine
[en] Highlights: • An in-house CFD code has been developed to simulate a β-type Stirling engine. • Temperature distribution is highly non-uniform across the engine volume. • Impingement is the major heat transfer mechanism in the engine. • The motions of displace and power piston are also important for heat transfer. - Abstract: A compressible CFD code has been developed to study the heat transfer characteristics of a β-type Stirling engine with a very simple design and geometry. The results include temperature contours, velocity vectors, and distributions of local heat flux along solid boundaries at several important time steps as well as variations of average temperatures, integrated rates of heat input, heat output, and engine power. It is found that impingement is the major heat transfer mechanism in the expansion and compression chamber, and the temperature distribution is highly non-uniform across the engine at any given moment. The results, especially the rates of heat transfer, are quite different from those obtained by a second-order model. The variations of heat transfer rates are much more complicated than the simple variations returned by the second-order model. This study sheds light into the complex heat transfer mechanism inside the Stirling engine and is very helpful to the understanding of the fundamental process of the engine cycle
[en] Highlights: ► A numerical model has been applied to study the performance of a gamma-type Stirling engine. ► A prototype engine has been built to correct the values of some factors in the model. ► The regeneration effectiveness is most prominent on efficiency. ► Engine speed is most effective on the engine power. ► The rotation arm and initial gas pressure are also influential factors on engine power. - Abstract: In this study, a prototype helium-changed twin-power-piston γ-type Stirling engine has been built, and some of its geometrical and operational parameters have been investigated by a numerical model. Data taken from the prototype engine have been used to correct the values of some factors in the numerical model. The results include volume and temperature variations in the expansion and compression chambers, p–v diagrams, and the effects of regeneration effectiveness, the crank radius of the power piston, the initial pressure of working gas, and the rotation speed on engine’s power and efficiency. It has been found that regeneration effectiveness poses the most prominent effect on efficiency, while engine speed is most effective on the engine power within the range of engine speed investigated in this study. This study offers invaluable guides for the design and optimization of γ-type Stirling engines with similar construction.
[en] In this study, the heat transfer characteristics of insulated cylindrical tanks are analyzed by using a numerical method and three one dimensional analytical methods, namely the RPSWT (Regular Polygon top Wedge Thermal resistance), PWTR (Plane Wedge Thermal Resistance) and the conventional models. It is found that in the situation of shorter cylindrical tanks where the ratio between height and radius H/R 2 < 10, the errors generated by the RPSWT model are positive in most cases, with only a few exceptions, and the errors generated by the conventional model are negative in all cases. Thus, a new CRPSWTC model is proposed, which combines the RPSWT and conventional models with appropriate proportion factors to neutralize the positive and negative errors. The combination allows the new model to obtain very accurate results in comparison with the numerical solutions within this H/R 2 range. Nevertheless, the CRPSWTC model is proven to be applied to cases with larger H/R 2 and still obtain satisfactory results. Alternatively, the RPSWT model obtains the best results when 10 ≤ H/R 2 < 16, while the PWTR model returns better solutions when H/R 2 ≥ 16
[en] In this study, a conjugate gradient method based on an inverse algorithm is applied to estimate the unknown space and time dependent forced convection heat transfer rate of two in line cylinders placed in a cross stream. While knowing the temperature history at the measuring positions near the surfaces of the cylinders, the heat transfer rate on the surfaces of both cylinders can be successfully computed. No prior information is needed on the functional form of the unknown heat transfer rate. A particular feature in this study is that the considered Reynolds number is high enough to result in an unsteady flow around the cylinders. In addition to the transient temperature field, this makes the flow field itself time dependent as well, thus adding more complexity to the numerical analysis. The accuracy of the inverse analysis is examined by using the simulated temperature measurement. Results show that an excellent estimation on the heat transfer rate and temperature distributions can be obtained for the case considered in this study
[en] Highlights: • A parametric study on a low-temperature-differential Stirling engine has been conducted by using CFD. • The effects of three geometric and two operational parameters on engine performance have been investigated. • It is found that each parameter produces different effects except power piston stroke and power piston radius. • The results are useful for guiding the design of new low-temperature-differential Stirling engines. - Abstract: An in-house CFD code has been applied to a low-temperature-differential (LTD) γ-type Stirling engine to understand the effects posed by several geometrical and operational parameters on engine performance. The results include variations of pressure, temperature, and heat transfer rates within an engine cycle as well as variations of engine’s power and efficiency versus these parameters. It is found that power piston stroke and radius influence engine performance very similarly, and power and efficiency both increase as these two parameters increase. In fact, the effects of the two parameters can be assimilated into those by the parameter of compression ratio. The stroke of displacer is observed to affect strongly on heat input but weakly on power, thus causing the efficiency to decrease as it increases. As expected, both power and efficiency increase as temperature difference between the hot and cold ends increases. Lastly, engine speed is observed to pose strong positive effects on power but exert weak effects on efficiency. This study reveals the effects produced by several important parameters on engine performance, and such information is very useful for the design of new LTD Stirling engines.
[en] Highlights: ► Time-dependent base heat flux of a functionally graded fin is inversely estimated. ► An inverse algorithm based on the conjugate gradient method and the discrepancy principle is applied. ► The distributions of temperature in the fin are determined as well. ► The influence of measurement error and measurement location upon the precision of the estimated results is also investigated. - Abstract: In this study, an inverse algorithm based on the conjugate gradient method and the discrepancy principle is applied to estimate the unknown time-dependent base heat flux of a functionally graded fin from the knowledge of temperature measurements taken within the fin. Subsequently, the distributions of temperature in the fin can be determined as well. It is assumed that no prior information is available on the functional form of the unknown base heat flux; hence the procedure is classified as the function estimation in inverse calculation. The temperature data obtained from the direct problem are used to simulate the temperature measurements. The influence of measurement errors and measurement location upon the precision of the estimated results is also investigated. Results show that an excellent estimation on the time-dependent base heat flux and temperature distributions can be obtained for the test case considered in this study.
[en] In this study, a conjugate gradient method based on an inverse algorithm is applied to estimate the unknown space and time dependent convection heat transfer coefficient of an annular fin. While knowing the temperature or strain history at the measuring positions of the fin, the convection heat transfer coefficient between the fin and the ambient fluid can be successfully computed. No prior information is needed on the functional form of the unknown convection heat transfer coefficient; and thus, the present study is classified as the function estimation inverse calculation. A particular feature in this study is that the thermal and strain fields are coupled, which makes solving the inverse problem a highly challenging task. The accuracy of the inverse analysis is examined by using the simulated temperature or strain measurements. Results show that excellent estimations of the convection heat transfer coefficient, temperature distributions and thermal stress distributions can be obtained for all the cases considered in this study
[en] In this investigation, the differences of heat transfer characteristics for insulated and non-insulated spherical containers between considering and neglecting the influence of heat radiation are studied by the simulations in some practical situations. It is found that the heat radiation effect cannot be ignored in conditions of low ambient convection heat coefficients (such ambient air) and high surface emissivities, especially for the non-insulated and thin insulated cases. In most practical situations when ambient temperature is different from surroundings temperature and the emissivity of insulation surface is different from that of metal wall surface, neglecting heat radiation will result in inaccurate insulation effect and heat transfer errors even with very thick insulation. However, the insulation effect considering heat radiation will only increase a very small amount after some dimensionless insulated thickness (such insulation thickness/radius ≥0.2 in this study), thus such dimensionless insulated thickness can be used as the optimum thickness in practical applications. Meanwhile, wrapping a material with low surface emissivity (such as aluminum foil) around the oxidized metal wall or insulation layer (always with high surface emissivity) can achieve very good insulated effect for the non-insulated or thin insulated containers.