Synthetical optimization of the structure dimension for the thermoacoustic regenerator

The quantitative investigation of parameters in the renegerator is essential for the optimization of thermoacoustic devices, while the majority of the previous research only considered parameters of the working field, working gas and the hydraulic radius. Based on the linear thermoacoustic theory, this paper extracts a normalized parameter for low-amplitude conditions, which is called the regenerator operation factor. By extracting the regenerator operation factor and relative hydraulic radius, the influence of frequency on the efficiency can be controlled and offset. It can be found that thermoacoustic devices with different frequencies can perform the same efficiency by adjusting the radius in proportion to the axial length.Finally, this paper synthetically optimizes the dimension of the thermoacoustic regenerator by taking the regenerator operation factor, relative hydraulic radius and acoustic field parameter as variables. Conclusions in this paper are of great significance for explaining the best working conditions of engines and directing the miniaturization and optimal design of thermoacoustic devices.

An experimental study of the incipient bed shear stress partition in mobile bed channels filled with emergent rigid vegetation

This paper investigates the bed shear stress based on the condition of the incipient motion of sediment in a uniform-flow flume covered with emergent rigid vegetation,which is represented by arrays of circular cylinders arranged in a regular pattern.A total of 148 tests are performed to observe the influence of the vegetation density,bed slope,flow depth and sediment size on the bed shear stress.The tests reveal that when the sediment is in incipient motion,the resistances acting on the flow passing the rigid vegetation contain the vegetation resistance and the bed shear stress.This shear stress could be divided into two parts:the grain shear stress and the shear stress caused by sand dunes,which are the deformed bedform with the sediment incipient motion.An empirical relationship between the shear stress of the sand dune and vegetation density,the Froude number,the apparent vegetation layer velocity is developed.

Resistance of the flow over rough surfaces

The mechanism of the flow resistance in open channels and pipelines is of vital importance for various critical issues related to the water flow.The Nikurade’s method of calculating the friction factor is not applicable in some cases.It is necessary to consider the influence of the vortex volume surrounding the vegetation and to study the hydrodynamic characteristics of the vegetated channels.This paper analyzes the variation of the vortices created by the surface roughness of different types and different sizes and proposes new definitions of the hydraulic radius and the equivalent roughness height.With consideration of the skin friction and the form drag,on the basis of the force balance equation,a novel calculation method is used for the friction factor,and this method is verified by experimental data.The hydrodynamic mechanism of the flow resistance revealed in this study may serve as a theoretical basis for hydraulic engineers to calculate the friction factor.