Effect of Radial Porosity Oscillation on the Thermal Performance of Packed Bed Latent Heat Storage

Owing to its high heat storage capacity and fast heat transfer rate,packed bed latent heat storage(LHS)is considered as a promising method to store thermal energy.In a packed bed,the wall effect can impact the packing arrangement of phase change material(PCM)capsules,inducing radial porosity oscillation.In this study,an actual-arrangement-based three-dimensional packed bed LHS model was built to consider the radial porosity oscillation.Its fluid flow and heat transfer were analyzed.With different cylindrical sub-surfaces intercepted along the radial direction in the packed bed,the corresponding relationships between the arrangement of capsules and porosity oscillation were identified.The oscillating distribution of radial porosity led to a non-uniform distribution of heat transfer fluid(HTF)velocity.As a result,radial temperature distributions and liquid fraction distributions of PCMs were further affected.The effects of different dimensionless parameters(e.g.,tube-to-capsule diameter ratio,Reynolds number,and Stefan number)on the radial characteristics of HTF and PCMs were discussed.The results showed that different diameter ratios correspond to different radial porosity distributions.Further,with an increase in diameter ratio,HTF velocity varies significantly in the near wall region while the non-uniformity of HTF velocity in the center region will decrease.The Reynolds and Stefan numbers slightly impact the relative velocity distribution of the HTF-while higher Reynolds numbers can lead to a proportional improvement of velocity,an increase in Stefan number can promote heat storage of the packed bed LHS system.

Effect of viscoelasticity on nonlinear vibration of single microbubble

Based on the Church-Hoff model, the nonlinear oscillations of a single encapsulated microbubble with a finite thickness shell are theoretically studied. The effects of viscoelasticity on radial oscillations and the fundamental and harmonic components are researched. The peaks of radial oscillations and magnitudes of power spectra of the fundamental and harmonic components all increase gradually with the shear modulus of shell varying from 0 to 10 MPa by an interval of 0. 1 MPa at the same shear viscosity, while they decrease as the shear viscosity increases from 0 to 1 Pa · s by an interval of 0. 01 Pa · s at the same shear modulus. The fluctuation ranges of subharmonic and ultraharmonic signals are much larger than both the fundamental and second harmonic components. It means that the effect of viscoelasticity on the subharmonic and ultraharmonic signals is greater than that on the fundamental and second harmonic components. So adjusting the viscoelasticity of the shell is a potential method to obtain a perfect microbubble contrast agent used for the subharmonic and ultraharmonic imaging. Four points with significant fundamental and harmonic components are chosen as an example： a shear viscosity of 0. 39 Pa · s with shear modulus of 3.9, 6. 6, and 8.6 MPa, respectively; a shear modulus of 6.6 MPa with a shear viscosity of 0.42 Pa · s.