Huiming Yin

Energy Conversion Efficiency of a Novel Hybrid Solar System for Photovoltaic, Thermoelectric, and Heat Utilization

A novel hybrid solar system has been designed to utilize photovoltaic (PV) cells, thermoelectric (TE) modules, and hot water (HW) through a multilayered building envelope. Water pipelines are cast within a functionally graded material layer to serve as a heat sink, allowing heat to be easily transferred into flowing water through an aluminum-rich surface, while remaining insulated by a polymer rich bottom. The theoretical energy conversion efficiency limit of the system has been investigated for documenting the potential of this hybrid solar panel design. Given the material properties of each layer, the actual energy conversion efficiency depends on the solar irradiation, ambient temperature, and water flow temperature. Compared to the traditional solar panel, this design can achieve better overall efficiencies with higher electrical power output and thermal energy utilization. Based on theoretical conversion efficiency limits, the PV/TE/HW system is superior to PV/HW and traditional PV systems with 30% higher output electrical power. However, the advantages of the PV/TE/HW system are not significant from experimental data due to the low efficiency of the bulk TE material. Thus, QW/QD TE materials are highly recommended to enhance the overall efficiency of the PV/TE/HW design. This design is general and open to new PV and TE materials with emerging nanotechnology for higher efficiencies.

Assessment of Existing Micro-mechanical Models for Asphalt Mastics Considering Viscoelastic Effects

ABSTRACT Micromechanical models have been directly used to predict the effective complex modulus of asphalt mastics from the mechanical properties of their constituents. Because the micromechanics models traditionally employed have been based on elastic theory, the viscoelastic effects of binders have not been considered. Moreover, due to the unique features of asphalt mastics such as high concentration and irregular shape of filler particles, some micromechanical models may not be suitable. A comprehensive investigation of four existing micromechanical methods is conducted considering viscoelastic effects. It is observed that the self-consistent model well predicts the experimental results without introducing any calibration; whereas the Mori-Tanaka model and the generalized self-consistent model, which have been widely used for asphalt materials, significantly underestimate the complex Youngs modulus. Assuming binders to be incompressible and fillers to be rigid, the dilute model and the self-consistent model provide the same prediction, but they considerably overestimate the complex Youngs modulus. The analyses suggest that these conventional assumptions are invalid for asphalt mastics at low temperatures and high frequencies. In addition, contradictory to the assumption of the previous elastic model, it is found that the phase angle of binders produces considerable effects on the absolute value of the complex modulus of mastics.

Effective thermal conductivity of two-phase functionally graded particulate composites

A multiscale modeling method is proposed to derive effective thermal conductivity in two-phase graded particulate composites. In the particle-matrix zone, a graded representative volume element is constructed to represent the random microstructure at the neighborhood of a material point. At the steady state, the particle’s averaged heat flux is solved by integrating the pairwise thermal interactions from all other particles. The homogenized heat flux and temperature gradient are further derived, through which the effective thermal conductivity of the graded medium is calculated. In the transition zone, a transition function is introduced to make the homogenized thermal fields continuous and differentiable. By means of temperature boundary conditions, the temperature profile in the gradation direction is solved. When the material gradient is zero, the proposed model can also predict the effective thermal conductivity of uniform composites with the particle interactions. Parametric analyses and comparisons ...

Micromechanics-based hyperelastic constitutive modeling of magnetostrictive particle-filled elastomers

Abstract An effective hyperelastic constitutive model is developed for particle-filled elastomer composites based on the microstructural deformation and physical mechanism of the magnetostrictive particles embedded in the hyperelastic elastomer matrix. Two types of loading conditions are considered––magnetic field and mechanical load. Magnetic eigenstrains are prescribed on the particles due to effect of magnetostriction. The effective constitutive relation of the composites during infinitesimal deformation can be established based on Eshelbys micromechanics approach. Since the elastomers normally exhibit finite hyperelastic deformation, the corresponding hyperelastic constitutive law of the composites is constructed in terms of the strain energy densities of the constituents.

ImageParser: a tool for finite element generation from three-dimensional medical images

BackgroundThe finite element method (FEM) is a powerful mathematical tool to simulate and visualize the mechanical deformation of tissues and organs during medical examinations or interventions. It is yet a challenge to build up an FEM mesh directly from a volumetric image partially because the regions (or structures) of interest (ROIs) may be irregular and fuzzy.MethodsA software package, ImageParser, is developed to generate an FEM mesh from 3-D tomographic medical images. This software uses a semi-automatic method to detect ROIs from the context of image including neighboring tissues and organs, completes segmentation of different tissues, and meshes the organ into elements.ResultsThe ImageParser is shown to build up an FEM model for simulating the mechanical responses of the breast based on 3-D CT images. The breast is compressed by two plate paddles under an overall displacement as large as 20% of the initial distance between the paddles. The strain and tangential Youngs modulus distributions are specified for the biomechanical analysis of breast tissues.ConclusionThe ImageParser can successfully exact the geometry of ROIs from a complex medical image and generate the FEM mesh with customer-defined segmentation information.

Elastic modelling of periodic composites with particle interactions

The mechanical properties of periodic composites containing identical spherical particles are investigated using the principles of micromechanics and homogenization procedures. The averaged strain and stress fields are derived in terms of an eight-particle interaction. The effective elasticity with the cubic symmetry tensor is explicitly obtained. If the interaction term is dropped, then one recovers the conventional Mori–Tanaka model. With further approximations, the dilute model and the self-consistent model can also be obtained within the proposed framework. It is observed that the particle interactions make no contribution to the effective bulk modulus, a result that is consistent with other models and experiments for composites with cubic lattices.

Magnetoelasticity of chain-structured ferromagnetic composites

A micromechanics-based model with particle interaction has been developed to study the effective elastic properties of chain-structured ferromagnetic composites subject to both magnetic and mechanical loading. Magnetomechanical coupled behavior is numerically simulated and magnetic-field-dependent elasticity is calculated using the Green’s function technique in a manner such that microstructure evolution is considered. Due to the magnetic angular momentum, the effective shear modulus of the composites increases much faster than the effective Young’s modulus as the magnetic field increases. Two mechanisms resulting in magnetic-field-dependent elasticity are demonstrated.

Effect of aligned ferromagnetic particles on strain sensitivity of multi-walled carbon nanotube/polydimethylsiloxane sensors

A strain sensor using chain-structured ferromagnetic particles (FPs) in a multi-walled carbon nanotube (MWCNT)/polydimethylsiloxane (PDMS) nanocomposite was fabricated under a magnetic field and its strain sensitivity was evaluated at different material proportions. When the proportion of MWCNTs that are well dispersed in PDMS is higher than the percolation threshold, the strain sensitivity reduces with the increase of MWCNTs, in general; whereas a higher volume fraction of FPs produces a higher strain sensitivity when the chain-structure of FPs sustains. The mechanisms causing this interesting phenomenon have been demonstrated through the microstructural evolution and micromechanics-based modeling. These findings indicate that an optimal design of the volume fraction of FPs and MWCNTs exists to achieve the best strain sensitivity of this type of sensors. It is demonstrated that the nanocomposites containing 20 vol. % of nickel particles and 0.35 wt. % MWCNTs exhibit a high strain sensitivity of ∼80.

Influence of Carbon Nanotube Clustering on Mechanical and Electrical Properties of Cement Pastes

Given the continued challenge of dispersion, for practical purposes, it is of interest to evaluate the impact of multi-walled carbon nanotubes (MWCNTs) at different states of clustering on the eventual performance properties of cement paste. This study evaluated the clustering of MWCNTs and the resultant effect on the mechanical and electrical properties when incorporated into cement paste. Cement pastes containing different concentrations of MWCNTs (up to 0.5% by mass of cement) with/without surfactant were characterized. MWCNT clustering was assessed qualitatively in an aqueous solution through visual observation, and quantitatively in cement matrices using a scanning electron microscopy technique. Additionally, the corresponding 28-day compressive strength, tensile strength, and electrical conductivity were measured. Results showed that the use of surfactant led to a downward shift in the MWCNT clustering size distribution in the matrices of MWCNT/cement paste, indicating improved dispersion of MWCNTs. The compressive strength, tensile strength, and electrical conductivity of the composites with surfactant increased with MWCNT concentration and were higher than those without surfactant at all concentrations.

Opening-mode cracking in asphalt pavements: crack initiation and saturation

ABSTRACT Opening-mode cracking has been commonly found in asphalt pavements and other layered materials with nearly uniform crack spacing. Due to the deformation mismatches between surface overlay and base layer of pavements, longitudinal tensile stress is induced in the surface course. When it reaches a certain level, transverse cracks will initiate at the surface to release the energy stored in the asphalt materials. When crack spacing reduces to a certain value, crack density is saturated and no new crack forms. This paper investigates the crack initiation and saturation for opening-mode cracking. Using elastic governing equations and a weak form stress boundary condition, we derive an explicit solution of elastic fields in the surface course and obtain the energy release rate, so that opening-mode cracking initiation can be determined by fracture energy criterion. Interestingly, the longitudinal stress between such cracks along the surface undergoes a transition from tensile to compressive with increasing applied tensile loading, which implies crack saturation. This explicit formulation is applicable to pavement structure design and opening-mode cracking analysis of asphalt pavements. If a loading condition is fixed, there exists a critical thickness of the surface overlay, below which no crack forms. Parametric analyses of opening-mode cracking are conducted considering the material stiffness, fracture toughness, interface conditions, and loading conditions. The viscoelastic effect of asphalt materials on the crack development is also discussed.