Analysis of contact problems of porous thermoelastic solids

In this article, we study a contact problem between a one-dimensional porous thermoelastic layer and a rigid obstacle. The mechanical problem consists of a coupled system of two hyperbolic partial differential equations and a parabolic one. By defining penalized problems, an energy decay property is obtained. Then, fully discrete algorithms are introduced to approximate both penalized and Signorini problems using the finite element method and the implicit Euler scheme. Stability properties are shown for both problems and a priori error estimates are proved for the penalized problem, from which the linear convergence of the algorithm is derived. Finally, some numerical simulations are performed to demonstrate the accuracy of the approximation and the behavior of the solution.     

Thermal rectification between two thermoelastic solids with a periodic array of rough zones at the interface

An analysis of the effective thermal contact resistance between two semi-infinite solids in the presence of a periodic array of rough zones at the interface is carried out on the basis of a solution of the corresponding thermoelastic contact problem. The effect of the roughness is modeled by localized thermal contact resistances varying inversely with the contact pressure. The contact problem is reduced to a nonlinear singular integrodifferential equation, and an iterative procedure is proposed for its solving. The results demonstrate that the periodic array of rough zones between two semi-infinite solids exhibits thermal rectification. It is also found that the effective temperature jump and the effective thermal contact resistance are nonlinear functions of a far field heat flux.     

Waves in nonlocal thermoelastic solids of type II

The present article deals with the investigation of the propagation of thermoelastic plane harmonic waves in a nonlocal thermoelastic medium. The Green and Naghdi theory II (without energy dissipation) of generalized thermoelasticity with elastic nonlocal effect is adopted to address this problem. The problem of reflection of thermoelastic waves due to an incident coupled longitudinal elastic wave from the rigid and thermally insulated boundary of a homogeneous, isotropic nonlocal thermoelastic half-space is also studied. The amplitude ratios of the reflected waves and their respective energy ratios are determined analytically. For a particular model, the effect of elastic nonlocality parameter on the variations of phase speeds, attenuation coefficients, amplitude ratios and corresponding energy ratios of the reflected waves is presented graphically and analysis of these results is given.     

Effect of phase-lag on thermoelastic vibration of Timoshenko beam

Abstract

An explicit formula of coupled three-phase-lag (TPL) thermoelasticity theory under the Timoshenko beam is constructed for microbeam resonators. The constructed mathematical model is based on the modified couple stress theory which implies a prediction of size-dependent effects in microbeam resonators. By using Hamilton’s principle, governing equations for motion and boundary conditions are derived. The thermal moment and thermal deflection of microbeam resonators are studied analytically and numerically. A comparison of the results between modified coupled stress theory and classical theory is executed for TPL, GN-II, and Lord–Shulman (LS) models. Also, a comparison of the results between TPL, GN-II, and LS models for modified coupled stress theory is done. Besides, the result is presented for silicon microbeam for different aspect ratios and phase-lags. It demonstrated the result corresponding to the behavior of thermoelastic frequencies of microbeam resonators.     

Turbine endwall film cooling with combustor-turbine interface gap leakage flow: Effect of incidence angle

This paper is focused on the film cooling performance of combustor-turbine leakage flow at off-design condition. The influence of incidence angle on film cooling effectiveness on first-stage vane endwall with combustor-turbine interface slot is studied. A baseline slot configuration is tested in a low speed four-blade cascade comprising a large-scale model of the GE-E³Nozzle Guide Vane (NGV). The slot has a forward expansion angle of 30 deg. to the endwall surface. The Reynolds number based on the axial chord and inlet velocity of the free-stream flow is 3.5 × 10⁵ and the testing is done in a four-blade cascade with low Mach number condition (0.1 at the inlet). The blowing ratio of the coolant through the interface gap varies from M = 0.1 to M = 0.3, while the blowing ratio varies from M = 0.7 to M = 1.3 for the endwall film cooling holes. The film-cooling effectiveness distributions are obtained using the pressure sensitive paint (PSP) technique. The results show that with an increasing blowing ratio the film-cooling effectiveness increases on the endwall. As the incidence angle varies from i = +10 deg. to i = ?10 deg., at low blowing ratio, the averaged film-cooling effectiveness changes slightly near the leading edge suction side area. The case of i = +10 deg. has better film-cooling performance at the downstream part of this region where the axial chord is between 0.15 and 0.25. However, the disadvantage of positive incidence appears when the blowing ratio increases, especially at the upstream part of near suction side region where the axial chord is between 0 and 0.15. On the main passage endwall surface, as the incidence angle changes from i = +10 deg. to i = ?10 deg., the averaged film-cooling effectiveness changes slightly and the negative incidence appears to be more effective for the downstream part film cooling of the endwall surface where the axial chord is between 0.6 and 0.8.     

The thermoelastic problem of a stretchable rigid line inhomogeneity at soft bimaterial interface

Abstract

The thermoelastic problem of a stretchable rigid line inhomogeneity at a bimaterial interface is considered. The closed-form solution is presented and the explicit expressions of the stress at the tip of the rigid line inhomogeneity are derived. The effects of thermal expansion of rigid line on the stress intensity factors are investigated. It is found that the singularity of stress maintains the same structure, and the stresses possess an apparent oscillatory character, as in the case of a rigid line without the effects of thermal expansion. However, there is a significant difference in the coefficient of the singular behavior of the stresses at the tip of rigid line, which is due to the temperature changes.     

An approximate solution to the melting of granular polymeric solids on a hot moving interface

The pertinent coupled equations describing the melting of granular polymeric solids on a hot moving interface were solved approximately with collocation method. A non-Newtonian boundary layer thickness function was defined, and the melting rate was evaluated in terms of this non-Newtonian boundary layer and the averaged velocity component in the moving direction. The results of this analysis were comparable to those by numerical simulation, successive iteration, and experimental data.     

Reflection and refraction of plane wave at the interface between elastic and thermoelastic media with three-phase-lag model

The problem of reflection and refraction phenomenon due to longitudinal and transverse waves incident obliquely at a plane interface between uniform elastic solid half-space and thermoelastic solid with three-phase-lag model half-space has been studied. In thermoelastic solid medium, potential functions are introduced to represent two longitudinal waves and one transverse wave. The amplitude ratios of various reflected and refracted waves to that of incident wave are derived. These amplitude ratios are further used to find the expressions of energy ratios of various reflected and refracted waves to that of incident wave. The graphical representation is given for these energy ratios for different directions of propagation. The law of conservation of energy at the interface is verified.     

Evaluation of thermal rectification at the interface of dissimilar solids by phonon heat transfer

Many investigations have suggested that the heat transfer coefficient at the interface between two dissimilar solids depends on the direction of heat flow across the interface. Although many factors that affect heat transfer across the interface are reasonably well understood, the directional dependence of the heat transfer coefficient, called thermal rectification, has not yet been completely explained. In this paper, we evaluate the thermal rectification from the results of linear response theory by considering scattering and transmission of phonons across the interface, which expresses the dependence of the temperature at the interface. This model explains the reported behavior of thermal rectification. © 2001 Scripta Technica, Heat Trans Asian Res, 30(2):164–173, 2001     

Effect of interface recombination on solar cell parameters

A model is presented for p–n hetero-junction solar cells in which interface recombination is the dominant diode current transport mechanism. The model explains the large diode ideality factor (n>2) and the increased saturation current density in terms of increased density of interface states N_ir. Furthermore, the model allows us to explain the non-translation between illuminated and dark J–V characteristics. The explanation is based on the assumption that, for high interface state density values, both the depletion layer width and the diffusion voltage in the p- and n-side of the junction are functions of N_ir. The interface recombination leads to lower values of the open-circuit voltage, short-circuit current density, and fill factor. These results are illustrated by numerical calculations of solar cell parameters and compared with experimental data achieved for ZnO/CdS/CuGaSe₂ single-crystal solar cells.     

Effect of contact angle on wetting limit temperature

The effect of surface wettability on evaporation of a water drop has been examined experimentally using surfaces with various contact angles. To greatly change the surface wettability, TiO₂ superhydrophilicity, plasma irradiation, and super‐water‐repellent surface are adopted as the heating surface. The range in contact angle achieved by these methods was between 0^° and 170^°. The relationship between the contact angle and the wetting limit temperature was obtained and it was found that the lifetime of a water drop dramatically decreases with contact angle in the lower temperature region, and that the wetting limit temperature increases with the contact angle. © 2006 Wiley Periodicals, Inc. Heat Trans Asian Res, 35(7): 513–526, 2006; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20128     

Effect of air gap on the performance of building-integrated photovoltaics

Ventilation of photovoltaic (PV) modules installed over or beside a building envelope can reduce the module temperature and increase the electrical conversion efficiency. A computational fluid dynamics (CFD) method has been used to assess the effect of the size of air gap between PV modules and the building envelope on the PV performance in terms of cell temperature for a range of roof pitches and panel lengths and to determine the minimum air gap that is required to minimise PV overheating. It has been found that the mean PV temperature and the maximum PV temperature associated with hot spots decrease with the increase in pitch angle and air gap. The mean PV temperature also decreases with increasing panel length for air gaps greater than or equal to 0.08 m whereas the maximum PV temperature generally increases with panel length. To reduce possible overheating of PV modules and hot spots near the top of modules requires a minimum air gap of 0.12–0.15 m for multiple module installation and 0.14–0.16 m for single module installation depending on roof pitches.     

Effect of gap thickness on a rectangular-cell compound-honeycomb solar collector

Compound-honeycomb solar collectors employ a honeycomb to suppress natural convection and air gaps between the honeycomb and the absorber plate or the glazing to reduce conductive and radiative heat losses. Results of an experimental study on the effect of the thickness of these gaps on the total heat transfer across a compound honeycomb confined between two isothermal and low emissivity boundaries are presented. The honeycomb consists of rectangular cells with elevation aspect ratio 6.3 and plan aspect ratio 82 (15.8 mm thick, 206 mm wide and 2.5 mm deep), constructed from parallel glass strips. Plots of Nusselt number versus Rayleigh number are given for compound honeycombs with air gaps of thickness 1.6 mm, 3.3 mm, 6.4 mm and 9.6 mm above and below the honeycomb and for a compound-honeycomb layer with an air gap of thickness 6.4 mm below the honeycomb. Measurements are reported for tilt angles of 0°, 30° and 60° where the long dimension of the rectangular cells in the honeycomb is horizontal. As the air gaps' thickness increase, coupled conductive-radiative heat transfer is reduced, while the critical Rayleigh number is also reduced and convective heat transfer increases. However, even for relatively thick air gaps, the decrease in critical Rayleigh number is moderate, and the rectangular-cell compound honeycomb is found to be an effective convection suppressor. A compound honeycomb with air gaps above and below the honeycomb is shown to be superior in suppressing convection to a compound honeycomb of equal total thickness with only one air gap below the honeycomb.     

The influence of interface thermal contact resistance on the heat transfer performance of prestressed Duplex tube

Duplex heat exchanger tubes consist of two concentric cylinders assembled in a state of prestress by shrink fitting. During operation, thermal expansion changes the interface pressure and this can cause a substantial increase in the thermal resistance of the tube. Under certain circumstances, more than one steadystate solution can be obtained. In this paper, the effect of this mechanism on the axial temperature variation in parallel or counterflow heat exchangers is analyzed. A method is developed for integrating the controlling equations for an arbitrary, non-linear, pressure-dependent contact resistance without iteration and the effect ofprestress and other parameters on the occurrence of in ultiple solutions is discussed.     

Effect of an ormosil-based filler on the physico-chemical and electrochemical properties of Nafion membranes

Composite Nafion-based membranes were prepared and characterized, using an organosilane derivative (sulfonated diphenylsilanediol, SDPSD) as a filler. The physico-chemical and electrochemical properties of the composite membranes were investigated using differential scanning calorimetry (DSC), field emission scanning electron microscopy (FE-SEM), water uptake (W.U.), and electrochemical impedance spectroscopy (EIS). Both conductivity values and thermal features of the composite demonstrated that the presence of SDPSD as a filler significantly modified the properties of the Nafion matrix, allowing to increase its performance in terms of proton conductivity as well as thermal stability. In particular, the interaction between Nafion and SDPSD was found to improve the mechanical stiffness of the Nafion matrix, leading to an increase in the temperature range at which the Nafion membrane maintains proper hydration requirements and hence satisfactory proton conductivity values, allowing its use as an electrolyte in Polymer Electrolyte Fuel Cells operating at intermediate temperatures.     

微细通道内蒸汽直接接触凝结汽液界面演变特征的实验研究

为探究微细尺度条件下蒸汽直接接触凝结汽液界面瞬时特征及演变行为,开展了圆形截面T型微细通道内蒸汽直接接触凝结的可视化实验研究。利用高速摄像系统获取了饱和蒸汽体积流量630μL/min、过冷水温度60℃及过冷水体积流量12 650μl/min工况下直接接触凝结过程的瞬时图像信息。研究发现,汽液界面演变历程不能简单的用一种凝结流型进行表述,其过程中包含了柱状、环状和间歇凝结流型的特征。蒸汽自支管入射至主管后其长度经历了相对较长时间(108.2 ms)的振荡增长,而消失过程则相对短暂(16.2 ms)。此外,水平主管内的蒸汽柱先后发生了脱离和断裂现象,且其与支管正下方的蒸汽泡和的凝结机理有所不同。     

Effect of solid surface properties on dynamic contact angles

An experimental investigation was conducted to understand the effect of solid properties on dynamic wetting. Using a liquid tank method, the wetting behavior of silicone oil over glass, aluminum, and stainless steel surfaces was measured. For all three surfaces, the dynamic contact angles disagreed with the universal function proposed by Hoffman. However, if a dimensionless parameter is introduced to describe the nature of the solid surfaces, the experimental results can be in good agreement with the model. Furthermore, the present experiments indicated that the Hoffman–Voinov–Tanner law should be valid for a wider range of dynamic contact angles or capillary numbers. © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 35(1): 1–12, 2006; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20099     

Thermal gap conductance at low contact pressures (<1 MPa): Effect of gold plating and plating thickness

Thermal contact conductance (TCC) measurements are made on bare and gold plated (?0.5 μm) oxygen free high conductivity (OFHC) Cu and brass contacts in vacuum, nitrogen, and argon environments. It is observed that the TCC in gaseous environment is significantly higher than that in vacuum due to the enhanced thermal gap conductance. It is found that for a given contact load and gas pressure, the thermal gap conductance for bare OFHC Cu contacts is higher than that for gold plated contacts. It is due to the difference in the molecular weights of copper and gold, which influences the exchange of kinetic energy between the gas molecules and contact surfaces. Furthermore, the gap conductance is found to increase with increasing thickness of gold plating. Topography measurements and scanning electron microscopy (SEM) analysis of contact surfaces revealed that surfaces become smoother with increasing gold plating thickness, thus resulting in smaller gaps and consequently higher gap conductance.