External Temperature Difference Research Articles

Formation of Temperature Cracks of Concrete in High-Rise Buildings and the Corresponding Measures

AbstractA lot of heat will generate in mass concrete after pouring to form temperature cracks, which will reduce structural stiffness. This paper briefly introduces the principle of solid heat conduction and the cause of temperature crack formation and then used COMSOL software to simulate and analyze the mass concrete. The results showed that the simulation model had enough reliability to analyze the temperature change; the internal and external temperature of concrete rose first and then decreased; the formation of temperature crack was related to the internal and external temperature difference; the internal and external temperature difference was inversely proportional to the heat conductivity coefficient of concrete and directly proportional to the pouring temperature. Then, according to the analysis results, two measures were put forward to prevent temperature cracks in mass concrete: selecting concrete materials with high thermal conductivity, i.e., selecting coarse aggregate and fine aggregate with larger heat conductivity coefficient and reducing concrete pouring temperature, i.e., selecting cement with lower hydration heat, paying attention to temperature reduction in the process of concrete stirring, and reducing the amount of cement.

(Invited) Cavity-Free Micro Thermoelectric Energy Harvester with Si Nanowires

Forthcoming trillion-sensors-network society demands the maturity of energy harvesting technology. Among various energy harvesting devices, thermoelectric generator (TEG), which utilizes the Seebeck effect of semiconductors, is anticipated as an ultimate one because it has a potential to provide a semipermanent power from almost unlimited heat energies. In a TEG, the efficiency of energy conversion from heat to electricity is determined by the thermoelectric figure of merit, ZT= S2σT/κ of the employed semiconductor, where S is the Seebeck coefficient, σ the electrical conductivity, κ the thermal conductivity, and T the average temperature between heat source and cold sink. Although higher ZT is favorable to achieve high conversion efficiency, only limited materials were known to have high ZT near the room temperature. Recently, silicon nanowires (Si-NWs) emerged as a promising thermoelectric material; the thermal conductivity of Si-NW was found to decrease drastically as the nanowire is thinned. [1,2] Since then, a number of fabrication examples of Si-based TEG has been reported. [3-5] Conventional demonstrations of planar type Si-based TEG employed long Si-NWs about 10-100 μm which were suspended on a cavity to avoid the bypass of the heat current, as shown in Fig 1a. The authors’ group proposed a new design of planar and short Si-NW TEG, which is illustrated as Fig. 1b. [6-9] Our proposed device is driven by a steep temperature gradient exuding around a heat flow perpendicular to the substrate, and the nanowires are not suspended on a cavity. Furthermore, the power density is scalable by shortening Si-NW to sub-μm length; the areal power generation density is inversely proportional to the square of the NW length. It has been experimentally confirmed that the thermoelectric power was enhanced by shortening the nanowire, and tens of µW/cm2-class power generation was achieved [6,7]. Fig. 2 shows an experimental Si-NW TEG fabricated on a silicon-on-insulator (SOI) wafer by the conventional CMOS process. The SOI layer was patterned into Si-NWs by ArF immersion lithography and reactive-ion etching. Both ends of Si-NW bundle are connected to Si-pads. Al/TiN/Ti electrode was deposited on the Si-pad. We prepared samples with different base Si-substrate thickness, by thinning with backside grinding. The hot side electrode was heated by a custom-made micro thermostat, and the base stage was maintained at room temperature by a water chiller. Fig. 3 shows the thermoelectric power density versus loading voltage curves, measured under an applied external temperature difference of 5 K. The measured power output is normalized by the area of the product of twice of the NW length and the NW pitch, supposing that the pads and NW arrays have a same footprint. The maximum power is obtained in the shortest NW device with 0.25 µm long. The thermoelectic power was drastically enhanced by reducing the Si-substrate thickness. Fig. 4 shows the dependence of the power density on the Si-substrate thickness. The power density of a sample with 50 µm thick Si-substrate reached 12 µW/cm2. The enhancement can be attributed to the increased temperature difference across Si-NWs by suppression of the series thermal resistance at the Si-substrate. Further improvement of thermoelectic power is expected by suppressing the thermal resistance at the Si-substrate. The temperature difference across Si-NW is expected to increase by partially replacing the Si substrate underneath the cold side edge of SiNW with Cu with higher thermal conductivity. According to our finite-element-simulation, [10] however, not only beneath the cold side edge of SiNW, replacing the whole substrate by Cu is the most effective way to improve the power generation density (Fig. 5). The results show that the thermal resistance of the entire substrate should be suppressed as a matter of first priority.

Effects of cooling process on the solid–liquid phase separation process in ultra-high-molecular-weight polyethylene/liquid paraffin blends

Ultra-high-molecular-weight polyethylene (UHMWPE) microporous membrane is generally prepared by thermally induced phase separation process. The phase separation process is closely related to the cooling process in practical production. In this paper, the phase separation temperature of UHMWPE/liquid paraffin blends was explored by the hot-stage-optical device and differential scanning calorimeter (DSC), and the result showed that the temperature was about 105–125 °C. The effects of cooling condition and crystallization ability of UHMWPE on the separation process of the blends were also investigated by DSC. The results showed that the phase separation was affected by the cooling rates rather than the initial cooling temperature. And the crystallization of UHMWPE was mainly limited by the nucleating at a low cooling rate, which could form larger porous structure. In a word, it should be inspirational for the actual production of the UHMWPE microporous membrane. The solid–liquid phase separation will occur in the cooling process of UHMWPE/LP blends, which is driven by the external temperature difference. It is significant to understand the phase separation process. In this paper, the effects of the melting properties and cooling condition on phase separation process were investigated, the S–L phase separation range is at 105–125 °C, and the speed of phase separation is mainly related to the external temperature difference and the UHMWPE content. It could be inspirational for actual production of the UHMWPE membrane with more uniform pore structure.

Simulated Calculation of Temperature Control and Anti-crack during Construction of Inverted Siphon

Inverted siphon, such a thin-walled structure, tends to have large internal and external temperature differences during the early stage of pouring, resulting in a large tensile stress on the surface. Once surface or penetrating cracks appear, it will have a great influence on the integrity, safety, impermeability and durability of the structure and reduce the water transfer function. The temperature control simulation of the Liyanghe inverted siphon in the middle line of South-to-north Water Diversion project was designed to select the temperature control measures of inverted siphon concrete during different month construction. Temperature control measures can provide references for similar projects at home and abroad.

An analytical solution of multi-layered thick-walled tubes in thermo-elasticity with application to gas-wells

Gas-wells are composed of several layers of different materials and undergo at least axial loads, internal and external pressure and temperature differences from inside to outside. In this article, an analytical solution for thick-walled tubes under internal and external pressure, constant axial deformation and a stationary heat distribution is provided. Here, we restrict ourselves to the case of small strain thermo-elasticity. As an example, a gas-well with several layers in the production region is investigated, where the temperature distribution can vary due to temperature cycles. The solution can also be used as verification example for finite element simulations or for comparing it to approximate solutions.

Thermal distribution analysis of inner defects in TSV

In order to uncover the external manifestations of TSV internal defects, the finite element models of typical internal defects, which were filling missing, axial cavity and end cavity, were established. The thermal analysis was carried out using thermoelectric coupling method. The temperature distribution of TSV with and without defects were obtained. And the temperature variation profiles on the defined paths of TSV layer were also analyzed. The analysis indicated that all the defective TSV showed distinct temperature distribution with the defect-free TSV. Among three typical defects, TSV with filling missing showed the most obvious difference on the temperature distribution and path variation. TSV with end cavity has relatively weak affect and the slightest defect was TSV with axial cavity. Therefore, it could be seen that the external temperature difference caused by the internal defects of TSV could provide effective information for the identification and detection in TSV with internal defects.

The Affected Analysis of Pipeline Thermal Stress Under Different Support Conditions and Wall Thickness Based on Finite Element Simulation

When gas transmission pipelines are working, due to being heated or cold and expansion and contraction, the pipes will produce thermal stress. Thus the stress poses a threat to pipeline working. Therefore, this article will adopt the method of model simulation and establish a channel space finite element model, then the ANSYS finite element analysis software will be applied to the calculation of thermal stress of the pipelines.Finally, we will do some research and analysis of the factors affecting the thermal stress in pipeline.The results show that the temperature had a greater influence on the thermal stress of pipeline, when the larger internal and external temperature difference of the pipeline get, the greater thermal stress the pipes produce. Secondly, the wall thickness of pipelines also affects thermal stress. Under the condition of the constant pipe diameter, thermal stress produced by pipes becomes small with the increase of pipe wall thickness.Lastly, with the fixed constraint on one end of the pipeline, the produced thermal stress is significantly larger than unconstrained condition and situation of constraints on two ends, etc.So it is suggested that the space between anchor blocks on pipelines cannot be too small, which means we should minimize that condition of the fixed constraint on one end of the pipeline.

Strongly correlated classical plasmas under external forcing and dissipation - an example using Molecular Dynamics

Systems with excess of average potential energy per particle than its kinetic energy develop strong correlations. It is well known that such systems are not amenable to standard procedures of BBGKY hierarchy. This results in failure of both kinetic and fluid models. Phenomenology is the normal way out. If such a strongly correlated system is further subjected to strong drive and/or dissipation, “near first principles” computational methods such as Molecular Dynamics become necessary. Formation of Rayleigh-Bénnard convection cells (RBCC), where a liquid is under the action of external gravity and external temperature gradient, is one such phenomena. We report here, the formation of RBCC in 2-dimensional strongly coupled Yukawa liquids, characterized by coupling strength Γ (ratio of average potential energy to kinetic energy per particle) and screening parameter κ (ratio of average inter particle distance to Debye length). We observe the existence of a critical external temperature difference, beyond which RBCC are seen to emerge. Beyond this critical external temperature difference, the strength of the maximum convective flow velocity is shown to exhibit a new, linear relationship with external temperature difference and with a slope independent of (Γ, κ). The time taken for the transients to settle down (ts) is found to be 10,000 to 20,000 ωpd-1, where ωpd is dust plasma frequency. At very high values of Γ and/or low values of κ, RBCC are seen to get suppressed.

Experimental measurement and numerical simulation of the thermal performance of a double glazing system with an interstitial Venetian blind

Venetian blinds, which were originally designed to provide sun shading and privacy, also have the potential to reduce heat transfer caused by internal and external temperature difference when integrated within the cavity between the two panes of a double glazing unit. In this paper, the thermal performance of a glazing system with and without a Venetian blind with various slat orientation angles under different temperature conditions is investigated through both experiment (undertaken in a large climate chamber) and numerical simulation (obtained via Computational Fluid Dynamic modelling). The thermal resistance of a Venetian blind glazing system varies with the change of slat inclination angle, and it also highly depends on the mean temperature of the glazing and the temperature difference between the indoor and outdoor environment. Inclusion of a Venetian blind modifies both the absolute and relative strengths of convection and radiation. Vertically oriented slats showed the most significant contribution to increasing radiative thermal resistance, which led to the best overall thermal performance. The system achieved up to 28% improvement of U-value when compared with a glazing unit without a Venetian blind. Empirical correlations generated based on simulations could be used for future building energy simulation.

Observation of the Rayleigh-Bénard convection cells in strongly coupled Yukawa liquids

Using “first principles” molecular dynamics simulation, we report for the first time the formation of Rayleigh-Bénard convection cells (RBCC) in two-dimensional strongly coupled Yukawa liquids, characterized by coupling strength Γ (ratio of average potential energy to kinetic energy per particle) and screening parameter κ (ratio of average inter-particle distance to Debye length). For typical values of (Γ, κ), existence of a critical external temperature difference is demonstrated, beyond which RBCC are seen to set in. Beyond this critical external temperature difference, the strength of the maximum convective flow velocity is shown to exhibit a new, hitherto unsuspected linear relationship with external temperature difference and with a slope independent of (Γ, κ). The time taken for the transients to settle down (τs) to a steady state RBCC is found to be maximum close to the above said critical external temperature difference and is seen to reduce with increasing external temperature difference. For the range of values of (Γ, κ) considered here, τs ≈ 10 000–20 000 ωpd−1, where ωpd is dust plasma frequency. As Γ is increased to very high values, due to strong coupling effects, cells are seen to be in a transient state without attaining a steady state for as long as 100 000 ωpd−1, even for a very high external temperature difference. Role of system size, aspect ratio, and dust-neutral collisions has also been addressed.

Preparation and characterization of a reflective and heat insulative coating based on geopolymers

A reflective heat insulation coating was prepared using a geopolymer, which was mainly made of sodium silicate solutions and metakaolin, as the primary film forming material before adding sericite powder, talcum powder, titanium dioxide and hollow glass microspheres as fillers. This coating is a new, environmentally friendly inorganic coating with many capabilities, such as good water-retention, simple spraying, the compacted paint, high durability, dirt resistance, high reflectivity and remarkable heat insulation. The results indicate a titanium dioxide content of approximately 12% yields 6% hollow glass microspheres yielded a reflective heat insulation coating with a reflectivity over 90% and thermal insulation performance (internal and external surface temperature difference) of up to 24°C. These mechanical and insulative properties could achieve the China National Standard (GB/T25261-2010).

Two-Sided Silicon Nanowire Array/Bulk Thermoelectric Power Generator

To improve the output power of n-and p-type Si nanowire array (NWA)/bulk thermoelectric power generators (TEGs) fabricated using metal assisted chemical etching, post-nanowire-etch spin-on-doping (SOD), and arrays on both sides of the bulk are used. Post-process SOD increases the power factor and removes surface depletion/inversion effects while maintaining crystalline NWs. A maximum output power of 3.5 μW for a p-NWA/bulk/NWA TEG with NWA length of 23 and 24 μm is obtained for an external temperature difference of 37 °C and a TEG area of 25 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> .

Effect of Antigravity-Suction-Casting Parameters on Microstructure and Mechanical Properties of Mg&amp;ndash;10Al&amp;ndash;0.2Mn&amp;ndash;1Ca Cast Alloy

We investigated the effect of the cooling rate and molten metal temperature on the microstructure and mechanical properties of a subrapidly solidified magnesium alloy, Mg­10Al­0.2Mn­1Ca, prepared by antigravity suction casting using a water-cooled steel mold. The microstructure of the antigravity-suction-cast material without water cooling consisted of coarse grains (grain size: 780μm), with networks of an Al­Ca compound at the grain boundaries. The higher cooling rate of the water-cooled steel mold promoted the formation of the Al­Ca compound and voids in accordance with increases in the internal and external temperature gap and differences in the solidification rate of the mold. The formation of voids and the shrinkage were suppressed, however, by adjusting the cooling rate and decreasing the molten metal temperature. The particle size of the Mg phase was refined to 135μm and the grain-boundary compounds were finely dispersed in the Mg phase. The as-cast alloy showed an ultimate tensile strength of 166MPa and an elongation of 8%. The microstructure and mechanical properties of the as-cast alloy were dependent on the cooling rate and molten metal temperature, but they were not dependent on the casting speed. [doi:10.2320/matertrans.MC201401]

Research on Hydration Heat Temperature Stress of the Pier by Finite Element Method

Hydration heat, one of the most severe factors, makes great effects on the mass concrete construction, especially on bridge structures and dam structures. Based on the experiment, this paper adopted large-scale finite element method with ANSYS software to analyze the transient temperature field during concrete placement. The atmosphere temperature, construction method, thermodynamics as well as material parameters (cement hydration heat, elastic modulus) were taken into account. The results clarity the distribution characters and process of the hydration heat temperature and stress. Besides, the computational and experimental results were compared. The results indicate that the surface cracks occur due to the large stress which is induced by the extensive internal and external temperature difference. As a result, references are provided to avoid harmful temperature cracks during pier construction period.

Failure mechanism of FC-PBGA devices under external stress

Flip chip plastic ball grid array (FC-PBGA) is unique and has been widely used. During FC-PBGA's practical application, the analysis of its failure mechanisms under high temperature, electricity, water vapor and other comprehensive environmental stress conditions is very important for improving its application reliability. In this paper, with 0.13-μm m/6-level copper-based FPGA with FC-PBGA package, failure mode of the device is exposed under external stress, which is dominated with thermal-mechanical stress generated by high temperature thermal reflow process. And the failure mechanism corresponding to the failure mode is analyzed. Results show that the basic cause of the failure is the thermal-mechanical stress, which is induced by the combination of internal and external reflow temperature difference and high temperature reflow process when assembled. The thermal-mechanical stress makes the solder bumps on the flip chip melt again and bridges the adjacent solder bumps to lead to the device short failure. It also induces crack or delamination in the underfill, crack in solder bump or solder bump fallen off to lead to device open failure. The residual stress on Cu/low-k interconnect structures damages the structure integrity and affects the reliability of FC-PBGA packages.

On the onset of dissipation thermal instability for the Poiseuille flow of a highly viscous fluid in a horizontal channel

The thermal instability of the plane Poiseuille flow as a consequence of the effect of viscous dissipation is investigated. No external temperature difference is assumed in the environment; the lower boundary is considered adiabatic, while the upper boundary is isothermal. Thus, the sole cause of the unstable thermal stratification is the flow rate, through the volumetric heating induced by the viscous dissipation. A linear stability analysis is carried out numerically by the analysis of normal modes perturbing the basic flow with different inclinations. The study of cases with different Prandtl numbers and Gebhart numbers suggests that the most unstable perturbations are the longitudinal rolls, namely the normal modes with a wave vector perpendicular to the basic flow direction. A possible comparison with the hydrodynamic instability of the plane Poiseuille flow, described by the Orr–Sommerfeld eigenvalue problem is proposed. This comparison, when referred to high values of the Prandtl number, reveals that the dissipation instability may be effective at a Reynolds number smaller than that needed for the onset of the hydrodynamic instability.

The Design of Cooling Water Pipe and Cooling Analysis in Mass Concrete

Inconstruction process of mass concrete is apt to form excessive temperature stress and cause mass concrete cracking, because of its high inner temperature, big internal and external temperature difference. In order to prevent their cracking, in mass concrete of the internal Settings with cooling water pipe cooling is one of the commonly used massive concrete construction method. This paper presents a massive concrete design of cooling water cooling and calculation theory, based on the cooling pipes in concrete heat transfer performance, through the cooling water pipe and concrete heat exchange principle.

Passive heat and moisture removal from a natural vented enclosure with a massive wall

Simultaneous transport of heat and moisture by conjugate natural convection in a partial enclosure with a solid wall is investigated numerically. Moist air motions are driven by the external temperature and concentration differences imposed across enclosures with different ambient moisture conditions. The Prandtl number and Schmidt number used are 0.7 and 0.6, respectively. The fluid, heat and moisture transports through the cavity and solid wall are, respectively, analyzed using the streamlines, heatlines and masslines, and the heat and mass transfer potentials are also explained by the variations of overall Nusselt and Sherwood numbers. The numerical simulations presented here span a wide range of the main parameters (heat and mass diffusion coefficient ratios, solid wall thickness and thermal Rayleigh numbers) in the domain of aiding and opposing buoyancy-driven flows. It is shown that the heat transfer potential, mass transfer potential, and volume flow rate can be promoted or inhibited, depending strongly on the wall materials and size, thermal and moisture Rayleigh numbers.

Effect of the Wavy permeable Interface on Double Diffusive Natural Convection in a Partially Porous Cavity

Two-dimensional, double diffusion, natural convection in a partially porous cavity satured with a binary fluid is investigated numerically. Multiple motions are driven by the external temperature and concentration differences imposed across vertical walls. The wavy interface between fluid and porous layer is horizontal. The equations which describe the fluid flow and heat and mass transfer are described by the Navier-Stokes equations (fluid region), Darcy-Brinkman equation (porous region) and energy and mass equations. The finite element method was applied to solve the governing equations. The fluid flow and heat and mass transfer has been investigated for different values of the amplitude and the wave number of the interface and the buoyancy ratio. The results obtained in the form of isotherms, stream lines, isoconcentrations and the Nusselt and Sherwood numbers; show that the wavy interface has a significant effect on the flow and heat and mass transfer.

CFD modelling of transparent bubble cavity envelopes for energy efficient greenhouses

Traditional greenhouses with single glazing have very poor thermal performance and the energy use for providing a productive internal environment is high. A cavity envelope formed by two transparent membranes and filled with transparent liquid bubbles can provide the greenhouse with super thermal performance. The liquid bubbles in the cavity transmit daylight while providing a thick blanket of insulation, thus reducing heat losses in winter and solar gains in summer. A computer model has been developed to simulate the thermal performance of a transparent bubble cavity envelope and the internal environment of the greenhouse. It has been found that the cavity convective heat transfer coefficient, equivalent thermal conductivity and thermal transmittance of a transparent bubble cavity envelope increase with bubble diameter and porosity as well as the internal–external temperature difference when bubbles are not compact in the cavity. The equivalent thermal conductivity and thermal resistance also increase with cavity width but the cavity convective heat transfer coefficient and thermal transmittance decrease with the increase in cavity width. A greenhouse with the transparent bubble cavity envelope has a better thermal environment with a significantly less energy requirement than does a traditional greenhouse.