Observation and Analysis of Water Temperature in Ice-Covered Shallow Lake: Case Study in Qinghuahu Lake

Water temperature serves as a key environmental factor of lakes and the most basic parameter for analyzing the thermal conditions of a water body. Based on the observation and analysis of the water temperature of Qinghuahu Lake in the Heilongjiang Province of China, this paper analyzed the variation trend of the heat flux, effective thermal diffusivity of the icebound water, and revealed the temporal and spatial variation law of the water temperature and the transfer law beneath the ice on a shallow lake in a cold region. The results suggested a noticeable difference existing in the distribution of water temperature beneath the ice during different periods of ice coverage. During the third period, the water temperature vertically comprised three discrete layers, each of which remained unchanged in thickness despite the alternation of day and night. Sediment–water heat flux and water–ice heat flux both remained positive values throughout the freezing duration, averaging about 3.8–4.1 W/m2 and 9.8–10.3 W/m2, respectively. The calculated thermal diffusivity in late winter was larger than molecular, and the time-averaged values increased first and then decreased with water depth, reaching a maximum at a relative depth of 0.5. This research is expected to provide a reference for studies on the water environment of icebound shallow lakes or ponds in cold regions.

Numerical Simulation of Thermal Diffusivity Measurements with the Laser-Flash-Method to Evaluate the Effective Property of Composite Materials

Laser Flash Method (LFM) is commonly used to measure the thermal diffusivity of homogeneous and isotropic materials, but it can be also applied to macroscopically inhomogeneous materials, such as composites. When composites present thermal anisotropy, as fiber-reinforced, LFM can be used to measure the effective thermal diffusivity (aeff) in the direction of heat flux. In the present work, the thermal behavior of composites during thermal diffusivity measurements with the LFM was simulated with a Finite Element Model (FEM) using commercial software. Three composite structures were considered: sandwich layered (layers arranged in series or parallel); fiber-reinforced composites; particle composite (spheres). Numerical data were processed through a non-linear least-square fitting (NL-LSF) to obtain the effective thermal diffusivity of the composite. This value has the meaning of "dynamic effective thermal diffusivity". Afterward, the effective thermal conductivity (?eff) is calculated from the dynamic effective thermal diffusivity, equivalent heat capacity and density of the composite. The results of this methodology are compared with the analytically calculated values of the same quantity, which assume the meaning of "static effective thermophysical property". The comparison of the dynamic and static property values is so related to the inhomogeneity of the samples, a deviation of the temperature vs time trend from the solution for the perfectly homogeneous sample gives information about the sample's lack of uniformity.

Thermophysical Properties of Temperature-Sensitive Paint

The complex thermophysical property of temperature-sensitive paint (TSP) research is discussed. TSP is used for visualization of the surface temperature distribution in wind tunnel aerodynamic tests. The purpose of this research was to provide reliable, experimental, thermophysical data of the paint applied as a coating. As TSP is applied as thin surface layers, investigation of its final properties is challenging and demands the application of non-standard procedures. At present, most measurements were performed on composite specimens of TSP deposed onto a thin metallic film substrate or on TSP combined with a cellulose sheet support. The studies involved gravimetric,, thermogravimetric, and microcalorimetric analyses, transversal thermal diffusivity estimation from laser flash data and in-plane effective thermal diffusivity measurements done by the temperature oscillation technique. These results were complemented with scanning electron microcopy analysis, surface characterization and the results of dilatometric measurements performed on the TSP bulk specimens obtained from liquid substrate by casting. Complex analysis of the obtained results indicated an isotropic characteristic of the thermal diffusivity of the TSP layer and provided reliable data on all measured thermophysical parameters—they were revealed to be typical for insulators. Further to presenting these data, the paper, in brief, presents the applied investigation procedures.

Application of Thermal Taylor Dispersion to Upscaling of Geothermal Processes in Heterogeneous Formations

<p>Well-logging data show that geothermal formations typically feature layered heterogeneities. This imposes a challenge in numerical simulations, in particular in the upscaling of geothermal processes. The goal of our study is to develop an approach to (1) simplify the description of heterogeneous geothermal formations and (2) provide an accurate representation of convection/dispersion processes for simulating the up-scaled system.</p><p>In geothermal processes, transverse thermal conduction causes extra spreading of the cooling front: thermal Taylor dispersion. We derive a model from an energy balance for effective thermal diffusivity, α<sub>eff</sub>, to represent this phenomenon in layered media. α<sub>eff</sub>, accounting for transverse heat conduction, is much greater than the longitudinal thermal diffusivity, leading to a remarkably larger effective dispersion. A ratio of times is defined for longitudinal thermal convection and transverse thermal conduction, referred to as transverse thermal-conduction number N<sub>TC</sub>. The value of N<sub>TC</sub> is an indicator of thermal equilibrium in the vertical cross-section. Both N<sub>TC</sub> and α<sub>eff</sub> equations are verified by a match with numerical solutions for convection/conduction in a two-layer system. For N<sub>TC</sub> > 5, the system behaves as a single layer with thermal diffusivity α<sub>eff</sub>.</p><p>When N<sub>TC</sub> > 5, a two-layer system can be combined and represented with α<sub>eff</sub> and average properties of the two layers. We illustrate upscaling approach for simulation of geothermal processes in stratified formations, by grouping layers based on the condition of N<sub>TC</sub> > 5 and the α<sub>eff</sub> model. Specifically, N<sub>TC</sub> is calculated for every adjacent two layers, and the paired layers with a maximum value of N<sub>TC</sub> are grouped first. This procedure repeats on the grouped system until no adjacent layers meet the criterion N<sub>TC</sub> > 5. The upscaled layer properties of the grouped system are used as new inputs in the numerical simulations. The effectiveness of the upscaling approach is validated by a good agreement in numerical solutions for thermal convection/dispersion using original and average layer properties, respectively (Figs. 1 and 2 in the Supplementary Data File). The upscaling approach is applied to well-log data of a geothermal reservoir in Copenhagen featuring many interspersed layers. After upscaling, the formation with 93 layers of thickness 1 – 3 meters is upscaled to 12 layers (Fig. 3 in the Supplementary Data File). The effective thermal diffusivity α<sub>eff</sub> in the flow direction is about a factor of 10 times greater than original thermal diffusivity of the rock. Thus, α<sub>eff</sub> should be used as simulation inputs for representing more accurately geothermal processes in the up-scaled system.</p><p> </p><p> </p>

Resonance Generation of Short Internal Waves by the Barotropic Seiches in an Ice-Covered Shallow Lake

Purpose. The observation measurements testify the fact that heat and mass transfer processes in the shallow ice-covered lakes are not limited to the molecular diffusion only. In particular, the effective thermal diffusivity exceeds the molecular one by up to a few orders of magnitude. Now it is widely accepted that the transfer processes, in spite of their low intensity, are controlled by intermittent turbulence. At the same time, its nature and generation mechanism are still studied insufficiently. The paper represents one of such mechanisms associated with resonance generation of short internal waves by the barotropic seiches. Methods and Results. The temperature measurements in a shallow lake in winter were used as an experimental base. Having been analyzed, the temperature profiles’ dynamics observed during a few weeks after freezing revealed the anomalous values of thermal diffusivity. At that the temperature pulsations’ spectra clearly demonstrate the peak close to the main mode of barotropic seiches. Counter-phase oscillations at the different depths and pronounced heterogeneity of the amplitudes of temperature pulsations over depth indicate presence of internal waves. Based on these data, the mechanism of energy transfer from the barotropic seiches to the internal waves similar to the “tidal conversion” (the latter governs resonance generation of internal tides in the ocean), is proposed. The expressions for heat flux, energy dissipation rate and effective thermal diffusivity are derived. Conclusions. Internal waves can play an essential role in the processes of interior mixing and heat transfer in the ice-covered lakes. Though direct wind-induced turbulence production is inhibited, baric perturbations in the atmosphere can give rise to barotropic seiches, which play the role of an intermediate energy reservoir and can generate short resonant internal waves resulted from interaction with the undulate lake floor. The internal wave field parameters strongly depend on the barotropic seiche amplitudes, buoyancy frequency and the bottom topography features.

Wool and Leather Waste Materials with Thermo-Insulating Properties

Seven new different thermo-insulating materials based on wool and /or skin wastes were obtained. To emphasize this capability, the effective thermal diffusivities were determined at a material moisture content of 10%, in a temperature range of 10 to 40oC. Depending on the material composition, the results showed that the effective thermal diffusivity varies between the limits of 6E-8 and 8.5E-8 m2/s. The smallest values were obtained for the untreated wool and for the material obtained from both untreated wool and finished leather powder. The obtained values underline the fact that the investigated materials can be used to obtain composites with good thermo-insulating properties.

Thermal properties and degradation kinetics of epoxy-γ-alumina and epoxy-zinc oxide light weight composites

Lightweight composite materials are the gold standard in aeronautical and aerospace applications due to their strength and low mass. To carry higher payloads and decrease launching costs, nanosatellites lightweight. Additionally, nanosatellites must also resist high thermal radiation loads while working in orbit. Polymer-based composite materials maintain low mass and added reinforcing ceramic fillers contributes to increasing radiation resistance, thus producing composites that meet both requirements. In this work, the effects of γ-alumina (Al2O3) and zinc oxide (ZnO) micro- and nanoparticles on the thermal properties and degradation kinetics of epoxy-based composites were investigated. The effective thermal conductivity improved up to 17.8 % for epoxy/γ-Al2O3 and 27.4 % for epoxy/ZnO. The effective thermal diffusivity values show a monotonic decreasing behavior as a function of the particle concentration for the epoxy/γ-Al2O3 composites; for the epoxy/ZnO composites, no correlation on the effective thermal diffusivity values with the ZnO-content was observed. Both oxide-based ceramic fillers increase the thermal stability of epoxy up to 250 °C; however, γ-Al2O3 decreased the maxima decomposition temperature of the epoxy matrix by 6°C. Zinc oxide did not affect the maxima decomposition temperature but decreased the activation energy of epoxy by ~ 45 %. These results provide a feasible manufacturing method for epoxy-based composite materials (i.e. nanosatellites) where efficient heat transfer, heat resistance, and low mass are required.

Thermal Diffusivity Measurement of Laser-Deposited AISI H13 Tool Steel and Impact on Cooling Performance of Hot Stamping Tools

Additive manufacturing is a technology that enables the repair and coating of high-added-value parts. In applications such as hot stamping, the thermal behavior of the material is essential to ensure the proper operation of the manufactured part. Therefore, the effective thermal diffusivity of the material needs to be evaluated. In the present work, the thermal diffusivity of laser-deposited AISI H13 is measured experimentally using flash and lock-in thermography. Because of the fast cooling rate that characterizes the additive process and the associated grain refinement, the effective thermal diffusivity of the laser-deposited AISI H13 is approximately 15% lower than the reference value of the cast AISI H13. Despite the directional nature of the process, the laser-deposited material’s thermal diffusivity behavior is found to be isotropic. The paper also presents a case study that illustrates the impact of considering the effective thermal conductivity of the deposited material on the hot stamping process.