Possibility of Advanced Modified-Silica-Based Porous Materials Utilisation in Water Adsorption Processes—A Comparative Study

Due to a high risk of power outages, a heat-driven adsorption chillers are gaining the attention. To increase the efficiency of the chiller, new adsorbents must be produced and examined. In this study, four newly developed silica–based porous materials were tested and compared with silica gel, an adsorber commonly paired with water. Extended sorption tests using mercury intrusion porosimetry, gas adsorption, and dynamic vapor sorption were performed. The morphology of the samples was determined using a scanning electron microscope. The thermal properties were defined using simultaneous thermal analysis and a laser flash method. Metal organic silica (MOS) nanocomposites analysed in this study had thermal properties similar to those of commonly used silica gel. MOS samples have a thermal diffusivity coefficient in the range of 0.17–0.25 mm2/s, whereas silica gel of about 0.2 mm2/s. The highest water adsorption capacity was measured for AFSMo-Cu and equal to 33–35%. For narrow porous silica gel mass uptake was equal about 25%. In the case of water adsorption, it was observed that the pore size of the sorbent is essential, and adsorbents with pore sizes higher than 5 nm, are most recommended in working pairs with water.

Method for dynamic measurement of thermal diffusivity during laser soldering of biological tissues

In this paper, we propose a method for dynamic measurement of the thermal diffusivity coefficient during laser soldering of biological tissues. The method is based on modelling the function of temperature dependence on time during cooling of biological tissue after exposure to laser radiation. The simulation is carried out by solving the heat equation for a homogeneous biological tissue and the absence of external heat sources. The desired value of the thermal diffusivity coefficient was determined by optimizing the residual functional of the temperature functions from time obtained experimentally and by solving the thermal diffusivity equation. Experiments were carried out to measure the thermal diffusivity coefficient by the proposed method for myocardial and skin tissues at maximum heating temperatures of 40, 50, 60 °C. The measured values of the thermal diffusivity coefficient for the myocardium are in the range from 2.3 to 2.7 m2/s*10-6, and for the skin from 1.5 to 1.7 m2/s*10-6.

On a problems related to a concept of soil thermal diffusivity and estimation of its dependence on soil moisture

Two problems in the theory of soil thermal conductivity are considered. First, the concept of the thermal diffusivity coefficient is discussed. It was shown that this coefficient can be used for model predictions only in a certain special cases. In the general case (when the soil thermal capacity and thermal conductivity vary in space and/or in time), the thermal diffusivity does not naturally appear. It could be artificially introduced into the heat equation but, in any case, to solve this equation (i.e., to calculate the dynamics of the soil temperature), this one parameter is not sufficient. It is necessary to set both the heat capacity and thermal conductivity as a functions of spatial and temporal coordinates or as a functions of environmental factors (e.g. soil moisture) depending on these coordinates. In this regard, the widespread misconception of the supposed sufficiency of one parameter (soil thermal diffusivity as a ratio of soil thermal conductivity to thermal capacity) for solving the heat equation using numerical methods is discussed. The examples of the common difference schemes used in computational practice show that this is not the case. Secondly, the condition number for the problem of parameters identification for the dependence of the soil thermal diffusivity coefficient on humidity for one well-known equation is considered. It is shown on real examples, that this problem is often ill-conditioned when solved by the least-squares method. However, sometimes its stability can be significantly improved if simple constraints are set for certain parameters (least-squares method with constraints). В работе рассматриваются две проблемы, возникающие в теории теплопроводности почв. Во-первых, обсуждается понятие коэффициента температуропроводности в свете того, что оно появляется только в отдельных весьма частных случаях, а в общем случае (когда теплоемкость и теплопроводность изменяются по пространству и/или с течением времени) коэффициент температуропроводности естественным образом вообще не возникает. Для такой среды с переменными (по пространству и во времени) свойствами он может быть искусственно введен в уравнение динамики температурного поля, но, в любом случае, для решения этого уравнения (т.е. для расчета динамики температурного поля) недостаточно одного параметра необходимо задать и теплоемкость, и теплопроводность как функции пространственной и временной координат или как функции факторов среды (например, влажности), зависящих от этих координат. В связи с этим обсуждается и распространенное заблуждение о якобы достаточности одного параметра (коэффициента температуропроводности как отношения теплопроводности к теплоемкости) при решении вышеуказанного уравнения численными методами. На примерах основных разностных схем, применяемых в вычислительной практике, показано, что это не так. Во-вторых, рассматривается число обусловленности задачи идентификации параметров одного изветного уравнения зависимости коэффициента температуропроводности от влажности. На конкретных примерах показано, что данная задача при ее решении обычным методом наименьших квадратов часто является плохо обусловленной. Однако иногда ее обусловленность удается существенно улучшить при наложении простейших ограничений на искомые параметры (метод наименьших квадратов с ограничениями). Текст статьи на русском языке см. на вкладке Дополнительные файлы

Определение температуропроводности материалов методом нестационарного точечного нагрева

Express thermographic method for measuring thermal diffusivity of thin flat objects is described in the paper. It could be considered as modification or a branch of laser flash method, however it does not require sample cutting or two side access to the object. The method comprises step-like heating of the small area (~ 0.1 mm2) at the object surface by focused laser beam and continuous recording temperature distribution at the surface by thermal camera. Developed data processing technique does not require information about absolute temperatures or absorbed energy for determination of thermal diffusivity coefficient.

New methods of thermographic control using multi-scale analysis of non-stationary thermal fields

A set of new approaches and techniques of non-destructive testing is described and implemented within a unified computer analysis of the patterns of multi-scale dynamic thermography. Depending on the size of the inspected area, nature, location, orientation and size of the defects, various energy sources were used for probe dynamic heating of the controlled article: air flow, focused laser beam, and point contact. The non-stationary thermal picture of the monitored area was recorded with a high resolution thermal imaging device and then analyzed using original model approaches and developed software. A set of discussed approaches allows detecting and quantitative characterizing of the defects of various types, size (from fractions to tens of millimeters) and orientation, including cracks, coating delamination or degradation, welding and glue seams defects, deposits, etc., both at the outer and inner surfaces of tubes, tanks, and reactors, etc. The developed methods provides determination of the thermophysical characteristics of the material, i.e., the thermal diffusivity coefficient with an accuracy better than ±3%.

Rigorous entropy formulation of the anelastic liquid equations in an ideal gas

The point of this short paper is to provide a useful set of equations governing stratified convection, expressed solely in terms of two thermodynamic variables, i.e. the pressure and the entropy, and the velocity field of the flow, free from any additional assumptions about the properties of turbulence. The pressure fluctuation is entirely eliminated from the energy equation and it appears only in the momentum balance, easily removable by taking its curl. This goal is achieved through the well-known anelastic approximation and an assumption of constant thermal diffusivity coefficient. The rigorously derived system of anelastic liquid equations constitutes a useful tool for modelling the dynamics of stellar interiors.

Thermochemical erosion and thermophysical properties of phenolic resin/carbon fiber/graphite nanocomposites

The main objective of this work is an experimental investigation and an analytical modeling of ablation and to analyze the thermophysical properties of nanocomposites based on novolac resin/short carbon fiber/graphite nanocrystalline powders in oxyacetylene flame test. The composite consisting of 40 wt.% carbon fiber was prepared as reference sample of which matrix was modified with three different percentages (6, 9 and 12 wt.%) of nano-sized graphite powders as reinforcement. Ablation is calculated by mass balance equation. Some parameters in the ablation modeling are evaluated by simultaneous thermal gravimetric analysis technique. Results of this work show that ablation rates decrease by the addition of graphite powders. The theoretical ablation rates are 33–38% less than the experimental data analyzed by oxyacetylene flame tests. This difference is reasonable because the effect of fluid stream force of oxyacetylene flame that causes the thermomechanical erosion of the surface is omitted in theoretical calculations. Therefore the model only calculates thermochemical erosion. Also, the thermophysical properties change due to heating is analyzed. Moreover, in nanocomposite with 9 wt.% graphite nanopowders, the rate of ablation and thermal diffusivity coefficient decreased by 10% and 50%, respectively, and thermal stability increased by 12% compared to the reference sample.