The present study is devoted to the experimental investigation of the boiling process of promising dielectric refrigerants HFE 7100 and Novec 649 on silicon substrate at atmospheric pressure. The purpose of the research is to determine the key heat transfer parameters, specifically nucleation temperature, heat transfer coefficients, critical heat fluxes, as well as the growth dynamics of irreversible dry spots, using the high-speed IR thermography method. It has been found that the boiling of Novec 649 is characterized not only by higher heat transfer coefficients and a lower nucleation temperature head, but also by lower critical heat fluxes (151 kW/m2) compared to HFE 7100 (183 kW/m2). The propagation velocity of irreversible dry spots during the boiling crisis development on a silicon substrate has been measured. For Novec 649, the velocity (6.8 mm/s) turned out to be lower than for HFE 7100 (10.4 mm/s). The obtained experimental data have been generalized and methods for calculating the basic thermal characteristics based on known theoretical models for silicon–dielectric fluid systems have been proposed. The research results are also of practical importance, providing engineers with specific data for designing highly efficient direct cooling systems for silicon wafers using dielectric fluids.
Thermal conductivity and enthalpy increment of the intermetallic compound LiK_3Pb_4 were measured for the first time using the laser flash method and drop calorimetry in the temperature range up to 1100 K. Based on the measurement results, the specific heat capacity and thermal diffusivity were calculated. It was found that the thermal conductivity of the liquid phase of LiK_3Pb_4 ranges from 2.5 to 4.3 W/(m K), which is an order of magnitude lower than the values calculated using the additive rule, indicating substantial localization of conduction electrons. This finding correlates well with the hypothesis of the formation and dominance of ionic complexes such as [Pb_4]^4- in liquid systems of lead with heavy alkali metals. Tables of recommended data for the investigated properties were created across the entire measurement range, together with an estimation of uncertainties in them.
Experiments were performed to study water pool boiling at atmospheric pressure on biphilic surfaces with grooves (open microchannels) produced by laser ablation and hydrophobized owing to chemisorption of fluorinated methoxysilane from vapor at a temperature of 100–110 ◦C. Surfaces with arrays of parallel grooves, as well as with arrays of grooves intersecting at right angles (groove grids with square cells), were used. The experimental data were analyzed and compared with literature and previous data. The influence of the size and location of hydrophobic regions on boiling heat transfer enhancement was studied. It has been shown that the main parameter determining the heat transfer enhancement for a surface with an array of parallel grooves is the pitch between them. For groove grids, this is also true, but only for large heat fluxes. Removal of the hydrophobizer from the grooves leads to a significant decrease in heat transfer. A comparison of the results of this study with previous data for arrays of hydrophobic cavities and arrays of hydrophobic round spots on a flat surface shows enhanced heat transfer on biphilic surfaces compared to a flat smooth surface. These surfaces are characterized by a strong dependence of heat transfer on the pitch between the hydrophobic regions. The maximum effect on heat transfer was observed on surfaces with a 3D biphilic pattern, namely on arrays of microcavities and arrays of intersecting grooves (groove grids with square cells).
Developing clean energy technologies to harness ocean waves remains a key engineering challenge. In this context, the influence of geometry on the performance of an oscillating water column (OWC) wave energy converter through the Constructal Design method is investigated. The objective is to maximize available hydropneumatic power when the system is subjected to full-scale regular waves. An inclined plate apparatus was inserted at the bottom of the OWC to evaluate its performance at inclinations of 0°, 20°, 40°, 60°, 80°, and 90° relative to the vertical direction. A validated and verified computational model based on the finite volume method was employed. The multiphase volume of fluid model was applied to handle air–water interaction. Results showed that the highest hydropneumatic power was achieved at 80°, which is 30.47
The paper presents a nonlinear elasticity theory (with plastic deformations) taking into account the second derivates of a metric tensor that describes failure of fluid-saturated porous rock. The theory shows that the invariance principles, conservation laws, and the first law of thermodynamics determine equations of the theory for a medium with a characteristic scale of failure. The simplest model describing the deformation of elastic layers under external stress demonstrates that the characteristic scale of failure is strongly affected by stresses applied to the elastic matrix and by the fluid content.
