Journal of Thermal Analysis and Calorimetry

The current article deals with a moving boundary problem describing solidification of a eutectic alloy in a semi-infinite medium. The process of solidification of a eutectic alloy is considered under imposed movement of material in the mushy zone in place of liquidus zone due to imposing an insulated boundary condition at liquidus front. It is assumed that solid fraction $$f_{\text {s}}$$ has a linear, quadratic and cubic relationship with distance within the mushy zone between the solidus and liquidus. An exact solution of the problem is obtained with the help of similarity transformation. A numerical example of the solidification of Al–Cu alloy is presented to demonstrate the application of the current analysis. Solidification of eutectic system is discussed in the absence of material movement and in the presence of material movement in each case of solid fraction distribution. Thus, the temperature profile and moving interfaces in each region are shown for different Peclet numbers Pe. In addition, heat extraction Q from the surface $$x=0$$ is shown with respect to the time for different Pe. The novelty of the current study is transition process becomes fast when material moves in the direction of freeze and hence time for complete freezing of the alloy reduces. Moreover, mushy zone becomes thinner when material moves in the direction of freeze. A comparative study and error analysis between the present work and Tien and Geiger (ASME J Heat Transf 89:230–233, 1967)[9] in linear case of solid fraction are presented in figure and tables. The application of the present analysis is useful for both eutectic and solid solution alloys.

As an important functional material, lanthanum fluoride (denoted as LaF3) shows promising application in infrared thermometry, infrared thermography and fluoride glass optical fiber, etc. Although great process has been obtained in preparing rare earth fluorides such as GdF3, YF3, and NdF3, the thermodynamic process and synthesis of LaF3 remain a challenge. Herein, the thermodynamic process is explored by theoretical calculation and cxs $$\Delta_{{\text{r}}} H$$ <0, $$\Delta_{{\text{r}}} G$$ <0, $$K_{{\text{p}}}$$ >109, indicating that the whole fluorination reaction is an exothermic reaction and can be fully carried out. Moreover, we now report an optimum fluorination process of LaF3. By exploring the effects of reaction temperature, holding time, material layer thickness and HF gas flow on fluorination rate k, the reliability of theoretical calculation has been verified, the best fluorination parameters have been obtained and the fluorination rate can reach more than 97%. This work may shed some light on the large-scale industrial production of high-grade LaF3, and promote its applications in infrared optical field, nuclear medicine and high-energy physics.

In this numerical study, we use two-dimensional scale-adaptive simulations to investigate the lift characteristics of two tandem airfoils in the globally unstable wake of a cylinder at a Reynolds number of 104, with and without cylinder heating. Both airfoils have NACA 0012 profiles and are at the same angle of attack (α = 10°), implying zero decalage. We find that the lift characteristics of both airfoils depend strongly on the streamwise distance (X) between the trailing point of the cylinder and the leading edge of the fore airfoil. At the conditions of this study (1 ≤ X/D ≤ 5, where D is the cylinder diameter), both airfoils experience periodic limit-cycle oscillations in their lift coefficients (Cl) at the same frequency. However, when X/D < 3, abrupt decreases occur in both the Cl amplitude of the fore airfoil and the Cl oscillation frequency of both airfoils. We attribute these decreases to the fore airfoil entering the wavemaker region of the self-excited cylinder wake and altering its global instability properties. Heating the cylinder to a temperature above that of the free-stream leads to monotonic decreases in the oscillation frequency and amplitude of Cl for both airfoils. For the purpose of harvesting the maximum energy from two tandem airfoils behind a cylinder, our findings suggest that (i) the airfoils should be positioned as close as possible to the cylinder without disrupting the wavemaker of the globally unstable wake and (ii) the cylinder should be prevented from heating up.

The nanofluids under magnetic fields show great potential in microchannel cooling and thermal absorption of the miniaturized devices. Studies about the lattice Boltzmann method (LBM) have been focusing on the effects of nanoparticle types, volume fractions, and magnetic field intensity at low Reynolds numbers. However, less effort has been made to elucidate the interactions between external forces at large Reynolds numbers. In this work, we firstly developed a preconditioned LBM (PLBM) to overcome the divergence problem of the original LBM caused by variable physical properties and Reynolds numbers, followed by investigating the thermo-hydraulic characteristics of Al2O3-water nanofluid in a microchannel with temperature-dependent physical properties and slip boundary conditions. The effects of the external magnetic field, buoyancy force, and volume fraction of nanoparticles are discussed, and the entropy generation is analyzed. When the magnetic field is applied, the average shear stress is twice that without a magnetic field and the average Nusselt number increases by about 10% and further by about 20% at higher buoyancy forces. Besides, the magnitudes of the entropy generation caused by magnetic field irreversibility are higher than that caused by the heat transfer irreversibility. The numerical study developed in this work elucidates the effects of external force and extends the simulation performance of the existing LB models.

This study aims to investigate the thermal conductivity, viscosity and thermal degradation of naphthenic-based mineral oil, palm oil methyl ester (POME) and its blend. Mineral oil and POME were mixed at various mass ratios (70:30, 80:20 and 90:10) via homogenisation for 10 min at 25,000 RPM. The thermo-physical properties of oil studied were thermal conductivity, viscosity, density, flash point and thermal degradation. Results indicated enhancement in thermal conductivity with an increase in temperature. The highest enhancement (0.185 W mK−1) was observed for the 70:30 blend ratio at 80 °C. The viscosity decreased as temperature increased, whereby POME resulted in lower values (2.3 mPa.s) than mineral oil (4.3 mPa.s) at 80 °C. Density also showed a decreasing trend with temperature with no significant difference between POME and mineral oil. However, POME (175.5 °C) resulted in a higher flash point compared to mineral oil (146.5 °C), resulting in the enhanced flashpoint of blend oils. Blend oil with a 70:30 ratio resulted in the highest thermal degradation of 274.56 °C in the air environment. Oxidation induction time (OIT) decreased with temperature from 160–180 °C where mineral oil resulted in higher induction time than POME. The results proved improvement in properties of mineral oil blend with POME, which can enhance the system performance and faster heat dissipation.

An energetic composite of an octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine with a polymer matrix is generally developed to reduce sensitivity, improve mechanical strength and furnish engineering shape. A hydroxyl-terminated polybutadiene (HTPB) and various curatives, namely methylene diphenyl diisocyanate (MDI), isophorone diisocyanate (IPDI), toluene diisocyanate (TDI) and trimethyl-hexamethylene diisocyanate (TMDI), were mixed in 1:1 [NCO]/[OH] equivalent ratio to form polyurethane, and these samples were designated by HTPB/MDI, HTPB/IPDI, HTPB/TDI and HTPB/TMDI, respectively. The four kinds of energetic composites were prepared by mixing of an energetic HMX with various HTPB/MDI, HTPB/IPDI, HTPB/TDI and HTPB/TMDI matrices. These composites were studied for the thermal stability by employing thermogravitry analysis (TGA) and found to be almost similar. The kinetic parameters of all kinds of composites were investigated through the isoconversional Flynn–Wall–Ozawa (FWO) and Kissinger–Akahira–Sunose (KAS) methods for the first and second stages of the thermal decomposition using TGA data. The results of the activation energy obtained from the FWO method were analogous to those values obtained from the isoconversional KAS method. In addition, the kinetic parameters were also investigated through isoconversional KAS method using DSC data. The overall values of the activation energy of the HMX/HTPB/MDI, HMX/HTPB/IPDI, HMX/HTPB/TDI and HMX/HTPB/TMDI samples were varied in a range of 168–263, 277–409, 210–421 and 206–324 kJ mol−1 at a constant value of conversion (α = 0.1–0.9), respectively. The results indicated that there was significant variation in the activation energy with varying curative groups in the HTPB-based PU matrices. The compensation effect method was used to calculate the pre-exponential factor. The energetic composites were also investigated for the thermodynamic parameters for formation of the activated complexes and are discussed.