A Visual Study of Two-Phase Flow Patterns of HFC-134a and Lubricant Oil Mixtures

The two-phase flow patterns of HFC-134a with lubricant oil mixtures inside a smooth horizontal tube were experimentally elucidated. Tests were performed in an inside diameter of 7.8 mm having a lubricating oil concentration of 5%. Tests were made of mass fluxes ranging between 150 and 590 kg/m 2 s. The most obvious difference from oil-free cases reported is the presence of froth flow pattern. Apparently, this flow pattern is related to the increase of surface tension and viscosity. With the presence of lubricant oil, the onset of transition from stratified flow region to annular flow regime shifted to a lower value of superficial gas velocity. In addition, the tearing phenomenon of the refrigerant-oil mixtures may be related to its relevant properties such as wettability and surface tension.     

A BEM-based domain meshless method for the analysis of Mindlin plates with general boundary conditions

In this paper, a BEM-based domain meshless method is developed for the analysis of moderately thick plates modeled by Mindlin’s theory which permits the satisfaction of three physical conditions along the plate boundary. The presented method is achieved using the concept of the analog equation of Katsikadelis. According to this concept, the original governing differential equations are replaced by three uncoupled Poisson’s equations with fictitious sources under the same boundary conditions. The fictitious sources are established using a technique based on BEM and approximated by radial basis functions series. The solution of the actual problem is obtained from the known integral representation of the potential problem. Thus, the kernels of the boundary integral equations are conveniently established and evaluated. The presented method has the advantages of the BEM in the sense that the discretization and integration are performed only on the boundary, and consequently Mindlin plates with general boundary conditions can be analyzed without difficulty. To illustrate the effectiveness, applicability as well as accuracy of the method, numerical results of various example problems are presented.     

Onset of flooding in a small diameter tube

The main concern of this work is to obtain and investigate the experimental data of the onset of flooding (OSF) for air and water in a small diameter tube. Flooding experiments are carried out in a tube having an inner diameter of 8.75 mm and the length of 1 m at inclination angles of 0, 30, 45, 60° from the vertical. The curves of the OSF are built and shown as a function of the dimensionless superficial velocity. The effects of the inclination angles of the small diameter tubes on the OSF are discussed. The present results are compared with existing limited data reported in literature.     

Characterization of copper sulfide nanostructured spheres and nanotubes synthesized by microwave-assisted solvothermal method

Copper sulfide nanostructured spheres and nanotubes were successfully synthesized, using a microwave-assisted solvothermal method, by the decomposition of [Cu(CH₃CSNH₂)₂]Cl₂ complexes, formed by the reaction of CuCl₂·2H₂O and CH₃CSNH₂ in ethylene glycol at different pH values, and identified by CHNS/O and FTIR analyses. The decrease in bonding energy of N-H revealed the coordination of copper ions and thioacetamide molecules. It was specified that nitrogen atoms of thioacetamide molecules were used to form Cu-thioacetamide complexes. XRD, SEM, TEM and SAED analyses show that the products were hexagonal CuS spheres in an extremely low pH solution, and hexagonal CuS nanotubes at a pH 13. Their Raman spectra show sharp peaks at 473 cm^− 1, identified as the S-S stretching mode of S₂ ions at the 4e sites.     

The Effect of the Electrohydrodynamic on the Two-Phase Flow Pressure Drop of R-134a during Evaporation inside Horizontal Smooth and Micro-Fin Tubes

This article concerns the pressure drop caused by using the electrohydrodynamic (EHD) technique during evaporation of pure R-134a inside smooth and micro-fin tubes. The test section is a counter-flow concentric tube-in-tube heat exchanger where R-134a flows inside the inner tube and hot water flows in the annulus. A smooth tube and micro-fin tube having an inner diameter of 8.12 mm and 8.92 mm, respectively, are used as an inner tube. The length of the inner tube is 2.50 m. The outer tube is a smooth copper tube having an inner diameter of 21.2 mm. The electrode, which is a cylindrical stainless steel wire having diameter of 1.47 mm, is placed in the center of the inner tube. The electrical field is established by connecting a DC high voltage power supply of 2.5 kV to the electrode while the inner tube is grounded. Experiments are conducted at saturation temperatures of 10–20°C, mass fluxes of 200–600 kg/m²s, and heat fluxes of 10–20 kW/m². The experimental results indicate that the application of EHD introduces a small pressure drop penalty. New correlations for the pressure drop are proposed for practical applications.     

Template-free synthesis of neodymium hydroxide nanorods by microwave-assisted hydrothermal process,and of neodymium oxide nanorods by thermal decomposition

Based on the 150 °C and 1 h microwave-assisted hydrothermal reaction of Nd(NO₃)₃ dissolved in deionized water with pH 10 adjusted by concentrated NH₄OH solution, the phase and morphology of the product, characterized by XRD, SEM and TEM analyses, were specified as hexagonal Nd(OH)₃ nanorods 50 nm in diameter and 700 nm long, growing along the [0 0 1] direction. TGA analysis showed the evaporation of adsorbed water and dehydration of Nd(OH)₃ to synthesize the final pure Nd₂O₃ product. With 500 °C and 2 h calcination of the Nd(OH)₃ self-template precursor, Nd₂O₃ nanorods were synthesized, retaining both the morphology and growth direction of the precursor.     

Fabrication of small size inner spiral ribbed copper tube by fluid mandrel drawing

This paper is concerned with the development of ultra-small inner spiral ribbed copper tubes with high-quality heat transfer. Since the demand for the ultra-small tubes in electrical appliances is currently high and will be greater in the future, the technology employed must enable the production of inner spiral ribbed fine tubes with various features, such as small size, high quality, high functionality, and low processing cost so as to meet the increasing demand. The conventional production method is suitable for large tubes with high drawability but is unsuitable for fabrication of long ultra-small tubes because of the difficulty to manufacture both an ultra-small spiral ribbed mandrel and a floating plug. This research paper has proposed four drawing methods as follows: tube sinking, water, oil, and wax as mandrels and presented the comparison of seven parameters, i.e., drawing stress ratio, wall thickness ratio, ribbed base width ratio, ribbed tip width ratio, ribbed height ratio, ribbed pitch ratio, and ribbed spiral angle ratio. It was found that tube sinking was unfit for making the ultra-small inner spiral ribbed copper tubes due to the resulting high ratio of wall thickness. The results of all the parameters were similar in the cases of oil and wax. Despite less impressive outcomes, water was easily removed from the inner spiral ribbed copper tube compared to oil and wax. Thus, the tube drawing using water as mandrel was most suitable for the production of the inner spiral ribbed copper tube.     

Effect of number of tube rows on the air-side performance of crimped spiral fin-and-tube heat exchanger with a multipass parallel and counter cross-flow configuration

The air-side performance of crimped spiral fin and tube heat exchangers at high Reynolds number (3000–13,000) is investigated in this study. The test heat exchangers have a new type of multipass parallel and counter cross-flow water flow arrangement which is a combination of parallel cross-flow and counter cross-flow. The test samples are made from copper and aluminium with different number of tube rows (N_row = 2, 3, 4 and 5). The effects of number of tube rows and fin material on the heat transfer and friction characteristics are studied. The results show that no significant effect for either number of tube rows or fin materials on the heat transfer performance is found at high Reynolds number. In addition, the correlation of the air-side performances of this type of the heat exchangers at high Reynolds number is developed for industrial applications.     

A correlation development for predicting the pressure drop of various refrigerants during condensation and evaporation in horizontal smooth and micro-fin tubes

This paper predicts the condensation and evaporation pressure drops of R32, R125, R410A, R134a, R22, R502, R507a, R32/R134a (25/75 by wt%), R407C and R12 flowing inside various horizontal smooth and micro-fin tubes by means of the numerical techniques of artificial neural networks (ANNs) and non-linear least squares (NLS). In its analyses, this paper used experimental data from the National Institute of Standards and Technology (NIST) and Eckels and Pate, as presented in Choi et al.'s study provided by NIST. In their experimental setups, the horizontal test sections have 1.587, 3.78, 3.81 and 3.97 m long countercurrent flow double tube heat exchangers with refrigerant flowing in the inner smooth (8, 8.01 and 11.1 mm i.d.) and micro-fin (4.339, 5.45, 7.43 and 8.443 mm i.d.) copper tubes and cooling water flowing in the annulus. Their test runs cover a wide range saturation temperatures, vapor qualities and mass fluxes. The pressure drops are calculated with 1485 measured data points, together with analyses of artificial neural networks and non-linear least squares numerically. Inputs of the ANNs of the best correlation are the measured values of the test sections, such as mass flux, tube length, inlet and outlet vapor qualities, critical pressure, latent heat of condensation, mass fraction of liquid and vapor phases, dynamic viscosities of liquid and vapor phases, hydraulic diameter, two-phase density and the outputs of the ANNs, which comprise the experimental total pressure drops of the evaporation and condensation data from independent laboratories. The total pressure drops of in-tube condensation and in-tube evaporation tests are modeled using the artificial neural network (ANN) method of multi-layer perceptron (MLP) with 12-40-1 architecture. Its average error rate is 7.085%, which came from the cross validation tests of 1485 evaporation and condensation data points. Dependency of the output of the ANNs from 12 numbers of input values is also shown in detail, and new ANN based empirical pressure drop correlations are developed separately for the conditions of condensation and evaporation in smooth and micro-fin tubes as a result of the analyses. In addition, a single empirical correlation for the determination of both evaporation and condensation pressure drops in smooth and micro-fin tubes is proposed with an error rate of 14.556%.     

Numerical investigation of an axi-symmetric laminar jet impinging on a dimpled surface under uniform heat flux using a finite element method

The purpose of this study is to numerically investigate heat transfer and flow field in a semi-confined axi-symmetric laminar air jet impinging on a concave surface, or dimple, with uniform heat flux. A commercial software package relying on the finite element method was used for the simulation, and mesh convergence was examined in order to minimize computational cost. Jet impingement on a flat plate was used as a baseline reference case, and flat plate results were validated against previously published experimental data with good agreement. The effects of various parameters involved in dimple impingement -such as Reynolds number (Re) between 100–1,400; jet-to-plate spacing (H/D_j) ranging from 2 to 6 jet diameters; dimple depths (d/D_d) of 0.1, 0.15, and 0.2; and the ratio of jet diameter and dimple projected diameter (D_j/D_d) from 0.25 to 1—were all studied. Comparisons show that heat transfer reduction occurs in the presence of dimples because of the larger impingement area, which results in less momentum flux. The dimple curvature lifts the post-impinging fluid and creates a backflow, instead of allowing it to maintain contact with the surface, as is the case with flat plate impingement.