Ignacio Carvajal-Mariscal

Experimental study on the local convective coefficient distribution on a pipe surface with inclined fins

Experimental results for the convective coefficient distribution in both the inside and conical end zones of the extended surface in a finned pipe are presented for three different flow velocities. The devices were located in a physical model of a staggered square pitch arrangement in which the pipes were disposed very close together and exposed to a transverse air flow. The fin inclination angle with respect to the axis was γ° = 20°, the fin height H = 15 mm, the pipe diameter D = 28 mm and the distance between fins s = 8 mm. The results show that the convective coefficient distributions in the conical region and in the inside region of the fin are quite different. For three velocities of flow (u = 10-40 m/s) higher values of the convective coefficient were obtained in the conical region of the fin, near the junction of the fin with the pipe. On the other hand, the higher values in the internal face of the fin were located in wider regions of the lateral extremes of the fin. Only for Re 20 000 the zones with higher values of convective coefficient were located near the junction of the fin with the pipe. The heat transfer coefficient distribution on the pipe surface is very similar to the one obtained for a flat cylinder. From the experimental results, the formula to calculate the heat transfer of a tube with fins inclined in an angle of γ = 20° was developed. This formula considers the separation between the tubes, the parameters of the fins and the arrangement of the bank of tubes.

Numerical Analysis of Turbulent Flow in a Small Helically Segmented Finned Tube Bank

Abstract A full finned tube bank is represented as a small finned tube bank in order to analyze numerically mean properties behavior in the streamwise direction. The main goal is to obtain criteria for implementing periodic boundary conditions in a single isolated finned tube module. The simulation is carried out with the Reynolds Averaged Navier–Stokes method and the turbulence effect is modeled with the Renormalization Group k-ϵ model. The complex geometry of finned tube is represented by means of a cut-cell method. Numerical results are compared with experimental data, experimental visualizations, and semi-empirical correlations. Predictions show an adequate hydrodynamics and heat transfer representation. Additionally, mean properties in the streamwise direction show quasi-sinusoidal behavior, and the heat transfer presents approximately identical values in every finned tube in the fully developed flow zone. Therefore, periodic boundary conditions for turbulent kinetic energy and its dissipation rate and a constant wall heat flux condition in the fully developed flow are proposed in numerical simulations on a single isolated finned tube module.

Development of High Efficiency Two-Phase Thermosyphons for Heat Recovery

Due to high fuel prices, it has become necessary to investigate new methods for saving and more efficient use of energy, emphasizing the use of energy remaining in the waste gases of combustion equipment. For this reason, in the last five decades there has been an important technological development in heat transfer equipment, to promote changes in configuration and applying heat transfer systems with high effectiveness. One example is the use of twophase thermosyphons (Azada et al., 1985; Faghri, 1995; Gershuni et al., 2004; Noie, 2005; Peterson, 1994; Reay, 1981).

Effect of Maximum Temperature and Heating-Cooling Repeated Cycles on Thermal Contact Resistance of a Composite Tube

Experimental research results of the operational parameter effect on Thermal Contact Resistance (TCR) in a copper-aluminum L-type finned tube are presented. The investigated operational parameters were the maximum operational temperature and the number of repeated heating-cooling cycles. The TCR was experimentally determined by measuring the total heat supply, core tube wall and inner fin surface temperatures for steady-state and natural-convection conditions. In addition, the specimen was tested through up to 200 heating-cooling cycles. The experimental results showed a TCR increase of 81% at the same time as the average temperature difference between the hot inner flow and cooling air increased from 30°C to 130°C; over the maximum operational temperature (120°C), the TCR increased faster than before; and, after the heating-cooling cycle testing the TCR presented an increase of 31% in respect with the initial value. Such findings may be useful as a reference for preliminary thermal design and as recommendations for optimal operation of heat exchangers based on copper-aluminum L-type finned tubes.

New Correlation for Two-Phase Flow in 90 Degree Horizontal Elbows

The comparison of experimental data and results obtained from four global models — homogeneous, Dukler, Martinelli and Chisholm, used to evaluate the two-phase flow pressure drop in circular 90° horizontal elbows — is presented in this paper. An experimental investigation was carried out using three galvanized steel 90° horizontal elbows (E1, E2, E3) with internal diameters of 26.5, 41.2 and 52.5 mm, and curvature radii of 194.0, 264.0 and 326.6 mm, respectively. According to the experimental results, the model proposed by Chisholm best fitted them, presenting for each elbow an average error of E1 = 18.27%, E2 = 28.40% and E3 = 42.10%. Based on experimental results two correlations were developed. The first one is the classical Chisholm model modified to obtain better results in a wider range of conditions; it was adjusted by a dimensionless relationship which is a function of the homogeneous volumetric fraction and the Dean number. As a result, the predictions using modified Chisholm model were improved presenting an average error of 8.66%. The second developed correlation is based on the entire two-phase mass flow taken as liquid and adjusted by the homogeneous volumetric fraction ratio. The results show that this last correlation is easier and accurate than the adjusted Chisholm model, presenting an average error of 7.75%. Therefore, this correlation is recommended for two-phase pressure drop evaluation in horizontal elbows.Copyright

Three-Dimensional Flow Behavior Inside the Submerged Entry Nozzle

According to various authors, the surface quality of steel depends on the dynamic conditions that occur within the continuous casting mold’s upper region. The meniscus, found in that upper region, is where the solidification process begins. The liquid steel is distributed into the mold through a submerged entry nozzle (SEN). In this paper, the dynamic behavior inside the SEN is analyzed by means of physical experiments and numerical simulations. The particle imaging velocimetry technique was used to obtain the vector field in different planes and three-dimensional flow patterns inside the SEN volume. Moreover, large eddy simulation was performed, and the turbulence model results were used to understand the nonlinear flow pattern inside the SEN. Using scaled physical and numerical models, quasi-periodic behavior was observed due to the interaction of two three-dimensional vortices that move inside the SEN lower region located between the exit ports of the nozzle.

Hydrodynamic Study of a Submerged Entry Nozzle with Flow Modifiers

The fluid flow modifier technology for continuous casting process was evaluated through numerical simulations and physical experiments in this work. In the casting of steel into the mold, the process presents liquid surface instabilities which extend along the primary cooling stage. By the use of trapezoid elements installed on the external walls of the submerged nozzle, it was observed that it is possible to obtain symmetry conditions at the top of the mold and prevent high level fluctuations. The flow modifiers have equidistant holes in the submerged surface to reduce the velocity of the liquid surface by energy and mass transfer between the generated quadrants. A flow modifier drilled with a 25 pct of the submerged surface provides stability in the mold and structural stability of the proposal is guaranteed.

Influence of Geometry and Velocity of Rotating Solids on Hydrodynamics of a Confined Volume

Three cylinder-based geometries were evaluated at five different rotating speeds ( = 20.94, 62.83, 94.25, 125.66, and 157.08 rad·s−1) to obtain the fluid flow pattern in nonsteady conditions. Two of the models were modified at the lower region, also known as tip section, by means of inverted and right truncated cone geometries, respectively. The experimental technique used a visualization cell and a Particle Imaging Velocimetry installation to obtain the vector field at the central plane of the volume. The Line Integral Convolution Method was used to obtain the fluid motion at the plane. In addition, the scalar kinetic energy and the time series were calculated to perform the normal probability plot. This procedure was used to determine the nonlinear fluid flow pattern. It was also used to identify two different flow regimens in physical and numerical results. As the rotation speed increased, the turbulent regions were placed together and moved. The process makes experimental observation difficult. The biphasic and turbulence constitutive equations were solved with the Computational Fluid Dynamics technique. Numerical results were compared with physical experiments for validation. The model with the inverted truncated cone tip presented better stability in the fluid flow pattern along the rotation speed range.

Calculation of Temperature Mode of Finned Tubes

To evaluate the reliability and to perform the strength design for transversely finned tubes, operating under the conditions of high heat loads and high temperatures of the heat-exchanging media, the calculation of their temperature mode is done by determining the following information: the temperatures of the fin base and tip, the mean integral temperature of the fin, and the temperature of the inside surface of the tube.

Heat-Transfer Calculations

The quantity of heat taken up by the heating surface in unit time depends on the overall heat-transfer coefficient from the area of the heating surfaces and the average temperature difference. The external convective heat transfer from gas to the bundles of finned tubes in cross flow in a wide range of parameters is presented in the form of generalized equations and nomograms. The convective heat-transfer coefficient with a transverse flow around staggered and in-line bundles of tubes with circular, helical, and square finning, referring to the total surface on the gas side, is calculated from the experimental equation, where coefficient C q and exponent n of Re number are a function of the shape parameter of the bundle X . The shape parameter of the bundle X depends on tube arrangement, on the spacing characteristics of bundles S 1 and S 2 , and fin coefficient Ψ r .