Resumo em Inglês:

This work presents the results of an experimental study carried out with R-134a to characterize heat transfer and pressure drop during flow boiling in a horizontal smooth tube of 2.6 mm ID. The experimental tests included (i) heat fluxes in the range from 10 to 100 kW/m², (ii) the refrigerant mass velocities set to discrete values in the range of 240-930 kg/m²s and (iii) saturation temperatures of 12 and 22°C. The study analyzed the heat transfer through the local heat transfer coefficient along the flow and pressure drop, under the variation of these different parameters. It was possible to observe the significant influence of heat flux on the heat transfer coefficient and mass velocity on the pressure drop, and the effects of saturation temperature. Moreover, it was possible to produce a complete database.

Resumo em Inglês:

The study of natural circulation phenomenon has been object of crescent interest in scientific community in recent years. The new generation of compact nuclear reactors uses natural circulation of the fluid as a residual heat removal cooling system in case of accident or shutdown. The objective of this paper is to present a comparison between experimental data and numerical simulation results for the natural circulation phenomenon in single and two-phase flow regime. Experimental data were obtained from a circuit built with glass tubes, through which thermal hydraulic phenomenon visualization was possible. The circuit is composed of an electric heater as the heat source, a heat exchanger as the heat sink and an expansion tank to accommodate fluid density changes. Instrumentation data acquisition is performed through thermocouples and pressure meters, and controlled by a computer interface developed using LABVIEW. Numerical modeling and simulations were done with the thermal hydraulic code RELAP5, which is widely used for this purpose. The cyclic reverse flow observed in the circuit was well represented by the numerical model. The comparison demonstrated that the numerical simulations are very close to the experimental data.

Resumo em Inglês:

The liquid film affects significantly the Venturi scrubber efficiency. Film fraction was measured in a vertically mounted laboratory scale Venturi scrubber with a 0.020 m throat diameter. The liquid was injected at the throat by one to six 0.001 m holes. The throat gas velocity varied from 50 to 90 m/s and the relative jet penetration varied between 0.05 and 0.85. A special test section with a slot was used to extract the film. The test section could be placed at different axial positions, allowing measurements at 0.0075, 0.0275 and 0.0675 m after the injection point. Results showed that film fraction varies as a function of the relative jet penetration and with distance from the injection point. There exists an optimal relative jet penetration (between 0.25 and 0.40) which minimizes film fraction. Film fraction increased with axial distance in the range studied, up to 3.375 throat diameters after the injection point.

Resumo em Inglês:

The monitoring and visualization of two-phase flow is of great importance either from technical/practical point of view for process control and supervision or from scientific/theoretical point of view, for the understanding of physical phenomenon. A wire-mesh sensor was applied to experimentally investigate two-phase horizontal pipe flow. Furthermore, some physical flow parameters were extracted based on the raw measured data obtained by the sensor. In this article, first, the work principle of wire-mesh sensors is revised and, second, the methodology of flow parameter extraction is described. A horizontal flow test section comprising of a pipe of 26 mm i.d. 9 m long was employed to generate slug flows under controlled conditions. An 8 × 8 wire-mesh sensor installed at the end of the test section delivers cross-sectional images of void fraction. Based on the raw data, mean void fraction, time series of void fraction and characteristic slug frequency are extracted and analyzed for several experiments with different liquid and gas superficial velocities.

Resumo em Inglês:

Twisted-tape inserts are frequently used in heat exchangers as a passive and inexpensive heat transfer enhancement method. However, their use results not only heat transfer coefficient increments, but also pressure drop penalties. The present study analyses the literature on single and two-phase frictional pressure drop inside tubes with twisted-tape inserts focusing on the physical mechanism and the effects of the use of twisted-tape in comparison to plain tubes. Experimental data were gathered from the open literature and compared against the available correlations developed in order to predict two-phase frictional pressure drop in tubes containing twisted-tape inserts. It was found that none of the correlations was able to predict such a database accurately. A new correlation to estimate the friction factor for two-phase flows inside tubes with twisted-tape is also proposed. Contrarily to previous studies, the proposed correlation presents reasonable predictions under single and two-phase flow conditions and obeys the trends when the twisted-tape ratio tends to zero and infinite.

Resumo em Inglês:

The present paper reports the results of an ongoing project aimed at providing statistical information on slugs in two-phase flow in a horizontal pipe. To this end, the flow was examined experimentally and numerically. On the experimental side, three non-intrusive optical techniques were combined and employed to determine the velocity field and bubble shape: particle image velocimetry (PIV), Pulsed Shadow Technique (PST) and Laser-Induced Fluorescence technique (LIF). Statistical information was provided by photogate cells installed at two axial positions. The flow was numerically determined based on the one-dimensional Two-Fluid Model. The tests were conducted on a specially built transparent pipe test section, using air and water as the working fluids. The velocity fields were obtained for flow regimes where the slugs were slightly aerated to facilitate the utilization of the optical methods employed. The main parameters for characterizing the statistically steady flow regime such as slug length and velocity obtained numerically were compared with the experimental data and good agreement was obtained.

Resumo em Inglês:

Stimulated by rapid progress in down-hole measuring techniques production engineers wonder whether in the near-future monitoring of oil/water production rates for horizontal wells can become possible on the basis of measured oil/water pressure losses and water hold-ups. A complicating issue is that these measured data depend on the oil and water flow patterns. The question then is if we use a flow-pattern-dependent model for pressure drop and water hold-up in an inverse mode, what then will be the accuracy by which production rates can be determined? In this review the progress with the inverse modeling will be investigated.