Phase Change Heat Transfer Process Research Articles

Pressure drop studies on two-phase flow in a uniformly heated vertical tube at pressures up to the critical point

Measurements of two-phase flow pressure drop have been made during a phase-change heat transfer process with refrigerant (R-134a) as a working fluid for a wide range of pressures right up to the critical pressure. The experiments were conducted in a uniformly heated vertical tube of 12.7 mm internal diameter and 3 m length over a heat flux range of 35–80 kW/m 2, mass flux range of 1200–2000 kg/m 2 s, exit quality range of 0.19–0.81 and for reduced pressures ranging from 0.24 to 1 with a fixed inlet subcooling of 3 °C. The measurements were compared with the predictions from the homogeneous flow model, a separated flow model using correlations drawn from the literature for void fraction and frictional pressure drop, and finally, using a flow pattern-based predictive method accounting specifically for bubbly, slug and annular flow regimes. It was found that the best results were obtained with the flow pattern-based approach with a mean deviation of ±20% over the entire pressure range.

Effect of multiple phase change materials (PCMs) slab configurations on thermal energy storage

Abstract The present work involves the use of a two dimensional control volume based numerical method to conduct a study of a combined convection–diffusion phase change heat transfer process in varied configurations of composite PCM slabs. Simulations were conducted to investigate the impact of using different configurations of multiple PCM slabs arrangements with different melting temperatures, thermophysical properties and varied sets of boundary conditions on the total energy stored as compared to using a single PCM slab. The degree of enhancement of the energy storage has been shown in terms of the total energy stored rate. The numerical results from the parametric study indicated that the total energy charged rate can be significantly enhanced by using composite PCMs as compared to the single PCM. This enhancement in the energy storage can be of great importance to improve the thermal performance of latent thermal storage systems.

Computation and validation of weld pool dimensions and temperature profiles for gamma TiAl

Previous research by the authors has shown that the welding current has a strong effect on the weld properties and microstructures of gamma TiAl. This article presents the results of experimentally measured and theoretically predicted temperature profiles of gas tungsten arc (GTA) welded gamma TiAl for welding currents of 75 and 100 A. The GTA welding model used in this study accounts for the fluid flow in the weld pool as well as conductive, convective, and phase change heat transfer processes in the solid, liquid, and mushy regions of a metal. The computed temperature fields predicted that as the welding current is increased, the maximum temperature reached in the weld pool also increases. Experimental validation of the computed temperature fields was determined by placing thermocouples at three locations on the specimen, to record the temperatures during welding using computer-based data acquisition hardware and software. The agreement between theoretical predictions and measurements was reasonably good, which provided a direct validation of the model.

Stability investigation on the thin films in capillary

The stability of the thin liquid film in a capillary is important to the phase-change heat transfer process in miniature or micro structures. From the basic equations for motion and heat transfer at the interface of the film, its stability is theoretically studied. With evaluation of the effects and relative magnitudes of various driving forces and with the use of long-wave theory in addition to linear stability analysis, the controlling equations are simplified and an evolution equation for the film’s thickness is obtained. Detailed analysis on the evolution equation shows that instability occurs first in the meniscus region and the instability condition varies with boundary conditions, geometrical scales and thermal properties. The numerical results agree well with earlier ones with some favorable extensions and improvements.

Two-phase pressure drop of refrigerants during flow boiling in small channels: an experimental investigation and correlation development

Two-phase flow pressure drop measurements were made during a phase-change heat transfer process with three refrigerants (R-134a, R-12, and R-113) at six different pressures ranging from 138 to 856 kPa, and in two sizes of round tubes (2.46 and 2.92 mm inside diameters) and one rectangular channel (4.06 × 1.7 mm). The data were compared with those from large tubes under similar conditions, and state-of-the-art correlations were evaluated using the R-134a data. The state-of-the-art large-tube correlations failed to satisfactorily predict the experimental data. The data were used to develop a new correlation for two-phase pressure drop during flow boiling in small channels. The correlation was then tested against the experimental data for the three refrigerants; the error was ±20%.

Experimental and analytical phase change heat transfer

An experimental and analytical study was performed to evaluate and correlate the heat transfer results during melting and solidification of a phase change medium. The experimental work included determination of the thermal conductivities for the solid as well as the liquid phases of a phase change fluoride salt. A two-dimensional, transient, axisymmetric analytical model was developed to study the phase change heat transfer process in an energy storage system. The results of previously conducted experimental tests on the system were compared and correlated with the numerical results. Agreement was very good in most cases.

NUMERICAL INVESTIGATION OF VAPOR-LIQUID FLOW AND HEAT TRANSFER IN CAPILLARY POROUS MEDIA

The transient two-phase flaw and phase change heat transfer processes in porous media are investigated. Based on an enthalpic approach, a one-domain formulation of the problem is developed, avoiding explicit internal boundary tracking between single- and two-phase regions. An efficient numerical scheme is applied to obtain the solution on a fixed two-dimensional grid. The transient response of a liquid-saturated, self-heated porous bed is examined in detail. A physical interpretation of the computed response to fast power transients is attempted. Comparisons with experimental data are made regarding the average void fraction and the limiting dryout heat flux. The numerical approach is extended, keeping the one-domain formulation, to include the surrounding wall structure in the calculation.