This paper investigates slug flow associated with condensation along a module containing 10 of 29.9-cm long micro-channels having 1 x 1-mm(2) cross section. Using FC-72 as condensing fluid, which is cooled by water counterflow, experiments are performed to measure pressure drop as well as detailed temperature variations along the condensation module. Using high speed video, the flow is shown to consist of a series of unit cells, each comprised of a liquid slug and an elongated bubble. New interfacial instability theory is developed to describe the transition from annular flow to slug flow and obtain analytical expressions for bubble and slug lengths for the most upstream unit cell. Also presented is a new theoretical model for slug flow that is used to determine axial variations of bubble, slug and unit cell lengths. These lengths are used to evaluate axial variations of the local heat transfer coefficient, which are accurately predicted using the new model. Additionally, pressure drop data show good agreement with predictions of a recent universal correlation approach for condensation in small channels. (C) 2017 Elsevier Ltd. All rights reserved.

This explores downflow condensation in a circular tube both experimentally and computationally using FC-72 as a working fluid. A highly instrumented condensation module is used to map detailed axial variations of both wall heat flux and wall temperature, which are used to determine axial variations of the condensation heat transfer coefficient. The experimental results are compared to predictions of a two-dimensional axisymmetric computational model using FLUENT. The study provides detailed construction of the model, including choice of interfacial phase change sub-model, numerical methods, and convergence criteria. The model is shown to yield good prediction of the heat transfer coefficient. The computed temperature profiles exhibit unusual shape, with steep gradient near the annular liquid film interface as well as near the wall, and a mild gradient in between. This shape is shown to be closely related to the shape of the eddy diffusivity profile. These findings point to the need for future, more sophisticated measurements of liquid film thickness, and both velocity and temperature profiles, to both validate and refine two-phase computational models.

This study explores the complex fluid flow behavior adjacent to the interface between parallel layers of gas and liquid. Using water and nitrogen as working fluids, the interface is examined experimentally using high-speed video, and the flow structure predicted using FLUENT. The computational model is used to analyze the gas flow near the interface by isolating and examining a domain that represents an instantaneous snapshot of the wavy interface. Both the observed and computed interfaces show appreciable interfacial waviness, which increases in intensity with increasing flow rates; they also show gas entrainment effects at high flow rates. The computed results show turbulence is completely suppressed along the interface by surface tension. Computed velocity vector plots, contour plots and flow streamlines show interfacial flow separation on the gas side, and these effects are amplified with increasing gas Reynolds number. This produces form drag along the wavy interface in addition to the viscous drag. The interfacial viscous and form drag components increase monotonically with increasing ratio of wave height to wavelength because of the increased frictional resistance and flow separation effects, respectively. A new relation for the interfacial friction factor is derived from the computational results, which agrees well with prior turbulent flow correlations.

This study examines the evolution of film thickness and interfacial temperature in turbulent, free-falling water films that are subjected to sensible heating. Measured temporal records of film thickness and interfacial temperature are subjected to thorough statistical analysis to understand the influence of interfacial waves on the distribution, periodicity and interdependence of these two parameters. A computational model of the film is constructed and its predictions subjected to similar statistical analysis. The statistical tools employed in this study include probability density, auto-covariance, cross-covariance, auto-spectrum and cross-spectrum. Probability density of film thickness shows an increase in substrate thickness and amplitude with increasing Reynolds number, while auto-covariance of thickness captures dominant frequencies corresponding to the large waves. Cross-covariance of film thickness and interfacial temperature difference captures a clear phase shift between the two parameters, with the temperature reaching a maximum in the relatively thin film region between the substrate and wave peak. Statistical results for both parameters exhibit clear dependence on axial location in the thermal entrance region, and point to fully developed wave structure downstream. The statistical results based on computed film thickness and interfacial temperature difference agree well with the results based on the measured, which demonstrates the effectiveness of the adopted computational tools at predicting the complex transport phenomena associated with wavy liquid–vapor interfaces.

This study explores the influence of interfacial waves on mass, momentum and heat transfer in turbulent, free-falling water films that are subjected to sensible heating. Measured temporal records of film thickness and temperature profile across the film are used to examine the film's thermal response to the passage of large waves. The temporal variations of liquid temperature and heat transfer coefficient are generally opposite to that of film thickness; the heat transfer coefficient is highest in the substrate regions upstream and downstream of large waves and lowest in the waves themselves. Increasing the film's Reynolds number increases the mean thickness and wave amplitude, and decreases the wave period, but results in appreciable attenuation in the measured liquid temperature response to the large waves. Using FLUENT, a computational model of the falling film is constructed and its predictions compared to the data. The computed results show good agreement with the measured mean film thickness, wave form and period, and both wall and mean film temperatures. The model captures the measured increase in liquid temperature in the film substrate and decrease corresponding to the large waves, but the predicted temperature response is less attenuated for higher Reynolds numbers than the measured. Velocity predictions point to acceleration of high temperature liquid from the upstream substrate toward the cold region within the large wave before losing the excess heat due to mixing downstream from the wave crest. Overall, the present study demonstrates the effectiveness of computational tools at predicting the hydrodynamic and thermal characteristics of separated flows involving a wavy liquid-vapor interface.

The primary objective of this study is to develop a numerical model for turbulent, free-falling liquid films subjected to sensible heating. The model is used to explore the influences of waves and interfacial dampening of turbulent eddies on fluid flow and heat transfer. The model represents two-dimensional axisymmetric film flow on a vertical circular tube, with both the computational domain and operating conditions matching those of an experimental database for water films. Interfacial waves are observed to be prevalent for all operating conditions and associated with a dominant repeated wave shape. Good agreement is achieved between the predicted axial variations of the heat transfer coefficient and experimental data, including an upstream decline in the upstream thermal development region, and slow downstream increase resulting from intensified turbulence and interfacial waviness. Predicted relations for both the film thickness and heat transfer coefficient are shown to agree well with popular experimental correlations. It is shown that turbulence is fully suppressed at the interface, with zero eddy diffusivity both at the wall and interface, and a maximum in between.

This paper explores the parametric influences of spray quenching for thick-walled metal alloy tubes. Using the point-source depiction of a spray, an analytical model is derived to determine the shape and size of the spray impact zone, as well as the distribution of volumetric flux across the same zone. This distribution is incorporated into heat transfer correlations for all spray boiling regimes to generate a complete boiling curve for every location across the impact zone. By setting boundary conditions for both the sprayed and unsprayed portions of the tube surface, a heat diffusion model is constructed for a unit cell of the tube for both aluminum alloy and steel. This model is used to construct spray quench curves for every point along the sprayed surface and within the wall. Increasing nozzle pressure drop or decreasing orifice-to-surface distance are shown to increase the magnitude of volumetric flux, which hastens the onset of the rapid cooling stages of the quench as well as improves overall cooling effectiveness. The sprayed surface is characterized by fast thermal response to the spray, while regions within the wall display more gradual response due to heat diffusion delays. With their superior thermal diffusivity, aluminum alloy tubes transmit the cooling effect through the wall faster than steel tubes. For steel, the cooling effect is more concentrated near the sprayed surface, causing the sprayed surface to cool much faster and locations within the wall much slower than for aluminum alloy. The predictive approach presented in this paper facilitates the determination of surface temperature gradients in the quenched part to guard against stress concentration. Also, when combined with metallurgical transformation models for the alloy, it may be possible to predict material properties such as hardness and strength.

Heat-treating of solid alloy cylinders is an important practical problem for which no optimal production methods have been developed, especially in terms of the most crucial quenching stage. This study explores the use of spray quenching as an alternative to the commonly used bath quenching, which is known to yield relatively slow quench rates and provide few options for spatial optimization of cooling rate. A carefully configured spray cooling system is examined, which provides maximum coverage of the surface of a solid alloy cylinder with full-cone pressure sprays. A new analytical model is derived to determine the shape and size of the spray impact zone, as well as the distribution of volumetric flux across the curved surface of the cylinder. This distribution is combined with heat transfer correlations for all spray boiling regimes to generate a local boiling curve for every location across the impact surface. Using these boiling curves as boundary conditions, a transient analysis is conducted for aluminum alloy and steel cylinders. Increasing the nozzle pressure drop or decreasing the orifice-to-surface distance are shown to hasten the exit from the poor film boiling regime to the more efficient transition boiling regime, resulting in a quicker quench. Relatively high thermal diffusivity causes faster transmission of the spray cooling effect through the cylinder and milder temperature gradients in aluminum compared to steel. This also causes the outer surface to cool earlier but deeper points much slower for steel. Large temperature gradients are encountered on the surface during the quench because of different boiling regimes occurring at different locations exposed to the spray. This study highlights several practical advantages of spray quenching compared with bath quenching, including the ability to achieve a wide range of fast quench rates, uniformity and predictability of quench rate, and the ability to predict and guard against imperfections caused by thermal stresses.