One-group interfacial area transport equation and its sink and source terms in narrow rectangular channel

The characteristics of two-phase flow in a narrow rectangular channel are expected to be different from those in other channel geometries, because of the significant restriction of the bubble shape which, consequently, may affect the heat removal by boiling under various operating conditions. The objective of this study is to develop an interfacial area transport equation with the sink and source terms being properly modeled for the gas–liquid two-phase flow in a narrow rectangular channel. By taking into account the crushed characteristics of the bubbles a new one-group interfacial area transport equation was derived for the two-phase flow in a narrow rectangular channel. The random collisions between bubbles and the impacts of turbulent eddies with bubbles were modeled for the bubble coalescence and breakup respectively in the two-phase flow in a narrow rectangular channel. The newly-developed one-group interfacial area transport equation with the derived sink and source terms was evaluated by using the area-averaged flow parameters of vertical upwardly-moving adiabatic air–water two-phase flows measured in a narrow rectangular channel with the gap of 0.993 mm and the width of 40.0 mm. The flow conditions of the data set covered spherical bubbly, crushed pancake bubbly, crushed cap-bubbly and crushed slug flow regimes and their superficial liquid velocity and the void fraction ranged from 0.214 m/s to 2.08 m/s and from 3.92% to 42.6%, respectively. Good agreement with the average relative deviation of 9.98% was obtained between the predicted and measured interfacial area concentrations in this study.     

Two-phase flow patterns in short horizontal rectangular microchannels

The two-phase flow in a short horizontal channel of a rectangular cross-section with the height of 100–500 µm and width of 9–40 mm was studied experimentally. The use of the Schliren and fluorescent methods made it possible to reveal the flow of liquid in the channel and to determine its characteristics quantitatively. The features of the churn, jet and drop flow patterns were studied in details. Two particular regimes that can be distinguished represent formation of immobile drops on the channel walls because of the liquid film or liquid bridges breakage and appearance of mobile drops due to the two-phase flow instabilities. It is found out that formation of various two-phase flow patterns and transitions between them are determined by instabilities of the liquid–gas flow in the side parts of a channel. Frontal instability has been observed during the liquid–gas interaction in the region of liquid output from the nozzle. It is shown that a change in the height and width of the horizontal channels has a substantial effect on the boundaries between the flow regimes. One of the results is that the region of the churn regime increases significantly with decreasing thickness of the channel.     

Numerical investigation of static flow instability in a low-pressure subcooled boiling channel

A three-dimensional two-fluid model to predict subcooled boiling flow at low pressure is presented. The model is adopted to investigate the two-phase flow and heat transfer characteristics in a heated channel. The presence of bubbles as a consequence of heating flow through a vertical rectangular channel has a significant effect on the overall pressure drop along the channel. Numerical results were compared against a series experimental data performed at various conditions – mass flux, heat flux, inlet temperature and exit pressure. Good agreement on the overall pressure drop was achieved. The onset of flow instability velocity was also accurately determined when compared against measurements. Predicted results of void fraction provided useful information towards a more fundamental understanding of the occurrence of onset of nucleate boiling, onset of significant voiding and onset of flow instability. The phenomenon of boiling onset oscillations was also predicted through the use of the two-fluid model.     

One-dimensional interfacial area transport of vertical upward bubbly flow in narrow rectangular channel

The design and safety analysis for miniature heat exchangers, the cooling system of high performance microelectronics, research nuclear reactors, fusion reactors and the cooling system of the spallation neutron source targets requires the knowledge of the gas–liquid two-phase flow in a narrow rectangular channel. In this study, flow measurements of vertical upward air–water flows in a narrow rectangular channel with the gap of 0.993 mm and the width of 40.0 mm were performed at seven axial locations by using the imaging processing technique. The local frictional pressure loss gradients were also measured by a differential pressure cell. In the experiment, the superficial liquid velocity and the void fraction ranged from 0.214 m/s to 2.08 m/s and from 3.92% to 42.6%, respectively. The developing two-phase flow was characterized by the significant axial changes of the local flow parameters due to the bubble coalescence and breakup in the tested flow conditions. The existing two-phase frictional multiplier correlations such as Chisholm, 1967, Mishima et al., 1993 and Lee and Lee (2001) were verified to give a good prediction for the measured two-phase frictional multiplier. The predictions of the drift-flux model with the rectangular channel distribution parameter correlation of Ishii (1977) and several existing drift velocity correlations of Ishii, 1977, Hibiki and Ishii, 2003 and Jones and Zuber (1979) agreed well with the measured void fractions and gas velocities. The interfacial area concentration (IAC) model of Hibiki and Ishii (2002) was modified by taking the channel width as the system length scale and the modified IAC model could predict the IAC and Sauter mean diameter acceptably.     

Dynamic film thickness between bubbles and wall in a narrow channel

The present paper describes a novel technique to characterize the behavior of the liquid film between gas bubbles and the wall in a narrow channel. The method is based on the electrical conductance. Two liquid film sensors are installed on both opposite walls in a narrow rectangular channel. The liquid film thickness underneath the gas bubbles is recorded by the first sensor, while the void fraction information is obtained by measuring the conductance between the pair of opposite sensors. Both measurements are taken on a large two-dimensional domain and with a high speed. This makes it possible to obtain the two-dimensional distribution of the dynamic liquid film between the bubbles and the wall. In this study, this method was applied to an air–water flow ranging from bubbly to churn regimes in the narrow channel with a gap width of 1.5 mm.     

Visualization of flow boiling of liquid nitrogen in a vertical mini-tube

The flow boiling patterns of liquid nitrogen in a vertical mini-tube with an inner diameter of 1.931 mm are visualized with a high-speed digital camera. The superficial gas and liquid velocities are in the ranges of 0.01–26.5 m/s and 0.01–1.2 m/s, respectively. Four typical flow patterns, namely, bubbly, slug, churn and annular flow are observed. Some interesting scenes about the entrainment and liquid droplet deposition in the churn and annular flow, and the flow reversal with the indication of negative pressure drop, are also presented. Based on the visualization, the two-phase flow regime maps are obtained. Compared with the flow regime maps for gas–water flow in tubes with similar hydraulic diameters, the region of slug flow in the present study reduces significantly. Correspondingly, the transition boundary from the bubbly flow to slug flow shifts to higher superficial gas velocity, and that of churn to annular flow moves to lower superficial gas velocity. Moreover, time-averaged void fraction is calculated by quantitative image-digitizing technique and compared with various prediction models. Finally, three kinds of oscillations with long-period and large-amplitude are found, possible explanation for the oscillations is given by comparing the instantaneous flow images with the data of pressure, mass flux and temperature recorded synchronously.     

Influence of the aspect ratio on boiling flows in rectangular mini-channels

This article presents experiments conducted with two single rectangular mini-channels of same hydraulic diameter (1.4 mm) and different aspect ratios for conditions of horizontal boiling flow. The Forane® 365 HX used was subcooled (ΔT_sub = 15 °C) for all the boiling curves presented in the paper. Local heat transfer coefficients were measured for heat flux ranging from 25 to 62 kW m⁻² and mass flux from 200 kg m⁻² s⁻¹ to 400 kg m⁻² s⁻¹. The boiling flows were observed with two different cameras (depending on the flow velocity) through a visualization window. The flow patterns in the two channels were compared for similar conditions. The results show that the boiling heat transfer coefficient and the pressure drop values are different for the two single mini-channels. For low heat flux condition, the channel with lowest aspect ratio (H/W = 0.143) has a higher heat transfer coefficient. On the other hand, for high heat flux condition, the opposite situation occurs, namely the heat transfer coefficient becomes higher for the channel with highest aspect ratio (H/W = 0.43). This is probably due to the earlier onset of dryout in the channel with lowest aspect ratio. For the two cases of heating, the pressure drop for the two-phase flow remains lower for the channel with lowest aspect ratio. These results show that the aspect ratio plays a substantial role for boiling flows in rectangular channels. As for single-phase flows, the heat transfer characteristics are significantly influenced (even though the hydraulic diameter remains the same) by this parameter.     

Flow boiling behaviors in hydrophilic and hydrophobic microchannels

Surface wettability is a critical parameter in small scale phenomena, especially two-phase flow, since the surface force becomes dominant as size decreases. In present study, experiments of water flow boiling in hydrophilic and hydrophobic rectangular microchannels were conducted to investigate the wettability effect on flow boiling in rectangular microchannels. The rectangular microchannels were fabricated with a photosensitive glass to visualize flow pattern. The hydrophilic bare photosensitive glass microchannel was chemically treated to obtain a hydrophobic microchannel. And, visualization of flow patterns was carried out. And boiling heat transfer and two-phase pressure drop was analyzed with visualization results. The boiling heat transfer coefficient in the hydrophobic rectangular microchannel was higher than that in the hydrophilic rectangular microchannel, which was highly related with nucleation site density and liquid film motion. And the pressure drop in the hydrophobic rectangular microchannel was higher than that in the hydrophilic rectangular microchannel, which was highly related with unstable motions of bubble and liquid film. Finally, we find out the wettability is important parameter on the flow pattern, which were highly related with two-phase heat and mass transfer.     

Stratification of boiling two-phase flow in a single horizontal channel

The objective of this study is to investigate experimentally the stratification phenomena of boiling two-phase flow in a uniformly heated horizontal channel. Two-phase flow stratification due to gravity effects, and consequently its thermal and hydrodynamic behavior, under steady state conditions, have been determined by measuring 16 top and 16 bottom wall temperatures. Six distinct wall temperature profiles are found, and the corresponding flow patterns are discussed. A dimensionless number has been formulated for the prediction of the occurrence of different flow patterns.     

Two-phase flow of R-134a refrigerant during flow boiling through a horizontal circular mini-channel

This research focuses on heat transfer to R-134a during flow boiling in a 1.75 mm internal diameter tube. Flow visualisation and heat transfer experiments are conducted to obtain heat transfer coefficients for different flow patterns. The measured data in each flow regime are compared with predictions from a three-zone flow boiling model. The calculations are in fair agreement with the experimental results which correspond in particular to slug flow, throat-annular flow and churn flow regimes under conditions of low heat flux.     

Evaporation and flow dynamics of thin, shear-driven liquid films in microgap channels

Thin and ultra-thin shear-driven liquid films in a narrow channel are a promising candidate for the thermal management of advanced semiconductor devices in earth and space applications. Such flows experience complex, and as yet poorly understood, two-phase flow phenomena requiring significant advances in fundamental research before they could be broadly applied. This paper focuses on the results obtained in experiments with locally heated shear-driven liquid films in a flat mini-channel. A detailed map of the flow sub-regimes in a shear-driven liquid film flow of water and FC-72 have been obtained for a 2 mm channel operating at room temperature. While the water film can be smooth under certain liquid/gas flow rates, the surface of an intensively evaporating film of FC-72 is always distorted by a pattern of waves and structures. It was found, that when heated the shear-driven liquid films are less likely to rupture than gravity-driven liquid films. For shear-driven water films the critical heat flux was found of up to 10 times higher than that for a falling film, which makes shear-driven films (annular or stratified two-phase flows) more suitable for cooling applications than falling liquid films.     

PTV experiments of subcooled boiling flow through a vertical rectangular channel

Time resolved Particle Tracking Velocimetry (PTV) experiments were carried out to investigate turbulent, subcooled boiling flow of refrigerant HFE-301 through a vertical rectangular channel with one heated wall. Measurements were performed with liquid Reynolds numbers (based on the hydraulic diameter) of Re = 3309, 9929 and 16,549 over a wall heat flux range of 0.0–64.0 kW/m². Turbulence statistics are inferred from PTV full-field velocity measurements. Quantities such as: instantaneous 2D velocity fields, time-averaged axial and normal velocities, axial and normal turbulence intensities, and Reynolds stresses are obtained. The present results agree well with previous studies and provides new information due to the full-field nature of the technique. This work is an attempt to provide turbulent subcooled boiling flow data for validation and improvement of two-phase flow computational models.     

Two-phase flow pattern identification in a tube bundle based on void fraction and pressure measurements,with emphasis on churn flow

Two-phase flow are frequently encountered in the industry. In particular, in steam generators of nuclear plants, water is heated so that at the top of the generator an important fraction of water flows as vapor. In this upper part, a rising co-current two phase flow transverse to the tube bundle takes place. Fluids exert significant forces on the tubes in this area which highly depend on the two-phase flow pattern. Thus, as a prerequisite, it is essential to gather information on the flow conditions associated with the different two-phase flow patterns, which can be bubbly, intermittent, or annular. Then we must analyze the potentially dangerous flow patterns. This paper presents an experimental campaign aimed at characterizing those flow patterns for a rising co-current transverse flow in a tube bundle representative of the geometry in a steam generator. A new methodology based on the understanding of key contributions to vertical two-phase flow pattern maps in tube bundles is proposed that leads to a more complete flow pattern map. Finally, the paper focuses on the churn flow, which is the flow pattern for which significant pressure fluctuations occur. For this pattern, important damages could be expected on the tubes of a steam generator. Different kinds of pressure fluctuations are observed at different frequencies depending on the flow rates and the location in the test section.     

Subatmospheric pressure boiling on a single nucleation site in narrow vertical spaces

Compact evaporators like plate heat exchangers can play a significant role in reducing the investment cost of low cooling power sorption systems. If water is used as refrigerant, their design remains mainly empirical. The objective of this paper is thus to investigate the specific characteristics of water pool boiling in narrow channel at subatmospheric pressure in order to acquire the fundamental knowledge needed to improve the design of compact evaporators in these sorption systems. An experimental test setup was thus designed and built to study water pool boiling in narrow channel at subatmospheric pressure (from 5 to 1.2 kPa) on a vertical heated copper disk. The influence of the thickness of the narrow channel and of the pressure on the heat transfer is discussed. As the pressure and the channel thickness decrease the occurrence of a specific subatmospheric pool boiling regime is observed, degrading heat transfer coefficient. Nevertheless, the general trends of evolution are in agreement with those generally observed in the literature: heat transfer is enhanced as the thickness of the narrow channel decreases but, depending on the pressure, decreasing too much the channel thickness could lead to a deterioration of the heat transfer coefficient. A particle image velocity (PIV) device was implemented to the experimental setup in order to highlight the effect of the wake-induced flow on the heat transfer.     

Nitrogen/non-Newtonian fluid two-phase upward flow in non-circular microchannels

This paper presents experimental investigations on nitrogen/non-Newtonian fluid two-phase flow in vertical noncircular microchannels, which have square or triangular cross-section with the hydraulic diameters being D_h = 2.5, 2.886 and 0.866 mm, respectively, by visualization method. Three non-Newtonian aqueous solutions with typical rheological properties, i.e., 0.4% carboxymethyl cellulose (CMC), 0.2% polyacrylamide (PAM) and 0.2% xanthan gum (XG) are chosen as the working fluids. The common flow patterns are identified as slug flow, churn flow and annular flow. The dispersed bubble flow is only found in the case with nitrogen/CMC solution two-phase flow in the largest channel. A new flow pattern of nitrogen/PAM solution two-phase flow, named chained bubble/slug flow, is observed in all the test channels. The flow regime maps are also developed and the results show that the rheological properties of the non-Newtonian fluid have remarkable influence on the flow pattern transitions. The geometrical factors of the microchannel such as the cross-section shape and hydraulic diameter of the channel can also affect the flow regime map. Finally, the results obtained in this work are compared with the available flow pattern transitions.     

Macro-to-microchannel transition in two-phase flow: Part 2 - Flow boiling heat transfer and critical heat flux

This part of the paper presents the current experimental flow boiling heat transfer and CHF data acquired for R134a, R236fa and R245fa in single, horizontal channels of 1.03, 2.20 and 3.04 mm diameters over a range of experimental conditions. The aim of this study is to investigate the effects of channel confinement, heat flux, flow pattern, saturation temperature, subcooling and working fluid properties on the two-phase heat transfer and CHF. Experimentally, it was observed that the flow boiling heat transfer coefficients are a significant function of the type of two-phase flow pattern. Furthermore, the monotonically increasing heat transfer coefficients at higher vapor qualities, corresponding to annular flow, signifies convective boiling as the dominant heat transfer mechanism in these small scale channels. The decreasing heat transfer trend at low vapor qualities in the slug flow (coalescing bubble dominated regime) was indicative of thin film evaporation with intermittent dry patch formation and rewetting at these conditions. The coalescing bubble flow heat transfer data were well predicted by the three-zone model when setting the dryout thickness to the measured surface roughness, indicating for the first time a roughness effect on the flow boiling heat transfer coefficient in this regime. The CHF data acquired during the experimental campaign indicated the influence of saturation temperature, mass velocity, channel confinement and fluid properties on CHF but no influence of inlet subcooling for the conditions tested. When globally comparing the CHF values for R134a in the 0.51-3.04 mm diameter channels, a peak in CHF peak was observed lying in between the 0.79 (Co ≈ 0.99) and 1.03 (Co ≈ 0.78) mm channels. A new CHF correlation has been proposed involving the confinement number, Co that is able to predict CHF for R134a, R236fa and R245fa in single-circular channels, rectangular multichannels and split flow rectangular multichannels. In summary, the present flow boiling and CHF trends point to a macro-to-microscale transition as indicated by the results presented in Ong and Thome (2011) [1].     

Effects of dynamic load on flow and heat transfer of two-phase boiling water in a horizontal pipe

An experimental investigation was performed to obtain the flow and heat transfer characteristics of single-phase water flow and two-phase pipe boiling water flow under high gravity (Hi-G) in present work. The experiments were conducted on a rotating platform, and boiling two-phase flow state was obtained by means of electric heating. The data were collected specifically in the test section, which was a lucite pipe with inner diameter of 20 mm and length of 400 mm. By changing the parameters, such as rotation speed, inlet temperature, flow rate, and etc., and analyzing the fluid resistance, effective heat and heat transfer coefficient of the experimental data, the effects of dynamic load on the flow and heat transfer characteristics of single phase water and two-phase boiling water flow were investigated and obtained. The two-phase flow patterns under Hi-G condition were obtained with a video camera. The results show that the dynamic load significantly influences the flow characteristic and boiling heat transfer of the two-phase pipe flow. As the direction of the dynamic load and the flow direction are opposite, the greater the dynamic load, the higher the outlet pressure and the flow resistance, and the lower the flow rate, the void fraction, the wall inner surface temperature and the heat transfer capability. Therefore, the dynamic load will block the fluid flow, enhance heat dissipation toward the ambient environment and reduce the heat transfer to the two-phase boiling flow.     

Experimental study of critical heat flux enhancement during forced convective flow boiling of nanofluid on a short heated surface

Enhancements of nucleate boiling critical heat flux (CHF) using nanofluids in a pool boiling are well-known. Considering importance of flow boiling heat transfer in various practical applications, an experimental study on CHF enhancements of nanofluids under convective flow conditions was performed. A rectangular flow channel with 10-mm width and 5-mm height was used. A 10 mm-diameter disk-type copper surface, heated by conduction heat transfer, was placed at the bottom surface of the flow channel as a test heater. Aqueous nanofluids with alumina nanoparticles at the concentration of 0.01% by volume were investigated. The experimental results showed that the nanofluid flow boiling CHF was distinctly enhanced under the forced convective flow conditions compared to that in pure water. Subsequent to the boiling experiments, the heater surfaces were examined with scanning electron microscope and by measuring contact angle. The surface characterization results suggested that the flow boiling CHF enhancement in nanofluids is mostly caused by the nanoparticles deposition of the heater surface during vigorous boiling of nanofluids and the subsequent wettability enhancements.     

Subcooled flow boiling of R-134a in vertical channels of small diameter

Subcooled flow boiling heat transfer for refrigerant R-134a in vertical cylindrical tubes with 0.83, 1.22 and 1.70 mm internal diameter was experimentally investigated. The effects of the heat flux, q″ = 1–26 kW/m², mass flux, G = 300–700 kg/m² s, inlet subcooling, ΔT_sub,i = 5–15 °C, system pressure, P = 7.70–10.17 bar, and channel diameter, D, on the subcooled boiling heat transfer were explored in detail. The results are presented in the form of boiling curves and heat transfer coefficients. The boiling curves evidenced the existence of hysteresis when increasing the heat flux until the onset of nucleate boiling, ONB. The wall superheat at ONB was found to be essentially higher than that predicted with correlations for larger tubes. An increase of the mass flux leads, for early subcooled boiling, to an increase in the heat transfer coefficient. However, for fully developed subcooled boiling, increases of the mass flux only result in a slight improvement of the heat transfer. Higher inlet subcooling, higher system pressure and smaller channel diameter lead to better boiling heat transfer. Experimental heat transfer coefficients are compared to predictions from classical correlations available in the literature. None of them predicts the experimental data for all tested conditions.     

Flow regime transition criteria for two-phase flow in a vertical annulus

In this work, a new flow regime transition model is proposed for two-phase flows in a vertical annulus. Following previous works, the flow regimes considered are bubbly (B), slug (S) or cap-slug (CS), churn (C) and annular (A). The B to CS transition is modeled using the maximum bubble package criteria of small bubbles. The S to C transition takes place for small annulus perimeter flow channels and it is assumed to occur when the mean void fraction over the entire region exceeds that over the slug–bubble section. If the annulus perimeter is larger that the distorted bubble limit the cap-slug flow regime will be considered since in these conditions it is not possible to distinguish between cap and partial-slug bubbles. The CS to C transition is modeled using the maximum bubble package criteria. However, this transition considers the coalescence of cap and spherical bubbles in order to take into account the flow channel geometry. Finally, the C to A transition is modeled assuming two different mechanisms, (a) flow reversal in the liquid film section along large bubbles; (b) destruction on liquid slugs or large waves by entrainment or deformation. In the S to C and C to A flow regime transitions the annulus flow channel is considered as a rectangular flow channel with no side walls. In all the modeled transitions the drift-flux model is used to obtain the final correlations. The final equations for every flow regime transition are easy to be implemented in computational codes and not experimental input is needed. The prediction accuracy of the newly developed model has been checked against air–water as well as boiling flow regime maps. In all the cases, the new developed model shows better predicting capabilities than the existing correlations most used in literature.