Inverted Annular Flow Research Articles

Experimental design and analysis for film boiling heat transfer in a circular tube

In order to study the heat transfer of water during film boiling, stable film boiling heat transfer experiments are conducted in a circular tube. As the temperature of the heater surface sharply increases in a short period of time when critical heat flux (CHF) occurs, hot-patch technology is adopted to prevent damage to the test section. A complete experimental design, including the experimental facility, test section, experimental procedures, and the measurement of parameters, is developed. Two experimental schemes are used for testing, namely the post-CHF test and the quench test, respectively. These two schemes have been verified to achieve stable film boiling and yield consistent results. The effect of various parameters, including heat flux, pressure, mass flow and inlet subcooling, on the heat transfer coefficient has been analyzed. The experimental parameters range from an inlet subcooling of 10–50 K, a pressure of 0.1–2.5 MPa, and a mass flux of 100–1000kg/m2s. Through the analysis of the distribution of heat transfer coefficient along the axial location, three distinct heat transfer regions can be identified. Combined with heat transfer characteristics, three types of flow are considered: smooth inverted annular flow, rough inverted annular flow and inverted slug flow. Most of the data in this study is concentrated in the first two regions, namely the inversed annular flow. The data obtained in these tests is expected to provide important information for the study of inverted annular flow.

An engineering stability technique for unsteady, two-phase flows with heat and mass transfer

This paper considers the stability of an unsteady, two-phase flow with heat and mass transfer. The model problem is motivated by loss of coolant accidents in nuclear power plants. For the example problem, two flow geometries are considered: inverted annular flow boiling and an annular mist flow. The model is comprised of coupled Mathieu equations so that stability can be determined using a Floquet analysis. The flow is found to be mathematically unstable to all perturbative wavenumbers, but for practical purposes there are regions of stability. Using the solution's growth behavior and doubling-time, the notion of practical stability, which is termed herein as "engineering stability," is quantified and a method is provided for application to other engineering stability problems.

Liquid hydrogen line chilldown experiments at high Reynolds numbers. II. Analysis

This paper presents in-depth analysis of the recently conducted high Reynolds (Re) number liquid hydrogen (LH2) transfer line chilldown experiments at NASA Glenn Research Center. Vertically upward trickle and pulse flow chilldown tests were conducted over a wide range of LH2 liquid temperatures (20.3 K – 24.2 K) and mass flow rates (0.0012 – 0.036 kg/s) typical of a proposed cryogenic propellant depot. Two-phase flow mapping is used to correlate flow visualization with LH2 stream temperature profiles. Thermodynamic state diagrams, reverse boiling curves, and heat transfer coefficients are used to explain the evolution of the chill down process at various locations. Analysis reveals stark differences in behavior between skin and stream temperature measurements due to annular flow and bubbly flow dominated cooling. Analysis also shows that hydrogen chilldown at high Re numbers promotes better mixing and thus more liquid contact along the wall earlier on, in comparison to chilldown with other fluids like nitrogen where classical inverted annular flow boiling is incurred.

Application of Hyperbolicity Breaking Model to the Prediction of Post-dryout Flow Regime Transition from Inverted Annular Flow

The post-dryout flow regime transition criterion from inverted annular flow (IAF) to agitated inverted annular flow (AIAF) is suggested based on the hyperbolicity breaking concept. The hyperbolicity breaking represents a bifurcation point where a sudden flow transition occurs. The hyperbolicity breaking concept is applied to describe the flow regime transition from IAF to AIAF by the growth of disturbance on liquid core surface. The resultant correlation has the similar form to Takenaka's empirical one. To validate the proposed model, it is applied to predict Takenaka's experimental results using R-113 refrigerant with four different tube diameters of 3, 5, 7 and 10mm. The proposed model gives accurate predictions for the tube diameters of 7 and 10mm. As the tube diameter decreases, the differences between the predictions and the experimental results slightly increase. The flow regime transition from AIAF to dispersed flow (DF) is described by the drift flux model. It is shown that the transition criterion can be well predicted if the droplet sizes in dispersed flow are evaluated appropriately.

Application of Hyperbolicity Breaking Model to the Prediction of Post-dryout Flow Regime Transition from Inverted Annular Flow.

The post-dryout flow regime transition criterion from inverted annular flow (IAF) to agitated inverted annular flow (AIAF) is suggested based on the hyperbolicity breaking concept. The hyperbolicity breaking represents a bifurcation point where a sudden flow transition occurs. The hyperbolicity breaking concept is applied to describe the flow regime transition from IAF to AIAF by the growth of disturbance on liquid core surface. The resultant correlation has the similar form to Takenaka's empirical one. To validate the proposed model, it is applied to predict Takenaka's experimental results using R-113 refrigerant with four different tube diameters of 3, 5, 7 and 10mm. The proposed model gives accurate predictions for the tube diameters of 7 and 10mm. As the tube diameter decreases, the differences between the predictions and the experimental results slightly increase. The flow regime transition from AIAF to dispersed flow (DF) is described by the drift flux model. It is shown that the transition criterio...

A phenomenological model of the thermal hydraulics of convective boiling during the quenching of hot rod bundles Part I: Thermal hydraulic model

In this paper, a phenomenological model of the thermal hydraulics of convective boiling in the post-critical-heat-flux (post-CHF) regime is developed and discussed. The model was implemented in the TRAC-PF1/MOD2 computer code (an advanced best-estimate computer program written for the analysis of pressurized water reactor systems). The model was built around the determination of flow regimes downstream of the quench front. The regimes were determined from the flow-regime map suggested by Ishii and his coworkers. Heat transfer in the transition boiling region was formulated as a position-dependent model. The propagation of the CHF point was strongly dependent on the length of the transition boiling region. Wall-to-fluid film boiling heat transfer was considered to consist of two components: first, a wall-to-vapor convective heat-transfer portion and, second, a wall-to-liquid heat transfer representing near-wall effects. Each contribution was considered separately in each of the inverted annular flow (IAF) regimes. The interfacial heat transfer was also formulated as flow-regime dependent. The interfacial drag coefficient model upstream of the CHF point was considered to be similar to flow through a roughened pipe. A free-stream contribution was calculated using Ishii's bubbly flow model for either fully developed subcooled or saturated nucleate boiling. For the drag in the smooth IAF region, a simple smooth-tube correlation for the interfacial friction factor was used. The drag coefficient for the rough-wavy IAF was formulated in the same way as for the smooth IAF model except that the roughness parameter was assumed to be proportional to liquid droplet diameter entrained from the wavy interface. The drag coefficient in the highly dispersed flow regime considered the combined effects of the liquid droplets within the channel and a liquid film on wet unheated walls. The heat-transfer and interfacial drag models used were based on the flow-regime map noted above with length averaging of the flow-regime length if more than one regime existed in a give hydraulic cell.

A phenomenological model of the thermal-hydraulics of convective boiling during the quenching of hot rod bundles Part II: Assessment of the model with steady-state and transient post-CHF data

After completion of the thermal-hydraulic model developed in a companion paper, we performed developmental assessment calculations of the model using steady-state and transient post-critical heat flux (CHF) data. This paper discusses the results of those calculations. The overall interfacial drag model predicted reasonable drag coefficients for both the nucleate boiling and the inverted annular flow (IAF) regimes. The predicted pressure drops agreed reasonably well with the measured data of two transient experiments, CCTF Run 14 and a Lehigh reflood test. The thermal-hydraulic model for post-CHF convective heat transfer predicted the rewetting velocities reasonably well for both experiments. The predicted average slope of the wall temperature traces for these tests showed reasonable agreement with the measured data, indicating that the transient-calculated precursory cooling rates agreed with measured data. The hot-patch model, in conjunction with the other thermal-hydraulic models, was capable of modeling the Winfrith post CHF hot-patch experiments. The hot-patch model kept the wall temperatures at the specified levels in the hot-patch regions and did not allow any quench-front propagation from either the bottom or the top of the test section. The interfacial heat-transfer model tended to slightly underpredict the vapor temperatures. The maximum difference between calculated and measured vapor temperatures was 20%, with a 10% difference for the remainder of the runs considered. The wall-to-fluid heat transfer was predicted reasonably well, and the predicted wall temperatures were in reasonable agreement with measured data with a maximum relative error of less than 13%.

Two-phase flow characteristic of inverted bubbly, slug and annular flow in post-critical heat flux region

Inverted annular flow can be visualized as a liquid jet-like core surrounded by a vapor annulus. While many analytical and experimental studies of heat transfer in this regime have been performed, there is very little understanding of the basic hydrodynamics of the post-CHF flow field. However, a recent experimental study was done that was able to successfully investigate the effects of various steady-state inlet flow parameters on the post-CHF hydrodynamics of the film boiling of a single phase liquid jet. This study was carried out by means of a visual photographic analysis of an idealized single phase core inverted annular flow initial geometry (single phase liquid jet core surrounded by a coaxial annulus of gas). In order to extend this study, a subsequent flow visualization of an idealized two-phase core inverted annular flow geometry (two-phase central jet core, surrounded by a coaxial annulus of gas) was carried out. The objective of this second experimental study was to investigate the effect of steady-state inlet, pre-CHF two-phase jet core parameters on the hydrodynamics of the post-CHF flow field. In actual film boiling situations, two-phase flows with net positive qualities at the CHF point are encountered. Thus, the focus of the present experimental study was on the inverted bubbly, slug, and annular flow fields in the post-dryout film boiling region. Observed post-dryout hydrodynamic behavior is reported. A correlation for the axial extent of the transition flow pattern between inverted annular and dispersed droplet flow (the agitated regime) is developed. It is shown to depend strongly on inlet jet core parameters and jet void fraction at the dryout point.

Experimental study on flow pattern and heat transfer of inverted annular flow

Experimental results are presented on flow pattern and heat transfer in the regions from inverted annular flow to dispersed flow in a vertical tube using freon R-113 as a working fluid at atmospheric pressure to discuss the correspondence between them. Axial distributions of heat transfer coefficient are measured and flow patterns are observed. The heat transfer characteristics are divided into three regions and a heat transfer characteristics map is proposed. The flow pattern changes from inverted annular flow (IAF) to dispersed flow (DF) through inverted slug flow (ISF) for lower inlet velocities and through agitated inverted annular flow (AIAF) for higher inlet velocities. A flow pattern map is obtained which corresponds well with the heat transfer characteristic map.

Experimental study of steady-state film boiling heat transfer of subcooled water flowing upwards in a vertical tube

A series of steady-state, post-DNB experiments have been performed on a vertically mounted Inconel 600 tubular test section of 0.28 m length (9 mm bore, 12 mm O.D.) with forced convective upflow of water at atmospheric pressure to investigate heat transfer characteristics at subcooled inverted annular flow film boiling (IAFFB) conditions. Using directly heated hot patches, empirical results have been obtained for mass flux ranging from 50 to 500 kg/(m 2 s) and inlet subcooling to 0–30°C. The accuracy of the heat transfer coefficient evaluated from the measurements was estimated at about ± 7%, except for the first few diameters from the point of initiation of film boiling. The experimental results have been correlated using a previously developed extended Bromley model that properly incorporates the effects of all relevant independent parameters on IAFFB heat transfer. The heat transfer coefficients predicted by this model are in good agreement with the experimental measurements, with an rms error of 12.2% and a mean deviation of 10.2%. This accuracy is significantly higher than that of other previously suggested heat transfer correlations.

Flow pattern transition and heat transfer of inverted annular flow

The characteristics of flow pattern transitions and heat transfer of a steady inverted annular flow (IAF) in a vertical tube are studied experimentally and analytically using freon R-113 as a working fluid. The heat transfer characteristics are divided into three regions and a heat transfer characteristic map is proposed. Flow patterns along the tube change from IAF to dispersed flow (DF) through inverted slug flow (ISF) for lower mass fluxes and through agitated inverted annular flow (AIAF) for higher mass fluxes. A flow pattern map is proposed and the boundaries of the flow patterns agree well with those of the heat transfer characteristic map. A turbulent boundary layer analysis is applied to IAF using modified Reichardt equations. Radial velocity and temperature profiles of both the gas and liquid phases are obtained. The predicted heat transfer coefficients agree well with the experimental results. The flow pattern transition criterion from IAF to AIAF is predicted by the analytical results and the Kelvin-Helmholtz instability and that from AIAF to DF is predicted by a previous drift flux model and a void fraction of 0.7. The predicted criteria agree well with the experimental results.

Two-phase flow regime transition criteria in post-dryout region based on flow visualization experiments

Abstract A visual study of film boiling is carried out to determine the flow regime transition in the post-CHF region. An idealized inverted annular flow is obtained by introducing a liquid jet of Freon 113 through a nozzle, precisely centered with respect to the internal diameter of the test section, with an annular gas flow. The respective ranges for liquid and gas velocities are 0.05–0.5 and 0.03–8.2 m ifs −1 . Nitrogen and helium are used in the study. For the present configuration, there are four flow regimes; namely, the smooth inverted annular flow section, the rough wavy, agitated and dispersed flow regimes.

Flow visualization study of inverted annular flow of post dryout heat transfer region

The inverted annular flow is important in the area of LWR accident analysis in terms of the maximum cladding temperature and effectiveness of the emergency core cooling. However, the inverted annular flow thermal-hydraulics is not well understood due to its special heat transfer condition of film boiling. In view of this, the inverted flow is studied in detail experimentally. A new experimental apparatus has been constructed in which film boiling heat transfer can be established in a transparent test section. Data on liquid core stability, core break-up mechanism, and dispersed-core liquid slug and droplet sizes are obtained using F 113 as a test fluid. Both high speed movies and flash photographs are used. The inlet section consists of specially designed coaxial nozzles for gas and liquid such that the ideal inverted annular flow can be generated. The roll wave formation, droplet entrainment from wave crests, agitated sections with large interfacial areas, classical sinuous jet instability, jet break-up into multiple liquid ligaments and drop formation from liquid ligaments have been observed in detail.

Reflooding With Steady and Oscillatory Injection: Part I—Flow Regimes, Void Fraction, and Heat Transfer

Simultaneous void fraction and wall temperature measurements were made during bottom-reflooding of a vertical Inconel tube under both constant and oscillatory injection rates. To support interpretation of these data, flow regime visualization experiments were also conducted by reflooding a heated quartz tube. With constant, high reflooding rates, inverted annular, transition, and dispersed flow regimes exist above the quench front, with typical chordal-average void fractions in the ranges of 10–30 percent, 30–70 percent, and 70–90 percent, respectively. Each regime exhibits different heat transfer rates. With lower injection rates or higher heating rates, annular droplet and dispersed flow regimes appear with void fractions above 80 percent. For reflooding with oscillatory inlet flow and high injection rates, large oscillations are seen in void fraction and wall temperature, indicating periodic changes in the flow regime near the quench front: The regime alternated between inverted annular (during an upstroke) and annular droplet flow (during a downstroke). These flow regimes were observed in the flow visualization experiments to be qualitatively similar to those for the constant injection cases. Heat transfer rates are substantially affected by the flow regime and increase (or decrease) as the void fraction falls (or rises). Compared to the constant-injection tests, increased rates of entrainment were observed during the forced-oscillation tests.