Film Dryout Research Articles

Liquid film dryout in a boiling channel under flow oscillation conditions

A simple theory was developed to elucidate the influence of sinusoidal oscillation of the inlet flow rate on the occurrence of liquid film dryout in an annular two-phase flow regime in a boiling channel. The theory assumes that the critical heat flux (CHF) under an oscillatory condition can be calculated from values in steady states provided that the effect of axial mixing of the liquid film is appropriately considered. The trends of CHFs calculated using a one-dimensional three-fluid model and those experimentally measured under atmospheric pressure were in reasonable agreement with the proposed theory. However, the CHF values measured under oscillatory conditions were usually higher in the experiment than in the numerical simulation, which indicated that axial liquid transport induced by disturbance waves might enhance axial mixing of the liquid film.

Liquid film dryout model for predicting critical heat flux in annular two-phase flow

Gas-liquid two-phase flow and heat transfer can be encountered in numerous fields, such as chemical engineering, refrigeration, nuclear power reactor, metallurgical industry, spaceflight. Its critical heat flux (CHF) is one of the most important factors for the system security of engineering applications. Since annular flow is the most common flow pattern in gas-liquid two-phase flow, predicting CHF of annular two-phase flow is more significant. Many studies have shown that the liquid film dryout model is successful for that prediction, and determining the following parameters will exert predominant effects on the accuracy of this model: onset of annular flow, inception criterion for droplets entrainment, entrainment fraction, droplets deposition and entrainment rates. The main theoretical results achieved on the above five parameters are reviewed; also, limitations in the existing studies and problems for further research are discussed.

Conditions of liquid film dryout during saturated flow boiling in microchannels

During saturated flow boiling in microchannels, two different trends of the dryout vapor quality ( x do ) have been observed experimentally: x do ∝ G - j and x do ∝ G j with j > 0 . In the literature, the first trend is experimentally observed for refrigerants and CO 2 whereas the second trend is detected so far only for CO 2 at high mass velocities. In this paper, the model of Revellin and Thome [A theoretical model for the prediction of the critical heat flux in heated microchannels. International Journal of Heat and Mass Transfer 51, 1216–1225] for the saturated critical heat flux in microchannels is used to analyze and explain this change of trend. Their theoretical model predicts three different trends of dryout vapor quality. Indeed, it is shown here that when increasing the mass velocity, the liquid film at the outlet of the channel passes through two different states: laminar flow ( Re δ < ψ = 2300 ) and transition flow ( Re δ = ψ ) . Turbulent flow ( Re δ > ψ ) has been omitted in this study since no experimental data are available for this regime. Laminar film dryout yields a decrease of the dryout vapor quality with respect to G ( x do ∝ G - j ) , transition film dryout is characterized by an increase of the dryout vapor quality with G ( x do ∝ G j ) . Furthermore, a new criterion is proposed here to identify the laminar-to-transition mass velocity, one for refrigerants and another one for CO 2 . The criterion gives the maximum value (upper limit) of the mass velocity for which the conventional microchannel correlations for the dryout vapor quality may be used. Notably, for a given pressure, the values of the laminar-to-transition mass velocities for refrigerants and CO 2 are approximately the same. This reinforces the idea that CO 2 is not a maverick fluid but just a normal fluid used at a higher reduced pressure in many boiling applications than conventionally used refrigerants.

OPTIMIZED NUMERICAL ANNULAR FLOW DRYOUT MODEL USING THE DRIFT-FLUX MODEL IN TUBE GEOMETRY

Many experimental analyses for annular film dryouts, which is one of the Critical Heat Flux (CHF) mechanisms, have been performed because of their importance. Numerical approaches must also be developed in order to assess the results from experiments and to perform pre-tests before experiments. Various thermal-hydraulic codes, such as RELAP, COBRATF, MARS, etc., have been used in the assessment of the results of dryout experiments and in experimental pre-tests. These thermal-hydraulic codes are general tools intended for the analysis of various phenomena that could appear in nuclear power plants, and many models applying these codes are unnecessarily complex for the focused analysis of dryout phenomena alone. In this study, a numerical model was developed for annular film dryout using the drift-flux model from uniform heated tube geometry. Several candidates of models that strongly affect dryout, such as the entrainment model, deposition model, and the criterion for the dryout point model, were tested as candidates for inclusion in an optimized annular film dryout model. The optimized model was developed by adopting the best combination of these candidate models, as determined through comparison with experimental data. This optimized model showed reasonable results, which were better than those of MARS code.

低圧・低質量流束条件下における周方向非均一加熱管の限界熱流束（加熱長さの影響について）

Critical Heat Flux (CHF) in a non-uniformly heated tube is a very important designing factor of actual boiling two-phase flow equipments. In this series of studies, the CHF in a non-uniformly heated tube was discussed by using the re-distribution of a liquid film caused by the wave interaction. In the present investigation, in order to estimate the influence of the entrainment of droplet under the high heat flux condition on the re-distribution of liquid film, the CHF experiment was conducted. The experimental apparatus was a similar to the forced convective boiling system described in the previous paper, i.e. the eccentric tube with 20 mm I.D., 24 mm O.D., but heated length was just only 450 mm to keep the high heat flux condition. The experimental result expressed the influence of the magnitude of the heat flux as the increasing in the entrainment. The CHF was occurred at the high quality condition, i.e. liquid film dryout and at the low quality condition, i.e. departure from nucleate boiling (DNB), respectively. This obtained CHF was evaluated by using the film flow model, and the empirical correlation of the spreading coefficient of liquid film re-distribution was proposed.

Interfacial area concentration in annular two-phase flow

Accurate prediction of the interfacial area concentration is essential to successful development of the interfacial transfer terms in the two-fluid model. The interfacial area concentration in annular flow and annular-mist flow is especially relevant to the transition process to the liquid film dryout, which might lead to fatal problem in the safety and efficient operation of boiling heat transfer system. However, very few experimental and theoretical studies focusing on the interfacial area concentration in annular flow region have been conducted. From this point of view, measurements of annular flow parameters such as one-dimensional interfacial area concentration of liquid film and local interfacial area concentration profile of liquid film were performed by a laser focus displacement meter at 21 axial locations in vertical upward annular two-phase flow using a 3-m-long and 11-mm-diameter pipe. The axial distances from the inlet ( z) normalized by the pipe diameter ( D) varied over z/ D = 50–250. Data were collected for preset gas and liquid flow conditions and for Reynolds numbers ranging from 31,800 to 98,300 for the gas phase and 1050 to 9430 for the liquid phase. Axial development of the one-dimensional interfacial area concentration and the non-dimensional local interfacial area concentration profile of liquid film were examined with the data obtained in the experiment. Total interfacial area concentration including liquid film and droplets was also discussed with help of the existing drift-flux model, entrainment correlation, and droplet size correlation.

Modelisation d’écoulement diphasique dans l’analyse du sous-canal de grappe d’assemblages

In order to practice a design-by-analysis of thermohydraulics design of BWR fuel rod bundles, the subchannel analysis would play a major role. There, the immediate concern is improvement in its predictive capability of CHF due in particular to the film dryout (boiling transition phenomena: BT) on the fuel rod surface. Constitutive equations in the subchannel analysis formulation are responsible for the quality of calculated results. The constitutive equations are a result of integration of the local and instantaneous description of two-phase flows over the subchannel control volume. In general, they are expressed in terms of subchannel-control-volume- as well as area-averaged two-phase flow state variables. In principle the information on local and instantaneous physical phenomena taking place inside subchannels must be counted for in the algebraic form of the equations on the basis of a more mechanistic modeling approach. They should include also influences of the multi-dimensional subchannel geometry and fluid material properties. Thermohydraulics phenomena of interests in this deed are: 1) vapor-liquid re-distribution by inter-subchannel exchanges due to the diversion cross flow, turbulent mixing and void drift, 2) liquid film behaviors, 3) transition of two-phase flow regimes, 4) droplet entrainment and deposition and 5) spacer-droplet interactions. These are considered to be five key factors in understanding the BT in BWR fuel rod bundles. In Japan, a university-industry consortium has been formed under the sponsorship of the Ministry of Economics, Trade and Industry. This paper describes an outline of the on-going project and, first, an outline of the current efforts is presented in developing a new two-fluid three field subchannel code NASCA being aimed at predicting onset of BT, and post BT phenomena in advanced BWR fuel rod bundles including those of the tight lattice configuration for a higher conversion. Then the current methodology adopted to improve the NASCA code capabilities for BT is described. There a combination of experimental and computational fluid dynamics approaches is undertaken to construct a two-phase fluid dynamics database. The experimental approach consists of 1) high-resolution airwater tests performed under the room-temperature and atmospheric pressure conditions for the intersubchannel exchanges, three-dimensional behaviors of liquid films, and spacer effects; and 2) integral steam-water tests performed at high-temperature and at higher pressure. In the integral tests, state-of the- arts of multi-phase flow measurement technologies are applied in order to obtain local and instantaneous data that reveal underlying detailed physical processes including high resolution void distributions inside a 4x4 bundle, liquid film thickness and two-phase flow regime. The analytical approach consists of computational multi-phase fluid dynamics (CMFD) applicable to two-phase flows. A physical interpretation of the equilibrium two-phase flow redistribution inside a rod bundle is discussed that is considered to closely be related to the void drift phenomena. Identification of interactions among dominant factors is a main objective of the integral test and acquired data will be utilized in verifying the improved subchannel code. Construction of a complete set of two-phase fluid dynamics database will be made by supplementing missing data regions with the aid of numerical analyses. Dependency on important state variables is extracted from the database and prototype constitutive equations are going to be proposed in the final stage of the project.

Development of the film-splitting look-up table applicable to mechanistic annular film dryout model in annulus geometry

Annular flow in annulus geometry is characterized as two liquid films flowing along the inner heated rod and outer unheated wall. Critical heat flux (CHF) occurs when the liquid film on the inner heated wall dries out, while there still exists the liquid film on the outer cold wall. In the safety analysis code, film dryout is calculated by a mechanistic model or CHF table look-up method. The mechanistic film dryout is a complex function of film flow rate, applied heat flux and entrainment/deposition rate, etc. and is determined by the hydrodynamic solution. However, both models were not able to distinguish the liquid films on the cold surface from that on the hot surfaces in a calculation cell, that is, the cold wall effect. This resulted in over-estimation of the calculated CHF in the single-channel modeling of annulus geometry, so it needs a new model that could consider the cold wall effect mechanistically in the single-channel modeling. In order to consider the cold wall effect, a mechanistic film-splitting model look-up table was developed, in which the inner and outer liquid film fractions are solved analytically. The new look-up table was assessed using Wurtz experimental data and was assessed indirectly using several annulus geometry CHF test data.

An experimental investigation on the critical heat flux enhancement by mechanical vibration in vertical round tube

In order to gain an understanding of the relationship between critical heat flux (CHF) and flow-induced vibration (FIV), an experimental investigation was carried out with vertical round tube at the atmosphere. In the both condition of departure from nucleate boiling (DNB) and the liquid film dryout (LFD), CHF increases up to 12.6% with vibration intensity, represented by vibrational Reynolds number ( Re v). CHF enhancement by tube vibration seems to come from the reinforced flow turbulent mixing and the increment of deposition of droplet into the liquid film. Based on the experimental results, an empirical correlation is proposed for the prediction of CHF enhancement ratio. The correlation predicts the CHF enhancement ratio ( E n ) with reasonable accuracy, with an average error rate of 4.5 and 26.5% for RMS. Vibration is an effective method for heat transfer enhancement as well as CHF. Nonetheless, the risk of system failure by FIV has made it very difficult to take advantage of vibration in heat transfer facilities. Therefore, it is necessary to find out optimal fuel design enhancing the CHF but preventing FIV damage in an acceptable vibration range.

Development of the critical film thickness correlation for an advanced annular film mechanistic dryout model applicable to MARS code

It is generally assumed in the mechanistic film dryout model that the critical heat flux (CHF) arises when liquid film calculated from evaporation, droplet entrainment and deposition gets dryout. The dryout of film is usually assumed when film thickness becomes zero. However, it was indicated that the complete dryout assumption can estimate CHF well for uniform heating case but cannot simulate accurately for non-uniform heating case. The critical film thickness concept may be an appropriate approach physically because there is a possibility of instantaneous disappearance of liquid film when it gets very thin. Therefore, a critical dryout film thickness correlation was developed to properly model dryout phenomenon together with MARS code based on experimental data. The modified version of MARS implementing a newly developed critical dryout film thickness correlation was assessed using various dryout data including those of non-uniform heating case and flow reduction transient test. The prediction results showed improved agreement with the experimental data.

Miniature-scale liquid-fuel-film combustor

A new concept is explained for the miniaturization of cylindrical direct-injection liquid-fueled combustors wherein the fuel is flowed in a wall film that reduces heat losses, optimizes vaporization rate, and inhibits quenching. A simple analysis indicates that if a combustor of this type were part of a miniature engine, power levels from 10 W to 10 kW would be achievable with combustor volumes varying from a few hundred cubic millimeters to a few cubic centimeters and fuel flow rates varying from about 1 mg/s to 1 g/s. The combustor takes advantage of the fact that a surface area advantage of film combustion over spray combustion occurs for small volumes as the surface-to-volume ratio of the combustor increases. In addition, the wall film of fuel prevents heat losses while cooling the combustor surfaces. Photographs of a laboratory demonstration of the miniature film combustor concept with methanol and hepatane fuel and a mechanically swirled airflow are presented. The methanol flame required some methane gaseous fuel for stability, but the heptane fuel burned successfully alone. The experiments demonstrated externally and internally anchored flame modes, and a transition between them. The stability of internal burning was affected by internal fluid, dynamics and film dryout.

Generation of Entrained Droplets from an Imitated Disturbance Wave and their Deposition on the Wall.

Gas-liqudid annular two-phase flow is an important flow pattern in terms of the liquid film dryout at the high quality region in various kinds of heat exchangers. In the past, many investigations on an annular-dispersed flow in horizontal pipelines were conducted, and the behaviors of entrained droplets have been discussed. They, however, focussed only on spatially averaged values of various properties of the droplets. In the present experiment, an isokinetic sampling probe technique was used for the measurement of the droplet flow rate generated from an imitated disturbance wave which constructed by injecting liquid through a slit mounted on the bottom of a horizontal duct. The experimental data on the distribution of entrained droplets and their deposition on the duct wall are presented. As a result, it is clarified that the distribution of the droplet flow rate is not uniform axially and in the direction of height and is actually nonsteady. Furthermore, the correlation of the entrainment fraction proposed by Paleev-Filippovich is superior to that by others.

1006 加熱円管内の環状流におけるドライアウト熱流束の予測に関する研究

Incorporating the recently developed correlations for the rates of droplet deposition and entrainment with the film flow model, critical heat flux due to liquid film dryout in annular flow was numerically evaluated. In the present calculations, the predicted critical heat flux is affected not only by the deposition and entrainment rates but also by the entrainment fraction at the transition to annular flow. For this reason, the entrainment fraction at the transition was correlated in terms of the equilibrium entrainment fraction and the dimensionless boiling length. It was demonstrated that the present method is to predict most data of dryout heat flux tested reasonably well.

Experiments for entrainment rate of droplets in the annular regime

Two-fluid model predictions of film dryout in annular flow are limited by the uncertainties in the entrainment rate of droplets from the liquid film. The main cause of these uncertainties is the lack of separate effects experimental data in the range of the operating conditions in industrial applications. Air–water and Freon-113 entrainment rate data have been obtained in 10 mm tubes using the double film extraction technique. These experiments have been scaled to approach high-pressure steam–water flow conditions. The effects of surface tension and density ratio, missing from most previous data sets, have been systematically tested. The entrainment rate mechanism is scaled by the Kelvin–Helmholtz instability at the film surface. Based on this analysis and the new data, a new correlation is proposed that is valid for low-viscosity fluids in small ducts in the ripple-annular regime.

An utilization of liquid sublayer dryout mechanism in predicting critical heat flux under low pressure and low velocity conditions in round tubes

From a theoretical assessment of extensive critical heat flux (CHF) data under low pressure and low velocity (LPLV) conditions, it was found out that lots of CHF data would not be well predicted by a normal annular film dryout (AFD) mechanism, although their flow patterns were identified as annular–mist flow. To predict these CHF data, a liquid sublayer dryout (LSD) mechanism has been newly utilized in developing the mechanistic CHF model based on each identified CHF mechanism. This mechanism postulates that the CHF occurrence is caused by dryout of the thin liquid sublayer resulting from the annular film separation or breaking down due to nucleate boiling in annular film or hydrodynamic fluctuation. In principle, this mechanism well supports the experimental evidence of residual film flow rate at the CHF location, which can not be explained by the AFD mechanism. For a comparative assessment of each mechanism, the CHF model based on the LSD mechanism is developed together with that based on the AFD mechanism. The validation of these models is performed on the 1406 CHF data points ranging over P=0.1–2 MPa, G=4–499 kg m −2 s −1, L/ D=4–402. This model validation shows that 1055 and 231 CHF data are predicted within ±30 error bound by the LSD mechanism and the AFD mechanism, respectively. However, some CHF data whose critical qualities are <0.4 or whose tube length-to-diameter ratios are <70 are considerably overestimated by the CHF model based on the LSD mechanism. These overestimations seem to be caused by an inadequate CHF mechanism classification and an insufficient consideration of the flow instability effect on CHF. Further studies for a new classification criterion screening the CHF data affected by flow instabilities as well as a new bubble detachment model for LPLV conditions, are needed to improve the model accuracy.

Critical heat flux of water in vertical round tubes at low pressure and low flow conditions

An experimental study on critical heat flux (CHF) has been performed for water flow in vertical round tubes under low pressure and low flow (LPLF) conditions to provide a systematic data base and to investigate parametric trends. Totally 513 experimental data have been obtained with Inconel-625 tube test sections in the following conditions: diameter of 6, 8, 10 and 12 mm; heated length of 0.3∼1.77 m; pressure of 106∼951 kPa; mass flux of 20∼277 kg m −2 s −1; and inlet subcooling of 50∼654 kJ kg −1, thermodynamic equilibrium critical quality of 0.323∼1.251 and CHF of 108∼1598 kW m −2. Flow regime analysis based on Mishima & Ishii’s flow regime map indicates that most of the CHF occurred due to liquid film dryout in annular-mist and annular flow regimes. Parametric trends are examined from two different points of view: fixed inlet conditions and fixed exit conditions. The parametric trends are generally consistent with previous understandings except for the complex effects of system pressure and tube diameter. Finally, several prediction models are assessed with the measured data; the typical mechanistic liquid film dryout model and empirical correlations of (Shah, M.M., 1987. Heat Fluid Flow 8 (4), 326–335; Baek, W.P., Kim, H.G., Chang, S.H., 1997. KAIST critical heat flux correlation for water flow in vertical round tubes, NUTHOS-5, Paper No. AA5) show good predictions. The measured CHF data are listed in Appendix B for future reference.

2832 模擬じょう乱波からの液滴発生と管壁への付着に関する実験的研究

Gas-liquid annular two-phase flow is an important flow pattern from the view point of the liquid film dryout at the high quality region in various kinds of Steam generators. In the past, many investigations on an annular-dispersed flow in horizontal pipelines were conducted, and the behaviors of droplets have been discussed. They, however, focussed only on spatially averaged values of various properties of the entrained droplets. On the other hand, it was reported that the generation of droplets from the top of disturbance waves is remarkable by the observation by a high speed camera. In the present paper, experimental data on the distribution of entrained droplets and their deposition on the wall are presented. In this experiment, the isokinetic sampling probe technique was used for the measurement of the entrained droplet flow rate generated from an imitative disturbance wave constructed by injecting liquid through a slit mounted on the bottom of duct.

Prediction of Liquid Film Dryout in Two-Phase Annular-Mist Flow in a Uniformly Heated Narrow Tube Development of Analytical Method under BWR Conditions

A method was developed based on the conservation lows to predict critical heat flux (CHF) causing liquid film dryout in two-phase annular-mist flow in a uniformly heated narrow tube under BWR conditions. The applicable range of the method is within the pressure of 3–9 MPa, mass flux of 500–2,000 kg/m2·s, heat flux of 0.33–2.0 MW/m2 and boiling length-to-tube diameter ratio of 200–800.The two-phase annular-mist flow was modeled with the three-fluid streams with liquid film, entrained droplets and gas flow. Governing equations of the method are mass continuity and energy conservation on the three-fluid streams. Constitutive equations on the mass transfer which consist of the entrainment fraction at equilibrium and the mass transfer coefficient were newly proposed in this study.Confirmation of the present method were performed in comparison with the available film flow measurements and various CHF data from experiments in uniformly heated narrow tubes under high pressure steam- water conditions. In the heat flux range (q“<2MW/m2) practical for a BWR, agreement of the present method with CHF data was obtained as, (Averaged ratio)±(Standard deviation)=0.984±0.077, which was shown to be the same or better agreement than the widely-used CHF correlations.

Entrainment Rate of Droplets in the Ripple-Annular Regime for Small Vertical Ducts

Two-fluid model predictions of film dryout in annular flow are limited by the uncertainties in the constitutive relations for the entrainment rate of droplets from the liquid film. The main cause of these uncertainties is the lack of separate-effects experimental data in the range of the operating conditions in nuclear power reactors.Air/water and Freon-113 entrainment rate data have been obtained in 10-mm tubes using the film extraction technique. These experiments have been scaled to approach high-pressure steam/water flow conditions. The effects of surface tension and density ratio, missing from most previous data sets, have been systematically tested.The entrainment rate mechanism is assumed to be a Kelvin-Helmholtz instability. Based on this analysis and two previous correlations, a new correlation is proposed that is valid for low-viscosity fluids in small ducts in the ripple-annular regime.

Prediction of Liquid Film Dryout in Two-Phase Annular-Mist Flow in a Uniformly Heated Narrow Tube Development of Analytical Method under BWR Conditions.

A method was developed based on the conservation lows to predict critical heat flux (CHF) causing liquid film dryout in two-phase annular-mist flow in a uniformly heated narrow tube under BWR conditions. The applicable range of the method is within the pressure of 3–9 MPa, mass flux of 500–2,000 kg/m2·s, heat flux of 0.33–2.0 MW/m2 and boiling length-to-tube diameter ratio of 200–800. The two-phase annular-mist flow was modeled with the three-fluid streams with liquid film, entrained droplets and gas flow. Governing equations of the method are mass continuity and energy conservation on the three-fluid streams. Constitutive equations on the mass transfer which consist of the entrainment fraction at equilibrium and the mass transfer coefficient were newly proposed in this study. Confirmation of the present method were performed in comparison with the available film flow measurements and various CHF data from experiments in uniformly heated narrow tubes under high pressure steam- water conditions. In the hea...