Film Dryout Research Articles

Ground tests for a future space experiment: Evaporation rates through a centimetric square chimney by vapor interferometry and simulation

In this paper, results of preparatory tests for a space experiment “Marangoni in Films” on board International Space Station are reported. The setup is thermalized at a setpoint temperature so that (thermal) Marangoni convection is exclusively a result of evaporative cooling of the film, here composed of a Hydrofluoroether (HFE)-7100, evaporating into nitrogen at a given setpoint pressure. The present paper is entirely devoted to the evaporation rates, whereas the Marangoni convection itself will be analyzed in future studies. Apart from the temperature and nitrogen pressure, various setpoints for the initial relative saturation in HFE-7100 vapor are explored. A special design of the test cell with a square chimney surrounding a square opening of the liquid layer permits to largely stabilize the evaporation rates during a longest-lasting middle stage (beyond the initial transients and the final film dry-out). In addition, it facilitates the use of vapor (digital-holographic) interferometry for measuring the evaporation rates, the results of which are compared with independent measurements by means of the pressure evolution within our sealed cell of a fixed volume. An excellent agreement is found before the final dry-out stage for all setpoint parameter combinations tested. The study is accompanied by numerical finite-element simulations in an axisymmetrized geometry, neglecting a possible influence of the Marangoni convection and verifying certain premises used in interferometric post-processing. Even if the simulation results might somewhat over-/underpredict in each particular case, the overall dependence of the evaporation rate on the setpoint parameters is well reproduced and the role of the buoyancy convection is thereby explored.

Mechanistic prediction of Westinghouse TRITON11® BWR fuel critical power with MEFISTO-T subchannel analysis code

The TRITON11® fuel design is the latest Boiling Water Reactor (BWR) fuel product developed by Westinghouse, based on an 11 × 11 optimized fuel rod lattice, including mixing vane spacer grids, three large water rods and 18 part-length rods. The design offers larger fuel cycle cost saving, improved fuel reliability and increased thermal margin over previous Westinghouse fuel products. The critical power performances of the TRITON11 fuel design were assessed at the Westinghouse thermal–hydraulic FRIGG loop using a full-scale test bundle covering a wide range of BWR core conditions (covering normal operation and Anticipated Operational Occurrences) with various radial and two axial power distributions. The resulting FRIGG steady state database was simulated with Westinghouse subchannel analysis code MEFISTO-T based on a two-phase three-field approach of annular two-phase flow, accounting for the drop deposition enhancement provided by the spacer grids. A new model of local film entrainment was introduced due to the liquid “scrapping off” effects provided by the frame and side vanes of the spacer grids along the fuel channel and water rods. After spacer grid effect calibration, the steady state critical power is simulated mechanistically by power iterations up to complete local film dryout. The MEFISTO-T code can successfully predict the critical power performance of the Westinghouse TRITON11 BWR fuel design, including on part-length rods, with almost no bias and trend, within a standard deviation of about 5 %.

Modelling of evaporative falling-film heat transfer over a horizontal tube

This article focuses on evaporative falling film heat transfer through numerical simulations. A two-dimensional symmetric liquid film flowing outside a single horizontal tube was simulated using the open-source CFD software OpenFOAM. The Tanasawa model was employed to consider mass transfer at the liquid–vapour interface, and the isoAdvector VOF method was used for interface tracking. The simulations explored the impact of film flow rate, tube surface heat flux, and tube diameter on the liquid film thickness and surface heat transfer coefficient. Additionally, the critical heat flux and operational limits were considered. The results demonstrate the accurate reproduction of evaporative falling film heat transfer performance by the present model. The film thickness exhibits a similar profile to adiabatic and sensible falling film heat transfer but provides smaller values. Similarly, the evaporative heat transfer coefficient exhibits significantly higher values and a gentle peripheral variation. The influence of heat flux on evaporative falling film heat transfer is contingent on the film flow rate; it can either enhance or weaken the process. Notably, a larger tube diameter negatively affects heat transfer, suppressing the impact of heat flux, and also increases the risk of liquid film dryout. For a given film flow rate, the average heat transfer coefficient increases initially, reaches a maximum value, and then decreases as the wall heat flux is increased. The peak heat flux, termed critical heat flux, increases with film flow rate but decreases with tube diameter. An operational limit exists where the heat flux is lower than the critical heat flux and the film flow rate is higher than the critical film flow rate; under these conditions, evaporative falling film heat transfer operates in a fully wetting status.

Utilization of XGBoost algorithm to predict dryout incipience quality for saturated flow boiling in mini/micro-channels

Critical heat flux (CHF) is arguably the most important design parameter for applications involving cooling of high heat flux devices. Despite prior limited successes, developing accurate predictive tools for CHF remains quite challenging, but key identifiable trigger mechanisms for CHF are intermittent liquid film dryout, complete liquid film dryout, and departure from nucleate boiling (DNB). For saturated flow boiling in mini/micro-channels, dryout incipience is a necessary condition for commencement of liquid film dryout, which is marked by a substantial deterioration in the heat transfer coefficient. The primary objective of this study is to predict system parameters for dryout incipience quality using machine learning, relying on a massive database for this parameter. Predictions are achieved using eXtreme Gradient Boosting (XGBoost), one of the supervised ensemble machine learning methods. This technique is applied to the Purdue University Boiling and Two-Phase Flow Laboratory (PU-BTPFL) consolidated database for dryout incipience quality for saturated flow boiling in mini/micro-channels. This database comprises 997 datapoints amassed from 26 sources and encompasses 13 different working fluids, hydraulic diameters from 0.51 to 6.0 mm, mass velocities from 29 to 2303 kg/m2s, liquid-only Reynolds numbers from 125 to 53,770, boiling numbers from 0.31×104 to 44.3×104, and reduced pressures from 0.005 to 0.78. Optuna, a supervised automated hyper-parameter optimization software, is used to set the best hyper-parameters in the learning process based on the consolidated database. A part of consolidated database is used to train the XGBoost algorithm, and the XGBoost machine, consisting of specific input parameters, is developed with appropriately determined hyper-parameter set after optimization. The trained XGBoost machine is shown to provide remarkable accuracy in predicting the dryout incipience quality, evidenced by a mean absolute error (MAE) of 2.45% and mean absolute deviation (MAD) of 3.57×10−2.

A critical heat flux model for IVR-ERVC application based on the experimental observation

It is essential to conduct theoretical and experimental research on the heat transfer limit for External Reactor Vessel Cooling (ERVC), which aims to implement In-Vessel Retention (IVR) severe accident mitigation strategy successfully. In this paper, a theoretical model for IVR-ERVC condition is proposed to predict the critical heat flux (CHF) based on the experimental observation. According to the experimental observation on the one-dimensional full height IVR-ERVC experimental facility, it is found that there is a thin liquid film between the heating surface and the vapor slug. And this film is evaporated completely when CHF occurs. In addition, a large number of bubble condensation collapses were observed under subcooling condition. According to these experimental observations, a CHF model based on the mechanism of liquid film dryout is developed. The CHF trigger mechanism of this model is that the supply of fresh liquid into the liquid film is not sufficient to prevent depletion of the liquid film due to the boiling. The influence of heating surface geometry and the heat flux shape are considered in this model. The predicted values of the model are compared with the CHF experimental result from different test facilities with different conditions, and the comparison result shows that the relative errors are basically within 20 %. The analysis results of the model indicate the increasing of coolant subcooled degree is conductive to the improvement of CHF. When the coolant subcooled degree increases by 15 ℃, more than 30 % maximum increase in CHF is obtained. In addition, this model can reflect the “exit phenomena” to a certain extent.

Parameter study for critical heat flux under external reactor vessel cooling condition based on a mechanism model

External reactor vessel cooling has been adopted an important method to implement In Vessel Retention severe accident mitigation strategy for many advanced passive pressurized water reactors. Critical heat flux is one of the important criteria to evaluate the success or failure of this strategy. The uncertainty of engineering factors in practical design and operation conditions may make influence on critical heat flux more or less. In this paper, a parameter study for critical heat flux was carried out based on a new critical heat flux mechanism model. This mechanism model is developed based on the mechanism of liquid film dryout, and the predicted results of this model are in good agreement with the experimental results. Five important parameters, namely, coolant subcooling degree, geometry of lower head, system pressure, flow rate and heating power shape are analyzed based on this mechanism model. The results of parameter study show that the decrease of inlet coolant temperature and lower head radius are beneficial to the increase of critical heat flux. And the critical heat flux increases with the increase of system pressure. While the change of flow rate and heating power shape have little effect on critical heat flux. Under the condition of same circumference, the critical heat flux of spherical lower head is higher than that of elliptical lower head in the middle and low inclination angle region, while the trend is opposite in high inclination angle region, but not significant.

A new paradigm for the role of disturbance waves on film dryout and wall heat transfer in annular two-phase flow

The entire liquid-film dryout process in vertical annular two-phase boiling flow is characterized experimentally, from inception to completion, in a newly defined flow regime between annular two-phase flow and mist flow, called ‘intermittent dryout’. Experiments are conducted using saturated R245fa flowing vertically through a heated transparent channel. Liquid-film thickness measurements are taken during dryout events. The state (wet or dry) of the heated surface is determined using a laser reflectance measurement, and the signal is used to calculate the time-averaged dry fraction, fdry. The local heat transfer coefficient (HTC) is characterized as a function of fdry. For all the investigated mass fluxes, optimum boiling conditions (OBC), corresponding to the heat flux that results in maximum HTC, consistently occur when the time-averaged dry fraction is fdry≈0.05. Subsequently, the critical boiling transition (CBT), corresponding to the heat flux that results in a significantly lower HTC, can occur for dry fractions of fdry>0.1. Further insight into the liquid film behavior within the intermittent dryout regime is obtained by combining analyses of high-speed videos, time-resolved liquid-film thickness signals, and statistics about the duration of and time between dryout events. In general, the dryout mechanism is dominated by disturbance waves.Mechanistic model frameworks are presented for the prediction of two-phase HTC and heat flux associated with dryout. The model inputs are local flow quantities (pressure, mass flux, vapor quality, and liquid-film thickness) and output a prediction for local heat transfer coefficient. The model framework accurately predicts the local two-phase HTC for a range of mass flux and vapor quality in the annular flow regime. The model framework captures the observed initial increase in the local HTC and subsequent decrease that is associated with liquid-film dryout. The mechanistic approach provides predictions for time-averaged HTC and OBC heat flux that both agree well with experimentally measured values.

Development of a critical heat flux correlation based on an annular film dryout mechanistic model under the annular flow conditions

ABSTRACT This study proposes a mechanistic model-based correlation for the critical heat flux to meet the practical requirements for the design and safety analysis of light water reactors. The correlation is based on an annular film dryout mechanistic model for a single tube, which needs numerical integration from the onset of annular flow to the dryout point, expressed as an explicit non-dimensional equation to ensure a wide applicable range of coolant conditions. The correlation predicts a critical heat flux for the annular flow region in the pressure range of 1–16 MPa, the mass velocity range of 500–3000 kg/m2s, and the thermal equilibrium quality range of 0.05–0.7. The prediction by the developed correlation agrees well with that of the existing mechanistic model without any numerical integration of the annular flow conditions.

Effects of a flow obstacle on liquid film thickness in relation with CHF enhancement

Abstract There is evidence in literature that the insertion of flow obstacles in the coolant flow of a boiler flowing in an annular regime postpones the film dryout emergence on the heated surfaces. Specifically, the obstacle is believed to force the deposition of droplets to the wall, hence to increase the film flow rate, and therefore to delay dryout. The two objectives of the present research are (i) to confirm the increase in the film flow rate due to the obstacle, by (ii) building an appropriate flow-loop equipped with suited techniques for film thickness and film velocity measurements. A solid cylinder is inserted in the center of a vertical water-air annular tube flow. Variations in liquid film characteristics, e. g. film thickness and interfacial parameters (disturbance waves velocity and frequency), are measured through a local conductance probes technique. This work shows that the presence of an obstacle in an annular flow leads to an increase in the film thickness, specifically the substrate thickness, and does not affect the interfacial behavior of the film. An increase in the droplet deposition constant is observed and agrees reasonably well with predictions from the Windecker et al. (1999) correlation. The flow-loop and the measurements techniques are suited for extensive droplet deposition measurements.

Growth of dry patches in an evaporating film flowing around a horizontal circular tube

The falling film flows over solid surfaces have been exploited extensively, but the modeling of film dry-out and the dry patches' formation poses a considerable challenge. Here, we presented a simple mathematical model that accounts for the evaporation to predict the evolution of dry patches in a falling film. Besides, we also analyzed the effect of flow rate, spray temperature, and tube diameter on the growth of dry patches. The distribution of liquid film thickness is asymmetrical, and the film thickness decreases firstly and then increases after a central angle of 90°. The liquid film covered the tube initially under the action of gravity, then evaporation thinned liquid film and induced small dry patches. With the increasing flow rate, decreasing spray temperature, or decreasing tube diameter, the dry-out time increases, and the dry patche area decreases. It is more effective to suppress dry patches' appearance by controlling both flow rate and spray temperature rather than tube diameter.

Program development and preliminary CHF characteristics analysis for natural circulation loop under moving condition

Critical heat flux (CHF) has traditionally been evaluated using look-up tables or empirical correlations for nuclear power plants. However, under complex moving condition, it is necessary to reconsider the CHF characteristics since the conventional CHF prediction methods would no longer be applicable. In this paper, the additional forces caused by motions have been added to the annular film dryout (AFD) mechanistic model to investigate the effect of moving condition on CHF. Moreover, a theoretical model of the natural circulation loop with additional forces is established to reflect the natural circulation characteristics of the loop system. By coupling the system loop with the AFD mechanistic model, a CHF prediction program called NACOM for natural circulation loop under moving condition is developed. The effects of three operating conditions, namely stationary, inclination and rolling, on the CHF of the loop are then analyzed. It can be clearly seen that the moving condition has an adverse effect on the CHF in the natural circulation system. For the calculation parameters in this paper, the CHF can be reduced by 25% compared with the static value, which indicates that it is important to consider the effects of moving condition to retain adequate safety margin in subsequent thermal-hydraulic designs.

Numerical study of the impact of the channel shape on microchannel boiling heat transfer

Flow boiling in multi-microchannel evaporators is recognised as one of the most efficient cooling solutions for high-performance electronics, and has therefore received increasing attention during the recent years. Despite the extensive literature, there is no general agreement yet about the effect of the channel cross-sectional shape on the boiling heat transfer performance, which results on a limited availability of thermal design guidelines and tools. This article presents the results of a systematic analysis of the impact of the channel shape on the bubble dynamics and heat transfer, under flow boiling conditions. Simulations are carried out using a customised version of OpenFOAM, and the Volume-Of-Fluid method is chosen to capture the liquid-vapour interface dynamics. A benchmark flow model is utilised, where a single isolated bubble is seeded at the channel upstream and transported by a liquid flow across the diabatic section, which is heated by a constant and uniform heat flux. Flow conditions that apply well to the flow boiling of water or refrigerant fluids in sub-millimetre channels at low heat flux ( ~ 10 kW/m2) are investigated, with cross-section width-to-height aspect-ratios ranging from 1 to 8, while the hydraulic diameter of the channel is fixed. This study emphasises that the heat transfer performances for different channel shapes are closely related to the perimetral distribution of the liquid film surrounding the very long bubbles. Square channels exhibit the highest heat transfer coefficients at low flow rates, due to a very thin liquid film that forms at the centre of the wall, but are more at risk of film dryout. High aspect-ratio rectangular channels may be beneficial at larger flow rates, as they promote the formation of an extended liquid film that covers up to 80 % of the cross-section perimeter. At larger aspect-ratios, the average heat transfer coefficient along the shorter wall becomes orders of magnitude smaller than the value detected along the longer wall, owing to a strong asymmetry in the liquid film perimetral distribution.

Development of a three-field mechanistic model for dryout prediction in annular flow

Critical heat flux (CHF) is one of the most important thermal criteria for nuclear power plants. It has traditionally been evaluated using look-up tables or empirical correlations. In this paper an annular film dryout (AFD) mechanistic model has been developed based on the interaction of three fields in annular flow region, i.e. the liquid film, entrained droplets and vapor core. The model describes the mass and momentum conservation equations of three fields together with a series of constitutive relations. The effect of some constitutive correlations (the entrainment and deposition of droplets and the onset of annular flow) on the prediction accuracy of the model is studied. The results are compared with CHF experimental data in flow boiling. Fairly good agreements are observed for the CHF in circular tubes with uniform and axially non-uniform heating, as well as rectangular channels with uniform heating. Through coupling with the subchannel analysis method, the model is used to predict the dryout-type CHF in the rod bundles with a good precision.

Phenomenological modelling of film-dryout ‘critical heat flux’

This paper addresses the topic of phenomenological analysis of film dryout in the context of high quality critical heat flux in a nuclear reactor. It seeks to record the major contribution made to the subject by Geoff Hewitt over many decades, to report some recent advances, and to identify areas where improvement is still needed. It closes with a discussion of what future advances are to be expected.

Prediction of rod film thickness of vertical upward co-current adiabatic flow in rod bundle

Prediction of rod film thickness and its distribution in annular flow in rod bundle is of essential importance to nuclear power industry. This is attributed to the possible film dryout on fuel rods under certain conditions, which will result in a drastically increasing cladding temperature and possible fuel damage. Though there exist a lot of models or correlations to predict film thickness for annular flow in pipes, there are few models to predict rod film thickness inside rod bundle. The one-dimensional models or correlations for pipe flow cannot be used for rod bundle directly due to different geometry and flow dynamics. In relation to this, a new approach to predict the rod film thickness inside rod bundle has been developed. This approach first divides the rod bundle cross-section into three regions according to their characteristic length scales. Then, the gas and liquid flow distribution inside rod bundle has been given with reasonable assumptions. Based on the flow distribution profile, the sub-channel area-averaged gas and liquid volumetric fluxes have been given. Finally, the correlation developed for pipe flow is adopted for each sub-channel based on the local two-phase parameters in the rod bundle. Compared to experimental data obtained in-house and found in the literature, the newly proposed approach can predict rod film thickness with satisfactory accuracy.

Critical heat flux model on a downward facing surface for application to the IVR conditions

A mechanistic model, which is widely applicable to the flow boiling regime, is developed to predict the critical heat flux (CHF) in the In-Vessel Retention (IVR) configuration. The model is based on five general equations describing the CHF mechanism and four primary CHF variables: vapor velocity, liquid velocity, microlayer thickness and the slug length. The CHF mechanism of liquid film dryout underneath the slug is considered. Velocities of vapor and liquid are determined by the Karman velocity distribution and the force balance between buoyancy and drag force. The microlayer thickness is defined by Cheung and Haddad (Cheung and Haddad, 1997) model, based on the Helmholtz instability for the vapor stem located in the microlayer. The slug length is postulated to be the critical Helmholtz wavelength. The solution is numerically obtained starting from seven scattered input parameters: mass flux, local quality, pressure, inclination angle, gap size, working fluid and heater material. In the model, four unknown constants are used for the premature CHF due to heater deformation, the minimum length of slug and the approximation of microlayer thickness and liquid velocity. To find the best-fitted values of the unknown constants, the URANIE code, developed by Commissariat à l'Energie Atomique (CEA), is used. The CHF predicted by the new model is compared with the integrated IVR-CHF database, including experimental data from KAIST, CEA (SULTAN experiments), UCSB (ULPU experiments) and MIT. Starting from 278 experimental data points, the CHF model is affected by a root-mean-square (RMS) error of 14.8%. The CHF predicted by the model is in good agreement with the experimental IVR-CHF database, except for the condition of high mass flux conditions (>500 kg/m2 s) and low inclination angle (<16°). Further improvement of the model is suggested to cover this range and reduce the RMS error based on future worldwide experimental series.

Prediction models of critical heat flux for vertical downward flow

Critical heat flux is one of the most important key parameters for various boiling systems, such as nuclear reactors and conventional boilers. Thus, many investigations have been reported, but most of these are concerned with the upward flow condition, and the phenomena under downward flow condition have not been fully understood. In this series of studies, the critical heat flux under the downward flow condition was classified into four mechanisms, i.e. dryout of falling liquid film, critical heat flux caused by the flooding, critical heat flux caused by the hydraulic instability and the liquid film dryout in annular flow regime. In addition, evaluation models of CHF were presented for each mechanism. Although these explanations showed the features of CHF under downward flow condition adequately, they were not suitable to explain the characteristics of CHF caused by the hydraulic instability. In this investigation, CHF of this region was formulated by two different models based on the comparison of the data in the experimental results obtained by using the tube with heated length of 200 mm and that of 400 mm. As a result, the model based on the Kelvin-Helmholtz instability was found to be suitable for relatively low mass flux condition. Under relatively high mass flux condition, the modified model of the departure from nucleate boiling (DNB) is proposed. These models show good agreement with the experimental results.

Dryout quality prediction for boiling two-phase flow in vertical helically coiled tubes

An analytical model for the prediction of the onset of dryout quality in annular flow regime in helically coiled tubes is present in this paper. According to the liquid film dryout mechanism, film dryout is primarily dependent on the competition between droplet entrainment and deposition with a certain heat flux. To this effect, it is necessary to define the net entrainment rate in the helical tube. The factors significantly affecting the droplet entrainment and deposition characteristics in the helical tube are analyzed theoretically. Three dimensionless numbers are used to modify the net entrainment rate in straight tube. A total of 252 experimental points from 6 different datasets are used to test the performance of the model. Comparisons between the calculated and experimental results in literatures indicate that the present model has good predictive capability in predicting onset of dryout in helically coiled tubes.

Dryout-type critical heat flux in vertical upward annular flow: effects of entrainment rate, initial entrained fraction and diameter

This study aims to reveal the effects of liquid entrainment, initial entrained fraction and tube diameter on liquid film dryout in vertical upward annular flow for flow boiling. Entrainment and deposition rates of droplets were included in mass conservation equations to estimate the local liquid film mass flux in annular flow, and the critical vapor quality at dryout conditions. Different entrainment rate correlations were evaluated using flow boiling data of water and organic liquids including n-pentane, iso-octane and R134a. Effect of the initial entrained fraction (IEF) at the churn-to-annular flow transition was also investigated. A transitional Boiling number was proposed to separate the IEF-sensitive region at high Boiling numbers and the IEF-insensitive region at low Boiling numbers. Besides, the diameter effect on dryout vapor quality was studied. The dryout vapor quality increases with decreasing tube diameter. It needs to be pointed out that the dryout characteristics of submillimeter channels might be different because of different mechanisms of dryout, i.e., drying of liquid film underneath long vapor slugs and flow boiling instabilities.

The effects of transient conditions on the onset of intermittent dryout during blowdown

For a given set of conditions in a boiling system the point of liquid film dryout or departure from nucleate boiling corresponds to the change from convective or nucleate boiling to transition or film boiling. This change is associated with a rapid deterioration of the heat transfer coefficient and the heat flux at this transition is denoted the critical heat flux (CHF). Computer models used to predict station transients and CHF rely heavily on empirical correlations to predict the CHF. Liquid film CHF data are usually obtained using a quasi-steady method wherein the heat flux is incremented in small steps with each step being allowed to reach a new equilibrium until an abnormal temperature increase is detected on the experimental surfaces. In applying a correlation derived from steady-state experiments to transient analyses these codes implicitly assume that dryout will occur for the same local conditions during transients as during steady state conditions. There is some disagreement in literature as to the validity of this hypothesis. This paper provides new steady-state and transient experimental data for CHF in water at intermediate pressures and demonstrates that for high-quality CHF that indeed CHF occurs at comparable local conditions during transients and steady-sate.