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1

Determination of dryout localization using a five-equation model of annular flow for boiling in minichannels

Wajs J., Mikielewicz D.

Archives of Thermodynamics

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2017

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Vol. 38, no. 1

123--139

EN

Detailed studies have suggested that the critical heat flux in the form of dryout in minichannels occurs when the combined effects of entrainment, deposition, and evaporation of the film make the film flow rate go gradually and smoothly to zero. Most approaches so far used the mass balance equation for the liquid film with appropriate formulations for the rate of deposition and entrainment respectively. It must be acknowledged that any discrepancy in determination of deposition and entrainment rates, together with cross-correlations between them, leads to the loss of accuracy of model predictions. Conservation equations relating the primary parameters are established for the liquid film and vapor core. The model consists of three mass balance equations, for liquid in the film as well as two-phase core and the gas phase itself. These equations are supplemented by the corresponding momentum equations for liquid in the film and the two-phase core. Applicability of the model has been tested on some experimental data.

2

Modeling of liquid film flow in annuli

Anglart H.

Journal of Power Technologies

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2014

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Vol. 94, nr 5

8--15

EN

One of the challenges in thermal-hydraulic analyses of BWRs is correct prediction of dryout occurrence in fuel assemblies. In practical applications the critical powers in fuel assemblies are found from correlations that are based on experimental data. The drawback of this approach is that correlations are valid only for these fuel assemblies on which the experiments have been conducted. Other restrictive factors are the limited ranges of experimental working conditions including pressure, mass flux and axial power distributions. To overcome the above-mentioned limitations, several different approaches have been proposed to predict the dryout occurrence. One of them is to employ a phenomenological model of annular flow, in which the mass transfer between the liquid film and the gas core is based on entrainment and deposition correlations. Most of these correlations are derived from water-air flows in vertical tubes and their applicability to other geometries in general, and rod-bundles in particular, should be analysed. This paper presents an analysis of the entrainment rate in vertical annuli. Using the standard approach to calculate the entrainment rate, one can demonstrate that the results deviate from measurements. It has been shown that modifying the entrainment correlation based on data obtained in the annulus geometry leads to an essential improvement in the predictive capability of the phenomenological model of annular two-phase flow.

3

Dry patch formation in diabatic annular two-phase flows

Anglart H.

Journal of Power Technologies

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2014

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Vol. 94, nr 5

85--95

EN

Conditions for the formation of a stable dry patch in vertical annular two-phase flows in heated channels are investigated. An analytical model of the force balance for the leading edge of the liquid film is developed. In addition to surface tension, evaporation thrust and capillary forces, the model includes the effect of turbulence, the pressure gradient and the interfacial shear stress. Numerical evaluations are performed to validate the model and to indicate the importance of various factors on the dry patch stability and on the resulting minimum wetting rate of the liquid film. The analyses indicate that good agreement with measurements is obtained in the case of a stagnant patch formed on liquid film flowing down a vertical surface. It is shown that for low and moderate mass flow rate of the gas phase in vertical co-current annular flow, the force balance is dominated by the stagnation and the shear stress forces. With growing mass flow rate of the gas phase, the pressure gradient and the interfacial shear stress become increasingly important. As a result, in accordance with measurements, the predicted minimum flow rate of the liquid film at which the patch is re-wetted decreases.

4

Experimental and theoretical study of dryout in annular flow in small diameter channels

Mikielewicz D., Gliński M., Wajs J.

Archives of Thermodynamics

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2011

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Vol. 32, no. 1

89-109

EN

In the paper the experimental analysis of dryout in small diameter channels is presented. The investigations were carried out in vertical pipes of internal diameter equal to 1.15 mm and 2.3 mm. Low-boiling point fluids such as SES36 and R123 were examined. The modern experimental techniques were applied to record liquid film dryout on the wall, among the others the infrared camera. On the basis of experimental data an empirical correlation for predictions of critical heat flux was proposed. It shows a good agreement with experimental data within the error band of 30%. Additionally, a unique approach to liquid film dryout modeling in annular flow was presented. It led to the development of the three-equation model based on consideration of liquid mass balance in the film, a two-phase mixture in the core and gas. The results of experimental validation of the model exhibit improvement in comparison to other models from literature.

5

Dryout in annular flow - theoretical analysis

Mikielewicz D., Wajs J., Gliński M., Zrooga A.

Archives of Thermodynamics

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2009

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Vol. 30, no. 2

77-90

EN

In the paper a two-equation model of mass balance of liquid in the film and in the core for calculation of dryout at high vapour quality is modified from the earlier version presented by Sedler and Mikielewicz [1]. Additionally, the mass balance equation for the liquid film contains a modified evaporation term. Modified is the heat flux density applied in evaporation of liquid film which now is variable within film thickness. The solution of such equations requires prior knowledge of rates of deposition and entrainment. The theoretical determination of there quantities involves serious difficulties. The relations for determination of deposition and entrainment rate are taken therefore from Okawa et al. [3], The results of calculations are validated against experimental data of Sedler and Mikielewicz [1], showing a satisfactory consistency.

6

Dryout predictions for liquid boiling in a thin hemispherical porous layer

Evdokimov I. A., Khoruzhii O. V., Likhanskii V. V.

Zeszyty Naukowe Politechniki Świętokrzyskiej. Mechanika

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1999

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Z. 67

205--214

EN

During TMI-2 accident a gap was formed between reactor vessel bottom and core debris bed. This gap is thought to provide high cooling rates. In the present paper additional heat removal mechanism due to gap is modeled as boiling of coolant in a thin hemispherical heated porous layer. Corresponding dryout power is found as a function of debris bed permeability and material properties of liquid and its vapor. Conditions are assessed at which heat removal by boiling in the gap is noticeable and should be taken into account in analysis of debris cooling inside reactor vessel.