Andrea Mariani

Rationalization of existing mechanistic models for the prediction of water subcooled flow boiling critical heat flux

Abstract This paper presents an analysis of the critical heat flux (CHF) of subcooled flow boiling based on the liquid sublayer dryout mechanism, i.e. the dryout of a thin liquid layer beneath an intermittent vapour blanket due to the coalescence of small bubbles. Starting from the same basic mechanism adopted in earlier models, a new model is derived for the analysis of the CHF in subcooled flow boiling under conditions of very high mass flux and liquid subcooling, typical of fusion reactors thermal hydraulic design. The model is characterized by the absence of empirical constants always present in earlier models. Predicted CHF values are compared with data of 1888 data points for water, showing a good agreement both in precision and in accuracy.

Experimental evaluation of the onset of subcooled flow boiling at high liquid velocity and subcooling

The knowledge of the onset of subcooled boiling in water forced convective flow at high liquid velocity and subcooling is of importance in thermal hydraulic studies of high heat flux components in fusion reactors. The present paper reports the results of an experimental research on the onset of subcooled boiling in water forced convective flow. From the measurement of the pressure drop along the heated test channel (D = 8 mm, L = 100 mm) it is possible to evaluate the heat flux at which the subcooled boiling occurs. As far as the coolant is in single-phase flow, it is possible to simply calculate its pressure drop, also taking into account the temperature effect on the friction factor, for the temperature variation along the heated channel and in the cross section (due to the steep thermal gradients). The classical corrections available in the literature have been used. Once bubble formation is established, the additional, significant contribution of the bubbles presence to the pressure drop leads to a deviation of the experimental pressure drop curve from the single-phase theoretical line. This latter identifies the onset of subcooled boiling. A comparison of the experimental heat flux at the subcooled boiling incipience with that provided by the major correlations available in the literature is given. A further comparison with measurements performed with an accelerometer device is provided in the paper. The accelerometer detects the additional noise due to bubbles formation close to the heated wall and subsequent collapse in the subcooled bulk of the liquid. The results given by the accelerometer and those obtained with the present evaluation method are in close agreement.

The prediction of the critical heat flux in water-subcooled flow boiling

Abstract The prediction of water-subcooled flow boiling critical heat flux (CHF) in peripherally non-uniform heated tubes with or without swirl flow promoters is accomplished using a model based on the liquid sublayer dryout mechanism recently proposed by the authors. Peripheral nonuniform heating and/or twisted-tape inserts are properly and simply accounted for in the model, originally developed for uniform heating and straight flow. Simultaneous occurrence of the two events is also well predicted by the model. Although initially formulated for operating conditions typical of the thermal hydraulic design of fusion reactor high heat flux components, the model is proved to give a satisfactory answer for the prediction of the CHF under more general conditions, provided local thermodynamic conditions of the bulk flow at the CHF are sufficiently far from the saturated state.

A Mechanistic Model for the Prediction of Water-Subcooled-Flow-Boiling Critical Heat Flux at High Liquid Velocity and Subcooling

A new model is presented for the prediction of the critical heat flux (CHF) of subcooled flow boiling based on a liquid-sublayer dryout mechanism, i.e., the dryout of a thin, liquid layer beneath an intermittent vapor blanket due to the coalescence of small bubbles. The model focuses on the analysis of the CHF in subcooled flow boiling under conditions of very high mass flux and liquid subcooling, typical of fusion reactor thermal-hydraulic design, and is characterized by the absence of empirical constants always present in earlier models. Peripheral nonuniform heating and/or twisted-tape inserts are accounted for in the model, which was originally developed for uniform heating and straight flow. The simultaneous occurrence of the two events is also well predicted by the model. Although initially formulated for operating conditions typical of the thermal-hydraulic design of fusion reactor high-heat-flux components, the model is proven to be able to satisfactorily predict the CHF under more general conditions, provided local thermodynamic conditions of the bulk flow at the CHF are sufficiently far from the saturated state. 60 refs., 11 figs.

Physical insight in the burnout region of water-subcooled flow boiling

Abstract The present paper reports the results of a visualization study of the burnout in subcooled flow boiling of water, with square cross-section annular geometry (formed by a central heater rod contained in a duct characterised by a square cross-section). In order to obtain clear pictures of the flow phenomena, the coolant velocity is in the range 3–9 m·s−1 and the resulting heat flux is in the range 7–13 MW·m−2. From video images (single frames were taken with a light exposure of 1 μs) the following general behaviour of vapour bubbles was observed: when the rate of bubble generation is increasing, with bubbles growing in the superheated layer close to the heating wall, their coalescence produces a sort of elongated bubble called a vapour blanket. One of the main features of the vapour blanket is that it is rooted to the nucleation site on the heated surface. Bubble dimensions, as well as those of the hot spots, are given as a function of thermal-hydraulic tested conditions.

Critical Heat Flux, Post-CHF Heat Transfer and Their Augmentation

The present work reports on the state-of-the-art review on the critical heat flux and the post-dryout heat transfer. The first two sections are somewhat tutorial, and are featured in a similar way. They provide, after a brief introduction, with information on parametric trends, i.e. on the influence of the thermal-hydraulic and geometric parameters on the thermal crisis. After that, the most widely used correlations are described in detail, either in terms of reliability and simplicity of use. Eventually, the various approaches for a modelling of the critical heat flux are reported. The third section describes correlations and models available for the prediction of the post-dryout heat transfer, trying also to highlight the main drawbacks. Finally, the fourth section describes the passive techniques for the enhancement of the critical heat flux and the post-dryout heat transfer, together with available correlations. The present work is a merge of original researches carried out at the Institute of Thermal Fluid Dynamic of ENEA and a thorough review of the recent literature.

High heat flux burnout in subcooled flow boiling

The paper reports the results of an experimental research carried out at the Heat Transfer Division of the Energy Department, C.R. Casaccia, on the thermal hydraulic characterization of subcooled flow boiling CHF under typical conditions of thermonuclear fusion reactors, i.e. high liquid velocity and subcooling.The experiment was carried out exploring the following parameters: channel diameter (from 2.5 to 8.0 mm), heated length (10 and 15 cm), liquid velocity (from 2 to 40 m/s), exit pressure (from atmospheric to 5.0 MPa), inlet temperature (from 30 to 80 °C), channel orientation (vertical and horizontal). A maximum CHF value of 60.6 MW/m2 has been obtained under the following conditions:Tin=30°,p=2.5 MPa,u=40 m/s,D=2.5 mm (smooth channel)Turbulence promoters (helically coiled wires) have been employed to further enhance the CHF attainable with subcooled flow boiling. Helically coiled wires allow an increase of 50% of the maximum CHF obtained with smooth channels.

Visual Investigation of Boiling Phenomena in CHF Subcooled Flow Boiling

The present work collects the main results obtained in an experimental research carried out at the laboratories of the National Institute of Thermal-Fluid Dynamics of ENEA. A photographic study of the burnout in highly subcooled flow boiling has been performed, in order to get a detailed description of the flow pattern under different conditions of boiling regime: ONB (onset of nucleate boiling), subcooled flow boiling and thermal crisis. In particular, the flow visualisation is focused on the phenomena occurring on the heated wall during the thermal crisis up to the physical burnout of the heater.