Elsevier

Nuclear Engineering and Design

Volume 238, Issue 9, September 2008, Pages 2460-2467
Nuclear Engineering and Design

Experimental and theoretical study on transition boiling concerning downward-facing horizontal surface in confined space

https://doi.org/10.1016/j.nucengdes.2008.02.015Get rights and content

Abstract

Experimental study has been conducted to examine the pool boiling occurs on a relative large downward-facing round surface with a diameter of 300 mm in confined water pool at atmospheric pressure. An artificial neural network (ANN) has been trained successfully based on the experimental data for predicting Nusselt number of transition boiling in the present study. The input parameters of the ANN are wall superheat, ΔTw, the ratio of the gap size to the diameter of the heated surface, δ/D, Prandtl number and Rayleigh number. The output is Nusselt number, Nu. The results show that: Nu decreases with increasing ΔTw, and increases generally with an increase of δ/D. Nu increases with increasing Pr when gap size is smaller than 4.0 mm. And Nu decreases initially and then increases with increasing Pr as gap size bigger than 5.0 mm. The results also indicate that the influence of Grashof number, Gr, could be negligible. Finally, a new correlation was proposed to predict the transition boiling heat transfer under the present condition. The comparisons between the prediction of the new correlation and experimental data show a reasonable agreement.

Introduction

During the TMI-2 severe accident, there were about 19 tons of molten core materials which relocated to the bottom of the lower plenum, and the dryout phenomenon occurred on the inner surface of the lower head of the reactor pressure vessel (RPV) where the temperature of the hot spot was estimated to be approximately 1100 °C for about 30 min. All severe accident analysis codes indicate that the RPV would be damaged under such high temperature condition. However, the RPV in the TMI-2 accident received no damage and the lower head kept its integrity even without the outside cooling of the pressure vessel wall. One proposed scenario is that the gap-cooling mechanism exists between the low head and molten corium (Henry and Fauske, 1993). According to the scenario, water or vapor penetrates into the gap or crack of the molten corium and offer the inherent cooling ability. Based on this knowledge, the strategy of the severe accident management concept well known as in-vessel retention is to flood the reactor cavity and submerge the RPV. Therefore, it will be very significant to investigate the characteristics of transition boiling heat transfer on downward-facing surface in confined space.

The transition boiling, a heat transfer mode commonly observed in high heat flux system with high wall superheat, is important to industrial applications such as the safety analysis of nuclear reactors and metallurgical processing. Research on transition boiling also lead to a broad understanding on nucleation and film boiling. Transition boiling heat transfer is very sensitive to several parameters including surface roughness, wettability and substrate thermal properties. However, it is difficult to observe visually the physical phenomena occurring at the interface between fluid and heating surface.

Literature review indicates that only a few theoretical studies have been conducted on transition boiling. Haa and Nob (1998) supposed the principal mechanism of transition boiling heat transfer is pure nucleate boiling which occurs over the wetted area in transition region and estimated the fractions of wetted area on the heating surface using wall superheat. They developed a theoretical model for transition boiling heat transfer using active site density and bubble departure diameter, etc. However, it is difficult to measure those parameters during transition boiling. In the study of Zhao et al. (2002), an analytical model was developed based on the microlayer model to predict the heat transfer characteristics for transient saturated pool boiling and theoretically examined the mechanism of transition from the non-boiling regime to film boiling. Chai and Peng (2005) claimed that dry patches play important roles during the whole transition boiling processes based on their previous experiment (Peng et al., 1992). Interactions between multiple dry patches with different sizes were analyzed in statistical way to make a renewed effort to the understanding of transition boiling. The effects of parameters on transition boiling heat transfer were seldom analyzed systematically. The existing understanding cannot provide a consistent physical basis for description of the contribution of various mechanisms during transition boiling.

In case of a downward-facing heated surface with small gap, some new phenomenon in the experiment was observed. Qiu and Dhir (2002) have studied sliding bubble dynamics and associated flow pattern and heat transfer over a downward-facing inclined heater surface in the subcooled and nearly saturated high performance liquid PF-5060. It was found that heat transfer could be significantly enhanced by the motion of the bubbles sliding along the heater surface during boiling.

In the present study, the pool boiling experiments have been conducted to examine the boiling process on the downward-facing heated surface under different gap sizes conditions. ANN method is developed to predict Nusselt number of transition boiling on the downward-facing heated surface under different gap sizes conditions in this study. And the effects of key parameters on transition boiling heat transfer are discussed by using the ANN. A correlation used to accurately predict the transition boiling heat transfer under the present condition is obtained which shows a good agreement with the experimental data.

Section snippets

Experimental apparatus

The experiment was performed in a thin water pool under atmospheric pressure condition. The experimental apparatus (Fig. 1) is consisted of the experimental section, the data acquisition system and auxiliary system used to control the level of the working fluid. The test section is composed of a circular stainless steel block with 300 mm in diameter, 30 mm in thickness and is located at the center of the acrylic box. The acrylic box, 900 mm in length, 750 mm in width, 150 mm in height, is far bigger

Data reduction

The transition boiling heat transfer in the present research is the post-critical heat flux (CHF) heat transfer region. The experimental data is reduced as follows: the bottom wall surface temperatures could be calculated according to the three temperatures measured in the vertical direction of the heated block by using the Fourier conduction law.

Fig. 3 shows the boiling curves with downward-facing heated surface at atmospheric pressure under different gap size conditions. In Fig. 3, qCHF,ZUBER

Analysis based on ANN

The ANN is capable to learn complex relationships from a set of associated input–outputs. It has been applied to a wide variety of different areas such as prediction, approximation, modeling, classification, etc. In the theoretical study of critical heat flux, the parametric trends of CHF were obtained by trained ANNs (Moon et al., 1996, Su et al., 2002, Vaziri et al., 2007). Zhang et al. (2006) proposed a simple, nondimensional CHF correlation by the application of the ANN and parametric trend

New correlation for transition boiling

The effects of parameters on heat transfer mentioned in the above section indicate that the wall superheat, nondimensionalized gap size and Prandtl number have significant effects on the Nusselt number, and the effect of Grashof number could be neglected in the present study. Supposing Nusselt number is in the derivation of power equation of the general form Nu = CTw)a(δ/D)bPrc, this equation is then transformed by taking its logarithm to yield log Nu = log C + a log(ΔTw) + b log(δ/D) + c log Pr. Thus, the

Conclusions

Experimental investigation on boiling heat transfer from a downward-facing horizontal heated surface in confined water pool have been carried out at atmospheric pressure. The boiling curves under different gap size conditions are obtained. The CHF and wall heat flux in transition boiling region presented in this study are far smaller than those of the upward-facing heated surface condition. An ANN for predicting Nusselt number is trained based on the experimental data with a mean of squared

Acknowledgements

This work is supported by program for New Century Excellent Talents in University (NCET-06-0837). And this work is also financially supported by the Japan Society for the Promotion of Science.

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