Recent achievements in the thermal hydraulics of high heat flux components in fusion reactors

Some components of fusion thermonuclear reactors, such as divertors, plasma limiters, or first-wall armor, are believed to be subjected to operating conditions characterized by extremely high thermal loads. It is therefore necessary to remove from the surface of these components very high heat fluxes, ranging from 2 to 60 MW/m². Water subcooled flow boiling, under conditions of high mass flux, high liquid subcooling, and small to intermediate channel diameter, can accomodate these very high heat fluxes. Further enhancement of the upper limit of cooling, the critical heat flux (CHF), can be obtained by making use of turbulence promoters such as twisted tapes and coiled wires even if coupled with a relevant increase in pressure drop. An overview is presented of recent achievements obtained in water subcooled flow boiling CHF under operating conditions of interest to the thermal hydraulic design of fusion reactors. Observed basic parametric trends—CHF as a function of mass flux, pressure, subcooling, and channel geometry—are outlined, together with findings on the use of CHF enhancement techniques. From experiments it was seen that water subcooled flow boiling allows CHF conditions as high as 228 MW/m² to be achieved under extreme geometric and thermal hydraulic conditions. On the other hand, design and engineering boundary conditions limit variation in these conditions, and a suitable compromise has not yet been reached. Predictive tools are presented for the evaluation of subcooled flow boiling CHF both in straight tubes and with twisted tapes, and are assessed with reference to recent available experimental data.

Although several indications for practical applications can be found in recent achievements, a full understanding of the basic mechanisms of heat transfer and CHF in subcooled flow boiling has not yet been achieved. Future research to overcome the present lack of knowledge in this field is suggested.     

Enhancement of the critical heat flux in saturated pool boiling using honeycomb porous media

Enhancement of the critical heat flux in pool boiling by the attachment of a honeycomb-structured porous plate on a heated surface is investigated experimentally using water under saturated boiling conditions. As the height of the honeycomb porous plate on the heated surface decreases, the CHF increases to 2.5 MW/m², which is approximately 2.5 times that of a plain surface (1.0 MW/m²). Automatic liquid supply due to capillary action and reduction of the flow resistance for vapor escape due to the separation of liquid and vapor flow paths by the honeycomb-structure are verified to play an important role in the enhancement of the CHF. A simplified one-dimensional model for the capillary suction limit, in which the pressure drops due to liquid and vapor flow in the honeycomb porous plate balances the capillary force, is applied to predict the CHF. The calculated results are compared with the measured results.     

Macro-to-microchannel transition in two-phase flow: Part 2 - Flow boiling heat transfer and critical heat flux

This part of the paper presents the current experimental flow boiling heat transfer and CHF data acquired for R134a, R236fa and R245fa in single, horizontal channels of 1.03, 2.20 and 3.04 mm diameters over a range of experimental conditions. The aim of this study is to investigate the effects of channel confinement, heat flux, flow pattern, saturation temperature, subcooling and working fluid properties on the two-phase heat transfer and CHF. Experimentally, it was observed that the flow boiling heat transfer coefficients are a significant function of the type of two-phase flow pattern. Furthermore, the monotonically increasing heat transfer coefficients at higher vapor qualities, corresponding to annular flow, signifies convective boiling as the dominant heat transfer mechanism in these small scale channels. The decreasing heat transfer trend at low vapor qualities in the slug flow (coalescing bubble dominated regime) was indicative of thin film evaporation with intermittent dry patch formation and rewetting at these conditions. The coalescing bubble flow heat transfer data were well predicted by the three-zone model when setting the dryout thickness to the measured surface roughness, indicating for the first time a roughness effect on the flow boiling heat transfer coefficient in this regime. The CHF data acquired during the experimental campaign indicated the influence of saturation temperature, mass velocity, channel confinement and fluid properties on CHF but no influence of inlet subcooling for the conditions tested. When globally comparing the CHF values for R134a in the 0.51-3.04 mm diameter channels, a peak in CHF peak was observed lying in between the 0.79 (Co ≈ 0.99) and 1.03 (Co ≈ 0.78) mm channels. A new CHF correlation has been proposed involving the confinement number, Co that is able to predict CHF for R134a, R236fa and R245fa in single-circular channels, rectangular multichannels and split flow rectangular multichannels. In summary, the present flow boiling and CHF trends point to a macro-to-microscale transition as indicated by the results presented in Ong and Thome (2011) [1].     

Nucleate boiling and critical heat flux of HFE-7100 in horizontal narrow spaces

The pool boiling heat transfer and critical heat flux CHF of saturated HFE-7100 at atmospheric pressure on a confined smooth copper surface were experimentally studied. The horizontal upward boiling surface was confined by a face-to-face parallel unheated surface. We analysed the effects obtained by changing the diameter of the unheated surface and the gap between the boiling surface and the adiabatic surface. The gap values investigated were s = 0.5, 1.0, 2.0, 3.5 mm. To confine the circular boiling surface (d = 30 mm), two different Plexiglas discs were used: one with a diameter D = 30 mm, equal to that of the copper boiling surface, and the other with a diameter D = 60 mm, equal to that of the overall test section support. For each configuration, boiling curves were obtained up to the thermal crisis. For both configurations, it was observed that, at low wall superheat, the effect of confinement was not significant if Bo > 1, while for Bo ? 1 the heat transfer coefficient increased as the channel width s decreased. By contrast, at high wall superheat, a drastic reduction in both heat transfer and CHF was seen when the channel width s decreased; this reduction was less pronounced when the smaller confinement disc (D = 30 mm) was used. CHF data were also compared with the values predicted by literature correlations.     

Experimental study of critical heat flux enhancement during forced convective flow boiling of nanofluid on a short heated surface

Enhancements of nucleate boiling critical heat flux (CHF) using nanofluids in a pool boiling are well-known. Considering importance of flow boiling heat transfer in various practical applications, an experimental study on CHF enhancements of nanofluids under convective flow conditions was performed. A rectangular flow channel with 10-mm width and 5-mm height was used. A 10 mm-diameter disk-type copper surface, heated by conduction heat transfer, was placed at the bottom surface of the flow channel as a test heater. Aqueous nanofluids with alumina nanoparticles at the concentration of 0.01% by volume were investigated. The experimental results showed that the nanofluid flow boiling CHF was distinctly enhanced under the forced convective flow conditions compared to that in pure water. Subsequent to the boiling experiments, the heater surfaces were examined with scanning electron microscope and by measuring contact angle. The surface characterization results suggested that the flow boiling CHF enhancement in nanofluids is mostly caused by the nanoparticles deposition of the heater surface during vigorous boiling of nanofluids and the subsequent wettability enhancements.     

Thermal characteristics of a high heat flux micro-evaporator

Our purpose is to design a high heat flux micro-evaporator that can remove more than 100 W/cm². For this purpose a thin liquid film is evaporized. The liquid film is stabilized in micro-channels by capillary forces. The micro-channels are fabricated by chemical etching on silicon to reduce thermal resistance. For the experiments, the channel plate is heated by an ITO thin film heater deposited on the opposite side of the channel plate. Influence of heat flux, coolant flow rate, and inlet temperature on the temperature of the heater element are investigated. Water is used as working fluid. A maximal heat flux of 125 W/cm² could be achieved for water inlet temperature of 90 °C and flow rate of 1.0 mL/min. The temperature of the heater element is kept constant at about 120 °C with fluctuations within 8 °C. The measured pressure drop is less than 1000 Pa.     

Theoretical analyses on boiling critical heat flux with porous media

Porous media has been widely applied to enhance boiling heat transfer in industry, especially for increasing the value of critical heat flux (CHF). Two cases were considered in the paper: boiling within porous bed and boiling above on porous coatings. For boiling within porous bed, simplified Rayleigh–Taylor stability was analyzed and parametric effects of porous media on boiling critical heat flux were revealed. For boiling above on porous coatings, a simple new critical heat flux model was proposed basing on the analysis of liquid film stability and parametric effect of porous coatings on CHF was elaborated.     

Critical heat flux and turbulent mixing in hexagonal tight rod bundles

Experimental and theoretical investigations have been performed on critical heat flux (CHF) and turbulent mixing in tight, hexagonal, 7-rod bundles. Freon-12 was used as working fluid due to its low latent heat, low critical pressure and well known properties. It has been found that the two-phase mixing coefficient depends mainly on mass flux. It increases with decreasing mass flux and ranges from 0.01 to 0.04 for the test conditions considered. More than 900 CHF data points have been obtained in a large range of parameters: pressure 1.0–3.0 MPa and mass flux 1.0–6.0 Mg/m²s. The effect of different parameters on CHF has been analysed. It has been found that the effect of pressure, mass flux and vapour quality on CHF is similar to that observed in circular tubes. Nevertheless, the CHF in the tight rod bundle is much lower than that in a circular tube of the same equivalent hydraulic diameters. The effect of wire wraps on CHF is mainly dependent on local vapour qualities and subsequently on flow regimes. Based on subchannel flow conditions, the effect of radial power distribution on CHF is small. Comparison of the test results with CHF prediction methods underlines the need for further work.     

Experimental study on critical heat flux in bilaterally heated narrow annuli

Critical heat flux (CHF) experiments using deionized water as working fluid have been conducted in a range of pressure from 0.6 to 4.2 MPa, mass flow velocity from 60 to 130 kg/m² s and wall heat flux from 10 to 90 kW/m² for vertical narrow annuli with annular gap sizes of 0.95 and 1.5 mm. We found that the CHF, occurring only on the inside tube, or on the outside tube or on both tubes of the annular channel, depends on the heat flux ratio between surfaces of the outside and inside tubes. The CHF, occurring on the surface of the inside tube, reaches the maximum value under the pressure of 2.3 MPa while it occurring on the surface of the outside tube keeps increasing with the increase of the pressure. The CHF, occurring on the inside or outside tubes, increases with the increase of the mass flow velocity and the annular gap size; and decreases with the increase of critical quality and the other tube wall heat flux. Empirical correlations, which agree quite well with the experimental data, have been developed to predict the CHF occurring on surfaces of the inside or outside tubes of the narrow annular channel on the conditions of low pressure and low flow.     

Towards understanding the critical heat flux for industrial applications

Understanding CHF is of an upmost importance in many industries, especially in the design and operation of boilers, nuclear power plants, cryogenic systems, etc. Due to safety issues related to the nuclear power plants, and the adaptation of CHF as the limiting criterion of power generation, it is important to understand the mechanisms of CHF relevant to nuclear systems operation. Moreover, CHF is expected to occur during transients than steady-state conditions. Therefore, knowledge of transient CHF is of great importance for the safety evaluation of nuclear reactors under transient condition. In this paper, the existing CHF experimental and modeling studies are discussed in order to understand the phenomena leading to CHF. Also, the effect of transient conditions on CHF for nuclear fuels has been evaluated.