Critical Heat Flux Enhancement

CRITICAL HEAT FLUX ENHANCEMENT SOON HEUNG CHANG*, YONG HOON JEONG and BYUNG SOO SHIN Department of Nuclear & Quantum Engineering Korea Advanced Institute of Science and Technology 373-1 Guseong-dong, Yuseong-gu, Daejeon, 305-701, Korea * Corresponding author. E-mail : [email protected] Received November 22, 2006

In this paper, works related to enhancement of the CHF are reviewed in terms of fundamental mechanisms and practical applications. Studies on CHF enhancement in forced convection are divided into two categories, CHF enhancement of internal flow in tubes and enhancement of CHF in the nuclear fuel bundle. Methods of enhancing the CHF of internal flows in tubes include enhancement of the swirl flow using twisted tapes, a helical coil, and a grooved surface; promotion of flow mixing using a hypervapotron; altering the characteristics of the heated surface using porous coatings and nano-fluids; and changing the surface tension of the fluid using additives such as surfactants. In the fuel bundle, mixing vanes or wire wrapped rods can be employed to enhance the CHF by changing the flow distributions. These methods can be applied to practical heat exchange systems such as nuclear reactors, fossil boilers, fusion reactors, etc. KEYWORDS : CHF Enhancement, Swirl Flow, Porous Coating, Surfactant, Nano-fluid, Mixing Vane, Wire Wraps

1. INTRODUCTION Critical heat flux (CHF) is the heat flux at which a boiling crisis occurs accompanied by a sudden increase of the heat transfer surface or deterioration of the heat transfer rate. The CHF imposes a limit in designing and operating boiling heat transfer equipment in power industries such as nuclear, fusion, and fossil power plants. Thus, an increase in the critical heat flux can increase the safety margins and allows for more economical design and operation at higher heat fluxes. As the capacity of nuclear power plants or fossil boilers increases, a higher heat load per unit heat transfer area is required. In fusion reactors, the maximum heat flux may be higher than 10 MW/m2 and special heat transfer measures are necessary. The most effective heat transfer mechanism is nucleate boiling, the upper boundary of which is dictated by the CHF. Therefore, efforts have been exerted to expand the regime of nucleate boiling. In this regard, enhancement of the CHF is an important topic in the area of heat transfer research and a considerable body of related work has been reported in the literature. Studies on CHF enhancement in forced convection can be divided into two categories, enhancement of the internal flow in tubes and enhancement of the CHF in nuclear fuel bundles. The main focus in fossil boiler and steam generator design has been CHF enhancement of the internal flow. Recently, CHF enhancement for effective cooling of high

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heat flux fusion equipment such as divertors has been attracting greater interest. Research on CHF enhancement in natural convection is mainly concentrated on the design of severe accident mitigation devices and strategies, such as in-vessel retention through external reactor vessel cooling. For practical applications such as nuclear reactors, fossil boilers, and fusion reactors, fundamental mechanisms and practical applications of the CHF have been studied. In the present paper, methods to enhance the CHF are introduced with a focus on fundamental mechanisms and practical applications.

2. CHF ENHANCEMENT IN AN INTERNAL FLOW 2.1 CHF Enhancement by Enhancing the Swirl Flow When the flow in a tube is directed by twisted tape, a helical coil or a grooved surface (rifled tube), a swirl flow can be generated. By the centrifugal force generated by the swirling motion of the fluid, liquid is forced to flow on the tube surface and vapor is forced to flow along center line of the tube. Consequently, local velocity of the flow is increased compared to that in a smooth tube under the same mass flux. Therefore, the swirl motion of the flow can contribute to enhancement of the CHF. Several methods to this end have been developed commencing in the 1950s [1]. Among these are insertion of twisted tape, insertion

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of a helical coil, and grooving the tube surface. Swirl flow generation by the insertion of twisted tape or twisted ribbon has been widely studied. The inserted twisted tape acts as a flow obstacle and increases the pressure loss. In order to apply the same mass flux, the pumping power should be increased. However, even at the same pumping power, the CHF obtained with twisted tape may be higher than that without tape having an optimized design. Notably, if the heat flux is concentrated at a certain point of the tube, the swirl tape can be placed at that point. Gambill et al. [1,2] were the first to study the effect of flow swirling on heat transfer and its effect on the CHF. They obtained high CHFs, 118 MW/m2 for water and 28 MW/m2 for ethylene glycol. From the mid-1980s a new approach to swirl flow boiling was introduced in connection with the problem of cooling the plasma facing components (PFC) of fusion reactors. Divertors can be subjected to extremely high energy fluxes, around 10-20 MW/m2. With the exception of the study of Gambill et al. [1,2], most early studies investigated CHF enhancement under saturated flow conditions (see Table 1). Later, CHF experiments under a subcooled boiling region were performed [3,4]. CHF experiments for one-sided(?) heating were also performed to investigate the effect of a one-sided(?) heat load in a PFS such as a divertor in fusion reactors [5]. Schlosser et al. [6,7] adapted the Tong-75 correlation [8] to fit their one-sided heat flux CHF database for smooth tubes, swirl tape, and hypervapotrons [6,7]. The correlation is as follows:

where where CHFw is the CHF at the tube wall (W/m2); f is friction factor; G is the coolant mass flux (kg/m2s); T is the local coolant temperature (°C); P is the local coolant pressure (MPa); Tsat is the saturation temperature (°C); Hfg is the latent heat of vaporisation of water at Tsat (J/kg); Pc is the critical pressure, 22.1MPa; Re is Reynolds number; dh is the hydraulic diameter (m); f is the water viscosity (kg/m s); Ja is Jakob’s(?) number; f is the water density (kg/m3); 3 g is the vapor density (kg/m ); cp is the water specific heat (J/kg-K); d0 is a reference diameter 12.7E-3 m; and Cf is a factor to account for the configuration. The exact value of Cf is dependent on the channel configuration and dimensions. From previous experimental data, 1.2 is appropriate for a smooth channel and 1.67 for a swirl tape tube with a twist ratio of 2 and swirl tape thickness of 0.8 mm. Swirl flow can be promoted by the insertion of helical wires. In early stage of helical wire inserted research, helical wires were used as turbulence promoters to enhance heat transfer in a single-phase flow (air, water, water glycerol solution, oil) both in laminar and in turbulent flows [12-16]. Only a few works related to the enhancement of the CHF in flow boiling can be found in the literature. Experimental CHF results on the uniform circumferential heating of a helical wire inserted tube shows very promising CHF performance (up to a 50% increase of the CHF), and in some cases the CHF surpasses that of a swirl tape inserted tube [17,18]. It has been widely reported many the literature that a

(1)

Table 1. Experimental Work on CHF Enhancement with Twisted Tape Source

L [m]

D [mm]

y

P [kPa]

G [kg/m2s]

X

Gambill et al. [2]

0.0381-0.442

3.45-10.25

4.2-24

172-5020

4500-47500

Subcooled-0.17

Viskanta [9]

0.457

8.0

2.5, 5.0

13800

670-2700

0.01-0.53

Moeck et al. [10]

1.016

11.43

5.55, 34.5

7000

390-1150

0.74-0.95

Matzer et al. [11]

4.88

10.16

15.0

6900

1260-4600

0.33-0.88

Araki et al. [3]

0.5

7.0

2.0, 2.5

< 1100

4300-12900

< 0.0

Nariai et al. [4]

0.1

6.0

> 2.6

100-1500

6000-17000

< 0.0

Boscary et al. [5]

0.085-0.105

10-18

2

1000-3600

5000-16000

< 0.0

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CHANG et al., Critical Heat Flux Enhancement

rifled tube yields significant improvement in heat transfer and CHF [19-29]. A rifled tube is a tube whose inner surface is machined like that of a nut. It can be fabricated on the basis of a simple mechanical drawing. The nut-like (grooved or rifled) surface can work as an enlarged heat transfer area and a swirl promoter, thereby enhancing heat transfer and the CHF. The improvement in the CHF is attributed to centrifugal fluid flow near the surface arising from the swirl flow. The centrifugal flow forces the water to the tube wall, retards re-entrainment of the liquid, and causes drift in the radial direction, which carries the vapor to the center region. Steam blanketing and film dryout at CHF conditions are thus prevented until substantially higher steam qualities are reached. The strength of the swirl flow can be quantified by the ratio of centrifugal acceleration to gravity:

(2)

where ar is the centrifugal acceleration, Vr is the radial velocity on the wall, R is the inside radius of the tube, Va is the axial velocity, and is the angle between the center line of the tube and the rifle or groove on the inner tube wall. The relationship between the acceleration ratio and the rifle angle is displayed in Fig. 1. Major applications of rifled tubes include coal fired boilers and fusion divertors. Experimental works for boiler applications are conducted at high pressure and high quality (~22 MPa, x>1 ). The CHF in the spiral internally rifled tubes can be enhanced by a factor of 1.3–1.6 as compared

to a smooth tube [19,20,26,29]. However, the enhancement ratio depends upon the various flow conditions and geometries. Iwabuchi et al. [30] studied a four-head rifled tube with an average inner diameter of 17 mm. They found that enhancement of the CHF disappeared at pressure exceeding 20.6 MPa. Cheng et al. [26] conducted a similar study and found that the enhancement effect is apparent from 10 MPa to 19 MPa. However, there was no obvious enhancement at 21 MPa. The ratio of liquid water density to vapor density was reduced and approached 1 as the pressure approached a critical pressure (~22 MPa). With a low (~1) density ratio, the swirl flow cannot lend strong centrifugal force to the liquid water and the effect of the swirl flow on the CHF is reduced. Works for fusion divertor applications are conducted at low pressure and low quality (~ 1 MPa, x