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Fusion Engineering and Design 87 (2012) 974– 978

Contents lists available at SciVerse ScienceDirect

Fusion Engineering and Design

jo ur nal homep age : www.elsev ier .com/ locate / fusengdes

umerical investigation of heat transfer enhancement in ribbed channel for therst wall of DFLL-TBM in ITER

iang Jina,c,∗, Songlin Liub,c, Min Lia,c, Weihua Wanga,b

Institute of Nuclear Energy Safety Technology, Chinese Academy of Sciences, Hefei, Anhui, 230031, ChinaInstitute of Plasma Physics, Chinese Academy of Sciences, Hefei, Anhui, 230031, ChinaSchool of Nuclear Science and Technology, University of Science and Technology of China, Hefei, Anhui, 230027, China

r t i c l e i n f o

rticle history:vailable online 13 March 2012

a b s t r a c t

As an important component of Dual Functional Lithium Lead-Test Blanket Module (DFLL-TBM), the firstwall (FW) must withstand and remove the heat flux from the plasma (q′′ = 0.3 MW/m2) and high nuclearpower deposited in the structure at normal plasma operation scenario of ITER. In this paper the transverse

eywords:FLL-TBMirst wallransverse ribeat transfer enhancement

ribs arranged along the plasma facing inner wall surface were used to enhance the heat transfer capability.After the validation compared with empirical correlations the Standard k–ω model was employed todo the numerical simulation using FLUENT code to investigate the heat transfer efficiency and flowperformance of coolant in the ribbed channel preliminarily. The perforation on the bottom of rib wasproposed near the lower heat transfer area (LHTA) to improve the heat transfer performance accordingto results of analyses.

. Introduction

China Liquid LiPb breeder blanket for DEMO [1] was definedccording to a series of LiPb breeder blanket concepts [2–9]esigned and evaluated by FDS team. Based on definition of DEMOlanket, a DFLL-TBM [10–13] was developed by China as one of theandidate concepts which is expected to be installed and operatedn half of an ITER equatorial port.

FW structure is one of the key elements for DFLL-TBM. It isesigned to withstand the heat flux from the plasma chambernd the volumetric nuclear heat while it needs to maintain struc-ure temperature and stress below the allowable limits, and toeep DEMO-blanket FW relevant. Thus, the heat transfer capabil-ty between the coolant and FW structural is critical to ITER-TBMevelopment.

As one of the well-developed technologies in the heat transfernhancement, the transverse rib arrays are used inside the internalhannel to enhance heat transfer by restarting the boundary layerfter flow reattachment between two neighboring ribs. In ordero evaluate the thermal and fluid performance of rib design in theoolant channel of FW, more detailed analysis of local values of heat

ransfer and friction loss should be investigated. It is also usefulo anticipate the LHTA where the effect of heat transfer is contraeteriorated compared with the corresponding smooth channel.

∗ Corresponding author. Tel.: +86 551 5591397; fax: +86 551 5591397.E-mail address: jinqiang@mail.ustc.edu.cn (Q. Jin).

920-3796/$ – see front matter © 2012 Elsevier B.V. All rights reserved.oi:10.1016/j.fusengdes.2012.02.072

© 2012 Elsevier B.V. All rights reserved.

This paper presents a physical model of FW coolant channel inDFLL-TBM where the solid rib geometries design is similar to thedesign in Chandra’s work [14]. Three turbulent models in predictionof heat transfer and friction loss were validated by the empiricalcorrelations and an appropriate turbulent model was employed tomake the Computational Fluid Dynamics (CFD) modeling of fluidflow and heat transfer in the coolant channel. A perforation wasproposed to reduce the LHTA according to results of analyses.

2. Physical model

In basic version, the structure of DFLL-TBM consisted of a484(Tor.) mm × 1660(Pol.) mm × 585(Rad.) mm rectangular steelbox. As shown in Fig. 1, the FW was a U-shaped structure madeof China Low Activation Martensitic (CLAM) steel [15,16] cooled byhelium coolant gas. After the scheme optimization of the FW chan-nel, each coolant sub-circuit consisted of 5 interconnected channels(Fig. 2), i.e. the frequency of channel passing through FW was 5. Theadjoining two channels remained counter-flowing arrangement inorder to achieve a uniform temperature distribution across the FWsurface.

The physical model used in this paper is shown in Fig. 3. Consid-ering the heat flux from the plasma facing side (q′′ = 0.3 MW/m2)was highest in all the region of the FW, the ribs were arranged

(AR = 1) facing inner wall surface to obtain a larger augmentationof heat transfer coefficient with a lower friction loss. Consideringthe limitation of compute resources, the periodic boundary con-dition was used for a small section of the FW channel assumed

Q. Jin et al. / Fusion Engineering and Design 87 (2012) 974– 978 975

Nomenclature

AR aspect ratio = W/HDh hydraulic diameter (m) 15 mme rib height (m)f friction factorh heat transfer coefficient (W/(m2 K))H channel height (m) 15 mmk thermal conductivity (W/mK)L channel length of test section (m)Nu Nusselt numberp static pressure (Pa)P pitch of rib (m)Pr Prandtl numberq′′ heat flux (MW/m2)Re Reynolds numberT temperature, Kum bulk mean velocity (m/s)W channel width (m) 15 mmy+ dimensionless wall distance� density (kg/m3)� thermal enhancement factor

tctsr

3

iff

ω turbulent specific dissipation (s−1)

he heat transfer and fluid flow were fully developed. Because theharacteristics of fluid flow and heat transfer were symmetrical tohe central plane of the channel, only half of the ribbed domain washown. The height-to-hydraulic diameter ratio e/Dh was 0.0625 andib pitch-to-height ratio P/e was kept at 8.

. Data reduction

To describe the fluid flow and heat transfer performances, three

mportant dimensionless parameters of interest are presented: (1)riction factor, (2) Nusselt number, and (3) thermal enhancementactor.

Fig. 1. 3D view of the DFLL-TBM.

Fig. 2. He flow scheme in sub-circuits.

The friction factor can be defined by:

f = (�p/L)Dh

2�u2m

(1)

Heat transfer coefficient used is defined by:

h = q′′/(Tw − Tb) (2)

where Tw is the wall temperature of the channel surface adjacent tothe fluid and Tb is the fluid bulk mean temperature in the channel.

The heat transfer effect measured by local Nusselt number canbe obtained by:

Nu = hDh

k(3)

The enhancement effect on heat transfer in channels with ribs isalways accompanied by large increase of pressure drop. The ther-mal enhancement factor � is used to evaluate the enhanced heattransfer performance:

� = Nu/Nus

(f/fs)1/3

(4)

where Nus is the Nusselt number and fs is the friction factor for thefully developed flow in the smooth channel, they can be defined byDittus–Boelter correlations (3000 ≤ Re ≤ 5 × 106, 0.5 ≤ Pr ≤ 200):

Nus = (fs/2)(Re − 1000)Pr

1 + 12.7√

fs/2(Pr2/3 − 1)

fs = 0.25 × (1.82 log Re − 1.64)−2 (5)

Fig. 3. The physical model of the transverse ribs.

9 ing and Design 87 (2012) 974– 978

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smooth channel because the stronger vortex was generated byrestarting the boundary layer after flow reattachment between twoneighboring ribs, which contributed to the increase of the Nu. But

76 Q. Jin et al. / Fusion Engineer

. Numerical simulation method

The governing equations are the three-dimensional incom-ressible steady-state Reynolds-averaged Navier–Stokes equa-ions. The thermophysical properties of fluid are assumed to beonstant. Three turbulent models with the potential predictivebility including the Renormalized Group (RNG) k–ε model, theealizable k–ε model and the Standard k–ω model were used forhe closure of the Reynolds Stress terms in momentum equation.

ore details of each model can be found in Ref. [17], including theransport equations and meanings of each term in the equations.

.1. Computational procedure

Enhanced wall treatment was adopted at adjacent wall regiono resolve the laminar sub-layer. To obtain grid independenceolution, the number of cells was varied by changing the y+

alue respectively. The numerical results of Nu indicated slight-ess when the boundaries near the ribbed walls was treated with+ < 2. The governing equations were solved using the commer-ial CFD code Fluent 12.1. The SIMPLE algorithm was used for theelocity–pressure coupling. A second-order upwind scheme washosen for energy and momentum equations.

.2. Performance evaluation of turbulent models

Based on the similarity law of momentum and heat respectivelyeveloped by the Nikuradse [18] and Dipprey/Sabersky [19], it wasnalogous in the flow and temperature field if the boundary con-itions in the FW channel were the same as that in the work ofhandra et al. [14]. The empirical correlations derived by Chandrat al. [14] could be used to verify the predictive ability of the turbu-ent models. It is reasonable when the range of Reynolds number ise > 1.2 × 104.

When working substance is air, all the turbulent models under-stimate the values compared with the results from the empiricalorrelation (Fig. 4(a)). It has been observed that the relative dif-erences between the turbulent models and empirical correlationsre less than 6% and 9% for Standard k–ω model and RNG k–ωodel, respectively. The Realizable k–ε model has the most devi-

tion from empirical correlation. It has been seen that the frictionactor decreases with increasing Reynolds number due to the sup-ression of viscous sub-layer (Fig. 4(b)). But the relative deviations

n the RNG k–ε model are generally approximately 25% comparedith 15% in the Standard k–ω model. It was acceptable that the

tandard k–ω model could be employed to investigate the heatransfer and flow performance of coolant (He) in the ribbed chan-el of FW taking into account that the thermophysical propertiesf Air and He were similar.

. Numerical results and discussions

.1. Numerical results

The minimum flow velocity needed to keep the FW temperatureithin the limit value (550 ◦C) was 45 m/s (Re = 1.265 × 105). Take

his situation for example, the normalized Nu (Nu/Nus) is higherhan 1 at most part of the FW (Fig. 6(a)). The augmentation of heatransfer is significantly high on the upwind side and the top of theib at the price of higher friction factor ratio (f/fs = 6.2). But the LHTAehind the rib can also be observed (Fig. 6(b)).

A perforation had been made at the center and bottom of the

olid rib to reduce the LHTA (Fig. 5). The friction factor ratio (f/fs = 4)s reduced with the decrease in the normalized Nu at the front wallFig. 6(c)) compared with that in the solid rib. It is observed thatHTA is reduced (Fig. 6(d)). The thermal and fluid performances in

Fig. 4. Validation of results in terms of (a) Nusselt number and (b) friction factor.

the solid rib (� = 1.01) are better than that in the basic design (� = 1)and (� = 0.89) integrally.

Figs. 7 and 8 exhibit the streamlines and fluid velocity in thesymmetry plane of ribbed-channel. As shown in Figs. 7 and 8, thestronger vortex and fluid stagnation were generated by the addedsolid rib compared with by the added perforated rib.

5.2. Discussion

Above two schemes were investigated, the heat transfer couldbe significantly enhanced in ribbed channel compared with in

Fig. 5. Spanwise shape of perforated rib vertical to flow direction.

Q. Jin et al. / Fusion Engineering and Design 87 (2012) 974– 978 977

Fig. 6. Heat transfer distribution in terms of normalized Nu for the front wall (a) and (b) solid rib; (c) and (d) perforated rib.

Fig. 7. Streamlines of symmetry plane in a single pitch (a) solid rib; (b) perforatedrib.

Fig. 8. Velocity magnitude of symmetry plane in a single pitch (a) solid rib; (b)perforated rib.

the pressure drop was higher due to the increase of the frictionfactor. The LHTA appeared behind the rid due to fluid stagnation.

The transfer heat effect in the perforated rib scheme was lowerthan that in the solid rib one, but the LHTA and the pressure dropwas significantly reduced. The main reasons were the variation ofthe flow separation/reattachment point and reduction in the inten-sity of vortex so as to improve the velocity magnitude of fluid whilea part of fluid can escape the perforation.

As a compromise between transfer heat and pressure drop andLHTA, the perforated rib scheme might be suitable for heat transferenhancement in ribbed channel for the FW. However, the thermaland fluid performance for rib scheme should be studied extensivelyto obtain the optimum parameters.

6. Conclusions

In present investigation, a numerical prediction has been con-ducted to study heat transfer and flow behaviors in ribbed coolantchannel in the FW of DFLL-TBM. The heat transfer could beenhanced although higher pressure loss of the fluid flow wasobtained. After the perforation was made, the pressure drop and theLHTA were reduced at the price of heat transfer effect decreased.As next step, the thermal and fluid performance at different ribgeometry including the angle, size, and spacing should be studiedextensively to obtain the optimum parameters. The detailed ther-mal structural analysis should be investigated and analyzed afterthe rib added in the future work.

Acknowledgments

This work was supported by National Special Project for Mag-netic Confined Nuclear Fusion Energy (No. 2009GB109001); andby the National Natural Science Foundation of China (No.10975157and No. 11175207).

9 ing an

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78 Q. Jin et al. / Fusion Engineer

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