H. Kobayashi

S. Kabashima

S. Satoh

T. Okamura

National Institute for Fusion Science (NIFS-Repository)

§26. Aspect Ratio Effect of He 11-Channels on Heat Transport Characteristics Kobayashi, H. (Nihon Univ. Sci. & Tech.) Kabashima, S., Okamura, T. (Tokyo lnst. Tech. Univ.) Satoh, S. The critical heat flux q A (W I wetted surface area) at which superfluidity is broken at the cen-ter of the channel length with a heat source dis-tributed along the inner wall (see Fig.l) has been observed to be smaller than the estimated value if the channel has short and wide aspect. This geome-try effect can be explained by the fact that the temperature difference between T2' _and T2 is built across the channel gap (Fig. 2). At q A the temper-ature T 3 near the heater side reaches the A -point while T2 in the liquid on the opposite side is yet lower. This aspect ratio effect should be taken in for the estimation of stability margin of super-conducting coils cooled with Hell because a large aspect ratio d/L occurs rather in the coils. The heat transport characteristics with dis-tributed heat source have been investigated in the framework of the Gorter-Hellink theory. A modifi-cation is required since the deviation of q A from the calculated value becomes noticable as the as-pect ratio increases. Because the heat flux q (x) (W /channel cross-sectional area) in the channel next to the heated surface deP,ends on the distance x along the surface q (x) becomes Q • x /(Ac • L) where Q(W) is the total heat and Ac the cross-sec-tional area of the channel. By integrating the Gorter-Mellink heat conduction dT/dx = -f(T) q (x) 3 from T b to the A -point, q A is given by q A (Calc.) = {(L4 / 3 /4d) 3 }- 1 / 3 • Z(Tb), where Z(Tb) is the integrated heat conductivity function. A factor y = qA(Exp.)/ qA(Calc.) was observed for channels with relatively short length L compared to their width d. Assuming a linear relation with constants a and b, q A (Exp.) becomes q A (Calc.) • {a - b(d/L)} Thus, q A is not proportional to the channel gap width d. As seen in Fig.3, the equivalent aspect ratio (d/L).Q as a function of d/L indicates how many portion of the actual channel size contrib-utes to the heat transport. I< L C)l j_l f.lT2 I d • .T2' • Tl r Yr 11 r f'r 11 r r r 1 Tb a a a d/L ; Aspect Ratio Fig. I. Channel arragment and the positions of· thermometers. 2 2 d=1.5mm L=25mm N"' s (.) ~ 1 1 c:r ~,, 1.8 2.0 2.2 P.6L.L L...I..--L1...L-.8~2--L.O....L.....L-2.JII.J.2 l. T2' [K] T2 [K] !Fig.2. Temperature changes at T2' and T2 as a !function of heat flux. 0.2 0" Q) ~ -;a 0.1 y--; ____ __ / / 0//\ ...... < / 0 ~ / //~(diL)eq=(d/L){ t-1.3(d/L)} ~ -0 0.1 0.2 0.3 d/L Fig.3. Equivalent aspect ratio (d/L)eq. 31 

Aspect Ratio Effect of He II-Channels on Heat Transport Characteristics

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