TAILIEUCHUNG - Two Phase Flow Phase Change and Numerical Modeling Part 15

Tham khảo tài liệu 'two phase flow phase change and numerical modeling part 15', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Nanofluids for Heat Transfer 409 Fig. 9. 3m circuit using a voltage divider and a dual phase DSP lock-in amplifier SD like model 7265 from Signal Recovery. by XT2M t insensitive to the influence of ỗTdc t . Using the t t usual approximation and introducing A ựa 2íC the thermal length of the fluid it can be shown Hadaoui 2010 that CT L qmax h Ỉ a .1 M_. qmax Fix í a 1 I _ r . zM-n ỖT2U t Ko -7-i2 cos 2m t - Ko -7-i2 sin 2o t 27 4nk L A -I 4nk L A -I XT m cos 2Mt 5Í2M sin 2Mt where p and A denote respectively the real and the imaginary part and Ko is the modified Bessel function. The formatting 27 is not easy to use to analyze experimental datas. Using the following approximation lim Ko x A ln2 Y in ln x 28 x o 0 4 it is possible to propose more suitable expressions of 5Í2M and ÕT2M as long as A a r-i-01 qmax 1 2a 1 T nk Ulna2 Y m 29 sTq qma 30 - 16k 30 To check the accuracy of the approximate expressions 29 and 30 we have gathered in Table 5 the values of A at room temperature for some common materials and excitation frequencies. As we can see in this Table in the case of most of the liquids here water and glycerol it is not possible to use 29 and 30 for excitation frequencies greater than 1 Hz. This is the main limitation of this technique for the thermal characterization of nanofluids because on the one hand low excitation frequencies require very stable external conditions and on the other hand 410 Two Phase Flow Phase Change and Numerical Modeling the measurements take a long time allowing to the convection and to losses due to electrical contacts to occur. materials water air glycerol silicon platinum V0 10 1 Hz A 335 3981 271 8433 4469 i- 0 100 Hz A 106 1259 86 2667 1413 i- 0 101 Hz A 33 398 27 843 447 V0 102 Hz A 10 6 126 8 6 267 141 Table 5. Values of the thermal length A in pm at RT for some common materials and usual excitation frequencies. Measurements As in the case of the Wheatstone bridge configuration the voltage divider Fig. 9 must be first .

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