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Heat Transfer Theoretical Analysis Experimental Investigations and Industrial Systems part 13

Tham khảo tài liệu 'heat transfer theoretical analysis experimental investigations and industrial systems part 13', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 470 Heat Transfer - Theoretical Analysis Experimental Investigations and Industrial Systems Approximating this equation by a finite differential equation the typical temperature drop over lx 0.9ẦW can be determined AT qW 09Ầ. dpes PCpUres 15 Where lịW is the heat flux at the wall Ures is the average liquid velocity in the residual layer P is the liquid density and cp is the specific heat. The remaining ten percent of lx correspond to the length of the wave front. The material properties are taken at inflow temperatures. From the thermographic pictures as presented in Fig. 7.c.1 the temperature difference in x and y direction can be evaluated. Thereby a proportionality can be determined. AT k AT. z x 16 As for current data the constant of proportionality is in the range between 1 k 5. For further considerations a value of k 3 was assumed. ATz 3ATx. 17 Now substituting expressions 16 and 15 into 11 we obtain do 1qW 0.9Ầw 1 dT n lz pcpUZZs 3 18 and 18 into 13 1 do 1 qW 0.9ẦW 1 Cw lz dT n lz Pcd-r. 3 ẦW 19 Solving Eq. 19 for qW - n _ q W 1 cw PcpUres nlZ 1 fW PcpUres 0.9 Ầ. do 3 0.9 cw do dT dT A- 1 n 0 -4 3 20 According to expression 20 the critical heat flux depends on the liquid properties the frequency of large waves and the typical transverse size of regular structures. If the following dimensionless numbers are used 2 q do f V1Ỵ f y qw JT II . Maq ---- Ầpy 2 Pr a and Ka Equation 21 can be presented in the form Ma 1 u 11 1 u q res_ res PrKA 0.9 cW 4 3 10.8 cW . 21 22 Heat Transfer Phenomena in Laminar Wavy Falling Films Thermal Entry Length Thermal-Capillary Metastable Structures Thermal-Capillary Breakdown 471 Therefore in Fig. 13 the experimental data for the dimensionless critical heat flux Maq PrKA are presented as a function of the Reynolds number. Only data which were obtained on excited falling films with the help of the loud speaker were used allowing to keep the major frequency fW at a constant value. As can be seen Eq. 22 depends on the .