REGULARITIES OF THE AVERAGE HEAT EXCHANGE FOR PIPES WITH ROUGH WALLS AND ROUGH FLAT CHANNELS UNDER ONE-SIDED THERMAL LOADING

Abstract

Abstract. Objectives The aim of the present work is to carry out mathematical modelling of heat exchange in rough flat channels and round pipes with rough walls during one-sided heating. Methods The calculation was based on the superposition principle of turbulent viscosity. The solution of the problem of intensified heat transfer in a flat channel and in a round tube with rough walls was obtained using the Lyon's integral. Results A methodology for the theoretical computational heat exchange determination for flat rough channels and round pipes with rough walls during one-sided heating is developed, in contrast to existing theoretical approaches, on the basis of the principle of full viscosity superposition in a turbulent boundary layer. The analysis of the calculated heat exchange and hydraulic resistance values for flat rough channels and round rough pipes under one-sided heating shows that the increase in heat exchange is always less than the corresponding increase in hydraulic resistance, which is a disadvantage as compared to the channels with turbulators, all else being equal. The results of calculating the heat exchange for channels with rough walls at one-sided heating in an extended range of determinant parameters differ significantly from the corresponding data for channels with turbulators and are used to determine the level of heat exchange intensification. Conclusion An increase in the calculated values of the relative average heat exchange Nu / NuGLduring one-sided heating for flat rough channels and rough pipes having very high relative roughness values is significantly affected by both an increase in the relative roughness height h/R0 and an increase in the Re Reynolds number. The main advantage of solutions for averaged heat transfer for rough flat channels and round pipes under symmetrical thermal load, obtained according to the developed theory, is that they allow the calculation of heat exchange in rough pipes to be made in the case of large and very large relative heights of roughness protrusions, including large Reynolds numbers, which is typical for pipes of small diameters and narrow flat channels. An increase in the relative heat exchange in air due to an increase in the relative height of the roughness or the Reynolds number is accompanied by an even more significant increase in the hydraulic resistance. Calculated data on averaged heat transfer obtained in the work indicated that in the range of determinant parameters for flat rough channels with one-sided heating, the average heat exchange is lower by (4  10)% as compared to round rough pipes, all other things being equal.

Keywords

• multilayer model
• flat channel
• round pipe
• roughness
• turbulator
• modelling
• heat exchange
• turbulent
• intensification
• heating
• heat supply
• one-sided