DOI: https://doi.org/10.20535/kpi-sn.2019.3.175745

TEMPERATURE FACTOR EFFECT ON THE FLOW STABILITY LOSS IN THE PIPE INITIAL SECTION

Gennadii O. Voropaiev, Oleksandra O. Baskova

Abstract


Background. Improving the energy efficiency of heat exchangers is possible by developing the heat exchange surface and changing the flow regime in order to reduce hydraulic losses. The processes of transition from laminar to turbulent regime, exactly as from turbulent to laminar, depend on a large number of factors, including temperature, that affecting the viscosity of the flow and, respectively, the Prandtl number. Another feature of the transitional flow regime is the sharp nature of the change in the heat exchange intensity even with small changes in the Reynolds number, which creates certain difficulties in working with it.

Objective. The purpose of the paper is: studying the processes of stability loss and transition to the turbulent regime in the initial section of the pipe with a partially developed surface; determining the influence of the temperature factor on the relationship of hydraulic and thermal flow parameters.

Methods. Numerical experiment using direct numerical simulation (DNS) and instruments of the Ansys Fluent software.

Results. The threshold values of the temperature gradient at which the flow in a pipe of a given length, for a given Reynolds number loses stability, are determined. The dependences of the temporal and spatial scales of arising disturbances and their growth rates from the combination of the Reynolds number and temperature gradient are determined. The interrelation of wall shear stress values with heat flux values is shown. Dynamic and thermal characteristics of the flow in the area of the corrugated surface were obtained, on the basis of which the threshold values of the temperature gradient at which the partial corrugation is energy efficient are determined.

Conclusions. An increase in temperature gradient leads to an earlier generation and intensification of the vortex motion, which causes an increase in convective heat transfer at the fixed Reynolds number. The location of the corrugated insert in relation to the place of the finite perturbations genesis, along with the geometry of the corrugation, significantly changes the dynamic and thermal characteristics of the flow.

Keywords


Flow in the pipe; Flow stability; Heat transfer intensification; Corrugation; Initial section of the pipe; Temperature gradient, Prandtl number

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