Integral characteristics of a non-stationary thermal boundary layer at thermal load release in the initial section of a cylindrical channel

THE PURPOSE. To study the development of a non-stationary thermal boundary layer of a turbulent gas flow in the initial section of a cylindrical channel.

METHODS. The study was carried out experimentally and by means of mathematical modeling. The experiments were carried out on a rig with plasma heating of the working fluid (air). The experimental rig is an open-loop wind tunnel. The experiments were carried out at Reynolds numbers Re₀₁= 44000. The gas temperature reached 1400 K, the wall temperature increased to 700 K. The fixation of temperature fields in the boundary layer is carried out by means of chromel-alumel thermoelectric sensors. The mathematical model represents the integral equations of the boundary layer. The laws of turbulent exchange are obtained in accordance with the Prandtl model on the length of the mixing path. A two-layer model of hydrodynamic and thermal boundary layers is adopted. The use of parametric methods for calculating the boundary layer of Kutateladze-Leontiev allows us to obtain relationships for calculating the velocity and enthalpy profiles. The parameters at the boundary of the thermal conductivity sublayer, enthalpy profiles, and integral characteristics of the thermal boundary layer are determined numerically. Within the framework of the adopted model, the integral characteristics are a function of the thermal non-stationarity parameter caused by the time variability of flow temperatures.

RESULTS. The development of integral flow thicknesses during the release of the thermal load is determined. Thermal non-stationarity during the release of the load along the main flow deforms the temperature profiles, they become less filled. The results are generalized within the framework of the boundary layer theory.

CONCLUSIONS. The energy loss thickness increases relative to its stationary isothermal analog with an increase in the thermal non-stationarity parameter. The effect of non-stationarity on the energy loss thickness is within 20% at z_h ≤ 2, Re_h**=1000 and Re**=1000. 

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