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Zeraati Dizjeh, Shahab

University of British Columbia

The interplay between turbulence and heat transfer in forced and natural convection is investigated through multiple high-fidelity numerical simulations. The effects of heat transfer on generating fluctuations and turbulence are addressed by analyzing linear stability and transition in vertical buoyancy-driven flows in solar chimneys employing a direct numerical simulation approach. The stability equations are derived, and a finite-difference method is proposed to solve them. Solutions of the equations show that a range of disturbance waves may be amplified in the channel, and those that sustain the largest amplification have phase velocities equal to the peak velocity of the flow near the heated wall. The non-linear amplification of these disturbances is studied, and it is shown that they can trigger transition. Based on the results, the transition may decrease the flow rate due to the increased friction caused by enhanced cross-stream momentum exchange. The spatial growth of the disturbances is studied, and different types of vortical structures are identified. It is shown that temperature fluctuations are dominant in producing turbulence, and the advection of the surplus kinetic energy makes the flow more turbulent as it rises. The effects of turbulence on heat transfer are addressed by large eddy simulations of convective heat transfer enhancement in turbulent pipe flows via patterned surface textures. The textures intended to enhance heat transfer consist of ellipsoidal inward-facing elements, a wire coil, and spiral corrugations. The pressure loss and heat transfer enhancement within each textured pipe are reported, and it is shown that the best balance of heat transfer enhancement and pressure penalty occurs for the ellipsoidal elements. The mechanisms for the observed performance are explored via order-of-magnitude analysis of the first and second laws of thermodynamics. It is concluded that the textures that most efficiently enhance the heat transfer induce strong vortical structures and turbulent radial fluctuations in near-wall regions. The study reveals that enhancing the turbulence does not guarantee an efficient heat transfer enhancement. An efficient heat transfer augmentation can be achieved if the turbulence induces strong wall-normal motions in regions where the temperature gradient is high such as in the thermal boundary layer.

Text

2023-02-01

Attribution-NoDerivatives 4.0 International

http://hdl.handle.net/2429/83774

Mechanical Engineering

University of British Columbia

UBCO

http://creativecommons.org/licenses/by-nd/4.0/