Doctoral Examination: Numerical investigations on the interplay between heat transfer and turbulence in forced and natural convection

Shahab Zeraati Dizjeh, supervised by Dr. Joshua Brinkerhoff, will defend their dissertation titled “Numerical investigations on the interplay between heat transfer and turbulence in forced and natural convection” in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Mechanical Engineering.

An abstract for Shahab’s dissertation is included below.

Examinations are open to all members of the campus community as well as the general public.

Registration is not required for in-person defences.

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ABSTRACT

The interplay between turbulence and heat transfer in forced and natural convection is investigated in this study 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 (DNS) approach. The two-dimensional stability equations for this flow type are derived, and a finite-difference method is proposed to solve them. Solutions of the linear stability equations show that a range of disturbance waves may be amplified in the channel, and those that sustain the largest linear 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 employing three-dimensional DNS, and it is shown that they are able to trigger transition. Based on the simulation results, the transition may decrease the solar chimney’s flow rate due to the increased friction caused by enhanced cross-stream momentum exchange near the walls. The spatial growth of the disturbances is also studied, and different types of vortical coherent structures in the flow are identified. It is shown that temperature fluctuations are dominant in producing turbulence in the flow, 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 means of large eddy simulations of convective heat transfer enhancement in turbulent pipe flows via patterned surface textures. The patterned surface textures intended to enhance convective heat transfer consist of ellipsoidal inward-facing elements, a circular wire coil, and spiral corrugations. The pressure loss penalty and heat transfer enhancement within each textured pipe are reported, and it is shown that the best balance of heat transfer enhancement and flow pressure penalty occurs for the inward-facing 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 surface textures that most efficiently enhance the convective heat transfer rate induce strong vortical structures and turbulent radial fluctuations in near-wall regions where the temperature gradient is high.

The study reveals that enhancing the turbulence does not guarantee a considerable increase in convective heat transfer. 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.