Transition of free convection boundary layer flow

Alzwayi, Ali Saad (2013) Transition of free convection boundary layer flow. PhD thesis, University of Glasgow.

Abstract

Transition of natural convection flow on a heated flat plate and inside a heated channel is studied numerically. Three different RANS based turbulent k-ε models namely standard, RNG and Realizable with an enhanced wall function are employed in the simulations. Additionally, a Large Eddy Simulation (LES) technique is also applied to particularly investigate the flow field and transition in a heated plate facing upward.

Numerous published papers presented the typical characteristic behaviour of natural convection flow inside a channel, however, none of these provided information about the transition behaviour of flow inside a channel, and importantly, how the transition is affected by the merging of two growing boundary layers which occur inside a channel. Therefore, taking the above important things into consideration, the aim of the study is to carry out in-depth investigations of the transition of the free convection flow inside a channel with an effect of its width, angular orientation and several important thermal and boundary conditions.

Moreover, the transition phenomena of the free convection flow developing in a heated channel facing both downward and upward are thoroughly investigated in the thesis. Numerically predicted results are compared with available experimental data in the published literatures. Fluid properties are assumed to be constant except for the density which changes with temperature and gives rise to the buoyancy forces and is treated by using the Boussinesq approach. Air with a Prandtl number of 0.7 is used as a test fluid in all the simulations.

In the RANS based models, the results show that the Realizable model with an enhanced wall function predicts numerical results well compared to the experiment than those obtained by the other two models (standard and RNG), therefore this model was selected to perform all the other RANS based numerical simulations in this work.

The results particularly indicate that the inclination of the channel has major effects on the transition stage. As the inclination angle is increased, the transition stage moves further downstream of the channel. However, the predicted local heat flux, reached its minimum further upstream of the channel, does not agree with that of the velocity and turbulent kinetic energy in a small channel width.

Natural convection from an isothermal plate is conducted by using a Large Eddy Simulation method. The results show that with an increase in the angle the peak of the thermal and velocity boundary layers move from the near outlet of the plate for the vertical case to the middle of the plate when θ = -70°. So the thermal and boundary layers become fatter which causes an early transition.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Keywords: Natural convection, boundary layers,the channel, the transition
• Subjects: A General Works > AC Collections. Series. Collected works
• Colleges/Schools: College of Science and Engineering > School of Engineering > Systems Power and Energy
• Supervisor's Name: Manosh, Dr Paul
• Date of Award: 2013
• Depositing User: Mr Ali Alzwayi
• Unique ID: glathesis:2013-4803
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 13 Jan 2014 14:24
• Last Modified: 13 Jan 2014 16:01
• URI: https://theses.gla.ac.uk/id/eprint/4803