Wake And Heat Transfer From Two-Dimensional Structures In Cross Flow

The wake and heat transfer topologies around isothermal circular and square cylinders have received a great deal of attention in many practical engineering applications such as electronic-chip cooling systems(processors and power chips),graphics processing units,micro heat exchangers,fuel cells,and data centers,etc.With technological advancement,thermal engineering researchers are seeking different mechanisms to enhance the thermal performance between interacting objects and surrounding fluid.The mechanisms of enhancing heat transfer are categorized as active and passive methods.The active methods require the external power to maintain the enhancement while passive methods do not require the additional energy source but require the surface extension,surface modification,object proximity,and additives in fluids(nano-fluids).The surface modification and proximity are the focus of the current study to find the optimized configurations/arrangements from the perspective of the heat transfer enhancement.In addition,the connection between heat transfer and flow structures is not well understood in the literature.Attention is,therefore,paid to fill these research gaps.Keeping in view these considerations,this thesis encapsulates numerical simulation of forced and mixed convection heat transfer around cylindrical structures,which have a wide spectrum of engineering applications.In forced convection,the objectives are to study the dependence of wakes and heat transfer on the corner radius ratio(r/d)of the single cylinder(changed from square to circular)and on the diameter ratio(d/D)of two tandem cylinders.Here,r is the corner radius,d is the cylinder width/diameter,and D is the downstream cylinder diameter.Similarly,in mixed convection,the objective is to systematically investigate the effects of Richardson number Ri(=0-2.0)on the flow and heat transfer topologies of a circular cylinder submerged in the wake of another cylinder of similar diameter in tandem arrangements in an unconfined flow.In mixed convection,Ri is the key performance controlling parameter,defined as Ri=Gr/Re~2,where Gr=g?(_?-_w)D~3/?~2 is the Grashof number,g is the gravitational acceleration,?is the volume expansion coefficient,D is the characteristic width of the bluff body,?is the kinematic viscosity of the fluid,and T is the temperature.The subscripts‘w'and‘?'represent the wall and freestream,respectively and Reynolds number,Re=U?D/?,where U?is the freestream velocity.A finite volume method-based commercial solver ANSYS FLUENT at low Re is employed to perform the numerical simulations.The pressure-velocity coupling for the finite volume formulation is achieved by SIMPLE(Semi-Implicit Method for Pressure-Linked Equations)scheme.The spatial discretization of convective terms is done using the second-order upwind scheme while the second-order central differencing scheme is used for the diffusive term in the conservation equations.The surface of the cylinder is maintained at a constant temperature with no-slip boundary conditions.For mixed convection,Boussinesq approximation is applied to the y-momentum equation which assumes that density is constant in all terms of momentum equation except the body force term because of density variation due to temperature differences which induces the buoyancy force.The flow structure around and heat transfer from a bluff body in cross flow is simulated at Re=150.The cylinder shape is modified from square to circular by modifying the corner radius as a governing parameter.The sharp corners of the square cylinder are rounded as r/d=0(square),0.125,0.25,0.375,and 0.5(circular).It is found that r/d has a profound effect on the flow structure from the perspective of flow separation,vortex strength,separation bubble,and wake bubble each playing a role in heat transfer from different surfaces of the cylinder.The increase in r/d from 0 to 0.5 leads to a 33%enhancement in the heat transfer from the cylinder due to a shorter wake bubble size and vortex strength which demonstrates that r/d=0.5 is the most efficient heat transfer configuration.The minimum time-mean drag and fluctuating forces are achieved at r/d=0.25 and 0.125,respectively.As evident from the dependence of heat transfer on r/d,the heat transfer is maximum for r/d=0.5(circular cylinder).Therefore,the particular focus is given to the circular cylinder.An isothermal cylinder(diameter D)in the wake of another smaller(diameter d)cylinder is numerically investigated for a fixed spacing ratio L/d=5.5,where L is the distance from the center of the upstream cylinder to the nominal front stagnation point of the downstream cylinder.The attention is given to how the size of the upstream cylinder influences the flow and heat transfer topology around the downstream cylinder when the diameter ratio d/D is varied from 0.15 to 1.0,where d/D is the governing parameter.The upstream-cylinder shear layers for d/D<0.3 reattach on the downstream cylinder and shed vortices behind it.They for 0.3?d/D?1.0,on the other hand,shed vortices in the gap,and the wake of the downstream cylinder features a primary vortex street followed by a secondary vortex street.The surface-averaged heat transfer enhances when d/D is decreased from 1.0 to 0.4,and maximum heat transfer occurs for d/D=0.4.A novel tertiary frequency in heat transfer fluctuations is identified which is found to be the difference between primary and secondary frequencies.A circular cylinder submerged in the wake of another cylinder in mixed convection regime is studied for Ri(=0-2).Three spacing ratios L/D=1.2,3.0,and 5.0 are selected,where L is the center-to-center distance between the two cylinders.The upstream cylinder is maintained at zero heat flux while the downstream cylinder is heated and maintained at a constant temperature.It is observed that,although the upstream cylinder is not self-heated,its temperature can reach upto 85%of the downstream cylinder temperature due to the reverse flow generated in the gap between the cylinders.Under the influence of Ri the symmetry of wake is broken and at sufficiently high Ri,the wake is characterized by a P+S(pair and single)vortex street with the P vortices becoming hotter than S vortices.When Ri is increased,downwash and upwash flows are generated before and after the heated cylinder,enhancing the heat transfer from the cylinder and engenders time-mean lift.L/D=5.0 effective heat transfer boosting configuration for the simulated ranges of Ri.