Engineering

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Turbulent Free ConvectionIntroduction

This study investigates the turbulent free convection in a tank of water with the bottom surface being a grooved surface and the top of the water surface exposed to ambient.

In natural convection, flow is caused due to density differences within a fluid. The density differences may be caused by gradients in temperature, concentration, or composition. Turbulent convection is ubiquitous in nature and in many industrial and natural situations. In nature, convection cell can be found in the oceans, earth’s atmosphere, earth’s mantle and outer core. A very common industrial application of natural convection is the solidification process in metal casting, crystal growth, and the solar collector as well as in heat transport and mass mixing in many engineering applications.

 

When the flow is turbulent, large gradients of temperature exist near the walls and a nearly isothermal core. The flow structure next to the wall is one of the boundary layers where momentum is transferred and heat is transferred mostly by molecular motions. Experimental evidence suggests the convective heat transfer near the wall is either from thermals or essentially line plumes which move randomly. Away from the wall rapid mixing leads to the near isothermal conditions.

 

Geometry Creation :

 

Open Cavity Geometry 

 

Meshing :

 

Mesh model

 

 

Fluent Setup :

In Fluent Setup Rayleigh number are generated in the software through scaling gravitational acceleration. Ra2gD3β TPr/µ2

Gravitational Acceleration used in this study are 4.479*10-6 for Rayleigh number 103. In material properties provide boussinesq density 1.205 for air and 1002.5 for water. In boundary condition provide Heat Flux to the grooved plate 150W/m2.In Solution method Gradient selection Green gauss node based was selected. In Convective Flux type Roe-Fdswas selected.

 

Result :

VELOCITY STREAMLINE

Fig. 3 Velocity Streamline

 

 

VELOCITY VECTOR

 Fig. 4 Velocity Vector

 

Discussion :

In the experimental setup conducted by various researchers before specially in paper titled “experimental study of the turbulent free convection over horizontal smooth or grooved surfaces in an open cavity.” it was observed that the heated fluid continuously rises along the plumes accelerating in the central stem of the plume. The rising material produces a stalk, while the deflected fluid at the end of the stalk produces a cap on top. As the push in and deflection continue, the edge of the cap may further fold over. Hence, plumes are of mushroom-shaped. It was also observed that structures of plumes are more inclined with increasing in heat fluxes due to the stronger large scale flow at the higher fluxes.

In this CFD simulation using ANSYS Fluent the velocity streamlines and velocity vectors reflected a similar phenomenon which validates the experimental observations. Further studies using different power inputs to the plate and also with different aspect ratios(= width of fluid layer/height of fluid layer) were further carried out in transient conditions to validate experimental studies and the results were found to be within acceptable limits.