| Summary: | 碩士 === 國立虎尾科技大學 === 機械與機電工程研究所 === 100 === The present thesis numerically investigates the three-dimensional, steady mixed convection of air inside a horizontal rectangular channel. A heated surface maintaining a uniform and constant temperature is located on the one of sidewalls of channel. Air flows into the channel with a two-dimensional, fully-developed velocity profile and a lower inlet temperature. Buoyancy force drives the air moving upward when flowing nearby the heated surface. Hence, complicated flow field and heat transfer are observed.
The important parameters are Reynolds number (50≦Re≦500) and Grashof number (5000≦Gr≦50000). Streamlines, velocity vector, local and average Nusselt number are illustrated to demonstrate the effects of Re and Gr.
Our results show that dramatically variations of hydrodynamic and heat transfer characteristics are obtained with different combination of Re and Gr. With a large Re and a small Gr, inertial apparently dominates these characteristics. The results seem similar to those of forced convection, and no recirculation is found. Although, the Nu distribution of the heated surface looks like the one of forced convection, the buoyancy force locally changes the behavior of heat transfer nearby the bottom plate. When a small Re and a large Gr, a strong recirculation exists close to the upper half of the heated surface and the top plate of the channel, reducing the local heat transfer. Main air stream is forced to move downward due to the recirculation, passing beside the lower half portion of the heated surface and consequently, a better heat transfer is observed locally.
Aspect ratio (A, channel width to channel height) of the channel is an important parameter, too. Without changing the channel width, a larger aspect ratio means a higher channel and a higher heated surface on the sidewall, resulting into a higher buoyancy force. Forced convection is the dominant effect, hence, average Nusselt number ( ) would not change with aspect ratio with Re=1000 and Gr=5000. However, seems to decrease with an increasing aspect ratio when Re=100 and Gr=50000. It is explained that a large aspect ratio induces the appearance of the recirculation around the top plate, therefore, decreasing the local heat transfer.
Finally, the mixed convection of an extruded heated surface is examined. The heated surface is moved to align along Z=0.5.The forward- and backward-steps are formed and assigned to keep the same higher temperature of the heated surface. Our results show the three-dimensional, open but not close swirl flows around these steps. With a sufficient large Re, streamlines and heat transfer appears symmetrically to middle high (Y=0) plane. On the other hand, the symmetry is broken due to a large upward buoyancy if Gr is large. increases with Re. also increases with Gr, but the enhancement is small for a large Re.
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