Numerical Study On Flow And Heat Transfer In The Concave-convex Flow Channel For Middle Reynolds Number

With the development of society and scientific technology, energy shortage becomes one of the world’s major problems. In order to adapt to the strategy of sustainable development, the urgent need is to improve the energy utilization and the service life of equipment. The structure of concave-convex wall is widely used in heat transfer equipment, so the study on the mechanism of flow and heat transfer in the channel with concave-convex wall in transition flow has an important significance for improving the energy utilization and the service life of equipment.Currently, the researches of wall heat transfer are focused on laminar flow and turbulent flow. It is less in transition flow. As the first step of this study, the research is mainly on the model of concave-convex wall with a single concave wall. The flow variation and heat transfer characteristics in the concave-convex flow channel for middle Reynolds number were analyzed by numerical simulation. And the paper shows the characteristics of flow and heat transfer in different Reynolds number and different geometries of the wall.In this study, FORTRAN is used to mesh and program. Then based on constant physical properties of fluid and two–dimensional flow, the discretization of continuous equation, unsteady N-S equations and energy equation are discreted by finite control volume method. When solving the finite difference equations, the upwind differencing is used for the convection terms, the central differencing is used for the diffusion terms. ADI( Alternating Direction Implicit) method is used for the finite differencing of time terms. SIMPLE( Semi-Implicit Method for Pressure-Linked Equations)method is used to correct the pressure so that it can satisfy continuity equation.This study is conducted by changing Reynolds number and the size of the concave-convex wall. The calculation results were treated by the post processor. Then the flow field, velocity field, temperature field within the calculate area and the Nu curve, Cf curve of the concave-convex wall are obtained. At last, by analyzing the results and graphs, the conclusions were gotten as follows:(1) When the entrance average speed is changed, the instability of the flow field and the flow velocity are increased with the increase of Reynolds number. And the vortexes above the concave wall are cyclical. The temperature gradient near the concave-convex wall and the convective heat transfer are also increased with the increase of Reynolds number.(2) When the characteristic length is changed, the structure of the flow field near the wall changes little and the instability of the mainstream increased with the increase of Reynolds number. But the mainstream flow line in the velocity field has no obvious fluctuations. It shows that the vortexes in the mainstream are without a stable cycle. With the increase of Reynolds number, the Nu value increased. So changing the characteristic length also has a significant impact on the heat transfer of the wall.(3) The structure of flow field upward the concave wall changes significantly and the mainstream is more stable with the increase of the width of the concave wall. The velocity field has a similar change with the flow field. The temperature gradient changes little and Nu value decreases slightly with the increase of the width of the concave wall. It shows that increasing the width of the concave wall can’t promote the convective heat transfer.(4) The instability of the flow field and the fluctuation of the flow line in the velocity field increased with the increase of the height of convex wall. The temperature gradient near the wall, the temperature fluctuation in temperature field and the Nu value also increased with the increase of the height of convex wall. The result shows that increasing the height of the convex wall can promote the convictive heat transfer of the wall.