管壁导热对管内流动流体热边界条件的影响

对层流管内受迫流动热入口段内流体与管壁间耦合传热进行了计算 ,研究了试件几何尺寸、试件与流体物性参数及流体流动状态对内管壁处流体热边界条件均匀性的影响。研究结果表明 ,即使在管外壁均匀加热 (或冷却 )的情况下 ,管壁导热也使得内管壁处流体的热边界条件出现明显的不均匀性。影响这种不均匀性的主要因素是Peclet数、管壁材料的相对导热系数、实验段相对加热长度及管壁相对厚度等无量纲参数。为了改善管内壁处流体的热边界条件以提高对流换热实验结果的精度和可靠性 ,在保证管外壁加热 (或冷却 )均匀性的前提下 ,一定要尽可能采用足够长的薄壁管做对流换热实验的加热段。此外 ,试件的材料与拟采用的加热方式有关。恒壁温加热时要选用导热系数大的材料 ,而恒热流加热时 (如外绕加热丝加热或管壁直接通电加热 )则应选用导热系数较小的材料

EFFECT OF WALL HEAT CONDUCTION ON FLUID THERMAL BOUNDARY CONDITIONS IN LAMINAR PIPE-FLOWS

The conjugate heat transfer problems of wall heat conduction and forced convection in the thermal entrance region of fully developed laminar pipe flows are studied numerically. The influences of pipe sizes, properties of fluid and pipe material and flow patterns on the homogeneity of the fluid thermal boundary conditions at the fluid wall interface are analyzed. Even if the outside wall of the testing pipe is heated or cooled uniformly, the boundary condition of the in tube fluid on the wall is still non uniform because of the inevitably existed wall heat conduction effects. The main dimensionless parameters that influence the homogeneity of the fluid thermal boundary conditions are Peclet number, the wall to fluid thermal conductivity ratio, the heating/cooling length radius ratio and the wall thickness radius ratio of testing pipe. The long thin walled testing pipe should always be used to improve the homogeneity of the thermal boundary conditions of fluid, which strongly influence the accuracy and the reliability of the in tube convection heat transfer measurements. A testing pipe with larger thermal conductivity should be adopted if the outside wall is heated or cooled under constant temperature condition, and a testing pipe with smaller thermal conductivity should be used if outside wall is heated or cooled under constant heat flux condition.