Improving student understanding and motivation in learning heat transfer by visualizing thermal boundary layers

The thermal boundary layer concept is an abstract topic due to the difficulty of direct observation. Three differentapproaches are used to allow students to visualize the thermal boundary layer for a geometry that is relevant to heatexchanger design. We focus on the case of a heated cylinder in cool water for both natural and forced convection. High-magnification videos of the boundary layer were made using a telecentric imaging system. Numerical simulations of theexperimental system agree with observations. A set of ultra-low-cost desktop learning modules based on shadowgraphywere constructed to allow students to directly see aspects of natural and forced convection from a cylinder. Numericalsimulations and telecentric imaging clearly show the initial diffusive growth of the boundary layer and subsequent onset ofbuoyant convection. When flow is initiated, both show a thinning of the boundary layer on the upstream side of thecylinder. The boundary layer itself may be impossible to see by eye for the low-cost experiment, however, the plume anddisruption of the boundary layer by laminar cross flow can be seen. We implemented the thermal boundary layervisualization activities in a fluid mechanics and heat transfer course with 48 students. Numerical simulations and videoswere shown after which the thermal boundary layer modules along with the worksheets were given to students. Statisticalanalysis of pre- and posttest results shows significant improvements for three out of five questions with moderate to verylarge effect sizes. Feedback is positive, with students finding the exercises interesting, helpful, informative, and well-explained. We believe that these visual representations aid learning and can actively engage students in the learningprocess.