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 different approaches are used to allow students to visualize the thermal boundary layer for a geometry that is relevant to heat exchanger 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 the experimental system agree with observations. A set of ultra-low-cost desktop learning modules based on shadowgraphy were constructed to allow students to directly see aspects of natural and forced convection from a cylinder. Numerical simulations and telecentric imaging clearly show the initial diffusive growth of the boundary layer and subsequent onset of buoyant convection. When flow is initiated, both show a thinning of the boundary layer on the upstream side of the cylinder. The boundary layer itself may be impossible to see by eye for the low-cost experiment, however, the plume and disruption of the boundary layer by laminar cross flow can be seen. We implemented the thermal boundary layer visualization activities in a fluid mechanics and heat transfer course with 48 students. Numerical simulations and videos were shown after which the thermal boundary layer modules along with the worksheets were given to students. Statistical analysis of pre- and posttest results shows significant improvements for three out of five questions with moderate to very large 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 learning process.