Optically Based Rapid Heat Transfer Measurements in Complex Internal Flows

An optically based technique was developed that involves fabrication of a thin-walled plastic model with laser heating applied to a small section of the outer surface. The heat flux distribution applied to the model by the laser was measured first using a short-duration, transient experiment. The external temperature distribution was then recorded using infrared thermography with steady laser heating. The measured heat flux and temperature distributions were used as thermal boundary conditions in a finite-element code to solve an inverse heat conduction problem for the heat transfer coefficient on the internal passage wall. Hydrodynamically fully developed turbulent flow in a round tube was used as a test case for the development of the new optical method. The Reynolds numbers used were 30,000 and 60,000. This flow was chosen because accurate computational tools were available to calculate the internal heat transfer coefficient for a variety of thermal boundary conditions. In addition, this geometry simplified both the model fabrication and the implementation of a finite-element model for the inverse heat conduction problem. Heat transfer coefficient measurements agreed with numerical simulations and semi-analytical solutions within 1.5% and 8.5% for the low and high Reynolds numbers, respectively. Additional simulations suggest that the method can be accurate with thermal boundary conditions more complex than in these experiments.