The Physical Mechanism of Heat Transfer Augmentation in Stagnating Flows Subject to Freestream Turbulence and Related Studies

Abstract

The mechanism of heat transfer augmentation due to freestream turbulence in classic Hiemenz stagnation flow was studied experimentally for the first time using time-resolved digital particle image velocimetry (TRDPIV) and a new thin film heat flux sensor called the Heat Flux Array (HFA). Unique measurements of simultaneous, time-resolved velocity and surface heat flux data were obtained along the stagnation line on a simple, rectangular flat plate model mounted in a water tunnel facility. Identification and tracking of coherent structures in the stagnation region lends support to the theory that coherent structures experience stretching and amplification of vorticity by the mean flow strain rate upon approaching the stagnation surface. The resulting flow field in the near-wall region is comprised primarily of high strength, counter-rotating vortex pairs with decreased integral length scale relative to the imposed freestream turbulence. It is hypothesized that the primary mechanism of heat transfer augmentation is the movement of cooler freestream fluid into the heated near-wall region by these coherent structures. Furthermore, the level of heat transfer augmentation is dictated by the integral length scale, circulation strength, and core-to-surface distance of the coherent structures. To test this hypothesis, these properties were incorporated into a mechanistic model for predicting the transient, turbulent heat transfer coefficient. The model was successful in predicting the shape and magnitude of the measured heat transfer coefficient over much of the experimental measurement time.

In a separate yet related set of studies, heat flux sensors and calibration methods were examined. The High Temperature Heat Flux Sensor (HTHFS) was designed and developed to become one of the most durable heat flux sensors ever devised for long duration use in high temperature, extreme environments. Extensive calibrations in both conduction and convection were performed to validate the performance of the sensor near room temperature. The measured sensitivities in conduction and convection were both very close to the predicted sensitivity using a thermal resistance model of the HTHFS. The sensor performance was unaffected by repeated thermal cycling using kiln and torch firing. Finally, the performance of Schmidt-Boelter heat flux sensors were examined in both shear and stagnation flow using two custom designed convection calibration facilities. Calibration results were evaluated using an analytical sensitivity model based on an overall sensor thermal resistance from the sensor to the heat sink or mounting surface. In the case of convection the model included a term for surface temperature differences along the boundary layer. In stagnation flow the apparent sensitivity of the Schmidt-Boelter sensors decreased non-linearly with increasing heat transfer coefficient. Estimations of the sensor's internal thermal resistance were obtained by fitting the model to the stagnation calibration data. This resistance was then used with the model to evaluate the effects of non-uniform surface temperature on the shear flow sensitivity. A more pronounced non-linear sensitivity dependence on heat transfer coefficient was observed. In both cases the main result is that convection sensitivity varies a great deal from standard radiation calibrations.