An experimental investigation of thermal boundary layer structure and heat transfer in oscillating flows using CARS spectroscopy and cold -wire anemometry

摘要

Single-phase convective heat transfer enhancement is important in a variety of residential and industrial end-use energy applications. Convective heat transfer enhancement can be achieved by passive (no external energy input) or active (requiring external energy input) means. Use of heat-transfer enhancement techniques often introduces complexity into the physics of the flow and temperature fields. This additional complexity can require the introduction of advanced, non-intrusive diagnostics for acquisition of detailed experimental velocity and temperature data. In this thesis, an experimental investigation of active heat transfer enhancement through periodic, forced flow oscillations is undertaken. The focus is on measurement of the time-resolved temperature field using both non-intrusive (CARS) and conventional (cold-wire) techniques.;A dual-broadband, pure-rotational coherent anti-Stokes Raman scattering (CARS) apparatus is developed for the acquisition of non-intrusive temperature measurements in both steady and oscillating thermal boundary layers. To our knowledge, this is the first application of CARS to the study of convective heat transfer. Low-temperature convection measurements push the CARS technique to the limits of its precision, but the method is desirable due to its ability to provide non-intrusive, spatially resolved temperature data with rapid frequency response. CARS temperature measurements from a tripped steady flow and a laminar, oscillating boundary layer flow are presented. The CARS apparatus developed for these studies provides a precision of +/-4K, which is a significant improvement over the precision obtained in typical CARS combustion applications. Temperature data are acquired within 50 mum of the heat transfer surface allowing for non-intrusive estimates of the convective heat flux with a precision of +/-15--20%.;A conventional, cold-wire technique is used to provide a more detailed description of the time-resolved structure of a thermal boundary layer in the oscillating flow. The cold-wire technique provides for increased data acquisition rates relative to CARS, at the expense of an ambiguous, systematic error associated with the intrusive nature of the probe. The cold-wire technique is used to investigate oscillating boundary layer flows with differing degrees of periodic flow reversal. Local, time-averaged Nusselt number results for the oscillating flow are a factor of two higher than accepted values for a laminar, developing channel flow. Periodic flow reversal did not provide a heat transfer advantage relative to non-reversed oscillating flows.