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 ofresidential and industrial end-use energy applications. Convective heat transferenhancement can be achieved by passive (no external energy input) or active (requiringexternal energy input) means. Use of heat-transfer enhancement techniques oftenintroduces complexity into the physics of the flow and temperature fields. This additionalcomplexity can require the introduction of advanced, non-intrusive diagnostics foracquisition of detailed experimental velocity and temperature data. In this thesis, anexperimental investigation of active heat transfer enhancement through periodic, forcedflow oscillations is undertaken. The focus is on measurement of the time-resolvedtemperature 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 inboth steady and oscillating thermal boundary layers. To our knowledge, this is the firstapplication of CARS to the study of convective heat transfer. Low-temperatureconvection measurements push the CARS technique to the limits of its precision, but themethod is desirable due to its ability to provide non-intrusive, spatially resolvedtemperature data with rapid frequency response. CARS temperature measurements from atripped steady flow and a laminar, oscillating boundary layer flow are presented. TheCARS apparatus developed for these studies provides a precision of ??4K, which is asignificant improvement over the precision obtained in typical CARS combustionapplications. Temperature data are acquired within 50 pm 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 ofthe time-resolved structure of a thermal boundary layer in the oscillating flow. The coldwiretechnique provides for increased data acquisition rates relative to CARS, at theexpense of an ambiguous, systematic error associated with the intrusive nature of theprobe. The cold-wire technique is used to investigate oscillating boundary layer flowswith differing degrees of periodic flow reversal. Local, time-averaged Nusselt numberresults for the oscillating flow are a factor of two higher than accepted values for alaminar, developing channel flow. Periodic flow reversal did not provide a heat transferadvantage relative to non-reversed oscillating flows.