A preheated-wall transient method using liquid crystals for the measurement of heat transfer on external surfaces and in ducts

摘要

This thesis deals with the development of a preheated-wall transient method using liquid crystals for the measurement of local heat transfer on external surfaces and in ducts. For external surfaces, local heat transfer measurements from a flat plate to an impinging jet were carried out as an example. For duct flows, the local heat transfer measurements in square ducts with transverse ribs on two opposite sides were carried out as an example.;For the external flow situation the preheated wall transient method involves the sudden exposure of a preheated test plate to a flow of ambient temperature air. For measurements inside ducts the preheated-wall transient method involves the sudden insertion of a preheated section of test duct on the end of a duct which already has an established flow at ambient temperature. The surface temperature response during the transient processes is measured using liquid crystals and recorded with a video system.;For the impinging jet measurements the Reynolds numbers used are 23,000, 50,000 and 70,000. The jet-to-plate distances are L/D = 2, 4, 6 and 10. The transient measurement results compared well with steady state measurement results. Correlations of Nusselt number with Reynolds number are given in terms of Re$sp{rm n}$. In the stagnation region (radial distance R/D $$ 2) n is 0.7.;For the measurements inside square ducts with transverse ribs on two opposite sides the Reynolds number (based on the hydraulic diameter of the square duct) was varied from 15,000 to 80,000. The square ribs had a height-to-diameter ratio (e/D) of 0.0625 and a pitch-to-height ratio (p/e) of 10. 3-D plots and contours are obtained showing the detailed heat transfer distribution within the rib segments. The result shows high heat transfer on the top of rib and in the region approximately 4 rib heights downstream of the ribs. The area average Nusselt number for rib segments was found to be independent of their position beyond X/D = 2.5, indicating rapid thermal development. The heat transfer enhancement was approximately 2.0 for the ribbed side and approximately 1.5 for the smooth side.