LOCAL FIN-SURFACE HEAT TRANSFER FOR FLOW AROUND A CIRCULAR CYLINDER WITH AND WITHOUT VORTEX-GENERATING WINGLETS

A direct comparison of local fin-surface heat transfer coefficients for flow around a circular tube without (a) and with (b) winglets at $ReH∼1200$ is presented in Fig. 1. The comparison reveals that, for this winglet location, the horseshoe vortex produced by the interaction of the flow with the circular cylinder is disrupted by the winglets. There is a reduction in the width of the low-heat-transfer wake region, but heat transfer coefficients directly downstream of the cylinder are actually slightly reduced for the winglet case compared to the no-winglet case. Stagnation-region heat transfer coefficients are slightly higher for the winglet case compared to the no-winglet case. The experiments were performed in a narrow rectangular duct designed to simulate a single passage of a fin-tube heat exchanger. A transient heat transfer measurement technique was employed for obtaining detailed local heat transfer measurements on the model fin surface. Using this technique, the room-temperature fin/tube model is suddenly exposed to a uniformly heated airflow, initiating a heat conduction transient in a lexan substrate. The transient localized heating is quantitatively recorded using an imaging infrared camera. Values of local heat transfer coefficient are then determined from a one-dimensional inverse heat conduction analysis. The winglets had a 1:2 height/length aspect ratio and were oriented at a 45 degree angle to the flow. The height of the winglets was 90 percent of the channel height. The quantitative thermal visualization images are obtained using a precision imaging infrared camera (FLIR PRISM DS). The camera detector has a 12-bit digital dynamic range and a minimum discernible temperature difference (MDT) of 0.1° C at 30° C. It is equipped with a 25-mm standard lens, which provides a 17 deg×13 deg field of view. Infra-red thermography has several advantages over thermochromic liquid crystals for surface temperature mapping, including wide available temperature range, high spatial resolution, excellent thermal resolution, and full-field direct digital data acquisition and processing.