Determination of local heat-transfer coefficient distribution on a vortex enhanced finned-tube heat exchanger fin using infrared thermography

Abstract

An innovative vortex-enhanced finned-tube heat exchanger geometry combining winglet longitudinal vortex generators and deflectors guiding the flow in tube wake has been experimentally studied by means of an infrared-based experimental method that allows the local heat transfer coefficient evaluation over the fin. The coefficient distribution is determined by using a transient technique and by calculating the energy balance during the fin cooling. The calculation model takes into account radiation with the surrounding and lateral heat conduction into the material. Results of local heat transfer coefficient distribution are presented for different Reynolds number values. The experimental convective heat transfer fields are first compared with CFD numerical results. A comparative analysis of heat transfer rates vs the smooth fin geometry is then presented. The present experiments were conducted in an open-circuit wind tunnel, see Fig. 1. The upstream part of the wind tunnel has a section with airflow conditioning elements (fibreglass screen, honeycomb flow straightener and converging section) to supply an incoming airflow with uniform cross-sectional distribution and low turbulent level at the inlet of the test section located immediately downstream. Moreover the air temperature in the enclosed upstream chamber is controlled, using an air-conditioning unit, to provide a constant inlet air temperature at the entry of the test section. A rectangular observation window has been air-tightly set into the top-wall of the test section right above the plate fin and tube assembly to allow the measurement of the fin surface temperature. The axial airflow in the wind tunnel is driven by a fan located downstream and its volumetric flow rate is determined using a micro-manometer measuring the pressure drop across an orifice plate flowmeter. A long wave IR camera (AGEMA® Thermovision 900) with a 10° lens is placed at its minimal focalisation distance right above the heat exchanger model which surface has been thinly coated with black coating having high emissivity (95%). The aforementioned rectangular observation window being transparent to infrared radiations, quantitative thermography measurement of the fin surface temperature is then made possible. This window, made of Zinc Selenide, has a direct transmission factor equal to 98 % for infrared wavelengths ranging from 8 to 12 m. The non-interlaced frames recorded by the infrared camera at a frequency equal to 15 Hz are post processed by a specific high accuracy signal-conditioning unit (ADDELIE ®). Finally, for a high accuracy, an in-situ calibration of the