Experimental investigation of surface radiation and mixed convection heat transfer in duct flows

Abstract

Mixed convection heat transfer in horizontal and vertical ducts with flow through the duct plays an important role in the design and operation of several industrial equipment involving heat transfer and fluid flow phenomena. The estimation of heat transfer requires consideration of both free and forced convective heat exchange mechanisms, in addition to surface radiation among the internal walls of the duct. Free convection leads to the onset and growth of secondary flows, which interact with the forced convection heat transfer rate, and thereby affecting the overall heat removal rates from a duct. Surface radiation in the presence of mixed convection is found to affect both free and forced convection heat transfer rates. Therefore, the combination of the convection (free and forced) and radiation heat transfer modes present an interesting situation. Due to the emissivity of the walls of the duct, surface radiation from the walls of the duct will affect the overall heat transfer phenomena.The present study therefore focuses on the interaction of surface radiation on the free and forced convective heat transfer phenomena occurring in airflow through a duct. Three configurations: mixed convection heat transfer in thermally developing flow in horizontal ducts (CS1), hydrodynamically developed thermally developing flow in horizontal ducts (CS2) and thermally developing flow in vertical ducts (CS3), are analyzed. Experiments are performed to study the mixed convection heat transfer in horizontal and vertical ducts with two differentially heated isothermal hot and cold walls and two adiabatic walls. Radiative heat transfer is estimated using a numerical scheme, with the help of experimentally obtained temperatures.The experimental work involved fabrication, assembly and instrumentation of test sections on horizontal and vertical ducts of suitable sizes. The major parameters considered in this work include wall emissivity, Reynolds number, thermal and geometric parameters and aspect ratios. To cover the required range of Reynolds numbers, two test sections were used in this study. In a series of experiments, the Reynolds number was varied from 800 to 2900, and the heat flux was varied from 250 W/m2 to 870 W/m2, for 2 aspect ratios of the duct cross section and the emissivity of internal walls were 0.05 and 0.85. The heated wall temperature ranged from 55 oC to 100 oC and the cold wall directly opposite to the heated wall was maintained at a uniform temperature.The flow field within the duct was made visible by a suitable smoke flow visualization method. The results showed that the flow conditions and surface radiation significantly affected the total Nusselt number.