Investigation of the effect of blockage ratio on flow and heat transfer in the wake region of a cylinder embedded in a channel using whole field dynamic measurements

Abstract

Non-intrusive field measurements in the wake region of a circular adiabatic cylinder fitted in a channel are reported. The top wall of the channel is subjected to constant heat flux while maintaining the bottom wall and the cylinder under adiabatic condition. The cylinder diameter has been varied to achieve three different blockage ratios (D/H) of 0.25, 0.38 and 0.5. Experiments are performed in the range of 63≤Re ≤ 165 with water as the coolant fluid under hydrodynamically fully developed and thermally developing conditions. Effect of blockage ratio on heat transfer enhancement capability, its capacity to alter the flow physics by modifying the size of vortices, the downstream distance they travel and the Reynolds number at which the vortices get initiated, is reported. Path-integrated temperature distribution in the channel has been mapped through the classical form of Mach Zehnder interferometer. The temperature distribution thus obtained has been used to determine the variations in local heat transfer coefficient along the length of the top horizontal surface of the channel. Schlieren deflectometry is used to quantify the vortex shedding frequency under three different blockage ratio regimes. The experimentally obtained results are benchmarked and validated by performing numerical simulations using FLUENT. It has been observed that a decrease in blockage ratio at a constant Re leads to alterations in the size of the generated vortices, which manifest as an increase in the vortex shedding frequency. Vortex shedding is seen to get initiated at relatively lower value of Re (= 65) when the blockage ratio is low (D/H = 0.25). However, when the wake transition to three-dimensionalities is considered, an increase in blockage ratio from D/H = 0.25 to 0.5 leads to an increase in the value of transition Re (165 for D/H = 0.5). It is observed that the vortices travel maximum downstream distance for blockage ratio of 0.38. For all blockage ratios, due to vortex shedding, the thermal boundary layer assumes a corrugated shape and aids in enhancing the heat transfer rates from the channel surface.