Thermal mixing enhancement of a free-cooling system with a fractal orifice plate

Abstract

The downstream hydrodynamic and thermal mixing performance of control and fractal orifice plates is numerically investigated. Each insert is positioned following a T-duct. Four blockage ratio plates σ=0.5, namely, square orifice (SO), circular orifice (CO), square fractal orifice (SFO), and the Koch snowflake orifice (KSFO), are employed to promote thermal mixing. In particular, orifice configuration effects that induced transverse and horizontal thermal convergence, turbulence kinetic energy, and pressure gradient changes are discussed. Numerical validations reveal good agreement between the experimental and numerical results for centerline velocity and temperature distributions along the channel. The results show that KSFO outperforms the rest with respect to effective hydrodynamic and thermal mixing. It is critical to note that the maximum cross-sectional temperature difference Δθ for KSFO is the lowest and decreases further downstream. Clearly, such low Δθ values along the channel ensure temperature uniformity. Furthermore, KSFO generated area-averaged turbulence kinetic energy levels approximately 37%, 48%, 371%, and 1454% higher than those of CO, SO, SFO, and the smooth channel without an insert, respectively, at x/H=1.04. It is also important to note that the studied fractal orifice pressure gradients are lower than those of CO and SO. These pressure drop observations are consistent with those of Nicolleau et al. (2011). Overall, the complex KSFO geometry forms a prominent balance between the pressure coefficient and thermal mixing at a Reynolds number of Reh=1.94×104. Most importantly, this finding may help guide the long-term sustainable development of heating, ventilation and free-cooling air conditioning systems.