Heat transfer enhancement in suddenly expanding annular shear-thinning flows

摘要

Heat transfer enhancement in suddenly expanding annular pipe flows of Newtonian and shear-thinning non-Newtonian fluids is studied within the steady laminar flow regime. Conservation of mass, momentum, and energy equations, along with the power-law constitutive model are numerically solved. The impact of inflow inertia, annular-diameter-ratio, k, power-law index, n, and Prandtl numbers, is reported over the following range of parameters:Re = {50, 100, 150}, k = {0, 0.5, 0.7}; n = {1, 0.8, 0.6}; andPr = {1, 10, 100}. Heat transfer enhancement downstream of the expansion plane, i.e., Nusselt numbers greater than the downstream fully developed value,Nu/Nufd > 1, is only observed forPr = 10 and 100. In general, higher Prandtl numbers, power-law index values, and annular-diameter-ratios, result in more significant heat transfer enhancement downstream of the expansion plane. Heat transfer augmentation, forPr = 10 and 100, increases with the annular-diameter-ratio. For a given annular-diameter-ratio and Reynolds numbers, increasing the Prandtl number fromPr = 10 toPr = 100, always results in higher peakNuvalues,Numax, for both Newtonian and shear-thinning flows. AllNumaxvalues are located downstream of the flow reattachment point, in the case of suddenly expanding round pipe flows, i.e.,κ = 0. However, for suddenly expanding annular pipe flows, i.e., κ = 0.5 and 0.7,Numaxvalues appear upstream the flow reattachment point. ForPr = 10 and 100, shear-thinning flows display two local peakNu/Nufdvalues, in comparison with one peak value in the case of Newtonian flows. The highest heat transfer enhancement,Numax/Nufd ≈ 5, is observed at κ = 0.7, n = 0.6, andPr = 100.