Shape memory alloys enabled smart heat exchangers

Abstract

Modern heat exchangers encounter the paradox between the high performance and low flow resistance while most of the tasks feature in wide operating conditions where great functional flexibility is required. In this paper, we propose to tackle the dilemma by integrating shape memory alloys into the design of heat exchangers, utilizing a two-way shape memory alloy (SMA) strip as the self-adjusting vortex generator (VG) for heat transfer enhancement. The SMA based vortex generators are trained with predetermined deformation according to the temperature of the working media. To be specific, at low temperature, the SMA strip is flat to reduce the flow resistance. While the temperature is high, the SMA strip is curved to form a rectangular wing vortex generator to enhance the heat transfer. This concept was firstly verified via the experimental study followed by detailed numerical investigation focusing on the mechanism explorations. The deformation characteristic was verified by water bath. With the increase of the temperature of fluid, the heat transfer is increased by 110 ∼ 125 %. On the other hand, the Darcy friction factor can be reduced by 74 ∼ 90 %, when the heat transfer demand decreases based on the concept proposed. The details of flow and heat transfer are given and analyzed by using numerical simulation. And the numerical results are in good agreement with the experimental ones with the relative error of 11 % ∼ 3 % (on heat transfer, Re=8004400) and the warping height ΔZ=4/5 hydraulic diameter (Dh)). The ratio of heat transfer and flow resistance are positively related to the warping height (ΔZ) of VG. Under six calculated Reynolds numbers (Re), the maximum heat transfer enhancement coefficient can be reached at Re = 400 and 800, respectively, with the values ranging from 1.18 ∼ 2.68 for different ΔZ. In the case that ΔZ equals to 4 / 5 hydraulic diameter (Dh), the best performance can be achieved with Re≤1600. If Re≤800, the design can always maintain better performance than the original flat channel, and the maximum performance evaluation criteria (PEC) achieved is 1.32. Based on our research, the smart heat exchanger is expected to satisfy the heat transfer requirements of specific variable working conditions in the future.