Heat recovery units are used for pre-heating the fresh air in waste heat plants. The nanofluid used as working fluid in the heat pipe will allow the heat recovery unit to benefit from the waste heat at lower temperatures, since it can evaporate at a temperature lower than the temperature of the base fluid. Therefore, the range of the operating temperature of the heat recovery unit will be increased. Different temperatures and flow rates of waste heat were used to find the optimum conditions for the evaporation of spinel nanofluid in the evaporator region of the heat pipe. Similar conditions were investigated for both of the cold fluid and optimal conditions on the condensation of nanofluid in the condenser region. In this study, an experimental setup was designed to improve the thermal performance of the air-to-air containing heat pipe heat recovery unit. The experiments were carried out using pure water-based nanofluids containing nano-sized ZnOAl2O3, MgOAl2O3, FeOAl2O3 particles, and the efficiencies of the heat transfer were investigated in this experiment. A thermosiphon mechanism that consists of five heat pipes, the length of each one is equal to 1 m, and the inner and the outer diameter of them is equal to 23.4 mm and 25.4 mm, respectively, those pipes are vacuumed, without wick and copper material. Heat pipes in the experimental setup; the evaporation zone is positioned to form 450 mm, the condensation zone 400 mm and the middle 150 mm parts to form the adiabatic region. At the beginning of the experiments, the working fluid charged to fill the 1/3 of the evaporation zone volumes of the heat pipes. Two different cooling air flow rates, 30 g·s−1 and 60 g·s−1, and three different heating air flow rates, 50 g·s−1, 70 g·s−1, 90 g·s−1 have been used, to calculate the heat obtained from the condensation zone. All the experiments were carried out by applying two different heaters with a power of, 1000 W and 2000 W, in the evaporation zone. Thus, the optimum temperatures and flow rates were found for all Reynolds numbers in the hot and cold fluid region, and its helps to determining the ranges of the operating temperature in the heat recovery unit. The experimental results that obtained from the two different working fluids (pure water and nanofluid) are compared. The best thermal performance of the system was found in the hot and cold air duct, when the Reynolds number was equal to 12 000. The thermal performance improvement rate was found as 92.8 % when the FeOAl2O3/water nanofluid used as working fluid, and 1000 W heater power and 60 g·s−1 cooling air flow applied.

中文翻译：

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使用尖晶石氧化物/水纳米流体在具有空气对空气热虹吸装置的热回收装置中改善热性能

热回收装置用于预热废热工厂中的新鲜空气。在热管中用作工作流体的纳米流体将允许热回收单元在较低温度下从废热中受益，因为它可以在低于基础流体温度的温度下蒸发。因此，热回收单元的工作温度范围将增加。利用不同温度和废热流量，寻找尖晶石纳米流体在热管蒸发区蒸发的最佳条件。研究了冷流体和冷凝器区域纳米流体冷凝的最佳条件的相似条件。在这项研究中，设计了一个实验装置来提高包含空气对空气的热管热回收装置的热性能。使用含有纳米尺寸的 ZnOAl2O3、MgOAl2O3、FeOAl2O3 颗粒的纯水基纳米流体进行实验，并在该实验中研究了传热效率。由五根热管组成的热虹吸机构，每根热管的长度等于1 m，它们的内径和外径分别等于23.4 mm和25.4 mm，这些管子是抽真空的，没有灯芯和铜材料。实验装置中的热管；蒸发区450mm，冷凝区400mm，中间150mm形成绝热区。在实验开始时，充填工作流体以填充热管蒸发区容积的 1/3。两种不同的冷却空气流量，30 g·s-1 和 60 g·s-1，以及三种不同的加热空气流量，50 g·s-1、70 g·s-1、90 g·s-1 具有用于计算从冷凝区获得的热量。所有的实验都是通过在蒸发区应用功率为 1000 W 和 2000 W 的两种不同的加热器进行的。因此，在冷热流体区域中找到了所有雷诺数的最佳温度和流速，这有助于确定热回收单元的工作温度范围。比较了从两种不同的工作流体（纯水和纳米流体）获得的实验结果。系统的最佳热性能出现在冷热风道中，