## 1 Introduction  
The use of nanofluids in many applications has received an increasing interest in recent years, mainly because their thermal properties are better than those of conventional fluids. Its use in the field of renewable and sustainable energy technologies allows for creating high-efficient and more economic systems. In this regard, many research studies [1; 2; 3; 4; 5; 6; 7; 8] have evaluated, theoretically and experimentally, the increase in performance in heat transfer applications and solar thermal systems associated with the use of alumina-water as working fluid. With the aim of providing an optimum design of the energy system, thermal properties of the nanofluid should be known beforehand, including thermal conductivity and viscosity, among other properties. Many researchers, in recent years, have evaluated these thermodynamic properties using different types of nanoparticles. Refs. [9; 10] presented extensive reviews on theoretical and experimental studies about nanofluids. In a more recent review, and related to the thermal conductivity of nanofluids containing nanoparticles of alumina and water as base fluid, ref. [11] summed up the results of [12; 13; 14; 15; 16], who assessed the influence of nanoparticle concentration on thermal conductivity. The same authors also summarized, in the review, the works of [17; 18; 19; 20] which evaluated the impact of nanofluid temperature on the thermal conductivity of the same fluid. In addition to the previous works included in the review of [11], refs. [21; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32] evaluated the way in which the temperature and nanoparticle volume fraction modify the thermal conductivity of nanofluids containing water and alumina. Table 1 summarizes experimental research found in the open literature evaluating the effect of temperature and nanoparticle concentration on the thermal conductivity of nanofluids composed of alumina and water. This table includes the ranges of the volume concentration and temperature used in their experiments, the nanoparticle phase and size and the test method used to measure the thermal conductivity. In the table, it can be observed that very large differences exist in the experimental conditions, and also in the size of the nanoparticles used. The effect of other variables has also been experimentally evaluated. For example, in [31], the authors have assessed the influence of surfactants on nanofluids thermal conductivity. The authors have shown that, after adding the surfactant to the aluminum oxide-water nanofluid, the stabilization of nanoparticles improves, and thermal conductivity rises when the nanoparticle volume fraction and the temperature increase. In their review, ref. [33] presented a summary of the published literature on the effects of the ultrasonication time on the thermophysical properties of nanofluids, including the thermal conductivity of aluminum oxide-water. Specifically, ref. [30] studied the effect of the sonication time on this nanofluid thermal conductivity using concentrations of alumina between 0.1 and 20 vol% and temperatures in the range from 25 to 65 \({}^{\circ}\)C. Sonication time was modified approximately between 25 and 115 min. The authors found no significant effect on the thermal conductivity of the nanofluid, with this property remaining almost constant for each alumina concentration. In a similar way, ref. [28] observed that for sonication times starting at 90 min and going up to 150 min, thermal conductivity remained approximately constant. However, in the range from 30 to 90 min, an increase in thermal conductivity is observed. In this case, the experiments were developed using 2 and 3 vol% of nanoparticles and the temperature changed from 15 to 45 \({}^{\circ}\)C. Higher values of thermal conductivities were reported by [28] in comparison to [30], using the same temperature and volume concentration of nanoparticles, and also without ultrasonication.