## 1 Introduction  
With the continuous advancement and development in the structure of the cities, demand for new technologies and, as a result, energy has increased [1]. With limitation in fossil fuel and environmental pollution due to the combustion of carbonaceous fuels, special attention has been paid to renewable energies as a reliable source of energy [2,3]. Solar energy is one of the potential renewable energy resources, which is available to a vast region of the world. However, the intermittent behavior of solar energy during the on-sun and off-sun periods dictates that the solar receivers and collectorsmust have plausible thermal efficiency to maximize the energy absorption from the sun [4; 5; 6]. A solar thermal collector is a two-phase apparatus embedded with heat pipes (HPs) to absorb the solar thermal energy and transport it to the top header using the HPs [7]. A HP is a heat-exchanging medium, which employs a working fluid with plausible thermal properties with low boiling point which can easily be evaporated inside the HP [8; 9; 10; 11]. A HP composed of an \"evaporator section\", in which the working fluid is evaporated using boiling mechanism and flows through an \"adiabatic section\" to reach a \"condenser section\". In the condenser section, the vapor phase loses its thermal energy and is condensed to the liquid phase. The formed liquid returns to the evaporator using a gravity-assisted falling film. The heat transfer in HPs does not need any external energy, thereby nominating HPs as one of the efficient technologies for heat transfer within a confined space. The current HPs employ alcohols and acetone as the working fluid; however, these working fluids have poor thermal conductivity, which strongly affects the thermal performance of the HPs. Since Aragon National Laboratory introduced the \"nanofluids\", a new direction of research was developed in thermal engineering science and in different research areas including single and two-phase heat transfer [8; 12; 13; 14; 15; 16; 17; 18; 19]. A nanofluid is a suspension of some conductive powders such as metal oxides uniformly dispersed within a common cooling fluid such as water or oil [20; 21; 22]. Hence, dispersion of nanoparticles requires specific techniques such as using ultrasonic waves and controlling the pH value of the nanofluid [21]. So far, many endeavors have been made to understand the effect of nanofluids on the thermo-physical properties of nanofluids. Thermal conductivity is one of the key physical properties of the nanofluids, which strongly influences the HTC of the system. Together with thermal conductivity, the heat capacity of the working fluid is another important parameter, which can improve the thermal characteristics of a system. Studies have also shown that the presence of the nanoparticles in the liquid adds some nano-scale phenomena into the heat-transfer mechanism. The first important contributor is the Brownian motion of the nanoparticles that enhances the heat transfer by random walk and random motion of the particles within the system [23; 24]. In a random movement, particles collide with the hot walls and absorb the thermal energy by conduction mechanism and carry the thermal energy to the cold region and release the thermal energy with the convection heat transfer. The thermophoresis phenomenon is another participant to the heat-transfer increment, in which the nanoparticles move from the hot wall to the cold wall due to the temperature gradient between the cold and hot wall [25]. Considering the above advantages of the nanofluids, many endeavors have been undertaken to use the nanofluids in thermal systems. The usage of the nanofluids in HPs has substantially been reviewed in the literature. For instance, [PERSON] et al. [26] numerically studied the HTC of three different nanofluids including Al\({}_{2}\)O\({}_{3}\), TiO\({}_{2}\), and CuO nanoparticles flowing through a cylindrical pipe. They modeled the system using a 2D model by assuming a steady state condition as well as incompressible and Newtonian nanofluids.