_Article_  
Convetion Heat Transfer in 3D Wavy Direct  
Absorber Solar Collector Based on Two-Phase  
Nanofluid Approach  
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environmental pollution and global warming [3]. All the factors, as mentioned above, have drawn attention to researchers all over the world to use renewable energy technologies [4], such as solar energy [5; 6]. Solar energy may be a solution to the current energy crisis, as the amount of energy in the solar flux landing on the Earth's surface in an hour is greater than all the energy consumed by humans in a year [7]. The difficulty extends toward collecting this energy efficiently and transforming it into something valuable. A solar thermal collector is a device which utilizes the solar energy via collecting and concentrating solar radiation [8; 9]. The collected thermal energy is transferred through a flowing fluid that can be used in various thermal applications, such as water heating, building heating, and other industrial applications [10]. Nevertheless, solar energy has a huge operation expense and low-efficiency [11], which motivates many researchers to optimize and enhance the performance of such devices [12]. In the case of a standard model, particularly concerning a solar thermal collector that employs a black cover as the absorber which later conveys heat toward the fluid, the efficiency by which the absorber gains and transfers heat into the working fluid remains inadequate. The direct absorption solar collector (DASC) is a current production of solar collectors into which solar radiation is directly absorbed via a transport medium [13]. In the 1970s, DASC was initially introduced for improving the efficiency of collectors; the system incorporates solar flux in the employed fluid [14; 15; 16] directly.  
Typical thermal liquids mentioned before such as water, ethylene/propylene glycol, and oil perform a vital purpose in numerous engineering sectors, including chemical production, electronic applications, power generation, heating and cooling processes, air conditioning, microelectronics, space and defense, and nuclear reactor cooling. These conventional fluids have weak thermal properties than the solids [17]. Various attempts for enhancing the heat transfer production of these liquids such as microchannels and fins with extended-surface, injection/suction of fluids, electrical/magnetic fields, and the vibration have been made; however, all these attempts have reached a dead end. New technologies such as nanotechnology and nanoscience utilizing the improved thermo-physical properties of conventional fluids have become a vital issue in heat transfer science [18; 19].  
An efficiency improvement of solar thermal collectors via employing nanofluids using DASC was demonstrated in various researches [20; 21; 22; 23; 24]. A comparison concerning the single- and two-phase procedures of nanofluids has been examined in several investigations [25; 26; 27]. A few authors also studied the three-dimensional behavior of nanofluids [28; 29]. Surface waviness is a very efficient way to enhance heat transfer in solar collecting devices [30; 31; 32].  
There is a significant number of achievements within the scope of heat transfer mode throughout direct absorption solar collectors; however, the 3-D model regarding the enhancement of collector efficiency, introducing surface waviness and nanofluid for such devices is yet to be studied. In this study, we focus on the two-phase nanofluid approach in a 3D wavy direct absorber solar collector. Moreover, the influences of various governing parameters as well as physical parameters are also discussed.