## 1 Introduction  
Thermal load estimation due to the release of latent heat, particularly in HVAC&R systems, is usually associated with a condensation process, which makes it an important topic to study and understand [1, 2]. Additionally, power industry and chemical processes are among many applications wherein the condensation process occurs. There are two common modes of condensation on solid surfaces, namely, dropwise condensation, DWC, and filmwise condensation, FWC [3]. Comparing both modes, low surface free energy and low wettability are features of the dropwise condensation mode. The wettability level, also known as the hydrophobicity level, is measured by the surface contact angle wherein the surface is deemed hydrophobic when the contact angle is equal to or greater than 90 degrees. Meanwhile, a surface is deemed hydrophilic when the contact angle is less than 90 degrees. The advantage of DWC mode over FWC is the presence of surface renewal where drops at a certain radius depart their original positions and sweep away smaller drops beneath them. The surface renewal allows the re-nucleation of small drops on the condensing surface, characterized by low thermal resistance and thus higher heat flux. Models determining dropwise condensation heat transfer involve the sub-modeling of heat transfer through a single drop and the sub-modeling of the drop size distribution function. Early researchers established theories and conducted experiments on DWC [4; 5; 6; 7; 8; 9]. They showed that the thermal performance of the dropwise condensation process depends on many influential factors such as total temperature drop, contact angle, contact angle hysteresis, nucleation density, minimum and maximum drop size, and others. Many researchers have contributed tremendous work to describing DWC phenomena and improving the prediction of thermal performance. [PERSON] and [PERSON] established a DWC theory to determine the heat flux for temperature drops due to conduction through a hemispherical drop, surface curvature, interphase temperature, and coating layer [5; 10; 11]. Furthermore, an average drop size distribution was estimated by [PERSON] and [PERSON] [12], which describes the growth cycle of drops as a packing problem. The growth process of smaller drops is due to direct condensation, whereas the growth of larger drops is due to drops' coalescence. By applying the population balance model, a drop size distribution is established for small drops that are swept away by departing drops [13]. An improved DWC model that also utilizes the population model for small drops is introduced by [PERSON], which considers all temperature drops in the DWC model of [PERSON] and [PERSON] in addition to the temperature drop due to coating thickness [14]. A drop-effective radius is defined to separate the distribution of drops that grow mainly by direct condensation and those that grow by coalescence. The consideration of the effect of surface free energy is included in the DWC model by approximating a drop with a contact angle as a cylinder of equal base area and volume [15]. The model concluded that DWC heat flux increases with a larger contact angle due to a smaller departing radius. An enhanced mathematical model was established by [PERSON] and [PERSON] to include the effect of contact angle in the expression of heat transfer through a single drop [16]. The influence of contact angle on DWC heat flux for the horizontal tube was presented by [PERSON] and [PERSON] [17], who estimated a tube surface as a series of flat plates with different inclination angles. Another DWC model, based on various populations of small drops on an inclined circular tube, is performed by [PERSON] et al. [18]. The authors suggest that DWC thermal performance is favorable for a more inclined surface. A simulation work for an improved drop size distribution was performed by [PERSON] et al., which shows the effect of drops overlapping and multiple nucleation [19].