## 1 Introduction  
The aerodynamic design of wind turbine blades plays an important role in determining the overall performance of the wind turbine. In order to increase the cross-sectional area through which the wind passes and to generate the maximum torque to drive the generator of a wind turbine, it is necessary to select a material with the appropriate weight and strength for the manufacture of large blades. Currently, most wind turbine blades are made of Fiber Reinforced Plastic (FRP), which can increase strength and efficiency to ensure lightness for design purposes [1, 2]. Among the various FRPs, Glass Fiber Reinforced Plastic (GFRP), a composite material mainly used for wind turbine blades, is twice as strong in strength as steel of the same volume, and Carbon Fiber Reinforced Plastic (CFRP) is up to five times stronger in strength than steel. There is a study result that shows that the stiffness is twice as good, by up to eight times [3, 4]. Although CFRP is superior to GFRP in many ways, it is relatively expensive, so it is common for blade manufacturers to mix the two materials to manufacture blades [5]. Wind turbine blades are manufactured using CFRP mainly for the spar-cap, which is the main structure of blades that requires high strength [6, 7, 8, 9, 10]. However, despite the fine mechanical properties of CFRP spar-caps, various defects that occur during the manufacturing process can cause blade failure. The spar-cap manufacturing process for wind turbine blades is mostly by hand, making defect prevention extremely difficult. Defects such as fiber misalignment [11] of the CFRP prepreg, the basic unit of the spar-cap, occur in the lamination process and make the spar-cap more brittle. As the content of this defect increases, fatigue resistance and corrosion resistance decrease, resulting in a significant decrease in shear strength and compressive strength [12; 13]. In particular, various defects caused by voids due to uncured epoxy are considered to be the main causes of adverse effects on the physical characteristics of composite materials [14; 15; 16]. The content and size of defects that occur in the epoxy, the matrix constituting the CFRP of the spar-cap, are greatly affected by process conditions and the environment. From the viewpoint of heat transfer in terms of various environmental and process factors, uneven curing of the epoxy can cause residual stress formation in composite materials [17; 18; 19]. Residual stress in the epoxy is responsible for the formation of voids and various defects, which can cause catastrophic failure of the spar-cap. In order to eliminate the temperature unevenness phenomenon of the curing process, it is necessary to specifically control the temperature of the spar-cap and use equipment such as an autoclave to maintain the same temperature at every position in the material. Due to the large size of wind turbine blades, the introduction of equipment is quite difficult with current technology [20]. Despite the fact that uneven curing is responsible for areas that have many effects on defect formation, heat transfer numerical analysis studies considering the thickness of the spar-cap are inadequate. Some blade manufacturers have made efforts to ensure the surface temperature unevenness of the spar-cap by installing a separate heat shield around the spar-cap mold to block a certain amount of heat loss, but the influence of the season is high. It is too large to alleviate the temperature unevenness phenomenon. In fact, there was a report that measured the mold temperature during curing, recognizing the problem of spar-cap quality deterioration due to temperature. It became clear that the temperature of the mold was high in the thick part of the spar-cap. In this temperature measurement experiment, the uneven temperature due to the thickness was checked, and heat transfer numerical analysis research was required to solve this phenomenon.