Effect of surface engineering methods on refrigerant evaporating flow characteristics in annular tubes: Experimental approach

Techniques such as mechanical, electrochemical, and chemical etching are recognized as functional strategies for modifying surface roughness, which in turn manipulates surface energy. This research aims to delve into the influence of these factors on the heat transfer and pressure drop characteristics of two-phase R134a refrigerant flow during boiling inside horizontal annular concentric tubes with internal heat flux. The study scrutinizes the heat transfer coefficient and frictional pressure drop for R134a evaporating flow in an annular tube featuring an inner diameter of 28.6 mm and an outer diameter of 42 mm. The exploration spans a broad range of parameters, which include vapor quality from 0.15 to 0.8, mass flux from 6.72 to 20.16 kg/m²s, internal heat flux from 0.608 to 2.432 kW/m², and varying surface characteristics. The results show that escalating surface roughness via mechanical techniques introduces greater resistance to flow shear stress in comparison to other methods. Moreover, an increase in surface roughness contributes to a reduction in contact angle and an expansion of the heating area, thereby enhancing both heat transfer coefficient and pressure drop. However, the implementation of low-energy coating holds an insignificant influence on R134a heat transfer characteristics, potentially due to its diminutive surface tension. This results in minor variations in the refrigerant contact angle in relation to the coatings. Performance evaluation criteria have been employed to assess the optimal thermal performance of the surface engineering method, with peak performance achieved at a mass flux of 20.16 Kg/m²s and an internal heat flux of 2.432 kW/m². Ultimately, the study corroborates the experimental procedure by comparing the observed outcomes with established empirical correlations for R134a flow boiling.