Parametric study of heat transfer coefficient and friction factor in a corrugated channel

In this paper, an experimental investigation of the heat transfer coefficient and friction factor in a vertical annular channel with an inner wavy (corrugated) wall is presented. Six inner elements were tested, five of them with different corrugation pitch and height and one element smooth. The corrugation height varies from 0.4 mm to 0.8 mm and the pitch varies from 4 mm to 8 mm. The hydraulic diameter varies from 7.1 mm to 10.7 mm and the Reynolds number, based on the hydraulic diameter, varies in the range of 7,500 to 37,000. Based on the empirical analysis, it was found that the most suitable characteristic parameter to define the effect of corrugation on the flow and heat transfer is the 'severity index' φ=e²/pD_h, where e is the corrugation height, p is the pitch and D_h is the hydraulic diameter. General empirical correlations were developed for the friction factor and heat transfer coefficient for an annular channel with a wavy inner wall under turbulent flow regime. Examining the thermal effectivity of a wavy element, commonly calculated with factors R1, R3, R5 to assess the real benefits offered by corrugation tubes, it is evident that by replacing a smooth wall with a wavy one, the thermal performance of the system can improve by 30–190%, depending on various fixed parameters, e.g., mass flow rate, pump power, pressure gradient and heat transfer surface. However, for the same elements the increase in friction factor was in the range of 70–210%. Regarding the size of the heat transfer area, it is shown that if a wavy element is used, the effective size for heat transfer area can be reduced up to 30–65%, depending on the Reynolds number and the severity index.