Cross-helix corrugation: The optimal geometry for effective food thermal processing

In the present paper, an innovative and effective heat transfer enhancement technique for food processing, cross-helix profile wall corrugation, is proposed and tested. In the food industry, the two most promising corrugation profiles are the transversal and single-helix ones because they both satisfy hygienic design principles. Among the available wall corrugation techniques, transversal corrugation allows for the highest heat transfer performance, and the spirally corrugated tubes guarantee the easiest manufacturing. For this reason, single-helix corrugated tubes are the most commonly employed in heat exchangers for food processing. The cross-helix profile presented in this work represents an intermediate solution between transversal and single-helix corrugation aimed at combining their positive aspects. One of the main goals of the current research is to identify an optimal geometry that maximises the heat transfer performance by limiting the pressure drop augmentation for this specific engineering application (i.e., food thermal processing). For this purpose, the effect of the geometrical parameters of the corrugation profile is investigated by varying two of the most influent quantities in terms of heat transfer performance and pressure drop: corrugation depth and corrugation pitch. Six pipes characterised by different cross-helix corrugations are tested. Their performance is evaluated by studying the forced convective heat transfer in the Reynolds and Prandtl numbers ranges (50–14,000 and 5–150, respectively), using ethylene glycol, water and a mixture of the two as the working fluids. The outcomes show that corrugation depth plays a crucial role in enhancing the heat transfer performance of the tested pipes. An optimal geometry is established, and correlations to describe its thermal and fluid flow behaviours are proposed. This optimal geometry shows superior performance to that of the most widely adopted types of corrugation. For the low/intermediate Reynolds number range (i.e., 200–2000), the efficiency of the proposed cross-helix profile is up to three times greater than that of the single helix and is also greater than that of transversally corrugated tubes. These outcomes make cross-helix a recommended option to be used in the design of optimised heat exchangers. Because the studied geometry is expressly developed for food industry application, a set of measurements is also performed in which apricot juice is adopted as the working fluid. The findings confirm the efficiency of the proposed correlations with non-Newtonian fluid foods and enable them to be extended to a wide range of real food industrial applications.