EXPERIMENTAL INVESTIGATION ON THE CONVECTIVEHEAT TRANSFER IN COILED CORRUGATED TUBES

In the present analysis, the forced convective heat transfer in coiled tubes having corrugated wall was experimentally studied in the Reynolds number range 50÷1100 by adopting Ethylene Glycol as working fluid. The primary aim of the investigation was to study the combined effect of the wall curvature and of wall corrugation in heat exchangers for highly viscous fluids. Two coiled tubes having a spirally corrugated wall with curvature ratios of 0.031 and 0.056, were investigated under the uniform heat flux boundary condition. The main conclusion is that the effect of the curvature ratio on the Nusselt number is small for Re<500. Moreover, in this flow regime the wall corrugation does not produce any significant beneficial effect. By increasing the Reynolds number the effect of the wall corrugation becomes instead more important and a sudden increase, over the smooth wall behavior, of the average Nusselt number is observed. Moreover the wall corrugation effect is amplified by the curvature effect. The enhancement is ascribable to an early transition to an unstable regime and, for the here considered corrugation profile, the critical Reynolds number value which demarcates the onset of this flow regime is close to the one expected for straight spirally enhanced tubes.