The performance of heat exchangers can be improved to perform a certain heat-transfer duty by heat transfer enhancement techniques. In general, these techniques can be divided into two groups: active and passive techniques. The active techniques require external forces, e.g. electric field, acoustic or surface vibration, etc. The passive techniques require fluid additives or special surface geometries. Curved tubes have been used as one of the passive heat transfer enhancement techniques and are the most widely used tubes in several heat transfer applications. This article provides a literature review on heat transfer and flow characteristics of single-phase and two-phase flow in curved tubes. Three main categories of curved tubes; helically coiled tubes, spirally coiled tubes, and other coiled tubes, are described. A review of published relevant correlations of single-phase heat transfer coefficients and single-phase, two-phase friction factors are presented.

A review of flow and heat transfer characteristics in curved tubes

The potential industrial applications of curved tubes for single- and two-phase flow are reviewed within the context of physics of flow, trends in the development of technology, and its laboratory to industrial-scale commercialization. Comparison of the performance of curved tube configurations demonstrates its edge over the conventional motionless mixers, heat exchangers, and reactors. Alongside, their respective advantages and limitations are also highlighted. Further, a compendium of the available correlations for single- and two-phase friction factor and heat- and mass-transfer coefficient in curved tubes has also been presented. Key issues regarding the design parameters governing the performance of the curved tubes for mixing and heat- and mass-transfer that impact the research, development, and scale-up or scale-down of such devices are also analyzed. Emerging trends for the development of a new class of curved tubes, namely, inverters and serpentine and chaotic devices are also presented.

A Review on the Potential Applications of Curved Geometries in Process Industry

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Pressure drop and heat transfer study in tube-in-tube helical heat exchanger

Numerical study on turbulent heat transfer and pressure drop of nanofluid in coiled tube-in-tube heat exchangers

Solutions for steady, fully developed, laminar flow of an incompressible Newtonian fluid through a helically coiled tube of finite pitch were obtained by finite difference calculations. Prior theoretical solutions have apparently been limited to toroidal flow (with zero pitch). The computed results are consistent with those for this limiting case and with prior experimental data for finite pitch. The behavior for helical coils is intermediate between that for a straight pipe and a torus, and, for a moderate degree of pitch differs only slightly from that for a torus. A correlating equation was developed for the friction factor for all Reynolds numbers, all ratios of coil radius to lube radius and all ratios of pitch to coil radius for which the flow remains laminar.

S. N. Gupta

Papers

Momentum Transfer in Curved Pipes. 2. Non-Newtonian Fluids