Experimental studies on heat transfer to Newtonian fluids through spiral coils

•R′ number, a new dimensionless number is introduced.•R′ number successfully characterizes the heat transfer phenomenon in curved pipes.•Great simplification of correlations in curved coils is achieved by use of R′ number.•Newtonian fluids subjected to isothermal and non-isothermal boundary wall conditions are studied.•The correlations developed can be used for spiral coil design of the thermal systems.

Spiral coils are investigated for flow of petroleum base oils SN70, SN150 and SN300 under isothermal steady state and a combination of isothermal steady state & non-isothermal unsteady state conditions for “Graetz problem”. The experiments were conducted with spiral coils with curvature ratios 0.01568, 0.01778, 0.019, 0.02466, 0.02525, 0.02666, 0.028055, 0.02952, 0.03011, under laminar flow condition. A new dimensionless number R′ number is introduced which is proved by the method of dimensionless analysis. The Nusselt number is correlated with R′ number for Newtonian fluids based on the experimental data. A number of correlations are developed for Newtonian fluids under isothermal steady state (total 130 tests) and a combination of isothermal steady state & non-isothermal unsteady state conditions (total 154 tests). It is found that R′ number characterizes the heat transfer phenomenon very well and it also simplifies the correlations on curved coils. The developed correlations are compared with the correlations developed by earlier investigators and are found to be in good agreement. These correlations can be used for the design of spiral coil heat exchangers.