Experimental Thermohydraulic and Second‐Law Assessment of Glycerol/Water Mixtures in a Counterflow Double‐Pipe Heat Exchanger

This study experimentally investigates the thermohydraulic performance of a compact counterflow double‐pipe heat exchanger using water and glycerol/water mixtures at 30 and 50 wt% concentrations. Experiments were conducted over a Reynolds‐number range of approximately 600–19,000 to evaluate the effects of fluid properties and flow conditions on heat transfer and hydraulic behavior. The axial temperature distributions showed enhanced thermal energy exchange with increasing Reynolds number, driven by improved convective mixing. The results demonstrated that both the overall heat‐transfer coefficient and Nusselt number increased significantly with Reynolds number for all investigated fluids. The maximum experimental Nusselt numbers were approximately 49, 53, and 60 for water, 30 wt% glycerol/water, and 50 wt% glycerol/water mixtures, respectively. However, the improvement in thermal performance was accompanied by a noticeable increase in pressure drop due to the higher viscosity of the glycerol mixtures. The friction factor showed the expected decreasing trend with Reynolds number for all cases. An empirical correlation was developed to predict the experimental Nusselt number as a function of Reynolds and Prandtl numbers. The proposed correlation fit the experimental data satisfactorily, yielding a coefficient of determination of 0.943 and a mean absolute deviation of approximately 4.38%. These findings provide practical guidance for the design and optimization of compact double‐pipe heat exchangers operating with viscous working fluids and offer a reliable predictive tool for improving thermal‐system efficiency in industrial applications.