Understanding drag enhancement through rheological measurements: Role of operating conditions

Drag reduction (DR) using polymer additives is typically assessed using large and costly flow loop systems. Recently, rotational rheometers have emerged as a quick and practical alternative. A Reynolds-number-based (Re-based) method, developed by our group using rotational rheometers, has shown strong agreement with conventional DR measurements. A challenge for low-Re testing platforms is drag enhancement. In this study, we examined how mixing conditions, additive concentrations, molecular weight, and temperature influence drag enhancement. To assess the impact of additive mixing intensity and time, we compared mildly mixed samples to samples subjected to turbulent mixing in a flow loop at different time intervals. At low concentrations, where drag enhancement was absent, mild mixing was sufficient, whereas turbulent mixing increased the risk of degradation, reducing DR performance. At higher concentrations, drag enhancement emerged under mild mixing, decreased with increasing mixing intensity, transitioned to DR, which peaked and then declined under excessive mixing due to degradation. Results suggest that drag enhancement can occur at high Re when the Weissenberg number (Wi) exceeds a threshold, likely due to strong elastic forces. A brief transient resistance to chain stretching may further amplify this behavior. Increased temperature delayed DR onset in samples without drag enhancement and reduced the magnitude of drag enhancement where it was present. Higher molecular weights were associated with higher drag enhancement. These findings clarify the interplay of mixing time, temperature, concentration, and molecular weight in causing drag enhancement, highlighting the capability of the Re-based rheometer approach for evaluating polymer performance under industrially relevant conditions.