Pool boiling heat transfer enhancement of aqueous solution with quaternary ammonium cationic surfactants on copper surface

Pool boiling is a widely encountered heat transfer phenomenon and the enhancement of boiling heat transfer performance is of vital importance for real applications. Adding surfactant into distilled water is regarded to be a promising and effective approach for heat transfer improvement. The present study selected four kinds of quaternary ammonium cationic surfactants with an increased molecular weight, namely, decyltrimethylammonium bromide (DeTAB), dodecyltrimethylammonium bromide (DoTAB), myristylmethylammonium bromide (MTAB) and cetyltrimethylammonium bromide (CTAB), to investigate the pool boiling performance of their aqueous solutions on a copper heater with and without electric field under different concentrations. Their dynamic surface tensions and viscosities were also measured. The bubble dynamics during pool boiling and apparent contact angle on copper surface before and after boiling test were evaluated. Experimental results reveal that the aqueous surfactant solution can significantly enhance the heat transfer coefficient (HTC) compared with distilled water. The HTC exhibits an increment with concentration until critical micelle concentration (CMC) and then levels off. The maximum relative HTC enhancement can reach up to 209.3%, which is ascribed to the increased nucleate site and bubble departure frequency. In addition to the reduction in surface tension, a repulsion force caused by electrostatic interaction is also responsible for the changes in nucleate site and bubble departure frequency. However, the critical heat flux (CHF) shows a continuous decrement with surfactant concentration. In a low concentration surfactant solution, the CHF is even slightly higher than that of water, which is caused by an improvement in surface wettability after boiling test. The CHF in moderate and high surfactant concentration will be lower. The surfactant with a smaller CMC tends to have a higher HTC and a lower CHF under a low concentration. However, for a relatively high concentration comparative to CMC, the four kinds of quaternary ammonium cationic surfactant solutions tend to have pretty similar boiling curves. The electric field is found to have a negligible influence on the pool boiling performance in aqueous surfactant solutions. Results in present study can provide valuable guidance for surfactant selection during the design of highly effective heat exchangers.