Effect of soluble additives, boric acid (H3BO3) and salt (NaCl), In pool boiling heat transfer

Abstract

The effects on pool boiling heat transfer of aqueous solutions of boric acid (H3BO3) and sodium chloride (NaCI) as working fluids have been studied. Borated and NaCI water were prepared by dissolving 0.5~5% volume concentration of boric acid and NaCI in distilled-deionized water. The pool boiling tests were conducted using 1 x 1 cm2 flat heaters at 1 atm. The critical heat flux (CHF) dramatically increased compared to boiling pure water. At the end of boiling tests it was observed that particles of boric acid and NaCI had deposited and formed a coating on the heater surface. The CHF enhancement and surface modification during boiling tests were very similar to those obtained from boiling with nanofluids. Additional experiments were carried out to investigate the reliability of the additives deposition in pure water. The boric acid and NaCI coatings disappeared after repeated boiling tests on the same surface due to the soluble nature of the coatings, thus CHF enhancement no longer existed. These results demonstrate that not only insoluble nanoparticles but also soluble salts can be deposited during boiling process and the deposited layer is solely responsible for significant CHF enhancement.