Abstract
This study explores an experimental investigation on single-phase and boiling heat transfer in micro- and minichannel heat exchangers (counter-current type), respectively, with molten salt heating, which is an application of a solar thermal power plant. For the present study, the heat of the solar thermal on molten salt is simulated by using electrical power supply. The high temperature liquefied molten salt (Hitec) is driven into the test section by the piston device. The working fluid in the hot side is molten salt and in the cold side is water (for the boiling experiment) or helium (for the single-phase flow experiment). The heat exchangers are prepared from aluminum alloy materials through electric discharge machine and computer numerical control machine processes. Both of micro- and minichannel heat exchangers contain five channels on each side and are covered with stainless steel. The depth and width of the microchannel on both sides are both 0.4 mm, and the length of the microchannel is 40 mm. The depth, width, and length of the minichannel are 2 mm, 1.5 mm, and 60 mm, respectively. During the single-phase flow experiment, i.e., the cold side fluid is helium, the mass flux of helium is varied from 16.9 to 30.5 kg/m2s, and the mass flux of molten salt is varied from 306 to 605 kg/m2s. Moreover, during the boiling experiments, i.e., the cold side fluid is water, the mass flux of water is varied from 3.9 to 5 kg/m2s. The results of the single-phase flow experiment demonstrate that heat transfer characteristics are significantly influenced by the mass flux of the cold side fluid. The higher mass flux of helium leads to the better heat transfer performance (i.e., heat transfer rate, heat transfer coefficient, and effectiveness). The effectiveness of the single-phase flow experiment is 24%-65%. On the other hand, the results of the boiling experiment show the transition boiling characteristics of water, indicating that heat flux decreases with an increase in the wall temperature. The boiling heat transfer coefficients, calculated by two methods, i.e., boiling model and log-mean-temperature-difference (LMTD), show an insignificant difference, which is within 20%. Moreover, the effectiveness of the boiling experiment is 75%-90%.