Study On The Characteristics Of Microchannel Heat Transfer And Dynamic Response Based On The Analysis Of The Characteristics Of Surface Energy

As a new high efficiency heat transfer working medium, nanorefrigerant has gain much attention from the researchers in the field of flow boiling heat transfer. It has much potential to be developed in the field of heat transfer enhancement. In order to explore the flow boiling heat transfer characteristics of nanorefrigerant in microchannels, the experiment platform has been constructed in this thesis. Based on the analysis of the characteristics of surface energy, the flow boiling heat transfer characteristics of Al2O3-R141 b nanorefrigerant in the steady state and the dynamic response characteristics of the systerm with different step signals have been studied through experiment in the 1mm×2mm rectangular microchannels.Comparative analysis of change regulation of the surface energy before and after the flow boiling experiment with nanorefrigerant of different concentration was conducted by measuring the contact angles of the microchannels surface, and it reveals the phenomenon that the nanorefrigerant deposits on the channel surface during the experiment. The experiment, on the one hand, shows that the heat transfer performance is enhanced because the massive amount of activation nucleuses and the earlier onset nucleate boiling caused by the deposition of the nanorefrigerant. On the other hand, the surface energy of the microchannels increases by 1.39 times, 1.89 times and 2.14 times respectively by using the nanorefrigerant with different concentration of 0.2%, 0.5% and 0.8% and this indicates that deposition of the nanoparticles intensified with the rise of the nanorefrigerant concentration which leads to the thermal resistance of the microchannels increases and then weakens the effect of nanorefrigerant’s heat transfer enhancement. Because of this, the impact factors of heat transfer enhancement diminish with the increase of the concentrations of the three kinds of nanorefrigerants.Dynamic response experiment with different step signals of three variables which stand for the flux of the working medium, flux of condensed water and heating voltage is conducted to analyze the influence the three kinds of step disturbances have on the dynamic heat transfer characteristics of the system and to build the dynamic response models of the outlet temperature of the system under the exactly three step signals. The experiment results indicate that under the three step signals of the flux of the working medium, flux of condensed water and heating voltage, the stability of the system decreases one by one and the system is more stable when it comes with the two signals of the flux. Under the three step signals of the flux of condensed water, heating voltage and the flux of the working medium, the responding hysteresis of the outlet temperature decreases one by one, this indicates that step signals’ site of action and acting effect are the key factors to the responding hysteresis.