Flow And Heat Transfer Characteristics Of Supercritical Pressure CO₂ In Channels

The rapid economic growth in our country prompts a great demand of the energy resources.Employing clean working medium and making full use of existing energy should be the main object in the future energy cycles.The supercritical CO2 cycle with compact layout and high efficiency has been widely used in many advanced power systems.Heat exchangers are important components in power cycles,and their performance has direct influence on the efficiency of the whole system.However,drastically changing thermophysical properties of supercritical CO2 and complex channels of some compact heat exchangers bring great challenges to the design and optimization of these heat transfer facilities.Hence,it is significant to clarify the complicated flow and heat transfer mechanism of supercritical CO2 in various heat exchangers.Based on both numerical simulations and experimental tests,the thermal-hydraulic performance of supercritical pressure CO2(sCO2)in channels were investigated and several novel channels were proposed in the present thesis.Firstly,straight channels with uniform heat flux were established to explore the flow and heat transfer mechanism of the sCO2.To obtain relatively larger heat transfer coefficient as well as ensure small pressure loss and entropy generation,the CO2 should operate in conditions with relatively small ratio of heat flux to mass flux and small pressure in single straight channels.Among three tubes with different cross section shapes and the same hydraulic diameter,the circular one can contribute to the largest heat transfer coefficient,and the square one has the smallest friction factor.According to both theory analysis and simulation results,the effective thermal conductivity in the viscous layer and buffer layer has determinant effect on the local heat transfer coefficient in channels.Heat fluxes in some heat transfer appliances,such as the solar collector in solar power systems and water wall in coal-fired power plants,are apparently non-uniform.Heat transfer characteristics under non-uniform heating conditions would be more complex than that with uniform heat flux.The investigation of the effect of the non-uniform heat flux was conducted in straight channels with the solid domain.Under most circumstances,the more non-uniform the heat flux is in the circumferential direction of the outer wall,the worse heat transfer performance the sCO2 has.For one-side heating conditions,it is better to place the heat source as closer as possible to the bottom of the tube for larger heat transfer coefficient.With four enhanced tubes proposed in this dissertation,the comprehensive performance of the sCO2 could have an improvement of about 23%,and the non-uniformity of the inner wall heat flux and temperature could be significantly reduced.The coordination distribution principle could interpret the heat transfer in enhanced tubes very well.The thermal-hydraulic characteristics obtained in single circular channels are apparently insufficient for the design and optimization of Printed Circuit Heat Exchanger(PCHE),whose channel structure and arrangements are more complicated.It is necessary to utilize coupled models to obtain more realistic flow and heat transfer performance in heat exchangers.Based on the analysis in horizontal semicircular straight channels,the ratio of the secondary flow number to Reynolds number(Se/Re)was innovatively employed to determine the effect of buoyancy resulting from the changing properties in horizontal channels.When the Se/Re is larger than 0.1,the buoyancy effect in horizontal channels cannot be ignored.This novel criterion could give more accurate predictions for the buoyancy effect on both overall and local heat transfer performance than existing ones.Under the conditions with relatively low Reynolds number,the axial heat conduction makes a great difference to the heat transfer performance in coupled models.The existing axial wall conduction number cannot provide exact prediction for the effect of the axial heat conduction on the local heat transfer performance of sCO2.Compared with the straight channels,zigzag ones can significantly enhance the heat transfer in PCHE,while at the same time,apparently increased flow resistance is observed.Investigations on the zigzag PCHE showed that zigzag channels with the bend angle between 110° and 130° have relatively good comprehensive performance in terms of the first and second laws of thermodynamics.The field synergy principle could provide great interpretation for the flow and heat transfer characteristics in zigzag channels with different bend angles.The reverse flows mainly occur near the corner region,and they could significantly promote the local field synergy and lessen the local entropy generation.The secondary flows get stronger with the decrease of the bend angle.They could result in relatively even temperature distribution and great field synergy between the velocity and the temperature gradient in cross sections.Finally,a novel airfoil fin(AFF)PCHE with a 100 kW class heat transfer capacity was experimentally tested as a cooler in a supercritical CO2 system which covers wide ranges of temperature and pressure.The novel AFF structure has superior hydraulic performance as the pressure drop in the AFF PCHE is only about 1/6 of that in the zigzag PCHE with a comparative heat transfer rate.Numerical investigations also showed that the broadening of the fin has no obvious improvement in heat transfer coefficient but increases the friction factor massively.At smaller temperature and mass flow rate,the periodic fluctuations of the heat transfer coefficient and comprehensive performance criterion could be weakened significantly,thereby ensuring the safe and stable operation of the heat exchangers.This study starts from simulations in single straight channels with uniform heat flux,then explores the effect of non-uniform heat flux on straight channels.Afterwards,more realistic coupled thermal-hydraulic characteristics of PCHE with different channel structures are obtained based on both simulations and experiments.We have revealed the complex heat transfer mechanism resulting from the fiercely changing properties and proposed optimized channels.In addition,a novel criterion was put forward to give more accurate prediction for the buoyancy effect on heat transfer enhancement in horizontal channels.The present work could provide significant guideline for the design and optimization of some novel compact heat exchangers whose working medium has changing thermophysical properties.