Theoretical And Experimental Study On Heat Transfer Enhancement Of Printed Circuit Heat Exchanger

The printed circuit heat exchanger(PCHE)has attracted more and more attention as a new type of micro-channel heat exchanger with high efficiency and compact feature in the field of industry.Hence,how to improve the heat transfer performance of PCHE more effectively has become one of the emphases for research.The complex convention heat transfer process inside the micro-channels is the key to study the heat transfer performance of PCHE.However,literature on the technique of heat transfer enhancement of heat exchangers in China is still lacking currently.This paper conducted a profound study to analyze the flow and heat transfer characteristics of the working medium which is supercritical LNG inside different geometries of micro-channels with the combination of a theoretical analysis and an experimental study,in order to explore the methods and principles of heat transfer enhancement inside the flow channels of the heat exchangers.Therefore,this fore-mentioned study is significantly important both in terms of theory and practice.First of all,the paper employed a numerical analysis to study the characteristics of flow and heat-exchange performance of the supercritical LNG inside the micro-channels in different conditions of mass fluxes with the semi-circular of 1.5mm diameter micro-channel and different bending angles(?=0°~45°)as the study subject.The study parameters include the changes of temperature,velocity,convention heat transfer coefficient(h),Nusselt number and pressure drop of the working medium caused by different working conditions in the process of convective heat transfer.The results showed that,compared with the straight channels,periodically bended channels could improve the capability of the convective heat transfer.When other conditions are kept consistent,the temperature rise and speed of supercritical LNG increase as the bending angles of the channels increase,resulting in not only higher convective heat transfer capability,but also greater flow resistance and local resistance to different degrees.By comparing the convective heat transfer coefficient and pressure drop,the study indicated that the optimal comprehensive flow and heat transfer characteristics of supercritical LNG occur when the bending angle of the channels is 15°.However,when the bending angles of the channels are constant,greater mass flux would be more likely to increase the convective heat transfer capability,although this may cause more pressure losses.As a result,within the study scope of this paper,greater mass flux would improve the comprehensive flow and heat transfer characteristics of supercritical LNG in different bending angles of channels.Then,the study aimed to explore the enhancement effects on the convective heat transfer by the traditional channels where airfoil fins were added while the working medium was constant.By comparing the flow and heat transfer performance of supercritical LNG between the straight channels and ones with added airfoil fins,it is found that the Nu/Eu value of straight channels is far less than that of the ones with added airfoil fins based on the same condition of flow rates.As the mass flow rate changed,the differences between them could range from 46.2% to 51.07%,which indicated that the increasement of the airfoil fins would enhance the enhancement effects of the convective heat transfer significantly.On this basis,a numerical simulation was performed on the flow and heat transfer characteristics of supercritical LNG in channels with different airfoil fin arrangements.The results revealed that the arrangement and the placement density of the airfoil fins would influence the flow and heat transfer capability of supercritical LNG.The staggered fin arrangement was favorable to the heat transfer,and a greater staggering interval Ls between the fins would result in a better comprehensive flow and heat transfer performance of the working medium.On this basis,the convective heat transfer coefficient(h)increased as the vertical dimension of airfoil fins Lv decreased,while the pressure drop increased significantly as well.According to the analysis of the ratio of Nu to Eu,it turned out that the greater the vertical dimensions of airfoil fins Lv were,the greater the comprehensive heat transfer capability of supercritical LNG would be.Accordingly,the supercritical LNG would have better flow and heat transfer characteristics in channels with a staggered and sparse arrangement of airfoil fins.At last,based on the theoretical analysis,an experiment system was designed and established to apply the printed circuit heat exchanger with airfoil fins to low-temperature and high-pressure vaporization system.Considering the flammable and combustible characteristics of LNG,supercritical nitrogen was selected as the substitute for supercritical LNG in this experiment system for safety concern.The experiment results indicated that the lower the system pressure was,the greater the turbulence intensity of supercritical nitrogen in printed circuit heat exchanger with airfoil fins and the convective heat transfer coefficient would be.In the meanwhile,the pressure drop would increase significantly as well.Under different conditions of pressures,the overall heat transfer coefficient of printed circuit heat exchanger with airfoil fins ranged from 850 W/m2 K to 2600 W/m2 K,and the heat transfer efficiency was up to approx.98%.Under the constant pressure condition,greater mass flux of supercritical nitrogen would be more likely to cause greater overall heat transfer coefficient U of the printed circuit heat exchanger with airfoil fins,which also indicated the better heat transfer performance of this heat exchanger would have.