Study On Enhanced Heat Transfer Characteristics Of A Wavy-walled Tube Heat Exchanger Under Nanofluids And Pulsating Flow Fields

With the rapid consumption of non-renewable energy,improving the heat transfer efficiency of shell-and-tube heat exchangers,using certain optimization methods to enhance heat exchanger performance,and reducing the power consumption during heat transfer have become an urgent problem to be solved.This study aims to improve the efficiency of shell-and-tube heat exchangers by using a composite enhanced heat transfer technique,replacing the straight-walled tube in conventional heat exchangers with the wavy-walled tube,using nanofluids as the cooling mass based on structural optimization of the heat exchanger,and combining the enhanced heat transfer method with the pulsating flow field to significantly improve the heat transfer efficiency of the heat exchanger.In this paper,the flow and heat transfer characteristics of the wavy-walled tube heat exchanger and the conventional straight-walled tube heat exchanger at different volume flow rates are compared using numerical simulation methods;the effects of particle type,volume fraction,particle size,and base fluid type of nanofluids on the flow and heat transfer characteristics of the wavy-walled tube heat exchanger are investigated;the pressure drop characteristics,heat transfer characteristics and comprehensive heat transfer performance of the wavy-walled tube heat exchanger with pulsating parameters（amplitude and frequency）under pulsating flow field are studied in depth;the heat transfer characteristics of the wavy-walled tube heat exchanger under the combined conditions of nanofluids and pulsating flow fields are comprehensively analyzed.The calculation results show that the comprehensive heat transfer effect of the wavy-walled tube heat exchanger has a greater improvement than that of the straight-walled tube heat exchanger,the maximum reduction in shell pressure drop is5.25%and the maximum increase in convective heat transfer coefficient is 9.67%compared to the straight-walled tube heat exchanger the in the study range;Cu nanoparticles have more prominent heat transfer advantages than Cu O,Al₂O₃ and Si O₂nanoparticles;the shell process pressure drop,convective heat transfer coefficient,and performance evaluation coefficient of the wavy-walled tube heat exchanger are proportional to the volume flow rate of the heat exchanger and the volume fraction of Cu-water nanofluids,inversely proportional to the nanoparticle sizes,and the performance evaluation coefficient of the wavy-walled tube heat exchanger under Cu-water nanofluids is up to 2.53.The cooling effect of the three alcohol-based nanofluids in the wavy-walled tube heat exchanger is also investigated,and the thermal conductivity of the three alcohol-based nanofluids is calculated using molecular dynamics simulations.The results show that the thermal conductivity of the three nanofluids increases with increasing volume fraction,and the largest increase in the thermal conductivity of the Cu-glycol-water nanofluid is 30.6%.It is also found that the shell-side pressure drop and convective heat transfer coefficient of the alcohol-based nanofluid in the wavy-walled tube heat exchanger are significantly higher compared with the base fluid.The comprehensive heat transfer performance of the three nanofluids is found to be better compared to the base fluid when the increase in pump work consumption and heat transfer performance of the wavy-walled tube heat exchanger is taken into account.Among them,the Cu-propylene glycol-water nanofluids show the strongest comprehensive heat transfer performance,Cu-ethanol-water nanofluids and Cu-ethylene glycol-water nanofluids show the peak comprehensive heat transfer performance in the studied range.The above results indicate that the synergistic application of the wavy-walled tube and nanofluids result in better comprehensive heat transfer performance of shell-and-tube heat exchangers.The study of the wavy-walled tube heat exchanger under pulsating flow fields shows that the flow field in the heat exchanger under the pulsating flow field is more fully developed and the vortex in the heat exchanger increases with the increase of amplitude and decreases with the increase of frequency.In terms of pressure drop characteristics,the instantaneous pressure drop of the heat exchanger shell process fluid increases with the increase of pulsation frequency and amplitude,but not strictly in accordance with the sinusoidal law of change;the average pressure drop of the heat exchanger under pulsating flow fields is positively correlated with the amplitude,and the average pressure drop gradually decreases with the increase of frequency under the same amplitude,but the change is small.It is also found that the heat transfer enhancement coefficient of the wavy-walled tube heat exchanger under pulsating flow field is greater than 1（1.76～2.24）,and is proportional to the pulsating amplitude,but increases first and then decreases with the increase of frequency and volume flow rate;the performance evaluation coefficient of the heat exchanger under pulsating flow fields increases gradually with the increase of frequency and then tends to be flat;with the increase of amplitude,the performance evaluation coefficient of the heat exchanger gradually decreases.Meanwhile,the field synergy analysis shows that the field synergy number of the wavy-walled tube heat exchanger is much less than 1,which indicates that the synergy between the heat exchanger shell process temperature field and velocity field is low.The results also show that the field synergy number is inversely proportional to the volume flow rate and positively proportional to the pulsation amplitude;the field synergy number shows a trend of increasing and then decreasing with the change in pulsation frequency.The degree of influence of nanofluids and pulsating flow fields on the wavy-walled tube heat exchanger shell process pressure drop varies at different volume flow rates:at low flow rates,the heat exchanger pressure drop decreases in the composite condition compared to the single condition instead,and with the increase of volume flow rate,the composite pressure drop is higher than the single condition.It is also found that the increase of the convective heat transfer coefficient in the shell process of the wavy-walled tube heat exchanger is in the order of composite>nanofluids>pulsating flow fields.The comprehensive heat transfer performance of the wavy-walled tube heat exchanger is greater than that of a single use of nanofluids or the application of pulsating flow fields in the case of combined nanofluids and pulsating flow fields enhanced heat transfer.Compared with the heat exchanger under the straight-walled tube,pure water,and constant flow field,the heat exchanger under the composite enhanced heat transfer conditions of the wavy-walled tube,nanofluids,and pulsating flow fields has the largest field synergy number and the best comprehensive heat transfer performance.