Effect Of Aqueous Perfluoroalkyl Surfactants On Pool Boiling Heat Transfer Of Water

Nucleate pool boiling heat transfer is widely applied in nuclear power,refrigeration,industrial manufacturing and aerospace,etc.Enhancing pool boiling heat transfer has always been a crucial field of engineering heat and mass transfer.Surfactants can reduce surface tension of based fluid and it is an innovative means for improving pool boiling heat transfer.Therefore,it is significant to explore promising surfactants.In this work,three novel perfluoroalkyl surfactants with different ionic nature were applied: Perfluoroalkyl quaternary ammonium iodide(Le-134),Perfluoroalkyl phosphate(Le-107)and Perfluoroalkyl polyether(Le-180).The main researches and conclusions are as follows:1.Research on interface properties of aqueous perfluoroalkyl surfactants solutions(1)Le-134,Le-107,Le-180 can reduce the surface tension of deionized water to 16 m N/m,19 m N/m,18 m N/m,respectively.The magnitude in ascending order for Critical Micelle Concentration(CMC)is Le-180(15ppm)<Le-134(40ppm)<Le-107(150ppm).The magnitude in ascending order for the saturated adsorption amount ?max is Le-107 <Le-134<Le-180.The hydrophilic and hydrophobic groups and counter ions have great influence on equilibrium surface tension of three aqueous perfluoroalkyl surfactants solutions.When the hydrogen atom in the hydrophobic group is replaced by fluorine atom that increases ?max and leads to high surface activity.(2)The dynamic surface tension(DST)of Le-134 decreases fastest,Le-180 in second and Le-107 is the slowest.The surface tension of aqueous Le-134 solution decreases to less than 20 m N/m within 10 s and reach the quasi-equilibrium in 30 s.The surface tension of aqueous Le-107 solution decreases to about 30 m N/m within 30 s and reaches the quasi-equilibrium in 70 s.The surface tension of aqueous Le-180 solution decreases to about 20 m N/m within 40 s and reaches the quasi-equilibrium in 50 s.The adsorption of three surfactants is diffusion-only mechanism at lower concentration.However,it changes to mixed diffusion-kinetic controlled adsorption at higher concentration.Apparent diffusion coefficients(Da)decreases and adsorption barriers(Ea)increases as increasing concentration.The variation of adsorption kinetics is mainly due to the difference in molecular structures and the presence of micelles that influence on adsorption barriers.Ea increases as the increasing electrostatic energy barrier and time scale of micelles break-up.2.Research on pool boiling heat transfer of perfluoroalkyl surfactant solutionsThe pool boiling experiment was conducted with 10-300 ppm concentration of aqueous Le-134,Le-107 and Le-180 solutions in heat flux range of 10-80W/cm2.Boiling curves were measured and the bubble behaviors were recorded by high-speed camera.The experimental results show:(1)Novel perfluoroalkyl surfactant additives significantly change bubble behaviors in pool boiling heat transfer.In aqueous Le-134 and Le-180 solutions,the number of bubbles generated on the boiling surface increase with the increasing concentration meanwhile the bubble size decreases.At lower heat flux,the trend of bubble coalescence is reduced and the shape of bubbles is closer to spherical during the departure.Furthermore,bubble jet phenomenon is found in Le-134 and Le-180 solutions.The main reason of bubble behavior variation is the impact of novel perfluoroalkyl surfactant additives on interface characteristics.Nucleate site density increases by reducing surface tension and the diameter of bubble departure decreases.The trend of bubble coalescence is reduced by amphiphilic properties and electrostatic repulsive forces.(2)Le-134 and Le-180 effectively enhance pool boiling heat transfer compared to deionized water,and the enhancement is more obvious as the increasing concentration of surfactant.300 ppm Le-134 solution has the most significant enhancement at heat flux of 10W/cm2,which reduces the superheat of boiling surface by 49.3% and increases HTCs by 109.1% in comparison with deionized water.However,when the concentration of Le-107 exceeds 80 ppm,the generated bubbles were difficult to depart from the boiling surface due to poor wettability.The new perfluoroalkyl surfactants have high efficiency of reducing surface tension is the reason that they can significantly enhance the pool boiling heat transfer.In addition,bubble jet effect brings more disturbances that enhances heat convection between vapor and liquid.Heat transfer of new perfluoroalkyl surfactant solutions are closely related to dynamic adsorption characteristics and wettability.The adsorption efficiency is higher and wettability is more improved so the enhancement of heat transfer is more significant as increasing concentration.3.Verification and analysis of pool boiling heat transfer models(1)Analyze and verify the experimental results with the HTC model proposed by Kolev.The model is in good agreement with the deionized water data and the discrepancy is within 10%.The discrepancy is about 30% between the model and the experimental data of aqueous Le-134 and Le-180 solution.The reason for the difference may be the variation trend of HTC is affected by dynamic adsorption characteristics of Le-134 and Le-180.(2)Predict the critical heat flux(CHF)of the new aqueous perfluoroalkyl surfactant solution with the CHF model proposed by Zuber.The predicted CHF value of aqueous Le-134 and Le-180 solution is about 80-90W/cm2,which is 21%-30% lower than deionized water.The addition of Le-134 and Le-180 increases the bubble flows coalescing at higher heat flux tends to reduce CHF.