Research On Boiling Heat Transfer With Surfactant Additives

Boiling and heat transfer enhancement have been playing an important role in industrial energy saving,safe operation of equipment,air conditioning refrigerators and heat dissipation of electronic products.Because of weak heat dissipation,CPU and GPU may be burnt out,the nuclear radiation disaster may occur.Therefore,it is necessary to further study the means of enhancing boiling heat transfer and the mechanism of boiling heat transfer.Subcooled pool boiling around heat wire,flow boiling in square channel,saturated plate pool boiling and LBM simulation of saturated pool boiling and flow boiling were researched in this thesis.The working fluid was an aqueous solution of cetyltrimethyl ammonium chloride?CTAC?with addition of sodium salicylate?NaSal?at the same mass concentration.The results showed that the experimental conditions affected the boiling heat transfer of surfactant solution.In the subcooled pool boiling of surfactant solution around heat wire experiment,a platinum wire was horizontally placed in the fluid for heating.It's found that the tested surfactant solution significantly augmented boiling heat transfer as compared with water,and the best heat transfer performance was found at the concentration ranging from 5 to 100 ppm.And the highest CHF was increased to 125% by CTAC/NaSal addition.With a high-speed camera and an inverted microscope,the nucleation boiling process on the heated wire was recorded.For the first time,we observed a new bubble explosion phenomenon around the heated wire in CTAC/NaSal solutions,which is apparently different from the previously reported boiling explosion due to homogeneous nucleation.Together with the observed strong jet-flow behaviors,bubble explosion strengthened the local disturbance and enhanced the boiling heat transfer.The velocity of bubble jet was about 0.5 m·s^-1.Comparative experiments of boiling around a heated wire in different types of surfactant solutions,ethanol and silicone oil were carried out in order to explore the mechanism of boiling heat transfer enhancement by surfactant addition.All these fluids had lower surface tensions than that of water.Cationic surfactant of CTAC,anionic surfactant of sodium dodecylbenzenesulfonate?SDBS?and nonionic surfactant of alkyl polyglycoside?APG?were used.Bubble jet and bubble explosion phenomenon were found in all three kinds of surfactant solutions.The nature of bubble jet and bubble explosion was the uneven surface tension distribution at the gas-liquid interface.Due to the heat sensitivity characteristics,APG solution had a lower critical heat flux?CHF?than water,while CTAC and SDBS solution had higher CHF.In flow boiling of surfactant solution experiment,tests were performed with 6.0×3.5 mm² cross-section channel and 8mm thick Nickel foil heating surface.The present experimental results show that,the flow boiling in CTAC/NaSal solutions at an appropriate concentration?100 ppm?behaved a good flow boiling performance.Besides,CHF was increased to about 125% by CTAC/NaSal addition when the solution concentration was beyond 100 ppm.And the flow boiling in CTAC/NaSal solution had stronger pressure fluctuations than that of pure water.Visualizations of the flow boiling process show that bubbles in CTAC/NaSal solution have smaller diameters longer existence time and larger population density than that in water.These bubbles increased the gas-liquid interface,which may be another reason of flow boiling heat transfer enhancement and stronger pressure fluctuations.And the number of nucleate sites in CTAC/NaSal solution was larger than that in water.Therefore,the nature of surfactant addition enhancing the heat transfer and CHF may be the nucleate site density and rewetting the heating surface.In saturated plate pool boiling of water experiment,indium tin oxide?ITO?transparent coating heat surface was used to observe the gas-liquid distribution and nucleate site distribution.Bubbles merging at the heating surface decreased the nucleate site density and two phase interface which reduced the boiling heat transfer.At CHF,the merged bubbles caused the vapor film and reduced the boiling heat transfer.In Lattice Bolzmann Method?LBM?numerical simulation of saturated pool boiling and flow boiling,LBGK model,D2Q9 model and P-R two phase equation of state?EOS?were used to simulate the saturated plate pool boiling and flow boiling.Nu number was highest at gas-liquid phase interface.Bubbles merging at the heating surface reduced two phase interface,decreased the effective nucleate sites and reduced the boiling heat transfer.Therefore,bubbles merging may be another factor of surfactant enhancing boiling heat transfer.