| Boiling heat transfer has been playing an important role in chemical industry,energy ector,nuclear power,air conditioning and refrigeration.The parameters for judging boiling heat transfer performance include onset of nucleate boiling(ONB),heat transfer coefficient(HTC)and critical heat flux(CHF).With the application of various advanced technologies and materials,researchers pay more attention to the reliability and safety of boiling equipment while trying to improve the boiling heat transfer performance.Because when the heat flux of boiling equipment exceeds the critical heat flux density,boiling deterioration will occur.In this paper,surfactant sodium dodecyl sulfate(SDS),a common boiling heat transfer additive,was used to enhance pool boiling heat transfer,and the critical heat flux of surfactant SDS solution was increased by combining with bi-conductive surface.In this paper,the boiling heat transfer characteristics of surfactant SDS solution on bi-conductive surface and smooth copper surface were studied by experiment and theoretical analysis.The effects of contact angle and surface tension on bubble departure diameter Dd,nucleation site density N and bubble departure frequency f,as well as heat flux distribution of surfactant solution and deionized water were analyzed by boiling PRI model.In addition,the bubble dynamics in the boiling deterioration stage was recorded by high-speed camera,and the heat flux and wall superheat were analyzed.The experimental results show that the wall superheat of surfactant SDS solution increases slowly in the boiling deterioration stage,which is due to the interaction between the large mushroom bubble generated in the center of the heater surface and the small bubble generated around the heater surface.The small bubble will enter and break the large mushroom bubble,which strengthens the disturbance of the liquid around the heater surface and makes the wall superheat rise slowly.In the pool boiling experiments with bi-conductive surfaces,it is found that the surfactant SDS solution can obtain higher critical heat flux.In addition,it is also found that the boiling heat transfer performance of surfactant SDS solution on the two surfaces is similar in the early stage of pool boiling.In the stage of boiling deterioration,the wall superheat on smooth copper surface increases obviously;on the bi-conductive surface,it enters into "quasi-static transition boiling".In this stage,even if the input power continues to increase,the wall superheat will not rise sharply,but will increase slowly in a state of intense fluctuation.The time required for the temperature rise of 25 ℃ on smooth copper surface and bi-conductive surface is recorded.It is found that the time required for temperature rise of 25 ℃ on bi-conductive surface is one order of magnitude higher than that on smooth copper surface.By recording the bubble dynamics and the change of superheat heat flux density in two kinds of surface boiling deterioration stages,combined with the analysis of thermal resistance,it is found that because of the existence of large bubbles in the center of the heating surface,the contact between the heater surface and the liquid is blocked,the axial heat transfer is obviously weakened,and the heat transfer from the center of the heater surface to the surrounding can not be ignored.In this case,the dry area on the smooth copper surface will expand rapidly,accelerating the deterioration of boiling;on the bi-conductive surface,due to the effect of epoxy resin with low thermal conductivity,the transverse heat transfer is obviously inhibited,and the surrounding of the heater surface can still be wetted by liquid,and the competition between the wetting area and the dry area causes the wall superheat to fluctuate violently. |
