Investigation Of Phase-changing Heat Transfer Characteristics Of Functionalized Nanofluid And Its Application In Gravity-assisted Heat Pipes

The past decade or more focused on exploring the heat transfer performance of nanofluids which are suspensions consisting of base fluids and nanoparticles. Most of the researches have demonstrated the potential of the nanofluid in the heat transfer enhancement and the nanofluid is predicted as an alternative working liquid in the heat transfer engineering which can meet the challenge of increasing demand of heat removal.Base fluids used are primarily common fluids such as water, engine oil, ethylene glycol and refrigerants. Nanoparticles used include metals such as Ag, Au, Cu, Fe, oxides such as CuO, SiO₂, Al₂O₃, TiO₂, ZnO, Fe₃O₄, carbonides such as SiC, TiC, nitrides such as AlN, SiN, carbon nanotubes and so on.Two ways are usually used to make nanofluids: the one-step method and the two-step method. The one step method simultaneously makes and disperses nanoparticles into base fluids. The two-step method first produces the nanoparticles and then disperses nanoparticles in base fluids. The one-step method can disperse nanoparticles uniformly. But its technical process is complicated and costly. Also, the one-step method lacks the ability to produce nanoparticles in large amounts. The two-step method is more widely used because of its convenience, low cost and large-amount producing capacity. Therefore most of the literatures reported use the two-step method. But the stability of nanofluids prepared by the two-step method is a key issue preventing their commercial application. Nanoparticles tend to aggregate and will settle out of the base fluids if severe aggregation happens. Two ways are usually used to improve the stability of nanofluids: (1) adding surfactants, (2) ultrasonication. The first way can increase the repulsion of nanoparticles, which restricts the aggregation. The second way can mechanically destroy the aggregation. However, the stability can only be available in a short period of time by doing this. Long time stability is still not easy to obtain.To solve this problem, the present study proposes a new way to prepare functionalized nanofluid by functionalizing silica nanoparticles. The functionalization is achieved by grafting silanes directly to the surface of silica nanoparticles. Disperse the obtained functionalized nanoparticles into water and keep the solution at the environmental temperature of 50 oC for 12 hours. Then the obtained solution is called functionalized nanofluid. Functionalized nanoparticles can still keep dispersing well after the nanofluid has been standing for 12 months and no sedimentation is observed. Functionalized nanoparticles disperse well in functionalized nanofluid. The functionalization can destroy the aggregation of nanoparticles. No surfactant is needed to make the functionalized nanofluid stable.Besides, research of phase-changing heat transfer of nanofluid is a hot topic in heat transfer field, such as heat transfer enhancement of nanofluid boiling, thermosyphon using nanofluid and refrigerant. Many researches reported the phase-changing heat transfer enhancement of nanofluid. Researches also reported that a deposition layer formed by nanoparticles forms on the heating surface. This layer will result in severe uncertainty for heat transfer and restrict the commercial application of nanofluids. Functionalized nanofluid, however, will not form deposition layer on the heating surface after the boiling experiment. This can guarantee the stable operation of euquipment using nanofluids as working fluids.Though many researches reported the heat transfer enhancement of nanofluids, there are still some references reporting that nanofluids have no effects on heat transfer or nanofluids deteriorate the heat transfer. Because of the large deviation of reported results, no agreement has been reached on the mechanism of phase-changing heat transfer of nanofluids. More experiment and mechanism exploration are needed to have a better understanding of the phase-changing heat transfer of nanofluids.Therefore, based on the good stability of functionalized nanofluid and no deposition layer forming on the heating surface by functionalized nanofluild, the present work studies the phase-changing heat transfer performance of functionalized nanofluid in two aspects: pool boiling heat transfer of functionalized nanofuid, and heat transfer of gravity-assisted heat pipe using nanofluid. The gravity-assisted heat pipe studied includes thermosyphon and loop thermosyphon. Meanwhile, the heat transfer performance of traditional nanofluid is also explored and compared with the functionalized nanofluid case to have a better understanding of the phase-changing heat transfer mechanism of nanofluids. Traditional nanofluid is prepared with unfunctionalized nanoparticles, which are the same nanoparticles with the ones prepraing functionalized nanoparticles. Mass concentrations of functionalized nanofluid and traditional nanofluid both range within 0.5%-2.5%. Thermoproperties of functionalized nanofluid and traditional nanofluid are measured including thermal conductivity, viscosity and surface tension. Also, the surface characteristics of the heating surface are explored before and after the experiment.Regarding to the boiling heat transfer of nanofluid, the present study designs and builds an experimental setup, measures the boiling heat transfer of water, functionalized nanofluid and traditional nanofluid, studies the boiling mechanism of nanofluid, and quantitatively calculates the heat transfer coefficient (HTC) and the critical heat flux (CHF) for nanofluids. The experimental pressures are 7.4 kPa, 20 kPa and 103 kPa respectively. Results indicate that:(1) A porous deposition layer exists on the heated surface during the boiling experiment using traditional nanofluid. The roughness of the heating surface with the deposition layer decreases compared with the water case. No deposition layer exists for functionalized nanofluid. The roughness of the heating surface using functionalized nanofluid increases compared with the water case. The pool boiling heat transfer of functionalized nanofluid and traditional nanofluid differs due to the differences of thermoproperties of nanofluids and the surface characteristics of heated surfaces.(2) Functionalized nanofluid can enhance the HTC comparing with the water case, but has nearly no effects on the CHF. The HTC enhancement results from changes of the thermophysical properties of the nanofluid, the contact angle and the surface roughness. Functionalized nanofluid doesn't show meaningful nano-scale characteristics. Equations predicting the HTC and the CHF for traditional fluids can also be used for those of functionalized nanofluid.(3) Traditional nanofluid can enhance the CHF, but conversely deteriorate the HTC. During the boiling experiment using traditional nanofluid, the deposition layer formed decreases the contact angle of functionalized nanofluid on the heated surface, and increases the microcavity number and the thermal resistance of the heating surface. The decrease of the contact angle mainly results in the CHF enhancement. Combing both effects of thermoproperties of nanofluids and the surface characteristics of the heating surface, equations predicting CHF of common liquid can also be used for that of traditional nanofluid.Regarding to the heat transfer of thermosyphon using nanofluid, the present study designs and builds an experimental setup, measures the evaporating and condensing heat transfer of water, functionalized nanofluid and traditional nanofluid, and the comprehensive heat transfer characteristics of the thermosyphon. The present study also investigates the boiling mechanism of nanofluid, and quantitatively calculates the HTC and the maximum heat flux (MHF) for nanofluids. The experimental pressures are 7.4 kPa, 15.75 kPa and 31.38 kPa respectively. Results indicate that: (1) Traditional nanofluid forms a deposition layer on the heating surface after the experiment, while no layer forms for functionalized nanofluid.(2) Functionalized nanofluid can enhance the evaporating HTC, while has generally no effects on the MHF. Since no deposition layer exists, the surface characteristic doesn't change much for functionalized nanofluid. The HTC enhancement of functionalized nanofluid results mainly from changes of thermoproperties of functionalized nanofluid.(3) Traditional nanofluid deteriorates the evaporating HTC of the thermosyphon, but enhances the MHF of the thermosyphon. The great decrease of the contact angle on the deposition layer corresponds to the MHF enhancement for traditional nanofluid.(4) Combining the data of contact angle and MHF of water, functioinalized nanofluid and traditional nanofluid, the present study modifies the exisiting predicting equation and can calculate the MHF more effectively. The deviation of experimental data and predicted ones by this equation ranges within 5%.(5) Functionalized nanofluid and traditional nanofluid both have no effects on the condensing heat transfer of the thermosyphon.Regarding to the heat transfer of loop thermosyphon using nanofluid, the present study designs and builds an experimental setup, and measures the evaporating and condensing heat transfer of water, functionalized nanofluid and traditional nanofluid, and the comprehensive heat transfer characteristics of the loop thermosyphon. The present study also investigates the heat transfer mechanism of nanofluid, and quantitatively calculates the HTC and the MHF for nanofluids in loop thermosyphon. The experimental pressures are 7.4 kPa, 15.75 kPa and 31.38 kPa respectively. Results indicate that: Functionalized nanofluid deteriorates the evaporating heat transfer (HTC and MHF) of the loop thermosyphon. Traditional nanofluid deteirorates the HTC, but enhances the MHF the of loop thermosyphon. Functionalized nanofluid and traditional nanofluid both have no effects on the condensing heat transfer of the loop thermosyphon. The heat transfer in loop thermosyphon is associated with the low-speed flow of working fluids. Therefore, the viscosity enhancement mainly results in the heat transfer deterioration. Also, both nanofluids have no effects on the condensing heat transfer. They both deteriorate the comprehensive heat transfer characteristics of the loop thermosyphon.