Study On Heat Transfer Mechanism And Application Of Nano-organic Working Fluid In Organic Rankine Cycle

Among the low temperature waste heat recover technologies,Organic Rankine Cycle(ORC)is considered as the most effective and matured one.However,due to the poor thermal conductivity and lower Prandtl number,the heat exchangers in ORC system are expensive and huge in size,which decrease the technological economy of the system to a great extent.Recently,several experimental studies showed that the heat transfer performance of organic fluid could be enhanced by adding nanoparticle.Unfortunately,systematical study on heat transfer characteristic of nano-organic fluid is limited.In this paper,the mechanisms of heat conduction,pool boiling heat transfer for nanofluid(nano-organic fluid)were studied theoretically,flow boiling heat transfer characteristic was experimentally investigated and the correlation was proposed,to provide basic data and design reference for the further research.The main research contents are as follows:(1)A theoretical model for thermal conductivity of nanofluids was developed incorporating thermal conductivity of nanoparticles/aggregates,interfacial layer,particle aggregation and Brownian motion-induced convection from multiple nanoparticles/aggregates.The predicated result using aggregate size,which represented the particle size in the actual condition of nanofluids,filed well with the experimental data for water-,ethylene glycol(EG)-based nanofluids and nano-organic fluid.Compared to serveral empirical models,the present model could be more precisely and generality in predicating thermal conductivity of nanofluids.In addition,the complicated and time consuming process for simulating the nanoparticle motion and random collision was eliminated,thus make the present model more compactly and clearly.A parametric analysis was conducted to investigate the dependence of effective thermal conductivity of nanofluids on nanoparticle concentration,temperature,interfacial layer thickness,aggregate size and fractal dimension.(2)According to the HFP model,the existing correlations for calculating bubble dynamic parameters of nucleate pool boiling were verified firstly.It is found that these correlations were limited in accuracy and scope.Based on the fractal distribution of active nucleation sites on boiling surface,a fractal model for nucleate pool boiling of nanofluids was established.Model results using nano-organic fluid and water based nanofluid were in good agreement with the available experimental data,thus the validity and versatility of model is verified.Effects of wall superheat,fractal dimension,contact angle and liquid properties on nucleate pool boiling characteristics of nanofluids were obtained from the present model.For water based nanofluid,the decrease of contact angle induced by nanoparticle deposition would result in a reduction of nucleate sites on heating surface.When the degree of decreased contact angle exceeded that of increased thermal conductivity,then heat transfer deterioration occurred.For organic fluid,the change in contact angle induced by nanoparticle deposition can be ignored due to the high wettability of organic fluid.It is speculated the enhanced thermal conductivity was one of the reasons for boiling heat transfer enhancement.(3)A flow boiling heat transfer experimental apparatus for nano-organic fluid was constructed,multiwalled carbon nanotube MWCNTs-R123 nano-organic fluids with volume concentration of 0.02%,0.05%and 0.2%were prepared.Flow boiling heat transfer characteristics and pressure drop are experimentally investigated using MWCNTs-R123 flowing inside a horizontal circular tube.The effects of particle concentration,mass flux,and vapor quality on the heat transfer coefficient and pressure drop of MWCNTs-R123-based nanofluid were analyzed.The research results showed that flow boiling heat transfer coefficient and frictional pressure drop increased with nanoparticle concentration as expected,increment of heat transfer coefficient was large than that of pressure drop.The maximum enhancement in heat transfer coefficient was 27.1%.The effect of nanoparticle on heat transfer coefficient and pressure drop was gradually suppressed with increasing mass flux.A modified correlation for predicting the flow boiling heat transfer coefficient of nanorefrigerants was proposed,and the proposed correlation predicts 95%of the points with a deviation of ±15%.In addition,frictional pressure drop can be predicted using the Miiller-Steinhagen and Heck correlation with a mean absolute error of 13.07%.(4)Combining with the analytical methods of heat transfer,thermodynamics and fluid mechanics,the effect of nano-organic fluid on the thermal performance of evaporator was emphatically investigated based on the thermodynamic model of ORC system.The effect of nanoparticle type,concentration and flow rate on the HTC of nanofluid,overall HTC of evaporator,total heat capacity,heat exchangers effectiveness and irreversible loss of heat exchanger were investigated.8123 was employed as the working medium.Variation trend of irreversible loss in evaporator with flow rate was revealed by introducing entropy generation number(Ns)and entropy impact factor(Fs).Research results showed that MWCNTs-R123 exhibits the best thermal performance compared with other nano-organic fluid.When the concentration of nanoparticle was low,the effect of nanoparticle on heat transfer enhancement was significant.With the further increasing of the nanoparticle concentration,effect of nanoparticle on pressure drop was strengthened causing the thermal index value increase at first then decrease.Entropy generation in evaporator decrease at first then increase.The results also shown that an optimal flow rate(near 540kg/h)which minimize the total entropy production of heat exchanger was existed under each nanoparticle concentration.