Study Of Thermal Property Enhancement Of Bromide Salts And Heat Transfer Performance Of Molten Salt Heat Pipe

Concentrating solar power?CSP?is one of the most promising solar energy utilization in renewable energy,and its heat transfer and thermal storage technology plays a key role in CSP.As a new type of heat transfer and heat storage working medium,molten salt has been widely used in CSP.At present,the scholars all over the world mainly focus on the study of carbonate and nitrate,but the research of bromide is less.Gravity heat pipe?GHP?has been widely used in solar energy,aerospace,waste heat recovery and other fields because of its excellent performance.The alkali,water,freon and organism were widely used as the working fluid of GHP currently,but the accidents like explosion,burning and leaks cause not only economic losses on the production,but also endanger the health and life of the staff.There are many characteristic of molten salt,such as widely operating temperature range,low vapor pressure and friendly environment,molten salt is a potiental working fluid for heat pipes.In this paper,the base salt used in the presented experiments were sodium bromide?AR?,potassium bromide?AR?,lithium bromide?AR?and calcium bromide?AR?.The SiO₂ nanoparticles with different size and different concentration were dispersed in bromide eutectic to prepare twenty-four kinds of nano-bromides.Heat of fusion,melting point and decomposing point were measured via a differential scanning calorimeter?DSC?.In result,it was observed that fusion of heat was enhanced by up to maximum 89.61 % and decomposing point was enhanced by up to 68.4 ?C by doping with 0.7 wt.% nanoparticles?10 nm in diameter?,and melting point was decreased by up to only 3.4 ?C by doping with 0.5 wt.% nanoparticles?20 nm in diameter?.In addition,possible mechanism involved in heat of fusion,decomposing point and melting point enhancement was discussed.And the mechanism study found that the SiO₂ nanoparticles,which dispersed in the molten salts formed an interfacial layer on the surface of molten salt.The nano-bromides needed a higher energy to melt because of the existence of higher thermal resistance,which caused by the interface layer.So,the nano-bromides could own higher fusion heat and better thermal stability.This paper developed a new type intermediate temperature molten salt gravity heat pipe?MSGHP?,the working fluid used in the intermediate temperature gravity heat pipes were aluminum bromide?AR?and titanium chloride?TiCl4,AR?respectively.The work performance of the MSGHP was tested by the GHP experimental system under the given lab experimental conditions.The start-up experimental of both the aluminum bromide GHP and the titanium chloride GHP were successfully,and all the MSGHP showed a consistency axial temperature in the isothermal performance experiment,and the reliable performance in long time steady operation.The analysis found that the heat transfer performance of MSGHP was good,and the maximum equivalent convective heat transfer coefficient was 16000W?m-2?K-1.Compared with naphthalene GHP,the start-up performance of AlBr3 GHP and TiCl4 GHP were better than that.Meanwhile,this paper studied the influence of the charging mass and the inclination angle under the consistent lab controlled conditions.The experimental results showed that the inclination angle and charging mass have significant influence on the working performance of MSGHP,and MSGHP achieved the maximum working performance at inclination angle of 60?.The GHP had a much shorter start-up time when they were charged with less working fluid;and a much higher equivalent convection heat transfer coefficient when they were charged with more working fluid.This paper found that the thermal properties of nanoparticles had a significant enhancement on thermal properties of bromide salts,and provided reference data for the study in this flied,which through experimental study and theoretical analysis;developed and tested a new type of molten salt gravity heat pipe,then achieved a good start-up performance and the reliability in long time steady operation.The overall research results were expected to provide certain guidance for further design and operation of molten salt in high-intermediate-temperature heat transfer and storage scenarios.