Flow Melting And Cold Storage Characteristics Of Clathrate Hydrate Slurry

Cold energy storage can be applied in the district and central air-conditioning systems,which has become more and more popular in the developed countries like US and Japan asthe energy crisis occurred all over the world in recent years. The cold energy storage is abeneficial energy saving technology, and it can provide both society and economy benefits.The phase change material slurries (PCMS) attract lots of attentions, since they can be used asboth cold storage material and transportation medium. The cold carry capacity of PCMS isseveral times as large as the single-phase coolant due to the phase change of the PCMparticles, which leads to the reduction of the pumping power consumption. Moreover, theusage of PCMS as secondary refrigerant minimizes the charging and leaking amount of theanti-environment primary CFC and HCFC refrigerant, resulting in a environmental protection.Nowadays, a new kind of PCMS, called clathrate hydrate slurry (CHS), is considered asa promising candidate as the cold storage and secondary refrigerant used in air-conditioningsystem due to its preferable properties, including the suitable temperature utilization range,high cold carry capacity and good flow and heat transfer performance. However, the presentresearches are not sufficient to support its application. The present article mainly investigateson the flow and heat transfer characteristics of tetrabutylammonium bromide (TBAB) CHSand its application to the cold storage air-conditioning system.The basic thermo-physical properties of TBAB CHS were measured before its utilization.The phase equilibrium temperatures of TBAB aqueous solution with different concentrationswere measured by differential scanning calorimetry (DSC), and the phase diagram wereconstructed; the enthalpy variation during the melting process of TBAB CHS was alsodetermined by the DSC curve; treating the density of water as the reference, density of thesolutions with different concentrations and at different temperatures were determined byhigh-precision balance and graduated flask, then the CHS density was calculated based on thedensity of hydrate crystal and its mass fraction; the specific heat of TBAB aqueous solution was determined by the experimental result of the heat exchange between the solution andwater in a plate heat exchanger; the thermal conductivities of TBAB solution and CHS wereboth measured by a transient hot-wire unit, while the corresponding physical and thermalconduction equations were developed.On the basis of the particular properties, the flow behaviour of TBAB CHS flowingthrough piping was investigated. The pressure drop and flow velocity of TBAB CHS wasmeasured, and then it was found that TBAB CHS was pseudo-plastic fluid and could bedescribed by power-law model, and the CHS in smaller tube deviated more from Newtonianfluid than that in larger tube; the modified Reynolds number was employed, based on whichPoiseuille equation and Dodge-Metzner equation were determined as the suitable frictioncorrelation for CHS; the constraint effect of tube wall on the flow of solid particle in CHS wasanalyzed, and it was believed that the pressure drop of CHS in smaller tube was enlarged dueto this effect; the laminarization effect was observed during the measurement, which resultedin that the pressure drop of CHS did not always increase as increasing slurry fraction.Moreover, the pressure drop of TBAB CHS flowing through pipe fittings, including elbow,sudden contraction, sudden expansion, tee distribution and convergence, were measured andthe corresponding friction factors and empirical correlations were obtained.The inlet/outlet temperatures and the flow velocity of TBAB CHS flowing throughheated tubes were measured and then the heat transfer characteristics in tubes were obtained.Temperature of TBAB CHS along the flow direction were calculated; it was found that theinfluence of heating flux on heat transfer was larger in smaller tube, and the heat transfercoefficient increased with slurry fraction in the case of laminar flow, while the heat transfercoefficient was dominated by flow velocity in the case of turbulent flow; the empirical heattransfer correlation was proposed based on the experimental results and the researches of iceslurry and MPCM slurry were used for reference, and the Stefan number was employed toreflect the influence of slurry fraction on heat transfer performance; the convection equation,momentum equation and continuity equation of PCMS in heated tube were also developed asthe general calculation method for calculating PCMS temperature variation in heated tube,and the temperature variation of TBAB CHS with time was then obtained.It is quite sure that the utilization of CHS in air-conditioning system involves the heatexchange in heat exchangers, therefore the flow melting performances of TBAB CHS inplated heat exchanger (PHE) and double-tube heat exchanger (DHE) were both studied. The corresponding empirical flow friction factor correlations were developed based on themeasured pressured drop; the TBAB CHS temperature variation along the flow direction wasobtained on the basis of the thermal balance with hot water in heat exchanger, and then thelocal heat transfer correlation was developed. Moreover, the generated hydrate crystal waseasy to adhere to the heat transfer surface and became a thermal resistance layer, which notonly enlarged the pressure drop and also deteriorated the heat transfer and resulted in a lowCHS generation rate. Therefore, the investigation on the flow and heat transfer behaviours ofthe generating TBAB CHS in heat exchanger was conducted. The pressure drops of TBABCHS in DHE without heat exchanger and that during the generation process were compared,and then the thickness of the crystal layer was determined, which was normally smaller than1mm and were the function of time, flow rate, slurry mass fraction and the temperaturedifference between slurry and coolant; the existence of crystal layer caused almost85%reduction of the heat transfer coefficient in DHE, and the recovery of the deteriorated heattransfer could be achieved by melting the crystal layer using hot water.Finally, the TBAB CHS air-conditioning system was designed, built and tested based onthe obtained properties and flow and heat transfer performances. The reverse operation of theair-conditioner from cooling to heating was treated as the effective and reliable CHSgeneration approach. The system COP was low due to the existence of crystal layer inside theheat exchanger, which decreased from about1.92~2.95at the beginning of slurry generationto about1.05~1.49at the end of generation; larger flow rate of slurry in the heat exchangerled to better heat transfer performance. Compared to water, the usage of TBAB CHS as thesecondary refrigerant was advantageous in pumping power reduction, and there were10.3%energy saving and19~37%cost saving, and the longer the higher saving ratio. Moreover, thecontrol equation was developed to calculate the crystal deposition while TBAB CHS wasstored in a long period, and it was found that larger storage height cause moreheterogeneously distribution, and larger viscosity of CHS with high mass fraction led to lowerdeposition velocity.