Clathrate Hydrate Slurry Research Articles

Experimental study on the rheological behavior of tetrafluoroethane (R-134a) hydrate slurry

ABSTRACTInformation on the rheological characteristics of clathrate hydrate slurry is vital due to its diverse applications including hydrate slurry transportation as in seawater desalination by gas hydrate process, gas delivery through slurry pipelines, cold thermal energy storage, and secondary refrigeration by hydrate slurries. The current study experimentally investigated the rheological behavior of Tetrafluoroethane (Freon) hydrate slurry formed from R-134a and water serving as a medium for sea water desalination. Experiments were performed in a flow loop with a volume of 5.68 L and an inner pipe diameter of 21.5 mm, which was immersed in a constant temperature bath to maintain hydrate stable condition. Experiments were conducted with two phases in the loop; solid hydrate particles and liquid water. The hydrate solid volume fraction ranged from 15.8 to 31.7 vol%. Pressure drops along the straight section of the pipe were monitored while temperature, solid volume fraction and flow rate were kept constant at desired values. The experimental results indicated that Freon slurry can be considered as a pseudo-plastic fluid. The shear-thinning characteristics of Freon slurry became more pronounced as the hydrate solid fraction increased. An empirical power law type equation that relates the apparent viscosity of the Freon slurry to the hydrate solid volume fraction and shear rate was developed and compared with experimental values. The experimental results well supported the values of the apparent viscosity calculated from the modeled equations.

Rheological and energy transport characteristics of a phase change material slurry

A phase change material slurry – TBAB (tetra-n-butyl ammonium bromide) CHS (clathrate hydrate slurry) has received intensive attractions in recent years due to its dual-function as thermal energy storage and transport media simultaneously in air conditioning and refrigeration applications. In the present study, the rheological characteristics of TBAB CHS were measured using a rheometer at various solid fractions and in a shear rate range of smaller than 1000 s−1. The results indicated that TBAB CHS was a pseudo-plastic non-Newtonian fluid which showed shear-thinning characteristics. The flow behaviour indices and fluid consistencies of type A and type B TBAB CHS were determined based on the power-law fluid model, which showed good consistency with the previous results obtained from the pressure drop measurements in straight tubes. The apparent viscosity of type A TBAB CHS was larger than that of type B TBAB CHS. The obtained apparent viscosities were compared to the calculated results by using the empirical equations, and the reason for the discrepancies was discussed. Based on the obtained rheological characteristics, the pumping power consumption of TBAB CHS as a secondary refrigerant was estimated and compared to that of chilled water at the same cooling capacity. The result showed a drastic reduction of pumping power when using TBAB CHS in lieu of chilled water.

Two-phase flow and heat transfer characteristics of tetra-n-butyl ammonium bromide clathrate hydrate slurry in horizontal 90° elbow pipe and U-pipe

Tetra-n-butyl ammonium bromide (TBAB) clathrate hydrate slurry (CHS) is commonly used as a cold storage medium and secondary refrigerant in the secondary-loop refrigeration. In the present study, the solid–liquid two-phase flow and heat transfer characteristics of TBAB CHS in horizontal 90° elbow pipe and U-pipe were numerically investigated using Eulerian–Eulerian multiphase model coupled with the interphase heat and mass transfer model. In order to validate the numerical models, the numerical pressure drops and wall temperatures as well as heat transfer coefficients were compared with the experimental evidences. After the model validation, the numerical models were firstly adopted to investigate the isothermal flow characteristics of TBAB CHS in the horizontal 90° elbow pipe and U-pipe. The numerical results showed that the primary flow was accelerated near the inner wall of the bending section and a secondary flow caused by the centrifugal force occurred on the cross-section of bending section, which made the solid particles agglomerate to the outer wall of the bending section. Meanwhile, vortex flow occurred beyond 45° of the bending section of 90° elbow pipe due to the boundary layer separation. Thereafter, the flow and heat transfer characteristics of TBAB CHS in horizontal 90° elbow pipe and U-pipe were also numerically investigated at inlet velocity of 1.0m/s and inlet solid volume fraction of 10.0vol%. The solid volume fraction distribution and local heat transfer coefficient were obtained, which indicated that the solid particles were melted more easily in the vortex region of the 90° elbow pipe and the inner side of U-pipe, resulting from the heat transfer enhancement by the secondary flow.

Conjugated heat and mass transfer during flow melting of a phase change material slurry in pipes

PCS (phase change materials slurries) are very useful for thermal energy storage. TBAB (tetra-n-butyl ammonium bromide) CHS (clathrate hydrate slurry) is a promising PCS, which is composed of TBAB hydrate crystal and TBAB aqueous solution. In the present study, the flow melting characteristics of TBAB CHS through pipes were numerically investigated. The solid–liquid two-phase nature of TBAB CHS was numerically investigated using the Eulerian–Eulerian multiphase model. And the interphase heat and mass exchange between TBAB hydrate crystals and TBAB aqueous solution was numerically modeled by considering the phase change phenomenon. Moreover, the diffusion of TBAB salt (solute) in TBAB aqueous solution was also numerically investigated. The numerical results showed that the liquid temperature increased more quickly than the solid temperature because of the latent heat involved. Moreover, the local heat transfer coefficient decreased in the thermally fully-developed region because of the increasing temperature difference between the wall and fluid. When the solid particles were almost fully melted, the fluid temperature increased more quickly than the wall temperature due to the absence of phase change, resulting in the increase of the local heat transfer coefficient.

Experimental investigation of forced flow and heat transfer characteristics of phase change material slurries in mini-tubes

Phase change material slurry can be used for thermal and cold storages, where the flow and heat transfer characteristics are very important and necessary for the design and construction of secondary-loop refrigeration and air conditioning so as to realize peak-load shifting and operation cost reduction. In the present study, two types of phase change material slurries: types A and B TBAB CHS (Tetra-n-butyl Ammonium Bromide clathrate hydrate slurry) were generated, and the forced flow and heat transfer characteristics in mini-tubes with 2.0 mm and 4.5 mm in diameters were experimentally investigated with solid fraction of 0–20.0 wt%. Flow behavior index and fluid consistency coefficient were obtained by considering TBAB CHS as a power-law fluid, where the tube diameter and solid fraction affected the fluidity of TBAB CHS. It has been clarified that the pressure drop of type B TBAB CHS was smaller than that of type A TBAB CHS. The main influential factors on the heat transfer characteristic of TBAB CHS were identified, and the heat transfer correlations for flow melting of both types A and B TBAB CHS were developed on the basis of the experimental data. The experimental results showed that the homogeneous model could depict flow melting of TBAB CHS in mini-tube more reasonably than that in large tube. The local heat transfer coefficient of type B TBAB CHS was larger than that of type A TBAB CHS under the similar thermal conditions, which implied that type B TBAB CHS was more favorable to be used as the secondary refrigerant.

Cold storage by tetra-n-butyl ammonium bromide clathrate hydrate slurry generated with different storage approaches at 40 wt% initial aqueous solution concentration

In the present study, the performance of cold storage by tetra-n-butyl ammonium bromide (TBAB) clathrate hydrate slurry (CHS) was studied. TBAB CHS was generated at 40 wt% initial aqueous solution concentration and the experiments were conducted with two different storage approaches, i.e., homogeneous storage and heterogeneous storage, in which the mass fraction of TBAB CHS was experimentally determined by melting experiments. It was found that the mass fraction of TBAB CHS and system COP of heterogeneous storage approach were much higher than those of homogeneous storage approach due to less crystal adhesion on the heat exchanger wall. Moreover, a filter screen was employed in the heterogeneous storage tank to impede the ascending of TBAB hydrate crystals to the top of storage tank. The results showed that the filter screen could reduce TBAB hydrate crystals being pumped into the heat exchanger, resulting in less crystal adhesion on the heat exchanger wall.

Thermal conductivity measurements of a phase change material slurry under the influence of phase change

Phase change material slurry is widely used in such applications as thermal energy storage and thermal management. Thermal conductivity of phase change material slurry is one of the most important thermo-physical properties that are necessary for system design and performance evaluation. In the present study, thermal conductivity of a phase change material slurry, tetra-n-butylammonium bromide (TBAB) clathrate hydrate slurry (CHS), is experimentally measured by using transient hot-wire method. The theoretical model of thermal conductivity measurement of phase change material slurry under the influence of phase change is proposed and numerically analyzed to obtain real thermal conductivity. It is found that phase change significantly affects the measurement of thermal conductivity in that the real thermal conductivity is smaller than that obtained directly from the experiments because phase change enhances heat transfer during the measurement. The smaller the solid fraction of TBAB CHS, the larger the influence of phase change on thermal conductivity is. Such effect is apparently ubiquitous, which should also be taken into consideration in thermal conductivity measurement of other phase change material slurries.

Experimental Investigation of Flow and Heat Transfer Characteristics in the Generation of Clathrate Hydrate Slurry

Tetra-n-butyl ammonium bromide clathrate hydrate slurry (CHS) is very promising for cold storage and energy saving in air-conditioning applications because of its appropriate phase-change temperature range of 0–12°C, reasonably large latent heat, and good fluidity. In this study, we experimentally investigate the flow and heat transfer characteristics during CHS generation in both an in-house-made annular channel heat exchanger and a commercial double-tube heat exchanger. Meanwhile, the visualization of the growth of the hydrate crystals is also carried out and the thickness of the crystal layer is estimated from the visualization results. It is concluded from the experimental results that the pressure drop and heat transfer coefficient play very important roles in CHS generation and the commercial double-tube heat exchanger is more effective for CHS generation.

Crystallization of tetra-n-butyl ammonium bromide clathrate hydrate slurry and the related heat transfer characteristics

Tetra-n-butyl ammonium bromide (TBAB) clathrate hydrate slurry (CHS) is a promising phase change material slurry for cold storage and transport in air-conditioning system. This slurry can be generated from the supercooled TBAB aqueous solution. In the present study, TBAB CHS was generated under different thermal conditions, i.e. different initial mass concentrations of TBAB aqueous solution and different supercooling degrees. The crystallization of TBAB CHS and the overall heat transfer coefficient under different thermal conditions were clarified. It was concluded that the crystallization characteristics of TBAB hydrate crystals mainly depended upon the thermal condition of the supercooled TBAB aqueous solution. In addition, the dropping of pre-produced TBAB CHS into supercooled TBAB aqueous solution could immediately induce the crystallization of TBAB hydrate crystals, and the initial type of TBAB hydrate crystals was only related to the status of the supercooled TBAB aqueous solution regardless of the type of the dropped TBAB CHS. Furthermore, the overall heat transfer coefficients before crystallization and during crystallization were also measured. It was found that more hydrate crystals would adhere to the vessel wall at larger supercooling degree and higher mass concentration of aqueous solution, which would deteriorate the heat transfer significantly. Moreover, images of TBAB hydrate crystals under different thermal conditions were recorded in order to help clarifying the crystallization characteristics.

Modeling the heat transfer characteristics of flow melting of phase change material slurries in the circular tubes

The heat transfer characteristics of phase change material slurries, e.g. tetra-n-butyl ammonium bromide (TBAB) clathrate hydrate slurry (CHS) and microencapsulated phase change material (MPCM) slurry, flowing through the heated circular tubes under constant heat flux are investigated in the present paper. The continuity equation, momentum and energy equations for the phase change material slurry in tubes are solved to obtain the variation of the temperature of the phase change material slurry with time and along the flow direction. The temperature variation of TBAB CHS can be divided into two regions, while there are three regions for MPCM slurry due to the existence of supercooling state. The comparison between the calculated and measured results reveals that MPCM slurry is possibly not fully melted during the flow melting, leading to a shorter melting region and higher outlet temperature than those of calculated values based on the heat balance equations.

Influence of crystal layer on the flow and heat transfer characteristics during TBAB CHS generation in a double-tube heat exchanger

The flow and heat transfer characteristics of tetra-n-butyl ammonium bromide (TBAB) clathrate hydrate slurry (CHS) during its generation process in a double-tube heat exchanger (DHE) and the growth of crystal layer were investigated experimentally. During the generation, the formed crystals were easy to adhere to the heat transfer surface, and the present study was focused on the influences of such crystal layer on the pressure drop and heat transfer coefficient between the generating TBAB CHS and coolant in the heat exchanger. Pressure drops of TBAB CHS flowing through DHE without heat exchange were measured in advance to develop the corresponding friction factor correlations, and the comparison of the pressure drops during TBAB CHS generation process to those without heat exchange was conducted to obtain the effective thickness of the crystal layer. It was found that the thickness of the crystal layer increased as the heat transfer process going on. The crystal layer caused the enlargement of pressure drop and the drastic reduction of heat transfer performance due to the narrowing flow passage and large thermal resistance, respectively. The effective thermal conductivity of the crystal layer was also determined for the prediction of the heat transfer performance in the heat exchanger.

A comparative study of different methods for the generation of tetra-n-butyl ammonium bromide clathrate hydrate slurry in a cold storage air-conditioning system

A cold storage air-conditioning system using tetra-n-butyl ammonium bromide (TBAB) clathrate hydrate slurry (CHS) as cold storage medium was built to investigate the high-efficiency method of TBAB CHS generation. In the present study, four kinds of different TBAB CHS generation methods were experimentally investigated and compared, and these methods included continuously cooling, turning off refrigerator while crystals appearing, supercooling release and accession of TBAB CHS into supercooled TBAB aqueous solution. The results showed that continuously cooling would lead to severe adhesion of crystal to the heat exchanger wall, and supercooling release took place with a big stochastic characteristic, hence the first and third method were concluded not reliable. Both the second and fourth methods could maintain the temperature of heat exchanger wall at a relatively higher level, therefore the crystal adhesion to the heat exchanger wall would be reduced significantly, which led to higher coefficient of performance (COP). In addition, accession of TBAB CHS into TBAB supercooled solution could shorten the time of supercooling release, resulting in about 21.8–35.4% shorter generation time than other methods. Moreover, the influence of flow rate on the CHS generation process was investigated, and the results showed that higher flow rate generally resulted in higher system COP.

Solid fraction determination in cold storage by tetra-n-butyl ammonium bromide clathrate hydrate slurry

Thermal or cold storage by phase change material slurry attracted many attentions in recent years, in which solid fraction distribution in the storage tank is an important issue due to different densities of the solid particle and carrier fluid. Tetra-n-butyl ammonium bromide (TBAB) clathrate hydrate slurry (CHS) has recently been intensively investigated for the purpose of the cold storage. In the present study, solid fraction distribution of TBAB CHS in a cold storage tank was both experimentally and numerically investigated. The different sedimentation characteristics of type A and type B TBAB CHS were identified. It was shown that high-capacity cold storage can be achieved by taking advantage of the sedimentation of type A TBAB CHS, based on which a procedure to realize it was also proposed.

State of the art on phase change material slurries

The interest in using phase change slurry (PCS) media as thermal storage and heat transfer fluids is increasing and thus leading to an enhancement in the number of articles on the subject. In air-conditioning and refrigeration applications, PCS systems represent a pure benefit resulting in the increase of thermal energy storage capacity, high heat transfer characteristics and positive phase change temperatures which can occur under low pressures. Hence, they allow the increase of energy efficiency and reduce the quantity of thermal fluids. This review describes the formation, thermo-physical, rheological, heat transfer properties and applications of four PCS systems: Clathrate hydrate slurry (CHS), Microencapsulated Phase Change Materials Slurry (MPCMS), shape-stabilized PCM slurries (SPCMSs) and Phase Change Material Emulsions (PCMEs). It regroups a bibliographic summary of important information that can be very helpful when such systems are used. It also gives interesting and valuable insights on the choice of the most suitable PCS media for laboratory and industrial applications.

Pressure drop and heat transfer characteristics of tetra-n-butyl ammonium bromide clathrate hydrate slurry during flow melting and generating in a double-tube heat exchanger

This study presents the flow and heat transfer characteristics of tetra-n-butyl ammonium bromide (TBAB) aqueous solution and clathrate hydrate slurry (CHS) in a double-tube heat exchanger (DHE). Both the TBAB aqueous solutions at the supercooling state and normal state were tested to understand the influence of supercooling state on the flow and heat transfer of TBAB aqueous solution. Pressure drops of 5–25wt% TBAB CHS flowing through DHE without heat exchange were measured and the corresponding flow friction factor was obtained, while the pressure drops of the flow melting and generating of TBAB CHS were also obtained. Single side heat transfer coefficients of the flow melting of 10–25wt% TBAB CHS in the heat exchanger were presented, and the local heat transfer correlation was developed with the consideration of the effect of latent heat on the heat transfer. Moreover, the heat transfer during TBAB CHS generation process was found to be deteriorated by the adherence of crystals on the heat transfer surface, and the heat transfer improvement by the melting operation was shown.

Experimental investigation on tetra-n-butyl-ammonium bromide clathrate hydrate slurry flows in a horizontal tube: Flow behavior and its rheological model

In the area of air conditioning, the tetra-n-butyl-ammonium bromide (TBAB) clathrate hydrate slurry (CHS) is a promising high-density latent-heat carrying medium. In this article, flow properties of two types of TBAB CHS were experimentally investigated in a horizontal tube with parameters of pressure drop ΔP, flow velocity of CHS V, and solid mass fraction χ. First, the Bingham rheological characteristics of TBAB CHS were confirmed in the range of solid mass fraction of χ ≤ 22% for type A and χ ≤ 25% for type B, respectively. Then, the re-laminarization phenomenon and region of weakened flow resistance were found according to the variation of pressure drop ΔP affected by the velocity and solid mass fraction χ throughout the flow regions, including laminar, intermediate, and turbulent flows. The deduction of the critical flow velocity and region of entire laminar flow velocity were useful for latent-heat transportation applications. Finally, the criterial relations between Fanning friction factor f and Metzner-Reed Reynolds number ReM were obtained for laminar-flow and turbulent-flow regions, respectively.

Pressure drops and loss coefficients of a phase change material slurry in pipe fittings

In this study, we investigated the pressure drops and loss coefficients of a phase change material slurry-tetrabutylammonium bromide (TBAB) clathrate hydrate slurry (CHS) in pipe fittings. Pressure drops of TBAB CHS in 6mm and 14mm straight tubes, 14mm 90° elbow, sudden contraction from 14mm to 6mm, sudden expansion from 6mm to 14mm, 14mm tee distribution and convergence were measured as the mass fraction of TBAB CHS varying from 0wt% to 30wt%. Power-law model was applied to describe the characteristics of slurry flow, and the fluid parameters including the flow behavior index and fluid consistence factor were obtained for 6mm and 14mm tubes respectively. Furthermore, the loss coefficients of TBAB CHS flowing through the pipe fittings were obtained from the experimental results and the corresponding correlations were also developed.

Experimental Investigation of the Hydraulic and Thermal Performance of a Phase Change Material Slurry in the Heat Exchangers

The application of phase change material slurry to the refrigeration and air conditioning system opens a new way for energy saving and reduction of the quantity of refrigerant in the system involved because it can serve as both the energy storage and the transportation media in the secondary loop, which is responsible for distributing the cooling power. In the present study, the experimental investigations of the forced flow and heat transfer characteristics of tetrabutylammonium bromide (TBAB) clathrate hydrate slurry (CHS) in both the plate heat exchanger (PHE) and the double-tube heat exchanger (DHE) are carried out. It is found out that the pressure drop in the PHE is about 3.0–50.0 kPa at the flow rate of 2.5–13.0 L/min (0.15–0.78 m3∕h)and is about 1.0–27.0 kPa at the flow rate of 3.0–14.0 L/min (0.18–0.84 m3∕h) in the DHE, which is nearly 2 times of that of the chilled water. The overall heat transfer coefficient is in the range of 2500–5000 W/(m2 K) for TBAB CHS in the PHE and is about 1500–3500 W/(m2 K) in the DHE, which are both higher than that of TBAB aqueous solution flow because of the involvement of the phase change of TBAB CHS.

Pressure drop and heat transfer characteristics of clathrate hydrate slurry in a plate heat exchanger

Pressure drop and heat transfer characteristics of 0–17.5 vol% tetrabutylammonium bromide (TBAB) clathrate hydrate slurry (CHS) as a secondary refrigerant flowing through a plate heat exchanger (PHE) were investigated in the present research. It was found that the pressure drop of TBAB CHS was about 3.0–50.0 kPa which was about 1.2–2 times of that of the chilled water at the flow rate of 2.5–13.0 L min −1. The pressure drop increased with the increase of the volume fraction. Variation of the pressure drop with the modified Reynolds number was discussed and compared with that of the ice slurry. Flow friction factor correlation and local heat transfer correlation for TBAB CHS flowing through PHE were both proposed based on the experimental data. In addition, the influential factors, such as the inlet water temperature and inlet CHS volume fraction, on the overall heat transfer coefficient were discussed.

Forced flow and convective melting heat transfer of clathrate hydrate slurry in tubes

In this study, we presented the forced flow and convective melting heat transfer characteristics of tetrabutylammonium bromide (TBAB) clathrate hydrate slurry (CHS) flowing through straight circular tubes. Pressure drops of TBAB CHS with the volume fraction in the range of 0–20.0 vol% were measured, and the influence of the volume fraction was analyzed. Power-law equation was used to describe the flow behavior of CHS, and the corresponding indicators including flow behavior index and fluid consistency coefficient indicated that TBAB CHS behaved as a pseudo plastic fluid. Based on the modified Reynolds number, it was found that the empirical correlation could be used to predict the flow friction factor of TBAB CHS. Heat transfer experiments were conducted to measure the local heat transfer coefficients of TBAB CHS flow under constant wall heat flux, which were found to be enhanced by the latent heat involved in the solid–liquid phase change. Local heat transfer coefficient decreased along the heating section until the volume fraction was smaller than a certain small value, then it increased in the following section due to the increase of Reynolds number. The effects of heating power, flow velocity, volume fraction on the local heat transfer coefficients were discussed. Correlations for predicting TBAB CHS heat transfer performance were proposed based on the experimental data in laminar, transitional and turbulent flow regions, respectively.