Supercooled Salt Research Articles

Isothermal compressibility and water self-diffusion coefficient in supercooled salt aqueous solutions using the Madrid-2019 model

The isothermal compressibility and self-diffusion coefficient of water in Li + , Na + , K + , Mg 2 + and Ca 2 + chloride and sulfate salts aqueous solutions is calculated by means of molecular simulations using the Madrid-2019 force-field, that uses TIP4P/2005 model for water and assigns scaled charges for ions. Our simulations predict that the isothermal compressibilities of these salts retain the anomalous behaviour of water, i.e. compressibility increases as temperature decreases, contrary to the behaviour of normal liquids. A maximum in the isothermal compressibility, analogous to that of pure water, is observed for some of the salt solutions. For the 1 m NaCl solution, simulations are performed at several pressures up to 1000 bar to estimate the location of the second critical point. We estimate that the liquid-liquid critical point is located at 190 K and 1000 bar, i.e. it is shifted to slightly higher temperatures and lower pressures with respect to the most accurate estimation of its location in pure TIP4P/2005 water (172 K and 1861 bar). Regarding the self-diffusion of water, all salts increase water mobility in the very supercooled regime (at temperatures within 200–230 K), with K 2 SO 4 producing the largest increase in water mobility, while MgCl 2 and MgSO 4 are the least effective in enhancing the self-diffusion coefficient of water.

Numerical investigation on thermal performance of a solar greenhouse with synergetic energy release of short- and long-term PCM storage

To address the problem of low greenhouse temperature at late night in north China, a new strategy of synergetic release of short- and long-term PCM heat storage is proposed in this paper, i.e. supercooled salt hydrate units are activated to release seasonally stored latent heat after the organic PCM north wallboard with daily storage cannot provide the required indoor temperature. To achieve this strategy, a microclimate model considering multiple greenhouse elements, including solar radiation, crops, indoor air, water vapor, soil and PCMs, is developed and 3D computational fluid dynamic (CFD) simulations are employed to examine the thermal performances of this strategy under Beijing climate conditions. The results demonstrated that the new strategy promotes the greenhouse temperature suitable for tomato growth (15 °C) throughout the whole night in winter. The organic PCM (Tm = 25 °C) wallboard can reduce the daytime indoor temperature by 0.5–1.5 °C and increase the nighttime indoor temperature by 4–5.5 °C than without PCM wallboard on January 1st. But the greenhouse temperature with this single daily storage is still lower than the required 15 °C after 4:00 a.m. With consideration of the unit numbers and heating effects, triggering two supercooled sodium thiosulfate pentahydrate (STP) units at one time (rather than sodium acetate trihydrate (SAT) and calcium chloride hexahydrate (CCH)) is the reasonable choice for greenhouse heating under present conditions. The system can achieve synergetic energy release across seasons, days and nights, regulate the thermal environment in the greenhouse, and promote the quantity and quality of crop output.

Hydrogel-stabilized supercooled salt hydrates for seasonal storage and controlled release of solar-thermal energy

Seasonal storage of solar-thermal energy within salt hydrate phase change materials (PCMs), which are known for their large latent heat capacity, suitable phase change temperature range and cost-effectiveness, has garnered...

Monitoring phase changes in supercooled aqueous solutions using an optical fiber Fresnel reflection sensor.

We report on a technique for monitoring the crystallization of water and aqueous solution of NaCl at atmospheric pressure, when cooled via liquid nitrogen, using a Fresnel reflection-based optical fiber sensor. The crystallization of distilled water and the associated changes in refractive index inferred from the sensor response comply with the previous reports on physical properties of supercooled water. The phase separation of NaCl.2H2O and the formation of eutectic mixture were inferred from the distinct features of the sensor signal during the cooling of NaCl solution. But the thermocouple did not detect the exothermic heat of crystallization due to rapid cooling. The influence of temperature gradients while interpreting the optical signals during this rapid cooling process and the effects of sensor debonding during the heating phase are discussed. The results demonstrate the potential of Fresnel sensors for monitoring the crystallization-induced phase changes in supercooled salt solutions and offers applications in areas where monitoring and controlling crystallization is important.

Validation of a heat conduction model for finite domain, non-uniformly heated, laminate bodies

Infrared thermographic validation is shown for a closed-form analytical heat conduction model for non-uniformly heated, laminate bodies with an insulated domain boundary. Experiments were conducted by applying power to rectangular electric heaters and cooled by natural convection in air, but also apply to constant-temperature heat sources and forced convection. The model accurately represents two-dimensional laminate heat conduction behaviour giving rise to heat spreading using one-dimensional equations for the temperature distributions and heat transfer rates under steady-state and pseudo-steady-state conditions. Validation of the model with an insulated boundary (complementing previous studies with an infinite boundary) provides useful predictions of heat spreading performance and simplified temperature uniformity calculations (useful in log-mean temperature difference style heat exchanger calculations) for real laminate systems such as found in electronics heat sinks, multi-ply stovetop cookware and interface materials for supercooled salt hydrates. Computational determinations of implicit insulated boundary condition locations in measured data, required to assess model equation validation, were also demonstrated. Excellent goodness of fit was observed (both root-mean-square error and R2 values), in all cases except when the uncertainty of low temperatures measured via infrared thermography hindered the statistical significance of the model fit. The experimental validation in all other cases supports use of the model equations in design calculations and heat exchange simulations.

The acoustical and structural changes of aqueous solutions due to transition to molten supercooled salts

The analysis of concentration dependencies of acoustical properties of aqueous electrolyte solutions has been carried out in a wide range of concentration. Among the large number of systems, including binary and multicomponent aqueous solutions of salts, some systems (∼20) are singled out where linear concentration behaviour of the square of sound velocity (u 2) is observed down to saturation. The linear extrapolation of u 2 up to 100% of saline component for solutions of halogens and nitrates of alkali metals yields a value of u 2 for the molten, supercooled salt. The interpretation of such acoustical dependencies is provided on the basis of a structural scheme including bulk water (1), water in hydration shells (2) and ionic clusters (3) in two concentration bands. The main principle of connection of these subsystems is the complementary organization of configuration (1), (2), (3). In the framework of this model the analytical expressions were deduced for a sound velocity of water in hydration shells in the first and second concentration bands. The condition of linearity of u 2 from 100% of water to 100% of salt was obtained. The deviations from linear concentration dependence are explained by the concentration variations of hydrophobic and hydrophilic hydration, or formation of ionic complexes. The calculated magnitudes of partial molar adiabatic compressibility of salts at infinite dilution ( k 2x 0 ) are in a good agreement with results obtained from concentration extrapolation of experimental data of compressibility coefficients and densities in dilute solution. The expression for the calculation of nonlinear acoustic parameters of solution and molten, supercooled salt (B/A) L was deduced using experimental data of dilute solution and density of molten supercooled salt. The packing factor of pure water and salt in molten, supercooled state was estimated.

Interactions of frazil and anchor ice with sedimentary particles in a flume

Frazil and anchor ice forming in turbulent, supercooled water have been studied extensively because of problems posed to man-made hydraulic structures. In spite of many incidental observations of interactions of these ice forms with sediment, their geologic effects remain unknown. The present flume study was designed to learn about the effects of salinity, current speed, and sediment type on sediment dynamics in supercooled water. In fresh-water, frazil ice formed flocs as large as 8 cm in diameter that tended to roll along a sandy bottom and collect material from the bed. The heavy flocs often came to rest in the shelter of ripples, forming anchor ice that subsequently was buried by migrating ripples. Burial compressed porous anchor ice into ice-bonded, sediment-rich masses. This process disrupts normal ripple cross-bedding and may produce unique sedimentary structures. Salt-water flocs were smaller, incorporated less bed load, and formed less anchor ice than their fresh-water counterparts. In four experiments, frazil carried a high sediment load only for a short period in supercooled salt water, but released it with slight warming. This suggests that salt-water frazil is either sticky or traps particles only while surrounded by supercooled water (0.05 to 0.1 °C supercooling), a short-lived phase in simple, small tanks. Salt water anchor ice formed readily on blocks of ice-bonded sediment, which may be common in nature. The theoretical maximum sediment load in neutrally-buoyant ice/sediment mixture is 122 g/l, never reported in nature so far. The maximum sediment load measured in this laboratory study was 88 g/l. Such high theoretical and measured sediment concentrations suggest that frazil and anchor ice are important sediment transport agents in rivers and oceans.

Thermal energy storage in supercooled salt mixtures

Mixtures of the two salts NaC 2H 3O 2.3H 2O and Na 2S 2O 3.5H 2O have been found to form supercooled melts which do not crystallize spontaneously even at temperatures below −10°C. The hydrated salts form a eutectic at a mole fraction of Na 2S 2O 3.5H 2O of 0.585 with a melting temperature of 40.8°C, and crystallization of the mixture by seeding results in a temperature rise to about 40°C even if the initial temperature was −20°C or below. The thermal energy storage capacity for supercooled 3:2, 1:1 and 2:3 molar mixtures of NaC 2H 3O 2.3H 2O and Na 2S 2O 3.5H 2O have been determined as a function of temperature, and a maximum storage capacity of 224 kJ kg −1 was observed for the 3:2 mixture at 39°C. The advantages of these supercooled salt mixtures as thermal energy storage media are discussed.