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Phase change dispersions have recently gained interest in isothermal cooling applications. So far, almost all of the investigated phase change dispersions consist of paraffins as the phase change materials. This paper presents a phase change dispersion with two fatty-acid esters as the phase change material, Crodatherm-53/Crodatherm-47 (50:50). The dispersion has a melting temperature of 50 °C and is foreseen in high-voltage direct current component cooling. The phase change dispersion was stabilised with emulsifiers to prevent phase separation and nucleation agents were added to supress supercooling. From thermal history calculations, the supercooling of the dispersion was reduced by 10 K with the addition of the nucleation agent. A monomodal particle size distribution was achieved. The viscosity of the dispersion at 20 wt.% and shear rate of 100 1s−1 was found to be 4.9 mPa۰s at 25 °C, and 2.0 mPa۰s at 60 °C .The viscosity at different temperatures, mass fractions and shear rates was also assessed. From DSC analysis, an apparent specific heat capacity of 8.5 kJ kg−1 K−1 was measured for the phase change dispersion. This value is double the apparent specific heat capacity of water within the desired melting range (47.5–50.0 °C). Thermal conductivity measurements showed in the emulsion form, at 25 °C the phase change dispersion had a thermal conductivity of 0.529 W m−1 K−1 and in suspension form at 60 °C was 0.561 W m−1 K−1. Both thermal conductivity values were lower than water at each temperature.

One of the main limitations concerning the implementation of heat transfer fluids in Concentrated Solar Power (CSP) plants, are their compatibility with the construction material. Hence, the study of this interaction over cycles is crucial for a proper material selection and life span forecast. In this work, the chemical compatibility of four commonly used metals in CSP plants; low carbon steel-A1045, stainless steel-304H and 316L, and nickel alloy-Inconel 600, with one of the most promising HTFs, Solar Salt, was evaluated. Two different methodologies (gravimetrical and descaled method) were compared and used to characterise the corrosion behaviour depending on the metal coupons analysed, concluding that the best method for most of the metals is the descaled one. The corrosion oxides were also characterized using a combination of different techniques: SEM, EDX, and XRD. Inconel 600 showed the best corrosion resistance among the metals evaluated. However, a further brief economical study concludes that a compromise between the overall metal loss cost per year and the price of the construction material over the lifetime of the plant must be found. The two stainless steels (304H and 316L) were identified as the best performing according to metal loss vs. material price and mechanical properties vs. corrosion rate.

A high-performance heat transfer fluid (HTF) plays a crucial role in the overall performance and efficiency of concentrating solar power (CSP) systems for utilizing solar energy. Molten salt-based nanofluids, which may offer a promising solution to help reduce the size and cost of CSP systems, have attracted increasing attention. However, there is still no comprehensive assessment strategy that considers the conflictive effects of adding nanoparticles in HTFs, such as the compromise between energy storage capacity increase and pumping cost increase. In this work, a methodology for nanofluids screening and selection is proposed and a novel parameter (R) is determined to assess the conflictive effect. The parameter (R) considers the ratio between the relative pumping power and the relative energy stored of the nanofluid compared to its base fluid. Three promising eutectics nitrate based nanofluids (NaNO3–KNO3, LiNO3–NaNO3–KNO3, LiNO3–NaNO3–KNO3–Ca(NO3)2) doped with 0.5 wt.% and 1 wt.% silica nanoparticles were selected and evaluated by the proposed methodology. As a result, adding nanoparticles into binary salts always present a negative effect (R = 1.03–1.22) when considering the ratio between the relative pumping cost and the relative energy stored. For ternary salt, adding 1 wt.% silica nanoparticles would be more preferable with a decrease of the parameter (R = 0.89–0.97, R

High voltage direct current converters require efficient cooling of thyristors via heat sinks. Currently, infrastructures use deionised water as a means of cooling the high voltage direct current converters, however, recent research has shown that other fluids have potential to offer more efficient cooling. Phase change dispersions are a new class of heat transfer fluids that employ the latent heat of phase change, thus offering isothermal cooling during melting. For cooling applications, the temperature increase during operation is thus lowered when using phase change dispersions (compared to water) and consequently, the heat sink and thyristors surface temperatures are reduced. In this investigation, a phase change dispersion with non-conductive components, high stability, high capacity and low viscosity has been developed and tested. An experimental setup of a real size heat sink has been installed and the heat transfer behaviour of both the formulated phase change dispersion and water have been investigated and a comparison has been presented. Using water as the heat transfer fluid, the temperature increase from inlet to outlet of the heat sink was 4 K and with the formulated phase change dispersion (at the same mass flow rate and heat input) the temperature increase was 2 K. The phase change dispersion caused a 50% reduction in the heat sink surface temperature. Furthermore, the global heat transfer coefficients obtained for the phase change dispersion were found to be independent of the heating input applied, unlike the trend found for water, additionally, the global heat transfer coefficients were found to be similar to those obtained for water at the same mass flow rates and reached a maximum value of 6100 W m2 K−1. Despite this, the pressure drops and viscosities obtained for the phase change dispersion were higher than for water. Overall, the current investigation demonstrates the ability of using a phase change dispersion as a cooling fluid for the cooling of electronic components, which thus far is limited to using air and water cooling and cannot reach the cooling capacity achieved by phase change dispersions.

The pressure drop that occurs during the falling film evaporation of dairy products is related to the downward co-flowing vapour and directly affects the saturation temperature of the product, resulting in a non-constant thermal driving force of the process along the evaporator. Frictional forces are the major contributor to the pressure drop, and the friction factor is a crucial parameter in reliably estimating the pressure drop. In the present study, the absolute vapour flow pressure losses during falling film evaporation of dairy products were measured using an experimental internal tube evaporator set-up. To simulate real industrial conditions, experiments were conducted by varying the co-flow inlet velocity (from 0 to 37 ms(-1)) over a wide range of product dry solid content DC (from DC = 0%-50%). Results obtained at DC = 0% show that the experimental approach is consistent when measurements are compared to the well-known Wallis correlation. A large number of experimental data has been obtained for DC between 13 and 40% for which the pressure losses have a strong dependence from the co-flowing vapour rate; in some specific cases, the influence of the surface bubbling phenomena has been put in evidence. The presented results, have been used to calibrate a new accurate correlation for the friction factor (4f(tv)) coefficient showing a squared residual of R-2 = 0.93.

Falling film evaporators constitute an essential part of dairy powder production facilities. Evaporation belongs to the thermal separation processes; consequently, the evaporation plant is fundamentally energy intensive. Hence, improvements to its operation can have a significant impact on the energy usage. A simulation tool that is based on experimental correlations developed at conditions similar to the industrial scale is presented to provide insight into the fundamental mechanisms occurring along the heat transfer surface of industrial evaporators. The model includes the complex interaction between the produced vapor and the liquid flow. The results demonstrate that the pressure drop, through its influence on the saturation temperature, is of high importance in the performance of evaporators. The tool can be used to investigate different design configurations, and it exemplifies how a change in the tube diameter for the same temperature difference and length of the surface will affect the maximum heat load per area as well as the pressure drop.

In the falling film evaporation of dairy products, the phase change process occurs via two major phenomena: surface convective evaporation and boiling evaporation. Previous studies have shown that under certain conditions, the heat transfer mechanism can be greatly improved when the evaporation is dominated by the presence of bubbles or foam clusters. In the present work, the influence of the surface bubbling phenomenon on the heat transfer coefficient has been studied. The effect of the co-flowing vapor rate inside the evaporative tube has been experimentally related to the presence of the surface bubbles for dairy products characterized by different dry solid contents (DC = 0%, 13%, 30%, 40%, 51%). The results show that at low dry solid contents (DC

The most evident aspects of a first order transition of a system from an old to a new phase, are the presence of a discontinuity at the interface between both phases and the thermal effects related to the latent heat exchanged with the surrounding environment. These effects are the result of a sequence of events promoted by thermodynamic conditions persisting over the equilibrium in a metastable state. The breakdown of metastability is promoted by infinitesimal energy fluctuations resulting in the germination of clusters of the new phase that can grow to a critical size (nucleus) and then develop or vanish. Examples of these sequences are common in various technological fields such as combustion, food processing, pharmaceutical manufacturing, condensation, and phase change heat transfer, etc. This work aims to highlight a logical path that leads the readers from the fundamental phenomenology to the most intricated aspects of the nucleation within dispersed systems such as oil-in-water emulsions. Differences between the homogeneous and heterogeneous mechanisms are, under the light of the Classical Nucleation Theory (CNT), presented in bulk and confined systems until defining a minimum confinement size. By collecting insights coming from a rich scientific literature mostly focused on the stability of emulsified systems, the discussion is then on the aspects related to the surface related mechanisms. Two main aspects are then considered: a) the wettability of the nucleating cluster by the surrounding melt; b) the affinity between the adsorbed layer, where a surfactant is located, and the oil melt phase (mainly n-alkanes and triacylglycerols with different moieties). In cases where nucleation is dominating over the dewetting of the nucleus, the contact angle can be considered as a constant value. The affinity in terms of molecular features between the surfactant and the oil phase can promote the template effect. Several factors seem to play a role in this interaction such as the thermal characteristics of the surfactant and comparable dimensions between the molecule (or fractions) of the dispersed compound and the tail of the surfactant.

The suitable phase transition temperature range and low electrical conductivity make fatty acid-based Phase Change Material (PCM) slurries an attractive heat transfer fluid for cooling of High–Voltage Direct Current (HVDC) electronic devices. However, their shape instability induced by external electrical field may alter their effective thermophysical properties and hence the heat transfer performance. In this work, the shape stability and flow behavior of a fatty acid droplet under a coupled fluid-thermo-electric coupled field are numerically investigated. The effects of droplet size, fluid velocity and temperature, and electrical field on the shape and energy of the droplet are evaluated. The results show that the droplet size is the major influencing factor for the shape stability. Given other conditions, the deformation ratio and the internal pressure difference of a 1 μm droplet are respectively 8 times higher and 5 times larger than a 7 μm droplet. An increase in the slurry velocity only slightly increases the particle interior pressure; given other conditions, an increase in the velocity from 0.22 m/s to 1.1 m/s only leads to an internal pressure increase by 25.77%. The electrical stress across the surface tends to squeeze the droplet into a prolate shape, offsetting the shape deformation by the shear stress and hence stabilizing the PCM slurry. The presence of the electrical field slows down the energy evolution and reduces the pressure difference inside the droplet.

A fatty acid based phase change dispersion (PCD) for efficient high voltage electrical devices cooling has been developed and investigated in this paper. Experiments were firstly conducted to characterise the PCD thermophysical properties and to measure the bulk fluid and inner wall temperature of PCD flowing in a circular pipe by a self-designed rig. A 3D modelling based on the Euler-Euler model and homogenous single-phase model were then performed and validated through experiments, and further employed to evaluate the effects of particle size, concentration and operating conditions on flow characteristics, pressure drop, heat transfer and energy transport performance of the PCD. The results showed that both the particle size and concentration presented significant influences on heat transfer between the two phases, and average friction factor as well as heat transfer coefficient of the PCD. A higher averaged Nusselt number and a larger pressure drop were observed when the PCM particle diameter decreased to 7 μm. For the investigated conditions in this work, an optimal set of cooling performance for the developed fatty acid PCD was proposed and a PCM particle size of 7 μm under a flow velocity of 4 m/s and a concentration of 35% was recommended for industrial cooling applications.

A large number of industrial processes use falling-film evaporation to concentrate liquid products. This technology allows for small temperature differences during operation and is often significantly more energy efficient than other techniques. When processing dairy products, a reduction in the solvent fraction results in an increased product viscosity and may thus result in non-Newtonian features. The interaction between a co-flowing vapor that is produced during the evaporation process and the falling film is an important feature of the process. Few studies have accurately studied the effect of co-flow on evaporative falling films at high solid contents. In this work, an experimental study of the influence of co-flowing vapor on the heat transfer coefficient for a dairy product is presented as a function of both the solid content (from 10 to 50%) and the mass flow rate of the feed. The experimental set-up, consisting of a unique industrial pilot-scale evaporator, provides the possibility of obtaining results useful for realistic industrial conditions. An analytical approach that enables the simultaneous evaluation of heat transfer in every experimental condition, e.g., for Newtonian or non-Newtonian fluids and with or without co-flowing vapors, is presented.

Combustion of water in oil μ -emulsions is still considered as a useful technology for the energy conversion of waste oil. One of the most relevant advantages is related to the phenomenon of micro-explosion ( μ – e ) that produces the secondary atomization of the oil. Several experimental approaches have been proposed in the last years with the aim to characterize the μ – e effect under different conditions. In this paper, an experimental comparison between the two useful approaches is presented. The results obtained with the technique of the Suspended droplet will be related to data present in the literature, obtained through the Leidenfrost technique . Quantitative thermal results such as the μ – e temperature and the fall temperature after μ – e show the most important differences. The important role played by the separation process as coalescence and creaming in both approaches is also discussed.

The micro-explosion phenomenon is involved in emulsified fuel droplets placed in a hot atmosphere, such as spray combustion. Droplets of water-in-sunflower oil emulsion are used, since they are representative of a class of emulsions used in practical applications of biofuels. Once the micro-explosion is triggered after a short delay, the rapid (

The burning of water in oil emulsions is considered as an effective alternative to bring out waste oil because of a significant reduction in carbon monoxide, nitrogen oxides and particulates in the exhaust. These advantages have different origins, an important contribution is provided by the phenomenon of micro-explosion. In this work, the influence of the size of the dispersed water droplets in the micro-explosion phenomenon is studied by the hot plate technique. The variation of the temperature and the evolution of the phenomenon have been investigated using a synchronized thermocouple/high speed imaging system. Three emulsions with the same amount of water (30% mass ) but with different distribution of the size of the dispersed water droplets have been tested. The results show that the size distribution of the dispersed water droplets plays an important role in the phenomenon of micro-explosion. Moreover, some internal phenomenon as the separation process between water and oil seems to affect significantly the phenomenon.

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