Fredrik Setterwall

Tetradecane and hexadecane binary mixtures as phase change materials (PCMs) for cool storage in district cooling systems

As the demand for refrigeration and air-conditioning has increased during the last decade, district cooling systems have been introduced in some major European cities. In a district cooling system, the combination of central cooling facilities and cool storage systems provides economic advantages over older conventional cooling plants. A cool storage system can meet the same total cooling load as a non-storage system over a given period of time with a smaller chiller. Cool storage systems using Phase Change Materials (PCMs) have a low temperature range and high energy density in the melting solidification of PCMs compared to sensible heat storage. Thus they are advantageous in reducing the storage volume, heat loss, and size of the chilling equipment. In this paper we describe some paraffin waxes and their binary mixtures. We discuss the thermal properties of laboratory-grade tetradecane, hexadecane and their binary mixtures, and we demonstrate their potential for use as PCMs for cool storage. The thermal properties include freezing point, the heat of fusion, thermal stability and volume expansion during the phase change process. In the study, a Differential Scanning Calorimeter (DSC) was used to determine the heat of fusion of these materials and to generate thermal data for study and analysis. The results show that these materials are attractive candidates as potential PCMs for cool storage in district cooling systems. However, because of the high cost of laboratory-grade materials, technical-grade materials must be used for cool storage.

Technical grade paraffin waxes as phase change materials for cool thermal storage and cool storage systems capital cost estimation

The present paper is confined to the discussion of technical grade paraffin waxes as phase change material (PCM) for cool storage and the cool storage systems capital cost investment. The objective of this study is to determine the potential for using cool storage systems. The thermal properties of technical grade paraffin waxes as PCM are investigated, and static and dynamic cool storage processes are discussed. The capital cost of the cooling storage system is estimated, which indicates the cooling storage system not only saves energy and other operation and maintenance costs, but also saves a significant fraction of the initial capital costs. Installing cool storage, at less cost than conventional nonstorage equipment, can often increase the cooling capacity of the existing system.

The coating of particles in a fluidized bed (residence time distribution in a system of two coupled perfect mixers)

Abstract A process of coating monosized seed particles in a fluidized bed is modelled. A concept of random surface renewal is applied to describe the exchange of particles between the active feed-zone and the inactive bulk of the bed. Three different methods of solving PBEs (population balance equations) for the system are presented and compared: (i) an analytical solution, (ii) a transformation of the PBEs into the moments of PDFs (population density functions), and (iii) a numerical simulation. It is shown that, with the assumptions adopted, the coating mass distribution in a batch fluid-bed coater can be described with sufficient accuracy by a normal distribution centered around the mean coating weight. A formula for the variance of the distribution is explicitly given. The numerical scheme presented here can be used as an effective tool for simulating the processes occurring in similar systems with internal circulation.

Liquid–solid phase equilibrium study of tetradecane and hexadecane binary mixtures as phase change materials (PCMs) for comfort cooling storage

Phase diagrams (equilibrium diagrams) are a convenient way of depicting the concentration-temperature-pressure relationships of a chemical system at equilibrium. They are invaluable to PCMs thermal storage researches and developers. In the present paper, the liquid-solid phase equilibrium of binary mixture system of tetradedcane and hexadecane has been studied. For theoretical evaluation of the thermodynamic equilibrium between the liquid phase and solid phase of tetradecane and hexadecane, several methods were compared: the UNIFAC group-contribution method, the model of Won, and Pedersens model. The temperature-composition phase diagram of the binary system has been obtained by the calculation. The diagram illustrates a binary, isomorphous system with a temperature minimum. Differential scanning calorimetry (DSC) was used to study the phase transformation of the binary system. DSC was run at different ramp rates and results indicate that these mixtures melt and freeze over a temperature range and the temperature range depends on the DSC ramp rate, and in a short mixture concentration interval appears low temperature solid-solid peaks. From the DSC results presented, it is concluded that the behaviour of binary mixtures of n-alkanes is far more complicated than considered in earlier studies. The phase diagram and all information from DSC are very important for designing PCM storage system.

On the instabilities of vertical falling liquid films in the presence of surface-active solute

~with a surface-active solute. It is assumed in the present model that the surfactant is soluble and volatile. In addition to the surface wave mode and the ‘wall wave’ mode which originate from the gravity-driven flow of the falling iilm itself, a new mode of instability related to the Marangoni effect induced by surface tension gradients is found for low Reynolds numbers and for moderate- or short-wavelength disturbances. The new mode is thought to be analogous to the thermocapillary instability examined first by Pearson (1958). The Marangoni instability of large-wavelength disturbances, revealed by Goussis & Kelly (1990) in a study of a liquid layer heated from below, may be completely suppressed in the present system by the effect of surface-excess concentration of the surfactant. The influence of the desorption of the solute and of its adsorption at the gas-liquid interface is determined for both the surface wave mode and the new wave mode. Desorption of the surfactant is shown to be responsible for the Marangoni instability of the new mode.

Experimental study of heat transfer additive influence on the absorption chiller performance

Abstract The influence of 2-methyl-1-pentanol (2MP) on the cooling effect of pilot absorption chiller has been studied experimentally. In one experimental series the additive was injected into LiBr solution. The enhancement ratio up to 20% was observed at the optimum additive concentration. In the second experimental series the additive was injected into the refrigerant. The enhancement ratio became 32% at higher additive concentration. Different additive concentrations have been tested in both series. These experimental results clearly showed that the presence of the additive in the vapour phase, even in very small amounts, favours the enhancement more than the additive in the LiBr solution. Also, it has been noticed, that the additive travels around the absorption cycle during long-term operation.

Gas-phase controlled convective drying of solids wetted with multicomponent liquid mixtures

The nonisothermal convective drying of an impermeable solid substrate covered with multicomponent solvent mixtures was studied. The aim was to simulate the nonsteady-state evaporation process under the control of the gas side heat and mass transfer. Diffusional resistances in the liquid layer were prevented by choosing conditions providing low evaporation rates. Evaporation fluxes were calculated by assuming either interactive or independent diffusion through the gaseous boundary layer. The interactive effects were accounted for by applying the linearized theory of Toor. Independent fluxes were calculated using the effective diffusivity method. In the experimental part, a Teflon substrate wetted with the binary mixture acetone—methanol, the ternary mixture ethanol—methylethylketone—toluene and the quaternary mixture ethanol—methanol—acetone—chloroform was dried using air flowing parallel to the evaporation surface as drying agent. Good agreement was obtained between experiments and simulations. Calculations show that liquid composition, liquid temperature, gas composition and diffusional interactions have a significant influence on the instantaneous selectivity while the effect of gas temperature and gas velocity is less important. Selectivity can be controlled by using a gas preloaded with an appropriate composition. The model can be applied to the drying, at an initial stage, of coated laminates with a gentle drying regime.

Model for dimensioning a falling film absorber in an absorption heat pump

Abstract A model for dimensioning a falling film absorber in an absorption heat pump is presented. The model considers both waviness and the coupled mass and heat transfer processes that take place in a falling film. Calculations demonstrate that the wave frequency has a strong impact on the heat flux in the absorber. Wave frequencies of fluids with viscosities and mass flow corresponding to the working media in absorption heat pumps were determined experimentally. The result of the calculations is in good agreement with experimental data.

An experimental study of the effects of surfactant on a falling liquid film

Abstract Addition of surface active agents to a falling liquid film changes the fluid mechanical behavior of the film. In this study we describe, both by observations and by calculations, what happens with the wave pattern after an addition of surfactant. The experiments were carried out in a 6-m high falling film column with a mixture of water and glycerol. The changes in wave pattern were detected by a video camera. Addition of surfactant will influence the film differently for different surfactant concentrations. Surfactant concentrations under the saturation limit destabilize the film and concentrations over the limit stabilize the film. The surface excess concentration, the concentration of surfactant in the surface per square meter, plays a key role in the influence of surfactant on the behavior of falling liquid films.

Effect of heat transfer additives on the instabilities of an absorbing falling film

Abstract The instabilities of an absorbing falling film in the presence of a surface-active agent are studied at low or moderate Reynolds numbers. This work is an extension of the previous stability analysis for a falling film in the presence of a surfactant (Ji and Setterwall, 1994, J. Fluid Mech.278, 297–323). The present model considers the absorption of water vapour which includes both mass transfer and heat transfer. There are three unstable modes for an absorbing falling film (without surface-active agent): one is the surface wave which is inherent to free surface flow, and the other two are associated with the Marangoni instability. One of these occurs for large positive absorption Marangoni numbers, being induced by the surface tension force resulting from the concentration gradient of water. The other one occurs for large negative absorption Marangoni numbers, and it is induced by the surface tension force resulting from the temperature gradient produced by the releasing heat of absorption. The effects of surface-active solute on these modes of instabilities are determined.