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Ice slurry is an energy-intensive secondary fluid, which may play an important role in various cooling purposes. From the system design perspective, it is of great importance to obtain detailed information of the ice slurry flow. A quantification of the complex two-phase flow characteristic is difficult due to the heterogeneous ice slurry flow. The paper includes the theoretical analysis of the ice slurry behaviour with different ice particle conditions (concentration, size) considering pressure drop and agglomeration. In addition to the mathematical model analyses of all important balance equations and other sub-equations (granular properties, phase interactions), various parameters influencing the behaviour of the ice slurry are considered. The CFD software has been successfully used to simulate the application of phase change material (PCM) in different technical applications, including cooling and air conditioning technologies. Based on the kinetic theory of granular flow, the ice slurry flow is described with and without consideration of melting. The Eulerian model provides granular properties and phase interaction parameters for the liquid-particle flow (ice slurry) to investigate particle collisions, drag, lift and turbulence dispersion. The numerical results were in good agreement with the experimental data from the literature.

Concerning the cold supply, ice slurry technology is a safe, environmentally friendly and efficient solution for energy storage. Previous studies have shown that ice slurry is potentially one of the most important phase change material (PCM) slurries used as a secondary refrigerant due to its high cooling capacity and flexibility in application. However, agglomeration phenomena are observed with ice particle slurry flows, which can lead to blocking of e.g. heat exchangers and pipes. The study applies an Euler-Euler approach based on the kinetic theory of granular flow to describe the melting of an ice slurry flow. The Eulerian model was used to simulate the ice slurry in new channel structures of specially manufactured plate heat exchangers. The mass flow rate, ice concentration, and industrial ice particle sizes were varied. The validation with experimental data is presented. The results show the pressure drop and agglomeration reduction with the new channel design relative to commercially available geometries of the plate heat exchanger. The simulation results show the velocity profiles, the volume fraction, and the pressure loss related to the ice concentration. The simulation results of improved heat exchanger plates to avoid high volume fractions at the inlet and outlet as well as the channels of specially designed plate heat exchanger are displayed. The results show the effect of changing the phase interaction parameter at high velocity.

The restrictions on the use of hydrofluorocarbons with a high global warming potential (GWP) are increasing on a global scale. They can be cost-driven or politically enforced. In Europe, the EU regulation 517/2014 restricts the refrigerants currently used in household appliances, such as R-134a, so new systems and working fluids must be investigated as replacements. The trend in domestic appliances with heat pumps, such as tumble dryers, is towards using propane as a refrigerant. This paper presents an experimental comparison of different refrigerants in a heat pump tumble dryer test rig. The original working fluid is R-134a, and the tests performed investigate a zeotropic refrigerant blend. The retrofit of the system for each new refrigerant is minimized. Modifications made to the system focus on adapting the throttling device to create a stable operating point. The cycle performance is evaluated using various parameters such as refrigerant mass flow, compressor input power, energy consumption and the duration of a standard drying cycle.

Carbon dioxide (CO2) is a working fluid, which is suitable for various applications. Excellent heat transfer properties, especially in the supercritical area, make it attractive for thermal systems. As it is an environmentally friendly and non-flammable fluid, it is used in larger stationary systems e.g. in supermarket refrigeration systems as well as in small cycles, e.g. for mobile air conditioning (A/C) systems. There is no risk of pollution or major environmental damage due to leakage or accidental release of fluid. In addition to power cycles, CO2 is suitable for cooling circuits for both subcritical (liquid-gas phase change) and transcritical applications. The latter includes supercritical heat rejection and subcritical heat absorption. Therefore, CO2 refrigeration systems and their components, e.g. compressors, have special design requirements. They need to be specially adapted to the fluid properties such as pressures above 10 MPa and pressure differences up to 8 MPa. The efficiencies of refrigeration systems must be competitive to those of conventional refrigerants. This requires advanced system designs, which will be presented. Finally, possible synergies between CO2 power systems and CO2 refrigeration and heat pump systems will be discussed., Conference Proceedings of the European sCO2 Conference4th European sCO2 Conference for Energy Systems: March 23-24, 2021, Online Conference, p. 131

The forced actuation of usually self-acting valves inside reciprocating compressors offers the possibility of a step-less capacity control and therewith the opportunity of energy savings. Differently from nowadays typically used electromagnetic actuated grippers for forced opening of the suction valve, the present paper deals with the pneumatically forced opening of the discharge valve. Pressure, which is provided by the compressor itself, is directly applied to the valves sealing elements to apply the opening force. Only few external energy and effort is necessary. The working principle and the numerical observation on the system was presented in former papers. The actual paper deals with the verification of the simulation results presented in a previous publication. For that purpose experimental testing was applied to the valve system components and the whole assembly. Component testing was necessary since only few information were available on some valve parts, however for example their temporal characteristics were crucial for the in-compressor-tests. The integral testing was performed by means of a test compressor. The instrumentation was chosen properly, being able to evaluate valve and overall compressor performance at different operation points. Beside pressures in cylinder, suction and discharge chamber, also the pressure supply for the valve control and the valve lift itself were monitored. Thus it was possible to compare the fundamental characteristics of the valve system to the results received by the simulation.

The optimization of refrigeration systems can be achieved by increasing the isentropic efficiency of the compressor. One possible way is the reduction of the friction and leakage losses which occur at the piston rings. The viscosity and the surface tension of the lubricant must be known for this purpose. From the literature only a small amount of measured values for the surface tension of commonly used lubricants are known. Additionally natural refrigerants like R-744 are more common according to the latest (European) environmental regulation. The thermophysical properties of lubricant-refrigerant-mixtures can currently only be calculated by generalized calculation methods. For specific mixtures, these equations need to be confirmed by measurements. In this paper the measurement results of the surface tension of different low-viscous lubricants are shown. A test bench was designed to measure the surface tension of different liquids especially under high pressure atmospheres. The investigation was performed by using the pendant drop method and analyzing the results with new algorithms for the solution of the Young-Laplace equation. Effects of the geometry of the used capillaries are discussed which leads to possible optimizations of this measurement method. Dilution effects of the surface tension of the lubricants in carbon dioxide atmospheres are shown and discussed. The effect of higher pressures and different temperatures on the surface tension is shown as well as a comparison of the measurements with generalized methods.

In principle, a reciprocating compressor stage consists of a cylinder, the working volume which varies periodically due to a piston moving inside a liner. Latter is sealed by two sets of valves. On the one hand the suction valve for admitting the gas to be compressed and on the other the discharge valve for allowing the high pressure gas to be delivered to the downstream process. The typical type of valves used within compressors for refrigeration plants are reed valves. The key feature of these mentioned valves - simultaneously affecting both their sealing performance as well as the reliability - is that they are not actuated. They are held close by elastic forces and they open and close automatically, in accordance to the balance of gas pressure forces as well as the previously mentioned elastic ones. The former is primarily characterized by the static pressure difference across the closed valve and the aerodynamic drag brought about by the gas flow impinging onto the sealing reed in a partially or fully open valve. Evidently, a high degree of coupling between the gas flow and the sealing element motion, today referred to as fluid-structure interaction (FSI), can be expected. The reed and the absence of a reliable guide that would provide for the plan-parallel motion give rise to non-parallel impacts with other parts of the valve assembly, leading to dynamic stress effects. This not only complicates the prediction of the reed valve dynamics, it also causes premature sealing element fracture, decreases the compressor efficiency and ultimately leads to unscheduled maintenance shutdowns. Consequently pursuing the optimization of the reed valve movement is a necessity. The measurements of reed valve dynamics as well as their movement in compressors can be performed by commercial proximity sensors, strain gauges or in house developed sensors. Although timing and lift information is available through these methods, other modes of fluttering such as torsional movement are more difficult to be observed. This work tries to address these aspects by providing real operation data obtained in a test rig. An in-depth comparative analysis of three measurement methodologies, represented by an optical, resistive as well as an eddy current sensor system will be carried out regarding their possible adoption in a state-of-the-art refrigeration compressor. Furthermore, the acquired data will be evaluated with respect to its potential application for collecting mentioned torsional movements. In the end a comparison against a numerical approach shall be conducted.

The heat transfer in the system piston - piston rod - crosshead of reciprocating compressors can be influenced by a fluid enclosed in an internal cavity. Thus, heat is transferred by the fluid flow inside the hollow parts utilizing the reciprocating motion. Subsequently, the high working frequency of such compressors can lead to an improved cooling of the high temperature compressor parts in comparison with conventional solid material designs.  For the experimental examination of this cooling concept a test rig has been set up at the Technische Universität Dresden which comprises a vertically oscillating hollow rod that is heated at its upper end and cooled at its lower end. Among other factors the heat transfer of this cooling concept is affected by the internal diameter of the piston rod. In order to quantify the influence of the internal diameter on the axial heat transfer three different hollow rods with varying internal diameters where investigated by means of temperature measurements.  The experimental results are presented and the influence of the size of the cavity is discussed. Furthermore, the measured temperatures are used for numerical simulations of the test rig in order to derive the axial heat flux of the fluid inside the cavity. Based on the resulting values the heat transfer capability of the investigated cooling concept can be examined with respect to the internal diameter and the benefit for the cooling of different compressor components can be estimated.

Perrichet-Thomas Christiane. La symbolique du sel dans les textes anciens. In: Mélanges Pierre Lévêque. Tome 7 : Anthropologie et société. Besançon : Université de Franche-Comté, 1993. pp. 287-296. (Annales littéraires de l'Université de Besançon, 491)

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