Alojz Poredoš

Concentration distribution and viscosity of ice-slurry in heterogeneous flow

Abstract Mathematical modeling of two phase flows, especially liquid–solid flows is very complex. Especially when a distribution of the solid phase in a carrier liquid is not homogenous but heterogeneous or even when a moving or stationary bed occurs. In this case, the rheological characteristics of suspension are changing and affect transport characteristics. Therefore, the slurry flow may present a Newtonian and non-Newtonian fluid as well, depending on the operation characteristics. In this paper the fully suspended ice-slurry flow in a horizontal pipe is analysed. The model allows us to avoid the definition on what kind of fluid ice-slurry is present. For the taken ice-particle diameter, the ice-concentration profiles depending on various average velocities and pipe diameters are shown. The viscosity of the ice-slurry is presented, depending on average concentration, velocity, pipe diameter and ice-particle size. The results of the analysis have shown that the ice slurrys can be treated as Newtonian-fluid at higher average velocities, and lower average concentrations as well. As the ice concentration increases and velocity decreases the viscosity depends not only on the ice concentration but also on the average velocity and the pipe diameter. The ice-slurry behaves then as a non-Newtonian fluid. The results show also the area where the safe operation of an ice-slurry-district-cooling system can be performed.

Bulk relaxor ferroelectric ceramics as a working body for an electrocaloric cooling device

The electrocaloric effect (ECE), i.e., the conversion of the electric into the thermal energy has recently become of great importance for development of a new generation of cooling technologies. Here, we explore utilization of [Pb(Mg1∕3Nb2∕3)O3]0.9[PbTiO3]0.1 (PMN-10PT) relaxor ceramics as active elements of the heat regenerator in an ECE cooling device. We show that the PMN-10PT relaxor ceramic exhibits a relatively large electrocaloric change of temperature ΔTEC > 1 K at room temperature. The experimental testing of the cooling device demonstrates the efficient regeneration and establishment of the temperature span between the hot and the cold sides of the regenerator, exceeding several times the ΔTEC within a single PMN-10PT ceramic plate.

Exergy loss as a basis for the price of thermal energy

Abstract The actual value of thermal energy can only be obtained by a qualitative or exergy analysis of its conversion, transport and distribution. The exergy value of heat, or its capacity to perform work, can contribute to formation of a good basis for determining a fair economic price for thermal energy. This paper presents the basic principles required to describe the exergy states of a district heating system. The parameters that determine the thermodynamic states at important individual points in the system are analysed. The exergy loss in a district heating system which supplies consumers with heat at different temperatures results from the transport and distribution of thermal energy at a constant ambient temperature. This exergy loss was taken as a very important part of the model for the differentiated pricing of thermal energy. The proposed model can serve as a basis for differentiated tariff regulations, which, in addition to the amount of consumed heat, will also take into account the temperature of that heat.

Influence of adsorption cycle limitations on the system performance

Abstract The economics of heat driven heat pumps are governed by two thermodynamic quantities: the performance on the one hand and the size of the heat exchangers, which is required to obtain this performance, on the other hand. The aim of this paper is to discuss the influence of the main features of adsorption cycles in comparison to absorption cycles on these quantities. In our case, a Zeolite UCC16×40-type 13X has been taken as the adsorbent. The analysis has already been used for absorption heat pumps. In the case of adsorption and other solid sorption chillers, additional limitations appear, e.g. identical design of all adsorbers, incomplete heat recovery between adsorbers, cycling of inert mass, etc., which all contribute to a lowering of the system performance or to an increase of the exchange area required. To show the basic differences between absorption and adsorption cycle optimization, and also to show the impact of physical or technical limitations on the adsorption chiller performance, a detailed analysis has been performed. It is confirmed that it is mainly the lack of a solution heat exchanger which lowers the adsorption system COP and not physical properties of the working pair, so there is still a lot of room for improvement in the solid-sorption of heat pumps.

Efficiency of cooled extended surfaces

Abstract A three-dimensional analysis of the combined heat and mass transfer between a moist air turbulent flow and a cooled solid surface was carried out. A turbulent boundary layer was modelled using the Wilcox k –ω turbulence model. Numerical results for the extended surface efficiency under various psychrometric conditions were obtained by solving simultaneously coupled equations of the fluid and the solid. The results show that variations in moist air parameters have a significant effect on the extended surface efficiency. It is shown that an increase in the inlet air humidity decreases the surface efficiency.

Overview of Existing Magnetocaloric Prototype Devices

Since Brown (J Appl Phys 47(8):3673–3680, 1976) built the first magnetic refrigerator prototype working near room temperature in 1976 there has been a large number of different prototypes (Int J Refrig 33:1029–1060) designed and built over the past 40 years.

Erratum: Electrocaloric cooling: The importance of electric-energy recovery and heat regeneration

Here we explore the effect of electric-energy recovery and heat regeneration on the energy efficiency of an electrocaloric-cooling system. Furthermore, the influence of the polarization-electric field hysteresis on the energy efficiency of the system is analysed. For the purposes of the analysis, the properties of (1 – x)Pb(Mg1/3Nb2/3)O3-x PbTiO3 (PMN-100xPT) with x = 0, , and are characterized. We show that if no heat is regenerated, even small irreversibilities in the electric circuit used to recover the electric energy can cause a significant drop in the achievable energy efficiency. On the other hand, when a heat regeneration process is considered and a realistic value for the degree of electric-energy recovery equal to 80% is assumed, the limit for the energy efficiency of a system employing PMN ceramics is estimated to be equal to 81% of the efficiency of a Carnot heat pump.

Magnetocaloric Materials for Freezing, Cooling, and Heat-Pump Applications

Magnetocaloric materials (MCM) are the ‘heart’ of every magnetic refrigeration or heat-pump application. Apart from having a crucial role in the heat-regeneration process, they also exhibit a special and vital phenomenon for magnetic refrigeration called the magnetocaloric effect.

Active Magnetic Regeneration

It is well known that the magnetocaloric effect of most magnetocaloric materials at moderate magnetic fields (up to 1.5 T) is limited to a maximum adiabatic temperature change of 5 K [1, 2]. This value is not sufficient for such materials to be directly implemented into a practical cooling or heating device where temperature span over 30 K are required. Therefore, in order to increase the temperature span, one and so far the best option is for a heat regenerator to be included in the magnetic thermodynamic cycle.

Magnetic Field Sources

In this chapter we describe some of the most important issues that relate to the sources for magnetic fields. We will try to provide information for engineers about the different possible magnetic field sources with respect to their application in magnetocaloric energy conversion.