Archives of Thermodynamics

The paper concerns a numerical analysis of cooling of the hybrid photovoltaic (PV) modules dedicated to Building-Inte-grated Photovoltaic/Thermal (BIPV/T) systems. Attention was focused on the photovoltaic roof tiles, using a jet impinge-ment technique, in which the intensification of heat transfer is ensured by streams of air hitting the heat exchange partition. A series of numerical simulations were carried out to assess an influence of the distance of the nozzle outlet from the absorber surface on the values of selected thermal-hydraulic performance indicators and the electrical parameters of the roof tile. The results confirmed the high effectiveness of the proposed method. The best effect was obtained for the case in which the relative distance of the nozzle from the partition to the nozzle diameter was equal to 1. For the mentioned configuration, an over 4 times increase in the value of the heat transfer coefficient was obtained in relation to the reference variant of cooling roof tiles. At the same time, the relative increase in the value of the generated electrical power was from 2.9 to 7.8%, depending on the value of the Reynolds number characterising the flow.

Flow structure maps are the most useful tool in the process of identifying structures formed in multiphase flows. They can be defined as a graphical way to present the transition boundaries of flow structures, depending on the characteristic pa-rameters of the phase transformation or flow, such as the vapor quality, the void fraction, the mass flow density, or the velocities of individual phases. Maps are usually two-dimensional drawings, described with a minimum of two selected parameters or quantities describing the phenomenon. The oldest maps of flow structures concerned adiabatic flows, mainly two-phase water-air systems or three-phase water-oil-air systems in conventional channels. Maps of nonadiabatic flow structures are simple and allow the selection of an appropriate model to determine the heat transfer coefficient as well as the flow resistance of the refrigerant. This has a major impact on the design of flow devices, where two- and even three-phase flows. This paper includes reviews of proposed maps of multiphase flow structures by various authors.

The article presents experimental results of boiling heat transfer during FC-770 flow in a group of five minichannels with a common heated wall. The flow orientation was changed from 0º to 180º, with a 15° increment. During the experiments, the temperature of its outer heated wall surface was measured by an infrared camera. At the same time, flow patterns were captured through the glass plate opposite the heated wall using a high-speed camera. The purpose of the calculations was to determine local heat transfer coefficients on the contact surface between the working fluid and the heated surface in the central minichannel, using a simplified 1D calculation method. The results in the form of dependences of the temperature of the heated wall and the heat transfer coefficient as a function of the distance from the channel inlet for various flow orientations were analysed. Furthermore, typical boiling curves and two-phase flow patterns were presented. The mean relative error of the heat transfer coefficient was determined for various flow orientation. The dependence of the void fraction as a function of heat flux was illustrated for various angles of minichannel inclination to the horizontal plane. It was observed that the void fraction increased with heat flux and with increasing angle of inclination of the minichannel to the horizontal plane.

The heat transfer measurements were conducted during pool boiling of water on surfaces with microchannels. Parallel grooves were made on a copper surface with widths ranging from 0.2 mm to 0.5 mm at intervals of 0.1 mm. The inclination angle of the grooves to the horizontal was set at 30° and 60°, and the depth of the microchannel grooves was 0.3 mm. The achieved heat flux ranged from 25 kW/m>² to 1730 kW/m², and the heat transfer coefficients ranged from 12 kW/(m²K) to 475 kW/(m²K). The influence of geometric parameters such as width, inclination angle of the microchannel, surface ex-tension, and Bond number on heat exchange efficiency was examined. A nearly sixfold increase in α (heat transfer coeffi-cient) and a twofold increase in critical heat flux were observed compared to a smooth surface.

Understanding thermodynamics can be challenging due to its dealing with abstract concepts such as entropy and energy transfer. This paper outlines six principles of thermodynamics, whose application necessitates a coherent overarching phi-losophy. The problems studied often entail complex mathematical equations, relying on a strong foundation in physics and mathematics. Moreover, comprehending thermodynamics requires a shift in thinking, focusing on macroscopic properties of matter rather than microscopic interactions, as in other branches of physics. Thermodynamics also introduces a new philosophy in science – the concept of irreversible phenomena, rooted in the heat flow theory, which is currently being extrapolated to other scientific domains. Notably, this involves extending the concept of work to systems performing var-ious types of work beyond volume change.

The paper presents the results of experimental investigation and the new statistical method for the determination of pre-liminary thermal characteristics of a prototype compact minichannel heat exchanger with laminar flows and significant heat transfer in the manifolds. The exemplary heat exchanger consists of 9 straight, parallel, square-shaped channels and two rectangular-shaped manifolds milled on both sides of the single aluminium plate. The design of the investigated heat exchanger is quite particular, as the heat transfer area of both pairs of manifolds provides almost 1/3 of the total heat transfer area. In the new statistical method presented in this paper, the manifolds’ and channels’ heat flows are considered separately. The heat exchanger’s thermal characteristic was obtained statistically on the basis of the experimental results and is presented in the form of the overall heat transfer rate. The developed thermal characteristic model accounts for two effects, among many others, which may affect heat transfer in the exchanger, i.e. the heat loss to the ambient and the significant heat transfer in the manifolds. It is proved that the heat transfer to the surroundings was negligible due to the suitable thermal insulation. In order to demonstrate that the heat transfer in the manifolds is significant, two calculation variants are presented. The relative differences (residuals) between the experimental and statistically corrected heat transfer rates and the coefficient of determination R² are determined in both variants. In the first variant the heat transfer in the manifold pairs is neglected and in the second model it is included. It was observed that the lack of consideration of the heat transfer in the manifold pairs provides drastic dispersion between the experimental and statistical results. In turn, in the second model, where the manifolds are accounted for, a significant enhancement in the consistency of the results is noticed. The relative residuals are much lower, and the corresponding coefficient R² is improved from R² = 0.8827 in the first variant to R² = 0.9335 in the second one, respectively.

During flow boiling in a system with small (mini/micro) channels, several instabilities may occur at the same time, which overlap each other  such a phenomenon complicates the analysis of boiling dynamics. The above mentioned processes cause that the fluctuation of recorded signals occur on various time scales. Although many criteria for the stability of two-phase flows are available, their practical application is limited (they need many recorded parameter of two phase flow). Methods which we are looking for should allow flow pattern identification based on a small number (or single) recorded signals. The paper presents a new approach to the recurrence plot method combined with Principal Component Analysis and Self-Organ-izing Map analysis. The single signal of pressure drop oscillations has been analyzed and used for flow pattern identification. New method of correlation analysis of flow patterns on video frames has been presented and used for flow pattern identifica-tion. The obtained results show that pressure drop oscillations and high speed video contain enough information about flow pattern for flow pattern identification.

Two-phase flow in channels of small dimensions is often a non-stationary process, the nature of such flow is oscillatory. Due to small channel dimensions, high heat flux, parallel channels interactions, pressure and temperature oscillations, the character of the phenomena occurring during boiling is complex. The changes of the measured signals are observed in different time scales. In order to examine in detail two-phase flow parameters changes, many acquisition devices are often installed. This solution becomes challenging concerning mini and microchannel heat-exchangers due to space limitation and modifications of an experimental setup. This paper presents a novel application of multiscale entropies for spatial and temporal analysis of two-phase flow based on only one registered parameter. This analysis is performed based on pixel brightness changes in photo frames registered by a high speed camera during two-phase flow. The spatial changes of pixel brightness are observed on single frames and temporal changes are examined using a set of frames (in time). The Composite Multiscale Sample Entropy is applied to identify two-phase flow patterns and to analyze the complexity of phase distribution. Using Multivariate Mul-tiscale Sample Entropy the most rapid changes of phase distribution in a multichannel heat exchanger are determined.

This paper presents the results of experimental investigations of the basic thermal property – the specific heat capacity of selected heterogeneous materials – graphene oxide rubber composites. The value of specific heat capacity was measured with a PerkinElmer DSC 8000 differential scanning calorimeter using modulated temperature mode of operation. The heterogeneous material under investigation was the graphene oxide/rubber composite, which is used in the production of roller bearing seals. Two types of rubber have been used as the basic matrix of composites: the hydrogenated acrylonitrile butadiene rubber and the fluoroelastomer. Graphene oxide reduced with sodium hypophosphite was applied as a compo-site filler. The main goals of the work was to expand the database of thermophysical properties of materials and to inves-tigate the influence of material heterogeneity on the results of specific heat capacity measurements obtained with small-sized test samples.