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Experimental investigation of latent thermal energy storage (LTES) charging and discharging has been performed. The LTES unit is a shell-and tube type tank with water as the heat transfer fluid (HTF), which flows through the tubes, and technical grade paraffin RT 25 as the phase change material (PCM), filling the shell side. The total of 19 longitudinally finned tubes are arranged concentrically in the tank. A set of 30 thermocouples have been placed at various radial and axial positions on the PCM side for monitoring transient temperature variations during charging and discharging processes. Timewise temperature variations of PCM during charging and discharging, for different HTF inlet temperatures, have been presented. Stored and released thermal energy have been calculated and analyzed.

In this study charging and discharging processes in a longitudinally finned shell-and-tube latent thermal energy storage (LTES) tank, with water serving as the heat transfer fluid (HTF) and paraffin being used as the phase change material (PCM), have been numerically analyzed. Numerical simulations including both phase change processes have been performed neglecting the influence of natural convection in liquid phase of the PCM and the results have been compared with experimental data. From the comparison, it can be observed that natural convection strongly influences the melting process, while its influence on the solidification process is much less significant. It can be concluded that neglecting natural convection in modeling solidification behavior in longitudinally finned shell-and-tube LTES may be beneficial, since an accurate enough solution could be obtained while significantly reducing computation time. Conclusions drawn from this study can be used in further numerical analyses of LTES charging and discharging processes, which could be expanded by including natural convection in the model to more accurately simulate phase change in longitudinally finned shell-and-tube type LTES.

This paper presents an experimental study about the effects of airflow nonuniformity on the thermal and hydraulic performance of fin-and-tube heat exchangers. Two fin-and-tube heat exchangers were tested. The first was a three-depth-row heat exchanger with a total of 38 tubes in 7 tube circuits and a total air side heat transfer area of 40 m2. The second heat exchanger has 32 tubes in 6 circuits for a total air side heat transfer area of 27 m2. In both heat exchangers, two circuits have four-tube arrangement while the remaining circuits have six-tube arrangement. The two heat exchangers were tested inside an open wind tunnel and the fluid on the tube side was hot water. The heat exchangers were placed vertically inside the wind tunnel. Nonuniform distribution of inlet air velocity was generated by partial blocking the entrance cross section, inducing airflow nonuniformity on the heat exchange surfaces. Inlet and outlet air and water temperatures were measured using thermocouples while air flow velocities were measured with hot-wire anemometers. The air pressure drop across the heat exchangers was measured with differential pressure sensors. The experiments revealed that airflow nonuniformity causes thermal effectiveness deterioration and pressure drop increase, the size of which depends on the degree of airflow nonuniformity. It was found that fin-and-tube heat exchangers with unbalanced tube circuitry may lead to nonuniform outlet air temperatures, as a consequence of uneven tube-side flow resistance due to circuits with different number of tubes.

Heat transfer simulations in a heat exchanger with the microchannel coil have been done and described in this paper. The aim of this study was to use a previously validated mathematical model and numerical procedure of compact heat exchangers type with straight microchannels and to investigate heat fluxes dependence on different geometry and operating parameters. According to dimensions of the heat exchanger, which is commonly used in HVAC units as a condensing unit, a 3D mathematical model has been defined and solved numerically. The objective function has been developed with selected input parameters of heat exchanger: fin pitch, flat tube transversal row pitch, and air inlet velocity. Response surface methodology has been applied to derive a polynomial function that describes power density change according to three selected input parameters. Finally, the created function has been subjected to genetic algorithm optimization procedure. Optimal setup for a given geometry and operating parameters has been acquired and followed with discussion and conclusions.

In this paper, optimization of a small size water-water shell and tube heat exchanger has been performed after a series of numerical calculations. Firstly, a mathematical model was defined, consisting of computational domain, conservation equations and boundary conditions. The problem was considered as steady-state, laminar and incompressible. Numerical method was the finite volume method, SIMPLE algorithm was used for pressure-velocity coupling and ANSYS Fluent was used as numerical solver. The numerical procedure was previously validated using experimental data. In order to perform optimization, a number of numerical calculations hasbeen performed for three different parameters ; tube diameter, shell diameter and shell-side inlet velocity. Specified objective functions were maximum heat flux, minimum mass and combined function of maximum heat flux with minimum mass. Using response surface methodology and Box-Behnken design, optimal values of parameters can be visually determined from plotted 3D surface plots.

The objective of the present numerical study is to investigate the air side thermal-hydraulic characteristics for water-to-air heat exchanger. For the purpose of numerical analysis, 3D mathematical model is developed and implemented, using governing equations of fluid flow and heat transfer, and boundary conditions. The fluid is considered incompressible with constant properties and the flow is assumed to be steady-state and laminar. All the computational work is carried out using the finite volume method by means of CFD software ANSYS FLUENT 18.2. The coupling between velocity and pressure is performed with the SIMPLE algorithm and the Power Law scheme is used for the treatment of convection- diffusion terms. Numerical mesh is validated by performing a grid independence study. A comparison of the numerical results with obtained experimental data shows good agreement. Numerical simulations have been performed for Reynolds number in a range 285 < Re < 690, with corresponding frontal air velocity ranging from 0.825 to 2 m/s. Inlet air and water temperatures are 288.7 K and 323 K, respectively. Numerically obtained temperature contours and velocity vectors in the mid-plane between two fins and near the fin surface have been presented. Finally, dependence of average air-side Nusselt number on Reynolds number has been analysed.

Latent heat that is absorbed or released during solid-liquid phase change at constant or nearly constant temperature presents thermal potential that can be exploited for various purposes, from thermal energy storage to thermal management. Materials that undergo phase change are called phase-change materials (PCM), and one of the most commonly used PCMs is paraffin. For a long time, modeling change of state has been extremely difficult because tracking of the solid-liquid interface was limited to extremely simple and simplified cases. With the application of the enthalpy-porosity method, that does not track the interface explicitly, modeling has become easier and applicable to a variety of problems. This paper deals with a two-dimensional transient numerical analysis of heat transfer with melting and solidification of paraffin encased in a rectangular enclosure. Effects of natural convection have been studied for both melting and solidification process. Computational model has been validated with published numerical results and adequate agreement has been observed. It is evident that natural convection ; a buoyancy-driven motion of fluid that results in heat transfer between liquid and solid, is the dominant form of heat transfer in melting process and should be taken into account, while has little effect on solidification process, where conduction is the dominant mechanism.

In this paper, an analysis of heat transfer and fluid flow in a wavy fin-and-tube heat exchanger has been performed followed with a comparative study of different algorithms used for the heat exchanger optimization. Three- dimensional (3D) numerical simulation results of a circular tube heat exchanger were used in multivariate optimization that included geometrical parameters that, as predicted, significantly influence thermodynamic and hydrodynamic behavior of the heat exchanger system. A nonlinear regression model (polynomial function) was derived for two developed objective functions. Several algorithms for the optimization of heat exchanger have been used: two evolutionary algorithms (Genetic Algorithm and Simulated Annealing) and Pattern Search algorithm. The influences of inlet air velocity, as well as fin pitch and wavy angle on heat transfer and pressure drop conditions have been studied. The comparison of results has been made along with the analysis and conclusions.

U ovome radu provedena je analiza energetske i eksergetske učinkovitosti cijevnog izmjenjivača topline voda-voda malih dimenzija. Prema postojećim dimenzijama izrađen je geometrijski model izmjenjivača te je provedena numerička analiza izmjene topline za četiri slučaja s različitim ulaznim temperaturama tople vode. Potvrda fizikalne ispravnosti modela dana je uspredbom bilanci topline izračunate iz numeričkih rezultata i analitički iz pogonskih uvjeta. Prikazane su izlazne temperature tople i hladne vode, kao i energetske i eksergetske učinkovitosti za svaki od četiri slučaja. Iz rezultata je vidljivo da se energetska učinkovitost vrlo malo mijenja, a eksergetska učinkovitost blago opada s porastom ulazne temperature tople vode. Slučaj s najmanjom ulaznom temperaturom ocijenjen je najpovoljnijim s energetskog i eksergetskog aspekta.

In this paper a multivariate optimization of a heat exchanger with microchannel coil has been done. Multivariate optimization included three geometry parameters of heat exchanger which, as expected, significantly influence hydrodynamic behaviour of the system. One more variable has been included in optimization process to compare gains in the heat transfer due to water inlet velocity change. Data set used for optimization has been numerically acquired and experimentally validated. Air/water side numerical model of heat exchanger composed of flat tubes with rectangular shapes has been used. Objective function that combines heat transfer rate and pressure drops of air and water has been developed and employed for an optimization procedure. Response Surface Method has been applied. Comparison of a results has been made accompanied by the commentaries and final conclusions.

U radu je dana usporedba rezultata proračuna vanjskih dobitaka topline kroz ostakljene plohe vanjske ovojnice zgrade primjenom satne i mjesečne metode proračuna. Proračun je proveden za različite tipove otvora, različite razine zaštite od sunčeva zračenja i različite orijentacije. Razlika vrijednosti dobivenih satnom i mjesečnom metodom mala je samo u slučajevima kada se radi o otvorima bez zaštite od sunčeva zračenja i orijentiranih prema sjeveru. Mjesečna metoda općenito precjenjuje utjecaj vanjskih toplinskih dobitaka od sunčeva zračenja pa se preporuča korištenje satne metode gdje god je to moguće.

U radu je dan osvrt na važnost energetske učinkovitosti u zgradarstvu kao i pregled važnije europske i hrvatske regulative iz tog područja. Daje se kratki pregled znanja, vještina i kompetencija koje bi između ostalog trebao posjedovati inženjer strojarstva kako bi se mogao suočiti sa zahtjevima poslova vezanih za energetsku učinkovitost u zgradarstvu. Dan je osvrt na trenutne mogućnosti i načine stjecanja tih znanja kao i moguće načine za uključivanje navedene tematike u nastavne planove studija strojarstva.

In the paper, an analysis of a refrigeration device operation under frost forming conditions has been performed. The results of mathematical modelling and numerical calculation of heat and mass transfer in the boundary layer have been used as a base for the calculation of an effective exchanged heat flux. Several cases with different operating conditions have been analysed and the resulting correlations for determining the optimal duration of the cooling cycle have been shown. The presented results could be used to control the device within the scope of maximising the cooling effect and reliability of a cooling energy supply. In the controlling algorithm hereunder, the starting moment of the defrosting cycle has been selected depending on real frosting conditions such as air temperature, humidity and velocity as well as temperature of the heat exchanger surface.

In this study experimental analysis of two compact heat exchangers of different construction types has been done. A fin-and-tube heat exchanger and heat exchanger with microchannel coil have been installed in an open circuit wind tunnel. Air and water were used as working fluids. From measured temperatures and flows of both fluids heat transfer and effectiveness have been calculated. Comparison of acquired results has been made accompanied by the commentary of advantages and disadvantages of both types of compact heat exchangers.

Numerical analysis of heat transfer and fluid flow in a rectangular offset strip-fin heat exchanger has been carried out. The chosen offset strip fin geometry is defined with a fin space of 1.7 mm, fin height of 2.2 mm, fin thickness of 0.3 mm and strip length of 6 mm. The commercial CFD package FLUENT 6 has been used for the numerical analysis. The SIMPLE algorithm has been used for velocity and pressure coupling in momentum equations. Both conduction in the solid sub-domain (separating plates and fins) as well as convection in the air sub-domain are considered. A constant temperature of 315 K is prescribed on the separating plates. The steady-state laminar and turbulent flow models have been chosen for Reynolds numbers, ranging from 100 to 4000. The resulting heat transfer and pressure drop numbers are compared with results of empirical correlations as well as results of experimental and numerical studies of different authors. The computed Nusselt numbers and pressure drops are over-estimated by an average of 20% with regard to the Manglik-Bergles correlation model. The discrepancy is even smaller when the results of the numerical analysis are compared against a numerical model. It has been concluded that the flow in the offset strip-fin is pure laminar only in the region of Reynolds numbers below 400. It becomes unstable laminar for Re > 400, when self-sustained unsteadiness is established from the interrupted fin surface. Recirculation zones or separation bubbles at fin leading edges and wake regions at fin trailing edges are visible at Reynolds numbers above 400. A change in the Fanning f-factor curve slope testifies that the flow becomes transitional at Re = 1000. The London goodness factor j/f has been used to evaluate the performance of the selected offset strip-fin geometry.

3D numerical study of transient heat transfer phenomenon on a solid plate with complex heat sources has been carried out. In order to validate the chosen numerical model, a set of thermographic measurements have been performed on a heating plate sample. The infrared camera provided a number of thermograms showing the development of transient temperature fields on the plate surface. A good agreement between thermograms and numerically obtained temperature fields has been achieved. Based upon validated numerical model, an approach involving thermographic measurements has been used to estimate the position of heat sources inside the plate. Numerically obtained temperature distributions have been used for calculation of effective transient heating output. The unsteady behaviour of heating plate with complex heat sources has been numerically studied for different plate materials. It has been concluded that the temperature fields and transient heating outputs depend on physical properties of the plate material. However, when a steady state has been achieved, different plate materials give equivalent steady heating outputs despite of different temperature distributions.

Radom su obuhvaćeni: sunčeva energija, energija vjetra, geotermalna energija, energija iz biomase i otpadaka. Uz kratki povijesni pregled, radom je prikazano stanje tehnologije obnovljivih izvora energije, analizirane su prednosti obnovljivih izvora energije, ukazano je na nedostatke i mane primjene energetskih sustava obnovljivih izvora energije. U segmentu zagađenja okoliša, posebno reduciranja emisije CO2 izvršena je usporedba s konvencionalnim izvorima energije. Korištenjem topline tla, vode i zraka prikazano je na primjeru sustava dizalice topline, toplinske crpke. Analizirane su cijene investicijskih ulaganja u sustave obnovljivih izvora energije, cijene proizvedene energije s ekonomskim pokazateljima isplativosti primjene. U zaključku rada, ukazano je na nužne poticajne mjere kao i na realne mogućnosti primjene tih obnovljivih izvora energije u našim prirodnim uvjetima, posebno priobalja i otoka.

Male i srednje tvrtke čine značajan dio gospodarskog sektora. Tvrtke iz proizvodnog i uslužnog sektora obično predstavljaju značajne potrošače energije i neminovno utječu na emitiranje stakleničkih plinova. Smanjenje emisije stakleničkih plinova može se postići odgovarajućim projektiranjem i odabirom energetskih instalacija i sustava osobito onih koji koriste obnovljive izvore energije. Kvalitetna suradnja poduzetnika-investitora i znanstvenika važna je za postizanje kvalitetnih rješenja uzimajući u obzir potrošnju energije i ekološke standarde. U radu je predstavljen međunarodni projekt pod nazivom "Smanjenje emisije stakleničkih plinova umreženom suradnjom tvrtki i istraživača" - REGENER. U projekt su uključeni austrijska, hrvatska i bugarska sveučilišta i poduzeća. Projekt financira Austrijska istraživačka agencija (FFG) u okviru CIRCE programa.

Temeljem nacionalnog energetskog programa Strategija energetskog razvitka RH ukazano je na dio programa o obnovljivim izvorima energije koji se može primijeniti u inženjerstvu morske tehnologije. Poseban naglasak dat je za segment primjene u području priobalja i otoka. Nacionalna energetska strategija obuhvaća uz konvencionalne i nove i obnovljive izvore energije, a s tim u svezi i nove tehnologije te legislativu države kojom se uvažavaju ekološki standardi. Radom su obuhvaćeni: sunčeva energija, energija vjetra, geotermalna energija, energija iz biomase i otpadaka, toplina tla, vode i zraka, energija plime i oseke te morskih valova. Uz kratki povijesni pregled, radom je prikazano stanje tehnologije obnovljivih izvora energije, analizirane su prednosti obnovljivih izvora energije, ukazano je na nedostatke i mane primjene energetskih sustava obnovljivih izvora energije. U segmentu zagađenja okoliša, posebno reduciranja emisije CO2 izvršena je usporedba s konvencionalnim izvorima energije. Korištenjem topline tla, vode i zraka prikazano je na primjeru sustava toplinske crpke. Analizirane su cijene investicijskih ulaganja u sustave obnovljivih izvora energije, cijene proizvedene energije s ekonomskim pokazateljima isplativosti primjene. U zaključku rada, ukazano je na realne mogućnosti primjene tih obnovljivih izvora energije u našim prirodnim uvjetima.

In the paper a numerical analysis of heat and mass transfer during frost formation on a fin-and-tube heat exchanger has been performed. Frost layer formation significantly influences the heat transfer on heat exchanger fins. The numerical analysis of the frost growth enables predictions of the heat transfer resistance and the decrease of exchanged heat flux. A transient two-dimensional mathematical model of frost formation has been developed. The applied mathematical model has been defined using governing equations for the boundary layer that include air and frost sub-domains as well as a boundary condition on the air-frost interface. The mathematical model with initial and boundary conditions has been discretised according to the finite volume method and solved numerically using SIMPLER algorithm for the velocity-pressure coupling. As result of numerical calculations, time-wise frost thickness variations for different air humidities have been presented.

In the paper, numerical and experimental analyses of heat and mass transfer during frost formation on a fin-and-tube heat exchanger have been performed. The modelling of frost formation on cold surfaces placed in a humid air stream requires a complex mathematical approach. Results have shown that frost layer growth is faster with higher inlet air humidity. Frost layer formation significantly influences the heat transfer between air and refrigerant. Using the developed mathematical model, the algorithm and the computer code, which have been experimentally validated, it is possible to predict a decrease of exchanged heat flux in the heat exchanger under frost growth conditions.