Frost Formation Phenomenon in a Fin-and-Tube Heat Exchanger

A transient two-dimensional mathematical model of frost formation on a fin-and-tube heat exchanger has been developed and numerically solved. The mathematical model and numerical procedure have been experimentally validated. The results have shown that frost layer formation significantly influences heat transfer between air and a refrigerant. Frost layer growth is faster with higher inlet air humidity. Using the developed mathematical model, the algorithm and the computer code, which have been experimentally validated, it is possible to predict frost layer growth on fin-and-tube heat exchangers under different operating conditions

Melting of PCM in a thermal energy storage unit: Numerical investigation and effect of nanoparticle enhancement

The present paper describes the analysis of the melting process in a single vertical shell-and-tube latent heat thermal energy storage (LHTES), unit and it is directed at understanding the thermal performance of the system. The study is realized using a computational fluid-dynamic (CFD) model that takes into account of the phase-change phenomenon by means of the enthalpy method. Fluid flow is fully resolved in the liquid phase-change material (PCM) in order to elucidate the role of natural convection. The unsteady evolution of the melting front and the velocity and temperature fields is detailed. Temperature profiles are analyzed and compared with experimental data available in the literature. Other relevant quantities are also monitored, including energy stored and heat flux exchanged between PCM and HTF. The results demonstrate that natural convection within PCM and inlet HTF temperature significantly affects the phase-change process. Thermal enhancement through the dispersion of highly conductive nanoparticles in the base PCM is considered in the second part of the paper. Thermal behavior of the LHTES unit charged with nano-enhanced PCM is numerically analyzed and compared with the original system configuration. Due to increase of thermal conductivity, augmented thermal performance is observed: melting time is reduced of 15% when nano-enhanced PCM with particle volume fraction of 4% is adopted. Similar improvements of the heat transfer rate are also detecte

Analiza prijelaza topline unutar grijaće ploče s mnogostrukim izvorima topline

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. Satisfactory agreement between thermograms and numerically obtained temperature fields has been achieved. Based upon the 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 behavior of the 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 different temperature distributions.Provedena je 3D numerička analiza nestacionarnog prijelaza topline u grijaćoj ploči s mnogostrukim izvorima topline. Valjanost odabranog numeričkog modela provjerena je usporedbom s termografskim snimcima koji su načinjeni na uzorku grijaće ploče. Infracrvena je kamera osigurala dovoljan broj termograma koji prikazuju nestacionarne temperaturne raspodjele na površini ploče. Usporedbom termograma i numeričkim putem dobivenih temperaturnih raspodjela utvrđena je dobra podudarnost termografskih mjerenja i numeričkih simulacija. Temeljeći se na provjerenom numeričkom modelu, razvijen je postupak za određivanje položaja izvora topline u grijaćoj ploči pomoću termografskih mjerenja. Numeričkim putem dobivene temperaturne raspodjele na površini ploče korištene su za određivanje toplinskog učina grijaće ploče. Nestacionarno ponašanje temperaturnih raspodjela na grijaćoj ploči s mnogostrukim izvorima topline ispitivano je numeričkim putem za različite materijale ploče. Zaključeno je da raspodjele temperatura i nestacionarni toplinski učini ovise o fizikalnim svojstvima materijala ploče. Međutim, kada se postigne stacionarno stanje, različiti materijali ploče daju jednake toplinske učinke usprkos različitim temperaturnim raspodjelama