Eksperimental in numerical analysis of fluid flow in a axial fan

Aksialni ventilatorji so zasnovani tako, da delujejo v stabilnem področju dušilne krivulje. Prostorske oziroma druge omejitve lahko povzročijo, da delovanje ventilatorja preide iz stabilnega v nestabilno področje, kjer se razvijejo kompleksni 3D časovno odvisni tokovni pojavi, ki negativno vplivajo na karakteristiko ventilatorja. Da bi razumeli kompleksne pojave v medlopatičnem kanalu ventilatorja, je bila najprej narejena eksperimentalna in numerična analiza toka okoli osamljenega krila. V nadaljevanju je predstavljena eksperimentalna in numerična raziskava tokovnih pojavov v aksialnem ventilatorju s poudarkom na nestacionarnem delovanju. V okvirju eksperimentalne raziskave je bila posneta dušilna krivulja, ki je bila razdeljena na stabilni in nestabilni del. S PIV merilno metodo so bila posneta tokovna polja pred rotorjem ventilatorja in v medlopatičnem prostoru v različnih delovnih točkah. Raziskovanje je usmerjeno v analizo vrtečega zastoja, ki se razvije pri prehodu iz stabilnega v nestabilno delovanje. Razvita je bila metoda rekonstrukcije vrtečega zastoja, s katero je bila v izbranih delovnih točkah opravljena rekonstrukcija vrtečega zastoja pred rotorjem in v medlopatičnem kanalu. Prav tako je bil izračunan delež povratnega toka v medlopatičnem kanalu. Rezultati meritev, integralni in lokalni, so bili primerjani z rezultati numeričnih simulacij. Izvedene so bile stacionarne in časovno odvisne simulacije z uporabljenima k-ε in SST turbulentnima modeloma. Stabilni del dušilne krivulje je bil primerjan z rezultati stacionarnih simulacij, nestabilni del pa s časovno odvisnimi simulacijami. Ugotovljeno je bilo, da stacionarne simulacije dobro napovejo časovno povprečena tokovna polja in potek stabilnega dela dušilne krivulje. Časovno odvisne simulacije z uporabljenima k-ε in SST turbulentnima modeloma slabo napovejo potek nestabilnega dela dušilne krivulje. Oba omenjena turbulentna modela uspeta napovedati pojav vrtečega zastoja, ne uspeta pa napovedati časovno odvisnega tokovnega polja v primeru osamljenega krila kot tudi v primeru ventilatorja.Designing of axial fans is done assuming their operation in stable part of characteristic curve. However, it is possible that due to some limitations the fan operation regime becomes unstable. In this case complex, time dependent, 3D flow phenomena occur resulting in decreasing of fan operational characteristics. A study of this phenomenon started with the numerical and experimental analysis of an isolated airfoil. The research continued with experimental and numerically analysis of flow phenomena in and axial fan with the focus on unstable operational regime and phenomena occurring in area between the rotor blades. Characteristic curve was recorded first and then split into stable and unstable part. Velocity profiles were recorded by PIV before fan blades and around them in various operating points. The research was focused on rotating stall which developed during the stable to unstable flow transition. A method for reconstruction of rotating stall has been developed and rotating stall reconstructed in several fan operating points before the rotor blades and around them. The backflow ratio around the fan blades was calculated. Global and local experimental results were compared to the numerical results. Steady state and transient numerical simulations were calculated using k-ε and SST turbulent models. The experimental results acquired during the fan operation on the stable part of the characteristic curve were compared to the results of the steady-state simulations while transient simulations were used for the comparison with the results of acquired during unstable operation. The agreement between the results of the steady-state simulations and averaged velocity profile was satisfactory, especially on the stable part of the characteristic curve. Transient simulations with k-ε and SST turbulent models did not provide good agreement with the experimental data when global characteristic (characteristic curve) was studied although fairly good agreement was observed when rotating stall was studied. Numerical simulations using both turbulent models failed to predict time dependent flow around the airfoil and in the case of an axial fan