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The aim of this paper is to estimate fin spacing and gas radiation effects on heat transfer in utility boiler economizers to achieve the highest feed water temperature, and the biggest amount of heat transferred from flue gases to economizer tubes. This study is carried out for five different cases including a bare tube economizer and four finned-tube economizers with different fin spacing values. A finite volume method is employed to discretize and solve the governing equations for all cases in 3D. As a result, for verification purpose, the present numerical result is compared to the experimental outputs and it shows that the average Nusselt number difference is 15%. The finned-tube economizer with fin spacing value of 21mm absorbs the biggest amount of heat, and it has the highest outlet feed water temperature; therefore, it has the best thermal performance among the other cases. It is also concluded that radiation heat transfer mechanism has a minor effect on heat exchange in utility boiler economizers.

U radu se istražuju karakteristike protoka i prijenosa topline mikropolarnog toka fluida preko elastičnog lima. Istražuju se utjecaji magnetskog polja, toka topline radijacije i poroznog lima, uz pretpostavku da su viskoznost i toplinska vodljivost ovisne o temperature. Primijenjene su jednadžbe formulirane aproksimacijom graničnog sloja i teorijom mikropolarnih fluida. Za rješenje problema, parcijalne diferencijalne jednadžbe transformirane su u obične diferencijalne jednadžbe primjenom rješenja sličnosti, a dobivene jednadžbe rješavane su metodom izbacivanja i metodom Runge-Kutta četvrtog reda (fourth-order Runge-Kutta). Rezultati pokazuju da djelovanje magnetskog polja i poroznosti rezultira smanjenjem vrijednosti brzine u graničnim slojevima, strujanje topline radijacije rezultira povećanjem debljine graničnog sloja. Nadalje, debljina toplinskog graničnog sloja smanjuje se porastom usisnog parametra i redukcijom učinaka radijacijskog i magnetskog polja., In this paper, the flow and heat transfer characteristics of a micropolar fluid flow over a stretching sheet are studied. The effects of magnetic field, radiation heat flux and porous sheet are investigated, and the viscosity and thermal conductivity are supposed temperature-dependent. The governing equations are formulated by boundary layer approximation and the theory of micropolar fluids. To solve the problem, the partial differential equations are transformed into the ordinary differential equations by using similarity solutions, and the achieved equations are solved by shooting method and fourth-order Runge-Kutta. The results show that the effects of magnetic field and porousness result in decrease of the velocity values in the boundary layers, but the presence of radiation heat flux results in the growth of the boundary layer thickness. Furthermore, the thickness of thermal boundary layer declines by increase of suction parameter and reduction of radiation and magnetic field effects.

Danas se metode brze izrade naveliko koriste u proizvodnji modela za ispitivanje u aerotunelima. Dimenzijska točnost, površinska hrapavost i čvrstoća materijala važni su parametri koji utječu na aerodinamička svojstva modela. U ovom radu ispitivane su primjene i potencijali dviju metoda brze izrade koje se koriste u konstruiranju i izradi modela za testiranje u aerotunelima, kako bi se izmjerili aerodinamički koeficijenti. Izabrani su AGARD-B model i konfiguracija nos-tijelo-rep vozila. Prvi model je bio izrađen od metala i ABS-M30 materijala strojnom obradom i oblikovanjem nanesenog rastaljenog metala. Drugi model, AGARD-B, izrađen je postupkom trodimenzijskog tiskanja. Za poboljšanje čvrstoće i krutosti modela i održavanje ravnoteže, korišteni su metalni umetci. Modeli su napravljeni što tanji kako bi se postigla prihvatljiva dimenzionalna točnost i površinska hrapavost. Mjereni su aerodinamički koeficijenti hibridnih modela uključujući silu otpora, silu uzgona i moment posrtanja pri različitim napadnim kutovima i određenim brzinama te uspoređeni sa standardnim modelima. Rezultati pokazuju da se hibridni modeli razvijeni metodama brze izrade mogu koristiti za mjerenje aerodinamičkih koeficijenata i kalibriranje aerotunela te tako smanjiti proizvodne troškove i vrijeme. Manje devijacije aerodinamičkih koeficijenata primijećene kod hibridnih modela u usporedbi s metalnim modelima mogu nastati zbog savijanja hibridnih modela kod velikog napadnog kuta., Nowadays, rapid manufacturing methods are widely used in order to produce wind tunnel test models. Dimension accuracy, surface roughness and material strength are important parameters affecting aerodynamic properties of the models. In this research, applications and capabilities of two rapid manufacturing methods used in design and development of wind tunnel test models, built to measure aerodynamic coefficients have been investigated. In this experience, the AGARD-B model and the nose-body-tail vehicle configuration have been chosen. The first model was fabricated from metal and ABS-M30 material by machining and fused deposition modelling. The other model, AGARD-B, was fabricated by the three dimensional printing process. To improve strength and stiffness of the models and maintain balance, metal inserts have been used. Models are built to the least thickness in order to achieve acceptable dimensional accuracy and surface roughness. Aerodynamic coefficients of hybrid models including drag force, lift force and pitching moment under different angles of attack and specified speed have been measured and compared with standard models. Results imply that hybrid models developed by rapid manufacturing methods can be used for measurement of aerodynamic coefficients and calibration of wind tunnels; hence save production cost and time. Minor deviation of aerodynamic coefficients observed in hybrid models compared to metal models, can be pertained to the bending of the hybrid models under large angles of attack.