Experimental and numerical analysis of flow field and ventilation performance in a traffic tunnel ventilated by axial fans

To investigate air flow in longitudinally ventilated traffic tunnels, a scaled model of a typical road-traffic tunnel with an appropriate ventilation system based on axial ducted fans, is designed and built in the Lab. The focus of this paper is the airflow in a bi-directional traffic, two-lane tunnel. At the scale ratio of approx. 1:20, at 20.52 m length it represents approximate to 400 m of a real-scale tunnel. The model consists of two parallel tunnel tubes, where the main tunnel (with a hydraulic diameter of D-h1 approximate to 0.4 m has the geometry of a scaled road traffic-tunnel. The second tunnel (D-h2 approximate to 0.16 m) has a smaller size and is circular in cross-section, used only to simulate airflow towards an evacuation tunnel tube. Thus the two tunnels are connected by the evacuation passages, equipped with adjustable escape doors. By a combination of experimental and numerical work, the air flow-field and the performance of the ventilation system are investigated. The velocity field and its turbulence properties exiting the fans were determined experimentally using hot-wire anemometry. These data were further processed to be used in the tunnel flow computations by CFD. The efficiency of momentum transfer (eta(i), Kempf factor) of the longitudinal tunnel ventilation is determined. The effect that the imposed boundary conditions and the level of their detail, have within a CFD computation of tunnel airflow, with respect to accuracy, velocity distribution and computed eta(i), Finally a traffic-loaded (traffic "jam'') case of flow is studied through experiment and CFD. The difficulty in assessing the required thrust of the plant in traffic-jam tunnel conditions is discussed, and the ventilation efficiency is estimated. Based on later results, the two limiting shapes of axial velocity distribution with respect to height above the road, in this type of tunnel and traffic, are estimated. The last result can be used as a realistic boundary condition (as inlet b.c. and/or initial condition) for numerical studies of flow and fire scenarios in such tunnels with the traffic load critical for design

Experimental and theoretical research of the structure of turbulent swirl flow in axial fan jet

У оквиру дисертацијe се истражује структура турбулентног вихорног струјања у млазу аксијалног вентилатора. Генерисано турбулентно струјање је тродимензионо, нехомогено и анизотропно. Примењени су сложени експериментални, нумерички и теоријски приступи. Експериментална инсталација је у раду детаљно описана, као и квантификација мерне несигурности трокомпонентног система за Ласер Доплер анемометрију (ЛДА). ЛДА систем је, при симултаном мерењу три компоненте брзине, захтевао прецизна подешавања мерне запремине у границама пречника до 100 μm. Остварено је прецизно померање ове запремине дуж млаза. Посебан изазов је представљала нумеричка обрада добијених, у времену неједнако распоређених, мерних резултата. Теоријским разматрањима, у оквиру дисертације, бачено је ново светло на класичне записе Навије-Стоксових и Рејнолдсових једначина у поларно-цилиндричним координатама. Детаљно су разматране емпиријске радијалноаксијалне расподеле укупне, аксијалне, радијалне и обимске брзине, измерене за две брзине обртања вентилатора и три угла лопатица. На овај начин је било могуће истражити утицај Рејнолдсовог броја, као и геометријских карактеристика вентилатора, тј. угла лопатица, на карактер дејства вихора на турбуленцију и еволуцију нивоа турбуленције, како у самом мерном пресеку, тако и у низструјном развоју млаза. Анализа генерисања турбуленције и продукције појединих Рејнолдсових напона указала је на битне карактеристике турбулентног преноса у слободном вихорном млазу. Уочено је да промена знака градијента брзине не изазива увек и промену знака припадајуће компоненте тензора турбулентног напона. У доменима струјног поља у којима се то дешава, механизам турбулентног преноса је нелокалног карактера, присутна је неградијентна турбулентна дифузија и јавља се негативна продукција кинетичке енергије турбуленције. Изложена је дискусија о утицају вихора на структуру турбуленције и механизам турбулентног преноса. Анализа је указала на сложена међудејства осредњених и флуктуационих поља у вихорном млазу. Закључено је да генерисање енергије флуктуационог кретања не настаје на основу кинетичке енергије осредњеног кретања, већ напротив. Смер преношења енергије се мења, тако да осредњено кретање „црпи” кинетичку енергију флуктуационог кретања, штоодговара сложеним структуралним својствима турбуленције. Зато су експериментална истраживања била усмерена на мерење и анализу статистичких момената вишег реда, и то овде до шестог реда...The dissertation deals with the structure of the turbulent swirl flow in an axial fan jet. The generated turbulent flow is three-dimensional, inhomogeneous and anisotropic. Complex experimental, numerical and theoretical approaches were applied. The experimental setup is described in detail, as well as the quantification of the measurement uncertainty of a three-component Laser Doppler Anemometry (LDA) system. The LDA system, while measuring the three velocity components simultaneously, requires precise adjustments of the measuring volume up to 100 μm in diameter. A precise displacement of this volume along the jet was achieved. A particular challenge was the numerical processing of the obtained unequally spaced data. Theoretical considerations, in the framework of the dissertation, have brought new light on classical notations of Navier-Stokes and Reynolds equations in polar-cylindrical coordinates. The empirical radial-axial distributions of total, axial, radial, and circumferential velocities measured for two fan speeds and three vanes angles are thoroughly analyzed. In this way, it was possible to investigate the influence of the Reynolds number as well as the geometric characteristics of the fan, i.e. angles of the blades, on the character of the swirl influence on turbulence and the evolution of the turbulence level, both in the measuring cross section and in the downstream development of the jet. An analysis of the generation of turbulence and the production of individual Reynolds stresses indicated the essential characteristics of turbulent transfer in a free swirl jet. It has been observed that a change in the sign of the velocity gradient does not always cause a change in the sign of the corresponding component of the turbulent stress tensor. In the fluid flow domains where this occurs, the turbulent transfer mechanism has nonlocal character, non-gradient turbulent diffusion and negative production of turbulence kinetic energy occur. Discussion of the effect of the swirl on the turbulence structure and the mechanism of turbulent transfer is presented. The analysis indicated the complex interactions of averaged and fluctuating fields in a swirl flow. It is concluded that the generation of fluctuating motion energy does not occur on the basis of kinetic energy of averaged motion, but on the contrary. The direction of energy transfer changes so that theaveraged motion "draws" the kinetic energy of the fluctuating motion, which corresponds to the complex structural properties of the turbulence. That is why experimental research has focused on measuring and analyzing higher order statistical moments, up to the sixth order here..

Integral and statistical characteristics of the turbulent swirl flow in a straight conical diffuser

The results of the experimental investigations of the turbulent swirl flow in a straight conical diffuser with inlet diameter 0.4 m and total divergence angle 8.6 degrees are presented in this paper. The incompressible swirl flow field is generated by the axial fan with outer diameter 0.397 m. The measurements were performed in one measuring section downstream the axial fan impeller in the conical diffuser in position (z/R-0 = 1) with original classical probes and an one-component laser Doppler anemometry (LDA) system, for four flow regimes. The comparative measurements of axial and circumferential velocities are presented. The Reynolds number, calculated on the basis of the average velocity, ranges from 149857 to 216916. Integral parameters, such as volume flow rate, average circulation and swirl number, are determined. Statistical characteristics, such as level of turbulence, skewness and flatness factors, are calculated. The highest levels of turbulence for axial velocity are reached in region 0.4 lt r/R lt 0.6, where D = 2R. The highest levels of turbulence for circumferential velocity are reached for the regimes with lower circulation in r/R approximate to 0.4, i.e., in the vortex core region for the cases with higher circulation

Ventilation performance and pollutant flow in a unidirectional-traffic road tunnel

To develop a reliable method for modeling fire case scenarios within the road tunnels and observing the effects of the skewed velocity, experimental and numerical approach is used. Experimental results obtained from a laboratory tunnel model installation, are used to define geometry and boundary conditions. The result for the overall ventilation performance is compared to the available cases, for empty tunnel and stationary bi-directional vehicle traffic. For a unidirectional traffic road tunnel, in traffic loaded conditions, with a ventilation system based on axial ducted fans, the numerical simulation is used to determine the flow and temperature fields, the ventilation efficiency (efficiency of momentum transfer), and to assess the shape of the velocity distribution. The effect that a skewed velocity distribution can have on the resulting thermal and pollutant fields (CO2), smoke backlayering and stratification, is evaluated using numerical simulations, for the model-scale tunnel fire conditions. The effect of two possible limiting shapes of the velocity distribution, dependent only on the location of the fire with respect to the nearest upstream operating fans, is analyzed. The numerical results for a fire are scenario are a starting point in assessing the feasibility of a laboratory model fire-scenario experiment, what is planned as the next step in this research

Turbulence Structure and Dynamics Investigation of Turbulent Swirl Flow in Pipe Using High-Speed Stereo PIV Data

Turbulent swirl flow, which exists in numerous turbomachinery systems, is the focus of this paper. It consumes a significant amount of energy, so it is a subject of investigation for many researchers. It is even more present in ventilation systems, as numerous axial fans are still installed without guide vanes. The experimental investigation of the turbulent swirl flow behind an axial fan in a pipe, installed in a test rig with a free inlet and ducted outlet, as defined in the international standard ISO 5801, is presented in this paper. Moreover, in this paper, the axially restricted case is studied. A designed axial fan generates a Rankine vortex with a complex structure, and research on the vortex turbulence structure and dynamics is presented. On the basis of the HSS PIV (high-speed stereo particle image velocimetry), measurement results are calculated using invariant maps. All states of turbulence anisotropy are thoroughly analyzed by applying the invariant theory on HSS PIV results. Vortex dynamics is observed on the basis of the total velocity minima positions and their repetitions. Both methods are correlated, and important conclusions regarding vortex behavior are deduced.Cite as: MDPI and ACS Style Čantrak, D.S.; Janković, N.Z. Turbulence Structure and Dynamics Investigation of Turbulent Swirl Flow in Pipe Using High-Speed Stereo PIV Data. Energies 2022, 15, 5417. https://doi.org/10.3390/en1515541

New Design of the Reversible Jet Fan

This paper presents two designs of the axial reversible jet fan, with the special focus on the impeller. The intention was to develop a reversible axial jet fan which operates in the same way in both rotating directions while generating thrust as high as possible. The jet fan model with the outer diameter 499.2 +/- 0.1 mm and ten adjustable blades is the same, while it is in-built in two different casings. The first construction is a cylindrical casing, while the second one is profiled as a nozzle. Thrust, volume flow rate, consumed power and ambient conditions were measured after the international standard ISO 13350. Results for both constructions are presented for three impeller blade angles: 28 degrees, 31 degrees and 35 degrees, and rotation speed in the interval n = 400 to 2600 rpm. The smallest differences in thrust, depending on the fan rotation direction, as well as the highest thrust are achieved for the first design with the cylindrical casing and blade angle at the outer diameter of 35 degrees. Therefore, it was shown that fan casing significantly influences jet fan characteristics. In addition, the maximum thrust value and its independence of the flow direction is experimentally obtained for the angle of 39 degrees in the cylindrical casing

Uticaj ugla lopatice aksijalnog ventilatora na karakteristike turbulentnog vihornog strujanja

U ovom radu je prikazano istraživanje turbulentnog vihornog strujanja u cevi na potisu aksijalnog ventilatora sa devet podesivih lopatica za uglove 22°, 26° i 30° i istovetni broj obrtaja (1500 min-1). Brzine su merene laser Dopler anemometarskom tehnikom u mernom preseku 3.35·D od ulaska u instalaciju. Postignuti su Rejnoldsovi brojevi Re=236784, 259151 i 277018. Prikazana je nehomogenost i anizotropnost izučavanog turbulentnog brzinskog polja. Profili vremenski osrednjenih obimskih brzina imaju karakter Rankinovog vrtloga, dok se u slučaju aksijalnih brzina primećuje povratno strujanje u oblasti vrtložnog jezgra za uglove 26° i 30°. Eksperimentalno određeni momenti drugog i viših redova prikazuju kompleksnost turbulentnog vihornog strujanja. Vizualizacija turbulentnog vihornog strujanja je prikazana za ugao 22°.The paper presents the investigation of the turbulent swirl flow in a pipe behind the axial fan with adjustable nine blades for the angles of 22°, 26° and 30° for the same rotation number (1500 rpm). Velocities were measured with laser Doppler anemometry (LDA) in the measuring section 3.35·D from the test rig inlet. The achieved Reynolds numbers are Re=236784, 259151 and 277018. The non-homogeneity and anisotropy of the turbulent velocity field are shown. The time averaged circumferential velocity profiles have shown the Rankine vortex structure and revealed a reverse flow in the vortex core region for the blade angles of 26° and 30°. The experimentally determined moments of the second and higher orders reveal complex mechanisms in the turbulent swirl flow. In addition, the visualization of the turbulent swirl flow for an angle of 22° is presented

New Design of the Reversible Jet Fan

This paper presents two designs of the axial reversible jet fan, with the special focus on the impeller. The intention was to develop a reversible axial jet fan which operates in the same way in both rotating directions while generating thrust as high as possible. The jet fan model with the outer diameter 499.2 +/- 0.1 mm and ten adjustable blades is the same, while it is in-built in two different casings. The first construction is a cylindrical casing, while the second one is profiled as a nozzle. Thrust, volume flow rate, consumed power and ambient conditions were measured after the international standard ISO 13350. Results for both constructions are presented for three impeller blade angles: 28 degrees, 31 degrees and 35 degrees, and rotation speed in the interval n = 400 to 2600 rpm. The smallest differences in thrust, depending on the fan rotation direction, as well as the highest thrust are achieved for the first design with the cylindrical casing and blade angle at the outer diameter of 35 degrees. Therefore, it was shown that fan casing significantly influences jet fan characteristics. In addition, the maximum thrust value and its independence of the flow direction is experimentally obtained for the angle of 39 degrees in the cylindrical casing

Influence of the axial fan blade angle on the turbulent swirl flow characteristics

U ovom radu je prikazano istraživanje turbulentnog vihornog strujanja u cevi na potisu aksijalnog ventilatora sa devet podesivih lopatica za uglove 22°, 26° i 30° i istovetni broj obrtaja (1500 min-1). Brzine su merene laser Dopler anemometarskom tehnikom u mernom preseku 3.35·D od ulaska u instalaciju. Postignuti su Rejnoldsovi brojevi Re=236784, 259151 i 277018. Prikazana je nehomogenost i anizotropnost izučavanog turbulentnog brzinskog polja. Profili vremenski osrednjenih obimskih brzina imaju karakter Rankinovog vrtloga, dok se u slučaju aksijalnih brzina primećuje povratno strujanje u oblasti vrtložnog jezgra za uglove 26° i 30°. Eksperimentalno određeni momenti drugog i viših redova prikazuju kompleksnost turbulentnog vihornog strujanja. Vizualizacija turbulentnog vihornog strujanja je prikazana za ugao 22°.The paper presents the investigation of the turbulent swirl flow in a pipe behind the axial fan with adjustable nine blades for the angles of 22°, 26° and 30° for the same rotation number (1500 rpm). Velocities were measured with laser Doppler anemometry (LDA) in the measuring section 3.35·D from the test rig inlet. The achieved Reynolds numbers are Re=236784, 259151 and 277018. The non-homogeneity and anisotropy of the turbulent velocity field are shown. The time averaged circumferential velocity profiles have shown the Rankine vortex structure and revealed a reverse flow in the vortex core region for the blade angles of 26° and 30°. The experimentally determined moments of the second and higher orders reveal complex mechanisms in the turbulent swirl flow. In addition, the visualization of the turbulent swirl flow for an angle of 22° is presented

Do-it-yourself microfluidics and possibilities for micro PIV

U odnosu na najčešće korišćene metode za proizvodnju mikrokanala, postupak koji je izložen u ovom radu ima sledeće prednosti: jednostavnost, niža cena potrebne opreme i velika ušteda u vremenu. Korišćenjem laserskog štampača moguće je ostaviti trag mastila na listu od termoplastičnog materijala, u obliku željenog mikrokanala. Zatim se list ubacuje u zagrejano ulje gde dolazi do izotropnog smanjivanja njegove dužine i širine, i istovremeno do povećavanja debljine. Na taj način se dobija kalup. Pripremljena PDMS masa se izliva na kalup i zatim peče. Nakon pečenja, kalup se odvaja od PDMS mase u kojoj se sada nalazi udubljenje koje predstavlja mikrokanal. Kada se uređaj pričvrsti za podlogu, i kada se naprave ulazni i izlazni otvori, spreman je za korišćenje. Na ovaj način moguće je napraviti veoma složene mikrokanale promenljivih dimenzijama, korišćenjem opreme koja nije skupa i u veoma kratkom vremenskom roku. U okviru rada su prikazane i vizualizacije strujanja u kanalima koji su napravljeni u okviru radionice 'Do-it-yourself Microfluidics Workshop' koja je održana na Mašinskom fakultetu Univerziteta u Beogradu. Dat je i kratak osvrt na mogućnosti primene mikro PIV merne tehnike.We present a method for microfluidic channel fabrication that has the following advantages compared to conventionally-used methods: technical simplicity, dramatically lower fabrication costs, and fabrication time. The method entails printing channel designs on a thermoplastic film from a LaserJet printer. Exposure to high temperatures elicits isotropic shrinkage of the film (in the x-y plane), in addition to thickening (in the z-direction), resulting in a positive relief mold. The microfluidic channel design of the mold is then transferred to a polydimethyl siloxane (PDMS) chip through soft lithography, resulting in a ready-to-use microfluidic chip. Through this approach, chips with complex channel geometries can be generated with low cost equipment and in as little as a couple hours. Flow visualizations from several chips from the 'Do-it-yourself Microfluidics Workshop', held at the Faculty of Mechanical Engineering Univeristy of Belgrade, are presented in this paper. We also discuss possibilities for adapting micro particle image velocimetry (PIV) measurements to channel designs on PDMS-based microfluidic chips using the fabrication method delineated here