Experimental and numerical analysis of eight different volutes with the same impeller in a squirrel-cage fan

Transcription

1 Experimental and numerical analysis of eight different volutes with the same impeller in a squirrel-cage fan SINA SAMARBAKHSH, JAVAD ALINEJAD Azmoon Pardazesh Research Institute, Malekloo St., Farjam St., Narmak,Tehran IRAN sina_samarbakhsh@yahoo.com Department of Mechanical Engineering Islamic Azad University Sary, Mazandaran IRAN alinejad_javad@iausari.ac.ir Abstract: - This article presents the experimental and numerical investigation on the effect of volute spread angle on efficiency, performance and flow pattern inside volute of squirrel cage fan. The flow in a volute is highly complex. It is strongly believed that an understanding of the detailed flow structure in a volute will provide insights into minimizing the losses by isolating the mechanism that contribute to entropy generation. For the study presented here, eight volutes just differing in volute profile equation are studied experimentally and numerically. In all cases volute dimensions were constrained in a specific domain. Attention was focused on the effect of different volutes with the same impeller on fans performance. The overall performances of fans as well as the velocity components were measured. Velocity components were measured across and around the impeller using Laser Doppler Anemometer. A computational model using the k-ε turbulence model and wall function has been used to predict the internal flows of all volutes. Solving a fully D viscous flow was carried out using commercial CFD code, FLUENT. The results showed that volute by spread angle of 5 has the highest head and efficiency in a limited space without losing the desired mass flow rate. Key-Words: - Squirrel cage fan, Volute flow, Volute spread angle, Effect of volute, Computational fluid dynamic, Laser Doppler anemometry Introduction Centrifugal fan includes an impeller which is forced to rotate inside a volute. The fluid is entered in this fan axially and after passing through the blades, it enters the container. During passing through the blades, both velocity and pressure of fluid increase. Squirrel cage fans include three major parts: inlet, impeller and volute. Regarding coupled flow field in three above-mentioned parts, the role of each part can not be clearly demonstrated in separation. However, generally it can be said that inlet has the role to direct the flow and its distribution into entrance to impeller, impeller energy transfer to the fluid and volute, collection and conduct of flow to exit throttle of fan. One of the important cases in designing squirrel cage fans is to design its entrance opening nozzle [, 5]. In these fans, entrance opening nozzle should be designed such that it would reduce flow separation in front region of impeller. The tested rotor in this research is according to the results of Ruth optimal experiments []. In order to collect the exit flow from the rotor inside the fan, volute container is used. Cross section surface of this kind of collector increases from fan tongue to its throttle repeatedly, proportional to the increasing outflow surrounding the impeller. Experiments results show that fan performance is strongly influenced by volute changes []. The goal of designing volute is to determine the optimal position of volute tongue, volute profile, exit throttle and volute width. Volute profile. Designing the volute of squirrel-cage fan One of the important problems in designing such turbomachines is the conformity of designing point for three main parts, inlet, impeller and volute. When these three point coordinate each other, outflow from impeller is symmetrical and ISBN:

2 consequently static pressuree will be also more stable in all-around the impeller. In the following geometric characteristics of inlet and impeller are presented. The procedure of designing and selection of different volutes will be discussed in the next section. r V ln = ϕ r = ϕ tanα () r V θ In equation, V r is radial velocity, V θ tangential velocity, φ is flow coefficient and α is volute spread angle. Fig : geometric characteristics of impeller and entrance nozzle Table : geometric characteristics of impeller Impeller D D βs β S b Z (mm) (mm) (mm) (DEG) (DEG) Table : geometric characteristics of fan inlet Entrance Nozzle d(mm) 85 d(mm) h(mm) 5 Regarding table D is impeller inner diameter, D is impeller outer diameter, βs is blade inlet angle, βs is blade outlet angle, Z is number of blades and b is impeller width. Base on table d is inlet minimum diameter, d is inlet maximum diameter and h is inlet width. Theoretical method used to design logarithmic spiral volute was obtained by using rules of mass and momentum conservation []. In this method, streamline angle or on the other hand; volute spread angle ( α s ) along all the direction length remains constants. To calculate the flow field direction in a simple volute, a vortex is considered as the flow production source. In this case if we replace the effect of this vortex presence by an impeller with diameter of D, each of streamline can be considered as volute curve boundary. In this research, a volute with parallel side walls was used for experimental and numerical models. Equation () is related to logarithmic spiral which can be considered as the volute curve [].. Characteristics of the studied volutes To construct profiles which are accompanied with faster increase in distance between volute and impeller and also they do not occupy a space more than the basis volute, a change is performed in curve equation of volute with parallel walls. In this way, an equation is considered as follows: r = EXP( θ tanα) () θ r α r = θ r 6 + ( rγ r6 ) EXP( m θ ) () γ = 65 Observing the equation (), it is clear that using of different magnitude of m, we are able to control the volute and impeller distance in different sections. In this way, next step in this research is to study the performance and flow field inside fans whichh their volute curves are formed by equations () and (). Spread angle in the basis profile is 5. Profile was obtained based on the equation and profiles, were obtained based on the equation and by values of m=.5, m=, respectively. As it is observed in Fig, all profiles are confined within a limited square which is one of the properties of new equation (eq. ). It is clear that the difference in volutes is just in boundary curve of volute and other parameters such as the beginning angle of volute spiral, exit throttle size (H) and volute width (B) are fixed. Therefore, the possibility to study several volutes which are different just in distance between impeller and volute in different sections, while other geometric parameters of them are the same was achieved. Fig : a schema of profiles,,. ISBN:

3 In continue it is addressedd to study simultaneously the fans which are constructed by these profiles, experimentally and numerically. There is the possibility to exploit the results of numerical model in order to observe phenomena and parameters which their measurement is difficult and very timethese two methods during preliminary steps provides the consuming. Moreover, performing possibility to produce a better computational model according to experimental facts. Predicting the performance of next fan and decision- making about the necessity of experimental study of them can be done by analysis the CFD modeling results of each fan. In this way, fans, and were constructed using volutes, and, respectively. Characteristicss of these volutes are presented in table. data was measured to determine the average magnitude of velocity components in each point. Laser Probe h L T Burst Spectrum Analysis D D Straightener Fig : schematic of experimental study setup [6, 7] Numerical modeling Pitot Tube Following conditionn were defined in FLUENT for solving fully D computational model of fans. First, SIMPLE algorithm for numerical solution of The Navier Stokes Equations was selected. Second, k-ε turbulence model and wall function has been used to predict internal flow [8]. Solving condition considered steady state. D Computer Pressure Measurement Section D Fig : Geometric characteristics of volute Table : Geometric characteristics of volutes, and Volute Profile B H α γ (mm) (mm) It should be mentioned that head coefficient, flow coefficient and efficiency of squirrel-cage fans for 5 Re are not dependent on Reynolds number []. Experimental setup. Methodology for performance experiment The overall performance of fans were measured based on ISO 58:997. The variables of flow rate and pressure have become dimensionless according to the equations, 5 [, ]. φ = V ( Q () π ND ) P ψ = F (5) ( ρπ N D ) To measure velocity components Laser Doppler Anemometry apparatus was used. 5 velocity (b) Fig 5: (a) Sketch of the fan unstructured mesh, (b) Mesh near the volute tongue 5 Analysis and discussion 5. The study of fans,, Fans, and are introduced as follows: Fan (a) Table : Characteristics of fans,, Entrance Volute Impeller N Nozzle (rpm) ISBN:

4 According to table, these fans are just different from each other in volute. Characteristics of these volutes are explained in table. SAIT.5.5 FAN-SAIT FAN-SAIT FAN-SAIT FAN It is observed that head coefficient of fan within operating domain is more than that of fans and and change in volute geometry results in decreasing performance. After observation of experimental data, the results of numerical simulation for fan performance are presented in Fig 7. SAIT ETTA Fig 6: the comparison of efficiency and performance curves in fans, and (experimental) FAN-SAIT(Num) FAN-SAIT(Num) FAN-SAIT(Num) FAN-ETTA FAN-ETTA FAN-ETTA Fig 7: the comparison of efficiency and performance curves in fans, and (Numerical). It is observed that performance curves of numerical model have also maintained the same procedure with experimental tests. In Fig8 velocity contours are compared to each other in the same flow rate and in three axial sections of the above-mentioned fans. FAN FAN Fig 8: comparison of velocity counters for fans, and in ϕ =.55 and Z =. B By observing velocity contours, it is clear that less change of volute geometry in fan in comparison to that of fan caused minimization of dead zone to that of fan. Considering the results of experimental study as well as CFD modeling of fans, and with focusing on velocity contours, it seems that in sections where the flow has less ability to exit from the impeller like sections near the inlet [], the existencee of additional zone in volute which is more than the amount of possible exit flow, creates regions with zero velocity which causes removing flow steadiness. Therefore in order to provide procurements which would reduce or eliminate the above-mentioned negative phenomenon, a planning was performed to produce volutes which have variable cross-section in lateral direction (conical volute). Based on this idea, near the back wall where the flow has more inclination to exit, more space is available and near the inlet zone where less flow is exited and according to the observed contours, the volume of dead zone is also bigger, trying to minimize the zone and removing the volume is unnecessary. To this purpose, two volutes of and 5 are introduced. Characteristics of these volutes are presented in table 5. ISBN:

5 5.. The study of fans, 5 The volutes and 5 have a similar profile to that of volute by this difference that cross-section of volute has changed in its lateral direction based on two different patterns. In first case, back wall diameter of volute is equal to the diameter of volute which its cross-section is reduced towards the fan inlet by an appropriate angle. A view of this volute is observed in Fig 9. In volute 5, by keeping the middle cross-section of volute constantt and extend by an angle similar to that of volute. It is obvious that in volute 5, the averagee cross-section is equal to that of volute. In both volutes, the given angle to reduce the cross-section in lateral direction is 6 degrees. The selection of this angle is based on moreover to study this new idea; the corresponding effect of volute on the performance is simultaneously studied in comparison to the conical impeller. In fans and 5, the volutes and 5 are used, respectively and their characteristics are presented in table. Fan 5 Table 5: the characteristics of fans,, 5 Profile Cross B(mm) Section Angle(θ) Variable 8 Variable 6 Fig 9: a view of volute, 5 The study and examinationn of fans and 5are just performed as numerical simulation. Of course, decision-making about the construction, installation and experimental measurements are timeconsuming. In Fig, performance curves of fans and 5 are compared to that of fan. sait FAN-SAIT FAN-SAIT FAN5-SAIT Fig : the comparison of performance curves for fans,, 5 based on numerical results Since the volutes and 5 are developed in order to improve the volute efficiency; their results are just compared to those of volute. As it is observed performance curve of volute is followed by an increase in head coefficient. 5. The study of fans 6, 7, 8 Regarding to the positive effect of conical volute, by modeling some fans, it is tried to repeat this process in respect of fan. In this way, fans 6, 7 and 8 were introduced. Characteristics of these fans are presented in table 6. All fans are obtained by exerting change on the volute of fan. Table 6: characteristics of fans, 6, 7, 8 Fan Profile Cross B( mm) Section Angle(θ) In order to study the effect of these geometrical changes on fan, performance curves of these fans are compared to that of fan. As it was mentioned these curves were obtained by numerical simulation..5 SAIT.5.. FAN-SAIT FAN6-SAIT FAN7-SAIT FAN8-SAIT Fig : performance curves for fans, 6, 7, 8 Regarding to the performance curves, it is clear that this new geometry has not significant effect on the performance of fan..8 ISBN:

6 It is determined that using of conical volute is effective in cases where due to the existence of extra space in the volute, we are encountered with dead zones which in this way, they can be omitted but since fan by expansion angle of 5 has the most uniform flow field, this method has not a significant effect on its performance and therefore, there is no necessity to perform experimental tests. 6 Conclusion After experimental and numerical study of 8 different fans which are confined in a specified space and just their volute profile is different the following conclusion is presented. First, efficiency and performance curves will change highly by a change in volute profile. Second, any change at the beginning of volute profile should be continued to the end. Third, Logarithmic profile by assuming the obtained ideal flow has the highest efficiency and performance among the similar profiles in the same space. Fourth, the fundamental conduction of flow to final sections causes an increase in efficiency. The last, fan with 5 expansion angle of volute has the highest head and efficiency in a specified space. This point is important that this optimal volute is obtained without flow rate loss. It means that in a specified space using of a bigger impeller and a volute with a smaller expansion angle, the same flow rate can be obtained. Acknowledgement: This research was supported by Amirkabir University of Technology to which the authors are most grateful. References: [] Eck B., Fans, Pergamon Press, st English Edition, Oxford, 97, pp. 7-. [] Gessner F.B, An experimental study of Centrifugal fan inlet flow and its influence on fan performance, ASME paper, 67-FE-, 967. [] ISO 58,Method For Testing Fans For General Purposes, :997 [] Montazerin N., Damangir A., Kazemi Fard A., A study of slip factor and velocity components at the rotor exit of forward-curved squirrel cage fans, using laser Doppler anemometry, Proc Instn Mech Engrs, Vol. 5,, 5-6. [5] Montazerin N., Damangir A., Mirian S., A new concept for squirrel-cage fan inlet, Journal of Power and Energy, Proceeding of Instn Mech Engrs, Vol., 998, -9. [6] Montazerin, N., Damangir, A., Flow Field Measurements in the Volute Section of a Squirrel Cage Fan, Proc. of the nd Biennial Conference on Engineering Systems Design and Analysis, Vol.8, France, 996. [7] Montazerin, N., Damangir, A., Stall Cells in the Volute of a Squirrel Cage Fan, Proc. of the rd Biennial Conference on Engineering Systems Design and Analysis, Germany, 998. [8] Nobari, M.R.H., Hekmatara, M., Damangir, E., A Comparison of different extended k-e Models for Flow and Heat Transfer in a Radially Rotating Duct, M.Sc. Thesis AmirKabir University of Technology 5. [9] Raj D., Swim W.B., Measurement of mean flow velocity and velocity fluctuations at the exit of an FC centrifugal fan rotor, ASME Journal of engineering for power, Vol., 98, [] Roth H.W., Optimierung Von Trommellaufer- Ventilatorn, Institut fur Stromungslhere und Stromungs Maschiner Universitat Karlsruhe (TH), 99. [] Samian R.S., Montazerin N., Damangir A., Nikkhoo M., An experimental study of squirrel cage fan rotor width on performance and velocity profiles, nd International Conference on computer and Automation Engineering (ICCAE), Singapore ISBN: