Flow Fan Research Articles

A Study on the Influence of Mobile Fans on the Smoke Spreading Characteristics of Tunnel Fires

Mobile fans, as flexible and convenient new longitudinal ventilation and smoke extraction equipment for tunnels, demonstrate more significant effectiveness in an emergency response to tunnel fires compared to traditional smoke extraction methods. This study employs computational fluid dynamics simulation methods, selecting two fire scenarios to investigate the effects of fan inclined angles and fan airflow volumes on the longitudinal temperature distribution and smoke back-layering length in tunnels. The results indicate that when using mobile fans for longitudinal ventilation in tunnels, at a lower fan airflow volume, the temperature distribution along the longitudinal axis is nearly symmetrical. The fire source and the fan installed in the upstream are within a certain range, and it is more effective to use the horizontal angle for longitudinal ventilation. As the fan airflow volume increases, the back-layering length significantly decreases (210,000 m3/h < V < 270,000 m3/h). However, as the fan flow volume continues to increase (270,000 m3/h < V < 300,000 m3/h), the reduction in the back-layering length becomes less pronounced, the smoke spread distance of the latter is only 11% of that of the former. Therefore, selecting appropriate fan airflow volumes and fan inclined angles them can effectively enhance the performance of tunnel smoke extraction systems. Moreover, by comparing with traditional fans, we find that mobile fans provide an alternative effective strategy during firefighting by allowing adjustments in distance from the fire source and fan inclination angles, enhancing fire suppression effectiveness while reducing energy losses. The research findings can serve as a reference for tunnel fire prevention design.

On Hysteresis In A Variable Pitch Fan Transitioning To Reverse Thrust Mode And Back

Abstract A novel hysteresis phenomenon during the transition to and back from the reverse thrust mode in a Variable Pitch Fan (VPF) is identified and characterised in this work. This is done by using a three-dimensional (3D) fully transient Unsteady Reynolds averaged Navier?Stokes (URANS) with the transitioning fan blade aerofoils simulated by an adaptation of the mesh displacement method. The VPF is modelled to be transitioning in a modern 40000 lbf geared high bypass ratio turbofan engine architecture at "Approach Idle" engine power setting in a typical twin-engine airframe with the flaps, slats, and spoilers set for an aircraft touchdown airspeed of 140 knots. The transition to reverse thrust mode involves flow starvation into the engine, formation of recirculation zones in the bypass duct and the establishment of the reverse stream, all of which occurs in the opposing presence of the free stream flow at aircraft touchdown velocity. The transition back to forward flow mode involves the gradual re-establishment of the free stream which is opposed by the presence of the reverse stream within the engine. It is quantified that in the transition to reverse thrust, the blockage develops with a larger time delay than the disappearance of the blockage during the transition back due to the interplay of the temporal dynamics of fan blade motion and flow field response. The hysteresis phenomena described in this work are critical in properly developing control schedules to adapt for potential bi-stable flow field development during the landing run.

Effect of inlet distortion on internal flow and performance of a contra-rotating fan operating with single-stage impeller

To establish a theoretical foundation for the rational selection of contra-rotating fans operating with a single-stage impeller under inlet distortion. Unsteady numerical simulations were conducted using the SST k-ω turbulence model to analyze the airflow fields, respectively, for the single-stage operation of front impeller (SSOFI) and the single-stage operation of rear impeller (SSORI) of the fan. The internal flow characteristics with single-stage operation under both distorted inlet condition (DIC) and uniform inlet condition (UIC) were studied. Furthermore, a comparison of the performance parameters of the fan, including total pressure and total pressure efficiency, was conducted. Focusing on the SSOFI with satisfactory operational performance of the contra-rotating fan, this study aims to reveal the propagation of inlet distortion inside the fan and reveal the influence mechanisms of inlet distortions on the performance of the fan. The results indicate that the impact of inlet distortion on the flow field is primarily concentrated in the upstream area of the front-stage impeller for SSOFI. However, the flow disturbance from the exit of the front-stage impeller to the diffuser is relatively minimal. For SSORI, the inlet distortion exacerbates the adverse effect of the front stage impeller as the front guide vane on the flow inside the rear stage impeller. The entropy loss in the two-stage impellers is significantly higher than that under the uniform inlet condition, and the turbulence level in the diffuser is intensified. While the efficiency of the fan is significantly influenced by the inlet condition and operating stages, their impact on the total pressure is small. Specifically, under uniform inlet conditions, the total pressure efficiency for SSOFI exceeds that of SSORI by 4.9 %, whereas the difference rises to 11 % under inlet distortion condition.

Optimization Based on Computer Fluid Dynamics Finite Element Method Automotive Cooling Fan Structure

With the continuous improvement of vehicle comprehensive technology, the heat load generated by automotive engine systems, hydraulic systems, and other devices also increases. Therefore, improving the heat dissipation efficiency of the cooling system is a key technology that cannot be ignored in the current vehicle development process. The car cooling fan is an important component of the cooling system, and its working characteristics will directly affect the cooling effect of the engine. This article uses the finite volume method of computer fluid dynamics to optimize the structure of a heat dissipation fan, analyze and define the flow field control volume required for its numerical calculation, use structured and unstructured grids to discretize the internal flow field region, select pressure inlet and pressure outlet boundary conditions, and simulate and analyze the fluid flow in the radiator based on fluid dynamics theory calculations. The orthogonal experimental method was used to analyze various performance indicators during the experimental process. The results showed that the comprehensive performance of the No. 3 optimized cooling fan was the best, with a 10.45% increase in static pressure efficiency, a 14.03% increase in dynamic pressure efficiency, and an 11.26% increase in maximum flow velocity. The optimized fan flow field streamline became more full and uniform, significantly improving the fluid dynamics performance of the air in the cooling system.

Effect of impeller structure on aerodynamic performance and noise reduction of a small multi-blade centrifugal fan

In this study, the numerical simulation and orthogonal test are combined to study the effects of blade outlet angle, blade inlet angle, and blade number on the centrifugal fan flow characteristics, stress distribution, and noise control. The experimental tests were carried out to confirm the accuracy of the computational model. The computational fluid dynamics (CFD) technology was used to establish 16 groups of impeller models and derive the optimal parameter combination form. The results show that the blade inlet angle and outlet angle have more significant effects on efficiency and flow rate of fans. Among different optimize structures, the A8 fan with an inlet angle of 60°, an outlet angle of 180°, and a blade number of 45, has the best aerodynamic performance, with a 17.4% increase in flow rate, a 3.3% increase in efficiency, a 7.6% reduction in noise and a 23.0% reduction in impeller stress, compared to the original fan. Reasonable adjustment of the inlet and outlet angles greatly improves fluid distribution and suppresses flow separation. Furthermore, the noise can be reduced by lessening the disturbance between the volute tongue and wake flow, so the flow state gets greatly improved at the volute tongue region.

Occupancy sensor-enabled demand control ventilation using virtual outdoor air flow meters in air handling units without an outdoor air flow meter

Nowadays, the outdoor air (OA) flow requirement for demand control ventilation (DCV) can be accurately determined using occupancy sensors in commercial buildings. However, in air handling units (AHUs) without precise OA flow measurements, the OA is often controlled by a fixed-damper position method, which cannot maintain the required OA flow rate. The objective of this study is to develop and evaluate occupancy sensor-enabled DCV OA control using a virtual OA flow meter in AHUs. Firstly, a virtual OA flow meter is derived from a virtual supply fan flow meter by applying the correlation between the OA ratio and damper position under appropriate building pressure control. Subsequently, experiments are conducted to calibrate the virtual OA flow meter in an AHU with the design OA flow rate of 120 L/s. Finally, occupancy sensor-enabled DCV OA control is implemented by using the calibrated virtual OA flow meter. The evaluation results show that the proposed DCV OA control can match the actual OA flow rate well with the required OA flow rate. The root mean squared error (RMSE) falls within the range of 16–20 L/s, demonstrating an improvement over the fixed-damper position control, which yields an RMSE of 36 L/s.

Resilience of Personalized Ventilation in Maintaining Acceptable Breathable Air Quality When Combined with Mixing Ventilation Subject to External Shocks

This work evaluates the ventilation resilience of the combined personalized ventilation (PV)-mixing ventilation (MV) system when implemented in a typical office space. This resilience is first evaluated by monitoring the ability of the PV devices when designed at different supply flow rates to maintain acceptable levels of CO2 at the occupant’s breathing zone when the MV system is subjected to a shock. The shock considers a malfunction of the MV system for periods of 3 h and 6 h, and at shutoff percentages of MV fan flow of 100% and 50%. This is followed by evaluating the resilience of the MV system when the PV air handling unit is shutoff for short periods. The following three aspects of resilience were calculated: the absorptivity, the recovery, and the resilience effectiveness. To monitor the CO2 temporal variation at the breathing zone, a computational fluid dynamic model was developed and validated experimentally. It was found that the resilience effectiveness varied between 0.61 (100% MV shutoff for 6 h and PV at 4 L/s) and 1 (50% MV shutoff for 3 h and PV at 13 L/s). Additionally, CO2 build-up and recovery took minutes during MV malfunctions and seconds during PV malfunctions.

Analysis of internal flow characteristics of squirrel-cage fan with multi-directional air intake impeller

The separate flow in the inlet region of the squirrel-cage fan, caused by airflow steering, hinders the further improvement of internal flow state and aerodynamic performance. This study proposes a multi-directional air intake design of the impeller to solve this problem. Specifically, a new parameter known as D* is introduced in this study to represent the axial air intake parameter of the impeller. Subsequently, the key parameters of the multi-directional air intake impeller (D*, entrance blade angle, exit blade angle) are optimally designed using an orthogonal experimental and CFD numerical simulation. These results are further validated through experimentation, demonstrating that a well-designed multi-directional air intake impeller can increase the fan's air volume, total pressure, and total efficiency by 27.3%, 12.6%, and 5.2%, respectively. Furthermore, it has been observed that the inclusion of an axial air intake in the impeller diminishes the impact of the entrance blade angle on fan efficiency. When evaluating different-sized models, the influence of the axial dimensionless parameter D* on the fan total efficiency shows a consistent pattern, which is worth noting that the efficiency does not necessarily increase with increasing values of D*, but rather the fan efficiency peaks when D* is approximately 0.5. The analysis of the flow field and entropy generation inside the fan reveal that the design can weaken the separation vortex at the inlet and shift the position of vortex core towards the impeller's leading edge, so as to eclectically enhance the uniformity of the impeller inlet and outlet, ultimately optimizing the aerodynamic characteristics within the fan. This study presents a novel approach to controlling the inlet separation flow of a squirrel-cage fan and provides a reference to elucidate the mechanism of internal flow evolution with the multi-directional air intake impeller.

Effects of pressure waves on the cooling-fan performance and indoor flow of air-conditioning units on the roof of a high-speed maglev train

Air-conditioning units (ACUs) are important components of trains, and their performance directly affects the thermal environment of train cabins. Their operational performance is affected by the ventilation capacity of its core component, the cooling fan. The compressible unsteady Reynolds-averaged Navier-Stockes (URANS) and the shear-stress transport (SST) k-ω turbulence model were used to simulate the flow field and pressure waves of high-speed maglev trains, both in open air and tunnels with different lengths, at 600 km/h. We validated the numerical method and settings using a scaled model test. The effects of pressure waves on the fan flow of the ACUs and the flow field around them on the roof of the train were analyzed. The results demonstrated reduced cooling-fan flow due to the high-speed flow generated by the train operation, with the flow of the downstream fan affected by the indoor flow from the upstream fan in the same ACU. The pressure waves from trains passing each other in the open air can cause significant fluctuations in the cooling-fan flow and are positively correlated with the pressure wave amplitude. Moreover, the cooling-fan flow of the ACUs was significantly affected by the tunnel pressure wave, particularly that coming from two trains passing each other in the tunnel. The impact of airflow caused by trains exiting the tunnel on the cooling-fan flow was significantly greater than that entering the tunnel, and the amplitude of the sudden change in the cooling-fan flow was closely related to the gradient of the tunnel pressure wave. Some Findings would help improve the cooling-fan performance and the internal construction design of ACUs.

Aerodynamic noise characteristics of a centrifugal fan in high-altitude environments.

In high-altitude areas, the air is thin and the atmospheric pressure is low, which can affect the performance of centrifugal fans and aerodynamic noise. In this paper, steady and unsteady simulations of a centrifugal fan flow field are performed at altitudes of 0, 1000, 2000, 3000, 4000, and 5000 m, and the Ffowcs Williams-Hawkings equation is used to predict the aerodynamic noise of the fan. The results indicate that the tonal and broadband noise generated by the fan decrease with increasing altitude, and the A-weighted sound pressure level of each frequency band of the fan decreases when the air volume is held fixed. The maximum sound power level Lwmax, sound pressure pulsation interval, and total noise sound pressure level Lp decrease linearly with increasing altitude. For every 1000 m increase in altitude, Lwmax and Lp decrease by 0.45 dB and 1.05 dB respectively. The fan noise characteristics, performance parameters, and human auditory perception are the main factors that affect the establishment of fan noise standards in high-altitude areas.

Chair Fans as Energy Efficient Strategy to Aid Localized Ventilation

Nowadays, the air conditioning industry is deviating from the conventional mixing ventilation technique towards localized ventilation. Displacement ventilation and ceiling personalized ventilation are localized systems presenting a high potential in insuring good indoor air quality while reducing the energy consumption compared to mixing ventilation. In this work it is proposed to enhance their performance by assisting them with chair fans controlling the behavior of occupants’ convective plumes. Computational fluid dynamics models were developed to simulate office spaces ventilated by displacement ventilation and ceiling personalized ventilation equipped with chair fans. The Lagrangian technique was adopted to track particle trajectories to determine particle behavior after generation from occupant respiratory activity. A parametric study was conducted to assess the effect of chair fan flow rate on each system performance in terms of indoor air quality. Recommendations were given to reduce cross-infection between occupants for both types of localized systems studied with reduced energy consumption. The total chair fans flow rate was optimized for both cases to insure acceptable indoor air quality resulting in significant energy savings. It was found that the optimal total chair fans flow rate per occupant was approximately 14 L/s when assisting displacement ventilation while it was 10 L/s when aiding ceiling personalized ventilation.

Design of the external forced air cooling control strategy for the bulb tubular turbine generator based on multi-objective optimization

The compact internal structure of the bulb tubular turbine generator (BTTG) leads to poor ventilation and heat dissipation capability (VHDC). It will result in high-temperature faults and lead to losses for the hydro-power station, if the staff at the hydropower station fail to promptly regulate the external forced air cooling for effective heat dissipation. Thus, a specific and effective ventilation control strategy (VCS) is of utmost importance for the operation of the BTTG. In this study, the finite element method (FEM) was employed to solve the magnetic-flow-thermal coupled field of a 24-MW BTTG. The distribution characteristics of electromagnetic losses and their impact on the temperature field were revealed. Experimental validation was conducted to verify the effectiveness of the numerical results. The response surface methodology (RSM) was employed to design an experimental plan by building upon this foundation. Integrated with a multiobjective genetic algorithm (MOGA), an optimal relationship between ambient temperature, fan flow rate, and heating components’ temperature was established. This relationship serves as the optimized ventilation control strategy for the BTTG. This research provides a theoretical framework for the formulation of operational guidelines to ensure the safe operation of generators.

Study of temperature distribution and smoke movement by mobile ventilation during emergency rescue in tunnel fires

Mobile ventilation unit (MVU), as a convenient and maneuverable ventilation method, can provide forced airflow for emergency ventilation to reduce tunnel fire risk. Compared with the conventional natural ventilation and mechanical ventilation, this mobile ventilation method plays a crucial role in the current tunnel fire emergency rescue. To explore the effects of MVU on the temperature or smoke spread in tunnel fires, Computational Fluid Dynamics simulation, validated by the previous experimental data, is applied in the current study. A series of tests are conducted with fire heat release rate of 1–30 MW and MVU fan flow being up to 3 × 105 m3/h. The results show the maximum tunnel ceiling temperature presents exponential attenuation with increasing fan flow and its relational expression with fan flow and fire size is proposed. Furthermore, the temperature decay distribution between upstream and downstream area is obviously different and a new coefficient γ is imported to represent the difference by connecting the fan flow and fire size. Moreover, the smoke backflow length theoretical equation in the upstream area is obtained, which can provide effective guidance to tunnel fire emergency rescue with MVU.

The multi-objective optimisation design of outlet guide vanes of diagonal flow fan based on sobol sensitivity analysis

Diagonal flow fans offer substantial energy-saving potential and find broad application across various sectors. Their performance relies heavily on factors like outlet guide vanes and spacing relative to moving blades. However, research into enhancing fan performance through optimized guide vanes and spacing remains limited. In this study, we focus on improving the accuracy of predicting the internal flow field of diagonal flow fans. This paper incorporate rotation and curvature effects using the Large Eddy Simulation (LES) model and introduce stress terms with helicity constraints to create a non-linear subgrid-scale model. This refined model enables more precise numerical simulations. By employing accurate simulations, we optimize the outlet guide vane configuration and conduct sensitivity analysis. We utilize a Radial Basis Function (RBF) model coupled with the Sobol method for this purpose. The optimized guide vane design exhibits enhanced resistance to airflow separation compared to the original, resulting in notable reductions in flow losses within the grille channel. Experimental tests are performed on the diagonal flow fan both before and after optimization. At the specified operating point, the second guide vane optimization leads to a 1.28 m3/min increase in fan flow, a 4.33% rise in total pressure efficiency, and a 2.2 dB noise reduction. These findings underscore the accuracy of the helicity correction model in predicting diagonal flow fan behavior. The multi-objective optimization approach, combining the RBF proxy model with the Sobol method, proves highly reliable. It offers valuable design insights for similar fans and establishes a credible design methodology.

Analysis on influence of self-fan cooling motor on fan aerodynamics

With the development of multi electric/all electric aircraft, the degree of aircraft secondary energy electrification is becoming higher and higher. As the core component of the electrical system, the motor presents the trend of small volume and high-power density. The heat dissipation problem of compact motor is becoming more and more serious. At present, the self-driven fan cooling system is widely used in aircraft motors. In this paper, the axial fan cooling system of an aero generator is taken as the research object, and the changes of fan flow field and aerodynamic characteristics in the fan motor coupling model of the isolated fan and the installed motor are analyzed by using three-dimensional computational fluid simulation. At the same time, the influence of the change of the axial distance between the fan and the motor on the aerodynamic performance is also studied. The analysis of the simulation results shows that the close coupling between the motor and the fan components greatly affects the flow structure in the fan, resulting in the distortion potential effect on the fan outlet section. The fan flow field appears the blade root reflux, the channel vortex in the blade and the expansion of the corner separation area. With the increase of the axial distance between the motor and the fan, the influence of the motor on the flow in the fan gradually approaches the flow blocking effect.

Effect of Airborne Pathogen Transmission Released by an Assailant in A Mosque Using CFD Simulation

This study investigated the aerosol particle spreading characteristic under transient state at different location released by an assailantinside a mosque. Particles deposited at receivers were used to determine the virus reproductive number (Ro) over time. The spreading during coughing process wasvalidated with previous literature review using Computational Fluid Dynamics (CFD) simulation study. Mesh sensitivity study was done on the model to get better accuracy results and optimum computational load. Themodel involved internal space of the mosque and 160 prayers during the congregation prayers. It was discovered that, the particle spreading characteristics wasfound to be influenced mostly by the velocity distribution and velocity vector inside the mosque. This is due to force flow generated by fan and air conditionerair flow. Particles size less than 10μm werethe most deposited on the wall and ceiling. The particles greater than 30μm deposited on the ground andthe prayers body. The location of assailant at the center wasfound to cause the most infection among the prayers which was52% of the total prayer with the Roof 0.83. The assailant at top right and bottom right produced high Roof 0.73 and 0.6 while top left produced the lowest which was0.32. The existence of partition wasfound to reduce the particle spreading from the assailant at bottom left.

Flow Field Simulations Over Reentry Modules at High Speed

Reentry capsule configurations significantly differ from each other due to entry conditions and mission requirements. This paper describes numerical simulations of the viscous flow over the Beagle and the OREX (Orbital Reentry EXperiments) configurations for freestream Mach numbers in the range of 1.2 - 5.0. The flow fields over the reentry modules are obtained by solving time-dependent axisymmetric compressible laminar Navier-Stokes equations. The fluid mechanics equations are discretized in spatial coordinates employing a finite volume method, which reduces the governing equations to semi-discretized ordinary differential equations. Temporal integration is carried out using a two-stage Runge-Kutta time-stepping scheme. A local time-stepping is employed to get the steady state solution. The numerical simulation is performed on a single-block structured grid. The flow field features around the reentry capsules such as bow shock wave, sonic line, expansion fan and recirculation flow in the base-shell region are well captured by the present numerical computation. The effects of the geometrical parameters of the module, such as aspect ratio, frontal segment bluntness, fore body cone angle and shoulder rounding radius on the wake throat width, distance from the wake throat to the base of the model, flow departure angle and aerodynamic drag are analyzed using the numerically simulated flow field.

Numerical Simulation of Flow Field Over a Sharp-Tipped Double Cone at High Speed

The flowfield over a sharp-tipped double cone axisymmetric configuration is carried out by solving time dependent axisymmetric laminar compressible Navier-Stokes equations for freestream Mach number range of 2.0 - 6.0. The fluid dynamics equations are discretized in spatial coordinates using integral formulation in conjunction with finite volume method which reduces the governing equations to semi-discretized ordinary differential equations. Temporal integration is performed employing multistage Runge-Kutta time-stepping scheme. A local time step is used to achieve steady-state solution. The numerical computation is carried out on a mono-block with structured grids. The flowfield features over the sharp-tipped double cone configuration such as conical shock wave, separation region, separation and reattachment shock wave and bow shock wave in the cone region, expansion fan, and recirculation flow in the base region are well captured at Mach number 3 and 6 which are identical to the Edney VI type shock interaction. The velocity vector, Mach contours plots and variations of surface pressure coefficient along the sharp-tipped double cone configuration are analyzed at various Mach numbers. The fore-body aerodynamic drag is calculated employing computed pressure distribution. The paper presents the influence of the freestream Mach number on the flows with shock interactions over the sharp-tipped double cone geometry.

Effect of tip winglet position on tip flow and noise of axial flow fan

In order to investigate the impact of blade tip winglet position on the internal flow and noise characteristics of axial flow fans,three different blade tip winglets,namely the TSW blade, the SSW blade, and the PSW blade,were hereby proposed. The accuracy of the simulation model was verified through experiments, and the circumferential internal flow characteristics and noise characteristics of these different blades were studied by numerical simulation using the blade tip partition method. The calculation results indicated the high-level efficiency of TSW, SSW and PSW blades in reducing the blade tip leakage flow in Z1 and Z2 regions and suppressing its circumferential propagation in the Z3 region. Besides, it was found that SSW blade and PSW blade had larger total pressure loss relative to Ori blade, and that TSW had higher total pressure difference at low blade height. Additionally TSW blades performed the best in improving the blade tip blockage,while TSW, SSW and PSW blades effectively reduced the leakage flow against the main stream, and blade tip winglet blades could effectively improve the fan discrete noise, SSW had the most significant noise reduction effect in the total sound pressure level, and in the overall blade,Z1, Z2 and Z3 were reduced by 2.16 dB,1.68 dB,2.24 dB and 3.74 dB.

Hierarchical Turbulence and Geometry Modeling: Case Study for Fan Stage Flows

Flow separations occur in the tip region when the fan is operating at the approach condition. To reduce computational cost without degrading accuracy, the hierarchical modeling approach has been developed to predict such fan flow. This allows both flow and geometry to be treated with various fidelity levels in different zones. To be specific, this is achieved by zonalizing large-eddy simulation (LES) in the fan tip region and modeling downstream stators with low-order blade body forces. This approach provides an accurate and economical prediction of separated fan flows in the stage environment. The predicted fan wake profiles show a reasonably good agreement with the hot-wire measurements considering passage-to-passage variations. Finally, the simulation data are used to assess the isotropic turbulence hypothesis that is made on fan wake in low-order acoustics models.