Air Crossflow Research Articles

Effect of cross airflow on the flame geometrical characteristics and flame radiation fraction of ethylene jet fires with carbon dioxide addition

Achieving carbon peaking and carbon neutrality is crucial to accelerating the construction of ecological civilization and promoting high-quality national development. Carbon dioxide as an additive in industrial combustion can effectively reduce soot production. The main background of the paper is derived from industrial flares, which are devices that convert unrecoverable combustible gasses produced in industrial production into environmentally friendly combustion products through combustion. The paper presents the effect of cross airflow on the flame characteristics and flame radiation fraction of ethylene jet fires with carbon dioxide addition. The experiments were carried out on the test bench at one end of a wind tunnel facility with size of 22 m (length) × 1.2 m (width) × 0.8 m (height). The flame geometrical features (i.e. flame length and lift-off height) and flame radiation fraction of ethylene jet fires with carbon dioxide addition under cross airflow were analyzed. It was found that the flame lift-off height of ethylene gas and carbon dioxide is a linear function with the fuel ejection velocity, which is consistent with the pure fuel phenomenon studied by previous researchers. For the same heat release rate, the flame radiation fraction decreases with the increase of the volume flow of CO2 added to ethylene. Moreover, for the given test conditions, the flame radiation fraction of ethylene jet fires with carbon dioxide addition decreases with the increase of cross airflow speeds. A model for the dimensionless flame length of ethylene jet fires with carbon dioxide addition was obtained. Finally, a new global model is also proposed to characterize the flame radiation fraction of ethylene jet fires with carbon dioxide addition by accounting for the effect of cross airflows.

Probable cross-corridor transmission of SARS-CoV-2 due to cross airflows and its control

A COVID-19 outbreak occurred in May 2020 in a public housing building in Hong Kong – Luk Chuen House, located in Lek Yuen Estate. The horizontal cluster linked to the index case’ flat (flat 812) remains to be explained. Computational fluid dynamics simulations were conducted to obtain the wind-pressure coefficients of each external opening on the eighth floor of the building. The data were then used in a multi-zone airflow model to estimate the airflow rate and aerosol concentration in the flats and corridors on that floor. Apart from flat 812 and corridors, the virus-laden aerosol concentrations in flats 811, 813, 815, 817 and 819 (opposite to flat 812, across the corridor) were the highest on the eighth floor. When the doors of flats 813 and 817 were opened by 20%, the hourly-averaged aerosol concentrations in these two flats were at least four times as high as those in flats 811, 815 and 819 during the index case's home hours or the suspected exposure period of secondary cases. Thus, the flats across the corridor that were immediately downstream from flat 812 were at the highest exposure risk under a prevailing easterly wind, especially when their doors or windows that connected to the corridor were open. Given that the floorplan and dimension of Luk Chuen House are similar to those of many hotels, our findings provide a probable explanation for COVID-19 outbreaks in quarantine hotels. Positive pressure and sufficient ventilation in the corridor would help to minimise such cross-corridor infections.

A novel active volumetric rotating disks solar receiver for concentrated solar power generation

• The complete analysis is composed by a CFD model and its experimental validation. • The experimental results fits with CFD model results. • Thermal performance shows promising results compared to the state of the art. • Next steps in the receiver development will be its upscaling. A new active, first of its kind, volumetric receiver prototype is used in this study to obtain the conclusions about its thermal performance. The rotating disks cooling, was studied first, numerically, using a CFD software and second, experimentally, using a laboratory-scale prototype. This study evaluates the convective heat transfer from rotating disks to a transverse air crossflow. The thermal energy exchange on the disk surfaces allows us to calculate the heat transfer coefficient from disk surface temperature and the thermal performance of the receiver prototype and local Nusselt number through the receiver depth are also a key point in the research. The influence of rotational speed and air mass flow on the heat transfer coefficient is also studied. Differences between numerical and experimental results show high concordance, with a relative error below 15% in the worst point. The thermal performance shown by the lab-scale receiver is promising for the further development of the active Open Volumetric Air Receivers (OVAR) technology. The obtained thermal efficiency results show an improvement of around 10% at 900 °C when compared to the current state of the art of volumetric receivers.

Design Optimization of a Cross-Flow Air Turbine for an Oscillating Water Column Wave Energy Converter

A cross-flow air turbine, which is a self-rectifying, air-driven turbine, was designed and proposed for the power take-off (PTO) system of an oscillating water column (OWC) wave energy converter (WEC). To predict the complicated non-linear behavior of the air turbine in the OWC, numerical and experimental investigations were accomplished. The geometries of the nozzle and the rotor of the turbine were optimized under steady-flow conditions, and the performance analysis of the model in bi-directional flows was conducted by commercial computational fluid dynamics (CFD) code ANSYS CFX. Experimentation on the full-scale turbine was then undertaken in a cylindrical-type wave simulator that generated reciprocating air flows, to validate the numerical model. The optimized model had a peak cycle-averaged efficiency of 0.611, which is 1.7% larger than that of the reference model, and a significantly improved band width with an increase in flow coefficients. Under reciprocating-flow conditions, the optimized model had a more improved operating range with high efficiency compared to the performance derived from the steady-flow analysis, but the peak cycle-averaged efficiency was decreased by 4.3%. The numerical model was well matched to the experimental results with an averaged difference of 3.5%. The proposed optimal design having structural simplicity with high performance can be a good option to efficiently generate electricity.

Cross-flow-induced transverse–torsional vibrations of slender structures mitigation via coupled controllers

This study aims to propose a coupled controller of tuned mass damping (TMD) and nonlinear energy sink (NES) for enhancing the stability and suppressing nonlinear vibrations of slender structures, such as tall buildings subjected to cross airflows. The nonlinear governing equations are established to describe the coupled vibrations of the slender structure with the controller. Subsequently, a linear analysis is performed to indicate the positive effect of proposed controllers on increasing the onset speed of galloping for the slender structure. A nonlinear analysis is conducted to state the mechanism of controllers on reducing oscillations of the slender structure. The results show that the coupled controller has better effects on whether enhancing the onset speed or oppressing the structure’s vibration amplitudes. A parametric analysis is then carried out to design an optimal coupled controller. The distributed controllers are designed in the structure, indicating that there is an optimal number for the controller where the onset speed is peak depending on the mass ratio. The present study can provide beneficial guidance and design for controlling the cross-flow-induced vibrations of slender structures in engineering applications.

The Effect of Weber Number on Spray and Vaporization Characteristics of Liquid Jets Injected in Air Crossflow

AbstractThis paper details a numerical investigation conducted to systematically evaluate the effects of aerodynamic Weber number, in the range from 68 to 136, on spray characteristics and gaseous fluid dynamics when liquid jets are injected in high-temperature air crossflow. The momentum flux ratio and air temperature for all the cases studied in this research effort are 9 and 573 K, respectively. The computations are conducted using an Eulerian–Lagrangian framework, where the gas phase is modeled by the compressible form of the Navier–Stokes equations and the liquid phase is treated in the Lagrangian frame with appropriate models to account for jet injection and breakup phenomena. A modified version of two-way coupling, which takes into account the finite size of the dispersed phase, is used to account for the exchange of mass, momentum, energy and species between the two phases. Turbulence closure is achieved using the large eddy simulation technique. As a first step, the framework is validated against measurements for non-vaporizing and vaporizing conditions—our results agree well with experimental data. Next, three computations in the range of Weber numbers mentioned above are conducted—the effect of Weber number is quantified in terms of the spatiotemporal evolution of the mass fraction of the vaporized liquid, detailed distributions of droplet sizes, their velocities, and volumetric fluxes. It is found that with an increase in the Weber number, the droplet sizes and the penetration depth monotonically decreased. As a result, at higher Weber number conditions, the vaporized liquid in the domain increases due to the overall enhancement in the effective surface area of the liquid phase.

Effects of cross airflow and burner distance on temperature profile and flame morphology of dual tandem pool fires

Understanding multiple fire sources interactions in cross airflow that frequently occur in wildland fire or industrial process accident scenario is helpful for establishing guidelines for fire protection engineering. This paper experimentally studies the thermal characteristic and flame behavior of dual tandem pool fires with equal- and unequal- heat release rates in varying separation distances (S) under cross airflow (u). Two square diffusion burners were used employing propane as fuel with varied heat release rates (HRR) at six separation distances, with a total of 216 experimental conditions were considered comprehensively. Flame merging condition, IR-measured flame temperature profile, flame height and flame horizontal projection length were quantified and analyzed. Results show that the flames tend to merge more easily in cross airflow when the HRR of the upstream flame is larger. The maximum temperature in the flame region increases with the increasing cross airflow velocity for both merging and non-merging flames, and the downstream pool fire's maximum temperature is lower than that of the upstream pool fire in the cases of non-merging. For merging cases, flame horizontal projection length decreases as S/D increases, while flame height changes little for varying S. For non-merging cases, flame height of downstream pool fire has an increasing trend with the increasing separation distance, while flame horizontal projection length declines in reverse due to the flame blockage effect. Three combustion states of dual fires are identified according to their force interactions: State I-pressure thrust dominated, State II-pressure thrust competes with cross airflow inertial force, State III-cross airflow inertial force dominated. Finally, a scaling analysis is performed to characterize the flame height and horizontal flame projection length based upon the competition mechanisms between total buoyant flow flux and cross airflow momentum flux for merging flame and non-merging flame in state III. This study provides experimental data and a scaling analysis into the fundamental physics of equal and unequal multiple pool fires interactions under cross airflow.

The Effect of Holes Number in Cylindrical Samples on the Forced Convection Heat Flow

In this article, an experimental examine changed into carried out on the effect of putting side holes in circular cylinders on the forced convection heat transfer process. Three circular cylindrical samples were made of aluminum. Sample No.1 without side holes, sample No.2 with two side holes, and sample No.3 with four side holes. These cylindrical samples were placed after being heated in an air duct (working section) and the position of samples in an air duct are same. Air cross-flow was applied to the cylindrical samples at different velocities while observing their temperature drop. The results showed that heat transfer of the cylindrical sample with four side holes is better than that of other samples at all air flow velocities. Also Nusselt number was calculated after obtained heat transfer coefficient and at Reynolds number range (7435-20697), where it become discovered that Nusselt number will increase with the Reynolds number in all cylindrical samples. Empirical relationships for Nusselt number with Reynolds number and Prandtl number had been get it for the three circular cylinders and liken with relationships of circular cylindrical forms exist in other references. These Empirical relationships given acceptable constants values.

Effect of Cross Airflow on the Flame Geometrical Characteristics and Flame Radiation Fraction of Ethylene Jet Fires with Carbon Dioxide Addition

Effect of Cross Airflow on the Flame Geometrical Characteristics and Flame Radiation Fraction of Ethylene Jet Fires with Carbon Dioxide Addition

Experimental Study for Counter to Cross Flow Air Cooled Heat Exchanger in Concentric Tube using Rectangular Copper Fins Spacing with Internal Spiral Grooving

In this manuscript we have presented eight variation of Air-Cooled Heat Exchanger (ACHE) design with internal spiral grooving, all of them are having variable number of rectangular copper fins with different distances between the fins. In the proposed design we get the value of heat transfer rate of a counter to cross flow ACHE is 7833.77 watt, 4068.13 watt, 2736.95 watt, 2161.49 watt, 1802.89 watt, 1546.44 watt, 1336.51 watt and 1165.74 watt in natural convection (without fan) for 0.5 cm, 1.0 cm, 1.5 cm, 2.0 cm, 2.5 cm, 3.0 cm, 3.5 cm and 4.0 cm respectively. Then again, value of rate of heat transfer in forced convection (with fan) are 8007.46 watt, 4084.81 watt, 2754.69 watt, 2205.98 watt, 1809.24 watt, 1555.39 watt, 1352.88 watt and 1172.78 watt for 0.5 cm, 1.0 cm, 1.5cm, 2.0 cm, 2.5 cm, 3.0 cm, 3.5 cm and 4.0 cm respectively.

Survey of air exchange rates and evaluation of airborne infection risk of COVID-19 on commuter trains.

To identify potential countermeasures for coronavirus disease (COVID-19), we determined the air exchange rates in stationary and moving train cars under various conditions in July, August, and December 2020 in Japan. When the doors were closed, the air exchange rates in both stationary and moving trains increased with increasing area of window-opening (0.23-0.78/h at 0m2, windows closed to 2.1-10/h at 2.86m2, fully open). The air exchange rates were one order of magnitude higher when doors were open than when closed. With doors closed, the air exchange rates were higher when the centralized air conditioning (AC) and crossflow fan systems (fan) were on than when off. The air exchange rates in moving trains increased as train speed increased, from 10/h at 20km/h to 42/h at 57km/h. Air exchange rates did not differ significantly between empty cars and those filled with 230 mannequins representing commuters. The air exchange rates were lower during aboveground operation than during underground. Assuming that 30-300 passengers travel in a train car for 7-60min and that the community infection rate is 0.0050-0.30%, we estimated that commuters' infection risk on trains was reduced by 91-94% when all 12 windows were opened (to a height of 10cm) and the AC/fan was on compared with that when windows were closed and the AC/fan was off.

Experimental and numerical investigation of evaporating fuel films in combustion

Fuel films evaporating throughout combustion can cause soot formation in spark-ignited direct-injection (DI) engines. In this work, laser-induced fluorescence (LIF), combustion imaging, 3D computational fluid dynamics (CFD), and a low-dimensional model (LDM) were used to investigate fuel-film evaporation, combustion, and soot formation in an atmospheric-pressure constant-flow facility that serves as a DI model experiment. A six-hole injector laterally injects isooctane doped with toluene as a fluorescent tracer. Most of the fuel evaporates into the preheated air crossflow, but some impinges on a quartz window on the opposite side, forming fuel films. The fuel/air-mixture can be ignited by spark electrodes protruding into the chamber. LIF was used to image the fuel-film thickness at different times after start of injection (aSOI). From that, the film mass and evaporation rate are calculated. The large eddy simulation (LES) CFD employs a Lagrangian treatment for the dispersed phase in combination with models for spray/wall interaction and the wall film. CFD and LIF consistently find that evaporation rates are highest for early times aSOI and remain constant from about 10 ms aSOI. At ambient temperature the evaporation rates from LIF and CFD are almost the same while at elevated temperatures the CFD predicts about a two times higher evaporation rate than measured. LIF, CFD, and LDM reveal a strong dependence of the evaporation rate on the wall temperature, while there is very little influence of combustion and thus convective heat transfer. The simulations show that the fuel-film temperature rapidly reaches the wall temperature that remains approximately constant throughout evaporation. High-speed combustion imaging and CFD show the inception of soot pockets at similar times, close to the evaporating fuel films, and consistent in spatial extent.

Effect of pulsed electric field treatment on the kinetics of rehydration, textural properties, and the extraction of intracellular compounds of dried chickpeas.

This study investigated the effect of pulsed electric fields (PEF) pretreatment on rehydration kinetics, firmness, and release of intracellular components of dried chickpeas during rehydration at 35 to 65°C. After soaking preconditioning, chickpeas were subjected to PEF treatments (2.5 and 3.3kV/cm, 0.2 to 12.0kJ/kg, 15 µs pulse width, 20Hz frequency). PEF treated and untreated chickpeas were dried in crossflow air dryer and their rehydration at constant seed/water ratio of 1:5 was studied for 24 hr. During rehydration, moisture, firmness, and concentration of released proteins, carbohydrates and raffinose family oligosaccharides (RFO) were determined and described using appropriate mathematical models. PEF treatment led to up to 70% higher rehydration rates of dried chickpeas. This increase corresponds to rehydration time of approximately 1.5 hr, as opposed to 5 hr for untreated samples. Firmness of PEF treated chickpeas (for energy inputs higher than 3kJ/kg) during rehydration decreased up to 30% compared to untreated samples. The firmness of untreated samples after 300min of rehydration was achieved at much shorter times (up to 30min) for PEF treated samples. At the end of 300min of rehydration, more than 47.7%, 76.1%, and 86.6% of total raffinose, stachyose, and verbascose, respectively has been extracted, but only 0.03% of nutritionally valuable proteins from PEF treated chickpeas. Consequently, this study demonstrates that PEF processing could be implemented in dried chickpeas processing as pretreatment, for the reduction of rehydration time prior to cooking and of intestinal discomfort caused by RFO.

Thermal performance augmentation of a semi-circular cylinder in crossflow using longitudinal fins

In the present work, the thermal/hydraulic characteristics of a semi-circular cylinder in crossflow of air have been numerically investigated at Reynolds number range between 103 and 104. Various attack angles between −90 and 90° were considered. The effect of finning was investigated at zero attack angle using three fin configurations (front fin, rear fin, and double fin) with different values of fin length and thickness. Flow patterns, local heat transfer coefficient, overall-averaged Nusselt number, and drag coefficient were reported and discussed. Correlations for Nusselt number and relative heat transfer rate were presented for plain and finned semi-circular cylinders, respectively. The irreversibility minimization approach was used to assess the performance of the semi-circular cylinder with/without fins compared to a plain circular cylinder with the same surface area, surface temperature, and heat transfer load. The semi-circular cylinder delivers the highest performance around an attack angle of 15 ° , while the worst performance occurs when the flat surface faces the flow. Additionally, finning significantly improves the performance by reducing the irreversibility. The front-fin and rear-fin configurations delivered relatively similar performances, while the best performance was attained using the double-fin configuration.

Bubble characteristics from injected air sheet through slots in a water cross-flow

While air injection has already been widely studied for stagnant water at low air injection rates for circular injectors, limited data is available on gas in liquid cross-flows injected through slots at high air flow rates. The key purpose of this work is to observe the evolution of bubble swarm characteristics for simple geometries such that, by correlation rules, these characteristics can be projected for more complicated geometries and environment. To that effect, an experimental water loop with both controllable water and air flow rates is used to generate bubble clouds with cross-flow air injection.Measurements with an optical probe allow the void fraction, the size and velocity of bubbles to be measured. The findings show that the general size of bubbles increases with an increase in the air flow rate for a constant water flow rate. Furthermore, bubbles pick up speed while moving away from the injection slot. Bigger bubbles travel faster than small ones ; they also tend to rise to the top of bubble plumes while small bubbles stay in the lower portion. Finally, the maps of void fraction indicate that its local values drop while heading away from the injection slot.

The aspect ratio effect on the performance of a cam-shaped cylinder in crossflow of air

ABSTRACT Heat transfer and flow over a cam-shaped tube with various aspect ratios (AR) have been investigated experimentally at two angles of attack, α = 0° and 180°, and 25000 ≤ Re≤ 55000 based on the equivalent diameter. The aspect ratios of the tubes are AR = 1.35, 1.7, 2.07, 2.51, and 3.74. The results show that with the increase in aspect ratio, the drag coefficient decreases. The AR = 3.74 has the lowest drag coefficient in both angles of attack, which is about 82–83% lower than the circular tube. Also, in terms of heat transfer, for tubes with AR = 1.35, 1.7, 2.51, and 3.74 the average Nusselt number at α = 0° is lower than that of the circular tube. At α = 180°, however, the mean Nusselt number for the tube with AR = 1.35 is 11% higher than that of the circular tube. As for the performance, the tube with AR = 2.51 at α = 0° works 4.83 times better than the circular tube with the equivalent diameter.

Heat transfer enhancement through a rectangular channel by DBD plasma actuators as vortex generators

This paper represents a three-dimensional numerical simulation to figure out the application of the DBD plasma actuators as an active vortex generator system to enhance heat transfer through a smooth rectangular channel. This system as an active flow control device consists of a pair of DBD plasma actuators, which have been implemented asymmetrically on the bottom wall by several angles of attack to induce the cross airflow. The investigation deals with the optimum angle of attack according to different Reynolds number (Re) ranging from 2300 to 3800 and alternating current voltages of 31–37 kV. The heat transfer rate, pressure drop, and overall performance have been assessed. The results show the optimum choice for the angles of attack was 30° among various flow conditions. By this design, at the voltage of 37 kV and Re of 2300, the average Nusselt number (Num) increased about 56% and heat transfer performance factor ( $$\eta$$ ) increased about 111%. Furthermore, at the lowest applied voltage (31 kV) and highest Reynolds number (3800), the value of Num rises to 41% and 59%, respectively. Thereupon, as the next optimum choices, angle of attack of 45° at high Re and angle of attack of 15°at low Re enhanced heat transfer performance.

Homogeneous and Automated Cooling of a Mold Segment by Multiple Water Jet Impingement and a Cross Air Flow

Abstract Multiple water jets and a cross air flow are used to cool a mold segment in a homogeneous and automated manner. The test segment was cooled from an initial temperature of 573 K. An average temperature difference of less than 3 K and a maximum temperature difference of less than 6 K were obtained along the length of the surface of the test segment during the entire duration of the cooling process as opposed to the traditional channel cooling approach where the mean and maximum temperature differences increase over time. The top surface of the test segment represents the mold/part interface which is of interest in this study. Using model predictive control (MPC) and a data-driven predictive model, the cooling speed of the test segment’s top surface was able to be maintained within ±5 K of the cooling ramp imposed. The results were compared to the results obtained when using a simpler On/Off algorithm for automated cooling. Compared to the simpler On/Off algorithm, there was an improvement in the accuracy of the cooling ramp with respect to its reference value of over 30% for most cooling ramps tested (5–25 K/min). A parametric study on the influence of the flowrates of the fluids on the cooling speed of the test segment’s surface was also conducted.

Statistical characteristics of spray autoignition of transient kerosene jet in cross flow

To study statistical characteristics of the random spray autoignition, aviation kerosene was injected transiently into non-vitiated air crossflow in a flow reactor with optical accesses. The operating conditions were relevant to gas turbine combustor: the air crossflow pressure and temperature were in the range of 1.4–1.7 MPa and 830–947 K, respectively, and the jet-to-crossflow momentum flux ratios were 20, 50 and 80. Statistical distributions of random ignition delay times with adequate convergence were estimated based on histograms. The dependences of the distributions on reactor pressure, temperature, and jet-to-crossflow momentum flux ratio were studied. The results show that the resulting distributions appear more concentrated with the increase of air temperature or jet-to-crossflow momentum flux ratio. And then the correlations for the mean and standard deviation of the ignition delay time sample data were developed based on the present results. Compared with the correlations of ignition delay time of homogeneous premixed gas-phase kerosene/air mixture reported in the literature, the results show a greater significance pressure dependence and lower temperature sensitivity of the ignition delay time of non-premixed kerosene spray.

Effect of Cross-Flow Swirl on the Trajectory of Spray in an Annular Passage

Abstract This paper presents the experimental analysis of the impact of swirl number of cross-flowing air stream on liquid jet spray trajectory at a fixed air flow velocity of 42 m/s with the corresponding Mach number of 0.12. The experiments were conducted for 4 different swirl numbers (0, 0.2, 0.42, and 0.73) using swirl vanes at air inlet having angles of 0 deg, 15 deg, 30 deg, and 45 deg, respectively. Liquid to air momentum flux ratio (q) was varied from 5 to 25. High-speed (at 500 fps) images of the spray were captured, and those images were processed using matlab to obtain the path of the spray at various momentum flux ratios. The results show interesting trends for the spray trajectory in swirling air flow. High swirling flows not only lead to spray with lower radial penetration due to sharp bending and disintegration of liquid jet but also result in spray with large spread and spray area. Based on the results, correlations for the spray path have been proposed, which incorporates the effects of the swirl number of the air flow.