Forced Air Flow Research Articles

Effect of Airflow Temperature on the Formation of Initial Flame Kernel and the Propagation Characteristics of Flame

Using liquid RP-3 aviation kerosene as the fuel to study, the effect of airflow temperature on the formation of initial flame kernel during the ignition of spray combustion and on the propagation characteristics of flame was investigated. Combining high-speed camera and dynamic temperature acquisitions at the outlet of combustor, the internal triggering mode was used under a constant fuel flow rate and airflow velocity. This combined system simultaneously recorded the formation of initial flame kernel, flame propagation, and outlet temperature variation of combustor under different airflow temperatures. MATLAB software was used to obtain the reaction zones at different moments and to analyze the effects of airflow temperature on morphological characteristics such as flame area, perimeter-to-area ratio, maximum length-to-height ratio, equivalent mean length-to-height ratio, mass center, and centroid. According to the growth rate in flame area, the ignition process can be divided into three stages: formation of flame kernel, rapid development of flame, and stable development of flame. Airflow temperature not only affects the formation time of flame kernel but also affects the growth rate of flame area. During the development of flame, the movements of mass center and centroid are irregular, and their positions do not coincide with each other. However, the overall moving trends are consistent. With the increase of the airflow temperature, the position, where the flame kernel is gradually formed, moves closer to the center of the end face of spark plug. The force of airflow on flame is the main factor that increases the flame area and heat-release rate. Therefore, the folds around the flame edge mainly result from the stretching under the action of airflow. With the increase in airflow temperature, the heat release of the initial flame kernel increases, and the ratio of perimeter to area as a characterization parameter increases by 8%, 86%, and 33%, respectively. In addition, the maximum outlet temperature rise increased by about 53%, 73.5%, and 0.65%, respectively. Meanwhile, the maximum rate of temperature rise increased by about 42.8%, 57%, and 5.1%, respectively.

A phase change material with enhanced thermal conductivity and secondary heat dissipation capability by introducing a binary thermal conductive skeleton for battery thermal management

In order to enhance the thermal conductivity and secondary heat dissipation capability of the phase change material (PCM) in battery thermal management (BTM) applications, a new kind of composite PCM (CPCM) is successfully prepared by constructing a binary thermal conductive skeleton of expanded graphite (EG)/copper foam (CF). The EG with porous structure can adsorb the PCM of paraffin and act as a micro-thermal-conductive framework to transfer the heat to the adjacent CF skeleton. The CF acts as a macro-skeleton to transfer the heat throughout the CPCM plate and enhance the heat transfer coefficient of the interface between the CPCM plate and air. In consequence, the obtained CPCM-based battery pack with EG/CF (CPCM-EG/CF) delivers much better cooling and temperature-uniformed performances than those without EG/CF or CF, especially under a secondary heat dissipation system of forced air convection. For example, the CPCM-EG/CF pack shows stable and lowest maximum temperature and temperature difference of 48.0 and 3.9 °C during the cycling charge-discharge tests under forced air flow, respectively.

Experimental characterization on eco-friendly micro-grinding process of titanium alloy using air flow assisted electrospray lubrication with nanofluid

In this paper, a new eco-friendly lubrication technique, which is referred to as the nanofluid air flow assisted electrostatic lubrication (AF-ESL), has been proposed and applied to the micro-grinding process of titanium alloy. In the nanofluid AF-ESL technique, nanofluid droplets can be injected into the grinding region by being driven by an electro-hydro-dynamic force and pressurized air flow. For the experimental performance evaluation, a series of micro-grinding experiments are designed and conducted by varying a size and a weight concentration of nanodiamond particles in the nanofluid. It is demonstrated that the nanofluid AF-ESL can significantly reduce the resultant grinding forces and grinding force ratios. It is also shown that the nanofluid AF-ESL improves the surface quality of ground workpiece and grinding tool condition significantly. In particular, larger nanodiamond particles (80 nm) are more advantageous for enhancing the nanofluid AF-ESL micro-grinding performances. In addition, a weight concentration of nanodiamond particles in the nanofluid is another important factor to influence on the micro-grinding performances.

Effects of ambient pressure on smoke back-layering in subway tunnel fires

This paper investigates the effects of ambient pressure on smoke back-layering in subway tunnel fires with and without train blockage. A series of numerical simulations were conducted in a 1/4 small-scale tunnel with different heat release rates (40–160 kW), longitudinal ventilation velocities (0.2–0.8 m/s) and ambient pressures (60–100 kPa). The smoke back-layering lengths under different conditions are analyzed, and the results show that under the same heat release rate and ventilation velocity, the back-layering length increases with decreasing ambient pressure due to the weak inertial force of longitudinal airflow led by the low air density. The Li’s and Zhang’s models, which can well predict the smoke back-layering length under regular pressure, are modified for the reduced pressure. The constant increment of back-layering length between adjacent ambient pressures, which mainly depends on the heat release rate, is used to correct Li’s model. The smoke back-layering length under the low ambient pressure can be predicted by this modified model without train blockage. With train blockage considered, new models are developed by introducing both equivalent and virtual fire sources for predicting the smoke back-layering length under the low ambient pressure, which is shown to well reproduce the simulation results.

Theoretical analysis of evaporative cooling of classic heat stroke patients

Heat stroke is a serious health concern globally, which is associated with high mortality. Newer treatments must be designed to improve outcomes. The aim of this study is to evaluate the effect of variations in ambient temperature and wind speed on the rate of cooling in a simulated heat stroke subject using the dynamic model of Wissler. We assume that a 60-year-old 70-kg female suffers classic heat stroke after walking fully exposed to the sun for 4h while the ambient temperature is 40°C, relative humidity is 20%, and wind speed is 2.5m/s-1. Her esophageal and skin temperatures are 41.9 and 40.7°C at the time of collapse. Cooling is accomplished by misting with lukewarm water while exposed to forced airflow at a temperature of 20 to 40°C and a velocity of 0.5 or 1m/s-1. Skin blood flow is assumed to be either normal, one-half of normal, or twice normal. At wind speed of 0.5m/s-1 and normal skin blood flow, the air temperature decreased from 40 to 20°C, increased cooling, and reduced time required to reach to a desired temperature of 38°C. This relationship was also maintained in reduced blood flow states. Increasing wind speed to 1m/s-1 increased cooling and reduced the time to reach optimal temperature both in normal and reduced skin blood flow states. In conclusion, evaporative cooling methods provide an effective method for cooling classic heat stroke patients. The maximum heat dissipation from the simulated model of Wissler was recorded when the entire body was misted with lukewarm water and applied forced air at 1m/s at temperature of 20°C.

Flame extension lengths beneath a confined ceiling induced by fire in a channel with longitudinal air flow

This paper investigates the length of flame extension for an impinging flame underneath the confined ceiling in a channel with longitudinal air flow. Previous works in this field have been primarily concerned with un-confined ceilings and no forced air flow conditions. Under longitudinal air flow conditions, the flame extension beneath the channel ceiling is non-symmetrical, that is, different in the upstream and downstream directions from the fire source. In this study, experiments were carried out with two channeled ceilings with widths of 1.5 m and 0.5 m. Square porous gas burners of different sizes were employed as the fire source, using propane as fuel, with various heat release rates and source-ceiling heights. The flame extension lengths beneath the ceiling, both upstream and downstream from the fire source, were measured. Their difference as well as their total length was quantified for different magnitudes of forced longitudinal air flow along the channel. Results show that the flame extension lengths beneath the ceiling increases with heat release rate, but decreases with source-ceiling height, channel width, burner size or longitudinal air flow speed. With a longitudinal air flow, the flame extension is longer downstream than upstream. Non-dimensional correlations are proposed for the flame extension lengths (upstream, downstream and their total length), based on the unburnt fuel distribution upstream and downstream, as well as considering air entrainment of the ceiling flow, which further consumes the unburnt fuel along the ceiling. These correlations are shown to fit the data well.

Fast and simple fabrication of SiO2/poly(vinylidene fluoride) coated cotton fabrics with asymmetric wettability via a facile spray-coating route

Cotton fabrics with hydrophilic-to-hydrophobic asymmetric surfaces are attractive as potential utilizable structures for functional garments. The spray-coating route could be deemed as a fast and simple way to achieve asymmetric surfaces. In this paper, SiO2 nanoparticles with size ∼ 205 nm were synthesized via the modified sol-gel method, and then modified with poly(vinylidene fluoride) (PVDF) to form a hydrophobic surface. The SiO2 nanoparticles modified with PVDF were uniformly deposited on the outer surface of cotton fabric aided with the robust air flow force from the sprayer. The morphology and chemical structures were characterized by scanning electron microscopy, mapping, atomic force microscopy, X-ray photoelectron spectroscopy and Fourier-transform infrared spectroscopy. The results indicated that SiO2 nanoparticles were evenly deposited on the surface of cotton fibers and stable interfacial interaction occurred between SiO2 and PVDF molecular chains. The existence of SiO2 could increase the roughness of the fabric surface, which could enhance the water-repellent property of the coated fabrics. Furthermore, the water-repellent property and thermal insulation properties were evaluated via the water contact angle and thermal conductivity tests, respectively, and the results showed that 20 wt.% SiO2/PVDF fabric achieved a desirable level of contact angle, 136.6°, which was the largest water contact angle among all the samples, and the lowest thermal conductivity of 0.033 W/mK, resulting from the existence of SiO2 nanoparticles. Such a fast and simple spray-coating strategy could be widely introduced into asymmetric fabric modification, and such asymmetric fabrics with reasonable water-repellent and thermal insulating outer surfaces could act as candidates in the field of functional garments.

NOTA TÉCNICA: MODELAGEM MATEMÁTICA DA CONTRAÇÃO VOLUMÉTRICA DE GRÃOS DE SOJA DURANTE O PROCESSO DE SECAGEM

A determinação e a quantificação do índice de contração volumétrica podem contribuir para a otimização do processo de secagem, visto que este índice permite predizer o volume ocupado pela massa de grãos. Assim, objetivou-se com o presente trabalho avaliar a contração volumétrica de grãos de soja durante o processo de secagem e ajustar diferentes modelos matemáticos aos valores experimentais obtidos em função do teor de água. Utilizou-se grãos de soja das variedades NS7901RR e TMG132RR, com média do teor inicial de água de 0,30 base seca (b.s.). Os grãos foram submetidos à secagem numa estufa com circulação forçada de ar, nas temperaturas de 40, 50 e 60 ºC, até atingirem um teor de água de pelo menos 0,15 (b.s.). A redução do volume foi analisada em função do teor de água por meio de diferentes modelos matemáticos que foram ajustados aos dados observados, por meio da avaliação da magnitude do erro cometido na estimação dos valores de contração volumétrica. Os modelos Bala e Woods adaptado e Polinomial, para a variedade NS7901RR, e Corrêa, Exponencial, Linear, Lozano, Mayor e Polinomial, para a variedade TMG132RR, foram os que apresentaram os melhores ajustes aos dados de contração volumétrica observados da tanto aparente quanto unitária nas diferentes temperaturas do ar de secagem utilizadas.

Empirical Parameter Identification for a Hybrid Thermal Model of a High-Speed Permanent Magnet Synchronous Machine

An accurate thermal model will commonly require empirical parameter identification, specifically for the convection coefficients and interface resistances. A high-speed permanent magnet synchronous machine test platform, equipped with various temperature and power measuring equipment, is used to determine these parameters. Specifically, two tests, a dc injection test and rotational test with no load connected, were performed. The results were compared with a lumped thermal model and the parameters updated until an acceptable match was achieved. There were significant differences in the temperature rise when activating forced air cooling, thus significantly influencing the convection coefficients. Also, a significant difference in the interface resistances showed that in these high-speed machines, doing only the dc injection test will not give accurate interface resistance values. The work is novel through combining systematic empirical parameter identification to determine the convection coefficients and interface resistances for a machine where the end windings are cooled by forced tangential air flow.

Simulation experiments on the controllability of dust diffusion by means of multi-radial vortex airflow

In order to control the diffusion of dusts towards the working area in an excavation face, a novel multi-radial vortex wind generator was designed and developed in this study. Using a self-designed simulation system, the multi-radial vortex airflow migration and dust diffusion rules were experimentally investigated. As to the airflow migration, it was easier to form the dust suppression air curtains with uniform airflow distribution at a longer distance away from the head-on section (Lvortex), under smaller forced-to-exhaust ratio (Y), forced air flow rate (Qforced) and axial-to-radial-ratio (Z), with the airflow pointing to the head-on section. As to the control of dust diffusion, the dust concentration decreased gradually with the increase of Lsection. As the distance away from the head-on section increased, the control capacity of dust diffusion firstly weakened and then strengthened. The smaller values of forced-to-exhaust ratio and axial-to-radial ratio and the larger forced air flow rate would be beneficial to the control of dust diffusion. When the ventilation parameters were reasonably set, the airflow distribution of the dust suppression air curtain, the airflow direction and the hindrance on dust diffusion tended to be stable. Additionally, the optimal ventilation parameters were determined in this study, i.e., Lvortex = 20 m, Y = 5:4, Qforced = 200 m3/min and Z = 1:9. Under the optimal condition, the average removal ratios of total dusts and respirable dusts were as high as 96.02% and 95.51%, respectively.

Numerical simulation of airflow around the evaporator in the closed space

The article deals with a numerical simulation of the forced airflow around a evaporator with the finned tubes in the electrotechnical box, by finite volume method in the program ANSYS Workbench. The work contains an analysis of the impact of forced airflow on the evaporator with the various seated the electrical components. The aim of the work is to find out the most effective way of heat dissipation by forced convection from the electrical components in the closed space with lowest pressure loss.

Combustion and flame spreading characteristics of diesel fuel with forced air flows

Research on combustion and flame spreading behaviors in a system with forced air flow is crucial for actual fire protection design and scientific meanings. Flame spread tests over sub-flash diesel fuel are conducted by using a self-developed cross flow system, in both opposed and concurrent configurations. Some key parameters to characterize the flame spread performance, namely, flame tilt, flame spread rate, temperature distribution and scale characteristic of subsurface flow are achieved and analyzed. The variations of flame tilt angle in opposed and concurrent flame spreads follow well with Thomas’s classical model, and the tangent value of the flame tilt angle is correlated with the wind Froude number with the power value of 0.8. The flame spread rate decreases monotonically with an increase in the opposed air flow velocity. By contrast, the flame spread rate decreases initially and increases afterwards as the concurrent air flow velocity increases, with the critical air velocity of 1.725 m/s. Furthermore, the step temperature rise remains stable in the range of 27 °C to 30 °C for opposed air flows, but it is not pronounced for concurrent air flows. The subsurface flow length increases with opposed air flow velocity, while it becomes irregular under concurrent air flows. The primary contribution of heat transfer to flame spread is liquid-phase thermal convection for opposed or low-speed concurrent air flow conditions, whereas it is flame radiation and gas conduction for high-speed concurrent air flow conditions. The finding of current study is favorable for the fire hazard assessment of spilling hydrocarbon fuel in air flow environments.

Fire hazard of titanium powder layers mixed with inert nano TiO2 powder

Metallic dust layers are highly sensitive to ignition from common ignition sources, even when mixed with high percentages of inert solids. In turn, dust layer fires are a potential ignition source for dust explosions or other damaging fires. Flame spread velocity (FSV), as a potential parameter for evaluating fire hazard, was investigated for titanium powder layers mixed with inert nano TiO2 powder in both natural convection and in forced airflow conditions. Increased mass percentage of nano TiO2 powder decreased FSV of Ti powder mixtures as expected. The mixing ratio of nano TiO2 to fully suppress layer fires was 80% and 90% for micro and nano Ti powder, respectively. Mechanisms governing flame spread across a layer of nano Ti powder differed from those of a layer of micro Ti powder. FSV in no airflow conditions was higher than in aided airflow for micro Ti powder because conduction was the dominant heat transfer mechanism. However, FSV in no airflow was lower than in opposed airflow for nano Ti powder because convection/radiation was the dominant heat transfer mechanism. A fly fire phenomenon contributed to greater FSVs and higher fire hazard with nano Ti powder mixtures under aided airflow conditions.

Influence of the method of implementing the forced air flow through the cooling system on the temperature of the coolant in heavy-duty engines

The article discusses the problem how forced air flow is implemented through the cooling module of heavy-duty engines. The kinematic connection between the coolant pump and the crankshaft of the engine, results in the fact that the pump performance often does not correspond to the engine demand at its part load. In conjunction with the fan drive method, the temperature of the coolant may be too low or too high for part load. The study was carried out in order to calculate the parameters of the cooling system in heavy-duty engines for maximum power and maximum torque, taking into account a mechanical fan drive, a hydraulic fan drive and an electric fan drive.

A shortcut algorithm for solving long-term conjugate heat transfer problems based on quasi-steady flow

PurposeThe time of tightly coupled transient calculation and the accuracy of conventional loosely coupled algorithm make it difficult to meet the engineering design requirements for long-term conjugate heat transfer (CHT) problems. The purpose of this paper is to propose a new loosely coupled algorithm with sufficient accuracy and less calculation time on the basis of the quasi-steady flow field. Through this algorithm, it will be possible to reduce the update frequency of the flow field and devise a strategy by which to reasonably determine the update steps.Design/methodology/approachIn this paper, the new algorithm updates the flow field by solving the steady governing equations in the fluid region and by calculating the transient temperature distribution until the next update of the fluid flow, by means of solving the transient energy equations in the entire computational domain. The authors propose a strategy by which to determine the update step, by using the engineering empirical formula of the Nusselt number, on the basis of the changes of the inlet and outlet boundary conditions.FindingsTaking a duct heated by an inner forced air flow heating process as an example, the comparison results for the tightly coupled transient calculation by Fluent software shows that the new algorithm is able to significantly reduce the calculation time of the transient temperature distribution with reasonable accuracy. For example, the respective computing times are reduced to 22.8 and 40 per cent, while the duct wall temperature deviations are 7 and 5 per cent, using the two flow update time steps of 100 and 50 s on the variable inlet-flow rate conditions.Originality/valueThe new algorithm outlined in this paper further improves the calculated performance and meets the engineering design requirements for long-term CHT problems.

Thermal management for energy storage system for smart grid

This paper is about the design and implementation of a thermal management of an energy storage system (ESS) for smart grid. It uses refurbished lithium-ion (li-ion) batteries that are disposed from electric vehicles (EVs) as they can hold up to 80% of their initial rated capacity. This system is aimed at prolonging the usable life of li-ion EV batteries by reusing them for energy storage. However, the high value of the internal resistance of refurbished batteries generates more heat and consequently accelerates further their aging process as their operating temperature plays a vital role on their reliability, lifespan, safety and performance. Battery Thermal Management System (BTMS) must keep the operating temperature of the battery pack between −10°C and 40°C in order to achieve good performances and long lifespan. Active BTMS consumes energy from the pack and reduces its available capacity. In order to reduce this adverse impact, this paper presents a novel approach that takes advantage of the non-uniform surface distribution of li-ion battery cell, which results from complex reactions inside the cell. First, li-ion hotspots were identified and found next to the positive and negative tabs. Then, thermoelectric coolers (TECs) are mounted next to the tabs and in the center of the li-ion battery. A control circuit is designed to turn on and off TECs in order to reduce the parasitic power feeding the proposed system, referred to as modular battery thermal management system (MBTMS). Experimental results show the performance of the MBTMS under normal conditions, such as an ambient temperature of 25°C and a discharge rate of C/1 to 2C, and under extreme conditions, such as 40°C and 3C discharge rate. Finally, its performance is compared against a compatible BTMS based on forced air flow. The experimental results show the feasibility of the proposed system.

2-D Wind Velocity Measurement Using a Vertically Suspended Optical Fiber Combined With a Photosensor Array

A novel low-cost 2-D wind velocity (speed and direction) sensor was designed based on the principles of statics and optics. A vertically suspended optical fiber was employed in the designed sensor as the sensitive element, which could be deflected by the applied force of airflow in 2-D horizontal planes. An optical signal emitted from the end of the fiber was projected onto a charge coupled device (CCD) array detector, which could recognize the offset magnitude and direction through image processing. The mathematic expression between the displacement of the light spot projected onto the CCD and the wind speed was derived. Preliminary wind tunnel experiments validated the correctness of the theoretical derivation, and the results also showed that the designed sensor had good precision (particularly in low wind speed).

Experimental investigation of the steady state behaviour of Breathing Walls by means of a novel laboratory apparatus

Breathing Walls are envelope components, based on porous materials, crossed by a natural or forced airflow. Since they behave both as recovery heat exchangers and active insulation, reducing the conductive heat flux, they represent a promising envelope technology, allowing to reduce energy consumption in buildings.From the modeling point of view, an analytical model can be found in literature, describing heat and mass transfer across Breathing Walls in steady state conditions. However, to the best of the authors' knowledge, the model lacks an exhaustive experimental validation. Therefore, in this paper, the novel laboratory apparatus named Dual Air Vented Thermal Box developed at Politecnico of Milano is presented. The apparatus is used to experimentally investigate the steady state behaviour of a 1 m2 Air Permeable Concrete sample, crossed by an airflow at different velocities up to 12 mm/s.The temperature profile inside the sample, measured in different positions, is compared with the model predictions. While in the central portion of the wall a very good agreement is found, the experimental results at the top and at the bottom of the wall suggest a non-uniform velocity field entering the sample. A qualitative confirmation of this hypothesis is provided by CFD simulations on the apparatus, clearly showing a mixed convection regime on both sides of the wall. The results lead to state the validity of the one-dimensional analytical model in literature, although a careful application should take into account adjusted boundary conditions, consisting in an airflow velocity possibly variable with height.

The preparation of continuous CeO2/CuO/Al2O3 ultrafine fibers by electro-blowing spinning (EBS) and its photocatalytic activity

CeO2/CuO/Al2O3 catalyst has attracted considerable attention due to its higher activity and better selectivity at lower reaction temperature in comparison with commercial catalysts, and it has exhibited potential applications in water gas shift reaction and exhaust gas treatment. In this paper, the continuous CeO2/CuO/Al2O3 ultrafine fibers were prepared by electro-blown spinning method followed by a calcination process. The as-spun fibers were formed with solvent evaporation under the drawing of air flow and electrostatic force. The diameters of fibers could be regulated by changing the parameters, such as PVP content, gas pressure, voltage and injection rate, etc. These fibers could be subsequently converted into CeO2/CuO/Al2O3 ultrafine fibers via calcination in air at 400 °C. The presence of Al2O3 made the more flexibility of CeO2/CuO/Al2O3 fibers and well inhibited the crystal growth of CeO2 and CuO during the calcination process. Most importantly, CuO could be highly dispersed into CeO2, which provide a higher content of surface oxygen vacancies and should empirically benefit the catalytic process. Therefore, CeO2/CuO/Al2O3 composite fibers exhibited the excellent catalytic efficiency in dye degradation, and the utilization efficiency of solar light was effectively enhanced by the loading CuO into the materials. In addition, the mass production of CeO2/CuO/Al2O3 ultrafine fiber catalysts might be realized by this novel method.

A matrix to evaluate the conjugate cooling of a heaters' array

Circuit boards are usually cooled by forced airflow and the heat transfer from the discrete electronic components may be enhanced by conductive boards. The conjugate forced convection-conduction cooling of an array of N heaters mounted on a conductive substrate was described by means of dimensionless conjugate coefficients g+kj grouped in a N-square matrix G+. Experiments were performed with one or two rows of protruding heaters mounted on the conductive lower wall of a rectangular duct, made of either Aluminum or Plexiglas. One end of the duct was closed and the heaters were cooled by two impinging airflows exiting from square holes at the duct upper wall. For each substrate plate, the conjugate coefficients were obtained from tests with a single active heater at a time and expressed as functions of a Reynolds number in the range from 2000 to 7000. Additional tests with two or three active heaters at a time were then performed and the measured temperature increase of each heater above the flow inlet temperature was predicted quite well using the previously obtained coefficients matrix G+. These results showed that the conjugate coefficients are invariant with the conjugate cooling rates from the heaters. The conjugate coefficients were also used to predict the allowable conjugate cooling rates for an array of heaters, in order to keep their temperature within reliable limits. Numerical simulations were performed for a duct configuration similar to that of the experiments, but with flush mounted heaters. The numerical results were thus distinct from those of the experiments with protruding heaters, but they showed similar trends of change with the Reynolds number and they indicated a perspective of the effects of the heaters height.