This paper mainly investigates the transient conjugate heat transfer characteristic that the high speed airflow invades into the gapacavityagap structure. For this purpose, a simplified and effective ...quasi-steady approach in order to improve the numerical computational efficiency in multi-domain transient conjugate problems is presented. In this quasi-steady approach the pretty difference of the characteristic time step between the fluid and solid domains is utilized, so the computational expense can be reduced by alternating the solution sequence between coupled convection/conduction analysis and conduction analysis. The control factors to impact on the accuracy of this quasi-steady state method are analyzed further, by analyzing the process that high speed airflow aerodynamically heats the solid cylinder. Based on this quasi-steady approach, the computational fluid dynamics model combined convection, conduction and surface thermal radiation is established. Then, the transient conjugate heat transfer for the gapacavityagap structure is discussed. The computational results and the experimental data are compared with each other and the relative errors between them are summarized. After that, during the transient process, the features of the temperature contour inside the structure are discussed. Finally, from viewpoint of initial pressure difference between high speed airflow outside the structure and the cavity, the transient conjugate heat transfer characteristics of this structure are analyzed.
Display omitted
Airflow limitation is a hallmark of chronic obstructive pulmonary disease, which can develop through different lung function trajectories across the life span. There is a need for ...longitudinal studies aimed at identifying circulating biomarkers of airflow limitation across different stages of life.
This study sought to identify a signature of serum proteins associated with airflow limitation and evaluate their relation to lung function longitudinally in adults and children.
This study used data from 3 adult cohorts (TESAOD Tucson Epidemiological Study of Airway Obstructive Disease, SAPALDIA Swiss Cohort Study on Air Pollution and Lung and Heart Diseases in Adults, LSC Lovelace Smoker Cohort) and 1 birth cohort (TCRS Tucson Children’s Respiratory Study) (N = 1940). In TESAOD, among 46 circulating proteins, we identified those associated with FEV1/forced vital capacity (FVC) percent (%) predicted levels and generated a score based on the sum of their z-scores. Cross-sectional analyses were used to test the score for association with concomitant lung function. Longitudinal analyses were used to test the score for association with subsequent lung function growth in childhood and decline in adult life.
After false discovery rate adjustment, serum levels of 5 proteins (HP, carcinoembryonic antigen, ICAM1, CRP, TIMP1) were associated with percent predicted levels of FEV1/FVC and FEV1 in TESAOD. In cross-sectional multivariate analyses the 5-biomarker score was associated with FEV1 % predicted in all adult cohorts (meta-analyzed FEV1 decrease for 1-SD score increase: −2.9%; 95% CI: −3.9%, −1.9%; P = 2.4 × 10−16). In multivariate longitudinal analyses, the biomarker score at 6 years of age was inversely associated with FEV1 and FEV1/FVC levels attained by young adult life (P = .02 and .005, respectively). In adults, persistently high levels of the biomarker score were associated with subsequent accelerated decline of FEV1 and FEV1/FVC (P = .01 and .001).
A signature of 5 circulating biomarkers of airflow limitation was associated with both impaired lung function growth in childhood and accelerated lung function decline in adult life, indicating that these proteins may be involved in multiple lung function trajectories leading to chronic obstructive pulmonary disease.
The purpose of this paper is to determine the dispersion and distribution characteristics of exhaled airflow for accurate prediction of disease transmission. The development of airflow dynamics of ...human exhalation was characterized using nonhazardous schlieren photography technique, providing a visualization and quantification of turbulent exhaled airflow from 18 healthy human subjects whilst standing and lying. The flow shape of each breathing pattern was characterized by two angles and averaged values of 18 subjects. Two exhaled air velocities, um and up, were measured and compared. The mean peak centerline velocity, um was found to decay correspondingly with increasing horizontal distance x in a form of power function. The mean propagation velocity, up was found to correlate with physiological parameters of human subjects. This was always lower than um at the mouth/nose opening, due to a vortex like airflow in front of a single exhalation cycle. When examining the talking and breathing process between two persons, the potential infectious risk was found to depend on their breathing patterns and spatial distribution of their exhaled air. Our study when combined with information on generation and distributions of pathogens could provide a prediction method and control strategy to minimize infection risk between persons in indoor environments.
•Noninvasive schlieren technique was applied to characterize human exhalation.•New methods were proposed to predict exhaled velocity um and up, respectively.•Potential infection risk depended on breathing patterns and spatial distribution of exhaled air.•New data was added to airflow dynamics of human breathing and CFD boundary settings.
•The combustion of methane hydrate under different airflow conditions was discussed.•The combustion flame under different airflow conditions has different stages.•Flame eruption exists under natural ...convection and horizontal airflow conditions.•The internal vertical airflow is beneficial to improve the combustion stability.•The external horizontal airflow is beneficial to increase the combustion rate.
Due to its large reserves and cleanliness, methane hydrate is widely regarded as a promising energy source. The study of the effect of airflow conditions on the combustion characteristics of methane hydrate can provide a theoretical basis for various engineering applications, such as realizing the method of in-situ combustion mining, improving the safety of mining and predicting the development of fire during the storage and transportation of methane hydrates. In this paper, the combustion process of methane hydrate under the conditions of natural convection, external horizontal airflow and internal vertical airflow was analyzed. This way, the effect of the form of airflow on combustion was studied, and the effect of airflow velocity on the characteristics of flame was documented. The results demonstrated that, under the conditions of natural convection and horizontal airflow, there was an “eruption phenomenon”, during which the ice layer broke and a large amount of methane gas instantly escaped, forming a high and bright flame. The vertical airflow reduced the mass transfer resistance of methane gas, whereas the combustion process was more stable, and the height of flame was more uniform. Under the condition of horizontal airflow, the tilted flame increased the heat transfer to the surface of hydrate. The flame tilt angles were different in different combustion stages. The flame tilt angle in the intensive combustion stage (61.26°) was smaller than that in the steady combustion stage (70.85°), whereas a higher combustion rate had a stronger capability to oppose the horizontal wind.
During the past few decades, both positive and negative aspects of indoor air movement on human comfort perception have been studied. However, most research has focused on collecting empirical ...evidence to support the potential application of airflows in practice. In this context, numerous studies have encouraged the use of moving air to promote comfort in warm environments, especially with regard to the application of dynamic airflows. Through these studies, the effects of different airflow parameters (such as air velocity, turbulence intensity, and fluctuation frequency) on human comfort perception have gradually been identified and even quantified. This article aims to summarize the progress in the literature concerning dynamic characteristics and comfort assessment of airflows in four main sections: demand for indoor airflows, parameters of indoor airflows, comfort assessment of indoor airflows, and dynamic airflows and their application. This paper will hopefully aid the understanding of human perception of indoor airflows and guide future research.
•This article reviews the researches in indoor air movement comfort assessment.•Air movement can improve occupants' thermal comfort in warm environment.•Different airflow parameters can influence human thermal perception.•Simulated natural wind has better comfort effects than constant mechanical wind.
•A function of the critical airflow velocity that droplet onset is established.•The process of droplet motion is divided into four stages.•The wetting length of droplet motion is analyzed in each ...stage.•The factors that affect the velocity of the center of droplet gravity are explored.•The change of dynamic contact angle of droplet on different surfaces is analyzed.
The movement and deformation of droplet is widely applied in many fields. In this study, to investigate the dynamics of droplets moving on surfaces with different wettability driven by airflow, a series of experiments are conducted by using a high-speed camera, under conditions of airflow speeds increased from 0m/s to 22.2m/s with the same pattern per case, droplet volume varied from 20 to 80 μL, and different wettability surfaces (aluminum, PMMA, titanium, PTFE, PAL). Based on the observation of experiments, the change of the morphology of the droplet motion is divided into 4 stages. Stage I: Droplet oscillates at the original position and its wetting line does not move. Stage II: The droplet still oscillates and its wetting line slightly moves on the surface. Stage III: Droplet deforms dramatically and accelerates forward, and its wetting line elongates sharply. Stage IV: Several sub-droplets are split from the mother droplet. The droplet separation usually occurs after its wetting length reaches the extremum value. Specifically, the droplet separation is not observed on the PTFE and PAL surface, and the four stages of droplet motion reduce to three in these cases. According to the observation, droplet shapes during their movement are classified into four categories including compact, tail formation, film formation, and droplet separation. The relationship between the critical airflow speed at which the wetting line begins to move and the droplet diameter is studied, corresponding to different surfaces. To provide an insight into the phenomenon of dynamics and deformation of airflow-driven sessile droplet spreading, quantitative analysis of the characteristic parameters of the droplet including the wetting length, the dynamic contact angle, and the velocity of the center of droplet gravity are conducted. It is shown that the wetting length ratio of droplets moving on different surfaces varied in the range from 0 to 3.63. The maximum value of the difference between cosines of the upstream contact angle and downstream contact angle usually approaches the cosines of the receding contact angle and the advancing contact angle at the end of Stage II. This study provides experimental reference data for the study of the droplet motion driven by airflow.
For the first time, in the current work, a dynamic stability analysis of a tilted curved pipe in a supersonic airflow under thermal loading is presented. The heat-transfer continuum problem is used ...for simulating the thermal environment conditions. The tilted pipe is reinforced by carbon nanotube agglomerations (CNTAs). For simulating the displacement fields of the current structure, Quasi-2D refined high order shear deformation theory is studied. The verification segment is divided into two parts. In the first and second sections, the credibility of the results of this study are confirmed by the results extracted using COMSOL multiphysics software and published articles in the literature, respectively. The results show that Mach number, hydrodynamic pressure, temperature change, yaw angle, and geometry properties of the current structure have a significant impact on the frequency information of the tilted curved pipe reinforced by CNTAs in supersonic airflow. One of the presented results is that the effects from volume fraction of clusters in the representative value element, amount of CNTs located outside the clusters, and the highest value related to the volume fraction of the CNT, parameters on the supersonic airflow characteristics of the system are more considerable at the higher value of the length to radius ratio of the pipe.
•The art on airflow distribution and management in data center is reviewed.•Experimental and numerical investigations of several typical cases are summed.•Energy efficiency of air distribution must ...be based on the reliability of data center.•HACS is a promising air management system due to its flexibility and extendability.•Improvement methods of vertical airflow uniformity needs to be developed.
Rapid expansions of high-density data centers have been triggered worldwide owing to the high demand of cloud computing over the past decade. The airflow distribution and management have great association with the thermal performance of the typical underfloor air distribution (UFAD) data center. Therefore, numbers of configurations and methods have been proposed to improve the thermal performance of data centers with various operation conditions. This paper reviews the ventilation configurations of data center and presents the geometric effects of underfloor plenum on airflow. The experimental and numerical investigations on typical cases as well as the relevant parameters are also summed up. Various air management methods including cold aisle containment system(CACS), hot aisle containment system(HACS) and vertical exhausted duct system(VEDS) are analyzed on the benefits and limitations. Based on the enormous amount of research on the traditional UFAD system with hot aisle/cold aisle (HA/CA) arrangement, it can be concluded that the practices of data centers have a tendency of being more flexible and extendable, while the literature on the corresponding cases which applied HACS or other air distribution systems remains scarce. Moreover, beside the improvement of the airflow uniformity along the cold aisle, methods for enhancing the airflow uniformity along the height direction are also encouraged to be developed.
•The full-scale measurements of airflow in a road tunnel are presented.•A CFD model of the spatio-temporal airflow structure is introduced.•A theoretical model of airflow dynamics is ...introduced.•Longitudinal and semi-transverse operation modes are compared.
This paper presents full-scale experimental, numerical, and theoretical research on the spatial and temporal structure of airflow for different modes of ventilation system operation in a road tunnel. The tunnel under investigation is equipped with a semi-transverse ventilation system. There are 19 pairs of axial jet fans in each tunnel tube and 164 air supply vents on both sides of a tube located just above the road surface. A set of seven stand poles with mounted vane anemometers were arranged along the tunnel tube axis. Different configurations of axial jet fans and air supply vents were employed, and changes in airflow velocity were recorded. Since the main objective of the study is to achieve a deeper understanding of the examined phenomena, a numerical model of the flows in the tunnel was built, and the acquired data was used to validate it. Some theoretical considerations are provided and also validated. All of this together reveals a very uneven distribution of longitudinal air velocity along the tunnel axis. Then, time constants when switching the fans on and off are determined. It is shown that the time needed to achieve a stable flow after the fans are turned on is in the order of five minutes, and the decay time after the fans are turned off is much longer; it takes about 15 min to calm down the flow. Next, it is determined that the transverse air supply significantly changes the airflow structure: the average longitudinal air velocity at the inlet portal was up to 25% lower than without it, while values at the outlet portal were similar. Finally, it is indicated that axial fans that operate in the suction mode are similarly efficient to those in the pumping mode.
PDF
