An under-aisle air distribution system facilitating humidification of commercial aircraft cabins

Air environment in aircraft cabins has long been criticized especially for the dryness of the air within. Low moisture content in cabins is known to be responsible for headache, tiredness and many other non-specific symptoms. In addition, current widely used air distribution systems on airplanes dilute internally generated pollutants by promoting air mixing and thus impose risks of infectious airborne disease transmission. To boost air humidity level while simultaneously restricting air mixing, this investigation uses a validated computational fluid dynamics (CFD) program to design a new under-aisle air distribution system for wide-body aircraft cabins. The new system supplies fully outside, dry air at low momentum through a narrow channel passage along both side cabin walls to middle height of the cabin just beneath the stowage bins, while simultaneously humidified air is supplied through both perforated under aisles. By comparing with the current mixing air distribution system in terms of distribution of relative humidity, CO₂ concentration, velocity, temperature and draught risk, the new system is found being able to improve the relative humidity from the existent 10% to the new level of 20% and lessen the inhaled CO₂ concentration by 30%, without causing moisture condensation on cabin interior and inducing draught risks for passengers. The water consumption rate in air humidification is only around 0.05 kg/h per person, which should be affordable by airliners.     

进风口对变压器室通风效果的影响

通风设计是户内变电站变压器室设计的难点问题。运用CFD方法,对某变压器室温度场和速度场进行模拟,并将模拟值与实测值对比,验证数学模型的有效性。在此基础上,以某变电站变压器室为模拟对象,通过改变进风口位置和面积,设计并模拟6种通风工况,通过对比各工况温度场、速度场和特征温度值的变化规律,重点研究进风口对变压器室通风效果的影响。模拟结果表明：进风口面积不变时,进风口应布置在散热器一侧且其中心高度宜控制在散热器中心高度或稍偏下位置,不宜高于散热器;进风口位置固定时,增大进风口面积改善通风效果时,宜选择沿高度方向增大进风口面积。所得结论可为变压器室通风设计提供技术参考。     

Dispersion and settling characteristics of evaporating droplets in ventilated room

Movement and evaporation of small droplets in the room air are investigated in this paper through CFD simulations. A modified drift-flux model is presented with the droplet evaporation rate and the drift velocity expressed as simple algebra functions of droplet diameter, which is integrated in the transport equations of droplet number density and droplet bulk density. Evaporating droplets are treated as a continuum phase with one way coupling with the carrier phase, i.e. air. Our numerical simulations reveal that the distribution of the large evaporating droplets in the ventilated room air is characterized by a combination of the settling feature when droplets are first generated and released and the dispersion feature after the droplets are evaporated to be either very fine or become droplet nuclei. For droplets less than 50 μm in diameter, the dispersion feature is dominant in the test room that we simulated, while for droplets larger than 100 μm in diameter, the settling feature dominates. For evaporating droplets between these two sizes, the spatial distribution of droplets tends to be located at the lower part of the test room than that of small neutral aerosol particles. Within this size range, a lower initial position of the droplets in the room results in a higher deposition rate of the droplets on the floor.     

The effect of air-conditioning parameters and deposition dust on microbial growth in supply air ducts

To investigate the effect of air-conditioning parameters (including temperature, relative humidity and air velocity) and deposition dust on microbial growth in supply air duct, a complete test facility according to ASHRAE Standard 62.1-2007 was constructed. A series of experiments for testing microbial concentration (including bacteria and fungus) were conducted under different working conditions (such as different temperatures and relative humidity). The air velocity was constantly kept at 2.0 m/s. Orthogonal design was employed for the analysis of test data. The results indicated that air velocity attenuation down the direction of the supply air affected dust distribution at the bottom of duct, to some extent, and the number of microorganisms was positively correlated with the quantity of dust. In the range of temperature 22-32 °C and relative humidity (RH) 40-90%, microbial growth significantly accelerated with higher temperature and RH increasing. The organic compounds composing the dust also had great impact on microbial growth. The basic researches are contributed to control the growth of microorganism and improve the indoor microenvironment in the air-conditioning room.     

列车单元式空调机组制冷量显著性试验分析

分析了蒸发器进风温度、进风湿度和冷凝器进风温度三个空气侧参数对列车单元式空调机组制冷量和压缩机电流的影响,通过正交试验发现:三个参数都是影响制冷量的显著性因素,其中,冷凝器进风温度影响最大,蒸发器进风相对湿度次之,进风温度最小;三个参数对机组压缩机电流的影响也具有显著性,但不如对制冷量显著,其中,冷凝器进风温度影响最大,蒸发器进风温度次之,进风相对湿度最小。     

冷却顶板-置换通风空调系统分析

介绍了一种冷却顶板设计样式,同时对冷却顶板进行了换热分析。冷却顶板只能除去显热负荷,无法除去湿负荷,置换通风的除湿能力受到送风量的限制,当整个制冷系统处于温度最低点时,冷却顶板表面温度可能会降到室内空气露点温度以下,而出现结露的危险。为了保证冷却顶板表面不结露特别是在湿负荷较高的状况下, 本文提出了在冷却顶板表面形成干燥空气保护层的建议,同时以计算流体力学(CFD)的模型为基础,采用有限容积法对具有干燥空气保护层的冷却吊顶一置换通风系统的气流分布进行了模拟分析。     

A simple two-layer zone model on mechanical exhaust in an atrium

Provision of make-up air is essential in designing mechanical exhaust system in a compartment. There are always problems in determining the inlet positions for supplying make-up air. In this paper, a zone model for studying the effect of different positions of make-up air supply on the performance of a mechanical exhaust system in an atrium will be developed. Traditional two-layer approach with an upper smoke layer and a lower air layer will be assumed.Three scenarios of extraction with different relative positions of the air inlet are studied. These are scenarios with the smoke layer interface lying above, within, and below the air inlet. Conservation of mass and energy are considered for each scenario to study the smoke filling process. Transient variations of smoke layer temperature and interface height will be predicted under different fire sizes, exhaust rates and make-up air conditions.Full-scale burning tests in an atrium were conducted to justify the predicted results. In addition, results predicted by this zone model will also be compared with those predicted by Computational Fluid Dynamics (CFD) with the software Fire Dynamics Simulator FDS version 3.1; and another zone model CFAST version 5.0.1. It is observed that the predicted results from this new zone model agreed well with experiments and CFD results. However, results predicted by CFAST deviated from experiments for the scenario with the smoke layer interface lying below the air inlet.     

Numerical study on temperature stratification in a room with underfloor air distribution system

In this study, numerical prediction using computational fluid dynamics (CFD) was utilized to investigate air temperature stratification in a room with an underfloor air distribution (UFAD) system. The numerical modeling using CFD computation was validated with physical test in a full size experimental room with an UFAD system. The different supply air conditions and heat loads were discussed. The results show that the effect of three parameters, heat load, supply volume flux and supply air velocity, on room air temperature would be expressed by the length scale of the floor supply jet. When the length scale increased from 0.8 to 1.56 m, the ratio of vertical temperature difference between 2.5 and 0.1 m at the occupied zone to the difference between return and supply air temperature decreased from 0.62 to 0.25. When there was only one local heat source in the room, there was a thermal stratified interface at the occupied zone. The interface height was about 1.42 times the length scale. The results may suggest ways to optimize UFAD design and operation.     

Evaluation of thermal comfort using combined CFD and experimentation study in a test room equipped with a cooling ceiling

This paper reports a full-scale experimental campaign and a computational fluid dynamics (CFD) study of a radiant cooling ceiling installed in a test room, under controlled conditions. This research aims to use the results obtained from the two studies to analyze the indoor thermal comfort using the predicted mean vote (PMV). During the whole experimental tests the indoor humidity was kept at a level where the condensation risk was minimized and no condensation was detected on the chilled surface of the ceiling. Detailed experimental measurements on the air temperature distribution, surface temperature and globe temperature were realized for different cases where the cooling ceiling temperature varied from 16.9 to 18.9 °C. The boundary conditions necessary for the CFD study were obtained from the experimental data measurements. The results of the simulations were first validated with the data from the experiments and then the air velocity fields were investigated. It was found that in the ankle/feet zone the air velocity could pass 0.2 m/s but for the rest of the zones it took values less than 0.1 m/s. The obtained experimental results for different chilled ceiling temperatures showed that with a cooling ceiling the vertical temperature gradient is less than 1 °C/m, which corresponds to the standard recommendations. A comparison between globe temperature and the indoor air temperature showed a maximum difference of 0.8 °C being noticed. This paper also presents the radiosity method that was used to calculate the mean radiant temperature for different positions along different axes. The method was based on the calculation of the view factors and on the surface temperatures obtained from the experiments. PMV plots showed that the thermal comfort is achieved and is uniformly distributed within the test room.     

A new formula for determining a minimum recommended value of inlet air velocity from UFAD system to prevent occupants from draught risk

During the winter period the natural convection flow along a cold window surface can be the cause of thermal discomfort. Warm air blown into the room near the window prevents occupants from draught risk. The paper presents a new formula for determining a minimum recommended value of inlet air velocity. A balance of a momentum flux of flow of the down draught currents and the warm air jet inflow from the UFAD (under floor air distribution) system was a base for the analytical solution. In order to estimate air movement in a typical room, numerical simulations are executed. A computational model of the room with the UFAD system is constructed within the CFD environment developed by Fluent Inc. Ten cases with varying inlet air parameters and a window U$U$-value are considered. The analytical estimation of minimum inlet air velocity is confirmed by simulation results of temperature and velocity distributions.     

Numerical study of a M-cycle cross-flow heat exchanger for indirect evaporative cooling

In this paper, numerical analyses of the thermal performance of an indirect evaporative air cooler incorporating a M-cycle cross-flow heat exchanger has been carried out. The numerical model was established from solving the coupled governing equations for heat and mass transfer between the product and working air, using the finite-element method. The model was developed using the EES (Engineering Equation Solver) environment and validated by published experimental data. Correlation between the cooling (wet-bulb) effectiveness, system COP and a number of air flow/exchanger parameters was developed. It is found that lower channel air velocity, lower inlet air relative humidity, and higher working-to-product air ratio yielded higher cooling effectiveness. The recommended average air velocities in dry and wet channels should not be greater than 1.77 m/s and 0.7 m/s, respectively. The optimum flow ratio of working-to-product air for this cooler is 50%. The channel geometric sizes, i.e. channel length and height, also impose significant impact to system performance. Longer channel length and smaller channel height contribute to increase of the system cooling effectiveness but lead to reduced system COP. The recommend channel height is 4 mm and the dimensionless channel length, i.e., ratio of the channel length to height, should be in the range 100 to 300. Numerical study results indicated that this new type of M-cycle heat and mass exchanger can achieve 16.7% higher cooling effectiveness compared with the conventional cross-flow heat and mass exchanger for the indirect evaporative cooler. The model of this kind is new and not yet reported in literatures. The results of the study help with design and performance analyses of such a new type of indirect evaporative air cooler, and in further, help increasing market rating of the technology within building air conditioning sector, which is currently dominated by the conventional compression refrigeration technology.     

Thermal effect of temperature gradient in a field environment chamber served by displacement ventilation system in the tropics

The effect of vertical air temperature gradient on overall and local thermal comfort at different overall thermal sensations and room air temperatures (at 0.6 m height) was investigated in a room served by displacement ventilation system. Sixty tropically acclimatized subjects performed sedentary office work for a period of 3 h during each session of the experiment. Nominal vertical air temperature gradients between 0.1 and 1.1 m heights were 1, 3 and 5 K/m while nominal room air temperatures at 0.6 m height were 20, 23 and 26 °C. Air velocity in the space near the subjects was kept at below 0.2 m/s. Relative humidity at 0.6 m height was maintained at 50%. It was found that temperature gradient had different influences on thermal comfort at different overall thermal sensations. At overall thermal sensation close to neutral, only when room air temperature was substantially low, such as 20 °C, percentage dissatisfied of overall body increased with the increase of temperature gradient. At overall cold and slightly warm sensations, percentage dissatisfied of overall body was non-significantly affected by temperature gradient. Overall thermal sensation had significant impact on overall thermal comfort. Local thermal comfort of body segment was affected by both overall and local thermal sensations.     

影响盘管干工况运行的各种因素

通过计算和试验,分析了进风参数、进水温度、进出水温差、水流程、迎风面积、盘管列数及风量等因素对盘管干工况运行的影响。探讨了降低干盘管出风温度的方法及其可行性。     

Impact of individually controlled facially applied air movement on perceived air quality at high humidity

The effect of facially applied air movement on perceived air quality (PAQ) at high humidity was studied. Thirty subjects (21 males and 9 females) participated in three, 3-h experiments performed in a climate chamber. The experimental conditions covered three combinations of relative humidity and local air velocity under a constant air temperature of 26 °C, namely: 70% relative humidity without air movement, 30% relative humidity without air movement and 70% relative humidity with air movement under isothermal conditions. Personalized ventilation was used to supply room air from the front toward the upper part of the body (upper chest, head). The subjects could control the flow rate (velocity) of the supplied air in the vicinity of their bodies. The results indicate an airflow with elevated velocity applied to the face significantly improves the acceptability of the air quality at the room air temperature of 26 °C and relative humidity of 70%.     

Experimental study on the frost characteristics of fin tube heat exchanger

The heat and mass transfer characteristics under frosting on surface of heat exchanger were experimentally investigated in different conditions of air temperature, relative humidity, and face velocity. The heat transfer and heat transfer coefficient decreased faster with the high relative humidity, low air temperature and initial face velocity. The air pressure drop rose faster with the high relative humidity and low air velocity.     

Effect of warm air supplied facially on occupants’ comfort

Human response to air movement supplied locally towards the face was studied in a room with an air temperature of 20 °C and a relative humidity of 30%. Thirty-two human subjects were exposed to three conditions: calm environment and facially supplied airflow at 21 °C and at 26 °C. The air was supplied with a constant velocity of 0.4 m/s by means of personalized ventilation towards the face of the subjects. The airflow at 21 °C decreased the subjects' thermal sensation and increased draught discomfort, but improved slightly the perceived air quality. Heating of the supplied air by 6 K (temperature increase by 4 K at the target area) above the room air temperature decreased the draught discomfort, improved subjects' thermal comfort and only slightly decreased the perceived air quality. Elevated velocity and temperature of the localized airflow caused an increase of nose dryness intensity and number of eye irritation reports. Results suggest that increasing the temperature of the air locally supplied to the breathing zone by only a few degrees above the room air temperature will improve occupants' thermal comfort and will diminish draught discomfort. This strategy will extend the applicability of personalized ventilation aiming to supply clean air for breathing at the lower end of the temperature range recommended in the standards. Providing individual control is essential in order to avoid discomfort for the most sensitive occupants.     

间接蒸发冷却用于空调新风预冷的实验研究

从温差换热驱动势和吸湿驱动势出发,分析了间接蒸发冷却换热效率的影响因素,实验研究了新风进口温度、排风进口温度和相对湿度对新风进出口温差和换热效率的影响。     

Comparison of underfloor and overhead air distribution systems in an office environment

This study compares thermal environment of two air distribution systems in an office setting. Airflow, heat and mass (water vapor and contaminant gas) transfer in steady-state condition are modeled for an underfloor air distribution system and an overhead air distribution system. The models include a typical cubicle in a large office floor with a chair, a desk with a personal computer on top, and heat sources such as seated person, desktop computer, and lights. For underfloor air distribution system, cool air enters the occupied zone through an inlet located at the floor level supplying a vertical upward inflow. Three different locations of the inlet diffuser are considered. For overhead air distribution, the inlet is located on the ceiling with slower and cooler inflow. Three inlet angles are considered. For both systems, the air return location is on the ceiling at the same place. Distributions of velocity, temperature, relative humidity, and contaminant concentration in various cases for both systems are computed. Thermal comfort factors are assessed for the two systems. The results are compared among cases of each system, as well as between two typical cases of the two systems and to experimental data for an actual office building given in literature. The results provide a detailed understanding of air transport and its consequence on thermal comfort and indoor air quality that are useful for office building air conditioner design. It is found that underfloor system gives better performance than overhead system in contaminant removal and significantly in energy saving while maintaining the same thermal comfort condition.     

Improved airflow around multiple rows of buildings in hot arid climates

This paper studies the natural wind environments in two locations of hot climate regions at Egypt. Aswan and Farafra in south and west Egypt were selected. Two proposed models are simulated by computational fluid dynamics (CFD) in two cases of wind velocities. The two models have different both of dimensions at passages and cavities between buildings and shapes of windward sides of buildings. The wide distances between passages of buildings at the same row and rectangular shapes that have slope exterior wall in two directions at the windward sides are devoted to Aswan. The narrow distances between passages of buildings at the same row and rectangular shapes that have trapezoid courtyards that face wind are devoted to Farafra. The results show that the Aswan model can achieve at the inlet surfaces (windward side of buildings) 1.8-2.4 m/s (60-80% of wind velocity) especially at the second and third rows of indoor air velocity which is the required indoor air velocity for comfort. The Farafra model which uses a courtyard can achieve at the inlet surface 1-1.2 m/s (33-41% of wind velocity) which is less than the required indoor air velocity for comfort. The results can achieve the high rate of wind velocity 38-70% and between two and three times comparing favorably with previous studies that report only 20% of wind velocity at the second row and 15% of wind velocity at the third row.