Minimum Air Flow Rate Research Articles

Optimised hydrodynamic parameters for the design of photobioreactors using computational fluid dynamics and experimental validation

A numerical simulation using computational fluid dynamics (CFD) was utilised to investigate the flow hydrodynamics of cylindrical bubble column type photobioreactors (PBRs) with a 30 l culture medium. To establish the reliability of the simulation study, the CFD model was validated using particle image velocimetry (PIV) computed data under various air flow rates. There were 32 simulation cases for the study comprising two PBR designs, four air flow rates and four nozzle size diameters. Hydrodynamic analyses such as % volume of dead zones, average circulation time and turbulence intensity inside the simulated PBRs were evaluated. Results have shown that the most appropriate PBR for microalgae cultivation was a design with internal baffle and an extended cone-shaped bottom section. In addition, the recommended nozzle diameter was found to be 10 mm and a minimum air flow rate of 0.10 vvm. To eliminate dead zones inside the PBR, the flow rate can be slightly increased but should not exceed 0.15 vvm. Practical evaluation through laboratory experiments has further confirmed the results of the study where the biomass concentration of Chlorella vulgaris from the proposed PBR was significantly higher compared to the standard PBR design. Based on the numerical investigation and practical evaluation, the improved PBR can be seen to be more effective in culturing microalgae particularly for larger scale mass production.

Stability and accuracy of variable air volume box control at low flows. Part 1: Laboratory test setup and variable air volume sensor test

Variable air volume systems with direct digital controllers have been widely adopted in the HVAC system of commercial, industrial, and large residential buildings because they provide better energy efficiency and occupant comfort. Normally, a variable air volume terminal unit defines a minimum airflow rate to satisfy the space ventilation requirement and/or the proper operation of a terminal heating coil, if so equipped. However, it has been found that variable air volume terminal units often fail to perform as expected at the minimum airflow range (below 500 fpm [2.5 m/s]). Under such a flow range, the embedded airflow sensor becomes inaccurate, and the designed minimum airflow rate is less than the minimum controllable airflow rate. This results in a series of problems, including lack of ventilation, uneven airflow control, reduced damper and operator life, and energy waste. Through designed laboratory and field tests, this study (ASRHAE Research Project RP-1353) aims to identify the major factors that cause inaccuracy and instability issues in variable air volume terminal units and the relationship between the major factors and performance of the airflow sensor, controller, and terminal unit system. Laboratory tests performed in this study included a variable air volume sensor test, controller test, and system test. Four variable air volume boxes from three manufacturers and four controllers from four manufacturers were tested systematically. Two identical test beds with high accuracy (±0.5%) reference airflow meters were designed and constructed in the test facility. The size of the reference airflow measuring stations was carefully selected to provide maximum airflow measuring accuracy and maximum available system pressure drop. This article describes the laboratory test setup and summarizes the variable air volume sensor test results. A companion article summarizes the controller test, system test, and field test results. From the variable air volume sensor test, three factors, namely, inlet conditions, low variable air volume damper positions, and low airflow rates, are identified as strongly impacting variable air volume terminal unit performance.

Expert-Based-Computer Aided Design and Component Selection for Dust Collection Systems

Environmental regulations regarding industrial pollution are getting tougher to keep high air quality. The design of proper system to meet specific requirements relays on selection of different filtration components. Components are available in a wide range of designs to meet various requirements of air filtration level, quantity, and characteristics of the contaminants to be removed. An expert system package for the selection of dust collectors' components is developed. The developed package utilizes database as well as knowledge base to aid the selection process based on practice and past experience. Industrial case studies are demonstrated to test the developed package and compare its results against the current systems design. The developed package in one case study estimated a minimum air flow rate of 60000 m 3 /hr, which is 30% higher than the existing value and a static pressure drop due to ducts of about 1900 Pa which is 30 % lower than the current value leading to improved air quality. In another case study the package results indicated the need to minimize the value of air flow to be 2300 m 3 /hr, which is 50% lower than the current value and increase the external static pressure to 1700 Pa which is 70% higher than the existing design value leading to better filtration efficiency.

A study on the control method of single duct VAV terminal unit through the determination of proper minimum air flow

The objective of this study was proposed a control method for the minimum air flow rate of a VAV terminal unit at an office building. The minimum air flow rate of the VAV terminal unit is the key factor affecting the thermal comfort, indoor air quality (IAQ), stratification and energy consumption, depending on the operating mode of the VAV system. Therefore, selecting the proper minimum air flow is very important. In this study, an algorithm was proposed considering the IAQ and stratification. The vertical air temperature was analyzed to find the supply air temperature that did not cause stratification. The integrated control algorithm with an air flow increase model in the VAV terminal unit and outdoor air intake rate increase model in the AHU was developed by comparing the energy consumption. Finally, the existing and proposed control algorithms were compared through a simulation. The proposed method was found to be more effective than the existing control method. The proposed VAV terminal unit control method satisfies all the conditions of indoor thermal comfort, IAQ and stratification issue. As a result of the energy comparison with the existing control method, the method satisfies not only the indoor thermal comfort, IAQ and stratification issue, but also reduces the energy consumption.

Development of a Terminal Control System with Variable Minimum Airflow Rate

A constant minimum airflow rate is used in conventional Single Duct Variable Air Volume Terminal Box control sequences. This control sequence can cause occupant discomfort or use excessive energy under partial load conditions. If the minimum airflow rate is higher than required; terminal boxes will have significantly more simultaneous heating and cooling; and AHUs will consume more fan power. Buildings will have indoor air quality problems if the minimum airflow rate is less than required. Many engineers and researchers have investigated advanced variable air volume terminal box control algorithms without a system retrofit for thermal comfort; indoor air quality and energy savings. In this study a developed control system with variable minimum airflow rate for Single Duct Variable Air Volume Terminal Boxes was applied and validated using an actual building and evaluated for comfort; indoor air quality and energy consumption. The energy consumption and thermal performance of terminal boxes using the conventional and proposed control algorithms were compared.

Designing and Evaluating an Innovative Solar Air Collector with Transpired Absorber and Cover

Applying solar collectors is a popular tool for harnessing solar energy. In this work, a flat plate solar air collector was investigated under direct solar radiation in an endeavor to enhance the thermal efficiency of solar air collector with a slatted glass cover, perforated absorber aluminum sheets (porosity 0.0177, 0.0314 and absorber thickness of 1.25, 2.5 mm) which is the most suitable for a solar dryer. The effects of porosity and thickness on absorber performance of collector were evaluated. Six levels of air mass flow rates (0.0056 to 0.0385 kg m−2 s−1) were adopted. The tests were conducted in three replications on very clear sky days in September and October. The experimental results showed that thermal efficiency of collector was increased by an increase in the porosity of the absorber. The absorber with lower porosity showed a better thermal efficiency at lower air mass flux. In the minimum air flow rate, absorber efficiency with porosity 0.0177 and 0.0314 was 0.31 and 0.29, respectively, whereas at the maximum flow, efficiency showed an enormous change of 0.83 and 0.88, respectively. This solar air heater can be used for drying agricultural products, heating the space of greenhouse, and so forth.

Amount of airflow required for olfactory perception in laryngectomees: a prospective interventional study

To determine minimum airflow rate required for olfactory stimulation in successfully rehabilitated laryngectomised patients after learning the polite yawning technique (PYT) and to confirm the hypothesis that sense of smell is rehabilitated once the nasal airflow is re-established. Prospective open interventional trial. Tertiary academic hospital. The study population comprised 100 laryngectomised patients. The control group consisted of 100 non-laryngectomised patients of similar age and sex. Rhinomanometry was used to measure air flow in the right and left nostrils, respectively, while the Smell Diskettes Olfaction test (SDOT) was used to test each individual's sense of smell. The primary endpoint was increasing the airflow, while the secondary endpoint was improvement in the Smell Diskettes Olfaction test score after learning the polite yawning technique. The difference in the Smell Diskettes Olfaction test results before and after introducing the polite yawning technique was statistically significant (F = 53.077; P < 0.001). The number of accurately identified odours increased with each measurement. There was a significant difference among rhinomanometric measurements of airflow through the right (F = 65.002; P < 0.001) and left nostrils (F = 75.465; P < 0.001). Nasal airflow improved with each measurement. The minimum airflow required for olfactory stimulation in successfully rehabilitated patients was approximately 60 cm(3) /s. The control group had considerably better airflow in both nostrils than the laryngectomised group. The difference between the total number of rehabilitated (normosmic) patients (48%) in the laringectomised group and normosmic participants (56%) in the control group (z = 1.132; P = 0.129) was not statistically significant. The number of odours identified by laryngectomised patients increased with the volume of nasal airflow. The number of patients with rehabilitated olfactory function approximated the percentage of normosmic individuals in the non-laryngectomised population. These findings confirm the hypothesis that sense of smell is rehabilitated once the nasal airflow is re-established.

The Merkel equation revisited: A novel method to compute the packed height of a cooling tower

In this work, a new methodology of analysis and computation is presented which simplifies calculation of the packed height in a counter current cooling tower, especially for design and cost optimization studies. An algorithm is presented with an implementation in MATLAB to compute the thermodynamic minimum air flow rate for the desired cooling. Combining the Merkel equation and a standard empirical mass transfer correlation, the packed height is shown to be independent of the water flow rate and tower diameter, and dependent only on the excess air flow. The relationship is unique for a given cooling range of water and inlet air wet bulb temperature. A simple power law regression is used to approximate this relationship and results are presented for Vertical Corrugated Packing.

Post-occupancy evaluation of 20 office buildings as basis for future IEQ standards and guidelines

The design of modern work environments must anticipate high levels of spatial and technological change by providing responsive thermal and air quality delivery systems. Building occupants will require indoor environmental conditions to support computer-intensive activities as well as paper-based tasks. However, the current standards and guidelines for indoor environments were predominantly developed without consideration for these modern office variables. This limitation may lead to an occupant's dissatisfaction with the indoor environmental quality (IEQ) as well as unnecessary energy use. With support from the U.S. General Services Administration, a research team at Carnegie Mellon University performed a wide range of post-occupancy evaluation studies in Federal office buildings across the U.S. over 7 years. IEQ conditions were determined with spot and continuous measurements, while the building occupants’ satisfaction with environmental attributes was surveyed simultaneously. Statistical analyses of over 400 workstations linked characteristics and environmental qualities to occupants’ satisfaction. The results challenged the validity of current IEQ standards and guidelines. Specific recommendations for improving current standards and guidelines are outlined here to help enhance environmental conditions in workplaces for future design projects. These include raising summer temperature set-points, separating task and ambient lighting requirements, and establishing minimum air flow rates at each workstation.

IMPROVED SULFUR DIOXIDE FUMIGATION OF FRESH LONGAN USING A VERTICAL FORCED-AIR TECHNIQUE

ISHS International Symposium Postharvest Pacifica 2009 - Pathways to Quality: V International Symposium on Managing Quality in Chains + Australasian Postharvest Horticultural Conference IMPROVED SULFUR DIOXIDE FUMIGATION OF FRESH LONGAN USING A VERTICAL FORCED-AIR TECHNIQUE

Specification of airflow rates in laboratories

People who work in research laboratories are potentially exposed to many kinds of hazards. These hazards may include airborne contaminants generated from processes involving chemical, biological and radioactive materials. Providing safe and productive laboratories requires ventilation to control hazardous airborne contaminants and provide comfortable working conditions. However, laboratory ventilation systems are expensive to operate with supply and exhaust of conditioned air being the largest consumer of energy in a laboratory building.(1) With energy costs continuing to increase, the desire to minimize airflow has placed pressure on laboratory safety managers to justify and reduce laboratory airflow rates. Criteria for laboratory airflow rates are generally expressed as air changes per hour (ACH) with rates ranging from 4 ACH to 15 ACH depending on the type of laboratory or operating mode. ACH is meant to express the number of room volumes exchanged in an hour. However, ACH is purely theoretical, too simplistic and fails to account for airflow patterns and the factors affecting contaminant accumulation and dilution within the lab. Specification of ventilation rates to ensure satisfactory dilution of contaminants in a laboratory requires understanding the lab procedures, identifying sources of contaminant generation, determining emission rates, knowing exposure limits and predicting the accumulation and dilution of hazardous concentrations. Reliance on a single airflow rate, or ACH, for protection of laboratory personnel is imprudent and can lead to a false sense of safety. Dilution is not the solution to laboratory safety. It may lead to the presumption of safety and increased energy consumption. Safe, productive and energy efficient laboratories require specifying minimum laboratory airflow rates that ensure proper performance of exposure control devices, provision of comfortable working conditions and training laboratory personnel to recognize laboratory hazards and take appropriate action when required. This paper describes some of the issues required to evaluate and determine appropriate ventilation guidelines for ensuring safe laboratories.

Evidence of a non-dimensional parameter controlling the flooding of PEMFC stack

Water management is a key issue to get satisfactory and stable Polymer exchange membrane fuel cell (PEMFC) performances. The work reported in the present paper focuses on the determination of the operational conditions when using PEMFC stack working with ambient air without extra humidification. The objectives are to reduce as much as possible the auxiliaries consumptions. As far as the reaction air blower is concerned, the specific goal of the present tests is to find the minimum air flow rate to feed the PEMFC stack in order to prevent flooding. Our particular interest concerns the control of a PEMFC stack to power a prototype vehicle for the Shell Eco Marathon race. Tests are then conducted on a wide range of stoichiometry, for different values of current and stack temperature using ambient air. Flooding is shown to depend on all these parameters. A water balance calculation is developed comparing the amount of water produced by the electrochemical reaction to the amount of water transported as vapour in the exit air flow minus the amount of water incoming the stack in the ambient air. A non-dimensional number called the Flooding Number is constructed. This balance is first considered in the ideal case with the theoretical flow rate of reactants and products. It is shown that the stack temperature and the stoichiometry are the main order parameters and that conditions of ambient air have only secondary effects on the water balance. In a second step, the Flooding Number is evaluated for all the experimental tests. A critical Flooding Number appears clearly delimiting the range of operational conditions for which stack flooding appears. This result allows us to control the air blower and the cooling fan during the runs at the Shell Eco Marathon 2007 race in order to reduce hydrogen consumption due to auxiliaries. The non-dimensional number exhibited in the present paper is believed to be relevant to stack flooding. It can be used for any PEMFC stack to make clear the limit of appearance of stack flooding.

Characteristics of flow in wet conical spouted beds of unequal-sized spherical Particles

Interparticle forces, developed in wet spouted beds composed of a mixture of spherical particles with different size distributions, intensify particle segregation mechanisms interfering in gas distribution inside the bed and, consequently, in the spouting flow characteristics. Therefore, this paper is aimed at describing the effect of interparticle forces on the air-solid flow distribution in conical spouted beds of unequal-sized particles coated by a thin glycerol film. Experimental results show that both the minimum spouting airflow rate and the minimum spouting pressure drop decrease as the amount of glycerol added to the bed increases. In addition, simulated results of the annular air velocity along the bed height showed that, at the base of the column, the radial component of the inertial force is high enough to break liquid bridges between particles and carry these particles out along the spout. Moreover, as the glycerol concentration increases, the spout diameter increases along the bed height. Such changes in the air-solid flow can maintain the spouting regime for higher glycerol concentrations as shown by experimental data.

ASCENT OF INDOOR HUMIDITY BY THERMAL STORAGE IN BUILDING MASS FOR COOLING AND THE EFFECT OF COUNTERMEASURES

Thermal storage in building mass for cooling, which stores thermal energy in building thermal mass, has been reported to bring changes in indoor thermal environment such as the improvement of radiative environment and the ascent of humidity. The field measurement in a building in Tokyo showed that the tendency of the humidity ascent. Several cases of simulation are conducted to study the effect of the storage hours on the degree of the ascent. The study shows that the more the humidity ascent is, the longer the storage hours are. Countermeasures are necessary to keep good thermal environment and to meet the legal regulation in Japan. The effect of two measures is compared: 1. decrease of the minimum air flow rate of variable air volume system, 2. independent treatment of outdoor air. The former might bring the humidity ascent when the indoor thermal load is small. Control parameters of AHU for outdoor air should be set appropriately for the latter.

취출구를 가진 덕트의 공기분배장치 설계

The purpose of this paper is to obtain design method of air-distribution system. Air-distribution system is composed of blower, duct, diffusers and measuring equipment. The air-flow rate from each diffuser is not equal. The air-flow rate is calculated with the combined equations which are Bernoulli's equation, continuity equation and minor loss equations. Inlet condition and outlet condition are adapted in each duct system. Then square difference between function of maximum air-flow rate and minimum air-flow rate is used as an object function. Area of diffuser and velocity are established as constraints. To minimize the object function, the optimization method is used. After optimization the design variables are selected under satisfaction of constraints. The air-distribution system is calculated again with the result of optimized design variable. It is shown that the air-distribution system has the equal air-flow rate from diffusers.

VAZÕES ESPECÍFICAS MÍNIMAS PARA SECAGEM DO MILHO COM AR NATURAL EM BOTUCATU-SP

Minimum airflow rates are recommended to ambient air grain drying to reduce fan power requirements and grain overdrying. Historical weather records from 1971 were selected to determine airflow rates for drying corn with ambient air in Botucatu, Sao Paulo State. 1971 was the poorest drying year in the 10-year period from 1971 through 1975 and l977 through 1981. In the simulation of ambient air corn drying initial moisture contents, wet basis used were 22%, 20% and 18% within allowable storage periods of 7, 11 and 29 days, respectively, for monthly average dry bulb temperature of 23.40C, the upmost in the 10-year period. The average dry bulb temperature and relative humidity were calculated from the hourly data recorded at the Faculdade de Ciencias Agronomicas, Botucatu county, Sao Paulo State, weather station. Due to impracticable continuous drying occurrence, only selective ambient air drying was considered, within daily periods from 9 a. m. to 5 p.m., 9 a. am. to 6 p.m. and 8 a.m. to 6 p.m. The simulation results showed that the minimum airflow rates decrease rapidly with decreasing grain equilibrium moisture content a few percentage points. Furthermore, the reduction in daily fan operation a few hours requires significant increases in minimum airflow rates to accomplish ambient air corn drying within the permissible storage time.

Reed vibration, pressure, and airflow in Western free-reed instruments

Western free reed instruments, including the reed organ, harmonium, harmonica, and accordion, use asymmetric free reeds that are able to maintain sustained oscillation in the absence of a pipe resonator. Studies of the motion of air‐driven American reed organ reeds and accordion reeds have been made that yield the volume air flow through the reed opening as a function of time. Reed vibration waveforms are obtained using a variable impedance transducer, and corresponding sound pressure waveforms are obtained from a probe microphone at positions near the reed opening on both sides of the reed tongue. The airflow waveform is obtained by integrating the pressure waveform and using a calculated area function and is calibrated using the measured average and minimum airflow rates. The results are compared with earlier preliminary studies as well as recent theoretical work. For pressure and airflow waveforms among different types of reeds or for the same reed at different blowing pressures, differences can be understood in terms of the configuration of the reed and reed frame system, as well as the amplitude of reed vibration. [Research supported by NSF REU Grant No. PHY‐0354058.]

Cooling performance of horticultural produce in containers with peripheral openings

Air pathways were investigated for peripheral and central opening configurations of package during horticultural produce forced-air cooling process using a research tool previously developed. Total opening areas of 0.67, 2, 4, and 8%, formed by combining three, four or eight holes of 0.67, 1, or 2% distributed at the bottom and top, corners or center line of the package surface, were tested for airflow rates ranging from 0.125 to 3.9 L s −1 kg −1 and compared to the fully open configuration. Air pressure drop, rates, and uniformities of cooling were measured. Enlarging opening area increased the cooling efficiency. The higher the airflow the greater the rate and the uniformity of cooling process. However, gravity force effect influenced the enhancement of air distribution uniformity at the minimum airflow rate studied. When the container design options are limited to central or peripheral openings, the bottom and top opening configuration is preferred for greater cooling performance.

A New Method to Investigate the Suspension of Solid Particles in Rectangular Tanks Agitated by a Source-Line Bubble Plumes

It is well known that when mechanically agitated vessels are used, the forces applied by the agitator moving in the fluid contained in the vessel maintain flow patterns in the fluid. In the tanks under study, the flow patterns are maintained by an air curtain rising within the vessel. The movement produced by this air curtain interchanges fluid between different parts of the vessel, maintains particle suspension and promotes gentle mixing that has applications in the field of biotechnology. The experimental work carried out in this article is focused on the study of mixing and the behavior of solid particles, using an air curtain as the means of mixing. The research included the fluid motion visualized by white polystyrene beads, video recording and a new method to investigate the suspension of solid particles. The new method introduced an index of deviation from acceptable suspension and was used as a tool to select the best design of tank-aerator combination suitable for suspension. Experimentally, the suspension was found to be a function of air flow rate and the type of aerator used. Two distinct factors were identified, namely, the aerator location and the shape of the aerated tank. It was confirmed that the best location was in the middle of the tank, and the minimum air flow rate to keep all the particles in suspension was reached when the aerated surface was a rectangular surface in which the length is twice the width. In addition, a relationship between fluid velocity, air flow rate, flow patterns and baffles disposition are discussed. Finally, a correlation for the velocity in the liquid was developed and found to be adequate in the scale-up of aerated tanks agitated by an air curtain.

An Experimental Study of Fluid Motion in a Bioreactor Agitated by Line-Source Bubble Plumes

The focus of this study is the movement of the fluid contained in a rectangular bioreactor agitated by line-source bubble plumes. The research included visualization of fluid motion by white polystyrene beads and supported by intensive video recording. Water velocity at the surface and at the bottom, as well as the velocity along the depth, have been measured in two aerated rectangular vessels (36 × 10−3 m 3 and 2 m 3 in volume) using tap water, as a function of the characteristic velocity (qg)0.33. It was confirmed that the best location of the perforated tube was in the middle of tank and the minimum air flow rate to maintain particle suspension was reached when the aerated surface was a rectangular surface in which the length is twice the width. In addition, the relationship between the fluid velocity, air flow rate, height of the liquid and flow patterns were discussed. Finally, a correlation for the velocity in the liquid was developed and found to be adequate in the scale-up of aerated tanks agitated by an air curtain.