scholarly journals Model of thermal buoyancy in cavity-ventilated roof constructions

2021 ◽  
pp. 174425912098418
Author(s):  
Toivo Säwén ◽  
Martina Stockhaus ◽  
Carl-Eric Hagentoft ◽  
Nora Schjøth Bunkholt ◽  
Paula Wahlgren
Keyword(s):  
Analytical Model ◽  
Flow Rate ◽  
Numerical Study ◽  
Air Flow ◽  
Wind Pressure ◽  
Air Flow Rate ◽  
Test Set ◽  
Air Cavity ◽  
Thermal Buoyancy ◽  
The Impact

Timber roof constructions are commonly ventilated through an air cavity beneath the roof sheathing in order to remove heat and moisture from the construction. The driving forces for this ventilation are wind pressure and thermal buoyancy. The wind driven ventilation has been studied extensively, while models for predicting buoyant flow are less developed. In the present study, a novel analytical model is presented to predict the air flow caused by thermal buoyancy in a ventilated roof construction. The model provides means to calculate the cavity Rayleigh number for the roof construction, which is then correlated with the air flow rate. The model predictions are compared to the results of an experimental and a numerical study examining the effect of different cavity designs and inclinations on the air flow rate in a ventilated roof subjected to varying heat loads. Over 80 different test set-ups, the analytical model was found to replicate both experimental and numerical results within an acceptable margin. The effect of an increased total roof height, air cavity height and solar heat load for a given construction is an increased air flow rate through the air cavity. On average, the analytical model predicts a 3% higher air flow rate than found in the numerical study, and a 20% lower air flow rate than found in the experimental study, for comparable test set-ups. The model provided can be used to predict the air flow rate in cavities of varying design, and to quantify the impact of suggested roof design changes. The result can be used as a basis for estimating the moisture safety of a roof construction.

scholarly journals Experimental Study and Performance Analysis of a Portable Atmospheric Water Generator

Energies ◽  
2019 ◽  
Vol 13 (1) ◽  
pp. 73 ◽  
Author(s):  
Wei He ◽  
Pengkun Yu ◽  
Zhongting Hu ◽  
Song Lv ◽  
Minghui Qin ◽  
...  
Keyword(s):  
Flow Rate ◽  
Reference Model ◽  
Air Flow ◽  
Operating Conditions ◽  
Water Yield ◽  
Air Humidity ◽  
Air Flow Rate ◽  
Thermoelectric Coolers ◽  
And Performance ◽  
The Impact

Found in some specific scenarios, drinking water is hard for people to get, such as during expeditions and scientific investigations. First, a novel water generator with only two thermoelectric coolers (Model A) is designed for extracting water from atmospheric vapor and then experimentally studied under a small inlet air flow rate. The impact of operating conditions on surface temperatures of cold/hot sides and water yield are investigated, including the air flow rate and humidity. Alternately, to determine the super performance of Model A, a comparative experiment between Model A and a reference model (Model B) is carried out. The results suggest that both the cold/hot temperature and water yield in Model A increases with the humidity and air flow rate rising. Seen in comparisons of Model A and Model B, it is found that, at an air humidity of 90% and air flow rate of 30 m3/h, the total water yield was increased by 43.4% and the corresponding value reached the maximum increment of 66.7% at an air humidity of 60% and air flow rate of 30 m3/h. These features demonstrate the advantage of Model A especially in low air humidity compared to Model B.

Prediction of alpha factor values for fine pore aeration systems

2008 ◽  
Vol 57 (8) ◽  
pp. 1265-1269 ◽  
Author(s):  
S. Gillot ◽  
A. Héduit
Keyword(s):  
Flow Rate ◽  
Oxygen Transfer ◽  
Air Flow ◽  
Domestic Wastewater ◽  
Aeration Tank ◽  
Air Flow Rate ◽  
Factors Affecting ◽  
Domestic Wastewater Treatment ◽  
Alpha Factor ◽  
The Impact

The objective of this work was to analyse the impact of different geometric and operating parameters on the alpha factor value for fine bubble aeration systems equipped with EPDM membrane diffusers. Measurements have been performed on nitrifying plants operating under extended aeration and treating mainly domestic wastewater. Measurements performed on 14 nitrifying plants showed that, for domestic wastewater treatment under very low F/M ratios, the alpha factor is comprised between 0.44 and 0.98. A new composite variable (the Equivalent Contact Time, ECT) has been defined and makes it possible for a given aeration tank, knowing the MCRT, the clean water oxygen transfer coefficient and the supplied air flow rate, to predict the alpha factor value. ECT combines the effect on mass transfer of all generally accepted factors affecting oxygen transfer performances (air flow rate, diffuser submergence, horizontal flow).

An analytical study to evaluate the impact of distributed zone fans on the air flow rate in a mechanical ventilation system

2019 ◽  
Vol 41 (4) ◽  
pp. 507-516
Author(s):  
Fa-Li Ju ◽  
Liying Liu ◽  
Xiaoping Yu
Keyword(s):  
Mechanical Ventilation ◽  
Flow Rate ◽  
Pressure Ratio ◽  
Air Flow ◽  
Ventilation System ◽  
Theoretical Expression ◽  
Air Flow Rate ◽  
Rate Testing ◽  
Fan Speed ◽  
The Impact

Based on air flow rate testing of each branch fan in a distributed fan ventilation system under different branch air duct inlet static pressures, the conclusion can be drawn that there is a branch fan air flow rate deviation phenomenon. The air flow rate of the branch fan increases with the branch air duct inlet static pressure at the same branch fan speed, and the branch fan hinders the air flow rate in some cases. In this study, a theoretical expression of the deviation of the branch air duct design air flow rate was established, and the influencing factors of the deviation were determined to include the branch air duct resistance characteristics, branch fan performance, and branch air duct inlet pressure ratio. A graphic analytical method for determining the deviation of the branch fan design air flow rate was also proposed. Both methods can provide a theoretical basis for calculating and analysing the deviation of the branch fan design air flow rate in a distributed fan ventilation system. Practical application: This paper provides new data on the performance of a distributed fan ventilation system. Our results could be used to evaluate the impact of distributed zone fans on the air flow rate in a mechanical ventilation system. Crucially, we not only propose two types of methods that can be applied to predict deviations of the air flow rate in a distributed fan ventilation system caused by the branch air duct inlet static pressures but also obtain the factors that are important for understanding the true impact of the deviation of the branch fan air flow rate. This study lays an important foundation for the design and operation of building mechanical ventilation systems.

scholarly journals Air Flow Model for Mixed-Mode and Indirect-Mode Natural Convection Solar Drying of Maize

2020 ◽  
Vol 10 (2) ◽  
pp. 1
Author(s):  
Isaac N. Simate
Keyword(s):  
Natural Convection ◽  
Flow Rate ◽  
Mixed Mode ◽  
Flow Model ◽  
Air Flow ◽  
Solar Drying ◽  
Air Flow Rate ◽  
Thermal Buoyancy ◽  
Early Stages ◽  
Grain Moisture

An air flow model for mixed-mode and indirect-mode natural convection solar drying of maize to help understand the factors that influence air flow in the dryer is presented. Temperatures at various sections of the dryer obtained from drying experiments were input to the air flow model to predict the respective thermal buoyancies. The air flow rate was determined by balancing the sum of the buoyancy pressures with the sum of the flow resistances in the various sections of the dryer. To validate the model, the predicted air flow was compared with measured air flow from experiments. For both the mixed-mode and indirect-mode, the biggest driver of the air flow is the thermal buoyancy created in the collector, while the grain bed is the dominant pressure drop. Thermal buoyancy on top of the grain bed is largely responsible for the variation in air flow, translating into low mass air flow during the early stages of drying when grain moisture is high, and higher air flow in the later stages when grain moisture is low. The heating of the grain bed by direct radiation in the mixed-mode translates into a slightly higher air flow rate than the indirect-mode. The implications are that a thinner grain bed results in shorter drying time as it has a higher air flow rate than a thicker one. To mitigate the low air flow at the early stages of drying, the collector length should be appropriately designed for a desired air flow.

A Numerical Study of the Effect of Bubble Layer on Propulsion Performance of Deep-Vee Hull Form Ship

2012 ◽  
Vol 479-481 ◽  
pp. 2551-2556
Author(s):  
Qi Yao Li ◽  
Ou Yongpeng
Keyword(s):  
Flow Rate ◽  
Liquid Mixture ◽  
Numerical Study ◽  
Air Flow ◽  
Open Water ◽  
Air Flow Rate ◽  
Hull Form ◽  
Propulsion Performance ◽  
Water Conditions ◽  
Bubble Layer

A numerical study of the effect of Bubble Layer on Propulsion performance is carried out by using the Air-Liquid Mixture Model. The effect of factors, such as air flow rate, air injected positions on propulsion performance are investigated. The following findings are obtained: (1) In the open water conditions, the air injected inlet is farther away from the propeller center, the little more effect on the propulsion performance; Along with the increase of air flow, the effect of bubbly layer on the propulsion performance increases slightly. (2)Because of the hull form and the clearance between the bottom and the blades tip, bubble layer has small effect on propulsion performance of deep-vee hull form ship propelled by propeller.

The Impact of Air Flow Rate on Photobioreactor Sparger/Diffuser Bubble Size(s) and Distribution

2013 ◽  
Author(s):  
Anil R. Kommareddy ◽  
Gary A. Anderson ◽  
Stephen P. Gent ◽  
Ghazi S. Bari
Keyword(s):  
Flow Rate ◽  
Bubble Size ◽  
Air Flow ◽  
Air Flow Rate ◽  
The Impact
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