Home
Extranet Login
Internal
CONTACT
FabricAir, Inc.
312-A Swanson Drive
Lawrenceville, GA 30043, USA
Phone (502) 493-2210
Fax (502) 493-4002
info@fabricair.com
04.06.2004
Documented Air Distribution in Trade Fair Centre
In the past, consultancy on air distribution was based on theoretical considerations going by standard situations and the personal experience of the consultant. To-day, CFD simulations may be used to move one step further and thus offer guidance based on the particular assignment and the specific conditions.
By Kristian Lykkemark
Messecenter Herning needs great flexibility when it comes to organizing and setup of fairs and other arrangements. Thus placing heavy requirements on the air distribution system which will distribute the large cooling volume and still work under all conditions.
A condition working optimally under all conditions is in practice virtually impossible to achieve but it is important to be aware of the need of flexibility and to allow for this when distributing the air. By examining the effect of the air supply for a number of different cases, we comply with and illustrate how the air distribution will behave under various conditions.
FabricAir A/S assumed the task of dimensioning and documenting an air distribution system for our client. Our choice was a textile system with nozzles. The nozzle throw length has been adapted so as to provide an acceptable indoor climate in the common zone.
Documentation through CFD
To achieve a good result, in addition to the experience gained by FabricAir A/S on cooling work, CFD (Computational Fluid Dynamics, see fact box) was applied to examine and illustrate the effect of the textile system on the indoor climate. Using CFD made it possible to optimize the solution within the framework and limitations of a project in which cooling is after installed.
Air Distribution in Hall D
Hall D used to be one of the halls having complaints about too high temperatures. A poor thermal indoor climate is highly important to exhibitors and visitors, and this contributes to determine whether a fair is successful or not.
Based on Hall D, it is examined how the air is distributed, and how this affects temperatures and air velocities.
To provide a good solution * and to throw light on any problem complexes, it was examined how the air was distributed based on three cases.
Case l has an evenly distributed thermal load of 45 W/m2.
Case 2 has a thermal load of 67.5 W/m2 distributed on zones l + 2.
Case 3 has a thermal load of 135 W/m2 distributed on zone 2.
The total thermal load is identical for the three cases, equivalent to 45 W/m2 which is the dimensioning thermal load. For the zone distribution for the thermal loads, please refer to Case l.
The simulations are limited to a section of hall D, allowing for the fact that the cut is made in symmetry lines in the building construction. Thus rendering it possible to use the results to evaluate the air distribution in the entire hall. This hall is 7.2 m high, and the cut is 12 m wide.
Case l velocities are in the common zone below 0.2 m/s, except for the area right under the channel where the velocities reach 0.3 m/s. The temperature is uniform, without any appreciable temperature gradients. The air is being equally distributed, without any "dead" zones. With an evenly distributed thermal load, the textile system works smoothly and is capable of lowering the temperature throughout the hall.
Case 2 velocities are in the common zone below 0.2 m/ s, and right under the channel below 0.25 m/s. For Case 2 for which the thermal load is mainly supplied at the left hand part of the hall, it may, as expected, be seen that the temperature is a couple of degrees higher than at the right hand side. Despite the asymmetric heat distribution, the whole hall is ventilated, and the temperature in the common zone is around 25 °C.
CFD - A Fine Though Time-Consuming Tool
CFD is a method by which a computer model is built up of the problem complex required to be illustrated. The model describes the aspects relevant to flow conditions and energy flow.
Flow conditions and energy flow are described by a number of transport equations, differential equations, which apply to the total flow field.
This model is divided into a number of cells (control volumes) with a point in each. The differential equations are converted by a numerical method into differential equations applying to each particular point in the control volume. For each point of the control volume, the equations controlling the flow will be solved.
The equation system is implemented in a CFD software handling the description of the computer model, the division of the model in control volumes, as well as computation of the equation system based on the marginal values required to solve the equation system. To reach a solution within a reasonable time horizon, heavy computer power will frequently be required though this will depend on the complexity and extent of the task.
CFD is time as well as resource demanding. To get a satisfactory benefit from CFD, it is based on the assumption that the company will already possess substantial know-how on air distribution.
Case 1:
Thermal load of 45 W/m
2
evenly distributed.
Case 2:
Thermal load of 67,5 W/m
2
distributed on zone 1 & 2
Case 3:
Thermal load of 135 W/m
2
distributed on zone 2
Case 1:
Figure 1, Velocities
Case 2:
Figure 3, Velocities
Case 3:
Figure 5, Velocities
Case 1:
Figure 2, Temperatures
Case 2:
Figure 4, Temperatures
Case 3:
Figure 6, Temperatures
The velocities for Case 3 are below 0.2 m/s in the common zone, and right under the channel a little higher, at appr. 0.3 m/s. The temperature does not have the same horizontal gradient as for Case 2 but is around 24° C. The air is distributed uniformly, and there are no areas which are not cooled. At a centred thermal load, the air distribution is uniform, and the textile system is capable of lowering the temperature in the entire hall.
Evaluation
For the three cases, the velocity level in the common zone is below 0.2 m/s, however, except for right under the channels where velocities of up to 0.3 m/s may occur. There is no significant difference in the velocities in the common zone or under the channels for the three cases. The temperature gradients are horizontally below 4 ° C and vertically below 3 ° C; the highest gradients are found for Case 2 for which the load is asymmetric. The air is distributed equally in the three situations, and there are no zones which are not cooled. The textile system thus ensures – at different thermal positioning – that the air will come down in the common zone without too high velocities and at a uniform temperature level. Thus providing for a satisfactory indoor climate, and the flexibility required in using the halls is maintained.
Putting Into Perspective
When it comes to consulting in ventilation and distribution of air, two projects will very rarely be the same. So advice allowing for the individuality of the project will be required. The traditional dimensioning form is based on standard situations documented by several lab tests. This knowledge is transferred to situations not necessarily like those from which the built-up theory emanates. Achieving really good results may thus prove difficult if no full-scale or model test of the project is run.
One alternative is that of using CFD which will allow for examining a considerable number of sketch proposals before selecting the final solution. This allows for meeting and illustrating any problem complexes and pay special attention to this at an earlier stage of the process. Combining experience, theory, and CFD will pave the way for getting a better starting point for the decision-making process and a better result for the building owner.
To FabricAir A/S, as a supplier of textile channels rarely two assignments will be alike. The textile system will always be adapted to the particular situation, and consequently each and every project will have to be handled as an individual task, and the consulting on this must be individually adapted.
CFD Increasing Understanding
CFD enables us to improve solutions and offer better guidance on the air distribution. For one thing, we may optimize the use of the cooling capacity, prevent short circuits between air supply and exhaust, and ensure that the air will come down in the common zone, thus resulting in a satisfactory indoor climate. To our client, this will entail a better result, to be achieved in an interaction between know-how/consulting and a close cooperation between the parties involved. Such close cooperation will enable us to deliver a synergetic result. Buyer will get a better understanding of what is supplied, why, and which problems there may be in relation to a solution. CFD will help to increase the understanding and improve the communication between the parties involved. Thus forming the basis of a discussion of problem complexes and solution proposals so that the right solution may be chosen.
CFD does, however, not inherently offer a guarantee for a better solution. It is important to know the function of the product/equipment, and it will take considerable validation based on lab measurements to quality assure the CFD simulations.
The application of CFD is best used by examining already at an early stage of the projecting phase the consequences of various initiatives. At a later stage, this may contribute to optimize the final solution and allow for the details.
Kristian Lykkemark
is a graduate engineer from Aalborg University, specializing in Indoor Climate and Energy Economy. With FabricAir A/S, Kristian is on full time handling of CFD simulations of ventilation solutions with textile channels.
