THERMOFLUIDS STUDY IN A SPACE HERIOT WATT UNIVERSITY – DUBAI
Table of Contents Abstract Literature Review Details of the test Test procedure Analysis and Results Conclusion and Recommendations Bibliography
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Abstract This study focuses on the topic of thermofluids and its application in the built environment. The task of recording various measurements was conducted by three groups, which were then collaborated to build the CFD model. The model was then simulated and analyzed to assess the performance of existing air distribution system installed in the test room. This research also appraises whether the examined space is reasonably designed for the specified activity or not. The main aims and objectives of this study are:
To evaluate whether the installed system provides satisfactory ventilation and air conditioning to the room. To combine the basic principles of airflow and heat transfer and redesign the HVAC system, to further enhance the efficiency and thermal comfort within a space. Not only will the HVAC system be redesigned, but even the room design characteristics will be optimized for better results. The criteria to appraise the performance of the existing air distribution system will also be specified.
Lastly recommendations to enhance user satisfaction and ventilation of the room have been considered.
Introduction This project involves assessing the suitability of the existing air distribution design serving the welding room, and consequently suggesting measures to improve the room conditions and effectiveness of the HVAC system. The testing for this study will be carried out using CFD modeling. A CFD model of the Welding room will be created using the ANSYS Fluent 16.2 software and simulated for the various design alternatives. CFD studies of air quality and thermal comfort levels of building occupants are an inevitable part of sustainable building design practices.
Literature Review HVAC consists of heating, ventilation, air-conditioning and controls. Controls determine how HVAC systems function to meet the design goals of comfort. Good HVAC design prevents air pollutants from negatively affecting the health of occupants. Ventilation is utilized to expel air contaminants from a worker's breathing zone, in a welding bay. (Canadian Centre for Occupational Health & Safety, 2015) The purpose of a suitable HVAC system is to create the proper mix of temperature, humidity, freshness and air velocity in the occupied space. Thermal comfort is maintained when the heat generated by human metabolism dissipates, therefore maintaining thermal equilibrium with the environment. (Bojić, n.d.)There are six fundamental factors that influence thermal comfort. The physical factors are air temperature, air velocity, radiant temperature and humidity while the personal factors include clothing and individual metabolism rate of the users. (KRUEGER, n.d.) If the air temperature is above 24°C it results into more uncomfortable thermal comfort level and thus the productivity decreases. (Google Books, 2015) Computational fluid dynamics (CFD), allows visualization of flow velocity, density, thermal impact and chemical concentrations for a space where the flow occurs. There are several advantages of CFD: short analysis time, lower test costs compared to lab experiments, availability of comprehensive statistics. (Mahu, 2012) The time to achieve a good estimation of the flow inside the occupied space using CFD method can be reduced by not modeling the small details, e.g. the interior of the diffusers or the exact shape of the objects inside a room. (Martinez, 2014) Using CFD, fluid flows can be simulated, and HVAC performance can be examined without in reality installing the HVAC system or building its model. The accuracy and speed of the simulations are controlled by the size of the mesh, the turbulent model used, and the numerical technique used to solve the equations. Therefore, critical problems can be identified, and solutions can be developed to boost the HVAC performance within a building. (Patel, 2014)
Details of the test Location: ‘Welding Room’, present in the ground floor mechanical lab of the Heriot Watt University – Dubai Campus. Date of the test: The measurements were recorded on 25th October 2015. Equipments used: FLIR-E40bx Thermal Camera was used to measure the ambient temperatures, by focusing the camera on the specific surfaces. A measuring tape was used to measure the room dimensions. An Anemometer was used to measure the air temperature and air velocity at the supply and extract vent, placed in the room. A heat stress meter was used to measure the ambient temperatures and relative humidity.
Thermal Camera
Measuring Tape
Anemometer
Heat Stress Meter
Problem description: The case modeled includes air flow inside and heat gains from outside into an enclosed space. The welding room is made of concrete, has interior and exterior glass windows and a wooden door. The room is illuminated through three light sources. Based on this initial data a geometric model was built. Welding Room
Test procedure Lab measurement: At first, measurements for various room parameters (like room dimensions, air velocity, air temperature and ambient temperature) were recorded by different groups of students undertaking this course. Ambient temperatures were taken at different points in the room and at various heights 0.5m, 1.5m and 2m.
Positioning for surface temperature measurement
Room Dimensions
Measurements for air, radiant & wet bulb temperatures & humidity
3D AutoCAD Model •Then depending on the room dimensions, a 3D model of the welding room was created in AutoCAD. This model was then exported to ANSYS Fluent Workbench 16.2, to assess the efficiency of the existing air distribution system as well as simulate the modified form of the welding room.
Schematic of inlet & outlet vents with measurement locations •
For the measurement of the air temperature and air velocity at the air distribution system 20 points were plotted along the supply and return vents each. All the recorded room parameters have been listed in the tables below.
Measurements recorded at the inlet & outlet vents
CFD Modeling: Simulations of the CFD welding room model were conducted as in the following steps: Existing Design • The overall geometry was generated in ANSYS Fluent 16.2. All the elements were grouped under appropriate names (like floor, ceiling, exterior and interior windows, exterior and interior walls, inlet, outlet and lights)
Overall CFD Geometry of current welding room
• This model was then meshed with the following settings; Physics preference = CFD Relevance = -80 Use Advances Size Function = On: Proximity Relevance Centre = Fine Smoothing = High Use Automatic Inflation = None Inflation Option = Total thickness, with 4 layers and maximum thickness of 0.003m Mesh metric = Orthogonal quality The meshing result is as follows:
Meshing Results Overall Mesh
The number of elements is below 500,000 and therefore the model is appropriate to be further used for the simulations. 
The model was then exported into Fluent and then a report quality was obtained.
Report Quality of existing welding room model
The minimum orthogonal quality which should be in the range of 0 and 1 is 0.202 for this model and the maximum aspect ratio which should be below 18 is 16.64 for this model. Since all the conditions are satisfied to start the simulation, the necessary changes were entered into the software: •Gravity model was enabled with the y-axis set as -9.81m/s 2 •Energy equation was enabled •Viscous model was changed to k-epsilon (2 eqn) The boundary conditions were changed for certain elements; • Exposed windows: Thermal Heat Flux was set to 200 w/m 2 and the material properties were changed for glass; Density – 2500 kg/m 3, thermal conductivity – 1.7 W/m-K and specific heat capacity – 840 J/kg-K (All About Glass, n.d.) • Exposed concrete walls: Thermal Heat Flux was kept at 0 and the material properties were changed for glass; Density – 2400 kg/m 3, thermal conductivity – 0.8 W/m-K and specific heat capacity – 880 J/kg-K • Inlet: The velocity magnitude was calculated by taking average of the velocities measured at the inlet vent of the welding room which came to 1.93 m/s. The hydraulic diameter was calculated for the duct by dividing four times the area of the duct by the perimeter of the duct which summed to 0.26 m. The temperature at the inlet was obtained by taking the average of air temperatures recorded at the inlet which totaled to 294K. • Lights: Thermal Heat Flux was set to 12 w/m2. • Outlet: The gauge pressure was maintained at 0 and the backflow total temperature was obtained by calculating the average of air temperatures recorded at the outlet which totaled to 297K. The Turbulent Kinetic Energy and Turbulent Dissipation Rate were changed to Second Order Upwind. Under Residual Monitors, the absolute criteria for the energy equation was changed to e -5. Standard Initialization was selected and finally the calculation was run for 500,000 iterations.
The model was completely converged as shown below;
Convergence statistics for the existing model
After the model was successfully converged, results were gathered for the existing air distribution system. So first three planes were made at 0.5m, 1.5m and 2m. Then for those planes, three contours were created with the variable selected as temperature and range as user specified. The min temperature was set at 20째C since the surface temperatures measured at various heights during experimentation did not go below 20째C and the max temperature was set at 30째C because there was no rise in surface temperature at any point beyond 30째C during testing. The number of contours was maintained at 100, so as to get an accurate representation. Temperature variable contours for the existing design
In this figure, it can be seen that there are some heat gains from the glass windows and therefore an increase in the temperature in that region. This results into thermal discomfort for the occupants working near that region. The air mixing pattern is not uniform in the welding room which is visible from the different shades of contours across the space and this is evidence of a poor design of the existing air distribution system. For visualizing the air velocity pattern, 3D streamlines were created to start from the inlet vent. Total 100 equally spaced points were selected for a more detailed form.
Velocity Streamlines for the existing design From the above figures it can be understood that all the air incoming from the inlet does not reach all parts of the welding room uniformly.
Next, lines were made at nine points throughout the welding room, where the surface temperature measurements were recorded at three different heights. Positioning of lines for surface temperature analysis
At each of these points charts (1 to 9) were created with temperature as the variable.
Modified Design Modifications were made to the model to optimize the design of the welding room in terms of ventilation and thermal comfort. The outlet vent was shifted from the vertical wall to the ceiling for better HVAC performance, using Solid edge software. CFD Simulations of the modified welding room model were conducted as in the following steps:
Displaced Outlet Vent (Modification)

The overall geometry was generated, and all the elements were grouped as before. Overall CFD Geometry of modified welding room

This model was then meshed with the same settings as for the previous model, but with only one change; Use Automatic Inflation = Program Controlled
The meshing result is as follows:
Overall Mesh
New Meshing Results The number of elements is below 500,000 and therefore the model is appropriate to be further used for the simulations. The report quality obtained for the modified model is as follows;
Report Quality of modified welding room model
The minimum orthogonal quality which should be in the range of 0 and 1 is 0.205 for this model and the maximum aspect ratio which should be below 18 is 17.58 for this model. Since all the conditions are satisfied to start the simulation, a few additional changes were entered into the software, apart from the original input data: The boundary conditions were changed for certain elements; • Exposed windows: Same conditions as before • Exposed concrete walls: Same set of properties as before • Inlet: The velocity magnitude and hydraulic diameter were assigned the same value as before, but the temperature at the inlet was changed to 291K since that’s the minimum surface temperature which would be appropriate to provide uniform cooling throughout the room. • Lights: Thermal Heat Flux was set at 12w/m 2 just like in the existing model. • Outlet: The gauge pressure was maintained at 0 like before, but the backflow total temperature was set at 300K, since the surface temperature should not exceed this value Standard Initialization was selected and finally the calculation was run for 500,000 iterations. No changes were made in General and Models setup, solution methods, residual monitors, solution initialization and number of iterations. The data in these parameters were kept same for the existing and modified model simulations. The model was completely converged as shown below;
Convergence statistics for the new model
After the model was successfully converged, results were gathered for the modified air distribution system, in the same way as was done for the previous simulation model. So first three planes were made at 0.5m, 1.5m and 2m. Then for those planes, three contours were created, but this time with altered temperatures. The min temperature was set at 15°C since this surface temperature will be able to nullify the heat gains through windows and lights, and thus create a uniform air distribution pattern throughout the room. The max temperature was set at 26°C because if there is an increase in surface temperature at any point beyond 26°C in an interior working environment, it results into thermal discomfort and user dissatisfaction. The number of contours was maintained at 100, to easily compare the data results.
Temperature variable contours for the new design
This figure depicts that the heat gains which were present due to the glass windows have been removed and therefore a uniformly cooled interior environment is achieved. Therefore the modification proved to be a successful design. This is evident from the illustration wherein only one shade of contours is obtained across the space.
For visualizing the air velocity pattern in the modified design, 3D streamlines were created to start from the inlet vent. Total 100 equally spaced points were selected for easy comparison with the existing simulation model. From the above figures it can be understood that the incoming air from the inlet reaches all parts of the welding room uniformly. Velocity streamlines for the new design
Next, lines were made at the same nine points throughout the welding room, where the surface temperature measurements were recorded at three different heights.
Same Positioning of lines for surface temperature analysis
At each of these points charts (1 to 9) were created with temperature as the variable.
Analysis and Results The following table shows the Simulated (S) and Average Recorded (Rec.) surface temperatures at nine different points, chosen at various locations throughout the welding room. These temperatures are in degree Celsius and for the existing air distribution system. Line 1 Line 2 Line 3 Line 4 Line 5 Line 6 Line 7 Line 8 Line 9 Point of measurement 0.5m 1.5m 2m
S 23 23 23
Rec. 23.6 24.2 24.4
S 23 22 22
Rec. 23.6 23.9 24.4
S 22 22 22
Rec. 23.3 23.8 23.6
S 21 21 22
Rec. 22.7 23.7 23.3
S 23 22 22
Rec. 23.4 23.7 23.9
S 24 24 24
Rec. 23.4 23.8 24.4
S 25 25 25
Rec. 23.2 23.6 24.5
S 24 23 23
Rec. 22.7 23.6 25.3
S 21 21 21
Rec. 22.7 22.5 21.5
This table illustrates that there is a difference in the surface temperatures obtained from simulating the model and manually measuring it in-situ. This variation might be due to inaccuracy of data input into the software or due to instrumental errors during taking readings. After analyzing the results from the modified model of the welding room it was found that the surface temperature at all the nine points and at all the three heights is the same i.e. 18°C. This means that by changing the positioning of the extract vent and altering the user specified surface temperatures, the air distribution system can be enhanced significantly. The lights and exterior windows are the main sources of heat gains and to nullify their effect the inlet air temperature was considerably reduced in the modified model. Discussion Return air vents are ideally positioned in stagnant areas and generally in ceilings and due to this basis the design has been optimized accordingly. (Elder, 2007) The proposed location of the extract vent is appropriate for this design since the HVAC system will be concealed in the ceiling. The location of an HVAC system affects its energy efficiency. The modified design allows adequate airflow around the vents and the outlet air does not feed to the inlet. (Australian Government Department of Industry, Innovation and Science, n.d.)
Conclusion and Recommendations
CFD modeling was used to predict airflow pattern in the welding room, which helped in identifying the optimal location of diffusers and return grilles for the HVAC system. Design Optimization techniques: • Changing air filter – Changing filters of the vents is the easiest way of improving the performance of the HVAC system. Blocked filters reduce the performance by more than 5 percent, since the airflow gets restricted. (WEBB'S , 2015) • Recalibrate thermostats – Recalibration is necessary in order to control surface temperature more precisely. • Adequate insulation, air-tightness of the doors and optimized glazing of the windows, which will allow limiting solar impact in hot climates. (Mitterer, 2012)
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