This assignment is designed to enable you to explore in some depth the use of Computational Fluid Dynamics (CFD) to analyse the air movement and temperature distribution within a simple room under a range of different scenarios.
Please produce a professionally presented academic report detailing your critical analysis of each of the five cases set out within this brief using phonexi CFD software.
MLO1 Select and defend an appropriate method for modelling a given energy system.
MLO2 Appraise the limitations and weaknesses of the method used.
MLO3 Successfully design, build and use a simulation model describing the dynamic response of an energy system.
MLO4 Critically evaluate the results obtained from a dynamic simulation model of an energy system.
MLO5 Critique the benefits and limitations of dynamic simulation modelling compared with conventional professional design practice.
This assignment is designed to enable you to explore in some depth the use of Computational Fluid Dynamics (CFD) to analyse the air movement and temperature distribution within a simple room under a range of different scenarios. Please produce a professionally presented academic report detailing your critical analysis of each of the five cases set out within this brief.
You are to use the PHEONICS CFD software available in all the Mechanical and Construction Engineering IT Laboratories for this assignment. This software will be demonstrated in the IT lab sessions and there is a set of four self-study tutorials available on the module eLP site which will be helpful in
building your CFD skills and understanding.
Please be aware that it takes time to build confidence with CFD software and that in order to maximise your learning potential you need to engage fully right from the early sessions and to dedicate significant additional time outside of class. You can then bring your questions along to the next class to discuss with the tutor to build your confidence and skills and doing so will prepare you to answer this assignment.
Five variants (A-E) of a simple case are presented below to allow you to simulate air and temperature distribution in a single rectangular room using CFD modelling. Two isothermal cases A and B focus on the air distribution in the room whilst cases C, D and E concentrate on the natural and mixed convection
heat transfer in the room.
Room size: 5.2m (L) x 3.6m (W) x 2.7m (H)
Supply grille size: 0.4m (W) x 0.15m (H)
Extract grilles size: 0.3m (W) x 0.3m (H)
Beam size: 3.6m (L) x 0.2m (W) x 0.4m (H)
Window size: 3.2m (W) x 1.5m (H)
Radiator size: 2.0m (W) x 1.0m (H) x 0.05m (D)
Module Learning Outcomes
In cases A, B & D, a supply grille is positioned at the centre of the west-facing wall, at a distance of 0.2m from the ceiling to the top edge of the diffuser. The discharge angle of the diffuser is horizontal. There are two extract grilles located in the two corners of the east wall, with a distance of 0.2m from the edges.
In cases C & D, there is a sash window located in the south-facing wall, at a distance of 0.8m above the floor. In case C, a single panel radiator is added to the room, which is located 0.1m in front of the southfacing wall and 0.1m above the floor.
Present and discuss the CFD results and include the key settings information such as the size of cells, the number of grids in each dimension, and the number of the iteration simulated. For cases A and B you need only present graphics of room air velocity. For cases C and D you should present graphics of both room air velocity and temperature. Discuss your results in the context of satisfactory room air distribution (cases A and B) and room air distribution and thermal comfort by using air distribution performance index (cases C, D and E).
You are asked to use the air diffusion performance index (ADPI) for cases (C, D & E) as it is helpful in your deliberations as to what constitutes ‘good’ room air distribution’ for the non-isothermal cases. The air ADPI concept and its implementation in the diffuser selection guideline (ASHRAE HandbookFundamentals, 2009) provide designers with a simple method to design overhead-air-distribution systems.
The following section provides more information and presents a sample of model of cases A-D (Fig 1-4).
The boundary conditions of the five cases are as follows:
Case A: Isothermal forced convection with a supply air grille and two extract grilles. Supply grille
discharge velocity = 3.0ms-1
Case B: As Case A but with a beam obstruction to the air flow. The beam is spans the width of the space and is located adjacent to the ceiling equidistant between the east- and west-facing walls.
Case C: Natural convection in winter heating by means of a radiator located beneath a cold window. The surface temperature of the radiator is 70°C. Wall / ceiling / floor surface temperatures = 17°C. Window surface temperature = 5°C.
Case D: Mixed convection in summer cooling condition with a supply grille and two extract grilles. Supply grille discharge velocity = 3.0ms-1. Supply air temperature = 12°C. Wall / ceiling / floor surface temperatures = 24°C. Window surface temperature = 35°C.
Case E: Choose one of the four cases above (A-D) and carry out further investigations and simulations to analyse the effect of changes upon the ventilation, thermal comfort or airborne transmission theme. The dimensions of room and window are fixed but all other objects and dimensions can be modified.
You may find the air distribution performance index (ADPI) helpful in your deliberations as to what constitutes ‘good’ room air distribution’ for the non-isothermal cases (C, D & E) only.
You can apply this method as follows. The effective draught temperature, TED, at any point in a room is obtained from:
(1) (where Tp is the dry bulb air temperature at some point p in the comfort zone of the room; is the mean room air dry bulb temperature in the comfort zone and up is the local air velocity at point p.) Observations suggest that a high percentage of people will be comfortable at:
(2) The air distribution performance index, ADPI, expresses the percentage of points in the comfort zone that lie in the above range:
(3) (where ΣnTED (-1.7,1.1) is the number of observed effective draught temperatures in the comfort zone whose values lie in the range (-1.7,1.1) and ΣnTED is the total number of observed effective draught temperature values in the comfort zone.) A good air distribution should exhibit a substantial percentage majority (e.g. 70% minimum).