What are the main factors determining air infiltration?
- Outdoor environment: wind speed, wind direction, surrounding buildings, location type (exposed, urban, city centre etc.) and outdoor air temperature.
- Indoor environment: indoor air temperature and over/under pressure relative to outside due to mechanical ventilation system.
- Building: shape of the building, amount, type and location of envelope openings and leakages, and amount, type and location of inside partition openings and leakages.
- Occupants: opening/closing of building openings (doors, windows etc.), control of vents and control of mechanical ventilation system(s).
Describe how the fluid flow within an energy system may be modelled by the nodal network (NN) method.
- Boundary conditions are represented by temperature and wind velocity (corrected for terrain roughness), and a pressure coefficient set to account for local obstructions and surface inclination/aspect ratio.
- The system is represented as a network of nodes where each node pressure is either internal (unknown) or external (known).
- Buoyancy effects are included by modifying the nodal fluid densities as a function of node temperature.
- Empirical flow models are introduced to determine the flow within the components that connect nodes as a function of the prevailing pressure difference.
- An iterative solution procedure is used to solve the mass balance at each node to yield the node pressures and branch flows.
Explain the steps involved in solving the equations relating to the flow of fluid within a distributed network using the nodal network method.
- A guessed value of pressure is assigned to each internal (unknown) node.
- The mass flow rate between nodes is evaluated by applying the pressure difference to the empirical equations representing connected components.
- A mass residual (imbalance) is determined for each node.
- Node pressure corrections are determined by establishing the system Jacobian matrix from knowledge of the nodal residuals, their rate of change with respect to nodal pressure change and the rate of change of the branch mass flow rates with branch pressure drop (all of which are known at the end of an iteration step).
- The pressure corrections are applied to the nodal pressures and the next iterative step commenced.
- The process is terminated when the nodal residuals fall below a prescribed value.
Describe how the fluid flow within an energy system may be modelled by the computational fluid dynamics (CFD) method.
- Boundary conditions are represented by the temperature of bounding surfaces and mass & momentum inputs.
- The system is represented by the discretised Navier-Stokes equations when applied to a finite volume lattice representing the flow domain.
- Buoyancy is represented by the Boussinesq approximation whereby the fluid density is held constant and the effects of buoyancy are included within the momentum equations.
- A turbulence transport model is normally used whereby the influence of turbulence on the time averaged motion of the fluid may be determined.
- An iterative solution procedure is used to solve the energy, momentum and mass equations to give the distribution of pressure, temperature and velocity (and perhaps other parameters such as humidity and contaminant concentration).
For each method, give one example of a problem that would be best suited to that approach.
NN (nodal network): best suited to the calculation of fluid flow in a network representing building leakage distribution or plant.
CFD: best suited to an assessment of room air quality and distribution of comfort parameters.
Why is it important, in an energy systems modelling context, to re-establish the parameters of the turbulence model at each time step?
Because energy systems are often non-steady, low Reynolds Number flows occur in which the flow regime may be characterised as weakly turbulent and be moving from laminar flow through the transition zone to fully turbulent.
The parameters are the turbulent kinetic energy (k) and its rate of dissipation (ε) throughout the domain; and the shear stress and convective heat transfer at surfaces.
