MODELLING AIR FLOW IN IRISH MUSHROOM TUNNELS

The entire growing process is influenced by the microclimate at the cropping surface and control of this is achieved by manipulating an air-conditioning system. The environmental factors of interest are air and compost temperatures, humidity, carbon dioxide concentration and air speed. As there is forced air movement throughout the growing cycle, research has focused on acquiring a better understanding of the flow and distribution of air in growing tunnels.

Lumped-parameter models of the environment do not describe air flow details that are of interest in mushroom growing. A mathematical model that describes air flows in the mushroom tunnels typical of the Irish mushroom industry has therefore been developed. The model has been built on a set of partial differential equations for the physical system.

The equation set includes air velocities, pressure/continuity and temperature. The k-e description of turbulence is included as measurements have shown the turbulent nature of the flow field. The model was discretized using control volume analysis. Calculations have been made for steady state, isothermal air flows. Operation of the heating system has a profound effect on air flow in mushroom tunnels and buoyancy effects due to the entry of heated air have been calculated.

It was found that convergence times for buoyant flows were too long for practical use. An alternative solver was implemented using the Couple Equation Line Solver method and the Additive Correction Multigrid technique. This was tested in two dimensions and found to give substantial reduction in convergence times.

Equations for mass flow have the same form as the current set. When these have been added the model will be used to examine the behaviour of the system under different control strategies.

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