A computational fluid dynamic (CFD) study of flow patterns, temperature distributions and CO2 dispersal in a tomato glasshouse

The aim of the project was to use computational fluid dynamics (CFD) to build a computer model of a commercial glasshouse and to use the model to study internal climate behaviour as a function of variables such as external weather, ventilation regime, heating regime and carbon dioxide injection regime.

In year 1, a computational fluid dynamics (CFD) model was constructed which was capable of simulating the internal climate of commercial tomato glasshouses. It was concluded that full size glasshouse modelling was required to answer questions raised by growers.

In year 2 a full size, but still two-dimensional model, was developed and used to study effects in a central section of the glasshouse. The model predicted the existence of a “dead” zone and its location, which was found to exist in practice and to be associated with poor production rates. The model was also used to conduct “what if” parametric studies showing the relationship between factors such as external wind speed, heating pipe temperature, CO² injection position and rate, ventilation regime and the presence of a crop canopy, on the internal climate characteristics.  A limited experimental study of internal flow speeds and directions confirmed some of the model predictions.

In year 3, the full-size two-dimensional model was used to model the internal glasshouse climate including carbon dioxide distributions and the sensitivity of changes in the model and boundary conditions was investigated. In addition, a full-size three-dimensional model was developed and the influence of wind direction on the internal airflow was studied.

The overall objective was to evaluate the usefulness and potential of CFD as a modelling tool for glasshouse research. The specific objectives were:

• Produce a description of the uniformity of CO₂ distribution in the glasshouse as affected by wind, sunlight and temperature and rates of CO₂ supply in order to answer questions relating to the correct positioning/ frequency of sampling points.
• Determine the relationship between pipe temperature and CO₂ concentration for the same glasshouse air temperature and rate of CO₂ addition in order to indicate when CO₂ increase by means of daytime pipe temperature increase is sensible.
• Determine the influence of position of layflats (height and frequency) and arrangement of holes in order to indicate if the current system layouts are optimal and if not how improvements may be accomplished.
• Analyse the movement of CO₂ from the layflat holes as a function of hole size, orientation, spacing, supply pressure (i.e. exit gas velocity) and supply gas concentration.
• Validate the modelling work using available and appropriate experimental data.