Développement d'un échangeur de chaleur air-air pour le milieu serricole

Abstract

In order to reduce the humidity in a greenhouse, while saving the cost of heating, air-to-air heat exchangers are used. The exchangers destined for greenhouse are voluminous. The objective of this work is to design a compact heat exchanger, resistant to icing in cold environments. A literature review of different exchangers allowed to choose the correct geometry for maximum exchange and reducing the risk of fouling. The current cross-type exchanger was selected. Numerical simulations by Fluent of the heat exchange, while varying the dimensions of the exchanger, were used to determine their influence on its efficiency. The exhaust warm and moist air from the greenhouse comes in contact with the wall cooled by the outside air. Condensation happens. This condensation is accompanied by a release of heat to the cold fluid, thus improving the efficiency of the exchanger. To take account of the effect of condensation in the energy equation in the CFD simulations, a user-defined function (UDF) has been added to Fluent software. This UDF adds a source term conservation equations in cells where condensation occurs. The first CFD simulations were used to determine the influence of the different dimensions of the exchanger on its efficiency. By combining the results found and the spécifications imposed for the exchanger to be compact and resistant to icing, preliminary dimensions were determined and the exchanger was simulated with Fluent. The efficiency obtained is 30%. Numerical simulations of the exchanger, in which the spacing between the plates where cold air flows was varied, helped to determine its pressure loss and efficiency. According to the pressure loss and the chosen supply fan, the final dimensions of the exchanger have been defined. The efficiency of the new heat exchanger is 40 %, without taking into account the condensation. From the simulation results when the condensation is account, studies were performed on the contribution of latent heat to the heat exchange process, and the amount of condensed water, depending on the humidity of the hot and the cold air temperature. According to the study carried out, for a hot air temperature equal to 293 K with a relative humidity of 80% and a temperature of the cold air equal to 261 K, the contribution of latent heat reaches 56 % and the efficiency of the heat exchanger passes from 40 % to 64%.

Date	22 Oct 2015
Original language	French
Awarding Institution	École de technologie supérieure
Supervisor	Daniel Rousse (Supervisor) & François Morency (Co-supervisor)