ABSTRACT?IN THE PRESENT WORK, A PARAMETRIC STUDY OF THE INFLUENCE OF EXTERNAL VENTILATION IN THE BEHAVIOR OF THE THERMAL CHARACTERISTICS OF A REAL GREENHOUSE IS DEVELOPED BY USING A NUMERICAL MODEL IMPLEMENTED IN THE COMPUTATIONAL FLUID DYNAMICS SOFTWARE ANSYS FLUENT 2019R3. SUCH MODEL ALLOWS TO CORRECTLY DESIGN AN AUTOMATIC CONTROL OF THERMAL PROPERTIES IN GREENHOUSES. TWO DIFFERENT MODELS ARE PRESENTED, THE FIRST ONE USES NATURAL VENTILATION AND THE SECOND MODEL USES FORCED VENTILATION BY USING 22 FANS OF 40 CM OF RADIUS EACH. ONLY THE FIRST MODEL INCLUDES THE EXTERNAL FLUID DOMAIN. THE CULTIVATED CROP IS LETTUCE AND ITS INFLUENCE IN THE MODEL IS INCLUDED AS A POROUS MEDIA AND A WATER VAPOUR SOURCE. SOLAR RADIATION IS ADDED USING THE SURFACE TO SURFACE MODEL (S2S) AND IN ORDER TO INCLUDE THE AIR HUMIDITY, THE LOCAL MASS FRACTION IS PREDICTED THROUGH THE SOLUTION OF A CONVECTION-DIFFUSION EQUATION FOR EACH SPECIES. RESULTS FROM THE FIRST MODEL SHOWS THAT THE TEMPERATURE DISTRIBUTION IS UNEVEN INSIDE THE GREENHOUSE AND MOST IMPORTANT THAT THE THERMAL CHARACTERISTICS ARE NOT THE RECOMMENDED FOR THE LETTUCE CROP. THE SECOND MODEL SHOWS THAT AFTER REACHING THERMAL EQUILIBRIUM THE AVERAGE TEMPERATURE IS REDUCED A 175 % AND THE RELATIVE HUMIDITY INCREASE 13.5 % BY VARYING THE FAN INLET VELOCITY FROM 1 TO 10 M/S, WHILE A CHANGE IN THE INLET TEMPERATURE FROM 2 C TO 20 C AND CONSTANT VELOCITY CHANGES THE AVERAGE TEMPERATURE IN 82% AND THE RELATIVE HUMIDITY IN 4.7%. FINALLY A VARIATION FROM 10 TO 100 % OF THE RELATIVE HUMIDITY AT THE INLET DOES NOT AFFECT THE AVERAGE TEMPERATURE BUT INCREASE THE RELATIVE HUMIDITY 24% INSIDE THE GREENHOUSE.