A modified greenhouse dryers with solar air heating collector were used for bitter gourd flakes drying under natural and forced convection modes till stagnation in their moisture evaporation. Experiments were performed simultaneously for open sun, natural and forced convection modes to validate the thermal modeling and compare drying kinetic results. The predicted temperatures of bitter gourd flakes, room air temperature and moisture evaporation rate show the fair agreement with the experimental observations within the root mean square of percentage deviation ranges from 6.10 to 10.41% and coefficient of correlation ranges from 0.96 to 0.99 under natural convection mode. Similarly, for the forced convection mode, coefficient of correlation ranges from 0.96 to 1 and root mean square of percentage deviation ranges from 3.74 to 7.55%. Logarithmic and Midilli-Kucuk models are selected as best curve fitting techniques for non-linear regression analysis for bitter gourd flakes drying under natural and forced convection modes respectively based on SSE, R-square, adjusted R-square and RMSE values. Proposed thermal models will be helpful in designing energy efficient greenhouse drying system for given crop mass and greenhouse dryer location. (C) 2017 Elsevier Ltd. All rights reserved.