Simulation of Heat and Mass Transfer in a Solar-Exhaust Gas Greenhouse Dryer for Black Nightshade Seeds

Abstract

Diesel engines, used in Kenyan farms for stationary operations including milling, threshing, winnowing, and chopping of agricultural produce, generate exhaust gas which is released with heat energy to the environment. Moreover, the central problem facing farmers who produce drought resistant vegetable crops such as black nightshade in Kenya, is lack of a suitable drying method for the seeds to enhance continued biodiversity preservation. As a solution, heat energy from exhaust gas can be recovered and potentially utilized to dry black nightshade seeds. The main objective of this study was to simulate heat and mass transfer in a solar-exhaust gas greenhouse dryer for black nightshade seeds in order to predict drying time, seed and inside greenhouse dryer temperatures, and moisture evaporated from seeds. The methodology involved an experimental setup; instrumentation and data acquisition; and performance of activities within the specific objectives of the study. The results for the first specific objective showed that at optimal engine speed of 2500 rpm, exhaust gas volumetric flow rate was 0.0167 m3/s; velocity was 8.52 m/s in connectors and 0.14 m/s in tubes, while Reynolds number was 10674 in connectors and 1368 in tubes. Consequently, 105 seconds elapsed for the 1.8 m3 heat exchanger to be filled with 1.3 kg of exhaust gas possessing kinetic energy of 39.79 kJ in connector number six and 0.01289 kJ in the sixth tube. In addition, exhaust gas temperature of 357.36℃ at a mass flow rate of 45.07 kg/h had 16002.56 kJ/h as available energy. The maximum frictional head loss reported was 71 m in the sixth connector and 0.00225 m in tube number six. For the second specific objective, the results showed that the heat exchanger raised the dryer temperature by an hourly average of 11.78℃ in the solar-exhaust gas mode and 8.04℃ when temperature differences between inside and outside were compared in the exhaust gas mode of drying. Moreover, the rate of heat energy utilized for the three modes of drying were: solar (37.33-683.3 J/m2‧s), solar-exhaust gas (40.49 to 685.94 J/m2‧s), and exhaust gas (21.69 to 668.11 J/m2‧s). The convective transfer coefficients of black nightshade seeds ranged between 2.48 and 2.55 W/m2‧℃ and those of evaporative heat transfer coefficients were found to be between 0.95 and 36.81 W/m2‧℃. The results for the third specific objective demonstrated that in the solar mode, seeds took 11 hours to reach a final moisture content of 7.13% (db) from an initial one of 89.34% (db). In the solar-exhaust gas mode, seeds dried from an initial moisture content of 92.57% (db) to a final one of 6.07% (db) in 10 hours. In the exhaust gas mode, it took 14 hours to dry black nightshade seeds from an initial moisture content of 88.84% (db) to a final one of 9.42% (db). Further, the Page model was found suitable for solar mode with the highest coefficient of determination (R2) of 0.9985 and the lowest root mean squared error (RMSE) of 0.0115. However, the Logarithmic model was found suitable for both solar-exhaust gas and exhaust gas modes of drying with RMSE of 0.0172 and 0.0232, and with the highest coefficient of determination (R2) of 0.9964 and 0.9933, respectively. Based on the results for the fourth specific objective, the exhaust gas drying mode had a difference of 12.5% when its mean germination percentage was compared to the solar mode. Moreover, a 16.2% difference in means of germination percentage was recorded when the solar-exhaust gas mode of drying was compared to the exhaust gas mode. The highest mean germination percentage was recorded at 89% for exhaust gas drying mode. Modified Giner’s model predicted germination changes of black nightshade seeds more accurately than modified Sharp’s model due to the higher R2 (0.6896 > 0.6853) and lower RMSE (6.1554 < 6.4519). The activation energy in the modified Giner’s model was found to be 7.034×〖10〗^3 Joule/mole through model fitting to experimental data. In conclusion, the concept of using a hybrid recuperative heat exchanger in a solar-exhaust gas greenhouse dryer was successfully applied in the recovery of exhaust gas energy from a diesel engine—heat energy that under normal circumstances is wastefully released to the environment—and it is, therefore, recommended that the solution studied in this work be extended to the recovery of waste heat energy from hammer mills operated on diesel engines in Kenya. Finally, the feasibility of exhaust gas heat energy use in drying can be expanded to seeds of other African vegetable crops.