Performance assessment of hybrid recuperative heat exchanger for diesel engine generated exhaust gas

Abstract

In this study, the use of supplemental heat energy from exhaust gas of a stationary diesel engine was assessed to explore a new method of drying black nightshade seeds in a solar-exhaust gas greenhouse dryer. The energy recovery potential of a hybrid recuperative heat exchanger (HRHE) was demonstrated with the objective of utilizing the recovered energy from an engine on milling operations to heat a fluid stream of drying air. The results show that 4.45 kW of thermal energy was available in exhaust gas of a diesel engine operated at 2500 rpm when mass flow rate was 45.07 kg/h at a temperature of 357.36 °C. The rate of heat utilized for solar-exhaust gas mode ranged from 40.49 to 685.94 J/m2.s and from 21.69 to 668.11 J/m2.s in exhaust gas mode of drying. The heat exchanger raised the dryer temperature by an average of 11.78 °C when temperature differences between inside and outside were compared in solar-exhaust gas mode of drying. The average hourly rise in temperature inside the dryer was 8.04 °C with a minimum rise of 3.7 °C and a maximum of 9.41 °C when exhaust gas was utilized to provide heat energy. The performance of the solar-exhaust gas greenhouse dryer improved when thermal energy was used as a supplement in drying and as a result the drying time for black nightshade seeds was significantly reduced from 11 h in solar mode to 10 h in solar-exhaust gas mode of drying. Moreover, the seeds were dried for 14 h when exhaust gas mode of drying was performed without utilizing solar energy. The percent internal uncertainty for experimental measurements of relative humidity (γ) was 4.1% and 17.5% for temperature (T) observations. The three proposed thermal models for temperatures and moisture evaporation performed better with low RMSEs in exhaust gas mode compared to the other modes of drying.