Abstract
This thesis presents an investigation of the utilisation of waste heat energy from an internal combustion engine for tri-generation. In articulating this, a preliminary experimental investigation was performed which involved employing waste heat from a diesel engine to drive a diffusion absorption refrigeration (DAR) system and a water heater system. Based on the experimental results it was found that the performance of this type of tri-generation system was severely limited by the narrow operating range and cooling capacity of the DAR cycle.
To understand the narrow operating range, the DAR cycle was modeled for steady state operating conditions. Using a sensitivity analysis, it was found that the generator temperature significantly influences the overall performance of the DAR system. It was hypothesised that this performance could be enhanced through an improved thermal driven bubble pump.
To better understand the DAR bubble pump, an airlift based bubble pump system was developed to produce controllable operating conditions. Experiments on the airlift based bubble pump system were performed to characterise different two-phase flow regimes inside the single lift-tube of the bubble pump. Four flow regimes were identified and mapped based on the superficial velocities of the air and water. Furthermore, it was found that the performance of the bubble pump was affected by the lift-tube diameter, submergence ratio, and airflow rate.
Having examined the operating characteristics of the bubble pump experimentally, a model was developed to predict its performance. To realise this model, empirical correlations were developed to determine the void fraction for each flow regime. The model reinforced the finding of limited pumping capacity from a single lift-tube.
However, to overcome the limited pumping capability of single lift-tube bubble pump, multiple lift-tubes were examined as a way to improve the pumping performance. It was found that using multiple lift-tubes is a viable solution as it improves the pumping capacity of the bubble pump. However, the flow rate of each lift-tube in a multiple lift-tube configuration was observed to be non-uniform. Analytical modelling of a multiple lift-tube bubble pump showed an inability to accurately predict the pumping characteristics. To overcome this, a novel artificial neural network (ANN) technique was used to predict their pumping capacity. The results showed that the ANN was capable of accurately predicting the performance of the multiple-lift tube bubble pump.
Finally, the influence of multiple lift-tubes on the performance of the DAR system integrated in a tri-generation system was examined. It was found that at high heat inputs, a DAR system with a multiple lift-tubes bubble pump showed an increase in performance and an improved operating range for an internal combustion engine driven tri-generation system using a diffusion absorption refrigeration.
