The temperature non-uniformity produced by energetic liquid reactions can cause unwanted products or otherwise uncontrolled reactions. On the other hand, operating a reaction at the highest acceptable temperature maximises the reaction rate. This work was designed to establish the relationship between flow conditions and temperature non-uniformity for a given reaction system in a particular structured flow passage. The work studied the temperature non-uniformity experimentally and computationally of the two reacting systems (HCl-NaOH and NaClO2- K2S4O6) in an F-element Reactor under different flow and thermal conditions.
A mathematical model was developed to show the approximate dependence of temperature non-uniformity on the Damköhler number (Da), reaction Enthalpy number (H), Reynolds number (Re) and Prandtl number (Pr). These parameters are non-dimensional numbers and they consist of physical variables. These variables include the dimension of the flow passage, species concentration, species velocity, enthalpy of reaction, etc. Substituting the non-dimensional parameters in terms of the physical variables shows the dependence of temperature non-uniformity on these variables.
Three small scale different structured reactors, i.e., C, F and S element Reactors, were used to investigate the temperature non-uniformity in the structured passages. These geometries were designed to have the same residence time. The experiments were implemented using a non-reacting flow system: this was, firstly, to discover the physical and thermal properties of the system through simulating with CFD; and secondly, to investigate the effect of gravity on the flow pattern and temperature distribution through the flow passage. The experiments were also implemented in two exothermic reacting flow systems, HCl-NaOH and NaClO2- K2S4O6, to investigate the effect of flow conditions on the temperature non-uniformity in the reacting flow. These experiments were only implemented in the F-element Reactor due to noise appearing in the temperature measurements in the C and S element Reactors. The experiments in both flow systems included a wide range of flow rates (Re) and thermal conditions (Ri). Ri represents the effect of buoyancy on the flow pattern and temperature uniformity within the flow passage. This effect decreases when the passage width and temperature difference between the reagent stream inlet temperature and the fluid temperature decrease and when the fluid velocity increases. The spatial temperature along the reactor domain was measured using the movable fine-wire thermocouple method.
A three-dimensional (3-D) computational model was used to study the temperature non-uniformity within the domain of flow passages. This model used numerical solutions for the flow, species and energy equations to reliably capture the flow and temperature field within the reactor domain.
The results showed that the agreement between the experimental data and the computational results was acceptable. The results also showed that buoyancy has a significant effect on the flow pattern and temperature profile in all geometries. This effect decreased with decreasing Ri and increasing fluid flow rate (Re) due to the decrease in the effect of gravity force. This effect can be ignored at Re ≥ 130 at the Ri ≥ 0.4, 0.3 and 0.4 for the C, F and S element Reactors respectively in the non-reacting flow and at the Ri ≥ 0.1 for the F-element Reactor in the reacting system. Furthermore, the results showed that the fluid temperature decreased along the flow passage due to the effective dissipation of the heat into the walls and the effect of the cooling system. Moreover, the temperature non-uniformity is affected by the Ri and the inlet temperature of the reagent stream. Temperature non-uniformity decreased with decreasing Ri. When the stream of reagent entering the reactor is cold, the temperature non-uniformity is higher than when it is hot. In addition, the temperature non-uniformity is also affected by the flow rate of fluid (Re): it decreased with an increasing fluid flow rate due to the enhancing of the dissipation of heat and a decrease in the effect of buoyancy. Finally, the temperature non-uniformity decreased with the decreasing enthalpy of reaction. The standard deviation of temperature for the HCl-NaOH reacting system was (1.9 oC) less than the NaClO2- K2S4O6 reacting system (2.2 oC) at the same Re and close Ri. This may be due to the large enthalpy of the NaClO2- K2S4O6 reacting system.
Generally, the experimental and computational works in this thesis demonstrate the potential for controlling the temperature non-uniformity within the flow passage using structured passages.
