A study on swirling plume enhanced natural draft dry cooling towers

Small scale power generation systems, i.e. in the sub-10MWe range, are being developed to serve mining sites and remote communities in Australia, providing the motivation for dry cooling technologies. The challenge, specifically, is that natural draft dry cooling towers (NDDCTs) have to be scaled down. Under such conditions, a 20-m-height natural draft dry cooling tower (NDDCT) has been designed and built on the Gatton campus at the University of Queensland. Meanwhile, swirling technologies have been adopted in engineering applications such as combustion chambers, cyclonic separators, tube and shell heat exchangers, etc. Swirls have also been proposed in NDDCTs to enhance the thermal performance. However, many details as well as the mechanism of heat rejection enhanced by swirls have not been clearly quantified and analysed. This thesis investigated swirl effects on short NDDCTs in terms of practical and theoretical interests. The former highlights the enhancement of thermal performance by swirls in the presence of cold air inflows (Chapter 3) and crosswinds (Chapter 5), while the latter focuses on the swirl influenced new draft equation (Chapter 4) and plume properties above the tower (Chapter 6). The main findings are summarized as follows:1. For Gatton tower on windless conditions, the transient simulations showed that the cold air inflow penetrates inside the tower after a short period of pseudo-steady state, and a steady state with the cold air inflow is finally formed. In addition, by reducing the local vortices caused by the specific tower structure, and thinning the boundary layer thickness, the swirl can decrease the cold air inflow effect. Finally, the feasibility of the suggested approach was verified by comparing the energy required to create the swirl with the extra heat transfer from the heat exchangers in the tower which would have not been materialized because of the cold inflow. It was observed that an extra 40kW thermal performance gain can be anticipated if only 1W is consumed to induce the swirl.2. In the absence of crosswind, a new draft equation involving swirling motions for natural draft dry cooling towers (NDDCTs) was derived and solved analytically. A 2-D axisymmetric model for a short NDDCT was built and computational fluid dynamics (CFD) simulations were carried out to cross-validate the theoretical predictions. Results showed that the optimized location for swirl input is at the tower outlet to avoid swirl decay. It was noted that the swirl influence on the airflow draft velocity gradually becomes more significant especially when the dimensionless input swirl ratio exceeds 2, i.e. the air draft velocity can be increased by around 20% with the input swirl ratio of 5. The theoretical predictions generally agree with the numerical results, but deviate when: (1) vortices adjacent to the wall occur in the presence of excessively strong swirl, in the case with swirl created by a thin source zone; (2) cold air inflow penetrates due to the significant heat exchanger resistance coefficient; (3) vortex breakdown appears when swirl fills the whole tower. All the aforementioned unfavourable phenomena are local effects and thus cannot be predicted by the draft equation unless proper local resistance terms are included.3. To quantify the swirl effects on the thermal performance of natural draft dry cooling towers (NDDCTs), 3-D simulations of a short NDDCT were carried out. The effects of crosswind were also investigated with no-wind as a special case considered here. It was assumed that the NDDCT is equipped with windbreak walls to mitigate the crosswind effects. In an attempt to optimize the swirl effects, the location of the swirl generator was varied parametrically. The results showed that, with no wind present, introducing swirling motions right above the heat exchangers is found to be the optimal location for improving around 40% of the reduced thermal performance, which is caused by cold air inflow penetration. On the other hand, as the swirl intensity further increases after the cold air inflow is eliminated, locating it at the tower outlet performs the best on the air draft speed enhancement, and thus further increases approximately 17% of the heat transfer rate at the angular frequency input of 2s−1. In the presence of crosswind and windbreak walls, air flows through the heat exchangers and tower non-uniformly. By mounting the swirl generator right above the heat exchangers, the uniform index of the heat flux can be improved by 5% with 1s−1 angular frequency input. More importantly, inducing swirls at the tower outlet is still the optimal choice for increasing the air draft speed through the tower. With 2s−1 angular frequency input, the thermal performance of the tower can be enhanced by 11 ∼ 17% in accordance with the crosswind speed.4. In the absence of crosswind, the system of equations describing the swirling plume was simplified but still yielded an accurate representation of the dynamics. A non-linear ordinary differential equation (ODE) system was derived, elucidating that the neck of the plume, as well as the axial velocity, are certainly influenced by swirling motions. However, the ODE system generally does not have exact analytical solutions. Thus, asymptotic approximations with first-order expansions on all swirling plume variables for both lazy and forced plumes were obtained through regular perturbation methods. Finally, the 4th order Runge-Kutta numerical evaluation of the non-linear swirling plume ODE system was conducted to compare with the asymptotic solutions, and the results generally showed good agreements.The current study provides quantitatively numerical results and a preliminarily fundamental understanding of swirl effects on short NDDCTs, demonstrating the feasibility of utilizing swirling technologies to enhance the thermal performance of the towers.