Modelling of Turbulent Flow and Heat Transfer Phenomena in Pipes of District Heating Systems Including Transient Temperature Propagation

Sammanfattning: Modern district heating systems, a carbon lean technology, involves dual usage of the energy due to the possibility to redistribute heat derived from numerous sources, such as industrial processes, refuse incineration, geothermal sources and combined heat and power plants. Such heat source flexibility facilitates the optimal usage of energy resources by varying the amount of purchased heat depending on demand, price and availability. As a result, the operating conditions are never stable and involve pronounced thermal and hydraulic transient regimes, in particular, temperature waves combined with temperature fluctuations. The prediction tools used for these phenomena have been applied for the optimization of the operational regimes. There is however, a need to identify the limitations of the prediction tools and to also improve the methods for more accurate simulations, to enable extended usage of the benefits of these tools in the understanding of transient heat transfer mechanics and to develop effective operational strategies. This work has been concerned with the modelling of turbulent flow and heat transfer phenomena in pipes in district heating systems, especially focusing on transient temperature propagation. Both simplified models and turbulence models have been used and their performances have been assessed against experimental data. In the simplified model, the fluid flow conditions were simplified by assuming a non-viscous fluid flow, contrary to the turbulence models, where the complex flow patterns occurring in pipe bends and T-connections were simulated in details. In both cases, the pipe flow was treated as a conjugated heat transfer problem, i.e., it included the effect of pipe wall heat conduction. The limitations of simplified approaches have been examined for the following cases: district heating pipelines, and the Neastved and Madumvej district heating networks located in Denmark. Simplified approaches provided a good approximation, particularly for representing the time delay in a system, as long as it is not used to predict the temperature value at a specific time during the emergence of the temperature changes. These values deviated from the measured values markedly for i) relatively large and sudden temperature changes at the network inlet and ii) low velocities combined with low turbulent Reynolds numbers. Other factors that can affect the temperature wave propagation throughout the network were also discussed in this work. The performance of turbulence models, including different versions of high Reynolds number two-equation models, (mostly k-epsilon type), have been investigated for a pipe-network fragment of a district heating system. Available experimental data has been used for validation. A methodology for analysing the transient temperature propagation was proposed, where transient values are in principle not linked with the average thermal level of the system. The strength and weakness of the above-mentioned models are discussed in this thesis. Moreover, heat transfer was modelled in a counter-flow double pipe arrangement (i.e., two adjacent pipes placed in a common insulation), which is practically applied in domestic hot water systems. The heat transfer coefficient and temperature distributions were predicted for typical operating regimes in such systems.