Stratification in hot water pipe-flows

In district heating and cooling networks, hot or cold fluid is transported in pipe systems over long distances. Heat loss (or gain) from the fluid-carrying pipes to the surrounding is a primary driving mechanism that can lead to various thermal dynamics effects in the fluid. Here, we investigate the thermal dynamics of low-Reynolds number hot water flow in horizontal pipes. In such flows, density differences between hot and cold water generate buoyancy effects. Further, the coupling between heat transfer and momentum transfer along with the temperature-dependent viscosity of water can result in stratified asymmetric flow profiles where fluid is transported with high velocities in the top region of the pipe cross-section and can be almost stagnant in the bottom region. In the flow laboratory at Kamstrup A/S, we carried out experiments to analyze thermally stratified flow profiles with laser-Doppler velocimetry. In this article, we present results for the downstream development of stratified flow profiles for different flow parameters including temperature, pipe diameter, and flow rate (or dimensionless Reynolds number and Rayleigh number, respectively.) Further, we discuss the impact of thermally stratified flow profiles on metering applications in district heating and cooling networks.