Hydraulic and thermal characterization of a family of thermo-hydraulic separators

Thermal-hydraulic separators (or hydraulic dispatchers) are flow collectors of relatively small size, connecting two or more hydraulic networks; they are mainly used to reduce the hydraulic interference of primary (e.g. heat supply) and secondary (e.g. heat consumer) circuits, thereby simplifying the system analysis, saving energy and operating complex hydraulic networks more safely. Hydraulic dispatchers are key components of modern district heating networks and of building water systems: nonetheless, little is known about their internal flow and temperature distribution and about their off-design performance. The transfer of thermal energy from the primary to the secondary circuit is governed by the secondary-to-primary flow rate ratio and affected by the turbulent mixing within the device. A simple thermal model is commonly used for design purposes: it disregards the actual flow and temperature pattern within the device and relates the inflow and outflow temperature by neglecting the mixing of supply and return streams. This simple approach is potentially inaccurate under certain operating conditions (e.g. relatively high flow rates) and deserves a validation study. Numerical simulations of flow and heat transfer are carried out, for a family of geometrically similar thermal-hydraulic separators under different operating condition. The reported numerical tests show that the aforementioned model is a reliable design tool under most operation conditions. Furthermore, it is verified that the device introduces a relatively modest pressure loss on the connected circuits, in particular when the flow rate ratio is close to one.