Momentum and Energy Transport

When solar radiation is converted into heat, thermal losses occur mainly due to passive heat transport modes as there are conduction through gases and solids, heat radiation and natural convection. In the latter one, already macroscopic mass transport driven by buoyancy effects can be observed. Convective heat transport is connected to momentum transport and movement of mass. Driven by mechanical pumps or ventilators the forced convective heat transport is supporting the thermal system by transporting heat with the help of a heat transfer medium from the place of optical conversion to the consumer. Effective heat and mass transport thus plays a key role in the design of a solar thermal system. Convective momentum and heat transport covers many aspects and is a-priori a nonlinear phenomenon. For example, flows can differ in the type of momentum transport, which can be forced, mixed or buoyancy-driven. In addition, the mass flow can have different flow patterns, for example laminar or turbulent. In the engineering treatment, a stability parameter plays a role in predicting the transition between the flow regimes. These regimes are even more complex in two-phase flows. Furthermore, the mass flow can be confined by walls, e.g. in tubes or between plates, often called bounded flows. On the other hand mass flow may fully include bodies as external flow for instance when an air flow occurs around a thermal storage tank. The interaction of mass flow and momentum transport within the media and with solid bodies not only leads to heat transfer but also to friction, which determines the hydraulic performance. External drives like pumps have to be dimensioned in a way that the pressure loss due to these hydraulic forces can be overcome. Last but not least, the effects of cold/heat can lead to phase changes, such as boiling, evaporation and condensation, which do not alter only the momentum characteristics but also the heat transfer characteristics of the transport. There is an extensive literature on convective momentum and energy transport in its various forms. The objective of this section is to elaborate the basic ideas of single-phase and multiphase momentum and energy transport, to show methods for classifying the flow type, and based on this, to present engineering approaches as well as relevant correlations for quantitative calculations, which can be used to carry out assessments.