Master's thesis Friedrich Schlieter

 

Investigation of heat transfer in air-to-water heat exchangers for active and passive chilled beams

Flow visualization downstream the fin-and-tube heat exchanger at an air velocity of 0.1 m/s. Copyright: EBC Flow visualization downstream the fin-and-tube heat exchanger at an air velocity of 0.1 m/s.

Thermal comfort is one of the most important factors influencing the overall comfort conditions for a human being. Thermal comfort and indoor air quality is maintained by heating, ventilation and air conditioning systems. In modern office environments there are an increasing number of heat sources due to technical equipment besides other heat loads from solar radiation and artificial light sources. To meet the cooling demand air-water systems can be used. The main advantage of water is its high specific heat capacity and thermal conductivity improving installation space demand and energy efficiency for the transport of the thermal loads. Additionally, this decentralized system offers a high degree of flexibility and improves individual and local thermal comfort. Air-water systems are divided into passive chilled beams and active chilled beams, the latter being able to supply primary air to the room. The basic component for both chilled beams is an ordinary fin-and-tube heat exchanger that is used with low air flow rates. Especially in the case of passive chilled beams, in which the air flow is only due to natural convection, air velocities across the heat exchanger are usually below 0.1 m/s.

The present numerical and experimental study has been carried out to investigate the air side heat transfer coefficient for a fin-and-tube heat exchanger in the realm of natural convection to forced convection. The computational fluid dynamics (CFD) software Ansys CFX is applied for the numerical calculations. The results of the experimental and numerical studies are compared to various Nusselt correlations available in the literature. Based on the experimental results a correlation is proposed describing the heat transfer coefficient in the velocity range from 0.02 m/s to 0.2 m/s. For higher air velocities Nusselt correlations from the literature are identified, that can be used in the case of the given heat exchanger geometry.