Abstract:
Storage tanks based on helical coil heat exchangers are an integral part of solar water
heating (SWH) systems and require optimal design configuration to achieve maximum
heat transfer and a longer storage period. This study is focused on thermal analysis of a
vertical helical coil-based water storage tank and derivation of inner and outer Nusselt
number correlations by considering the storage tank as well as helical coil and catering to
both forced and natural convection effects. A fluid-fluid conjugate heat transfer transient
state model is validated with experiments performed with 50/50% water-glycol mixture
as HTF at flow rates of 2 and 3 L/minute in a solar water heating system, while water
remains at static flow condition inside the tank. To reduce the computational cost, a
symmetrical downsized model is also designed and validated, which showed a mean
absolute percentage error of 3.25%. In order to derive the mathematical correlations,
alterations were made to the validated model by using coils with curvature ratios ranging
from 0.09-0.184 with HTF flow rates in laminar flow regimes, ranging from 30 to 100
liters per hour at temperatures from 40 C. Furthermore, Nui mathematical correlations based on M number, curvature ratio, and Prandtl number showed good agreement with correlations derived by past investigators. Similarly, the outer Nusselt number of the coil was related to Rayleigh’s number and the derived relations showed good agreement with the past studies, while Nuo= 0.1996 (Rado 0.326o C to 80) showed the least error of 2.06%. Later on, thermal stratification analysis helped to understand that coil position and aspect ratio determine the thermocline developed inside the storage tank and hot water storage for longer periods. Finally, a comparative study was carried out to determine the variation in inner and outer HTC with different geometric parameters of helical coil.
