Suggestion of Correlations to Obtain Shell and Tube Side Nusselt Numbers in Coiled Heat Exchangers

Abstract

In coiled tube heat exchangers increase in curvature ratio and effective contact of shell fluid with coil surface led to better thermal characteristics. This work addresses study of thermal performance, shell and tube heat transfer coefficients (h_o and h_i). Also, mathematical correlations are predicted to obtain shell and tube Nusselt numbers (Nu_o and Nu_i). In the designing of compact coiled heat exchangers consisting of straight helical coil (SHC), conical coil (CC) and spiral coil knowledge of Nu_o and Nu_i is necessary whereas mathematical correlation to calculate Nu_o considering variations in shell geometry is not found. Five heat exchangers consisting of three cone coils (angle, ϴ = 30°, 50° and 70°), spiral coil (ϴ = 0°) and SHC (ϴ = 90°) are tested. Using Wilson plot method h_o and h_i are obtained for wide range of tube side Reynolds numbers (Re_i = 3700 - 21000). It is observed that, highest h_o is obtained for ϴ = 30°CC with better thermal performance. Lowest h_o is obtained for ϴ = 90°SHC. Ratio of height of shell, H_s and diameter of shell, D_so is considered in prediction of single correlation as: Nu_o = 16.55.Re_o^0.55.Pr_o^0.4 *(H_s/D_so)^0.096. Comparison with experimental findings shows variation of 0-14%. Similarly two correlations are proposed as: Nu_i = 0.0512.Re_i^0.80.Pr_i^0.4.CR_ave^0.18 and Nu_i = 0.0365.De_i^0.815.Pr_i^0.4.CR_ave^-0.3. A fair agreement is found with existing researchers.