SINGLE-PHASE CONVECTIVE HEAT TRANSFER AND PRESSURE DROP COEFFICIENTS IN SMOOTH CONCENTRIC ANNULI

Varying diameter ratios associated with smooth concentric tube-in-tube heat exchangers are known to have an effect on its convective heat transfer capabilities. Linear and nonlinear regression models exist for determining the heat transfer coefficients; however these are complex and time consuming, and require much experimental data in order to obtain accurate solutions. A large dataset of experimental measurements on heat exchangers with annular diameter ratios of 0.483, 0.579, 0.593 and 0.712 was gathered. Using the modified Wilson plot technique, a nonlinear regression scheme, and the log mean temperature difference method, both local and average Nusselt numbers were determined. Local wall temperature measurements were made using a novel method. Friction factors were calculated directly from measured pressure drops across the annulus. Both heated and cooled horizontal annuli with Reynolds numbers based on the hydraulic diameter varying from 10 000 to 45 000 with water as the working medium were investigated. INTRODUCTION Convection heat transfer is the main heat transfer mechanism in most heat exchangers. It is thus essential that the convection heat transfer and friction factors are thoroughly understood in order to optimally design heat exchangers. The tube-in-tube heat exchanger with a counter flow configuration operating in the turbulent flow regime is one of the simplest types of heat exchangers. Correlations for the heat transfer coefficients and friction factors for such heat exchanger types have been found to be inconsistent with each other. The annular diameter ratio, which is defined as the ratio of the inner tube’s outer diameter to the outer tube’s inner diameter have been reported to influence both the heat transfer and pressure drop within the annulus. Due to the complexities of turbulent flow it is required that experimental data is used to determine correlations for heat transfer coefficients and friction factors within the annulus. NOMENCLATURE a [-] Annular diameter ratio As [m 2 ] Surface area Ao [m 2 ] Cross sectional area of annulus Ci [-] Inner tube correlation coefficient Co [-] Outer tube correlation coefficient Cp [J/kgK] Specific heat CV [-] Control volume Dh [m] Hydraulic diameter eb [%] Energy Balance f [-] Friction factor Fann [-] Factor taking into account the dependence on a h [W/mK] Convective heat transfer coefficient j [-] Index number k [W/mK] Thermal conductivity K [-] Factor to take into account the temperature dependence of fluid properties Ldp [m] Pressure drop length Lhx [m] Heat exchange length [kg/s] Mass flow rate Ni [-] Number of thermocouples on inner tube Nu [-] Nusselt number n [-] Exponent P [-] Reynolds number exponent Pr [-] Prandtl number ∆p [Pa] Pressure drop [W] Heat transfer rate [W] Average heat transfer rate Re [-] Reynolds number Re* [-] Modified Reynolds number, Equation (2) and (15) T [°C] Temperature [°C] Logarithmic mean temperature difference [°C] Average temperature V [m/s] Fluid velocity x [m] Axial length along the heat exchanger Special characters μ [m/s] Kinematic viscosity ρ [kg/m 3 ] Density φ [-] Factor in Equation (2) Subscripts 1 Inner tube outer wall b Bulk property Dh Based on hydraulic diameter i Inner tube