Local resistance characteristics of elbows for supercritical pressure RP-3 flowing in serpentine micro-tubes

Based on the demands of compact heat exchangers and micro cooling channels applied for aviation thermal protection on aero-engines, the elbow local flow resistance characteristics for supercritical pressure aviation fuel RP-3 flowing in adiabatic horizontal serpentine tubes with the inner diameter of 1.8 mm and the mass flux of 1179 kg/(m²·s) were experimentally studied. The long-short-tube method was used to obtain the elbow pressure drop from the total serpentine tube pressure drop, and the effects of system pressures (P/P_c = 1.72–2.58) and geometry parameters including bend numbers (n = 5–11), bend diameters (D/d = 16.7–27.8), and bend distances (L/d = 20–60) on elbow pressure drops and local resistance coefficients are analyzed on the basis of the thermal physical property variation. The results show that both the increase in the elbow pressure drop and the decrease in the local resistance coefficient with temperatures speed up at the near pseudo-critical temperature region of T > 0.85T_pc. And the growth of the elbow local pressure drop could be inhibited by the increase of system pressures, while the local resistance coefficient is slightly affected by pressures. The influence of bend diameters on the local resistance coefficient is mild when D/d is larger than 22.2 in the premise of fully developed flow in straight tubes. Furthermore, a piecewise empirical correlation considering the bend diameter and physical property ratio is developed to predict the elbow pressure drop of the serpentine tube and optimize the layout of the cooling tube system on aero-engines.