Abstract
Rotating machines are widely encountered in industrial applications. However, it is very hard to measure their internal flow fields. As a result, the relevant fluid flow information is scarce in the open literature and their designs often rely on trial and errors. Moreover, although heat transfer results based on thermocouple readings have provided valuable information for reference, they gave actually regional averaged values instead of local values. In view of this fact, non-intrusive and full-plane heat transfer measurement techniques are preferred to reveal heat transfer inhomogeneity and in turn thermal stress concentration information in rotating coolant channels. This study is concerned with the characteristics of fluid flow as well as heat transfer enhancement and uniformity. Transient thermochromic liquid crystal thermography (TLCT), a laser-Doppler velocimeter, and pressure transducers have been used to measure the local heat transfer, velocity, and pressure drop distributions, respectively, in a rotating two-pass square duct with 90° ribs on the leading and trailing walls. Three types of rib configurations are considered: attached, detached, and detached/attached rib cases. All ribs were square in cross-section and the ratio of detached-distance to rib-height was 0.38. The rib-height/duct-height ratio and the pitch/rib-height ratio were 0.136 and 10, respectively. These values of parameters were selected according to previous results of stationary coolant ducts. The duct Reynolds number was varied from 2500 to 40000 and rotation number ranged from 0 to 0.8 for heat transfer experiment. Results are presented of local and regional averaged Nusselt number, mean and turbulent velocity components, turbulent kinetic energy, pressure coefficient distributions and variation of friction factor with rotation number. For detached ribbed case, the competition between convection effect of the wall jet and downwash effect of the rib-top separated shear layer on the heat transfer augmentation is addressed in detail. Rib detachment is found to enhance heat transfer on the leading wall of the first outward pass and on the trailing wall of the second inward pass over as compared to the attached rib case. The trend is reversed on the other two walls. Nevertheless, detached ribs provide more uniform heat transfer distributions on the leading and trailing walls than attached ribs but higher pressure loss. According to the measured results of attached and detached ribbed cases, this study suggests a better rib configurations (detached/attached case) installed in the rotating internal coolant duct for practical reference of designing a turbine blade internal coolant channel. The case of detached/attached ribs can attain passage averaged heat transfer augmentation of Nup/Nu0 = 4.57 under a constant flow rate and Nup/Nu0* = 2.58 under a constant pumping power. For engineering reference, the passage averaged Nusselt number ratios on the leading and trailing walls of the first and second passes can be correlated as linear functions of rotation number for attached, detached, and detached/attached ribbed cases in the parameter range examined. Moreover, simple expressions are also developed to linearly correlate the friction factor with rotation number for smooth wall, attached, detached, and detached/attached ribbed cases. Power spectral analysis of the fluctuating velocity demonstrates that the vortex shedding only occurs behind the first two rib pairs for the stationary case. The predominant shedding frequency is 520 Hz correspond to a Strouhal number 0.2 which is in good agreement with the bluff-body wake flow results of the previous researchers. When the coolant duct is rotated, there is no predominant frequency can be found in the fluctuating velocity spectrum, suggesting that vortex shedding does not play a role in the heat transfer enhancement of rotating coolant duct. It should be pointed out that there are no measurements of fluctuating velocity power spectrum under rotating condition in the previous studies. The spectral analysis further indicates that the integral time scale near the leading and trailing walls increases and decreases linearly with increasing rotation number. A decrease in the turbulent integral time scale denotes a decrease in the lifetime of eddy breakup and, in turn, the enhancement of turbulent mixing or heat transfer. This observation provides the rationale for the increased heat transfer augmentation on the trailing wall of the first pass with increasing rotation number.