The turbulent pulsating flow and heat transfer in straight and 90° curved square pipes are investigated in this study. Both experimental temperature field measurements at the cross-sections of the pipes and conjugate heat transfer (CHT) simulation were performed. The steady turbulent flow was investigated and compared to the pulsating flow under the same time-averaged Reynolds number. The time-averaged Reynolds number of the pulsating flow, as well as the steady flow, was approximately 60,000. The Womersley number of the pulsating flow was 43.1, corresponding to a 30 Hz pulsating frequency. Meanwhile, the Dean number in the curved pipe was approximately 31,000. The results showed that the local heat flux of the pulsating flow was greater than that of the steady flow when the location was closer to the upstream pulsation generator. However, the total heat flux of the pulsating flow was less than that of the steady flow. Moreover, the instantaneous velocity and temperature fields of the simulation were used to demonstrate the heat transfer mechanism of the pulsating flow. The behaviors, such as the obvious separation between the air and pipe wall, the low-temperature core impingement, and the reverse flow, suppress the heat transfer.

The objective of this study is to investigate experimentally the effect of pulsation frequency on the heat transfer characteristics and the mechanism of the pulsation flow, which is representative of the operating conditions of the engine exhaust flow. The experimental apparatus consists of a rotating disk with holes that converts steady hot air flow rate into a pulsating flow to exchange heat energy with external air. The fluid temperature is measured by thermocouples, and the wall temperature is measured by thermography. It is found that heat transfer enhancement due to pulsation does not occur at frequencies below 25 Hz, even though the velocity amplitude is large. In order to investigate the cause of this phenomenon, the flow field is measured by PIV(Particle Image Velocimetry) and the turbulent kinetic energy is evaluated. It is clarified that the turbulent kinetic energy near the wall is small at frequencies below 30 Hz, despite the large velocity amplitude. From the time series of velocity data, it was observed that the turbulence is extremely small during the acceleration phase of the fluid. As a result, the turbulent mixing during the acceleration phase is suppressed, and the time-averaged turbulent kinetic energy becomes small, which is thought to have suppressed heat transfer enhancement. This is the first attempt to experimentally link heat transfer and flow structure fluctuations in a pulsating flow, which is achieved by unsteady measurement of the flow field using PIV and calculation of the turbulent kinetic energy.

This paper discusses the turbulent flow and heat transfer from a uniform air flow with high temperature to the outside through a 90° curved square pipe. Both conjugate heat transfer (CHT) simulation and experiments of temperature field measurements at cross sections of the pipe are performed. A straight pipe is investigated and compared with the 90° curved pipe. The temperature of the air flow at the inlet of the pipe is set at 402 K, and the corresponding Reynolds number is approximately 6 × 104. To obtain the spatial average temperature at each cross section, the temperature fields are measured along the streamwise of the pipes and in the circumferential direction using thermocouples at each cross section from the inlet to the outlet of both the straight and curved pipes. Furthermore, the simulation is performed for turbulent flow and heat transfer inside the pipe wall using the Re-normalization group (RNG) k-ε turbulence model and CHT method. Both the experimental and numerical results show that the curvature of the pipe result in a deviation and impingement in the high-temperature core and a separation between the wall and air, resulting in a secondary flow pattern of the temperature distribution.

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