Thermal and flow characteristics under oscillating acoustic field in a channel

• Main Author: Harikumar, Govind
• Other Authors: Fei Duan
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2021
• Subjects: Science::Physics::Heat and thermodynamics; Engineering::Mechanical engineering::Fluid mechanics
• Online Access: https://hdl.handle.net/10356/152685
• Institution: Nanyang Technological University

Thermal and fluid flow processes undergone by a gas in an oscillating acoustic field can convert sources of heat energy into acoustic power or use acoustic power to generate or pump heat from low to high temperatures. Thermoacoustic systems utilize this process for heating, cooling or power generation. They are environmentally friendly devices that use inert gases and have no or minimal moving parts, thereby offering high reliability and a long life span. The stack is the key component in thermoacoustic systems where energy conversion takes place. However, the underlying flow and heat transfer mechanisms of the oscillating fields in the internal channels of stacks is not well understood and therefore warrant further exploration. In this work, two different thermoacoustic systems were investigated. First, the standing wave system with a straight duct was designed and constructed. For this system, porous steel wool which was characterized based on the ratio of hydraulic radius to the thermal penetration depth was investigated and found to generate considerably high temperature difference which illustrates the thermoacoustic effect and the viability of using porous media as an alternative to regular geometries of stacks in standing wave systems. Additionally, to investigate the flow field behind channels of the stack, time-resolved particle image velocimetry measurements (PIV) were conducted and the phase averaged results revealed two pairs of counter-rotating dominant and residual vortices. Non-linear vortex shedding phenomenon was also observed which may affect the heat transfer between the fluid and stack. The second system investigated was the looped thermoacoustic system, where a part of the straight duct is replaced by a looped tube enclosing the stack. The phase averaged results of the time resolved PIV measurements showed the evolution of dominant and residual vortices in the flow field in which the dominant vortices remained attached to the ends of the stack and the corresponding residual vortices propagate with the mean flow. Here, due to the complex geometry of the loop, proper orthogonal decomposition was applied to identify the small-scale fluctuations from the prominent flow structures. Temporal coefficients revealed that the vortical flow structures varied from cycle to cycle. Finally, a transient computational fluid dynamic model was developed to observe the velocity, temperature and flow fields within channels of the stack. The simulation results showed good agreement with the experimental measurements at several frequencies and generated similar flow structures. A third pair of counter-rotating inner vortices formed during suction and a residual vortex layer were observed within the channel. Velocity profiles inside the channel showed the existence of peaks near the walls and a reduction of velocity at the channel core which was most prominent at resonance frequency. The temperature profile was also found to have analogous peaks and dips which were dependent on the velocity profile and the direction of heated gas flow. The distribution of peaks or dips in the channel varied with the viscous and thermal penetration depths which decreased with increasing mean pressure. These variations could affect the heat transfer between the stack and fluid where the average heat flux in a cycle was found to increase with Reynolds number for different working fluids.