Experimental and CFD studies of a thermoacoustic apparatus

This paper reports a study of a thermoacoustic system by both experimentation and simulation. A small scale thermoacoustic refrigerator prototype was built for experimentation purposes. In addition, a two-dimensional transient thermoacoustic model was solved by using computational fluid dynamics (CFD) software. In experimentation, a small thermoacoustic refrigerator (TAR) rig operated at a frequency of 133.45 Hz was built and the system was tested using two different types of stack; acrylonitrile butadiene styrene (ABS) and a stainless-steel scrubber. At atmospheric pressure and a relatively low frequency, a small temperature drop is recorded. Similar result was obtained using a simple CFD model that was designed based on the actual operating parameters of the experimental rig with ABS as a stack. The study shows that thermoacoustic principles can be used as a sustainable and green alternative technology for a refrigerator

Oscillatory Flow Across Plates With Different Shape Of Edges

Oscillatory flow is the type of flow found in the greener thermoacoustic based technologies.Understanding the behavior of the less understood oscillatory flow of this kind is one of the key feature for the success of the system.Heat exchanger is one of the important part of the system.In this study,oscillatory flow across pile of hot and cold parallel-plates heat exchanger with three different shape of edges (i.e. rectangular,round and triangular shape of edges) were investigated.A suitable computational model was created in ANSYS.The results were compared to theoretical predictions and a good match was found.The study shows that the shape of the edge affects the flow and heat transfer of the system.A triangle-shaped edges with shorter length provides the higher heat transfer between plates and the oscillating fluid compared to plates with round and square edges.The results indicated that the entrance effect could be the reason for the change of heat transfer performance as the shape of edge changes

Oscillatory flow across plates with different shape of edges.

Oscillatory flow is the type of flow found in the greener thermoacoustic based technologies. Understanding the behavior of the less understood oscillatory flow of this kind is one of the key feature for the success of the system. Heat exchanger is one of the important part of the system. In this study, oscillatory flow across pile of hot and cold parallel-plates heat exchanger with three different shape of edges (i.e. rectangular, round and triangular shape of edges) were investigated. A suitable computational model was created in ANSYS. The results were compared to theoretical predictions and a good match was found. The study shows that the shape of the edge affects the flow and heat transfer of the system. A triangle-shaped edges with shorter length provides the higher heat transfer between plates and the oscillating fluid compared to plates with round and square edges. The results indicated that the entrance effect could be the reason for the change of heat transfer performance as the shape of edge changes

Entrance and exit effects on oscillatory flow within parallel-plates in standing-wave thermoacoustic system with two different operating frequencies

Thermoacoustics is about conversion between thermal and acoustical energies to provide alternative green technology for power cycle and cooling system. The oscillatory flow across porous structure inside the system is playing the role of energy conversion between the acoustic wave and the surface of the porous structure. Better understanding of fluid dynamics of oscillatory flow inside thermoacoustic system is therefore important for the thermoacoustic based energy conversion system. This paper presents the ‘entrance’ and ‘exit’ effects of the oscillatory flow within a parallel-plate structure that is placed inside a standing-wave thermoacoustic environment. Two-dimensional SST k-ω CFD models which were validated using experimental data and theoretical predictions were used for this investigation. Two different operating frequencies of 14.2 Hz and 23.6 Hz were studied for flow with five different amplitudes that were represented using drive ratios of 0.3%, 0.83%, 1.5%, 2.1% and 3%. These correspond to cases with Reynolds numbers between 5936 and 62926. Due to the cyclic nature of the flow, a region defined as an ‘exit’ region was observed in addition to the usual ‘entrance’ region and the fully developed region for flow inside a channel. The change of shape of velocity profiles from the ‘m’ shape profile, to the ‘slug-like’ profile and ‘parabolic-like’ profile was discussed in relation to the ‘entrance’ and ‘exit’ effects on flow inside the channel. The ‘entrance’ and ‘exit’ effects become bigger as drive ratio increases. The effect of ‘entrance’ and ‘exit’ are slightly reducing as frequency increases from 14.2 Hz to 23.6 Hz. This may be related to the shorter travel distance of fluid as the frequency increases. The results shown in this paper suggest that the flow within a 200 mm parallel-plate structure should be treated as developing flow especially for flow with low resonance frequency at drive ratio higher than 1%

Numerical And Experimental Investigations Of The Oscillatory Flow Inside Standing Wave Thermoacoustic System At Two Different Flow Frequencies

Energy crisis has led to the search of sustainable and green technology and thermoacoustics have been recognised as one of them. One of the challenging issue with this emerging technology is the difficulty in understanding the complex fluid dynamics phenomena of the oscillatory flow of the acoustic wave inside the system. In this paper, computational fluid dynamics (CFD) models of a standing wave thermoacoustic flow conditions are solved using ANSYS Fluent and the CFD results are validated with experimental data from a similar setup; standing wave flow conditions with two resonance frequencies of 13.1 Hz and 23.1 Hz. Good match of velocity amplitude data was found between the CFD and the experimental results, particularly for the low flow frequency of 13.1 Hz. Similar trend of velocity results between numerical models and experimental results of higher frequency of 23.1 Hz is also observed. As frequency increases, the velocity amplitude did not change much but the displacement of fluid becomes smaller. This causes the vortex to travel rapidly but at a shorter distance into and out of the channel when the fluid flows at higher frequency of 23.1 Hz. The amplitude of annular flow also becomes closer to the wall because the viscous penetration depth becomes thinner as frequency increases. These results help in understanding the impact of frequency variations on fluid dynamics aspect of the standing wave inside the emerging technology of thermoacoustic systems

Thermal analysis of microchannel heat sink

Microchannel heat sink is now one of the most effective cooling techniques. As micropump works under pulsation regime and influenced by the possibility of heat transfer enhancement through pulsation, the goal has been to study the effect of pulsation to thermal behavior of microchannel heat sink. A computational model for studying pulsatile flow in microchannel had been developed using FLUENT. The meshes generated had been tested for grid independency and the results numerically iterated by FLUENT had been validated and compared to various published data. The pulsating flow pressure amplitudes were 50%, 70% and 90% of mean pressure and the flow regime is laminar. Pulsation tested was with frequencies in the range 500 Hz to 1.5 kHz. The results of pulsating flow simulations had been analysed and compared with the steady flow simulations. The values of the augmentation factor of heat flux along the flow direction were found to be less than unity. The values of the augmentation factor of heat transfer coefficient along the flow direction were less than unity at the entrance region and increased above unity further downstream. Pulsation had resulted in a lower wall temperature distribution compared to steady flow. The pulsation amplitude and frequency investigated has no significant effect on wall temperature. Heat flux ratio and heat transfer coefficient ratio however varies at frequencies and amplitudes investigate

Experimental and numerical studies of one-directional and bi-directional flow conditions across tube banks heat exchanger

Bi-directional flow condition imposes different fluid dynamics and temperature changes compared to that of the usual one-directional flow condition. Bi-directional flow can be found in applications like thermoacoustic systems that offer a green technology for at least two major applications: refrigeration and power production. The technology is appealing as an alternative to traditional systems as it offers the replacement for the use of harmful working media and exhausted resources with the use of inert gaseous with relatively fewer moving mechanisms. As the fluid dynamics and heat transfer of bi-directional flow in a thermoacoustic working environment is less known, it is difficult to estimate losses and gain, especially during the design stage. This paper reveals the differences to be expected in the behaviour of flow and heat transfer through experimental as well as Computational Fluid Dynamics (CFD) results of one-directional and bi-directional flow conditions. Two different drivers were used to create the two different flow conditions: a loudspeaker for the bi-directional flow and a centrifugal blower for the one-directional flow. Both conditions were monitored based on flow amplitude that is calibrated between the two drivers. Results of velocity, temperature and, vorticity are recorded for Reynolds number that ranges between 270 and 1700. Analyses are supplemented with data from validated two-dimensional computational fluid dynamics models that were solved using the Shear-Stress-Transport (SST) k-ω turbulence model with second-order accuracy for all equations. Interesting features of differences in temperature and velocity changes between the one-directional and the bi-directional flows are reported. The temperature and velocity at upstream and downstream locations of the tube banks heat exchanger are almost the same for bi-directional cases but are significantly different when a one-directional flow is flowing over the heated tubes. In addition, the interplay between natural and forced convections is seen to affect the results that were recorded for the two flow conditions. The presence of thermally developing and fully developed regions is also discussed. The results indicate that the heat transfer behaviour of bi-directional flow is not the same as in the one-directional flow and the future calculation for heat transfer for bi-directional flow conditions of thermoacoustic must be carefully done with consideration of changes of flow conditions between the one-directional and the bi-directional flow conditions so that error could be minimized in the evaluation of the system’s performance

Numerical and experimental study of flow behaviours in porous structure of aluminium metal foam

This study conducted a simulation and an experimental study on a channel that was partly filled with open-cell metal foam block. Different blockage ratios have been considered, where the foam height could occupy more than half of the channel size. The aim is to investigate the flow behaviours across the complicated structure of the porous medium. Results show that the use of Ergun and Forchheimer models showed a similar stagnant region and recirculation zone in the partially filled channel, which agreed with the experimental results. However, the deviation in pressure drop performance at a high blockage ratio is noticeable

Microstructural properties and surface roughness of 3D printed open cell-foam

In this study, the microstructure of open-cell metal foam was generated and reconstructed, to produce a new generation of open-cell foam, which is called 3D printed open-cell foam. At the current stage of research, nylon powder and plastic acid are utilized as the materials for two different 3D printing technologies: Selective Laser Sintering (SLS) and Fused Deposition Modelling (FDM), respectively. The microstructural properties and surface roughness of the 3D printed open-cell foam are investigated using CAD files and microscope images. The surface smoothness and structure strength are found to be dependent on the printing technologies, material employed, and foam size. However, the SLS technology produced smoother ligament surfaces with fewer residues than using the FDM. The ligaments of the small-size 3D printed open-cell foam at the exact size of the metallic foam, on the other hand, are weak and easily shattered. This study also found that the trends of pressure drop from additive manufacturing methods agreed to the original metallic open-cell foam, which are decreasing with the increase of pore sizes.</p

Microstructural properties and surface roughness of 3D printed open cell-foam

In this study, the microstructure of open-cell metal foam was generated and reconstructed, to produce a new generation of open-cell foam, which is called 3D printed open-cell foam. At the current stage of research, nylon powder and plastic acid are utilized as the materials for two different 3D printing technologies: Selective Laser Sintering (SLS) and Fused Deposition Modelling (FDM), respectively. The microstructural properties and surface roughness of the 3D printed open-cell foam are investigated using CAD files and microscope images. The surface smoothness and structure strength are found to be dependent on the printing technologies, material employed, and foam size. However, the SLS technology produced smoother ligament surfaces with fewer residues than using the FDM. The ligaments of the small-size 3D printed open-cell foam at the exact size of the metallic foam, on the other hand, are weak and easily shattered. This study also found that the trends of pressure drop from additive manufacturing methods agreed to the original metallic open-cell foam, which are decreasing with the increase of pore sizes.Process and Energ