Temperature And Velocity Changes Across Tube Banks In One-Directional And Bi-Directional Flow Conditions

The back-and-forth movement of flow in oscillatory flow condition that can be found in blood flow, thermoacoustic energy system and ocean wave can be categorized as bi-directional flow condition and heat transfer in this flow condition is not well understood. This paper reports an experimental investigation that compares temperature and velocity values between the onedirectional (the usual flow condition) and the bi-directional flow conditions. The experiment was done using thermoacoustic’s standing wave rig with two different drivers to drive the one-directional flow and bi-directional flow conditionsin the test rig. Results, that were recorded using piezoresistive pressure sensor, type-K thermocouple and hotwire anemometer, indicate that care should be exercised when calculating heat transfer in bi-directional flow conditions as the temperature and velocity changes are different compared to the one-directional flow condition. Differences were recorded to be within the range of 77% for temperature and 59.5% for velocity, presumably due to the different behavior of forced and natural convection effect as flow conditions change

The impact of stack parameters on the temperature difference of a thermoacoustic cooler

Thermoacoustics offer alternative solution for cooling needs where a method that is safer to environment is used. The thermodynamic process that needs to be completed by using interaction between inert gaseous and porous material must be made efficient so that the system works properly. This paper reports numerical and experimental investigations of the use of several porous material in air at atmospheric pressure to provide cooling effect. Experimental investigation was also conducted by using cheap and abundant materials as the porous media. Results were collected at two different frequencies and with two different stack lengths. The study showed that thin-walled honeycomb porous structure made of polycarbonate offers the best temperature for thermoacoustic cooler with air at atmospheric pressure. The best COP of 4.73 was recorded. Disparity between numerical and experimental results is expected to be the result of losses that need to be carefully addressed in the future especially when long stack is used in the system

The effect of porous materials on temperature drop in a standing wave thermoacoustic cooler

Thermoacoustics is a principle of sciences that offers an alternative solution for cooling system with a technology that is green and sustainable. The thermoacoustic energy conversion takes place mostly within the area of the porous structure that forms the core of the system. In this study, the effect of changing the material of the porous structure on the performance of the thermoacoustic refrigerating system is reported. Experiments were performed under standing wave environment with two different resonance frequencies with air at atmospheric pressure. The porous stack was chosen to be with three different materials of polycarbonate, ceramic and stainless steel. The results show that the use of ceramic celcor as the porous material provides the biggest temperature difference which means that thermoacoustic performance is better. The performance is even better when the system is working with higher resonance frequency. At atmospheric pressure condition with air as working medium, the thermoacoustic cooler with ceramic porous material is capable of producing temperature difference of 39.16C when operating at a frequency of 202.1 Hz

Numerical investigations of fluid flow and heat transfer processes in the internal structures of thermoacoustic devices

Thermoacoustic devices are built based on interactions between sound wave and a solid boundary, within a well-controlled environment, to produce either power or cooling effect. Recent studies on two important internal structures, namely regenerator and heat exchangers, are reviewed. Furthermore, the need for detailed investigations on a pressure drop condition in the flow through the regenerator and heat transfer condition in heat exchangers working in a thermoacoustic environment is also addressed. A two-dimensional porous medium model is developed based on the pressure drop measurement of a regenerator working in a well-controlled travelling-wave time-phasing, wherein the pressure and velocity of the oscillatory flow across the regenerator are controlled to be in-phase. A friction correlation is proposed based on Darcy’s law. The model is developed in a commercial software ANSYS FLUENT to determine a permeability coefficient for the model. The findings suggest that a steady-state correlation is applicable provided that the travelling-wave time-phasing is met. Otherwise, a phase-shift effect should be considered and the steady-state approximation may no longer hold true. A pair of adjacent plate heat exchangers in the oscillatory flow is studied. It is shown that the application of the temperature difference between “cold” and “hot” plates leads to interesting asymmetries within the flow field. Also a need for a turbulence model at a drive ratio lower than suggested in current literature is discovered and discussed. It is found that the heat absorbed by the cold plate is lower than the heat supplied by the hot plate and heat accumulation is observed in the system. The vortex structures and viscous dissipation change with operating conditions. The combined effect of flow amplitude, natural convection and the “annular effect” of velocity profiles near the channel wall on the flow are discussed. A good agreement with experimental results obtained previously is shown

Cooling Enhancement through Pulsation Flow in 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 a commercial Computational Fluid Dynamics, (CFD) package FLUENT. The meshes generated had been tested for grid independency and the result numerically iterated by FLUENT had been validated and compared to various published data. The pulsating flow 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. Pulsation had resulted in a lower wall temperature distribution, therefore enhanced cooling, compared to steady flow

DeltaE modelling and experimental study of a standing wave thermoacoustic test rig

Thermoacoustics is a principle of sciences that could be used to create an alternative green and sustainable technology for a cooler or a generator. Unfortunately, the fluid dynamics of the oscillatory flow within thermoacoustic environment is less understood especially as the flow conditions change to higher values of operating conditions. This leads to difficulties in design practices of the system. In this paper, a test of an experimental rig for the investigation of fluid dynamics of an oscillatory flow inside a standing-wave thermoacoustic rig with two different flow frequencies are reported. An experimental setup was build and numerical modelling is also solved using a thermoacoustic software known as DeltaE. The rig consisted of a quarter wavelength resonator attached to a loudspeaker that acts as an acoustic driver. A structure known as ‘stack’ is located at a location of approximately 0.19 from the pressure antinode. Experimental results showed that the resonance frequency of the two setups are 14.2 Hz and 23.6 Hz, respectively. Measured velocity and pressure at several locations are analysed and the results indicated that the thermoacoustic flow conditions are achieved. The rig could be used for further and deeper investigations of fluid dynamics behaviour for oscillatory flow of thermoacoustic

Application of 3D printing technology in thermoacoustic stack fabrication

A thermoacoustic refrigerator uses an inert gas as the working fluid instead of refrigerants; thus, considered environmentally friendly. However, its performance is still low because the desired cooling at the stack is much lower than that it was designed for. The factor has been attributed mainly to the stack, the “heart” of the system. Stack finished product has been inconsistent due to the methods available to obtain the desired geometry and dimensions even with available optimized design parameters. This paper presents performance results from stacks fabricated using 3D printing technology which minimizes the error, disposes of irregularities and can reduce production time of the stack. The temperature difference across the stack is measured to determine the performance of the thermoacoustic refrigerator. Experiments were done at 400 Hz frequency with different stack plate spacing and plate thickness, placed in a 21-mm diameter resonator. Results show that the 0.7 mm stack plate spacing with a 0.5 mm plate thickness performed better compared to those with smaller spacing at the same thickness or with the same spacing but larger thickness. The outcomes of this study have shown the need for the fabrication technology to keep pace with optimized design to realize global efforts towards a sustainable environment