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

Investigation on standing wave thermoacoustic generator using DeltaEC

There is currently an urgent demand to reuse waste heat from industrial processes with approaches that require minimal investment and low cost of operation. Thermoacoustic generator (TAG) is a device that converts heat energy into useful work through the use of acoustic wave, porous media (honeycomb ceramic celcor) and heat exchangers that are all enclosed in a custom-defined resonator. This paper reports the basic design of thermoacoustic generator that is tested using a design software known as a Design Environmental for Low-amplitude Thermoacoustic Energy Conversion (DeltaEC). Many studies have highlighted the relationships between the geometry of the stack and the performance of the device. In this study, attention is given on the impact of the length of stack which was found to be the best at a length of 0.6 m when the frequency of the flow is at 127.4 Hz. Performance indicators like the acoustic power and the temperature difference across the stack have been used to analyse the results. The result shows that the highest acoustic power can be achieved when the generator that work with air at an atmospheric pressure is designed with a resonator of 2.14 m long and a stack with a length of 0.6 m. The maximum value for acoustic power is predicted to be as much as 24.01 kW

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

Oscillatory Flow and Heat Transfer Within Parallel-plate Heat Exchangers of Thermoacoustic Systems

Oscillatory flows past solid bodies are a feature typical for thermoacoustic systems. Understanding such flows is one of the keys for improving the system performance. This work investigates oscillatory flows around parallel-plate heat exchanger through numerical modeling developed based on the experimental data obtained in-house within a standing-wave thermoacoustic setup. Attention is given to developing a model that can explain the physics of phenomena observed in the experimental work. Four drive ratios (defined as maximum pressure amplitude to mean pressure) were investigated: 0.3%, 0.45%, 0.65% and 0.83%. The suitability of selected turbulence models for predicting the flow phenomena at varied drive ratios has been tested. Discussion of results is based on the velocity profiles and vorticity contours within the flow. Associated heat transfer phenomena are also discussed

CFD modelling of flow and heat transfer within the parallel plate heat exchanger in standing wave thermoacoustic system.

Thermoacoustics is a sustainable technology for novel energy conversion applications. The performance of future thermoacoustic systems can be improved through a better understand- ing of the complex fluid flow and heat transfer phenomena that form the fundamental basis for the system construction. Here, a particular attention is focused on heat exchangers. This paper investigates the flows and heat transfer within parallel plate heat exchangers working in a thermoacoustic environment characterised by an oscillatory flow induced by a standing wave. A two-dimensional computational fluid dynamics (CFD) model was developed and validated using earlier experimental data obtained within our group. The natural convection effect which is commonly neglected in most numerical analyses was included in this compu- tational investigation to account for temperature-driven buoyancy effects observed in exper- iment. The flow and heat transfer characteristics were investigated by obtaining the velocity and temperature profiles over twenty periods of a flow cycle. The velocity profile was found to be distorted due to the presence of temperature, indicating a change in the flow structure. Temperature profiles produced by the computational model agreed qualitatively with the ex- perimental model, but with differences in magnitude particularly noticeable in the area of the hot heat exchanger. Thus the temperature profile appears to have the same trend and pattern over the whole phases investigated, apart from the slight differences in the aforementioned area. Accordingly, the space average wall heat flux was discussed for different phases and lo- cations across both cold and hot heat exchanger. Discussion includes the effect of gravity and device orientation to the flow and heat transfer. The results thus contributed toward a better understanding of the hydrodynamic and thermal performance of the flow investigated and eventually it will assist in experimental design for future research

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.EThOS - Electronic Theses Online ServiceMinistry of Higher Education MalaysiaUniversiti Teknikal MalaysiaMelakaGBUnited Kingdo