1. HEAT TRANSFER ENHANCEMENT DUE TO ACOUSTIC EXCITATION Presented by Ross Tuite Supervisors: Dr. Gareth Bennett Prof. Darina Murray
2. Introduction and Background Situation European electronics industry – €45bn, 177,000 jobs Transistor density doubling every two years
3. Introduction and Background Situation European electronics industry – €45bn, 177,000 jobs Transistor density doubling every two years Problem Heating exceeds Cooling Heat Transfer Bottleneck Fans required to remove heat Added noise
4. Introduction and Background Situation European electronics industry – €45bn, 177,000 jobs Transistor density doubling every two years Problem Heating exceeds Cooling Heat Transfer Bottleneck Fans required to remove heat Added noise Solution Constructive use of fan noise Heat transfer enhancement using thermo-acoustic effect
5. Thermo-acoustic Effect Classical Approach Phase-lock sound wave with thermal input (Rijke tube) Generation of high-amplitude sound energy
6. Thermo-acoustic Effect Classical Approach Phase-lock sound wave with thermal input (Rijke tube) Generation of high-amplitude sound energy Current Approach Superposition of acoustic field on turbulent flow Increased mixing in the flow increased heat transfer
7. Thermo-acoustic Effect Classical Approach Phase-lock sound wave with thermal input (Rijke tube) Generation of high-amplitude sound energy Current Approach Superposition of acoustic field on turbulent flow Increased mixing in the flow increased heat transfer Two Mechanisms Observed: Added particle velocity (acoustic fluctuations) and mixing
8. Thermo-acoustic Effect Classical Approach Phase-lock sound wave with thermal input (Rijke tube) Generation of high-amplitude sound energy Current Approach Superposition of acoustic field on turbulent flow Increased mixing in the flow increased heat transfer Two Mechanisms Observed: Added particle velocity (acoustic fluctuations) and mixing Acoustic streaming at high amplitudes
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11. Investigation and Main Aims Novel Approach: Examine physics of process Understand and optimise effect Novel testing conditions: Turbulent flow Low acoustic amplitude Open-ended duct
12. Rig and Set-up Hydrodynamic/ Acoustic Fields Schematic of Rig
13. Acoustic Theory Introduction of Standing Waves 63Hz and 167Hz: Maximum pressure, zero velocity 113Hz and 223Hz: Zero pressure maximum velocity
16. Rig and Set-up PMMA duct held vertically between fan and speaker Flow temperature (Tm) – Cold-wire Surface temperature (Ts) – Thermocouple Surface Heat Flux (q’’) – Hot-film All measurements taken along same plane Heat transfer at surface: Newton’s Law of cooling
17. Experimental Procedure Cross-wire Measurements Stepper Motor Axial mid-point 15 Temperature and velocity measurements Four resonant frequencies at each point
The results above were found by Dr. John Mahon on the same rig last year and are evidence of the presence of acoustic streaming patterns in the flow. The graphs show a significant increase in mean and fluctuating velocity in the vicinity of the hydro-dynamic boundary layer. Acoustic streaming increase the mean velocity in the flow locally and can therefore be used to increase heat transfer from a heated surface
The current study follows on from these observations. The goal is to try and understand the mechanisms involved in these results.
The study is also different from many others in that it is applied to a turbulent flow. Most other studies deal with laminar conditions. Secondly, it considers the presence of acoustic mechanisms at low amplitudes which are more applicable to real-world situations. Finally, most studies perform tests in a closed volume such that the heat is circulated around a small area. In this case, there is a NET heat loss from the system, which is important in applications for enhanced cooling
The four figures above show the pressure and velocity profiles for the four standing wave frequencies of the duct. This is the duct section here, with the central heated section, and two PMMA sections. The study is looking at the heat transfer at this point. Note that the top two figures correspond to frequencies for which a maximum pressure and zero velocity exists at the sensor, while the bottom two show frequencies at which a maximum velocity and zero pressure is located at the measurement plane
The local flow behaviour is measured using a cross-wire probe, which combines the principles of cold-wire and hot-wire anemometry. Cold-wire anemometry records the flow temperature by relating it to resistance at a constant current (CCA) and hot-wire anemometry records flow velocity by relating this to voltage at constant temperature (CTA)The hot-film works very like a hot-wire, recording the heat flux by measuring the voltage required to keep the film at a constant temperature.
The cross-wire measures temperature at the axial mid-point of the duct, corresponding to either a velocity or pressure anti-node in the duct. It takes 15 discrete readings along the radius of the pipe. At each position, the four resonant frequencies are played by the speaker so that the effect of this sound on the results can be compared
The results above correspond to mean flow temperature with and without forced convection. 0mm above refers to the centre of the pipe and 50mm refers to the temperature at the wall. The effect of an added particle velocity is easier to see without a mean flow through the duct. Each resonant frequency has an effect on the temperature inside the duct, increasing the temperature of the main flow and decreasing the temperature at the wall. The highest deviation in temperature was found at the highest acoustic amplitude. There are also signs of acoustic streaming in the main flow and in the vicinity of the thermal boundary layer.The drop in temperature at 63Hz can be explained using the mean velocity results.
At high velocity, the mean flow rate also changed with frequency through the duct, which explains the cooling effect at 63Hz. There is no obvious effect of acoustic streaming, which would result in a peak in the mean velocity near the wall. However, the fluctuating results verify that frequencies corresponding to velocity anti-nodes have a significant influence on the turbulent behaviour of the flow
The heat transfer coefficient could not be found quantitatively but is instead represented by the bridge voltage required to keep the hot-film at a constant temperature.The heat flux results show that increasing the mean flow, although disguising the affect of an added particle velocity, does not affect the overall enhancement in heat transfer due to the acoustic field. The results show that frequencies corresponding to particle velocity anti-nodes (113Hz and 223Hz) are more effective at removing heat from the surface and have a larger influence on heat transfer than the frequencies corresponding to velocity nodes.In general, the bridge voltage is higher for the forced convection case since a higher voltage is required to keep the sensor temperature constant. Assuming that acoustic streaming has had no effect, these results show that higher frequencies increase the local fluctuations which would result in a steady increase in heat flux.
On the left is the surface temperature during a single iteration. The graph shows that the temperature decreases dramatically at acoustic pressure anti-nodes and recovers during when exposed to a pressure node.For forced convection, the drop in surface temperature at the fundamental frequency is similar to that for free convection. On the right, the average flow temperature is graphed as a function of standing wave frequency.
The main conclusion that can be drawn from the results is that signs of both mechanisms of heat transfer enhancement can be identified using the set-up and parameters described in this study. These results are significantly in furthering the understanding of these mechanisms as tests done with turbulent flow conditions under low acoustic amplitudes are limited.
The project offers significant industrial value and scope. Further study in the area will look at enhancing heat transfer using only the acoustic energy of the fan. The noise can also be minimised by introducing non-planar acoustic fields down the pipe below the cut-off frequency which will prevent the sound from propagating to the surroundings. Thus we get an increase in heat transfer with virtually no noise.
