IR technology and image processing applied to flow investigation
IR - thermography is a useful contactless instrument for detecting very fast temperature changes with adequate accuracy. Our investigations of heat transfer processes in pipe flows show that we can determine the moment of the loss of stability and transition to turbulence of the accelerated laminar flow.
The IR - thermography has proved to be a useful instrument in fluid flow research, especially for investigation of heat transfer processes. Series of experimental measurements of suddenly accelerated and pulsating pipe flow were made at Satakunta Polytechnic, Technology in Pori, Finland, with this aim. Transition from laminar to turbulent flow in start-up pipe flow can be detected by changes in the thermal regime on the surface of the pipe. Therefore, the IR - thermography can be used to determine the loss of stability of incompressible laminar basic flow.
The heat transfer of pulsating flow in pipes and ducts has actively been investigated in the recent years. The interest in this problem is due to possible applications, mainly in industry, to increase heat exchange efficiency, and in biomechanics. Experiments with different pulsating flow frequencies have been carried out, where heat transfer processes in pipe were measured by the IR - thermography.
The experimental results of the thermally developing pulsatile flow in pipe, with constant heat flux at the wall, show an unexpected phenomenon of cutting off the temperature pulsating curves. From the solution of energy equation we can conclude that the phenomenon of cutting off the peaks in temperature pulsating curves is due to the developing character of downstream temperature.
The application of the IR - thermography enables us to measure processes related to energy changes of the flow outside the pipe. This method does not require transparent pipe walls or any flow disturbing probe in the flow.
Applied experimental methods
Picture 1 shows a thermograph from a metal element where hot water travels inside a buried copper tube. The heat transfer at the boundary layer of the tube is very difficult to analyse quantitatively. Reason to this is the fact that mathematical models are very complicated and it is almost impossible to verify results by measuring internal temperatures near tube wall. Picture 2 shows finite element method from 2D calculation of temperature distribution near tube. The calculation results can be compared in qualitative way with thermography, but information about dynamics of physical phenomena is not achieved here.
Earlier investigations of high-speed temperature scanning of paper tracks (made by Satakunta Polytechnic) have proved out that thermal radiometer can detect very fast temperature changes with adequate accuracy. Thermal data is collected from thermal images with normal image acquisition and results are converted to temperatures from pixel data. - An EU funded take-up project EUTIST-IMV, covering this application is going on with paper industry.
At the same time flow and pressure parameters were measured with different transducers to gain more data from heat exchange phenomena in tube boundary layer. All experiences were controlled with proportional valve to create different oscillation waveforms.
When sinusoidal flow was applied to tube the wall temperature shows temperature curves from radiometer measurement (PICTURE 3). The unexpected and interesting feature of detected temperatures is cutting off the peaks, which can be used to verify mathematical models.
Let us use the slug-flow assumption that the velocity of the flow at every moment of time is constant in any cross-section of the pipe, equalling the average flow velocity. Then the problem of pulsating flow with constant heat flux at the wall is defined by the energy equation
By solving the wall temperature from this equation with boundary constraints, it can be found out that tube wall temperature with contour plot looks like in picture 4 (x ^ position in tube, y ^ flow position, time depends on both x and y). Good qualitative agreement is observed between the experimental and theoretical results. From the results we can also conclude that the phenomenon of cutting off the peaks in temperature pulsating curves is due to the developing character of downstream temperature.
It is shown that the IR - thermography is a useful instrument for the investigation of heat transfer processes in pipe flows. With the IR - thermography we can determine the moment of the loss of stability and transition to turbulence of the accelerated laminar flow. This method enables us to verify the mathematical models for describing the heat transfer processes in accelerated and pulsating flows. In future image processing will be used to provide more accurate models how to calculate heat transfer parameters near tube wall boundary layer.