The use of liquid crystal thermography and particle image velocimetry in the exploration of heat transfer measurements
Generation of vortices enhances heat transfer by swirl, flow destabilization and development of viscous layers. They may increase heat transfer by several hundred percent. Prior to the use of vortices to influence heat transfer it must be known how different vortices are generated and controlled, and how they interact with the base flow and temperature field. To select the most appropriate vortex generators (VG) for a given task it is necessary to know the heat transfer and flow losses associated with the generation of a specific vortex system. The aim of the paper is to asses the state of art and to encourage exploration of heat transfer control by vortices. Flow visualization and heat transfer experiments were conducted using an open low-speed wind tunnel equipped with Liquid Crystals Thermography (LCT) and Particle Image Velocimetry (PIV). In this work we consider five types of transverse vortex generators (TVGs) which where brought to required temperatures by the hot film method. Heat transfer measurements were carried out by LCT.
Liquid crystal thermography and true-colour digital image processing
In the last decade thermochromic liquid crystals (TLC) and true-colour digital image processing have been successfully used in non-intrusive technical, industrial and biomedical studies and applications. Thin coatings of TLCs at surfaces are utilized to obtain detailed temperature distributions and heat transfer rates for steady or transient processes. Liquid crystals also can be used to make visible the temperature and velocity fields in liquids by the simple expedient of directly mixing the liquid crystal material into the liquid (water, glycerol, glycol, and silicone oils) in very small quantities to use as thermal and hydrodynamic tracers. In biomedical situations e.g., skin diseases, breast cancer, blood circulation and other medical application, TLC and image processing are successfully used as an additional non-invasive diagnostic method especially useful for screening large groups of potential patients. The history of this technique is reviewed, principal methods and tools are described and some examples are also presented.
Thermochromic liquid crystals applied for heat transfer research
Liquid crystals are highly anisotropic fluids that exist between the boundaries of the solid phase and the conventional, isotropic liquid phase. The TLC based temperature visualisation is based on the property of some cholesteric and chiral-nematic liquid crystal materials to reflect definite colours at specific temperatures and viewing angle. The colour change for the TLC ranges from clear at ambient temperature, through red as temperature increases and then to yellow, green, blue and violet before turning colourless (isotropic) again at a higher temperature. They appear colourless above and below the active range. The colour-temperature play interval depends on the TLC composition. It can be selected for the bands of about 0.5 C to 20 C, and working temperature of –30 C to above 120 C. These colour changes are repeatable and reversible as long as the TLC’s are not physically or chemically damaged. The response time of TLC’s equals about 10 ms. It is short enough for typical thermal problems in fluids. Since the colour change is reversible and repeatable, they can be calibrated accurately with a proper care and used in this way as temperature indicators. They can be painted on a surface or suspended in a fluid and used to indicate visibly the temperature distribution. Thus, liquid crystals have been successfully applied to heat transfer and fluid flow research. During the past, liquid crystals have been extensively applied to the qualitative visualisation of entire steady state, or transient temperature fields on solid surfaces. Since quantifying colour is difficult and somewhat ambiguous task, application of thermochromic liquid crystals initially was largely qualitative. Application of the colour films or interference filters was tedious and inaccurate. First application of true-colour digital image processing gave impact to qualitative and fast temperature measurements. Rapid development of the hardware and software computerised image processing techniques made possible now to set-up inexpensive system capable for real-time transient full-field temperature measurements using TLCs. In this paper, we review the above issues and use illustrative examples from our own work in applying TLC to the study of forced and natural convective heat transfer on a cooled surface heated by air flow disturbed by a number of complex geometrical configurations and for temperature and flow visualisation in closed cavities also with phase change. Significance of the full-field temperature and flow measurements for verification of numerical results becomes evident. We found that only with help of the full-field measurements it was possible to identify serious discrepancies between numerical predictions and the experimental observations. With the help of experimental feedback, it was possible to introduce necessary modifications to numerical codes and in such a way to improve its output in the correct direction.