HEAT TRANSFER ENHA THE USE OF TRANSVERSE

Vortices and generation contain three passive elements to influence heat transfer: swirl, flow destabilization and developing viscous layers. They 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 original or 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 art and to induce others to join in the 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 hot film method. Heat transfer measurement were carried out by TLC. Pressure losses were also measured. To use vortices to influence heat transfer it must be known how different vortices are generated and controlled and how they interact with the original or base flow and temperature field. Special attention will be given to the influence of geometrical VG variations on self-sustained oscillations, flow visualization, heat transfer and pressure drop penalty. Liquid crystals were used to determine the distribution of the surface temperature, and then evaluation of the heat transfer coefficient or the Nusselt number. The flow pattern produced by transverse vortex generators was visualized using a planar beam of double-impulse laser tailored by a cylindrical lens and oil particles. Sequential images of the particles in a cross sectional plane taken with CCD video camera from the downstream side the flow ware stored on a personal computer to obtain distributions of velocity vectors by means of the PIV method. LIQUID CRYSTAL THERMOGRAPHY Thermochromic liquid crystals (TLC) and true-colour digital image processing have been successfully used in nonintrusive technical, industrial and biomedical studies and applications. Thin coatings of TLC's at surfaces are utilized to obtain detailed temperature distributions and heat transfer rates for steady or transient processes. Liquid Crystals Themography (so called LCT) have been extensively applied to the qualitative visualization of entire steady state, or transient temperature fields on solid surfaces. Since quantifying colour is a 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. The first application of true-colour digital image processing gave impact to qualitative and fast temperature measurements. Rapid development of image processing techniques has made it now possible to set-up inexpensive systems capable of real-time transient full field temperature measurements using TLCs. Before the execution of a thermal or flow visualization experiment, we should recognise the characteristics of the overall combination of the TLC, the light source, the optical and camera system, and make a rational plan for the total measurement system. The relationship between the temperature of the crystal and the measured Hue of the reflected light defines the calibration curve for the specified liquid crystal [7]. The distribution of the colour component pattern on the liquid crystal layer was measured by RGB colour camera and a series of images at different temperatures defines the calibration. Two main methods of surface temperature measurements are performed involving steady state and transient techniques. A brief history of these is given by Baughn et al [1] and Stasiek [7]. Steady State Analyses Constant Flux Method The steady state techniques employ a heated model and the TLC is used to monitor the surface temperature Tw, it gives the local heat transfer coefficient, h,