Heat transfer in rectangular channels with transverse and V-shaped broken ribs

Liquid crystals have become an accurate and convenient means of measuring surface temperature and heat transfer for the gas turbine and heat transfer research communities. The measurement of surface shear stress using liquid crystals is finding increasing favour with aerodynamicists and developments in these techniques ensure that liquid crystals will continue to provide key thermal and shear stress data in the future. The increasing use of three-dimensional finite element computational models has allowed industry to capitalize on the advantages of the full surface data generated. The paper reviews the use of these complex materials in research with a special emphasis on recent developments in the field. The aim is to provide the reader with an up to date background in this measurement technology and allow the researcher to decide whether liquid crystals would be suitable in specific applications.

https://iopscience.iop.org/article/10.1088/0957-0233/11/7/313/pdf

Liquid crystal measurements of heat transfer and surface shear stress

This paper presents in detail the transient liquid crystal technique for convective heat transfer measurements. A historical perspective on the active development of liquid crystal techniques for convective heat transfer measurement is also presented. The experimental technique involves using a thermochromic liquid crystal coating on the test surface. The colour change time of the coating at every pixel location on the heat transfer surface during a transient test is measured using an image processing system. The heat transfer coefficients are calculated from the measured time responses of these thermochromic coatings. This technique has been used for turbine blade internal coolant passage heat transfer measurements as well as turbine blade film cooling heat transfer measurements. Results can be obtained on complex geometry surfaces if visually accessible. Some heat transfer results for experiments with jet impingement, internal cooling channels with ribs, flow over simulated TBC spallation, flat plate film cooling, cylindrical leading edge and turbine blade film cooling are presented for demonstration.

https://iopscience.iop.org/article/10.1088/0957-0233/11/7/312/pdf

A transient liquid crystal thermography technique for gas turbine heat transfer measurements

The heat transfer enhancement in cooling passages with dimpled (concavity imprinted) surface can be effective for use in heat exchangers and various hot section components (nozzle, blade, combustor liner, etc.), as it provides comparable heat transfer coefficients with considerably less pressure loss relative to protruding ribs. Heat transfer coefficients and friction factors were experimentally investigated in rectangular channels which had concavities (dimples) on one wall. The heat transfer coefficients were measured using a transient thermochromic liquid crystal technique. Relative channel heights (H/d) of 0.37, 0.74, 1.11 and 1.49 were investigated in a Reynolds number range from 12000 to 60000.The heat transfer enhancement (NuHD) on the dimpled wall was approximately constant at a value of 2.1 times that (Nusm) of a smooth channel over 0.37≤H/d≤1.49 in the thermally developed region. The heat transfer enhancement ratio Display FormulaNu¯HD/Nusm was invariant with Reynolds number. The friction factors (f) in the aerodynamically fully developed region were consistently measured to be around 0.0412 (only 1.6 to 2.0 times that of a smooth channel). The aerodynamic entry length was comparable to that of a typical turbulent flow (Xo/Dh = 20), unlike the thermal entry length on dimpled surface which was much shorter (xo /Dh<9.8). The thermal performance Display FormulaNu¯HD/Nusm/f/fsm1/3≅1.75 of dimpled surface was superior to that Display Formula1.16<Nu¯HD/Nusm/f/fsm1/3<1.60 of continuous ribs, demonstrating that the heat transfer enhancement with concavities can be achieved with a relatively low-pressure penalty.Neither the heat transfer coefficient distribution nor the friction factor exhibited a detectable effect of the channel height within the studied relative height range (0.37≤H/d≤1.49).© 1999 ASME

Channel Height Effect on Heat Transfer and Friction in a Dimpled Passage

Illuminant invariant calibration of thermochromic liquid crystals

A new image processing based color capturing technique for the quantitative interpretation of liquid crystal images used in convective heat transfer studies is presented. This method is highly applicable to the surfaces exposed to convective heating in gas turbine engines. It is shown that, in the single-crystal mode, many of the colors appearing on the heat transfer surface correlate strongly with the local temperature. A very accurate quantitative approach using an experimentally determined linear hue vs temperature relation is found to be possible. The new hue-capturing process is discussed in terms of the strength of the light source illuminating the heat transfer surface, the effect of the orientation of the illuminating source with respect to the surface, crystal layer uniformity, and the repeatability of the process. The present method is more advantageous than the multiple filter method because of its ability to generate many isotherms simultaneously from a single-crystal image at a high resolution in a very time-efficient manner.

/pdf/a-new-hue-capturing-technique-for-the-quantitative-2qcoasutsr.pdf

A New Hue Capturing Technique for the Quantitative Interpretation of Liquid Crystal Images Used in Convective Heat Transfer Studies

Accurate determination of convective heat transfer coefficients on complex curved surfaces is essential in the aerothermal design and analysis of propulsion system components. The heat transfer surfaces are geometrically very complex in most of the propulsion applications. This study focuses on the evaluation of a hue capturing technique for the heat transfer interpretation of liquid crystal images from a complex curved heat transfer surface. Impulsively starting heat transfer experiments in a square to rectangular transition duct are reported. The present technique is different from existing steady-state hue capturing studies. A real-time hue conversion process on a complex curved surface is adopted for a transient heat transfer technique with high spatial resolution. The study also focuses on the use of encapsulated liquid crystals with narrow color band in contrast to previous steady-state hue based techniques using wide band liquid crystals. Using a narrow band crystal improves the accuracy of the heat transfer technique. Estimated uncertainty for the heat transfer coefficient from the technique is about 5.9 percent. A complete heat transfer map of the bottom surface was possible using only seven liquid crystal image frames out of the 97 available frames during the transient experiment. Significant variations of heat transfer coefficientsmore » are quantitatively visualized on the curved surfaces of the transition duct. 33 refs.« less

Evaluation of a Hue Capturing Based Transient Liquid Crystal Method for High-Resolution Mapping of Convective Heat Transfer on Curved Surfaces

Convection heat transfer at the curved bottom surface of a square to rectangular transition duct using a new hue capturing based liquid crystal technique

This paper deals with the experimental and computational analysis of flow and heat transfer in a turning duct simulating the overall three-dimensional flow and heat transfer characteristics of gas turbine passages and internal cooling channels. The numerical results obtained for three different Reynolds numbers (790, 40,000, and 342,190) are compared to measured flow characteristics. Extensive measurements of the mean flow structure using a sub-miniature five-hole-probe and a hot-wire provide a quantitative assessment of the computational model used in the analysis. An in-house developed three-dimensional viscous flow solver is the main computational tool of the present study. A well known pressure correction method (SIMPLE) is used for the solution of 3-D incompressible, steady Navier - Stokes equations in a generalized coordinate system. The k - ∈ turbulence model of Launder and Spalding including a curvature correction scheme is utilized to describe the turbulent flow field. A non-staggered grid system is used in an upwind scheme with additional numerical dissipation terms. It is clearly shown that reducing the artificial dissipation terms in a central differencing scheme reduced the numerical dissipation error. Part II of this paper deals with the convective heat transfer aspects of the specific flow near the endwall surface where three-dimensional viscous flow structures are dominant. The measured fluid mechanics data presented in this paper also provide a reliable data set that can be used in the future validation of new computational methods.

/pdf/turbulent-flow-and-endwall-heat-transfer-analysis-in-a-90-2c8ofs9eho.pdf

Turbulent Flow and Endwall Heat Transfer Analysis in a 90∘ Turning Duct and Comparisons with Measured Data: Part I: Influence of Reynolds Number and Streamline Curvature on Viscous Flow Development

Fundamental character of forced convection heat transfer to the endwall surface of a gas turbine passage can be simulated by using a 90° turning duct. Elevated operating temperatures in gas turbines require a thorough understanding of the turbulent thermal transport process in the three-dimensional end-wall boundary layers. The current study uses an in-house developed three-dimensional viscous flow solver to computationally investigate the heat transfer character near the endwall surfaces. Extensive heat transfer experiments also illuminate the local heat transfer features near the endwall surface and form a baseline data set to evaluate the computational method used. Present experimental effort at Re=342,190 employs a prescribed heat flux method to measure convective heat transfer coefficients on the end-wall surface. Local wall temperatures are measured with liquid crystal thermography. Local convective heat flux is determined by solving the electric potential equation with constant uniform potential that is applied along the inlet and exit boundaries of the endwall. The viscous flow and heat transfer computation algorithms are based on the same methods presented in Part I. The influence of the two primary counter-rotating vortices developing as a result of the balance of inertia and pressure forces on the measured and computed endwall heat transfer coefficients is demonstrated. It is shown that the local heat transfer on the endwall surface is closely related to the structure of the three-dimensional mean flow and the associated turbulent flow field. A comparison of the molecular/ turbulent heat diffusion, wall shear stress, visualization of local turbulent kinetic energy are a few of the tasks that can be performed using a numerical approach in a relatively time efficient manner. The current numerical approach is capable of visualizing the near wall features that can not be easily measured in the flow field near the end-wall surfaces.

/pdf/turbulent-flow-and-endwall-heat-transfer-analysis-in-a-90-2jgadabgdq.pdf

Turbulent flow and endwall heat transfer analysis in a 90° turning duct and comparisons with measured Data - Part II: Influence of secondary flow, vorticity, turbulent kinetic energy, and thermal boundary conditions on endwall heat transfer

Kuisoon Kim

Papers

A New Hue Capturing Technique for the Quantitative Interpretation of Liquid Crystal Images Used in Convective Heat Transfer Studies

Evaluation of a Hue Capturing Based Transient Liquid Crystal Method for High-Resolution Mapping of Convective Heat Transfer on Curved Surfaces

Convection heat transfer at the curved bottom surface of a square to rectangular transition duct using a new hue capturing based liquid crystal technique

Turbulent Flow and Endwall Heat Transfer Analysis in a 90∘ Turning Duct and Comparisons with Measured Data: Part I: Influence of Reynolds Number and Streamline Curvature on Viscous Flow Development

Turbulent flow and endwall heat transfer analysis in a 90° turning duct and comparisons with measured Data - Part II: Influence of secondary flow, vorticity, turbulent kinetic energy, and thermal boundary conditions on endwall heat transfer