Transient Liquid Crystal Technique Research Articles

Influence of Geometric Parameters of Alternate Axis Twisted Baffles on the Local Heat Transfer Distribution and Pressure Drop in a Rectangular Channel Using a Transient Liquid Crystal Technique

This paper reports the effects of alternate axis twisted baffle geometric parameters on the heat transfer and flow characteristics within rectangular channels. In our experiments we used modified shapes of alternate axis twisted baffles according to relative pitch ratios (s/w) equal to 2–12 and twist ratios (y/w) equal to 1–5, under conditions where the angle of attack (α) was 90° and the relative blockage height (e/Dh) was at a constant value of 0.095. The results for the Reynolds numbers based on the duct hydraulic diameter ranged from 9000 to 24,000 at a constant Prandtl number, Pr = 0.707, using air as a working fluid. A 0.05 mm thick stainless-steel foil was used as a heater, and a thermochromic liquid crystal technique was used to obtain the local temperature distribution on the heated surfaces. Images were captured in areas with periodic, fully developed regions in the channel. The results show that rectangular channels equipped with alternate axis twisted baffles demonstrated 80%–185% greater heat transfer than rectangular channels with no baffles. Channels with alternate axis twisted baffles at higher twist ratios (y/w) and smaller relative pitch ratios (s/w) showed increased heat transfer, as well as pressure loss within the system, compared with other types of twisted baffles. The thermal enhancement factor of the rectangular channels equipped with alternate axis twisted baffles was higher than that for transverse baffles and smooth channels under similar operating conditions.

On correlated measurements in the transient thermochromic liquid crystals technique with multiple indicators

This work demonstrates that it is possible, and with relative ease, to determine unique characteristic temperatures for the red & green light intensity signals of a narrowband thermochromic liquid crystal mixture. It is proposed to utilize additional indicators in future transient thermochromic liquid crystals measurements to increase data throughput, improve robustness, and reduce uncertainty in the calculated heat transfer coefficient. Potential algorithmic implementations for a multi-color technique are discussed. Optimum conditions for minimizing the uncertainty in the calculated heat transfer coefficient are discussed and compared to that of the traditional single-color technique. The multi-color technique is also extendable to multi-band techniques.

Experimental and numerical research on heat transfer and flow characteristics in two-turn ribbed serpentine channel with lateral outflow

This paper experimentally and numerically investigates the heat transfer and flow characteristics of a two-turn ribbed serpentine channel with lateral outflow. The heat transfer coefficient was measured by a transient liquid crystal technique. Experiments were carried out at Reynolds numbers between 5,000 and 20,000 and rotation numbers of 0 and 0.03. The results indicate that with increasing inlet Reynolds number, the high-Nusselt number regions of the middle and lateral outflow channels gradually move to upstream ribs. The inlet channel shows the highest increase rate of the Nusselt number; whereas, the rate is lowest at downstream turning area. The rotation increases the trailing surface Nusselt number of the inlet and lateral outflow channels, and the increase rate is more prominent for higher values of the Reynolds number. However, the rotation has negative effects on the averaged area Nusselt number of the middle channel in cases of Re ≤ 17,000. The pressure coefficients decrease the in inlet and middle channels along the flow direction; whereas, they slightly increase in the lateral outflow channel. There is a long low-velocity vortex formed in the lateral outflow channel, and the vortex size is considerably reduced by the rotation. Accordingly, the rotation has the most positive effects on the pressure coefficient of the lateral outflow channel.

Application of the Transient Heat Transfer Measurement Technique Using Thermochromic Liquid Crystals in a Network Configuration With Intersecting Circular Passages

The present study deals with the application of the transient thermochromic liquid crystal (TLC) technique in a flow network of intersecting circular passages as a potential internal turbine component cooling geometry. The investigated network consists of six circular passages with a diameter d = 20 mm that intersect coplanar at an angle θ = 40 deg, the innermost in three, the outermost in one intersection level. Two additional nonintersecting passages serve as references. Such a flow network entails specific characteristics associated with the transient TLC method that have to be accounted for in the evaluation process: the strongly curved surfaces, the mixing and mass flow redistribution at each intersection point, and the resulting gradients between the wall and passage centerline temperatures. All this impedes the choice of a representative fluid reference temperature, which results in deviations using established evaluation methods. An alternative evaluation approach is introduced, which is supported by computational results obtained from steady-state three-dimensional (3D) Reynolds-averaged Navier–Stokes equations (RANS) simulations using the shear-stress transport (SST) turbulence model. The presented analysis uncouples local heat transfer (HT) coefficients from actually measured local temperatures but uses the time information of the thermocouples (TC) instead that represents the fluid temperature step change and evolution along the passages. This experimental time information is transferred to the steady-state numerical bulk temperatures, which are finally used as local references to evaluate the transient TLC experiments. As effective local mass flow rates in the passage sections are considered, the approach eventually allows for a conclusion whether HT is locally enhanced due to higher mass flow rates or the intersection effects.

Experimental Investigation on the Effect of Mainstream Turbulence on Full Coverage Film Cooling Effectiveness for a Turbine Guide Vane

Experiments have been performed to investigate the effect of mainstream turbulence on the three-dimensional distribution of the full coverage film cooling effectiveness for two enlarged actual twisted vanes with cylindrical or shaped holes. The film cooling effectiveness was measured by transient liquid crystal technique at mainstream turbulence intensities of 2%, 9% and 15%. The mass flow rate ratios range from 5.5% to 12.5%. There are 3, 8 and 7 rows of film holes on the suction side, leading edge and pressure side, respectively. Results show that for the cylindrical hole vane the high mainstream turbulence intensity decreases the film cooling effectiveness in the top region and down region of pressure side in the low mass flow rate ratio of 5.5%, while the effect is opposite in the high mass flow rate ratio of 12.5%. The film cooling effectiveness in the middle region of pressure side decreases obviously with the increase of the turbulence at the low mass flow rate ratio of 5.5%, while the influence of increasing turbulence weakens gradually with the increase of mass flow rate ratio. Moreover, the high mainstream turbulence improves the film cooling effectiveness in the further downstream of the holes on suction side at the high mass flow rate ratio of 12.5%. For the shaped hole vane, the increase of mainstream turbulence decreases the film cooling effectiveness at all mass flow rate ratios. This study reveals the influence rule of the mainstream turbulence on the film cooling effectiveness in the different regions of the three-dimensional vane surface. The results would guide the designs of engineering heat transfer with application in gas turbine blade/vane cooling.

Enhanced film cooling performance of a row of cylindrical holes embedded in the saw tooth slot

The film cooling performance of a row of cylindrical holes embedded in the saw tooth slot on the flat plate was experimentally investigated using the transient thermochromic liquid crystal technique at blowing ratios of 1.0, 1.5 and 2.0 respectively. The density ratio of the coolant to the main stream was 0.92. An injection angle of 30° relative to the cooled surface in the streamwise direction and a length to diameter ratio of the hole of 6 were used. The saw tooth slot at the hole exit was used to enhance the spanwise spread of the coolant. The effects of the hole pitch, slot height and corner angle on film cooling performance were examined. The standard cylindrical hole was also tested for comparison. The emphasis of this study was on the analysis of the local and spanwise averaged values of the film effectiveness and heat transfer coefficient. Numerical simulation was also conducted to determine the velocity and thermal field to explain the enhanced mechanism of the film cooling by the saw tooth slot configuration. The results showed that the influence of the configurations on the film cooling was strongly dependent on the blowing ratio. A small hole pitch resulted in the significantly larger values of the film effectiveness. The opposite effect of the slot height on the film effectiveness was observed at low and high blowing ratios. A 60° corner provided the significantly larger values of the film effectiveness, especially at high blowing ratio. The heat transfer coefficients were similar for all configurations of the saw tooth slot at blowing ratio of 1.0. Increases in the hole pitch and the corner angle both resulted in a decrease in the heat transfer coefficient at blowing ratios of 1.5 and 2.0.

Development of a heat sink module for a near-term DEMO divertor

A heat sink module has been developed for high-heat flux applications in the divertor region of a near term DEMO divertor. The concept employs water jet impingement, in a cascading flow path, together with CuCrZr as the structural material. This fluid-flow architecture maximises the heat transfer coefficient obtained for a given coolant flow rate. Combined with high operational pressures, it facilitates increased coolant outlet temperatures. A conjugate CFD study was performed to compare its performance to the ITER divertor under a representative heat flux boundary condition of 10 MW/m2. A transient thermochromic liquid crystal technique was used to experimentally validate the CFD heat transfer data, using air as the working fluid at matched Reynolds numbers. These results showed that area averaged heat transfer coefficients could be accurately predicted by the CFD to within ∼12%.

Effect of pin fin arrangement on the heat transfer characteristics in a convergent channel with impingement

The protection of the inlet components of an aircraft engine from the adverse effects of ice accretion has been a crucial design problem since the very early years of flight. Therefore, the configuration with efficient heat transfer is the focus on the design of a hot-air anti-icing system in an aero-engine. This paper studies experimentally on the heat transfer in a convergent channel with pin fins within the strut. Experiments are carried out by using a transient liquid crystal technique. The pitch ratio (D/d, where D is the diameter of the pin fins) of the pin fins and impingement hole, the dimensionles lateral distance (S/d, where S is the distance between the leading edge and pin fins, d is the diameter of the impingement hole) as well as the dimensionless vertical distance (S1/d, where S1 is the distance between pin fins) are respectively investigated to study the heat transfer on the surface of convergent channel. The Reynolds number based on the hydraulic diameter of the impingement hole ranges from 6300 to 12700. Within the experimental range, the result shows that the increasing pitch ratio D/d leads to a lower Average Nusselt Number, the Average Nusselt Number has the highest value when D/d = 1/3. The Average Nusselt Number increases at first and then decreases as the dimensionless lateral distance S/d increases, the Average Nusselt Number has the highest value when S/d = 2.5. When the dimensionless vertical distance S1/d increases, the Average Nusselt Number goes up at first and then reduces, the Average Nusselt Number has the highest value when S1/d = 3. The Average Nusselt Number increases as the Reynolds number increases.

Turbine Blade Leading Edge Cooling With One Row of Normal or Tangential Impinging Jets

Jet impingement cooling has been extensively used in the leading edge region of a gas turbine blade. This study focuses on the effect of jet impinging position on leading edge heat transfer. The test model is composed of a semicylindrical target plate, side exit slots, and an impingement jet plate. A row of cylindrical injection holes is located along the axis (normal jet) or the edge (tangential jet) of the semicylinder, on the jet plate. The jet-to-target-plate distance to jet diameter ratio (z/d) is 5 and the ratio of jet-to-jet spacing to jet diameter (s/d) is 4. The jet Reynolds number is varied from 10,000 to 30,000. Detailed impingement heat transfer coefficient distributions were experimentally measured by using the transient liquid crystal (TLC) technique. To understand the thermal flow physics, numerical simulations were performed using Reynolds-averaged Navier–Stokes (RANS) with two turbulence models: realizable k–ε (RKE) and shear stress transport k–ω model (SST). Comparisons between the experimental and the numerical results are presented. The results indicate that the local Nusselt numbers on the test surface increase with the increasing jet Reynolds number. The tangential jets provide more uniform heat transfer distributions as compared with the normal jets. For the normal jet impingement and the tangential jet impingement, the RKE model provides better prediction than the SST model. The results can be useful for selecting a jet impinging position in order to provide the proper cooling distribution inside a turbine blade leading edge region.

Film cooling performance of a row of dual-fanned holes at various injection angles

Film cooling performance about a row of dual-fanned holes with injection angles of 30°, 60 ° and 90° were experimentally investigated at blowing ratios of 1.0 and 2.0. Dual-fanned hole is a novel shaped hole which has both inlet expansion and outlet expansion. A transient thermochromic liquid crystal technique was used to reveal the local values of film cooling effectiveness and heat transfer coefficient. The results show that injection angles have strong influence on the two dimensional distributions of film cooling effectiveness and heat transfer coefficient. For the small injection angle of 30 degree and small blowing ratio of 1.0, there is only a narrow spanwise region covered with film. The increase of injection angle and blowing ratio both leads to the enhanced spanwise film diffusion, but reduced local cooling ability far away from the hole. Injection angles have comprehensive influence on the averaged film cooling effectiveness for various x/d locations. As injection angles are 30 and 60 degree, two bands of high heat transfer coefficients are found in mixing region of the gas and coolant. As injection angle increases to 90 degree, the mixing leads to the enhanced heat transfer region near the film hole. The averaged heat transfer coefficient increases with the increase of injection angle.

Heat transfer of impinging jet arrays onto half-smooth, half-rough target surfaces

Detailed heat transfer distributions from arrays of impinging jets on a half-smooth, half-rough target surface were experimentally investigated using a transient liquid crystal technique. The target surface was roughened through the creation of rectangular grooves aligned with the jet holes. The grooved regions were designed either parallel (longitudinal grooves) or orthogonal (transverse grooves) to the exit flow direction. Jet-to-jet spacing and jet-to-surface spacing (Z/d) were 4 and 3, respectively. In this experimental test, the effect of crossflow was investigated for three exit flow directions, each with a jet Reynolds number ranging from 2500 to 7700. Heat transfer was enhanced near the edge of grooves, whereas the heat transfer was degraded inside grooves. For the half-smooth, half-rough surface, the sudden change in surface geometry broke the flow development and caused intensified flow mixing in the impingement flow channel. Compared with traditional fully roughened surfaces, the half-rough surface is more effective for heat transfer, and an enhancement of more than 50% was achieved for the longitudinal grooves. The idea of partially roughened surfaces may be further extended to the other internal flow channels with different roughness elements.

Heat transfer in rectangular channels with porous wire mesh under impinging jet conditions

Detailed heat transfer distribution in a rectangular channel under an array of impinging air jets was experimentally investigated using transient liquid crystal technique. The jet-to-jet spacing was 4 and the jet-to-surface spacing was 3. The rectangular channel was fully or partially filled with porous wire meshes (filling ratios = 100%, 50% and 25%) and the jet Reynolds numbers ranged from 2500 to 7700. Although the literature data indicated that the porous material enhanced the overall heat transfer rate, the regional heat transfer on the target surface was degraded because of the blockage and breakup of the core jet flow. The heat transfer coefficient was further reduced with an increase in the filling ratio. For the partially filled channel, the porous wire mesh located near the target surface contributed to even lower heat transfer. All the cases demonstrated that the porous material decreased regional heat transfer on the target surface, and the average Nusselt number for the fully filled case was only 50% of the unfilled case. Although the impingement heat transfer was hindered, the reduced variation in Nusselt number could prevent the development of local hot or cold spots in engineering applications.

Heat Transfer Measurements inside Narrow Channels with Ribs and Trenches

ABSTRACTDetailed heat transfer coefficient distributions are obtained for high aspect ratio (width/height = 12.5) duct with rib and trench enhancement features oriented normal to the coolant flow direction. A transient thermochromic liquid crystal technique has been used to experimentally measure heat transfer coefficients from which Nusselt numbers are calculated on the duct surface featuring heat transfer enhancement features. Reynolds number (calculated based on duct hydraulic diameter) ranging from 7100 to 22400 were experimentally investigated. Detailed measurements of heat transfer provided insight into the role of protruding ribs and trenches on the fluid dynamics in the duct. Experimentally obtained Nusselt numbers are normalized by Dittus-Boelter correlation for developed turbulent flow in circular duct. The triangular trenches provide heat transfer enhancement ratios up to 1.9 for low Reynolds numbers. The in-line rib configuration shows similar levels to the trench whereas staggered rib configuration provides heat transfer enhancement ratios up to 2.2 for a low Reynolds number of 7100.

Experimental Investigation of Film Cooling Performance for a New Shaped Hole at the Leading Edge

An experimental investigation has been performed to study the film cooling performance of a smooth expansion exit at the leading edge of a gas turbine vane. A two-dimensional cascade has been employed to measure the cooling performance of the proposed expansion using the transient Thermochromatic Liquid Crystal technique. One row of cylindrical holes, located on the stagnation line, is investigated with two expansion levels, 2d and 4d, in addition to the standard hole. The air is injected at 90o and 60o inclination angle relative to the vane surface at four blowing ratios ranging from 1 to 2 at a 0.9 density ratio. The Mach number and the Reynolds number based on the cascade exit velocity and the axial chord are 0.23 and 1.4E5, respectively. The detailed local heat transfer coefficient over both the pressure side and the suction side are presented in addition to the lateral-averaged normalized heat transfer coefficient. The proposed expansion provides a lower heat transfer coefficient compared with the standard cylindrical hole over the investigated blowing ratios. Combining the heat transfer coefficient with the corresponding cooling effectiveness, previously presented, the smooth expansion shows a significant reduction in the heat load with more uniform distribution of the coolant over the leading-edge region. The strong confrontation between the coolant jet and the mainstream, in case of 90o injection, yields a strong dispersion of the coolant with higher heat transfer coefficient and high thermal load over the vane surface.

Effect of sidewall slots and pin fins on the performance of latticework cooling channel for turbine blades

Abstract Latticework channel is a typical structure applied in turbine blade cooling. This paper aims to study the convective heat transfer in latticework cooling channels experimentally and numerically. Firstly, heat transfer in a test latticework channel is measured using transient liquid crystal (TLC) technique under different Reynolds numbers and detailed Nusselt number distribution is obtained. Pressure drop is also measured. Secondly, a comparison between experimental data and numerical solution is presented, which shows a good agreement in terms of pressure drop and heat transfer. Lastly, hybrid structures of latticework channel and slots or pin fins are studied by numerical simulation, the effects of slot width, slot shape and pin fin diameter and layouts on the thermal performance of latticework channel are investigated. The result shows that slot can reduce overall heat transfer and pressure drop, while pin fin can increase heat transfer and pressure drop. Both of them improve the heat transfer uniformity markedly. The thermal performance of latticework channel can be improved significantly with the introduction of sidewall slot and pin fin.

Secondary flow and heat transfer coefficient distributions in the developing flow region of ribbed turbine blade cooling passages

This paper reports an experimental and numerical study of the development and coupling of aerodynamic flows and heat transfer within a model ribbed internal cooling passage to provide insight into the development of secondary flows. Static instrumentation was installed at the end of a long smooth passage and used to measure local flow features in a series of experiments where ribs were incrementally added upstream. This improves test turnaround time and allows higher-resolution heat transfer coefficient distributions to be captured, using a hybrid transient liquid crystal technique. A composite heat transfer coefficient distribution for a 12-rib-pitch passage is reported: notably the behaviour is dominated by the development of the secondary flow in the passage throughout. Both the aerodynamic and heat transfer test data were compared to numerical simulations developed using a commercial computational fluid dynamics solver. By conducting a number of simulations it was possible to interrogate the validity of the underlying assumptions of the experimental strategy; their validity is discussed. The results capture the developing size and strength of the vortical structures in secondary flow. The local flow field was shown to be strongly coupled to the enhancement of heat transfer coefficient. Comparison of the experimental and numerical data generally shows excellent agreement in the level of heat transfer coefficient predicted, though the numerical simulations fail to capture some local enhancement on both the ribbed and smooth surfaces. Where this was the case, the coupled flow and heat transfer measurements were able to identify missing velocity field characteristics.

Heat Transfer Performance of Internal Cooling Channel With Single-Row Jet Impingement Array by Varying Flow Rates

The current detailed experimental study focuses on the optimization of heat transfer performance through jet impingement by varying the coolant flow rate to each individual jet. The test section consists of an array of jets, each jet individually fed and metered separately, that expel coolant into the channel and exit through one end. The diameter D, height-to-diameter H/D, and jet spacing-to-diameter S/D are all held constant at 9.53 mm, 2, and 4, respectively. Upon defining the optimum flow rate for each jet, varying diameter jet plates are designed and tested using a similar test setup with the addition of a plenum. Two test cases are conducted by varying the jet diameter within 10% compared to the benchmark jet diameter, 9.53 mm. The Reynolds number, which is based on hydraulic diameter of the channel and total mass flow rate entering the channel, ranges from approximately 52,000 up to 78,000. The transient liquid crystal technique is employed in this study to determine the local and average heat transfer coefficient distributions on the target plate. Commercially available computational fluid dynamics software, ansys cfx, is used to qualitatively correlate the experimental results and to fully understand the flow field distributions within the channel. The results revealed that varying the jet flow rates, total flow varied by approximately ±5% from that of the baseline case, the heat transfer enhancement on the target surface is enhanced up to approximately 35%. However, when transitioning to the varying diameter jet plate, this significant enhancement is suppressed due to the nature of flow distribution from the plenum, combined with the complicated crossflow effects.

Impingement Heat Transfer on a Cylindrical, Concave Surface With Varying Jet Geometries

Jet impingement is often employed within the leading edge of turbine airfoils to combat the heat loads incurred within this region. This experimental investigation employs a transient liquid crystal technique to obtain detailed Nusselt number distributions on a concave, cylindrical surface that models the leading edge of a turbine airfoil. The effect of hole shape and differing hole inlet and exit conditions are investigated. Two hole shapes are studied: cylindrical and racetrack-shaped holes; for each hole shape, the hydraulic diameter and mass flow rate into the array of jets is conserved. As a result, the jet's Reynolds number varies between the two jet arrays. Reynolds numbers of 13,600, 27,200, and 40,700 are investigated for the cylindrical holes, and Reynolds numbers of 11,500, 23,000, and 34,600 are investigated for the racetrack holes. Three inlet and exit conditions are investigated for each hole shape: a square edged, a partially filleted, and a fully filleted hole. The ratio of the fillet radius to hole hydraulic diameter is set at 0.25 and 0.667 for the partially and fully filleted holes, respectively, while all other geometrical features remain constant. Results show the Nusselt number is directly related to the Reynolds number for both cylindrical and racetrack-shaped holes. The racetrack holes are shown to provide enhanced heat transfer compared to the cylindrical holes. The degree of filleting at the inlet and outlet of the holes affects whether the heat transfer on the leading edge model is further enhanced or degraded.

Research on the thermal performance of matrix cooling channel with response surface methodology

In this investigation, three structural parameters are defined for matrix cooling channel: rib angle, rib-to-subchannel ratio and rib density, in a certain typical rectangular cooling space. A carefully designed wind tunnel experiment using transient liquid crystal technique is carried out to obtain surface Nusselt number distribution and pressure drop. The measured data is used to validate the computational method. Turbulence SST model is adopted to study the performance of 13 different matrix channels in Reynolds number range of 5000–90,000. A response surface method is used to fit the CFD solutions and polynomial expressions are established to calculate friction factor ratio (f/f0) and heat transfer enhancement (Nu/Nu0). Variation of thermal performance factor (TPF=(Nu/Nu0)/(f/f0)1/3) versus rib angle, rib-to subchannel ratio and rib density is plotted. The result shows that different structural parameters have to be considered simultaneously in order to improve matrix thermal performance. The comparison of different cases indicates that although smaller rib angle or narrower subchannel can produce very strong local heat transfer, the uniformity is sacrificed.

Endwall heat transfer and pressure loss measurements in staggered arrays of adiabatic pin fins

In many applications, pin fin arrays are used for heat transfer enhancement. Due to additionally induced turbulence in the flow field, the heat transfer is increased. This study focuses on different pin fin arrays with thermally inactive pin fins. The configurations include short and long elements. Transient experiments are conducted to determine the heat transfer characteristics on the endwall. Therefore, the transient liquid crystal technique is applied to obtain locally resolved heat transfer data. Three configurations with a relative spacing of 2.5⩽S/D⩽5 and aspect ratio of 2⩽H/D⩽4 are investigated. Hereby, the Reynolds number of the air flow, based on the pin fin diameter, varies from 3·103 to 3·104. First, an overview of the experimental setup and the transient measurement technique is given. Then, heat transfer results on the endwall are presented in terms of Nusselt number. Finally, the pressure drop is evaluated by the friction factor and the thermal performance of the investigated configurations is discussed.