Transient Liquid Crystal Technique Research Articles

Method for analysis of showerhead film cooling experiments on highly curved surfaces

The transient liquid crystal technique has been extensively used for measuring the heat transfer characteristics in gas turbine applications. Thereby, the time evolution of the surface temperature is usually evaluated using the model of a semi-infinite flat plate. For experiments on cylinders, Wagner et al. [G. Wagner, M. Kotulla, P. Ott, B. Weigand, J. von Wolfersdorf, The transient liquid crystal technique: influence of surface curvature and finite wall thickness, ASME Paper GT2004-53553, 2004] showed, that curvature and finite thickness effects can have an influence on the obtained heat transfer coefficients. The aim of this study is to develop a time effective data reduction method that accounts for curvature and that is applicable to film cooling experiments with time varying adiabatic wall temperatures. To verify this method, transient liquid crystal experiments have been carried out on a blunt body model with showerhead film cooling. The experimental data was evaluated with the traditional semi-infinite flat plate approach and with the curvature correction using regression analysis.

Design criteria for ribbed channels: Experimental investigation and theoretical analysis

The present study investigates heat transfer and pressure drop in flows through ribbed channel for application to turbine blade cooling. The experiments are conducted for different cross-sections, for Reynolds number from 20 to 60 × 10 3. Local heat transfer coefficients are obtained using a transient thermochromic liquid crystal (TLC) technique. Detailed knowledge of the local heat transfer coefficient is essential to analyze thermal stresses in turbine components, while the combined effect of heat transfer and pressure drop should be taken into account for a proper cooling system design. As a compromise has always to be found, a new design criteria to choose the most appropriate solution for typical turbomachinery parameters is inferred and shown. Entrance effects for ribbed channels are also studied, as the common hypothesis of fully developed flow is rarely satisfied in real engine geometries; relevant results are revealed.

가스터빈 블레이드 끝단 형상에 따른 블레이드 끝단 및 그 주변에서의 열전달 계수 변화

Detailed heat transfer coefficient distributions on two types of gas turbine blade tip, plane tip and squealer tip, were measured using a hue-detection base transient liquid crystals technique. The heat transfer coefficients on the shroud and near tip regions of the pressure and suction sides of the blade were also measured. The heat transfer measurements were taken at the three different tip gap clearances of 1.0%, 1.5%, and 2.5% of blade span. Results show the overall heat transfer coefficients on the tip and shroud with squealer tip blade were lower than those with plane tip blade. By using squealer tip, however, the reductions of heat transfer coefficients near the tip regions of the pressure and suction sides were not remarkable.

Experimental study of impinging heat transfer along rib-roughened walls by using transient liquid crystal technique

The objective of this work is to examine the detailed heat transfer coefficient distributions over a ribbed surface under impingement of in-line and staggered jet arrays by using a liquid crystal thermograph technique. In-line and staggered jet arrays with different exit flow orientations were considered. Three jet-to-target spacing Z of 3, 6 and 9 with in-line and staggered jet arrays were considered at jet Reynolds numbers of Re = 1500, 3000 and 4500 with three different exit flow orientations. In addition, the effects of rib configuration on the heat transfer distributions were discussed in detail. Results show that the local heat transfer rates over the ribbed surface are characterized by obvious periodic-type variation of Nusselt number distributions. The downstream peaks are diminished for increasing cross flow effect. Compared to the results without ribs, the heat transfer over the ribbed surface may be enhanced or retarded. Whereas, among the test angled-rib arrangements, the best heat transfer performance is obtained with a surface with 45° angled ribs.

Thermal Performance Investigation for Confined Heat Sinks By Using a Modified Transient Liquid Crystal Technique

A series of experimental investigations with a new modified transient liquid crystal method on the studies related to the fluid flow and heat transfer characteristics in a channel installed with a heat sink have been successfully performed. The parametric studies on the local and average effective heat transfer characteristics for confined heat sinks have been explored. The influencing parameters and conditions include air preheating temperature at channel inlet, flow velocity and heat sink types. Besides, a concept of the amount of enhanced heat transfer (AEHT) is introduced and defined as the ratio of j∕f. The j∕f ratio is almost independent of Reynolds number for a specific confined heat sink. The j∕f ratios are 0.0603 and 0.0124 for fully-confined and unconfined heat sinks, respectively.

Heat Transfer on Internal Surfaces of a Duct Subjected to Impingement of a Jet Array with Varying Jet Hole-Size and Spacing

One significant issue concerning the impingement heat transfer with a jet array is related to the so-called “crossflow,” where a local jet performance is influenced by the convection of the confluence from the impingement of the jet∕jets placed upstream. As a result, the heat transfer coefficient may vary along the streamwise direction and creates more or less nonuniform cooling over the component, which is undesirable from both the performance and durability standpoints. Described in this paper is an experimental investigation of the heat transfer coefficient on surfaces impinged by an array of six inline circular jets with their diameters increased monotically along the streamwise direction. The local heat transfer distributions on both the target surface and jet-issuing plate are measured using a transient liquid crystal technique. By varying the jet hole-size in a systematic manner, the actual distribution of jet flow rate and momentum within a jet array may be optimally metered and controlled against crossflow. The effects on the heat transfer coefficient distribution due to variations of jet-to-target distance and interjet spacing are investigated. The varying-diameter results are compared with those from a corresponding array of uniform jet diameter.

The Transient Liquid Crystal Technique: Influence of Surface Curvature and Finite Wall Thickness

The transient liquid crystal technique is currently widely used for measuring the heat transfer characteristics in gas turbine applications. Usually, the assumption is made that the wall of the test model can be treated as a flat and semi-infinite solid. This assumption is correct as long as the penetration depth of the heat compared to the thickness of the wall and to the radius of curvature is small. However, those two assumptions are not always respected for measurements near the leading edge of a blade. This paper presents a rigorous treatment of the curvature and finite wall thickness effects. The unsteady heat transfer for a hollow cylinder has been investigated analytically and a data-reduction method, taking into account curvature and finite wall-thickness effects has been developed. Experimental tests made on hollow cylinder models have been evaluated using the new reduction method as well as the traditional semi-infinite flat-plate approach and a third method that approximately accounts for curvature effects. It has been found that curvature and finite thickness of the wall have, in some cases, a significant influence on the obtained heat transfer coefficient. The parameters influencing the accuracy of the semi-infinite flat-plate model and the approximate curvature correction are determined and the domains of validity are represented.

A Transient Infrared Thermography Method for Simultaneous Film Cooling Effectiveness and Heat Transfer Coefficient Measurements From a Single Test

In film cooling situations, there is a need to determine both local adiabatic wall temperature and heat transfer coefficient to fully assess the local heat flux into the surface. Typical film cooling situations are termed three temperature problems where the complex interaction between the jets and mainstream dictates the surface temperature. The coolant temperature is much cooler than the mainstream resulting in a mixed temperature in the film region downstream of injection. An infrared thermography technique using a transient surface temperature acquisition is described which determines both the heat transfer coefficient and film effectiveness (nondimensional adiabatic wall temperature) from a single test. Hot mainstream and cooler air injected through discrete holes are imposed suddenly on an ambient temperature surface and the wall temperature response is captured using infrared thermography. The wall temperature and the known mainstream and coolant temperatures are used to determine the two unknowns (the heat transfer coefficient and film effectiveness) at every point on the test surface. The advantage of this technique over existing techniques is the ability to obtain the information using a single transient test. Transient liquid crystal techniques have been one of the standard techniques for determining h and η for turbine film cooling for several years. Liquid crystal techniques do not account for nonuniform initial model temperatures while the transient IR technique measures the entire initial model distribution. The transient liquid crystal technique is very sensitive to the angle of illumination and view while the IR technique is not. The IR technique is more robust in being able to take measurements over a wider temperature range which improves the accuracy of h and η. The IR requires less intensive calibration than liquid crystal techniques. Results are presented for film cooling downstream of a single hole on a turbine blade leading edge model.

Spray cooling characteristics of water and R-134a. Part II: transient cooling

Spray cooling of a hot surface was investigated in the second of two papers to determine the effect of mass flux, Weber number, and degree of subcooling on two different working fluids such as pure water and R-134a. Full cone circular sprays were used to cool a circular surface of diameter 80 mm with an initial temperature about 250/70 °C for pure water/R-134a, respectively. Both the transient liquid crystal technique (only for R-134a) and thermocouple wire temperature measurements were conducted. Cooling curves were obtained in a wide range of the above-mentioned parameters.

Measurement of detailed heat transfer on a surface under arrays of impinging elliptic jets by a transient liquid crystal technique

Detailed heat transfer characteristics on a flat surface under arrays of impinging elliptic jets were measured by a transient liquid crystal technique. The elliptic jet holes of five different aspect ratios, AR = 4, 2, 1, 0.25, and 0.5, jet Reynolds numbers Re = 1500, 3000, and 4500, and three exit flow conditions are considered to investigate impingement heat transfer performance and the associated flow structure at various conditions. Results show that effects of the aspect ratio and crossflow have significant influences on the axial shift of the impingement/touchdown locations. The present thermographs with the high- Nu spots are very useful for understanding of the flow characteristics and jet structure deformation at various conditions. The axis-switchover phenomenon is found with the elliptic jets of aspect ratio AR > 1, but it does not occur in the cases of AR ⩽ 1. Among the five aspect ratios considered, the mean heat transfer rates with the elliptic jets of AR = 0.5 are the highest at Re = 3000 and 4500; while, in the low- Re case of Re = 1500, the jet array with jet arrays with AR = 2 and 1 perform better than that with AR = 0.5. In addition, among the three, the two-way exit flow condition is most beneficial to the heat transfer characteristics of an impinging jet array.

Heat Transfer Augmentation of a Co-Rotating Disk Assembly with Special Emphasis on the Lower Disk

Experiments were performed to map local heat transfer coefficients on an unobstructed co-rotating disk system simulating the internal component of a computer disk drive assembly. The disk diameter was twice as large as a hard drive, and the rotational speed up to 5000 rpm was studied. The effects of two parameters, on the heat transfer phenomena the Reynolds number from 6.587 × 10 4 to 3.096 × 10 5 and the non-dimensional disk-to-disk ratio (L/D) from 0.02 to 0.06, were observed. The disks were positioned horizontally, and heat transfer was measured for the top faces of both disks. Emphasis is given to the top face of the lower disk in this research, as less experimental data are available in the literature. A transient liquid crystal technique was employed for the measurements, and the results reveal an interesting pattern of the heat transfer rate in the gap for the upper surface of the lower disk. The overall heat transfer is much lower for the surface of the lower disk as compared to that of the top disk. The maximum average heat transfer rate was established to be at L/D = 0.04 for the case with the highest rotational speed.

Effect of Tip Gap and Squealer Geometry on Detailed Heat Transfer Measurements Over a High Pressure Turbine Rotor Blade Tip

The present study explores the effects of gap height and tip geometry on heat transfer distribution over the tip surface of a HPT first-stage rotor blade. The pressure ratio (inlet total pressure to exit static pressure for the cascade) used was 1.2, and the experiments were run in a blow-down test rig with a four-blade linear cascade. A transient liquid crystal technique was used to obtain the tip heat transfer distributions. Pressure measurements were made on the blade surface and on the shroud for different tip geometries and tip gaps to characterize the leakage flow and understand the heat transfer distributions. Two different tip gap-to-blade span ratios of 1% and 2.6% are investigated for a plane tip, and a deep squealer with depth-to-blade span ratio of 0.0416. For a shallow squealer with depth-to-blade span ratio of 0.0104, only 1% gap-to-span ratio is considered. The presence of the squealer alters the tip gap flow field significantly and produces lower overall heat transfer coefficients. The effects of different partial squealer arrangements are also investigated for the shallow squealer depth. These simulate partial burning off of the squealer in real turbine blades. Results show that some partial burning of squealers may be beneficial in terms of overall reduction in heat transfer coefficients over the tip surface.

Local heat transfer in rotating square-rib-roughened and smooth channels with jet impingement

An experimental investigation was carried out to examine the effects of jet impinging positions on heat transfer from rib-roughened (square and semi-circular) channels with rotational speeds of up to 600 rpm. The rib height ( e) was fixed at 1.5 mm and pitch-to-height ratio ( p/ e) kept at 6.67. The study covered jet Reynolds number in the range 5000–9000 based on jet hole diameter. Results were presented for rotational number, jet impinging positions, and roughened surfaces effects on local Nusselt numbers. Significant heat transfer enhancement was found for the present physical geometry and within the ranges of operating parameters considered in the study. Moreover, detailed heat transfer coefficients on target wall in a rotating square smooth channel using transient liquid crystal techniques for the rotational speeds of 30 and 60 rpm at Re j=7000 and 9000 were also examined.

과도 액정 기법을 이용한 와동발생기 하류의 유동장 및 열전달 측정

The effects of the interaction between flow field and heat transfer caused by the longitudinal vortices are experimentally investigated using a five hole probe and a transient liquid crystal technique. The test facility consists of a wind tunnel with vortex generators protruding from a bottom surface and a mesh heater. In order to control the strength of the longitudinal vortices, the angle of attack of vortex generators used in the present experiment is 20, and the spacing between the vortex generators is 25mm. The height and cord length of the vortex generator is 20mm and 50mm, respectively. Three-component mean velocity measurements are made using a f-hole probe system, and the surface temperature distribution is measured by the hue capturing method using a transient liquid crystal technique. The transient liquid crystal technique in measuring heat transfer has become one of the most effective ways in determining the full surface distributions of heat transfer coefficients. The key point of this technique is to convert the inlet flow temperature into an exponential temperature profile using the mesh heater set up in the wind tunnel. The conclusions obtained in the present experiment are as follows: The two maximum heat transfer values exist over the whole domain, and as the longitudinal vortices move to the farther downstream region, these peak values show the decreasing trends. These trends are also observed in the experimental results of other researchers to have used the uniform heat flux method.

Heat Transfer Coefficients and Film Cooling Effectiveness on the Squealer Tip of a Gas Turbine Blade

Experimental investigations were performed to measure the detailed heat transfer coefficients and film cooling effectiveness on the squealer tip of a gas turbine blade in a five-bladed linear cascade. The blade was a two-dimensional model of a first stage gas turbine rotor blade with a profile of the GE-E3 aircraft gas turbine engine rotor blade. The test blade had a squealer (recessed) tip with a 4.22% recess. The blade model was equipped with a single row of film cooling holes on the pressure side near the tip region and the tip surface along the camber line. Hue detection based transient liquid crystals technique was used to measure heat transfer coefficients and film cooling effectiveness. All measurements were done for the three tip gap clearances of 1.0%, 1.5%, and 2.5% of blade span at the two blowing ratios of 1.0 and 2.0. The Reynolds number based on cascade exit velocity and axial chord length was 1.1×106 and the total turning angle of the blade was 97.9 deg. The overall pressure ratio was 1.2 and the inlet and exit Mach numbers were 0.25 and 0.59, respectively. The turbulence intensity level at the cascade inlet was 9.7%. Results showed that the overall heat transfer coefficients increased with increasing tip gap clearance, but decreased with increasing blowing ratio. However, the overall film cooling effectiveness increased with increasing blowing ratio. Results also showed that the overall film cooling effectiveness increased but heat transfer coefficients decreased for the squealer tip when compared to the plane tip at the same tip gap clearance and blowing ratio conditions.

Heat Transfer Coefficients on the Squealer Tip and Near Squealer Tip Regions of a Gas Turbine Blade

Detailed heat transfer coefficient distributions on a squealer tip of a gas turbine blade were measured using a hue detection based transient liquid crystals technique. The heat transfer coefficients on the shroud and near tip regions of the pressure and suction sides of a blade were also measured. Tests were performed on a five-bladed linear cascade with a blow-down facility. The blade was a two-dimensional model of a first stage gas turbine rotor blade with a profile of a GE-E3 aircraft gas turbine engine rotor blade. The Reynolds number based on the cascade exit velocity and axial chord length of a blade was 1.1×106 and the total turning angle of the blade was 97.7 deg. The overall pressure ratio was 1.2 and the inlet and exit Mach number were 0.25 and 0.59, respectively. The turbulence intensity level at the cascade inlet was 9.7 percent. The heat transfer measurements were taken at the three different tip gap clearances of 1.0 percent, 1.5 percent, and 2.5 percent of blade span. Results showed that the overall heat transfer coefficients on the squealer tip were higher than that on the shroud surface and the near tip regions of the pressure and suction sides. Results also showed that the heat transfer coefficients on the squealer tip and its shroud were lower than that on the plane tip and shroud. However, the reductions of heat transfer coefficients near the tip regions of the pressure and suction sides were not remarkable.

Closure to Discussion of “Heat Transfer in Rotating Rectangular Cooling Channels (AR=4) With Dimples” (2003, ASME J. Turbomach., 125, p. 564)

The authors appreciate Dr. Ligrani’s comment on the paper. The discrepancy of heat transfer enhancement on the dimpled surface reported by different investigators might be due to the use of different measurement technique for the data taken and reduction. In addition to the aforementioned studies, for example, Zhou and Acharya [15] reported that the heat transfer enhancements of the dimpled surface for nonrotation could be about 1.5–2 times the smooth channel values for a dimple depth to print diameter ratio of 0.29 by using the naphthalene sublimation mass-transfer technique. Moon and Lau 22 showed that the dimpled surface heat transfer for nonrotation enhanced about 1.6–1.8 times the smooth-channel values for the dimple depth to print diameter ratios around 0.2–0.23 by using the standard aluminum plate with heaters and thermocouples. These heat transfer enhancement ratios are close to the present study’s values for nonrotation by using the standard copper plate with heaters and thermocouples. But they are lower than the aforementioned values reported by Chyu et al. [12] using the transient liquid crystal technique, Moon et al. [13] using the transient liquid crystal technique, and Burgess et al. [21] using the IR camera technique. It is likely that using different measurement technique and data analysis could produce 10–20% different heat transfer enhancement values for turbulent channel flow through such a complex dimpled surface. The present study used the traditional copper plate with heaters and thermocouples technique. The purpose was to obtain the regionally averaged heat transfer coefficient per copper plate. According to Fig. 3 in the paper, each copper plate is 2.54 cm by 2.54 cm facing to the cooling flow and with 0.3175-cm thickness. The estimated maximum Biot number of the copper plate is around 0.0022 for the highest heat transfer coefficient case at Re=40,000 of the present study. This means that the temperature gradient within the copper plate is small as expected by using this kind of standard measurement technique. On the other hand, in order to produce 20% heat-transfer coefficient’s discrepancy in the copper plate, it requires 7°C of temperature gradient within each copper plate. This unlikely would happen in the present test condition (Tw=67°C,Tb=32°C). In addition, the high-conductivity copper plate with dimples represents the true span-averaged heat transfer coefficient including the potential end-wall effect from the channel smooth-side walls. For example, the potential smooth-side wall effect on the span-averaged heat transfer coefficient could not be included by viewing only the central 5-dimple area using the IR camera technique shown in Fig. 2 by Mahmood et al. [14]. The central 5-dimple area only might potentially produce higher heat transfer enhancement than the truly span-averaged values including the potential smooth end-wall effect. To solve the issue, the authors plan to further investigate this topic. Again, the authors do appreciate Dr. Ligrani’s insightful comment on the important dimple cooling technology for turbine blade cooling designs.

Heat-Transfer Coefficients of a Turbine Blade-Tip and Near-Tip Regions

The detailed distribution of heat-transfer coefficients on a gas turbine blade tip was measured using a hue-detection-based transient liquid crystals technique. The heat-transfer coefficients on the shroud and near-tip regions of the pressure and suction sides of a blade were also measured. Tests were performed on a five-bladed linear cascade with a blowdown facility. The Reynolds number based on the cascade exit velocity and axial chord length of a blade was 1.1 x 10 6 , and the overall pressure ratio was 1.2. The heat-transfer measurements were made at the three different tip gap clearances of 1.0, 1.5, and 2.5% of blade span

Flat Surface Film Cooling from Cylindrical Holes with Discrete Tabs

The effect of discrete delta-shaped tabs on film cooling performance from a row of cylindrical angled holes is investigated. The holes are inclined at 35 deg along the streamwise direction. Four tab locations are investigated: 1) tabs placed along the upstream edge of the hole covering 33% of the hole, 2) tabs placed along the upstream edge covering 11% of the hole, 3) tabs placed along the downstream edge of the hole, and 4) tabs placed along the lateral edges of the hole. They are compared to the baseline case without tabs. Measurements were carried out in a low-speed wind tunnel using the transient liquid crystal technique. The mainstream velocity and freestream turbulence intensity in the low-speed wind tunnel are 8.5 m/s and 6%, respectively, and the Reynolds number based on hole diameter is 6375. Three blowing ratios of 0.56, 1.13, and 1.7 are tested

A Novel Transient Liquid Crystal Technique to Determine Heat Transfer Coefficient Distributions and Adiabatic Wall Temperature in a Three-Temperature Problem

Transient liquid crystal techniques are widely used for experimental heat transfer measurements. In many instances it is necessary to model the heat transfer resulting from the temperature difference between a mixture of two gas streams and a solid surface. To nondimensionally characterize the heat transfer from scale models it is necessary to know both the heat transfer coefficient and adiabatic wall temperature of the model. Traditional techniques rely on deducing both parameters from a single test. This is a poorly conditioned problem. A novel strategy is proposed in which both parameters are deduced from a well-conditioned three-test strategy. The heat transfer coefficient is first calculated in a single test; the contribution from each driving gas stream is then deduced using additional tests. Analytical techniques are developed to deal with variations in the temperature profile and transient start time of each flow. The technique is applied to the analysis of the heat transfer within a low aspect ratio impingement channel with initial cross flow.