Liquid Crystal Sheet Research Articles

Experimental and Computational Study on Effect of Vanes on Heat Transfer and Flow Structure of Swirling Impinging Jet

The study focuses on heat transfer performance and flow structure associated with swirling jet on a flat target surface. The analysis is carried out with helicoid inserts of swirl number S = 1.3 by varying the number of vanes with Reynolds number between 11200 and 35600. The comparison of swirling jet with circular jet is carried out on its heat transfer performance. The heat transfer and flow structure are visualized using thermo-chromic liquid crystal sheet and oil film technique respectively. The numerical simulation is also performed at Re = 24700 for H/D distance between 1 and 4 using computational fluid dynamics. The heat transfer results reveal that the presence of axial recirculation zone at Re = 29800 and 35600 for the triple helicoid affects the uniformity of heat transfer distribution at 0 < X/D < 1.5 at H/D = 3. The axial component of velocity with respect to swirling jet is less than zero in the stagnation area and it increases at 0.57 < r/D < 0.97 for single vane and 0.63 < r/D < 0.97 for double and triple vanes. While the steep increase in tangential velocity of the triple vane jet is apparent at 0 < r/D < 0.5 at H/D = 2 and 3, the maximum value of point radially shifts inward towards the jet. The location of maximum turbulent kinetic energy approaching the surface at about r/D = 0.9 - 1.2 which characterizes the swirling jet at H/D = 2.

Thermal evaluation of flow channels with perforated-baffles

The influences of perforated-baffles on the local Nusselt number (Nu) and thermo-hydraulic behaviors were comprehensively studied using thermochromic liquid crystal sheet. The perforated-baffles were designed in two forms: perforated-baffle (PB) and perforated-baffle with square wings (SW-PBs). Transverse solid baffles (TBs) were also tested for an assessment. All baffles had an identical height of 12 mm (e/H = 0.3). Experimental results showed that SW-PBs offered better Nu than PBs. It is also seen that PBs and SW-PBs caused lower pressure loss than TBs by around 20.49% and 13.98%, respectively. The reduction of friction loss was primarily due to the baffle perforation. In addition, the PBs yielded the thermal performance factors (TPF) up to 1.01 at the lowest Reynolds number of 6000.

EXPERIMENTAL INVESTIGATIONS FOR EFFECTIVENESS OF COMBINED IMPINGEMENT FILM COOLING

Protecting the gas turbine blades from high-temperature gases and increasing the specific work output is one of the most challenging tasks for most gas turbine design engineers. Combined impingement and film cooling can be used as a hybrid cooling technology to improve the film cooling performance over the gas turbine blade surfaces. In the present work, experimentations were performed on a flat plate with combined impingement film cooling for two thermally conductive materials (Perspex and stainless steel) at different blowing ratios and compared with the computational results. Thermo chronic liquid crystal sheets were used for measuring the temperature distribution. Effectiveness is the parameter that plays a vital role in determining thermal performance. Uniform effectiveness distribution was observed in stainless steel material because of its more thermal conductivity. Combined impingement film cooling showed higher effectiveness compared to film cooling. Uncertainty of ± 7% present in TLC data and experimental results agreed with the computational results.

Thermal Visualization and Performance Analysis in a Channel Installing Transverse Baffles with Square Wings

The experimental examination of local heat transfer, thermal intensification, friction factors, and thermal performance factors (TPF) in a rectangular channel with square-winged transverse baffles (SW-TB) are presented in this paper. The purpose of this study is to modify the typical transverse baffles (TB) into square-winged transverse baffles (SW-TB) in order to improve the thermal performance and heat transfer rate of the channel. The effects of SW-TBs with various wing attack angles and Reynolds numbers on the heat transfer performance characteristics were examined using a thermochromic liquid crystal sheet. In the experiments, the SW-TBs were attached to the bottom wall of the channel, which had an aspect ratio (W:H) of 3.75:1. The SW-TBs had a width (w) of 150 mm, a square perforated cross-sectional area (a × b) of 8 × 8 mm2, and attack angles (θ) of 0° (solid transverse-baffle), 22.5°, 45°, 67.5°, and 90°. The bottom wall of the channel was evenly heated, while the other walls were insulated. The temperature contours on the heated surface were plotted using temperatures obtained through using the thermochromic liquid crystal (TLC) image-processing method. Experimental results revealed that the SW-TBs created multiple impinging jets, apart from the recirculation. At the proper attack angles (θ = 22.5° and 45°), the SW-TBs offered greater heat transfer rates and caused lower friction losses, resulting in higher TPFs than the solid transverse baffles. In the current work, channels where the SW-TBs display a θ = 45° presented the greatest TPF, as high as 1.26. The multiple impinging jets issuing by the SW-TBs suppressed the size of the recirculation flow and allowed better contact between the fluid flow and channel wall.

Flow and heat transfer studies of multijet impingement cooling for different configurations: A review

AbstractIncreasing the gas turbine engine's turbine intake temperature has long been a potential strategy for increasing the specific work output of the engine. However, the melting temperature of the turbine blades and vane material limits the maximum intake temperature. As a result, internal and external cooling techniques are commonly used to maintain the vane material in a safe condition. This study provided an overview of internal impingement cooling to highlight the significance of geometrical variations, such as flat plate, curve plate, and actual vanes. It was observed that flat and curved plate impingement heat transfer studies were reported extensively, whereas limited studies were found on the conjugate effects on airfoil surfaces. The importance of conjugate heat transfer studies and their impact has recently been described in the literature. In most of the literature, a wide range of instruments, such as Laser Doppler Velocimeter, Particle Image Velocimeter, liquid crystal sheets, and so forth, were used for experimental investigations. According to most studies, the local value of internal surface temperature and heat transfer coefficient are vital factors of local flow behavior. Jet‐to‐jet spacing, jet‐to‐plate spacing, jet hole diameter, and jet Reynolds numbers played a crucial role in both numerical and experimental analyses. Different geometric variations strongly influence flow behavior. Therefore, the usual method for determining interior temperature distributions and heat transfer coefficients by considering generalized geometries like the flat and curved plate may not produce accurate conjugate solutions. Most of the computational studies on the flat and curved plate indicate the usage of κ−ω shear stress transport and κ–ε realizable model to predict the heat transfer coefficient.

Characterization and driving of polymer-dispersed liquid-crystal sheet for smarthome applications

A commercial polymer-dispersed liquid-crystal (PDLC) sheet sample was characterized for its basic optical and electrical properties under DC and AC excitation. The results give driving voltage for 90% of transmittance for the PDLC of 10 V for DC excitation, and 9.5 Vrms for 50 Hz AC excitation. The measured contrast ratio of the PDLC is 2.55 and 2.47 under DC and AC excitation, respectively, which is suitable for smart window in a smarthome application. While it has rather similar optical properties under DC and AC excitation, it has significantly different electrical properties for DC and AC excitation. The PDLC exhibits a capacitive property which has capacitance per unit area of 18.5 μF/m2, indicating that DC excitation will give benefit compared to AC excitation which will scale with area of the PDLC. Based on these results, a DC driving circuit employing boost converter was designed to make a microcontroller-compatible driving circuit. The system was then integrated into a miniature smarthome demonstrator model exhibiting contrast ratio of 2.05 under switching from 0 to 100% of duty cycle of pulse width modulation (PWM) control signal from the microcontroller executed through voice command in an internet of things (IoT) platform.

A continuum model for solitons in the twisted magnetic liquid crystals

We systematically explore the solitons in the twisted magnetic liquid crystals using a continuum model. Firstly, we derive the dynamic equation of the twisted magnetic liquid crystal sheet under the control of magnetic field. It is proved to be a Sine-Gordon equation. Solving the equation, we get the kink and anti-kink soliton solution and breather solution. Finally, we proved that under the control of the magnetic field, the twist motion of the magnetic liquid crystal, which is regarded as the continuum model, can produce solitons and breathers.

Liquid crystal thermography based study on melting dynamics and the effect of mushy zone constant in numerical modeling of melting of a phase change material

Melt front tracking is vital in a solid–liquid phase change process to understand the melting process and quantify the system’s liquid faction, sensible and latent energies. This study presents a method to track the solid–liquid interface during the melting of a phase change material using liquid crystal thermography. The melt front movement is captured simultaneously through temperature influenced color variation in Thermochromic Liquid Crystal (TLC) sheet and direct visualization. Experiments have been performed for the cases of (i) heating from the bottom and (ii) heating from the top. Further, three-dimensional numerical simulations have been performed to validate the mushy zone temperatures and understand the effect of the mushy zone constant in the mushy zone temperature and liquid fraction. The results show that the TLC easily captures the dynamic motion of the solid–liquid interface during melting. From numerical simulations, it is found that the mushy zone constant affects the numerical prediction of melting in the bottom heating case with the low and high mushy zone constants, respectively, over predicting and under predicting the mushy zone temperatures during melting. Additionally, in this case, it is seen that the Rayleigh–Benard convection creates a wavy interface and accelerates the melting rate of the PCM. In the case of heating from the top, the numerical prediction of melting is independent of the mushy zone constant. Besides, a flat interface is observed in this case due to the absence of convection in the PCM during melting. Additionally, in the case of heating from the top, the combination of the PCM’s low thermal conductivity and heat diffusion through conduction leads to the self insulation effect of the PCM during melting. The convection-driven melting of PCM maintains the heater temperature lower by up to 35%, reduces the melting thermal resistance up to 78%, and increases the average volumetric melting to 92% over the case of melting without convection in the PCM.

Heat transfer enhancement and flow characteristics in a rectangular channel having inclined pin arrays mounted on the endwall surface

This study describes heat transfer and flow characteristics for various angles of inclined-pin staggered arrays in a rectangular channel. The pin array consists of 10 staggered rows of cylindrical pins. Influence of the pin inclined angle is studied at θ = 30o, 45o, 60o, 90o, 120o, 135o, and 150o. A thermochromic liquid crystal sheet is used for temperature measurement. Flow field and heat transfer characteristics are predicted by numerical simulation. From the study, certain pin inclined angles have shown a favorable effect on the heat transfer enhancement with an array of pins. Only pin arrays with inclined angles at θ = 120o and 135o could enhance heat transfer on the surface when compared with conventional right-angle pin. Jet-like flows behind the pin move the counter-rotating vortices with high velocity (CRH) that induce airflow attachment to the heat transfer surface down to the lower surface. So, CRH has a significant effect on heat transfer promotion. Pin inclination angles at θ = 120o, 135o and 150o could greatly reduce the pressure drop and enhance the thermal performance factor. The greater number of rows in the test section has been proven crucial for some inclination angles in promoting heat transfer performance.

Thermal properties of polymer networks used in light control liquid crystal sheets for cars

Thermal properties of polymer networks used in light control liquid crystal sheets for cars

Characteristic of Color-Play on Liquid Crystal Sheet Based on Image Analysis

In this research, we compare two samples of Liquid Crystal sheet (LCs), R30C5W and R35C5W to get information about the characteristic of LCs based on the color-play of the LCs surface caused by increasing of temperature. We analize the color-play of the samples quantitatively by image analysis using hue parameter, texture analysis method by choosing entropy and energy parameter, and color-play area measurement. According to the experiment results, both samples show the similar characteristic in color-play behaviour, illustrated by similar curve for each sample, the different only in the starting temperature of the color-play and the clearing point temperature. The samples also show the same behavior in increasing of the color-play area linearly by increasing of temperature. Based on the graph of hue, entropy and energy parameters, it can be measured the clearing-point temperature of samples at 43 °C and 49 °C for LCs R30C5W and LCs R35C5W respectively. Moreover, the changes of color from red to blue cause the irregularity of pattern illustrated by increasing of entropy values and decreasing of energy values. Study the characteristic of LCs in various types is important particularly before it is used for many application purposes, for instance in heat transfer observation.

Mathematical model of physical process of heat transfer in liquid crystal

The process of heat transfer by conduction is relevant in different context of engineering and sciences. An example of this situation is confirmed by the study of the thermal properties of liquid crystal sheets to measure the effects of heat transfer on metal surfaces. The aim of this work is to establish a mathematical model by numerical methods to determine the physical process of heat transfer in liquid crystal. For the data acquisition, an experiment that measures thermal properties on an aluminum plate was designed. We proceed to propose a mathematical model that uses the Laplace equation. Also, we proceed to calculate the solution of the equation by the finite difference method and then make a comparison with the analytical solution and the experimental data.

A novel method to detect hot spots and estimate strengths of discrete heat sources using liquid crystal thermography

This paper reports the results of an investigation to solve the inverse problem of estimating the strengths of different strip heat sources embedded in a flat plate under laminar steady-state forced convection and by way of this propose a novel method to detect hot spots from remote measurements. A Bayesian framework is adopted to infer the strength of the heat sources from thermochromic liquid crystal (TLC) temperature measurements. This framework consists of the forward model, the measured data, and the inverse model. The forward model simulates the conjugate three-dimensional heat transfer problem with the specified thermophysical properties, and the boundary conditions. The input data for the forward model is a combination of different heat source strengths, and the output is temperature data obtained at the bottom surface of the cork. The input-output data of the numerical simulations are used to build a proxy or surrogate (artificial neural network, ANN) that acts as a replacement for the actual forward model to increase the computational speed and decrease the computational time while solving the inverse problem. Calibrated thermochromic liquid crystal sheets are attached at the bottom surface of the cork for mapping the temperature data, so that the top surface where the convection takes place is undisturbed. In the inverse model, Bayesian statistics, along with the Gibbs sampling algorithm is adopted for analyzing the posterior distribution to estimate the mean, the maximum a posteriori and the standard deviation of the heat source strengths. Validation and robustness of the inverse methodology have been examined. The estimated heat source values are input to the forward model to determine the hot spot temperatures on the flat plate. This is a key spin-off from the present study, wherein based on temperature measurements at a convenient place, the hot spot in a geometry can be remotely ‘estimated.’ A comparison of the simulated and the measured values of the hot spot temperatures is reported for different flow Reynolds numbers.

Research on the Heat Transfer Characteristics of the Three-Dimensional Backward-facing Step Flow Using Liquid Crystal Sheet Based on SVR Method

Research on the Heat Transfer Characteristics of the Three-Dimensional Backward-facing Step Flow Using Liquid Crystal Sheet Based on SVR Method

Effect of pin inclination angle on flow and heat transfer characteristics for a row of pins in a flow channel

The aim of this research was to study flow and heat transfer characteristics with a row of inclined pins in a rectangular channel. The cylindrical pins of diameter D = 10 mm were mounted on the heat transfer surface in spanwise direction of the channel. The pin-to-pin distance was fixed at S = 2D. Two of pin heights H were studied, namely short pins or long pins with H = 2D or H = 3.2D, respectively. The effects of pin inclination angle were investigated for θ=30o, 45o, 60o, 90o, 120o, 135o, and 150o in the range of Reynolds number from 14,000 to 32,860. The local temperature on endwall surface was measured using a thermochromic liquid crystal sheet coupled with image processing. Numerical simulations were used to obtain three-dimensional flow fields and heat transfer rates on pin surfaces and endwall surface. Results for the case H/D = 2 show that pins inclined by θ=30o, 45o and 60o can enhance heat transfer downstream of the pin row, from the baseline case of pins with θ=90o. The heat transfer was improved by a pair of counter-rotating vortices near the heat transfer surface. These vortices induced the main flow to attach on the heat transfer surface. The pin inclinations θ=120o and 135o somewhat improved heat transfer behind the pins. However, the inclination θ=150o gave the lowest heat transfer rate on the endwall surface. The heat transfer on endwall surface with H/D = 3.2 was poorer than with H/D = 2 because the counter-rotating vortices did not reach the heat transfer surface. The inclination θ=30o with H/D = 2 gave the best thermal performance. The local Nusselt number on the rear side of pin surfaces with H/D = 3.2 and inclinations θ=30o, 120o, 135o and 150o tended to increase from the case H/D = 2, due to a jet-like flow attachment on the pin surfaces. However, the local Nusselt number on pin surfaces with H/D = 3.2 and inclinations θ=45o and 60o tended to be lesser than with H/D = 2, due flow separation downstream far from the pins.

An improved technique for visualizing microwave wavelengths in microwave ovens

There have been a few methods described in this journal and elsewhere for measuring the wavelength of the standing electromagnetic waves in a microwave oven. Typically, these involve melting chocolate, cheese, or some other substance on a plate that is prevented from rotating. In this article I describe a more dynamic and colorful technique that utilizes a temperature-sensitive liquid crystal sheet. With this method one can not only measure the wavelength of the microwaves, but also calculate the speed of these waves, which intriguingly turns out to be the speed of light. It also has the advantage of looking more like a wave than the other methods.

An Impingement Cooling Using Swirling Jets Induced by Helical Rod Swirl Generators

Abstract The heat transfer performance of swirling impinging jets (SIJs) was investigated and compared with that of the conventional jet (CIJ). The swirling jets were induced by helical rod inserts (HRs) fitted with pipe nozzles. The helical rod inserts with two different rod diameter to nozzle diameter ratios (d/D) of 0.46 and 0.64 were employed for comparison. Jet-to-plate distance to nozzle diameter ratio (L/D) was varied from 1 to 5 while Reynolds number was fixed at 20,000. The temperature and Nusselt number distributions on the impinged plate were measured and evaluated using thermochromic liquid crystal (TLC) sheet and image processing technique. The experimental results showed that the swirling jet (SJ) at d/D=0.64 gave higher average heat transfer rate than the SJ at d/D=0.46 up to 6.41 % and the conventional jet (CJ) up to 35.05 %. In addition, Nusselt number peak of swirling jets increased as jet-to-plate distance decreased.

Estimation of local heat transfer coefficient from natural convection experiments using liquid crystal thermography and Bayesian method

In this work, an inverse methodology is developed for estimating the local heat transfer coefficients on a vertical plate embedded with the three discrete heat sources, under steady state natural convection, with the temperatures measured at the adiabatic surface without disturbing the fluid flow, using simple conduction/surrogate model and Bayesian inference. Liquid crystal thermography (LCT), an optical measurement method based on the colour-temperature relationship of thermochromic liquid crystal sheet (TLC) is used to determine the temperature field of the adiabatic surface. Bayesian framework with Metropolis Hastings-Markov chain Monte Carlo (MH-MCMC) sampling method is considered for exploring the posterior distribution to estimate the parameters in terms of point estimates like mean, Maximum a posteriori (MAP) and standard deviation. A parity plot between simulated (using retrieved parameters) and measured TLC temperatures shows good agreement.

Mean shear flow in recirculating turbulent urban convection and the plume-puff eddy structure below stably stratified inversion layers

The mean and random components of the velocity field at very low wind speeds in a convective boundary layer (CBL) over a wide urban area are dominated by large eddy structures—either turbulent plumes or puffs. In the mixed layer at either side of the edges of urban areas, local mean recirculating flows are generated by sharp horizontal temperature gradients. These recirculation regions also control the mean shear profile and the bent-over plumes across the mixed layer, extending from the edge to the center of the urban area. A simplified physical model was proposed to calculate the mean flow speed at the edges of urban areas. Water tank experiments were carried out to study the mean recirculating flow and turbulent plume structures. The mean speed at urban edges was measured by the particle image velocimetry (PIV), and the plume structures were visualized by the thermalchromic liquid crystal (TLC) sheets. The horizontal velocity calculated by the physical model at the urban edge agrees well with that measured in the water tank experiments, with a root mean square of 0.03. The experiments also show that the pattern of the mean flow over the urban area changes significantly if the shape of the heated area changes or if the form of the heated urban area becomes sub-divided, for example by the creation of nearby but separated “satellite cities.” The convective flow over the square urban area is characterized as the diagonal inflow at the lower level and the side outflow at the upper level. The outflow of the small city can be drawn into the inflow region of the large city in the “satellite city” case. A conceptual analysis shows how these changes significantly affect the patterns of dispersion of pollutants in different types of urban areas.

Combined numerical and experimental investigations of heat transfer of a highly loaded low-pressure turbine blade under periodic inlet flow condition

This paper experimentally and numerically investigates heat transfer characteristics of a low-pressure turbine blade under steady/unsteady flow conditions. Generally, the low-pressure turbine blades are not exposed to excessive temperatures that require detailed heat transfer predictions. In aircraft engines, they operate at low Re-numbers causing the inception of large separation bubbles on their suction surface. As documented in previous papers, the results of detailed aerodynamic simulations have shown significant discrepancies with experiments. It was the objective of the current investigation to determine the discrepancies between the experimental and numerical heat transfer results. It is shown that small errors in aero-calculation results in large deviations of heat transfer results. The characteristics of the blades mentioned above, make low-pressure turbine blades suitable candidates for evaluating the predictive capability of any numerical method. Documenting the scope of these discrepancies defines the framework of the current paper. The periodic flow inside the gas turbine engine was simulated using the cascade facility at the Turbomachinery Performance and Flow Research Laboratory (TPFL) of Texas A&M University. In this study, the wakes that originate from stator blades were simulated by moving rods. The instrumented blade was covered with a liquid crystal sheet and it was used to measure heat transfer coefficient. Reynolds-averaged Navier–Stokes equations were used for numerical investigation purposes. Measurements and simulations were conducted at three different Reynolds numbers (110,000, 150,000, and 250,000). Furthermore, for unsteady flow condition, reduced frequencies of the incoming wakes were varied. The current paper includes a comprehensive heat transfer assessment of the predictive capability of Reynolds-averaged Navier–Stokes based tools. The effect of the separation bubbles on heat transfer is thoroughly discussed in this paper. Comparisons of the experimental and numerical results detail the differences and identify the sources of error that leads to in accurate calculations in terms of predicting heat transfer calculation results.