Transient Heat Transfer Experiments Research Articles

Assessment of Binary Pressure-Sensitive Paint for Temperature and Heat Transfer Coefficient Measurement of Leading-Edge Film Cooling

Abstract Film cooling is a common technique for protecting gas turbine components from the hot combustor exhaust. Highly resolved film cooling effectiveness distributions are often obtained by measuring the mass transfer of a foreign gas coolant in mainstream air using pressure-sensitive paint (PSP). However, PSP is not able to measure the heat transfer coefficient, which is necessary to fully quantify the impact of film cooling. Instead, binary pressure-sensitive paint (BPSP) has an additional luminophore that is sensitive to temperature and can be used to measure the heat transfer coefficient. In this experiment, the film cooling effectiveness and heat transfer coefficient were measured using BPSP on the leading edge of a cylinder. The cylinder had a 7.62-cm diameter with two rows of cooling holes at ±15 deg from the leading edge. Each row contained ten holes with a 0.475-cm diameter, spaced 4 diameters apart in the spanwise direction, and angled 30 deg from the cylinder axis. The mainstream Reynolds number was 100,000 based on cylinder diameter with a turbulence intensity of 7.1%. The coolant-to-mainstream density ratio was 1.0, and the blowing ratio was 0.8. The heat transfer coefficient was measured in a transient heat transfer experiment using the reference signal from the BPSP. Despite the high uncertainty of the measurement, ranging from 24.0% to 71.1%, the results demonstrate the feasibility of the method and identify the best test methodology to minimize conduction errors.

Numerical and experimental investigations on intense pulsed light sintering of silver nanoparticle inks for printed electronics

Intense pulsed light (IPL) sintering has the advantages of high efficiency and selective heating for silver nanoparticles (AgNPs) inks, which could be widely used in printed electronics. In this contribution, the heat transfer and diffusion mechanisms of AgNPs during IPL sintering are quantitatively investigated with a mathematic model combined transient heat transfer and molecular dynamics (MD) method and experiments. The results show that only 51.522% of the IPL radiation energy is absorbed by the AgNPs. During IPL sintering, the temperature of AgNPs rises significantly, whereas the temperature of polyamide substrate keeps almost unchanged, demonstrating that IPL is feasible for thermal-sensitive substrates. The MD modeling results show that sintering neck between AgNPs forms and grows rapidly at the beginning of sintering, then remains relative stable. Accordingly, the electrical resistivity of the AgNPs drops rapidly to a stable value. In addition, the effects of IPL sintering parameters are studied and the results show that increasing IPL energy and reducing IPL duration could increase the sintering performance of AgNPs inks. Finally, an antenna is fabricated using AgNP inks and IPL sintering technology. The experimentally measured performance of the antenna agrees well with the theoretical analysis. Our simulation and experimental results demonstrate that IPL is suitable for sintering of AgNPs inks for flexible electronics.

A Monte Carlo approach to evaluate the local measurement uncertainty in transient heat transfer experiments using liquid crystal thermography

This work presents an improved estimation of the local measurement uncertainty of transient heat transfer experiments using Thermochromic Liquid Crystals (TLC). Additional signal analysis steps and a Monte Carlo (MC) approach are added to the evaluation routine. As a result, the local uncertainty in the indication time of the maximum green intensity is determined. All other input quantities for the heat transfer coefficient calculation are provided with their characteristic distribution function introducing additional uncertainties. As a final result, lower and upper boundaries for a confidence interval with a user defined confidence level for the heat transfer coefficient are calculated locally for single pixels. The new method is applied to own measurement data from a transient TLC experiment. The local measurement uncertainty is compared to the traditional approach based on error propagation. Hence, the advantages of taking the non-linearity and the local uncertainty in the peak detection into consideration are shown.

Conjugate Heat Transfer Study of Innovative Pin-Fin Cooling Configuration

The Paper involves experimental and numerical heat transfer analyses of four pin-fin cooling configurations in a rectangular channel. Two design concepts were studied with two separate pin-fin (cylindrical and triangular) geometries. The first design includes a single-wall pin-fin configuration where the coolant flows through an array of full-length cylindrical (PF) and triangular pin fins (TF). The second configuration involves a double-wall pin-fin–jet configuration where the coolant jet impinges on the target wall, then flows through the array of partial-length cylindrical pin fins with jets (PFC) and partial-length triangular pin fins with jets (TFC). IR thermography was used to measure the steady state wall temperature of the target surface to estimate the overall cooling effectiveness . A transient heat transfer experiment was conducted to evaluate the internal heat transfer coefficient from the transient wall temperature. It is found that the double-wall pin-fin–jet (PFC and TFC) configurations show higher cooling effectiveness compared to the single-wall pin-fin (PF and TF) designs and the triangular pin fin (TF and TFC) showed a higher heat transfer augmentation compared to the cylindrical pin fin (PF and PFC) design. Numerical study later confirmed that the triangular pin fin creates a pair of streamwise vortices that augments the local heat transfer. The presence of the pin fin in the double-wall configuration prevents the development of a strong crossflow that negatively affects the heat transfer of the impinging jet. In addition, the triangular pin fin not only prevents the strong crossflow but redirects the coolant in the lateral direction that eventually contributes to the formation of a counterrotating vortex pair in the streamwise direction and thus augments local heat transfer and improves overall cooling effectiveness. Pressure drop measurement showed that the full-length triangular pin fin has a higher-pressure loss but the partial-length pin fin can lower the pressure loss considerably without compromising additional cooling performance.

Synthesis of structure-controlled hematite nanoparticles by a surfactant-assisted hydrothermal method and property analysis

The impact of the type and concentration of surfactant on the synthesis of iron oxide nanoparticles (IONPs) has been investigated. Three types of surfactant — anionic, cationic, and nonionic surfactants (sodium dodecyl sulfate, SDS; cetyltrimethylammonium bromide, CTAB; and Triton X-100, respectively) — have been selected to control the phase, shape, and size of the IONPs. The synthesis was carried out with varying concentrations at 2, 4, and 6 times the critical micelle concentration (CMC) of each surfactant in a hydrothermal system at 180 °C. XRD and FE-SEM with the Microsoft Visio 2010 program have been applied for characterization and nanoparticle analysis. All synthesized IONPs, without and with the different surfactants, were characterized as spherical hematite with diameters in the range 15–205 nm, depending on the surfactant type and concentration. Surfactant type and concentration influenced the nature of the ferrous hexahydrate intermediate through acid-base equilibria and the Le Châtelier principle. The magnetic properties of the hematite nanoparticles have been examined by means of a vibrating sample magnetometer (VSM). The highest magneticity of synthesized hematite occurred at the transition from a single monolithic structure to one with multiple domains at a diameter of approximately 70 nm with a coercivity (Hc) of 225 Oe. The smallest hematite particles were chosen to prepare water-based nanofluids using the surfactants as stabilizing agents. Colloidal stability of the nanofluids has been studied by sedimentation, turbidity, and ζ-potential analyses. Nanofluid thermal conductivity has been determined by transient heat-transfer experiments. A nanofluid prepared in 2 CMC SDS solution showed the highest stabilization and a thermal conductivity of 0.4787 W/m.K.

Numerical and experimental determinations of contact heat transfer coefficients in nonisothermal glass molding

AbstractHeat transfer at the interfacial contact is a dominant factor in the thermal behavior of glass during nonisothermal glass molding process. Recent research is developing reliable numerical approaches to quantify contact heat transfer coefficients. In most previous studies, however, both theoretical and numerical models of thermal contact conductance in glass molding attempted to investigate this factor by either omitting surface topography or simplifying the nature of contact surfaces. In fact, the determination of the contact heat transfer coefficient demands a detailed characterization of the contact interface including the surface topography and the thermomechanical behavior of the contact pair. This paper introduces a numerical approach to quantify the contact heat transfer by means of a microscale simulation at the glass‐mold interface. The simulation successfully incorporates modeling of the thermomechanical behaviors and the three‐dimensional topographies from actual surface measurements of the contact pair. The presented numerical model enables the derivation of contact heat transfer coefficients from various contact pressures and surface finishes. Numerical predictions of these coefficients are validated by transient contact heat transfer experiments using infrared thermography to verify the model.

Semi-infinite solid heat transfer limitation

One-dimensional semi-infinite heat transfer solution is a common solution for transient heat transfer experiments. This solution is valid for a short certain amount of time before the semi-infinite solid became invalid. Crank Nicolson solution has been chosen to address this issue. This paper reports the time limitation for semi-infinite solid solution and justify the usability of Crank Nicolson solution given the same boundary conditions. The flat plate heat transfer experiment has been conducted. With the same boundary conditions, at Fourier number 0.1, the resultant heat transfer coefficient and adiabatic wall temperature have shown a good agreement between the semi-infinite solid solution and the Crank Nicolson solution. Beyond this Fourier number, both solutions have given inaccurate results. The inaccurate results are due to unsuitable boundary conditions. Future work will involve modification of the back face boundary conditions to address the time limitation of the one-dimensional semi-infinite solid heat transfer solution.

Quantifying ageing effects in thermochromic liquid crystal thermography as applied to transient convective heat transfer experiments

Thermochromic liquid crystal (TLC) thermography is used in transient heat transfer experiments to determine distributions of convective heat transfer coefficient (HTC) inside models of internally cooled gas turbine engine components. As these components become more geometrically complex, the application of TLC thermography becomes increasingly challenging and additional sources of experimental uncertainty grow to be significant. The present work quantifies the uncertainties introduced by TLC ageing using a state-of-the-art imaging system and a new post-processing methodology that are optimised for the intensity-based method of analysing TLC data. A coating comprising multiple TLCs with different active temperature ranges is considered and subject to 33 repeated thermal cycles. These repeated cycles are shown to increase the random and systematic uncertainties in the TLC measurements, resulting in consequent increases in the uncertainties associated with calculated HTCs. Increases in systematic uncertainty are caused by reflectance in the measured wavelength band moving to different temperatures, while increases in random uncertainty are related to changes in individual crystals or crystal clusters with ageing. Approaches to calibrating out increases in systematic uncertainty are proposed and recommended, but increases in random uncertainty will always persist unless the TLC coating is removed and reapplied.

Comparison of the Subchannel Code CTF to Steady-State and Transient Heat Transfer Experiments at Intermediate Pressures in Water

The CTF code is a subchannel thermal-hydraulic code developed based on the COBRA-TF code. In this work, the CTF code is used to predict the single- and two-phase heat transfer, pressure drop, onset of nucleate boiling, and dryout heat flux in water at several temperatures and pressures under steady-state and transient conditions. The conditions cover a range of pressures from 2 to 6 MPa, flows from 1000 to 2500 kg/(m2∙s), and inlet subcooling from 40°C to 70°C. Experimental heat balance tests show agreement between coolant enthalpy change and the electrical power with a difference of no more than 1.0%. Steady-state experiments were performed at constant inlet conditions in a cylindrical directly heated Inconel test section where the wall temperatures were measured at each power level. For each steady-state test, the experimental boiling curve is compared to CTF predictions. Transient experiments were performed by initiating a blowdown from the test section outlet plenum using a fast-acting valve with an open time of less than 100 ms. The time of dryout in these transient experiments is compared with the CTF results to clarify the pressure transient effect on the dryout prediction.

Inverse approach for estimating boundary properties in a transient fin problem

A solution methodology is proposed for an inverse estimation of boundary conditions from the knowledge of transient temperature data. A forward model based on prevalent time-dependent heat conduction fin equation is solved using a fully implicit finite volume method. First, the inverse model is formulated and accomplished for time-invariant heat flux at the fin base, and later extended to transient heat flux, base temperature and average heat transfer coefficient. Secondly, the Nusselt number is then replaced with Rayleigh number in the forward model to realistically estimate the base temperature, which varies with respect to time, based on in-house transient fin heat transfer experiments. This scenario further corroborates the validation of the proposed inverse approach. The experimental set-up consists of a mild steel \(250 \times 150 \times 6\, \hbox {mm}^3\) fin mounted centrally on an aluminium base \(250 \times 150 \times 8\, \hbox {mm}^3\) plate. The base is attached to an electrical heater and insulated with glass-wool to prevent heat loss to surroundings. Five calibrated K-type thermocouples are used to measure temperature along the fin. The functional form of the unknown parameters is not known beforehand; sensitivity studies are performed to determine suitability of the estimation and location of sensors for the inverse approach. Conjugate gradient method with adjoint equation is chosen as the inverse technique and the study is performed as a numerical optimization; subsequently, the estimates show satisfactory results.

Understanding the temperature dependence of thermo-physical properties of nanofluid suspensions using non-intrusive dynamic measurements

Thermal diffusivity dependence on temperature and concentration of aqueous colloidal suspensions of Al2O3 in deionized water has been investigated by conducting transient heat transfer experiments. The study has been performed in a purely non-intrusive manner using Mach Zehnder interferometry. Through the dependence of refractive index on temperature, the time sequence of recorded interferograms has been used to generate highly sensitive data of evolving temperature distribution in the thermal diffusion field. Determination of thermal diffusivity from temperature-position data has been carried out by the method of least square. In comparison with deionized water, substantial enhancement in thermal diffusivity with increasing temperature and concentration of nanofluid suspension of Al2O3 is observed through the time sequence of the recorded interferograms as well as through analytical measurements. Additionally, the measurement of ζ-potential, pH and effective viscosity of dilute Al2O3 suspension at various temperatures explains the possible mechanisms at micro-scale levels that are responsible for such an anomalous behaviour of thermal diffusivity. The competing interaction of these factors reveals dependence of thermal diffusivity on particle cluster formation and thereby in the resultant stability of the suspension. The experimental findings indicate that, at specific size and shape, the higher conductivity of solid alumina particles and their cluster formation plays a major role in enhancing the thermophysical properties of the nanofluid Al2O3 suspension over the considered range of temperature and concentration. Also, a reasonably close agreement of the obtained trends of thermal diffusivity values with those reported in the literature and clear visualization of enhancement in thermal transport through interferograms signifies the importance of the proposed non-intrusive measurement technique for reliable measurement of thermal properties of nanofluids.

Characterization of heat transfer enhancement and frictional losses in a two-pass square duct featuring unique combinations of rib turbulators and cylindrical dimples

Transient heat transfer experiments using liquid crystal thermography have been carried out on a two-pass square channel for testing several unique combinations of ribs and cylindrical dimples. Four different rib shapes, viz. 45° angled, V, W and M have been studied. In compound channels, the rib pitch accommodates cylindrical dimples arranged in the shape of the rib, hence following the direction of rib induced secondary flows. For each rib shape, three different configurations – rib alone, dimple alone and rib–dimpled compound cases were studied. The rib-height-to-channel hydraulic diameter ratio was 0.125 and rib-pitch-to-rib-height ratio was 16. The dimple-depth-to-print diameter ratio was 0.3. The experiments were carried out for a wide range of Reynolds number (19,500–69,000), covering a spectrum typically found in both land-based and air-breathing engines. A total of 52 experiments were carried out to measure detailed heat transfer coefficient on the bottom wall of the two-pass channel. A transient liquid crystal thermography technique was used for heat transfer measurement. Static pressure measurements were carried out to measure the overall pressure drop in the two pass channel. From globally averaged Nusselt number and overall pressure drop, thermal-hydraulic performance of the 13 configurations were determined, compared and analyzed. It has been observed that 45° angled and V compound configurations resulted in higher heat transfer augmentation as well as higher thermal hydraulic performance when compared with their corresponding ribs alone and dimples alone configurations.

On the measurement and analysis of data from transient heat transfer experiments

This paper describes a new method to determine the heat transfer coefficient, h, and the adiabatic-surface temperature, Tad, from transient measurements of the surface temperature of a test piece. Maximum Likelihood Estimation (MLE) is used in conjunction with Fourier’s 1D equation to determine the optimum values of h and Tad, and also their 95% confidence intervals, without having to measure the air temperature. Validation experiments are conducted in a small purpose-built wind tunnel, and a novel infra-red (IR) sensor is used to measure the surface temperature of the test piece. A mesh heater is used to generate either a step-change in the air temperature or a ‘slow-transient’ in which the air temperature – and consequently Tad – increases slowly with time. Numerical simulations, using ‘noisy data’, show that the computations give accurate estimates of h and Tad for both the step-change and slow-transient cases. The values of h and Tad determined from the measurements in the wind-tunnel are in good agreement with empirical correlations for turbulent flow over a flat plate.An advantage of the new method is that it can be used for all transient experiments, even those slow transients that violate the assumption of a semi-infinite solid, an assumption that is used in most existing analysis methods. The new method, which was applied here to boundary-layer flow with one stream of fluid, could also be applied to ‘three-temperature problems’, like film cooling, which involve two streams of fluid. The significant advantage of using the method for these problems is that bothh and Tad could be determined accurately from a single experiment.

Analysis of film cooling performance of advanced tripod hole geometries with and without manufacturing features

The present study evaluates the film cooling performance of a set of manufacturable tripod hole designs, with and without shaped exits. The tripod holes with realistic manufacturing features included rounded corners at the hole inlet and outlet, as well as a webbing between the tripod holes. Standard cylindrical and shaped cylindrical (10° fan+laidback) holes were also studied for comparative analysis. Transient heat transfer experiments with a mainstream Red≈3200 were conducted on a flat plate test rig. Different hole geometries were tested at equal mass flow rates, corresponding to a range of blowing ratios equal to 0.5, 1.0, and 2.0 for the cylindrical hole. IR (Infrared) thermography was used to evaluate adiabatic film cooling effectiveness, heat transfer coefficient, and the normalized heat flux on the flat surface. Results showed that the presence of rounded corners or webbing did not lower the performance of the tripod cooling holes. Both tripod hole geometries, with and without manufacturing features, yielded higher film cooling effectiveness compared to the cylindrical holes and slightly higher effectiveness than the shaped holes, while consuming 50% less coolant when operating at the same blowing ratio. The heat transfer coefficient measurements and the overall heat flux ratios further corroborated the thermal advantages of the tripod hole design over traditional cylindrical and shaped holes.

Full surface heat transfer characteristics of rotor ventilation duct of a turbine generator

The present work reports the investigation on the cooling performance of rotor ventilation duct of turbine generator. An investigation using experimental and analytical approaches was carried out to understand the flow and heat transfer characteristics of a single rotor ventilation duct. For the investigation, the stationary experiments are carried out for the three different conditions of (1) no cross flow (2) inlet cross flow (3) inlet and outlet cross flow followed by CFD analysis for each case. Transient heat transfer experiment using thermochromic liquid crystal (TLC) technique was carried out for measuring heat transfer characteristics. Furthermore, pressure losses are measured. A series of CFD analysis is carried out for validating modeling approach. The CFD results show good agreement with experimental data. Since the experiments were carried out on the stationary condition, the effect of rotation is examined using validated CFD model. Reynolds number is varied in the range of 13100 ~ 21000.

On the effects of coating thickness in transient heat transfer experiments using thermochromic liquid crystals

Transient heat transfer experiments typically employ thermochromic liquid crystals to temporally map surface temperatures. The desired heat transfer coefficient is then calculated from the solution of Fourier’s 1D transient heat conduction equation which is set to model the wall temperature at the solid–fluid interface. However, the experimental conditions do not always justify this assumption due to occurring layers of additional paint shielding the actual liquid crystal from the immediate exposure to the working fluid. The disregard of these additional layers with respect to their thicknesses in the evaluation process produces biased heat transfer results.In order to systematically assess the effect of coating thickness on the evaluated heat transfer, the present investigation reports on the application of three different liquid crystal types in layers in transient experiments. These were conducted for two different flow regimes using separate test facilities, i.e. a flow over a tetrahedra-shaped vortex generator and jet flows from an in-line row of orifices within a low aspect ratio impingement channel. Reynolds numbers of 100,000 and 50,000 based on hydraulic and jet orifice diameter were investigated, respectively. Upon consideration of the actual liquid crystals’ coating thicknesses from measurements, the investigations show that disregarding the layer thicknesses can lead to a significant underestimation of the resulting heat transfer, particularly for large thicknesses. By taking into account the respective coating thicknesses the experimental discrepancies could be reduced from 14% to less than 5%, accomplishing high data redundancy.

Local heat transfer characteristics of a slot nozzle array for batch drying of thin films under industrial process conditions

For optimizing the drying process of thin functional films, knowledge of the relevant influencing parameters is needed. One paramount parameter for design of experimental conditions and the accurate interpretation of experimental data is the heat transfer coefficient inside a dryer. Information on the heat transfer in a dryer also facilitates a reliable transfer of results obtained in laboratory set-ups to industrial production. In this work, the heat transfer in a batch drying set-up with an array of slot nozzles is investigated. The measurement arrangement consists of a heat flux sensor mounted on the surface of a temperature-controlled plate. For validation of this arrangement, heat transfer measurements with a single round nozzle were compared to transient heat transfer experiments using thermochromic liquid crystals. The data measured in the batch drying set-up show a periodically homogenous distribution of heat transfer coefficients for the investigated 16 nozzles. This gives the possibility to dry thin films with a coating length of up to 800 mm in the set-up at constant conditions along the coating length. A prerequisite is a periodical movement with an amplitude of at least one nozzle-to-nozzle spacing.

Heat transfer performance and transport properties of ZnO–ethylene glycol and ZnO–ethylene glycol–water nanofluid coolants

Experiments were carried out on preparation and characterization of ZnO–ethylene glycol (EG) and ZnO–ethylene glycol–water nanofluids and analysis of their performance as coolants. Favorable interactions between ZnO nanoparticles and ethylene glycol molecules ensured superior transport properties of ZnO–EG nanofluids. These interactions were utilized during formulation of ZnO–EG–water nanofluids with preservation of ethylene glycol molecules over ZnO nanoparticles’ surface rendering them with better transport properties. ZnO–EG nanofluids containing 4vol.% nanoparticles showed thermal conductivity enhancement of 33.4% and viscosity reduction of 39.2% at 27°C. Similarly, 2vol.% ZnO–EG–water nanofluids showed thermal conductivity enhancement of 17.26% and viscosity reduction of 17.34% at 27°C. Disturbance of hydrogen bonding network of ethylene glycol by ZnO nanoparticles resulted in reduced dispersion viscosity. Empirical models were developed to predict the thermal conductivity enhancement and viscosity reduction of the nanofluids apart from elucidating mechanisms for the same. Transient heat transfer experiments showed that ZnO–EG and ZnO–EG–water nanofluids had better heat absorption characteristics compared to their respective base fluids. The enhancements in heat transfer were proportional to thermal conductivity enhancements, which showed that superior thermal conductivity of nanofluids could be harnessed for cooling applications.

Coupled Zero-Dimensional/One-Dimensional Model for Hybrid Heat Transfer Measurements

This paper covers the application of an improved model to address errors associated with transient heat transfer experiments, which also include the application of lumped capacitance. Using transient thermochromic liquid crystals techniques, and applying thermochromic liquid crystals underneath lumpable features, it is possible to calculate the heat transfer using a lumped heat capacitance approach. In previous studies using the classical lumped capacitance model, the heat loss into the surface underneath the lumped features was not accounted for. In this paper, an exact, closed-form analytical solution to the enhanced lumped capacitance model is derived for discrete bodies for the case of perfect thermal contact. To validate the model and its exact solution, the transient heat conduction in a representative two-dimensional ribbed surface is simulated numerically using the finite volume method. The modeled behavior of the coupled zero-dimensional/one-dimensional model has reasonable agreement with the numerical simulation. The solution for perfect contact can also be extended for imperfect contact.

Solutions of Fourier’s equation appropriate for experiments using thermochromic liquid crystal

In transient heat-transfer experiments, the time to activate the thermochromic liquid crystal (TLC) can be used to evaluate h, the heat transfer coefficient. Most experimenters use the solution of Fourier’s equation for a semi-infinite substrate with a step-change in the temperature of the fluid to determine h. The ‘semi-infinite solution’ can also be used to determine Tad, the adiabatic surface temperature, but this is an error-prone method suitable only for experiments with relatively large values of Bi, the Biot number. For Bi > 2, which covers most practical cases, more accurate results could be achieved using a composite substrate of two materials. Using TLC to determine the temperature–time history of the surface of the composite substrate, h and Tad could be computed from the numerical solution of Fourier’s equation. Alternatively, h and Tad could be determined analytically from a combination of the semi-infinite and steady-state solutions.