Heat Transfer Measurement Technique Research Articles

A review on thermal conductivity of unsaturated bentonite

As unsaturated bentonite is used a buffer/backfill material in the construction of engineered barriers for high-level nuclear waste disposal, understanding its thermal properties is crucial for maintaining the operational stability of the repository. This review paper synthesizes research on the thermal conductivity of unsaturated bentonite, encompassing aspects of heat transfer mechanisms, measurement techniques, influencing factors, and prediction models. The results highlight that the transient and steady-state methods have emerged as the predominant techniques for measuring the thermal conductivity of unsaturated bentonite owing to their efficiency and the prevention of water redistribution within the sample throughout the testing phase. Factors such as dry density, degree of saturation, additive content, and temperature were observed to positively influence the thermal conductivity of unsaturated bentonite, whereas an increase in the ion concentration of the pore fluid decreased the thermal conductivity. The prediction models for the thermal conductivity of unsaturated bentonite are categorized into empirical, normalized, and theoretical models. Although empirical and normalized models provide some insight, they suffer from a lack of theoretical foundation, and the parameters they incorporate often lack clear physical significance, complicating their practical application despite their capacity to represent the multifield coupling characteristics of thermal conductivity. The advancements in microscopic testing methods and computational technology herald the potential of mesoscale models and machine learning as formidable tools for predicting the thermal conductivity of unsaturated bentonite, suggesting a promising direction for future research.

Development and Application of an Internal Heat Transfer Measurement Technique for Cooled Real Engine Components

Abstract The aim of this work is to present the development and application of a measurement technique that allows to record internal heat transfer features of real components. In order to apply this method, based on similar approaches proposed in previous literature works, the component is initially heated up to a steady temperature, then a thermal transient is induced by injecting cool air in the internal cooling system. During this process, the external temperature evolution is recorded by means of an infrared (IR) camera. Experimental data are then exploited to run a numerical procedure, based on a series of transient finite element analyses of the component. In particular, the test duration is divided into an appropriate number of steps and, for each of them, the heat flux on internal surfaces is iteratively updated as to target the measured external temperature distribution. Heat flux and internal temperature data for all the time steps are eventually employed in order to evaluate the convective heat transfer coefficient via linear regression. This technique has been successfully tested on a cooled high-pressure vane of a Baker Hughes heavy-duty gas turbine, which was realized thanks to the development of a dedicated test rig at the University of Florence, Italy. The obtained results provide sufficiently detailed heat transfer distributions in addition to allowing to appreciate the effect of different coolant mass flow rates. The methodology is also capable of identifying defects, which is demonstrated by inducing controlled faults in the component.

Development of fast response heat transfer measurement technique with thin-film thermocouples

A transition boiling on a high temperature solid beyond the Leidenfrost temperature is characterized by a strong unstable heat transfer. This unstable one brings a rapid temperature change beneath the surface in the solid and causes a repetition in time as well as a space of wet and dry on the surface. Therefore, the development of a fast temperature measurement technique is indispensable to understand this transition phenomenon. The developed temperature sensor has a junction of the end face of chromel fine wire of ϕ0.32 mm diameter, whose position is 3 μm away from the surface of nickel disk of 5 mm thickness and can follow a rapid temperature change within 100 kHz. The heat transfer can be calculated from the measured temperature by using an inverse problem analysis for one-dimensional unsteady heat conduction. Besides, the calculated heat flux is also confirmed to be sufficient to capture the characteristics of unsteady boiling heat transfer in the experiment that a sessile ethanol droplet impacts onto the hot surface.

Effect of nonuniform isoflux heating on natural convection heat transfer in a prismatic modular reactor

A high temperature and pressure dual-channel facility has been designed and developed at Multiphase Reactors Engineering and Applications Laboratory (mReal) at Missouri University of Science and Technology (Missouri S&T) to investigate the thermal hydraulics of natural circulation under the loss of flow accident scenario (LOFA) in the core of a prismatic modular reactor (PMR). An advanced heat transfer measurement technique consisting of a series of heat flux foil sensors and thermocouples is adapted and implemented along the coolant flow channels (riser and downcomer channels). This study presents the effect of nonuniform isoflux heating (1.432 kW/m2 to 3.152 kW/m2, in form of stepwise-reducing and stepwise-increasing) on natural circulation heat transfer using air as a coolant at 413.7 kPa. The results showed a reversal in the direction of heat transfer and reduction in temperature fields as well as observed within the riser flow channel close to the exit (Z/L = 0.773) due to the end effect. A significant reduction of the average field temperatures along the riser channel is observed to be 24.5% and 22.4% for inner surface and air centerline temperatures, respectively for stepwise-reducing. Moreover, for stepwise-increasing, a significant reduction of the average riser channel temperature fields is observed with decreasing stepwise-increasing heating by 25.6% and 23.1% for the inner surface and air centerline temperatures, respectively. The average values of the heat transfer coefficient(h¯L) and the average Nusselt number (Nu¯L) along the riser channel were decreased with reducing the heating by 38% and 19% for stepwise-reducing heating, respectively, and by 32% and 13% for the stepwise-increasing heating, respectively.

Heat transfer measurement techniques in microchannels for single and two-phase Taylor flow

Accurate measurement of heat transfer in experiments of single, two-phase liquid-liquid and gas-liquid Taylor flows without phase change in microchannels is a challenging task. In this paper we review experimental studies of these flows, and the methods of heat transfer measurement used, in order to gain insight into the effect of some critical dimensionless groups on the accuracy of the estimates of the heat transfer. It was found that experimental measurements of Nusselt number often disagree with theoretical predictions, particularly at low Reynolds numbers. We identify the major causes of error in experimental estimation of the Nusselt number, and propose solutions to decrease the error in experimental heat transfer measurement. Measurements obtained from a new microchannel experimental setup are reported upon, to demonstrate these sources of error.

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.

Application of the Transient Heat Transfer Measurement Technique in a Low Aspect Ratio Pin Fin Cooling Channel

This study investigates on heat transfer enhancement in pin fin cooling channels. Experiments are conducted in a staggered pin fin array consisting of 15 rows. Heat transfer measurements are conducted in the pin fin cooling channel using the transient liquid crystal technique. The reference temperature is approximated by the fluid bulk temperature, acquired by thermocouples at specific positions. Thermal inertia of the used thermocouples is considered. One other problem that occurs while using relatively long thermocouples in short aspect ratio ducts is the heat conduction along the wires, the so-called stem effect. This can lead to erroneous temperature measurements. The impact of the thermocouple immersion length on the temperature measurement is investigated. A detailed assessment of the space and timewise varying temperature distribution is conducted for the appropriate reference temperature. This paper gives an overview about the experimental setup and the used transient measurement technique. Results are represented in terms of temperature distribution, heat transfer distribution, and averaged Nusselt number at the endwall.

Characterizations of aerothermal performance of novel cross-corrugated plate heat exchangers for advanced cycle aero-engines

The shape of the surface of cross-corrugated plate heat exchangers were parameterized, producing geometries with different surface profiles (sinusoidal and asymmetrical) and corrugation furrow shapes (straight and sinusoidal). A total of seven geometries were tested at laboratory scale using a Transient Liquid Crystal (TLC) heat transfer measurement technique, which produced data at high resolution (2.68×10-4m interval). Pressure drop as well as heat transfer measurements were carried out at Reynolds numbers ranging from 1300 to 13,000 with uncertainty of 5.5% and 8.5% respectively, where the range of Reynolds numbers tested were representative of the conditions seen in the aero-engine cycle. High resolution heat transfer distribution data for these novel heat exchange surfaces obtained in the current experimental campaign are presented for the first time in the literature. This allows local Nusselt number in the heat exchangers to be examined in great details, from which the flow patterns could be inferred; thus enabling further improvements to be made in the design iteration. Correlations for Nusselt number and Fanning friction factor as a function of Reynolds number were also developed; the data were further summarized as volume goodness and area goodness factors to allow sensible comparison between different heat exchanger surfaces. The results of the current study provide extremely useful reference data for future design optimization of plate heat exchanger.

Experimental investigation on the leading edge film cooling of cylindrical and laid-back holes with different radial angles

Film cooling performances of the cylindrical film holes and the laid-back film holes on the turbine blade leading edge model are investigated in this paper. Experimental measurements have been carried out to investigate the influence of the inclined angle in the spanwise direction (i.e. radial angle for a blade in the engine) on the film cooling performances of these two kinds of holes. Three rows of holes are arranged in a semi-cylinder model which is used to model the blade leading edge. Two inclined angles and three blowing ratios are tested. Transient heat transfer measurement technique with double thermochromic liquid crystals is employed in the present experiment. The results show that the trajectory of the film jets in the leading edge region deviates from the mainstream direction to the spanwise direction gradually as the blowing ratio increases. Under large blowing ratio, more area can benefit from the film protection and the film cooling effectiveness distribution is more uniform than those under small blowing ratio, while the heat transfer coefficient is also higher. The basic distribution features of heat transfer coefficient are similar for all the tested models. The heat transfer coefficient in the region where the jet core flows through is relatively lower, while the heat transfer coefficient in the jet edge region is relatively higher. Compared with the cylindrical holes, the jets from the laid-back holes have better film coverage and meanwhile make more area have relatively higher heat transfer coefficient, especially under large blowing ratio. Under the same blowing ratio, the jets from film holes with small radial angle can attach on the wall surface better and give higher film cooling effectiveness in the region close to the hole exit than the film holes with large radial angle, while they also produce relatively higher heat transfer coefficient.

Towards convective heat transfer enhancement: surface modification, characterization and measurement techniques

In this work, heat transfer surface modification and heat transfer measurement technique is developed. Heat transfer investigation was aimed to study the effect of carbon nano fibers (extremely high thermal conductive material) on the enhancement level in heat transfer. Synthesis of these carbon nano structures is achieved using thermal catalytic chemical vapor deposition process (TCCVD) on a 50 μm pure nickel (Ni270) wire. The micro wire samples covered with CNF layers were subjected to a uniform flow from a nozzle. Heat transfer measurement was achieved by a controlled heat dissipation through the micro wire to attain a constant temperature during the flow. This measurement technique is adopted from hot wire anemometry calibration method. Synthesis of carbon nano structures, heat transfer surface characterization and measurement technique are evaluated. Preliminary results indicate that an average enhancement in Nusselt Number of 17% is achieved.

Experimental investigation on the leading edge film cooling of cylindrical and laid-back holes with different hole pitches

Experimental investigation has been performed to study the film cooling performances of cylindrical holes and laid-back holes on the turbine blade leading edge. Four test models are measured for four blowing ratios to investigate the influences of film hole shape and hole pitch on the film cooling performances Film cooling effectiveness and heat transfer coefficient have been obtained using a transient heat transfer measurement technique with double thermochromic liquid crystals. As the blowing ratio increases, the trajectory of jets deviates to the spanwise direction and lifts off gradually. However, more area can benefit from the film protection under large blowing ratio, while the is also higher. The basic distribution features of heat transfer coefficients are similar for all the four models. Heat transfer coefficient in the region where the jet core flows through is relatively lower, while in the jet edge region is relatively higher. For the models with small hole pitch, the laid-back holes only give better film coverage performance than the cylindrical holes under large blowing ratio. For the models with large hole pitch, the advantage of laid-back holes in film cooling effectiveness is more obvious in the upstream region relative to the cylindrical holes. For the cylindrical hole model and the laid-back hole model with the same hole pitch, heat transfer coefficients are nearly the same with each other under the same blowing ratios. Compared with the models with large hole pitch, the laterally averaged film cooling effectiveness and heat transfer coefficient are larger for the models with small hole pitch because of larger proportion of film covering area and strong heat transfer region.

Experimental Heat Transfer Investigation of an Aggressive Intermediate Turbine Duct

In most designs of two-spool turbofan engines, intermediate turbine ducts (ITDs) are used to connect the high-pressure turbine (HPT) with the low-pressure turbine (LPT). Demands for more efficient engines with reduced emissions require more “aggressive ducts,” ducts which provide both a higher radial offset and a larger area ratio in the shortest possible length, while maintaining low pressure losses and avoiding nonuniformities in the outlet flow that might affect the performance of the downstream LPT. The work presented in this paper is part of a more comprehensive experimental and computational study of the flowfield and the heat transfer in an aggressive ITD. The main objectives of the study were to obtain an understanding of the mechanisms governing the heat transfer in ITDs and to obtain high quality experimental data for the improvement of the CFD-based design tools. This paper presents and discusses the results of the experimental study. The duct studied was a state-of-the-art “aggressive” design with nine thick nonturning structural struts. It was tested in a large-scale low-speed experimental facility with a single-stage HPT. In this paper measurements of the steady convective heat transfer coefficient (HTC) distribution on both endwalls and on the strut for the duct design inlet conditions are presented. The heat transfer measurement technique used is based on infrared thermography. Part of the results of the flow measurements is also included.

A hot-wire method based thermal conductivity measurement apparatus for teaching purposes

The implementation of an automated system based on the hot-wire technique is described for the measurement of the thermal conductivity of liquids using equipment easily available in modern physics laboratories at high schools and universities (basically a precision current source and a voltage meter, a data acquisition card, a personal computer and a high purity platinum wire). The wire, which is immersed in the investigated sample, is heated by passing a constant electrical current through it, and its temperature evolution, ΔT, is measured as a function of time, t, for several values of the current. A straightforward methodology is then used for data processing in order to obtain the liquid thermal conductivity. The start point is the well known linear relationship between ΔT and ln(t) predicted for long heating times by a model based on a solution of the heat conduction equation for an infinite lineal heat source embedded in an infinite medium into which heat is conducted without convective and radiative heat losses. A criterion is used to verify that the selected linear region is the one that matches the conditions imposed by the theoretical model. As a consequence the method involves least-squares fits in linear, semi-logarithmic (semi-log) and log–log graphs, so that it becomes attractive not only to teach about heat transfer and thermal properties measurement techniques, but also as a good exercise for students of undergraduate courses of physics and engineering learning about these kinds of mathematical functional relationships between variables. The functionality of the experiment was demonstrated by measuring the thermal conductivity in samples of liquids with well known thermal properties.

A study on the flow field and local heat transfer performance due to geometric scaling of centrifugal fans

Scaled versions of fan designs are often chosen to address thermal management issues in space constrained applications. Using velocity field and local heat transfer measurement techniques, the thermal performance characteristics of a range of geometrically scaled centrifugal fan designs have been investigated. Complex fluid flow structures and surface heat transfer trends due to centrifugal fans were found to be common over a wide range of fan aspect ratios (blade height to fan diameter). The limiting aspect ratio for heat transfer enhancement was 0.3, as larger aspect ratios were shown to result in a reduction in overall thermal performance. Over the range of fans examined, the low profile centrifugal designs produced significant enhancement in thermal performance when compared to that predicted using classical laminar flow theory. The limiting non-dimensional distance from the fan, where this enhancement is no longer apparent, has also been determined. Using the fundamental information inferred from local velocity field and heat transfer measurements, selection criteria can be determined for both low and high power practical applications where space restrictions exist.

High-resolution hot-film measurement of surface heat flux to an impinging jet

To investigate the complex coupling between surface heat transfer and local fluid velocity in convective heat transfer, advanced techniques are required to measure the surface heat flux at high spatial and temporal resolution. Several established flow velocity techniques such as laser Doppler anemometry, particle image velocimetry and hot wire anemometry can measure fluid velocities at high spatial resolution (µm) and have a high-frequency response (up to 100 kHz) characteristic. Equivalent advanced surface heat transfer measurement techniques, however, are not available; even the latest advances in high speed thermal imaging do not offer equivalent data capture rates. The current research presents a method of measuring point surface heat flux with a hot film that is flush mounted on a heated flat surface. The film works in conjunction with a constant temperature anemometer which has a bandwidth of 100 kHz. The bandwidth of this technique therefore is likely to be in excess of more established surface heat flux measurement techniques. Although the frequency response of the sensor is not reported here, it is expected to be significantly less than 100 kHz due to its physical size and capacitance. To demonstrate the efficacy of the technique, a cooling impinging air jet is directed at the heated surface, and the power required to maintain the hot-film temperature is related to the local heat flux to the fluid air flow. The technique is validated experimentally using a more established surface heat flux measurement technique. The thermal performance of the sensor is also investigated numerically. It has been shown that, with some limitations, the measurement technique accurately measures the surface heat transfer to an impinging air jet with improved spatial resolution for a wide range of experimental parameters.

Comparison of Heat Transfer Measurement Techniques on a Transonic Turbine Blade Tip

The present study considers spatially resolved surface heat transfer coefficients and adiabatic wall temperatures on a turbine blade tip in a linear cascade under transonic conditions. Five different measurement and processing techniques using infrared thermography are considered and compared. Three transient methods use the same experimental setup, using a heater mesh to provide a near-instantaneous step-change in mainstream temperature, employing an infrared camera to measure surface temperature. These three methods use the same data but different processing techniques to determine the heat transfer coefficients and adiabatic wall temperatures. Two of these methods use different processing techniques to reconstruct heat flux from the temperature time trace measured. A plot of the heat flux versus temperature is used to determine the heat transfer coefficients and adiabatic wall temperatures. The third uses the classical solution to the 1D nonsteady Fourier equation to determine heat transfer coefficients and adiabatic wall temperatures. The fourth method uses regression analysis to calculate detailed heat transfer coefficients for a quasi-steady-state condition using a thin-foil heater on the tip surface. Finally, the fifth method uses the infrared camera to measure the adiabatic wall temperature surface distribution of a blade tip after a quasi-steady-state condition is present. Overall, the present study shows that the infrared thermography technique with heat flux reconstruction using the impulse method is the most accurate, computationally efficient, and reliable method to obtain detailed, spatially resolved heat transfer coefficients and adiabatic wall temperatures on a transonic turbine blade tip in a linear cascade.

Water hammer and condensation hammer scenarios in power plants using new measurement system

This contribution presents experimental results of the investigation of water hammer and cavitational hammer at the pipeline test facility at Fraunhofer UMSICHT in the context of the EURATOM project WAHALoads. The main objective of this running project is the prediction of the strain on equipment and support structures caused by water and condensation hammer. Thus, some typical scenarios of power plants are conducted. For this purpose the test facility at Fraunhofer UMSICHT (PPP) was modified in order to simulate a piping system and associated supports that are typical for a power plant. For a better understanding of thermohydraulic processes, new wire mesh sensors with thermocouples for the measurement of condensation heat transfer and other innovative transient measurement techniques were implemented into PPP additionally.

The WAHA Code : a numerical tool for water hammer in two-phase flow – some simulations of experiments

The WAHA computer code has been developed during the WAHALoads project in the frame of the 5th European Program. The main purpose of the WAHA code is to predict various mechanisms of single- and two-phase water hammer transients in piping systems. The WAHA code can simulate thermal-hydraulic transients with one-dimensional six-equation two-fluid model approximations and calculate the hydraulic forces on the piping system due to water hammers in two-phase flow. In the first part of the present paper, the main features of the WAHA code and its capabilities are presented : basic equations, specific closures laws, numerical scheme, possible boundary conditions, some basic results of water hammers induced by a rapid valve closure. In the second part of the paper, a description of the UMSICHT test facility PPP which has been used for water hammer tests inducing bubble condensation is given. The installation has been equipped with a new measurement technology including new wire mesh sensors with thermocouples for the measurement of condensation heat transfer and other innovative transient measurement techniques. From the UMSICHT data bank,some basic data have been chosen as reference case for validating the WAHA code. The third part deals with the simulation of several test cases using the WAHA code : Edwards pipe - Discharge of a hot liquid from a horizontal pipe. PPP water hammer tests – Depressurisation inducing cavitation water hammer initiated by a fast valve closure. Super Moby Dick experiment – Critical two-phase flow through a long nozzle. The results of these simulations are systematically compared with the experimental data, when available. It is concluded that the WAHA code has the abilitity to correctly predict various mechanisms of transients flows occurring in piping systems.

Surface Flow Visualization Using Encapsulated Thermal Tufts for Steady Flow and a Dynamic Heat Transfer Measurement Technique for Unsteady Flows

Surface Flow Visualization Using Encapsulated Thermal Tufts for Steady Flow and a Dynamic Heat Transfer Measurement Technique for Unsteady Flows

Aerospace applications of luminescent paint

AbstractA heat transfer measurement technique has been developed, which utilised a laser to heat a spot of the ‘standard’ luminescent paint on an insulated metal wind tunnel model. The convective heat transfer coefficient was determined from the experimental quasi steady-state surface temperature, and solutions obtained from radial and axial conduction in a numerical heat transfer model. The convective heat transfer coefficient variation over both a flat plate and a NACA 0012 aerofoil have been measured in transonic flow. Measurements obtained from the flat plate were seen to agree well with correlation data from the literature. Measurements on the NACA 0012 aerofoil indicated the point of transition from laminar to turbulent boundary layer as well as the location of shock boundary layer interaction. The luminescent paint provided simultaneous measurements of pressure and temperature (see Part 1). The distribution of pressure over the NACA 0012 aerofoil was shown to be in excellent agreement with conventional transducer data, although the luminescent paint data provided greater spatial resolution. The position of the shock determined from the heat transfer measurements was shown to be in excellent agreement with the pressure measurements.