Miniature Thermocouples Research Articles

Experimental observations of the onset of unsteadiness for buoyant airflow along smooth and rough vertical isothermal walls

ABSTRACT The buoyant airflow along a vertical isothermal wall, under conditions close to transitional, is studied. The schlieren technique and miniature thermocouples are employed for qualitative (flow unsteadiness) and quantitative (heat transfer coefficient and local air temperature) observations. For the smooth surface, the phenomena are found to be sensitive to the enclosure configuration surrounding the plate. Roughening the heated surface with staggered rib segments of proper dimensions leads to increased heat transfer, ascribed to the onset of unsteadiness close to the segments’ edges, together with a better redistribution of the flow thanks to a favorable hydrodynamic interaction between fluid and protrusions.

Beyond the surface: Understanding temperature polarisation in DCMD membranes through varied operational parameters

Conventionally, a single Temperature Polarisation Coefficient (TPC) value is calculated to quantify Temperature Polarisation (TP). In this research, the extent of polarisation is investigated by capturing temperature profiles at specific points along a MD membrane using miniature thermocouples, eliminating the need for TPC calculations. The extent of polarisation at a point is affected by two contributory factors, namely the proximity of flow inlets and the difference in vapour pressure across the membrane at that point. Under this direction, this work examined the influence of permeate temperature, feed salinity, and flow direction on the development of the temperature profiles. Our analysis revealed an elevation in TP on both sides of the membrane when the permeate temperature was increased. In addition, changes in feed salinity had a very minute impact on the development of the temperature profiles. By comparing the cocurrent and counter-current flow, the influence of the two contributory factors was further proved, with counter-current flow working better for long membrane modules. Furthermore, an investigation on the symmetricity of polarisation across the membrane revealed asymmetricity depends on the operating conditions, especially direction of flow. The asymmetricity was infinitesimal at low inlet temperature differences for cocurrent flow.

Measurement and simulation of voltage differences under ribs and channels and temperature variations within channels in PEMEC

The internal voltage inhomogeneity in a proton exchange membrane electrolyzer cell (PEMEC) is experimentally investigated by using customized gaskets. Three miniature thermocouples are strategically placed inside the channels, specifically in the inlet region, middle region, and outlet region, to monitor temperature variations along the channels. Local voltage measurements are conducted in 13 regions corresponding to the flow channels and 12 regions corresponding to the ribs, under various operating conditions. Additionally, a three-dimensional simulation is performed to analyze the underlying causes of non-uniform voltage distribution and temperature variation. The results reveal contrasting trends in temperature changes along the flow direction, attributable to the varying magnitudes of heat loss and electrochemical heat at different current densities. At low current densities, temperatures increase from the inlet to the outlet along the channels, while at high current densities, temperatures decrease in the same direction. Furthermore, the local voltages of the flow channels (Vchan) and the ribs (Vrib) exhibit different increases along the flow direction at various current densities with the Vchan consistently smaller than Vrib. The inhomogeneity of Vchan is found to be greater than that of Vrib due to bubble disturbances in the channels. The proposed measurement principle for local voltage distribution in PEMECs provides insights into the performance discrepancies between the channel and rib scales in the flow field, thereby enhancing our understanding of electrical property inhomogeneity within the cell.

Experimental investigation of temperature polarisation by capturing the temperature profile development over DCMD membranes

Temperature polarisation (TP) is a major drawback limiting the global acceptance of membrane distillation (MD) technology. TP is typically quantified using a dimensionless index known as Temperature Polarisation Coefficient (TPC). TPC has significant limitations, whereby it cannot be used to compare different MD configurations or design conditions, nor to analyse the TP phenomenon along the membrane. In this research, the temperature profile over and along a lengthy DCMD membrane has been measured under various operational conditions, where its impact on TP has been explored for the first time. A specialised DCMD membrane cell was manufactured to capture temperature profiles, both along and over the membrane surfaces, using miniature thermocouples. The effects of flow rate and feed temperature were investigated on the temperature profiles. The results showed that the extent of TP was not constant along the membrane, and that the temperature profile was not symmetrical across the feed and permeate side, predominantly due to the effects of the inlet and outlet on the flow. The TPC value calculated using the conventional method was not able to accurately reflect the TP phenomenon along the membrane, indicating TPC to be an ineffective tool to study TP along the membrane.

Inverse determination of sliding surface temperature based on measurements by thermocouples with account of their thermal inertia

This study developed an inverse heat conduction algorithm to determine temperature at a sliding surface taking account of thermocouple thermal inertia. The direct heat conduction problem was solved analytically based on the Laplace integral transform approach. The inverse algorithm was applied to the problem of friction of a brake material against a steel. The experiments were conducted on a pin-on-disc tribometer for three short-time sliding regimes: velocity step, acceleration and deceleration. Temperature in the pin sample was measured by two identical miniature thermocouples installed at different distances from the friction surface. It was found that the two inverse surface temperatures agree well between each other. The inverse algorithm allows predicting the contact temperature measured by infrared thermography with accuracy 5–7%.

Influence of hydrodynamic interactions among multiple bubbles on convective heat transfer in nucleate boiling

To clear the effects of the hydrodynamic interactions among neighboring vapor bubbles on the saturated pool boiling heat transfer, artificial nucleation sites with several cavity arrangements were manufactured on the substrate surface and measured liquid temperatures near heated substrate by a miniature thermocouple probe with 10 kHz. The surface temperature of heated substrate was measured by an infrared camera with 1 kHz and bubble dynamics were simultaneously visualized by a high-speed video camera with 5 kHz and 10 kHz, respectively. The liquid temperature fluctuations, measured with micro thermocouple 100 μm and 10 μm above the substrate surface, were compared among each other and with the transient temperature fields obtained through IR measurements. Results showed that the hydrodynamic interaction among neighboring bubbles causes the liquid convection, that was only detected 100 µm above the boiling surface. Generally, the degree of liquid temperature fluctuations at 10 μm height and different arrangements of nucleation sites was under 1 K. Here the superheated sublayer in the vicinity of the boiling surface was generally not affected by the convection. Only when the coalescence from two neighboring nucleation sites occurred and the liquid-vapor interface nearly reached the substrate, a steep temperature drop over 2 K was detected. It was concluded that the viscous sublayer stably exists during boiling of water in lower heat flux conditions. To enhance the convective heat transfer in nucleate boiling, coalesced bubble needs to peel off the viscous sublayer during its deformation. Based on the measurements in this study, this peel-off effect only happens when the coalescence point is less than 1 mm above the heated surface.

Конвекция магнитной жидкости в замкнутом гидродинамическом контуре

Thermal convection of ferrofluid in a closed side-heated hydrodynamic loop is investigated experimentally when a nonuniform magnetic field is applied to the tube section near the electric heater. The nonuniform magnetic field with a strength of up to 24 kA/m is created by a permanent magnet of the “neodymium-iron-boron” type, equipped with ferrite pole pieces. For temperature measurements, miniature copper-constantan thermocouples are used. The temperature distribution along the circuit and temperature differences on both sides of the heater is measured. The tubes of the loop are cooled by a stream of thermostatic air which ensures a constant heat transfer coefficient at the outer surface of the tubes and an exponential temperature distribution along the circuit. The exponent determined in the experiments is used to provide information about the integral axial heat flux (Nusselt number). The experiments were performed with undecane in the ordinary gravitational convection regime and with medium concentrated magnetic fluid in the combined (gravitational and thermomagnetic) convection regime in the range of Rayleigh numbers 103–104. The estimation of the characteristic magnetic Rayleigh numbers was carried out taking into account the demagnetizing fields. For all modes, the dependence of the Nusselt number normalized to the heat transfer coefficient on the thermal Rayleigh number is plotted. It is shown that thermomagnetic convection increases the intensity of heat exchange by 4–6 times.

Optimization Algorithm and Joint Simulation to Micro Thermal Deformation Using Temperature Measurement in the Orifice of Hydraulic Valve

When exposed to viscous heating, hydraulic valve orifices experience thermal deformation, which causes spool clamping and actuator disorder. Quantitative research on thermal deformation can help reveal the micro-mechanism of spool clamping. In this study, miniature thermocouples are embedded into a valve orifice with an opening size of 1 mm to measure temperature distribution. An optimization algorithm based on measurement data (M-OA) for the thermal deformation of the valve orifice is proposed. The temperature and thermal deformation of the valve orifice are calculated through Fluent and Workbench joint simulation, with the measurement data serving as boundary conditions. Results show that, for a valve orifice with a valve wall length of 18 mm, when the temperature of the sharp edge is at 60 °C, thermal deformation measures 7.7 μm via observation and 7.62803 μm via M-OA, indicating that the M-OA method is reliable. The results of the joint simulation can be accepted because measurements of temperature reached an accuracy rate of 95%, and that of deformation reached 82.7%. A large drop in pressure led to a rapid increase in temperature, causing serious thermal deformation of the valve orifice. With an inlet pressure of 3 MPa, the temperature of the sharp edge reached 72.9 °C within 110 min, and radial thermal deformation can reach 8.3 μm. Such deformation poses great risk of spool clamping for a spool valve of Φ36 mm.

Development of Nanoscale Thermocouple Probes for Local Thermal Measurements

We developed thermocouple probes consisting of constantan and chromel segments with nanoscale sharpness. The ends of wires made of these alloys were polished utilizing a single step drop-off electrochemical procedure in a H3PO4 solution. The macroscopic shapes of the etched wires were designed to detect ultra-small heat inputs with high sensitivity after assembling a thermocouple. After electrochemical treatment, ultimately smooth, mirror-polished wire surfaces were observed. The etched wires were then assembled in a transmission electron microscope (TEM) to create a miniature thermocouple by using which we successfully detected a small temperature increase induced by focused TEM electron beam irradiation. Judging from high- resolution TEM imaging, a thermocouple with the tip-end of only ~5 nm in diameter was fabricated.

The gel fuel ignition at local conductive heating

An experimental study of ignition of a gel fuel at local conductive heating has been performed. The initial temperatures of the fuel samples were 120, 260, and 290 K. Metal particles in the shape of a sphere, a disk, and a parallelepiped with a characteristic size of 10 mm were used as ignition sources. The initial temperature of the heating sources varied in the range of 1050–1350 K. The use of software and hardware high-speed video recording (4200 frames per second at a maximum resolution of 1280 × 800 pixels) allowed analyzing the regularities of physical and chemical processes, occurring during the induction period. Limit conditions necessary for stable ignition of a gel fuel and dependences of ignition delay time on parameters of a local heating source have been established. A low-inertia (sampling rate of 100 Hz) measuring system based on miniature thermocouples and an oscilloscope served to establish the velocities of the gel fuel melting front motion from the contact boundary between the metal particle and the fuel in the direction of the thermal wave propagation during induction period and under stationary combustion. The mathematical model of ignition of the gel fuel has been developed within the framework of the mathematical apparatus of continuum mechanics and the theory of chemical kinetics. The results of numerical simulation are in good agreement with the experimental data obtained. The applicability limits of the developed model within which it reliably describes characteristics of ignition have been established. The mathematical model may be used to develop advanced energy-efficient technologies for ignition of new fuels in wide ranges of parameters of the system gel fuel/local heating source.

Phase detection aided thermometry (PDaT) for two-phase flow

Since temperature and void (or phase) fraction are important parameters to characterize and grasp multiphase flow behaviors, various methods to measure these quantities have been developed and applied to. However, these multi-phase flow parameters have not been simultaneously measured at the same time and position until now because they need the individual sensors, such as an optical fiber void sensor, a conductance probe and a sheath thermocouple (TC), and so on. This paper describes the development of a new thermometry to detect the phase for two-phase flow and simultaneously measure the liquid/gas temperature with a miniature TC with high temporal-spatial resolutions; hereafter, this method calls a Phase Detection aided Thermometry (PDaT). The principle of PDaT is that a miniature TC with 10 kHz of the sampling rate is used not only as a thermometer with the high temporal-spatial resolution, but also as an electrical conductance probe for phase detection. In statistically, PDaT can acquire the void fraction, the average temperature and that phase at the same time and position. In addition, temperature of the interface between liquid and gas phases can be locally and instantaneously measured, besides the bubble cord length measurement as same as the ordinary void sensor in case of the bubbly flow. To demonstrate the applicability of PDaT, a proof of principle experiment was conducted with liquid water and a static air bubble, simultaneously visualized by a high speed camera.

Application of the planar laser-induced fluorescence method to determine the temperature field ofwater droplets under intensive heating

Presented are results of experimental investigations concerned with formation of a nonstationary and essentially nonuniform temperature field of a water droplet (initial radius of 1 mm to 2 mm) under intensive heating in a flow of heated air (from 50◦C to 1000◦C). The method used for this purpose was a noncontact optical planar laser-induced fluorescence (PLIF) method. It is shown that temperature distribution in a water droplet is essentially inhomogeneous even under prolonged heating (to several tens of seconds). Reliability of the results of measurements by the noncontact PLIF method was analyzed by applying a group of fast miniature thermocouples. Restrictions of using the PLIF method for studying temperatures fields of evaporating droplets under high-temperature heating (over 800◦C) were marked out. Characteristic times of droplet existence (complete evaporation) were determined. It was analyzed how the temperature difference in a water droplet affects this parameter during heating and intensive phase transitions. It was substantiated that it is expedient to consider essentially inhomogeneous and nonstationary temperature field of a water droplet inmathematical modeling of the heat andmass transfer processes in high-temperature gas–vapor-droplet systems (corresponding, e.g., to burning or heat cleaning of liquids, firefighting, production of composite and gaseous fuels, their combustion, etc.).

Effect of external lighting on biopotential of maize leaves caused by pulsed temperature stimulation

Study of electrophysiological indicators of the condition and behavior of plants has become more important in the development of farming activities and the search for effective ways to improve the productivity of crops. The influence of external light on the adaptive ability of corn leaf cells to rhythmic cold stimulation was determined experimentally. The method of rhythmic cold stimulation is not adequate for the studied plants, but its application allows us to evaluate the stability of plant cells to external stimuli. The method consists in repeating irritation during the time period of less duration than the relative refractory phase, which causes a response of less than the previous amplitude. Because of this in the system there is a negative feedback that leads to stabilization of the amplitude of biopotentials that are registered. Rhythmic cold stimulation was applied to the leaf with the help of a quick-response thermostimulator. Rhythmic cold stimuli and settings of pulses were set by computer software. Cooling temperature was controlled using miniature differential thermocouple. Potentials of the leaf surface were diverted by an unpolarized macroelectrode and after a preamplifier fed to the input of the USB oscilloscope connected to the computer. Analysis of the results of experiments was performed using automated developed software. As a result, we experimentally established that rhythmic stimulation of leaves by cold leads to stabilization of responding potential. The level of stabilization depends on the frequency of cold stimuli and describes the adaptive properties of the system causing the biopotential. We found that the absence of photosynthesis when there is a deficit outdoor lighting leads to a significant increase in the average level of stabilized responses, indicating increased stability of the system to external influences. The maximum of this increase fell on the fourth day. This increase is likely to be due to the restructuring of functional ion transport through cell membranes, generating potentials registered. In the interval 4–9th days there was a significant decrease in stabilization, probably due to adaptation of plant cells to a lack of light, or depletion of ATP, which provides the active transport of ions across the cell membrane.

An Investigation of Coolant Within Serpentine Passages of a High-Pressure Axial Gas Turbine Blade

Cooling flow behavior is investigated within the multiple serpentine passages with turbulators on the leading and trailing walls of an axial gas turbine blade operating at design-corrected conditions with accurate external flow conditions. Pressure and temperature measurements at midspan within the passages are obtained using miniature butt-welded thermocouples and miniature Kulite pressure transducers. These measurements, as well as airfoil surface pressure field data from a full computational fluid dynamics (CFD) simulation, are used as boundary conditions for a model that provides quantitative values of film-cooling blowing ratio for each film-cooling hole on the blade. The model accounts for the continuously changing cross-sectional area and shape of the channels, frictional pressure loss, convective heat transfer from the solid portion of the blade, massflow reduction as coolant bleeds out through film-cooling or impingement holes, compressibility effects, and the effects of blade rotation. The results of the model provide detailed coolant ejection information for a film-cooled rotating turbine airfoil operating at design-corrected conditions and also account for the highly variable freestream conditions on the airfoil. While these values are commonly known for simpler experimental geometries, they have previously either been unknown or estimated crudely for full-stage experiments of this nature. The better-quantified cooling parameters provide a bridge for better comparison with the wealth of film-cooling work already reported for simplified geometries. The calculation also shows the significant range in blowing ratio that can arise even among a single row of cooling holes associated with one of the turbulated passages, due to significant changes in both coolant and local freestream massfluxes.

Multilayer thick-film thermoelectric microgenerator based on LTCC technology

Purpose The purpose of this paper was to develop the methodology of thick-film/low temperature co-fired ceramic (LTCC) multilayer thermoelectric microgenerator fabrication including the procedure of silver-nickel thermocouples integration with LTCC. Design/methodology/approach To miniaturize the structures and to increase the output parameters (generated voltage, electrical power), the microgenerator was designed as multilayer systems. It allows to reduce size of the system and to increase the number of thermocouples integrated inside the structure. It also protects buried thermocouples against exposure to harmful external factors (e.g. moisture, oxidation and mechanical exposures). As a substrate, LTCC was used. For the thermocouples fabrication, thick-film pastes based on silver and nickel were chosen. Ag/Ni thermocouple has nearly three times higher Seebeck coefficient and 30 per cent lower electrical resistance than the combination of Ag/PdAg used in previous works of the author. Findings A multi-layer thick-film thermoelectric generator based on LTCC and Ag, Ni pastes was fabricated. Thirty Ag/Ni thermocouples were precisely screen-printed on few layers. Thermocouples’ arms are 15 mm long and about 150 μm wide. Interlayer connections (via-holes filled with conductive paste) provided the electrical contact between the layers. The biggest fabricated harvester consisted of 90 miniature thermocouples buried inside the LTCC. Originality/value The paper presents the results of research that provided to optimize the co-firing process of the LTCC/Ni set. In the result, the methodology of co-firing of silver-nickel thermocouples and LTCC ceramic was elaborated. Also, the methodology of fabrication of miniature thermoelectric energy harvesters was optimized.

Application of the surface renewal technique in two types of screenhouses: Sensible heat flux estimates and turbulence characteristics

While the usefulness of screenhouses in crop improvement and water savings is rarely disputed in semi-arid regions, reliable evapotranspiration (ET) estimates in screenhouses remain a challenge. The surface renewal (SR) technique utilizes high frequency readings of air temperature near or above the vegetation by low-cost fine-wire thermocouples, to indirectly estimate the sensible heat flux from which the latent heat flux can be estimated assuming energy conservation. The SR technique had been successfully used in open fields, and this research investigates its applicability in screenhouses. Experiments were carried out in two field campaigns, in banana and pepper screenhouses covered with shading and insect-proof screens, respectively. The SR technique was employed by installing several miniature thermocouples at different heights. Reference measurements of sensible heat flux were conducted using the eddy covariance (EC) technique. A calibration coefficient, α, was calculated using linear regressions between SR and EC outputs. In agreement with observations in open fields, in both experiments, α was found to decrease with height, and values were smaller for stable than unstable conditions. In a separate validation period, R2 reached 0.93 and 0.65 for measurements above the shading and insect-proof screen, respectively. For both the shading and the insect-proof screens, best coefficients of determination (R2) of these regressions were obtained just above the screen, at 1.02 and 1.12 of the screen height, respectively. Turbulence characteristics were observed to be affected by the porosity of the screen, which is conjectured to be the main cause for reduced performance of the SR technique in the insect-proof than the shading screenhouse. In addition, it was concluded that in screenhouse conditions sampling frequency can be relaxed to as low as 2Hz with little effect on performance. The specifications provided here allow for a low cost deployment of the SR system in shading screenhouses. Such systems can be available for day-to-day use by farmers to improve irrigation management.

Optical fiber sensors-based temperature distribution measurement in ex vivo radiofrequency ablation with submillimeter resolution.

Radiofrequency thermal ablation (RFTA) induces a high-temperature field in a biological tissue having steep spatial (up to 6°C∕mm) and temporal (up to 1°C∕s) gradients. Applied in cancer care, RFTA produces a localized heating, cytotoxic for tumor cells, and is able to treat tumors with sizes up to 3 to 5 cm in diameter. The online measurement of temperature distribution at the RFTA point of care has been previously carried out with miniature thermocouples and optical fiber sensors, which exhibit problems of size, alteration of RFTA pattern, hysteresis, and sensor density worse than 1 sensor∕cm. In this work, we apply a distributed temperature sensor (DTS) with a submillimeter spatial resolution for the monitoring of RFTA in porcine liver tissue. The DTS demodulates the chaotic Rayleigh backscattering pattern with an interferometric setup to obtain the real-time temperature distribution. A measurement chamber has been set up with the fiber crossing the tissue along different diameters. Several experiments have been carried out measuring the space-time evolution of temperature during RFTA. The present work showcases the temperature monitoring in RFTA with an unprecedented spatial resolution and is exportable to in vivo measurement; the acquired data can be particularly useful for the validation of RFTA computational models.

Comparative thermal conductivity measurement of chemical vapor deposition grown graphene supported on substrate

We develop a comparative thermal conductivity measurement technique based on miniature differential thermocouples and measure the temperature dependence of the thermal conductivity of chemical vapor deposition grown graphene supported on glass or SiO2 substrate in the temperature range of 180–420 K. For all 28 investigated samples, the obtained room temperature values of the thermal conductivity are bounded between 1800 and 2200 W/m K. Compared with other techniques, we find that a significant improvement in accuracy of our measurements is because of the absence of chemical doping/contamination from micro-fabrication procedures. Our results are confirmed using scanning thermal microscopy.

Time-Averaged and Time-Accurate Aerodynamic Effects of Forward Rotor Cavity Purge Flow for a High-Pressure Turbine—Part I: Analytical and Experimental Comparisons

The effect of rotor purge flow on the unsteady aerodynamics of a high-pressure turbine stage operating at design corrected conditions has been investigated, both experimentally and computationally. The experimental configuration consisted of a single-stage high-pressure turbine with a modern film-cooling configuration on the vane airfoil and the inner and outer end wall surfaces. Purge flow was introduced into the cavity located between the high-pressure vane and the high-pressure disk. The high-pressure blades and the downstream low-pressure turbine nozzle row were not cooled. All of the hardware featured an aerodynamic design typical of a commercial high-pressure ratio turbine and the flow path geometry was representative of the actual engine hardware. In addition to instrumentation in the main flow path, the stationary and rotating seals of the purge flow cavity were instrumented with high frequency response flush-mounted pressure transducers and miniature thermocouples in order to measure the flow field parameters above and below the angel wing. Predictions of the time-dependent flow field in the turbine flow path were obtained using FINE/Turbo, a three-dimensional Reynolds-averaged Navier–Stokes computational fluid dynamics CFD code that had the capability to perform both a steady and unsteady analysis. The steady and unsteady flow fields throughout the turbine were predicted using a three blade-row computational model that incorporated the purge flow cavity between the high-pressure vane and disk. The predictions were performed in an effort to mimic the design process with no adjustment of boundary conditions to better match the experimental data. The time-accurate predictions were generated using the harmonic method. Part I of this paper concentrates on the comparison of the time-averaged and time-accurate predictions with measurements in and around the purge flow cavity. The degree of agreement between the measured and predicted parameters is described in detail, providing confidence in the predictions for the flow field analysis that will be provided in Part II.

Microfabrication of low thermal mass heated nebulizer chips for mass spectrometry

A low thermal mass Si–glass heated nebulizer chip for mass spectrometry is presented. The chips are used to mix a liquid sample (about 10 µL min−1) with a nebulizer gas (N2 at about 100 mL min−1) and to vaporize the mixture. They are suited to work with multiple atmospheric pressure ionization methods. Miniaturization of heated nebulizer chips increases sensitivity, performance and flexibility for mass spectrometry. The microsystem includes a sample channel with an in-plane converging exit nozzle and a resistive copper heater, located on the channel roof, enabling operation temperatures above 450 °C; needed for the complete vaporization of the sample. Excessive Si is etched away for the reduction of heat losses due to conduction and improvement of the heater efficiency. The chip is designed to ensure low temperatures at the side of fluidic inlet, allowing an easy connection of a nebulizer gas with a polymeric sealant. The jets are characterized by scanning with a miniature thermocouple perpendicularly to the stream direction. Jet temperatures and shape can be evaluated with the acquired cross-sectional two-dimensional temperature maps. Jets are found to stream without spreading, creating small spot sizes depending on nozzle dimensions.