Instantaneous Local Heat Transfer Research Articles

Local instantaneous heat transfer characteristics around a rising single Taylor bubble and the temperature response of relatively thick pipe wall under local constant heat flux

A novel non-intrusive thermal diffusion measurement (TDM) method is proposed to measure the flow rate of gas–liquid two-phase flows under the slug flow state in our previous research. To improve the measurement accuracy of TDM by clarifying the real-time correspondence between wall temperature change and Taylor bubble (TB) location, the local instantaneous heat transfer characteristics around a rising single TB and the corresponding temperature response of the pipe wall are further investigated in detail by using the CFD tool in this paper. During the passage of TB, the local heat transfer between the pipe wall and fluid was analyzed based on the dynamic velocity field and temperature boundary layer, and the temperature on the fluid–solid interface presents the opposite trend with the heat transfer coefficient. For the zone upstream of the heat flux center, the lag time and increase of the wall temperature become large and weak along the radial direction, respectively, whereas the lag time is independent of the axial location, TB velocity, and TB length. Meanwhile, the increase in wall temperature shows an upward trend with the TB length for short TBs but is the same for long TBs.

Local instantaneous heat flux measurements in an internal combustion engine using a MEMS sensor

• Local instantaneous heat flux was clearly measured using a resistance-type sensor. • Local instantaneous heat flux exhibited a strong cycle-to-cycle variation. • Influence of turbulent eddy on heat flux was detected experimentally. • Local heat transfer coefficient also exhibited a strong cycle-to-cycle variation. Wall heat transfer is one of the most important aspects of internal combustion engines. In this study, the local instantaneous heat transfer characteristics were investigated using an originally developed thin-film resistance-type sensor. The sensor, which was fabricated by micro-electro-mechanical systems (MEMS) technology, had advantages in the sensitivity, the spatial resolution, and the number of measurement points compared with conventional sensors. The MEMS sensor could clearly detect the cycle-by-cycle heat flux. Consequently, it was found that the local instantaneous heat flux had a strong cycle-to-cycle variability, the magnitude of which was in the order of 0.1–1 MW/m 2 , not only in fired operations but also a motored operation. The heat transfer coefficients also exhibited a strong cyclic variability with a magnitude of 0.1–1 kW/(m 2 ⋅K). These results indicated that the heat transfer characteristics could not be properly evaluated only by the conventional analysis based on the ensemble-averaged data. Additionally, it was found that the heat fluxes at the adjacent points exhibited similar but different values, indicating that there were turbulent eddies of sub-millimeter scale and they affected the local heat transfer. Since the local instantaneous heat transfer had quite different characteristics from the average, advanced design and control based on the understandings of the local instantaneous characteristics will be need for the further improvement of the engine’s thermal efficiency.

Application of a MEMS heat flux sensor to heat transfer research on an impinging diesel jet

To clarify the mechanisms of heat transfer on an engine wall is an important challenge because the heat loss on the wall is one of the dominant factors that limit the thermal efficiency. A thin-film resistance type heat flux sensor fabricated with Micro-Electro-Mechanical Systems (MEMS) technologies was applied to research on the heat transfer of an impinging diesel jet. The MEMS sensor had a high sensitivity and multiple measurement points with a scale comparable to the turbulence of in-cylinder flow for the investigation of local instantaneous heat transfer characteristics in engines. Measurements of wall temperature and heat flux were conducted in a constant volume chamber under the reference conditions of Engine Combustion Network (ENC) Spray A with variations in injection pressure. As a result, the maximum heat flux and heat transfer coefficient reached about 5 MW/m2 and 3 MW/(m2 K), respectively. The results were compared to those obtained with a commercially available thermocouple, and both sensors produced similar trends. In addition, fluid motion near the wall was estimated from the heat flux fluctuations using a cross-correlation analysis. The relationship between heat transfer and flow was investigated in a dimensionless number fashion, and it was compared to a semi-theoretical prediction that was based on a steady state laminar flow heat transfer model.

Features of heat and mechanical characteristics of pulsating flows in gas-air paths of piston engines with turbocharging

This article provides a comparative analysis of unsteady gas dynamics and instantaneous local heat transfer of pulsating flows in the intake and exhaust systems of reciprocating internal combustion engines in the case of a turbo-compressor installed without it and based on the results of experimental studies. Experimental studies were carried out on full-scale laboratory stands under the conditions of gas-dynamic nonstationarity. The article provides an original method for determining the instantaneous values of the local heat transfer coefficient in pipes, and describes the procedure for conducting experiments. It has been established that the presence of a turbo compressor in the gas-air system of a piston engine leads to significant differences in the patterns of changes in the gas-dynamic and heat exchange characteristics of pulsating flows. The obtained new data can be used to improve engineering methods for calculating the quality indicators of gas exchange processes, to refine the working processes of the engine when installing a turbocharger, as well as to develop advanced gas-air ICE systems with turbocharging.

Local instantaneous heat transfer around a single elongated bubble in inclined pipes

The instantaneous convective heat transfer rate resulting from the passage of a single elongated bubble propagating in an inclined pipe with a constant translational velocity is examined experimentally for various pipe inclinations and liquid flow rates. Local instantaneous heat transfer rates in the upper and lower parts of the pipe were determined using wall temperature measurements by an IR camera. The ensemble-averaged values of the heat transfer rate are presented as a function of the distance from either the bubble tip or bottom. Optical sensors synchronized with the IR camera were used for this purpose. Possible mechanisms that govern the enhancement of the heat transfer rate during the passage of an elongated bubble are discussed.

Measurements of Local Instantaneous Convective Heat Transfer in a Pipe - Single and Two-phase Flow.

This manuscript provides step by step description of the manufacturing process of a test section designed to measure the local instantaneous heat transfer coefficient as a function of the liquid flow rate in a transparent pipe. With certain amendments, the approach is extended to gas-liquid flows, with a particular emphasis on the effect of a single elongated (Taylor) air bubble on heat transfer enhancement. A non-invasive thermography technique is applied to measure the instantaneous temperature of a thin metal foil heated electrically. The foil is glued to cover a narrow slot cut in the pipe. The thermal inertia of the foil is small enough to detect the variation in the instantaneous foil temperature. The test section can be moved along the pipe and is long enough to cover a considerable part of the growing thermal boundary layer. At the beginning of each experimental run, a steady state with a constant water flow rate and heat flux to the foil is attained and serves as the reference. The Taylor bubble is then injected into the pipe. The heat transfer coefficient variations due to the passage of a Taylor bubble propagating in a vertical pipe is measured as function of the distance of the measuring point from the bottom of the moving Taylor bubble. Thus, the results represent the local heat transfer coefficients. Multiple independent runs preformed under identical conditions allow accumulating sufficient data to calculate reliable ensemble-averaged results on the transient convective heat transfer. In order to perform this in a frame of reference moving with the bubble, the location of the bubble along the pipe has to be known at all times. Detailed description of measurements of the length and of the translational velocity of the Taylor bubbles by optical probes is presented.

Three-point MEMS heat flux sensor for turbulent heat transfer measurement in internal combustion engines

A heat flux sensor was developed with micro-electro-mechanical systems (MEMS) technologies for investigating turbulent heat transfer characteristics in engines. The sensor has three thin-film resistance temperature detectors (RTDs) of a square 315 µm on a side on a 900 µm diameter circle in rotational symmetry. The performances of the MEMS systems sensor were tested in an open combustion chamber and a laboratory engine. In the open chamber tests, it was revealed that the MEMS sensor can measure the wall heat fluxes reflecting flow states of gas phase. In addition, the noise was evaluated as 3.8 kW/m2 with the standard deviation against the wall heat flux of a few hundred kW/m2. From these results, it was proved that the MEMS sensor has the potential to observe turbulent heat transfer on the order over 10 kW/m2 in the engine. In the laboratory engine test, the wall heat flux for continuous 200 cycles was measured with a good signal-to-noise ratio. The noise was evaluated as 13.4 kW/m2 with the standard deviation despite the noisy environment. Furthermore, it was proved that the MEMS sensor has the comparable scale with the turbulence in the engine because the three adjacent detectors measured similar but different phase oscillations in the local instantaneous heat fluxes. In addition, a heat flux vector reflecting the state of the local instantaneous heat transfer was visualized by the adjacent three-point measurement. It is expected that the three-point MEMS sensor will be a useful tool for the engine heat transfer research.

The gas-dynamic unsteadiness effects on heat transfer in the intake and exhaust systems of piston internal combustion engines

This article presents the results of experimental research into the gas-dynamic and heat-exchange parameters of gas flows in the intake and exhaust pipelines of piston ICEs (internal combustion engine) taking into account the nonstationary nature of the gas exchange processes. Some characteristic times of transient processes (recovery time and relaxation time) are established during the gas flow recovery in round pipelines. Based on the obtained data, the degree of non-stationarity or unsteadiness of the gas-dynamic processes in piston ICE pipelines can be established. It is demonstrated that there are two types of resolution for gas-dynamic unsteadiness. It is established that unsteadiness reduces the instantaneous local heat transfer intensity in the pipelines of piston ICEs by 1.3–2.5 times. A method is proposed for taking the thermomechanical unsteadiness into account in thermal calculations by applying the heat transfer mobility correction coefficient for classical equations that are used to calculate the local heat transfer coefficient.

Features of the gas dynamics and local heat transfer in intake system of piston engine with supercharging

Comparison of experimental research results of gas dynamics and instantaneous local heat transfer in the intake pipes for piston internal combustion engines (ICE) without and with supercharging are presented in the article. Studies were conducted on full-scale experimental setups in terms of gas dynamic nonstationarity, which is characteristic of piston engines. It has been established that the turbocharger installation in a gas-air system of piston internal combustion engine leads to significant differences in the patterns of change in gas-dynamic and heat transfer characteristics of flows. These data can be used in a modernization of piston engines due to installation of a turbocharger or in a development of gas-air systems for piston ICE with supercharging.

The Influence of Piston Internal Combustion Engines Intake and Exhaust Systems Configuration on Local Heat Transfer

The improvement of work processes of piston engines to boost the technical and economic parameters is the main objective in the field of power engineering. The improvement of the gas exchange processes (intake and exhaust processes) quality is one of the possible profiles to improve the efficiency and reliability of piston internal combustion engines. The study of the instantaneous local heat transfer at flow unsteadiness is one of the objectives of the experimental studies of the thermomechanical processes in the intake and exhaust systems of the engines. The influence of configuration of the intake and exhaust pipes on the heat transfer intensity is a relevant engineering problem. The local heat transfer intensity data serves as the basis to determine the basis for determination of heating of the air (at intake), cooling exhaust gas (at exhaust), calculation of thermal stresses in the nodes and details of intake and exhaust systems. The article presents the results of the experimental investigation of the instantaneous local heat transfer (taking into account the hydrodynamic nonstationarity) in the intake and exhaust pipes of different configurations for piston internal combustion engines. It is established that the cross profiling of the intake and exhaust pipes of piston engines leads to 5-20% decrease in the local heat transfer intensity depending on the crankshaft speed.

Local instantaneous heat transfer around a rising single Taylor bubble

Due to the complexity and intermittent nature of gas–liquid slug flow, the existing data on the local instantaneous heat transfer in this flow regime is quite limited. To gain a better understanding of the heat transfer mechanism in vertical slug flow, transient local heat transfer measurements were performed around a single rising Taylor bubble. A part of the pipe wall was replaced by a thin electrically heated metal foil. Infrared video camera was used to determine the temporal variation of the temperature field along the foil. The camera was synchronized with the passage of the Taylor bubble using an optical sensor. This arrangement made it possible to obtain ensemble-averaged data on the heat transfer augmentation as a function of the location relative to the Taylor bubble’s bottom for a range of operational conditions. The effect of the local hydrodynamic parameters on the heat transfer rate is discussed.

Statistical analysis of instantaneous turbulent heat transfer in circular pipe flows

Turbulent heat transfer in circular pipe flow with constant heat flux on the wall is investigated numerically via Large Eddy Simulations for frictional Reynolds number Re τ = 180 and for Prandtl numbers in the range 0.1 ≤ Pr ≤ 1.0. In our simulations we employ a second-order finite difference scheme, combined with a projection method for the pressure, on a collocated grid in cylindrical coordinates. The predicted statistical properties of the velocity and temperature fields show good agreement with available data from direct numerical simulations. Further, we study the local thermal flow structures for different Prandtl numbers. As expected, our simulations predict that by reducing the Prandtl number, the range of variations in the local heat transfer and the Nusselt number decrease. Moreover, the thermal flow structures smear in the flow and become larger in size with less sharpness, especially in the vicinity of the wall. In order to characterize the local instantaneous heat transfer, probability density functions (PDFs) for the instantaneous Nusselt number are derived for different Prandtl number. Also, it is shown that these PDFs are actually scaled by the square root of the Prandtl number, so that a single PDF can be employed for all Prandtl numbers. The curve fits of the PDFs are presented in two forms of log-normal and skewed Gaussian distributions.

Heat transfer characteristics in a pressurized fluidized bed of fine particles with immersed horizontal tube bundle

The effect of pressure on the average and local heat transfer coefficients between a submerged horizontal tube (25.4mm-O.D.) and a fluidized bed has been determined in a fluidized-bed-heat-exchanger (FBHE; 0.20×0.26×0.58m-high) of silica sand particles. The heat transfer coefficients were measured around the tube circumference by thermocouples. The average heat transfer coefficient (havg) exhibits a maximum value with variation of gas velocity (Ug) irrespective of pressure. The havg increases with increasing pressure at a given fluidizing number (Ug/Umf) due to increase of gas density and improvement of fluidizing quality with indication of low standard deviation of solid holdup fluctuation. Variation of instantaneous local heat transfer coefficient (hi) is a function of bubble behavior around the tube surface. The hi exhibits higher values at the bottom than top location of the tube, and the highest value at the side of the tube (0°) at the given bed pressure. Instantanous hi variations at the top regions (+45°, +90°) becomes much uniform with high peak frequency at higher pressures. The obtained maximum heat transfer coefficients (hmax) in terms of the maximum Nusselt numbers (Numax) have been correlated with Archimedes, Prandtl and Froude numbers.

Eulerian–Eulerian simulation of heat transfer between a gas–solid fluidized bed and an immersed tube-bank with horizontal tubes

A two dimensional Eulerian–Eulerian simulation of tube-to-bed heat transfer is carried out for a cold gas fluidized bed with immersed horizontal tubes. The horizontal tubes are modelled as obstacles with square cross section in the numerical model. Simulations are performed for two gas velocities exceeding the minimum fluidisation velocity by 0.2 and 0.6 m/s and two operating pressures of 0.1 and 1.6 MPa. Local instantaneous and time averaged heat transfer coefficients are monitored at four different positions around the tube and compared against experimental data reported in literature. The effect of constitutive equations for the solid phase thermal conductivity on heat transfer is investigated and a fundamental approach to modelling the solid phase thermal conductivity is implemented in the present work. Significant improvements in the agreement between the predicted and measured local instantaneous heat transfer coefficients are observed in the present study as compared to the previous works in which the local instantaneous heat transfer coefficients were overpredicted. The local time averaged heat transfer coefficients are within 20% of the measured values at the atmospheric pressure. In contrast, underprediction of the time averaged heat transfer coefficient is observed at the higher pressure.

Numerical modeling of laminar annular film condensation for different channel shapes

This paper presents a theoretical and numerical model to predict film condensation heat transfer in mini and micro-channels of different internal shapes. The model is based on a finite volume formulation of the Navier--Stokes and energy equations and it includes the contributions of the unsteady terms, surface tension, axial shear stresses, gravitational forces and wall conduction. Notably, interphase mass transfer and near-to-wall effects (disjoining pressure) are also included. Dimensional analysis and characteristic numbers of the process are proposed and simulation results are shown both in dimensionless and dimensional representations. Isothermal, iso-heat flux and variable heat flux external wall boundary conditions have been implemented and their effects on the distribution of the heat flux are shown and compared. The instantaneous local and perimeter-averaged heat transfer coefficients, the liquid condensate film thickness distribution, the cross sectional void fraction and the mean vapor quality can be obtained for different channel shapes. Results obtained for steady state conditions are presented for circular, elliptical (with different eccentricities), flattened (with different aspect ratios) and flower shape cross sections for R-134a and ammonia, for hydraulic diameters between 10 μm and 3 mm. A time dependent simulation with variable heat flux is presented for a copper channel having a length of 4 cm and a rectangular cross section with a hydraulic diameter of 133 μm and an aspect ratio of 2, showing the importance of axial conduction at this length scale. The model has been validated versus various benchmark cases and versus experimental data available in literature.

Estimation of instantaneous local heat transfer coefficient in spark-ignition engines

Optimizing heat transfer for internal combustion engines requires application of advanced development tools. In addition to experimental method, numerical 3D-CFD calculations are needed in order to obtain an insight into the complex phenomenas in-cylinder processes. In this context, fluid flow and heat transfer inside a four-valve engine cylinder is modeled and effects of changing engine speed on dimensionless parameters, instantaneous local Nusselt number and Reynolds number near the surface of combustion chamber are studied. Based on the numerical simulation new correlations for instantaneous local heat transfer on the combustion chamber of SI engines are derived. Results for several engine speeds are compared for total heat transfer coefficient of the cylinder engine with available correlation proposed by experimental measurements and a close agreement is observed. It is found that the local value of heat transfer coefficient varies considerably in different parts of the cylinder, but it has equivalent trend with crank angle position.

Experimental evaluation of local instantaneous heat transfer characteristics in the combustion chamber of air-cooled direct injection diesel engine

An experimental analysis is conducted investigating the differences between the variations of overall and local instantaneous heat transfer coefficients, during the engine cycle, in the combustion chamber walls of a direct injection (DI), air-cooled diesel engine located at the authors’ laboratory. For this purpose, a novel experimental installation is developed, which separates the engine transient temperature signals into two parts, namely the long- and the short-term response ones, processed in two independent data acquisition systems. Moreover, a new pre-amplification unit for fast response thermocouples, appropriate heat flux sensors and an object-oriented control code for fast data acquisition have been designed and applied. Experimentally obtained cylinder pressure diagrams are used as a basis for the calculation of the overall heat transfer coefficients, whereas one-dimensional heat conduction theory with Fourier analysis techniques, combined with an iterative procedure between calculated and measured temperature data, are implemented in order to calculate the instantaneous local heat transfer coefficients in the engine cylinder. Analysis of the experimental results reveals interesting aspects of transient engine heat transfer. Significant differences are disclosed between the overall and local heat transfer coefficient variations, with the importance of the latter one on engine design being emphasized. The local heat transfer coefficient on the cylinder head is quantified based on the experimental data. The effect of engine speed and load as well as of the air swirling motion on the heat transfer variations are presented. From the analysis results it is concluded that the instantaneous heat transfer variation is non-uniform, unlike its values calculated from standard correlations that assume spatial uniformity, noting that such information, especially for air-cooled diesel engines, seems to be very scarce in the open literature.

Generalized dynamic modeling of local heat transfer in bubble columns

Instantaneous local heat transfer rates were measured by using a hot-wire probe in three bubble columns of different diameters of 200, 400 and 800 mm. The time series of heat transfer rates were analyzed by means of rescaled range ( R/ S) and deterministic chaos analyses. Due to the influence of highly chaotic bubble motions, the instantaneous local heat transfer exhibits low-dimensional chaotic features. The dependences of Hurst exponents and Kolmogorov entropy on the column scale consistently suggest different nonlinear hydrodynamic behaviors exist in bubble columns of different scales. Based on the measurement of instantaneous heat transfer rates, an artificial neural network (ANN) was applied to correlate instantaneous local heat transfer with dynamic motions of bubble and liquid. The ANN was optimized and trained by only using the experimental data measured at one location of 200 mm column. The trained ANN model shows good performance for the generalized use to predict the dynamic heat transfer rate in three columns over whole experimental conditions studied, indicating the ANN is capable of capturing the universal relation between instantaneous heat transfer and local bubble dynamics.

Generalized dynamic modeling of local heat transfer in bubble columns

Instantaneous local heat transfer rates were measured by using a hot-wire probe in three bubble columns of different diameters of 200, 400 and 800 mm. The time series of heat transfer rates were analyzed by means of rescaled range (R/S) and deterministic chaos analyses. Due to the influence of highly chaotic bubble motions, the instantaneous local heat transfer exhibits low-dimensional chaotic features. The dependences of Hurst exponents and Kolmogorov entropy on the column scale consistently suggest different nonlinear hydrodynamic behaviors exist in bubble columns of different scales. Based on the measurement of instantaneous heat transfer rates, an artificial neural network (ANN) was applied to correlate instantaneous local heat transfer with dynamic motions of bubble and liquid. The ANN was optimized and trained by only using the experimental data measured at one location of 200 mm column. The trained ANN model shows good performance for the generalized use to predict the dynamic heat transfer rate in three columns over whole experimental conditions studied, indicating the ANN is capable of capturing the universal relation between instantaneous heat transfer and local bubble dynamics.

Scale-up effects on the time-averaged and dynamic behavior in bubble column reactors

Scale-up effects on the dynamic and time-averaged behavior are investigated in bubble columns with diameters of 200, 400 and 800 mm , respectively. Using a hot-wire probe, the local instantaneous heat transfer rates and the local time-averaged gas holdups were measured at the different locations of the columns over the gas velocity range from 20 to 90 mm/ s . The dyanamic behavior of local heat transfer was characterized by frequency domain and state-space analysis in terms of power spectrum, correlation dimension and Kolmogorov entropy. The power spectra of heat transfer rate fluctuations were found to be insensitive to the bubble column size. The state-space analysis indicates that heat transfer rate exhibits a chaotic behavior with the correlation dimension varying from 2.0 to 3.0. No significant effect of column size on the correlation dimension was observed, whereas the Kolmogorov entropies are apparently dependent on the column diameter. With the increase in column diameter, Kolmogorov entropy decreases significantly and exhibits rather flat radial distribution in the large-scale column. With respect to the time-averaged behavior of local gas holdup, the effects of column size on the radial distribution of gas holdup was examined. It was found that the 800 mm column exhibited a relatively uniform distribution of gas holdup. The dependence of both dynamic and time-averaged behavior on the column scale is considered to be closely related to the different macroscopic flow structure, which is a function of the scale of the column.