Internal Convective Heat Transfer Coefficient Research Articles

Wind-tunnel experiments on cross-ventilative cooling in a generic isolated building with one heated wall: Impact of opening size

This paper presents wind-tunnel experiments of cross-ventilative cooling in a generic isolated building with an interior heated side wall. Two different sizes of openings are considered: large and small openings. Particle image velocimetry (PIV) is used to determine velocities in the vertical centerplane. Air temperatures in the vertical centerplane are measured using negative temperature coefficient (NTC) sensors. Surface temperatures on the heated wall are measured using an infrared camera. Surface heat fluxes are obtained using heat flux sensors. In both cases the indoor airflow is dominated by the jet through the openings, with higher velocities in the building with large openings. The air temperatures measured with small openings are up to 7.5 % larger than those with large openings. The surface heat fluxes are up to 20 % higher in the building with large openings. The interior convective heat transfer coefficients vary considerably across the heated wall for both opening sizes and can be very different (up to 5 times higher) from those obtained by existing internal convective heat transfer coefficient correlations. The measurement results give insight into the complexity of ventilative cooling and can be used to validate computational fluid dynamics (CFD) simulations of cross-ventilative cooling.

Structure optimization and operation characteristics of metal gas storage device based on compressed air energy storage system

Compressed air energy storage (CAES) is a key technology for promoting the replacement of fossil fuels with renewable energy. Currently, CAES systems typically require an underground salt cavern, which is limited by geological and geographical conditions. To promote the rapid popularization of CAES, there is an urgent need to develop new gas storage devices with flexible layouts. Metal pressure vessel as a gas storage device is concerned with the benefits of high storage pressure and reliable operation. Considering both strength and fatigue, this study conducted a structural optimization of Tank-D2000 and Pipe-D400 to achieve the design operating life with the minimal quality. Furthermore, the optimized structure was used to study the operating characteristics of the gas storage device by the multi-domain fluid–solid thermal coupling method and the real gas model. The results indicated that the internal convective heat transfer coefficient of the gas storage device was a linear function of time during operation, whereas the external natural convective heat transfer coefficient almost remained constant. The pressure distribution was uniform; however, the temperature difference was large, particularly for D400. Analysis and prediction based on the theory of the polytropic process showed that the polytropic index of D400 was lower than that of D2000, so the heat transfer of D400 was more adequate and the effective capacity of gas storage device under the designed operating pressure range was larger. With an increase in flow, polytropic index increased and was lower at energy storage than at energy release. The pressure prediction error was basically within the range of ±4 %, and the temperature prediction error was within the range of ±3 %.

Development and experimental validation of a CFD model for the thermal behaviour assessment of Complex Fenestration Systems

Complex Fenestration Systems with integrated shading systems are becoming widely used to enhance the building's envelope performance and increase the occupants' thermal and visual comfort. Their peculiarity of integrating a complex shading system makes it difficult to assess their thermal and optical performance. The complexity of the shading system itself, in addition to the interaction with the fluid flow in the window cavity, calls for the use of adequate modelling and simulation techniques. In this framework, a novel modelling approach for the thermal characterization of Complex Fenestration Systems with integrated venetian blinds at different tilt angles has been investigated and validated against onsite measurements under dynamic conditions. The numerical assessment includes a preliminary optical calculation to determine the fraction of solar radiation absorbed by the glazing system and a subsequent Computational Fluid Dynamic simulation to which the absorbed solar radiation is assigned. On the other side, the experimental characterization is based on heat flux, solar irradiance, and temperature measurements. After the model calibration - through the selection of the most appropriate models for the internal and external convective heat transfer coefficients, the numerical simulation showed a good correspondence with experimental data for all analyzed blind tilt angles with a Coefficient of Variation of the Root Mean Square Error in the open blind configuration (18° from horizontal) of 22.75%, 7.27% and 4.74% and in the intermediate blind configuration (37° from horizontal) of 21.14%, 6.02% and 2.86%, respectively for heat flux, external and internal surfaces temperatures.

Thermal Performance and Heat Dynamics Energy and Exergy of Integrated Asphalt Collector Storage: Sources of Thermal Energy, and Thermoelectric Energy

Having kilometers of asphalt road, yet with this heat going to waste, an attempt has been made in this research to extract the road's renewable energy heat. The purpose of the experiment is to compare the energy and exergy efficiency of various materials of asphalt solar water heaters (ASWH), as well as heat transmission through the water tube and how friction affects exergy destruction. The water flow rate of one ASWH was 0.01 kg/s, while that of the other was 0.02 kg/s. Each ASWH has an area of 0.5 square meters. The copper tube is buried 10 mm deep in the asphalt. 15 degrees is the angle of inclination. The results indicate that the energy and exergy efficiencies are reasonably high for the water flow rate of 0.02 kg/s. Depending on the water flow rate, asphalt temperature, and sunlight intensity, the energy and exergy efficiencies changed from 32% to 65% and 5.8% to 16%, respectively. The water flow rate is an essential parameter for estimating the internal convective heat transfer coefficient and Reynolds number in order to calculate the friction factor in the copper tube based on internal convection heat transfer. In contrast, the friction factor is a consequence of the pressure loss and exergy degradation induced by friction.

The performance evaluation for thermal protection of turbine vane with film cooling and thermal barrier coating

• An expression of temperature-independent overall cooling effectiveness is obtained. • The external convective heat transfer is effected by the internal cooling flow. • Increasing convective heat transfer can increase overall cooling effectiveness. • Four parameters need to be similar when conducting heat transfer experiments. When conducting wind tunnel heat transfer test to evaluate the cooling performance of the turbine, it is difficult and costly to simulate the extremely high temperature gas of the real working conditions, and small temperature difference and room temperature test is usually used instead. In order to evaluate the cooling performance under real working conditions from the experimental results at room temperature, the corresponding evaluation method needs to be studied. In this paper, a turbine vane with film cooling and thermal barrier coating (TBC) is used as the study object. By introducing an improved driving temperature and adopting one-dimensional heat transfer analysis, an improved analytical expression of overall cooling effectiveness, which is independent of temperature, is derived and analyzed. Through numerical calculation, the overall cooling effectiveness defined by different reference temperatures are compared. The results show that the heat transfer of the turbine vane with film cooling and TBC is complicated. The influence of the internal cooling should be considered when calculating the external convective heat transfer. The temperature-independent overall cooling effectiveness on the TBC surface and the superalloy surface are obtained, respectively. The improved overall cooling effectiveness is more sensitive to the thermal resistance of the superalloy compared with the thermal resistance of the TBC. The overall cooling effectiveness can be improved by increasing the internal or external convective heat transfer coefficient. In order to obtain an accurate overall cooling effectiveness (dimensionless wall temperatures), it is necessary to satisfy similarities of four dimensionless parameters, including the Biot numbers ( B i TBC , B i w ), the ratio of external and internal convective heat transfer coefficients ( h e / h i ) and the ratio of gained convective heat transfer coefficient and external convective heat transfer coefficient ( h e , i / h e ).

Experimental and Theoretical Study on the Internal Convective and Radiative Heat Transfer Coefficients for a Vertical Wall in a Residential Building

Experimental studies on internal convective (CHTC) and radiative (RHTC) heat transfer coefficients are very rarely conducted in real conditions during the normal use of buildings. This study presents the results of measurements of CHTC and RHTC for a vertical wall, taken in a selected room of a single-family building during its everyday use. Measurements were performed using HFP01 heat flux plates, Pt1000 sensors for internal air and wall surface temperatures and a globe thermometer for mean radiant temperature measured in 10 min intervals. Measured average CHTC and RHTC amounted to 1.15 W/m2K and 5.45 W/m2K, compared to the 2.50 W/m2K and 5.42 W/m2K recommended by the EN ISO 6946, respectively. To compare with calculated CHTC, 14 correlations based on the temperature difference were applied. Obtained values were from 1.31 W/m2K (given by Min et al.) to 3.33 W/m2K (Wilkes and Peterson), and in all cases were greater than the 1.15 W/m2K from measurements. The average value from all models amounted to 2.02 W/m2K, and was greater than measurements by 75.6%. The quality of models was also estimated using average absolute error (AAE), average biased error (ABE), mean absolute error (MAE) and mean bias error (MBE). Based on these techniques, the model of Fohanno and Polidori was identified as the best with AAE = 68%, ABE = 52%, MAE = 0.41 W/m2K and MBE = 0.12 W/m2K.

Accurate numerical modeling of convective heat transfer coefficient for a high power PLA-core toroidal electromagnetic coupler subject to cooling

The toroidal electromagnetic coupler has many advantages over the classical transformer. It is widely used in low-power applications. However, there are some limits that hamper its use in modern and high-power applications, such as the cost, the difficulty of winding and, especially, the limited number of studies carried out on the thermal aspect. In this article, we look at the problem of cooling a high-power and very compact toroidal electromagnetic coupler used in smart grid systems. The cooling is modeled thanks to heat transfer coefficients considering the windings as a surface heat source. A mathematical model which is quick and useful for a first approach is developed, and the results obtained show the temperature distribution for convective boundary conditions inside and outside the hollow torus. In addition, a parametric study is carried out. It aims to determine the optimal internal and external convective heat transfer coefficients for which the temperature of the coupler remains below the material’s destruction threshold. Additionally, a 3D numerical model taking into account the wires of the toroidal coupler is studied and compared with the mathematical model. The good consistency between the two methods ensures that this simplified thermal modeling can be trusted for the study of thermal management during the design of future electromagnetic couplers.

Preliminary analysis of the influence of environmental boundary conditions on convective heat transfer coefficients

In this contribution, a hot box apparatus was used to investigate convective heat transfers through an experimental setup composed of a heat-flow sensor, air and surface temperature probes installed on the wall tested in the hot box and a hot-wire anemometer. The air near the wall was indirectly moved by means of a forced convection system through which the air velocity can be modified to simulate different environmental conditions. The hot box apparatus was used in order to investigate the influence of the air velocity on internal convective heat transfer coefficients (hc), under natural/mixed convection conditions. The Standard ISO 6946 suggests a typical internal convective heat transfer coefficient equal to 2.5 W/m2K when horizontal heat fluxes occur. No detailed information about this value are reported in the Standard. So, the final aim of this study is to assess the applicability of the internal convective heat transfer coefficient value proposed by ISO 6946 under different air velocity values, quantifying how hc varies as air velocity changes.

Simultaneous estimation of fluid temperature and convective heat transfer coefficient by sequential function specification method

In pressurized water reactor (PWR) with multiple loops, abnormal working conditions occur when coolant pumps do not work in some loops. In these closed-loop pipelines, thermal stratification is formed under the effect of natural convection cooling and this poses a threat to the safe operation of the reactor. In this paper, the sequential function specification method (SFSM) is implemented to simultaneously predict the spatially and temporally varying internal fluid temperature and convective heat transfer coefficient of two-dimensional pipe under thermal stratification. In the direct problem, the finite volume method (FVM) is adopted to solve the governing equations subjected to initial and boundary conditions. In the inverse problem, the least-square method is employed to obtain the heat flux of the inner wall, and then Newton's law of cooling and the natural-convection correlation are adopted to obtain the fluid temperature and convective heat transfer coefficient. Numerical experiments of natural convection cooling are carried out under different outer wall boundary conditions to demonstrate the effectiveness of the proposed inversion method. The results of temperature estimation are consistent with the results of natural cooling numerical experiment and the relative error is below 4%, whereas the convective heat transfer coefficient is slightly worse due to the difference in the extraction positions. The effects of the number of involved future time steps and the measurement noise on the accuracy of the inverse analysis are also investigated in detail. Our results show that the fluid temperature and convective heat transfer coefficient of natural cooling may be effectively estimated using the proposed inverse method.

Effect of Internal Heat Generation on Solidification of Molten Fuel Droplet during its Interaction with Coolant in a Nuclear Reactor

One of the important issues in safety analysis of severe accidents in fast reactors is Molten Fuel Coolant Interaction (MFCI). MFCI is the phenomenon in which fragmentation of the molten fuel takes place when it comes in direct contact with liquid sodium coolant. For complete understanding of MFCI, we need to study the solidification of the molten fuel happening along with fragmentation. Solidification / melting of any material initially held at its melting temperature is the classic Stefan’s problem. In the case of a severe reactor accident the molten fuel might have a significant amount of decay heat, even though nuclear fission reactions have been stopped by reactor shut down systems. The presence of decay heat in this case, makes the transient Stefan problem more difficult to solve analytically. Here, an investigation of this special case of Stefan’s problem with internal heat generation in nuclear materials has been carried out considering a typical droplet size of 4 mm diameter. The numerical heat transfer analysis has been carried out using the commercial software Fluent. The validation of the problem is done with the steady state analytical solution available for the position of phase change front. Parametric studies have been carried out by varying internal heat generation rates and convective heat transfer coefficient at the surface of the droplet. Temperature profiles and liquid fraction are obtained at different instants. Solidification time and final equilibrium temperature are estimated from the results. It is observed that there is significant increase in drop solidification time for heat generation rates (HG) greater than 100 MW/m3 for the drop size of 4 mm. The maximum HG which is tolerated by the droplet without reaching its boiling point is about 6 GW/m3 when the heat transfer coefficient from its surface is fixed at h=56000W/m2K.

Simulation performance of single slope solar still by using iteration method for convective heat transfer coefficient

In the present paper, an attempt has been made to simulate the performance of solar still by using iteration method for an internal convective heat and evaporative (mass) transfer coefficient which are used in basic energy balance equations. The energy balance is based on the first law of thermodynamics as a function of design and climatic parameters respectively. The internal convective heat transfer coefficient plays a significant role in the performance of solar still. The iteration method based on Kumar and Tiwari model (KTM), which has not been considered earlier by any authors, has been adopted for numerical computation for the convective heat transfer coefficient (hcw). Based on numerical computation for New Delhi climatic condition, it has been observed that the numerical values of hourly change of an internal convective heat transfer coefficient (hcw) vary between 1.3 to 1.5 W/m2 0C, after fourth iteration process which is reduced by 11.7% in comparison with the first iteration.

A numerical prediction and potential control of typical icing process on automobile windshield under nocturnal radiative cooling and subfreezing conditions

In this work, a simplified mathematical model, concerned with transient heat conduction as well as convective and radiative heat transfer, was developed to predict the variations of temperature and supercooling of the windshield during practical nocturnal cooling processes of a car. Final supercooling [Formula: see text] was introduced as an indicator to evaluate the probability of occurrence of frosting. Following that, the Taguchi statistical method was used to conduct a parameter sensitivity analysis and then figure out the potential control strategies for frosting suppression. The results showed that relative humidity had the most significant influence on the distribution of supercooling during the nocturnal cooling period, whereas the initial temperature as well as the thickness and thermal conductivity of the windshield played a minor role in it. An increase in relative humidity resulted in a significant increase in [Formula: see text], which might be expected to trigger an earlier initiation of frosting. The emissivity of the windshield, concerned with the nocturnal radiation potential, showed a considerable effect on the response of [Formula: see text], whereas the influence of the total opaque cloud amount appeared to be largely limited. In addition, through a potential control of the thermal conductivity of the windshield, [Formula: see text] just exhibited a very limited decline, thus contributing little to frosting mitigation. However, with a moderate potential control of the internal convective heat transfer coefficient, the frosting behavior might be effectively suppressed under a severe condition that favored the occurrence of icing. Besides, by introducing a combined control of the emissivity of the windshield and the internal convective heat transfer coefficient, [Formula: see text] could be well reduced to a value below zero even as the relative humidity increased up to 90%, which was supposed to prevent the occurrence of frosting under a far severer condition.

Influence of design parameters on the night ventilation performance in office buildings based on sensitivity analysis

Overheating and energy-extensive consumption in buildings, especially in office buildings, are emerging challenges. Night ventilation (NV) is a promising technique. The performance of NV can be evaluated by a series of performance indicators. As many design parameters affect those indicators, it is beneficial to choose suitable indicators and identify the most important design parameters to develop more efficient design solutions at the early design stage. Sensitivity analysis makes it possible to identify the most important design parameters in relation to NV performance and to focus design and optimization of NV on these fewer, but most important, parameters. A holistic approach integrating sensitivity analysis and parametric simulation analysis is developed to explore the key design parameters on night cooling performance indicators and evaluate the applicability and limitations of those indicators. The results show that the climatic conditions and NV modes strongly affect the influence of design parameters on the performance indicators. The window-wall ratio, internal thermal mass level, internal convective heat transfer coefficient, and night mechanical air change rate are the most important design parameters. The indicators of ventilative cooling advantage, cooling requirement reduction, and percentage outside the range are recommended for the night cooling performance evaluation.

Influence of ICHTC correlations on the thermal characterization of façades using the quantitative internal infrared thermography method

The thermal characterization of façades of the existing building stock is essential to establish optimal energy conservation measures. There are different methods to characterize thermal properties of façades. The quantitative internal infrared thermography method is among those most developed. Given the existing differences in the scientific literature among the proposals of the method, this study analysed the influence of the internal convective heat transfer coefficient (ICHTC). In total, 25 correlations of temperature differences (temperature of the wall and internal air temperature) were analysed, as well as 20 correlations of dimensionless numbers. To do this, an experimental campaign was performed in 3 façades belonging to the most representative building periods of the building stock in Spain. First, a cluster analysis was carried out to determine similarities among the equations analysed, using the Ward method as an agglomerative hierarchical method and the Euclidean distance as an association measurement. In total, 12 and 8 groups were obtained for correlations of temperature difference and of dimensionless numbers, respectively. Afterwards, results associated with each approach were obtained. These results showed that a better adjustment was obtained for correlations of dimensionless numbers by using the approach of convection and radiation, with an average value of representative results higher than 80%.

Filtered reciprocity functional approach to estimate internal heat transfer coefficients in 2D cylindrical domains using infrared thermography

Performance estimation of heat exchanger apparatuses requires a proper knowledge of the thermo-fluid dynamic interaction between fluid and device. One of the aspects to take into account is related to the estimation of the internal convective heat transfer coefficient. This requires the knowledge of the internal temperature and the internal wall heat flux, although these quantities are complicated to be measured. Therefore, the use of inverse problem techniques may be very useful for estimating these quantities. This approach deals with the estimation of the local internal properties, given some external temperature measurements, possibly by means of contactless experimental methodologies (i.e. infrared camera imaging).The solution strategy here presented, for a 2D model, is based on the Reciprocity Functional approach (RF), which requires the solution of two auxiliary problems that are solved, in this paper, by means of the Classical Integral Transform Technique (CITT). The RF approach coupled with CITT, presents some advantages over other techniques since its solution is completely analytical and therefore computationally fast, and it does not require any numerical simulation. Moreover, it deals with the noise content of the experimental data, filtering it and overcoming the documented limits of the classical RF approach. This original method, named Filtered Reciprocity Functional (FRF) approach, was applied to different test cases using simulated measurements, and the results were compared to both the exact and the reconstruct solutions obtained by the Truncated Singular Value Decomposition (TSVD) method, showing a good agreement. Finally, the FRF approach was applied to an experimental case in order to test its robustness when dealing with real data.

Energetic and Exergetic Performance of a Solar Flat-Plate Collector Working With Cu Nanofluid

The evaluation of the performance and characteristics of a solar flat-plate collector (FPC) are reported for domestic and industrial requirements in the existing literature. A computer code was developed using matlab to model and evaluate the energetic and exergetic performance of a nanofluid-based FPC for steady-state and laminar conditions. The analysis was performed using practical geometry data, especially the absorber emittance, for a standard collector. Linear pressure losses in manifolds were taken into account, and a more accurate exergy factor corresponding to a correct value of 5770 K for the sun temperature was employed. The results demonstrate that copper–water nanofluid has the potential to augment the internal convection heat transfer coefficient by 76.5%, and to enhance the energetic efficiency of the collector from 70.3% to 72.1% at 4% volume concentration, when compared to the values with water. Additionally, it was revealed that copper nanofluid is capable of increasing the collector fluid's outlet temperature and decreasing the absorber plate's mean temperature by 3 K. The addition of nanoparticles to the water demonstrated a reduction in the total entropy generation by the solar FPC. Furthermore, increasing the nanoparticle size reflected a reduction in the overall performance of the solar collector.

Identification of heat transfer coefficient through linearization: explicit solution and approximation

We derive an explicit reconstruction formula for the heat flux on the internal surface of a duct based on external temperature data. With the heat flux at hand we determine the internal convective heat transfer coefficient. Technically, by determining explicitly the singular system of a compact operator and assuming noise-free data, the heat flux is expressed as an infinite singular value expansion, whereas for noisy data, the truncated expansion is used as regularized solution. An error bound is derived for the case where the truncation parameter is determined by the Morozov’s discrepancy principle. The method has high flexibility in that it is readily adapted to the finite data case without any substantial modification. Moreover, it does not require any linear system solver and it is therefore easy to implement and extremely inexpensive when compared to existing heat transfer coefficient estimation techniques. The method is illustrated by means of numerical results using both synthetic and experimental data.

Thermal modelling and experimental validation of a semi-transparent water wall system for Sydney climate

A transient heat balance model (THBM) based on energy conservation is developed for predicting the thermal performance of a semi-transparent water wall system. In order to validate the THBM against field measurements, real climate conditions in Sydney, Australia are adopted and the time variations of both internal and external convective and radiative heat transfer coefficients are calculated using empirical correlations from the literature. An alternative concrete wall model is also developed for further validation of the THBM. Good agreements between the THBM simulation and field data are achieved for both the water wall and concrete wall systems. The validated THBM is then adopted in a sensitivity analysis in order to assess the thermal performance of the semi-transparent water wall system under different configurations. It is found that the daily maximum water and air temperatures decrease significantly with the decrease of the transmissivity of the semi-transparent panels, whereas an increase of the attenuation coefficient of solar radiation in water has an insignificant impact on the water and air temperatures. The present results also indicated that increasing the thickness of the water column can reduce the fluctuations of the water temperature and the diurnal peak air temperature. From the practical application point of view, reducing the transmissivity of the external panel provides the most effective way to mitigate over-heating in the semi-transparent water wall system.

Bayesian inference for estimating thermal properties of a historic building wall

In this paper, the use of Bayesian inference is explored for estimating both the thermal conductivity and the internal convective heat transfer coefficient of an old historic building wall. The room air temperature, as well as the temperatures at the surface and within the wall have been monitored during one year and then used to solve the identification problem. With Bayesian inference, the posterior distributions of the unknown parameters are explored based on their prior distributions and on the likelihood function that models the measurement errors. In this work, the Markov Chain Monte Carlo method is used to explore the posterior distribution. The error of the inadequacy of mathematical model are considered using the approximation error model. The distribution of the estimated parameters have a small standard deviation, which illustrates the accuracy of the method. The parameters have been compared to the standard values from the French thermal regulations. The heat flux at the internal surface has been calculated with the estimated parameters and the standard values. It is shown that the standard values underestimate the heat flux of an order by 10%. This study also illustrates the importance of the preliminary diagnosis of a building with the estimation of the thermal properties of the wall for model calibration.

Heat Transfer Augmentation Due to Coolant Extraction on the Cold Side of Active Clearance Control Manifolds

Jet array is an arrangement typically used to cool several gas turbine parts. Some examples of such applications can be found in the impingement cooled region of gas turbine airfoils or in the turbine blade tip clearances control of large aero-engines. In the open literature, several contributions focus on the impingement jets formation and deal with the heat transfer phenomena that take place on the impingement target surface. However, deficiencies of general studies emerge when the internal convective cooling of the impinging system feeding channels is concerned. In this work, an aerothermal analysis of jet arrays for active clearance control (ACC) was performed; the aim was the definition of a correlation for the internal (i.e., within the feeding channel) convective heat transfer coefficient augmentation due to the coolant extraction operated by the bleeding holes. The data were taken from a set of computational fluid-dynamics (CFD) Reynolds-averaged Navier–Stokes (RANS) simulations, in which the behavior of the cooling system was investigated over a wide range of fluid-dynamics conditions. More in detail, several different holes arrangements were investigated with the aim of evaluating the influence of the hole spacing on the heat transfer coefficient distribution. Tests were conducted by varying the feeding channel Reynolds number in a wide range of real engine operative conditions. An in depth analysis of the numerical data set has underlined the opportunity of an efficient reduction through the local suction ratio (SR) of hole and feeding pipe, local Reynolds number, and manifold porosity: the dependence of the heat transfer coefficient enhancement factor (EF) from these parameter is roughly exponential.