Internal Heat Transfer Coefficient Research Articles

Optimizing wall structure for enhanced thermal performance: a layer-specific dimensionless parameter analysis in multi-layer walls

Enhancing building energy efficiency and thermal comfort crucially involves optimizing the phase shift (PS) and decrement factor (DF) of exterior walls. Using the auxiliary function method, this paper presents analytical solutions for the DF and PS in double-layer walls, subsequently extending these to multi-layer walls. Insulation and structural material properties (density, thermal conductivity, specific heat capacity), thickness, internal and external convective heat transfer coefficients, outdoor temperature fluctuations, and insulation layering in multi-layered walls were taken into consideration in acquiring analytical solutions. Our findings in double-layer walls and analogous structures show that the DF and PS are largely governed by two dimensionless parameters per layer: thermal diffusivity index and Biot number. Regardless of Biot numbers (ε1 and ε2), an increase in thermal diffusivity indexes (δ1 and δ2) consistently reduces the DF and enhances the PS. Moreover, a larger ε1 with a smaller ε2 generally associates with a lower DF, but their impact on PS is less significant compared to thermal diffusivity index. In multi-layer walls, especially with mirror symmetry, we observe a dual extremum phenomenon, marked by prolonged PS and reduced DF. This underlines the importance of structural configuration in wall thermal performance. Thus, in double-layer structures, while the DF decreases and PS increases with increasing thermal diffusivity index, the influence of Biot numbers is comparatively minimal, highlighting the paramount role of thermal diffusivity index in optimizing thermal behavior. This discovery provides valuable guidance for identifying wall structures with minimal DF and maximal PS during the architectural design phase.

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.

Heat and Mass Transfer of Water During Activated Carbon Adsorption

AbstractIn order to investigate the dynamic process of the heat and mass transfer phenomenon of water vapor on activated carbon, a fixed‐column adsorption unit and two kinds of activated carbon were used as adsorbent for water vapor adsorption under the adsorption temperature 293.15 K, 303.15 K and 313.15 K, respectively. A coupled model was developed by mass and energy balance, mass transfer and simplified DO adsorption equilibrium equation. The effects of physical parameters on heat and mass transfer were theoretically investigated. Results show that the numerical model match well with the experimental data (R2>0.993). The axial diffusion coefficient (DL) increases from 1.226×10−6 m2 s−1 to 4.062×10−6 m2 s−1 for RAC and 1.425×10−6 m2 s−1 to 4.030×10−6 m2 s−1 for MAC, mass transfer coefficient (k) increases from 8.171 s−1 to 27.083 s−1 for RAC and 9.499 s−1 to 26.864 s−1 for MAC. Concurrently, the breakthrough time of adsorption water vapor gradually shorten from 1000 s to 780 s with temperature increasing from 293.15 K to 313.15 K. The effect of axial diffusion coefficient (DL) on mass transfer is not obvious. Furthermore, the influence of the mass transfer coefficient (k) on temperature variation rate surpasses that of the internal heat transfer coefficient (h).

Heat flux measurement approach for an enhanced thermometric method: preliminary tests

In situ tests are suitable to confirm the real thermal performance of building components, and several significant on-site measurement techniques have been studied in literature. However, among them the Thermometric (THM) method has been poorly examined by the scientific community, thus having opportunities for improvement, being a quite a new and non-standardized technique. The theory behind this technique is the Newton’s law of cooling and the main issue is associated to the heat transfer coefficient for which there is no agreement about the value to use. Here, a simple experimental apparatus characterized by a vertical heated sample, suitably thermally insulated was realized. Sensors were installed and direct heat flux measurements through a heat flux plate were performed and compared with (i) the heat flows obtained through the THM method (test conducted using the internal heat transfer coefficient recommended by the ISO 6946) and (ii) the heat fluxes obtained through the proposal of an enhanced THM method based on dimensionless groups analysis, thus requiring data processing based on convective and radiative components.

Scaling Overall Effectiveness in Low-Temperature Experiments

Abstract The design of film-cooled engine components requires an understanding of the expected temperature distributions while in service, thus requiring accurate predictions through low-temperature testing. Overall effectiveness, ϕ, is the integrated indicator of overall cooling performance. An experiment to measure ϕ at low temperature requires appropriate scaling through careful selection of not only the coolant and freestream gases but also the model material itself. Matching ϕ requires that the experiment has matched values of the adiabatic effectiveness, Biot number, coolant warming factor, and ratio of external to internal heat transfer coefficient. Previous research has shown the requirements to match each of those four parameters individually. However, matching all those parameters simultaneously presents an overconstrained problem, and no comprehensive recommendations exist for the practical experimentalist who wishes to conduct an appropriately scaled, low-temperature experiment truly suitable for determining ϕ. Four fluidic parameters are identified, which in an experiment must be as close as possible to their values at engine conditions. A normalized root-mean-square difference (NRMSD) of the residuals of those parameters is proposed to quantify how well a proposed wind tunnel experiment is likely to yield engine-relevant ϕ values. We show that this process may be used by any experimentalist to identify the appropriate fluids, conditions, and materials for a matched ϕ experiment. Several case studies were performed using computational fluid dynamics (CFD) to show the utility of this process. Of the common experimental techniques examined here, a matched Biot number experiment with 500 K freestream air and 250 K coolant appears to be particularly adept at simulating engine conditions, even better than experiments that make use of CO2 coolant.

Linear and Nonlinear Stability Analysis of Molten Salt Natural Circulation Loop

Abstract A molten salt natural circulation loop (MSNCL) has been setup to study the steady-state, transient, and stability characteristics of molten nitrate salt, mixture of sodium nitrate, and potassium nitrate in 60:40 ratios by weight. Natural circulation experiments in a temperature range of 290 °C–600 °C have been performed. A semi-analytical linear model was derived for stability analysis of rectangular natural circulation loops with conventional localized surface heating and cooling. The developed model includes the effect of wall thermal inertia, variable internal heat transfer coefficient, finite secondary side heat transfer coefficient at cooler, and heat losses for predicting the stability map. These effects are incorporated considering their significant role in modeling high-temperature molten salt-based natural circulation systems. The developed model has been first validated with the experimental data of water loop available in literature. The developed model is then validated with the experimental data generated in MSNCL. Validation of in-house developed nonlinear model has also been performed against the same experimental data. The comparison of both linear and nonlinear stability analysis with the experimental data shows good agreement and articulates the importance of various parameters which have been included in the developed model.

Математичне моделювання пристінної оливної плівки в камері підшипника ГТД

The most preferable result of modeling the flow in the BC is the determination of the heat transfer coefficient to the inner wall. The complexity of solving this problem is due to both the complex geometry of the BC and the presence of a two-phase flow, the structure of which changes from air-droplet in the core to a nearly liquid in the wall oil film. Available research results show that even three-dimensional CFD modeling of such a flow does not completely solve the problem. At the same time, the calculation time is long, and the results require, at a minimum, selective experimental validation. At the same time, it can be considered proven that the main mechanism of heat transfer from the core to the wall region of the BC is related to the radial flow of droplets, and the thermal resistance of the wall oil film has a decisive effect on the value of the internal heat transfer coefficient. It is advisable to model these media on the basis of a two-dimensional problem with the averaging of phase parameters along the axis. Considering the small volume fraction of droplets, the Lagrangian approach can be used to model the two-phase flow in the core of the BC. This allows consideration of not only the polydispersity of the droplets, but also the creation and movement of secondary droplets during the formation of the wall oil film. One of the main problems in the modeling of the wall film is the definition of its flow regime and the corresponding criterion equations for calculating the coefficients of friction and heat transfer. Most of the equations use the longitudinal coordinate of the plate as a geometric parameter and cannot be applied to the bearing chamber. In this study, the possibility of converting the film flow into a flow similar to that occurring in a flat pipe is substantiated. This allows not only to consider the geometric features of the BC, but also to use the corresponding Reynolds numbers and similarity equations for the equivalent flow. Along with the use of a two-layer model for the boundary region and the concept of the analogy of transfer processes, this made it possible to form a mathematical model of the film, which considers all the components that determine the formation, movement, and heat transfer of the wall oil film. In addition, the model does not contain restrictions on the appearance of the parameters transverse profile, which are, for example, for the EWF model of the oil film in ANSYS FLUENT. The obtained results along with the previously developed air-droplet flow model in the core provide a complete two-dimensional model of the gas-liquid flow in the GTE bearing chamber, which allows the main geometric and all mode parameters to be used to determine the heat transfer coefficient to the inner wall of the chamber. Given the short calculation time, the proposed model allows for a detailed investigation of each factor’s contribution and element-by-element identification of the model based on the results of more detailed modeling and by comparing calculated and experimental data.

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 %.

Chemical potential of different phases inside the pyramid stepped basin solar still through Gibbs free energy

Chemical potentials of the temperature components of the pyramid stepped basin solar distiller (PSBSD) have been evaluated to illustrate the behavior of the water vapor and condensed water droplets during the process of solar distillation. Chemical potential is one of the main criteria in terms of chemical and phase equilibrium which is obtained from the Gibbs phase rule. The application of the Gibbs phase rule established a good relationship between the design, climatic and experimental parameters of the PSBSD. The internal heat transfer coefficients of the distiller are influenced by the intensity of solar radiation and ambient temperature which in turn explains the intensive state of the PSBSD. It is found that the chemical potentials and the intensive states of the different phases of the system established a relation between the equilibrium of liquid and vapor mixture with specifications of temperature components of the PSBSD. It is also found that the efficiency of the PSBSD is 38.135% with a distillate yield of 4.280 l/m2day over 24 h cycle.

Experimental and theoretical thermal investigation of bio-composite panels based on sawdust particles

This work focuses on the valorization of sawdust waste in order to develop a new bio-sourced insulation material. To assess the dynamic thermal behavior of a multi-layer external wall insulated with these panels, their thermal characteristics, the time lag, and the decrement factor, were investigated. The results obtained show that when the density increases, the thermal conductivity, the thermal diffusivity, and the volumetric heat capacity increase while the thermal diffusivity decreases. For a thickness of 20 cm of the composite material, the time lag increases, and the decrement factor decreases with increasing density. Moreover, the results obtained for an external wall show that the decrement factor decreases and the time lag increases with the thickness of the thermal insulation. It was also found that the position of the insulation layer has a considerable effect on thermal inertia properties. The wall with insulation on the outside is the most desirable. In addition, the internal global heat transfer coefficient affects the external wall's time lag and decrement factor, while the external global heat transfer coefficient does not affect its inertia properties.

Boundary element method for contactless estimation of spatially varying internal heat transfer coefficient in circular pipes

In the present study it is proposed a solution approach of the Inverse Heat Conduction Problem (IHCP) intended at assessing the local heat transfer coefficient on the inner wall of a tube, under a forced convection problem. The estimation method is established on the Boundary Element Method (BEM) combined with the Truncated Singular Value Decomposition (TSVD) methodology, employed to manage the ill-conditioned nature of the problem. The numerical results of the direct problem, built by the BEM, are firstly validated, and consequently adopted as synthetic data inputs to resolve the IHCP and corroborate the whole assessment procedure. This approach is also tested with experimental data about forced convection problem in coiled pipes. In these geometries, the convective heat transfer coefficient changes considerably along the wall periphery and for this reason, it constitutes a perfect example to test the ability of the presented method to infer a spatially varying internal heat transfer coefficient distribution.

Experimental and numerical investigation of the use of new generating refrigerant R513A in the gravity-assisted heat pipe

In this study, theoretical–experimental and life cycle climate performance analyses of the gravity-assisted heat pipe filled with R513A were carried out. The heat carried by the heat pipe and the internal heat transfer coefficients of the evaporator and condenser were investigated. As the temperature set value (30, 35, 40, 45, 50 °C) increased, the heat transferred by the heat pipe increased and the averaged experimental data were obtained as 27.65, 49.30, 74.95, 99.40, 114.90 W, respectively. For the given temperature set values, the experimental internal heat transfer coefficient of the evaporator was obtained as 946, 1125, 1360, 1652, and 1215 Wm−2 °C−1. Experimental condenser internal heat transfer coefficients were obtained as 1385, 1460, 1527, 1563, and 1417 Wm−2 °C−1. According to the life cycle climate performance analysis of the given system, it has been determined that 82.22 % of the total emissions are caused by direct emissions and 17.78 % by indirect emissions. Direct emissions are related to leaks of the working fluid. Choosing a low global warming potential working fluid will significantly reduce emission and thus reduce the overall emission of the gravity-assisted heat pipe.

Investigating Numerically the Effect of Wind on Fire Spread Between Two Informal Settlements Dwellings

AbstractPrevious full-scale fire studies revealed that the role of wind on fire spread between informal settlement dwellings was critical. However, the influence of wind conditions on informal settlement dwellings fire spread is currently understudied in the literature. This study aimed to investigate the effect of external wind conditions on fire spread between two informal settlement dwellings with a distance of 1 meter between them. A parametric numerical analysis was performed using the computational fluid dynamics code Fire Dynamics Simulator. The numerical models were benchmarked through laboratory experiments. The investigation included an analysis of the fire spread mechanism, flashover conditions, and heat transfer processes at the boundaries of the dwellings. Simulations were conducted with burning wood cribs as fuel and three wind speeds (6 m/s, 10 m/s, and 14 m/s) with four wind directions (East, West, South, and North). Results showed that wind speed and direction had a significant impact on the fire dynamics of the origin dwelling and its spread to neighboring dwellings. The wind direction also influenced the time to flashover in both dwellings, with a delay observed when the wind flowed through the alley between the two dwellings. The total heat transfer coefficient was found to be directly proportional to the wind speed for all directions. The internal radiative heat transfer coefficient of one wall was found to represent the total heat transfer coefficient in different scenarios. This study highlights the complexity of determining the role of wind in urban fire spread and underscores the need for further research in this area.

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.

Inverse problem estimation of internal heat transfer coefficients in ducts through a time-dependent reciprocity functional method

Inverse problem estimation of internal heat transfer coefficients in ducts through a time-dependent reciprocity functional method

Математичне моделювання теплопередачі у повітряно-крапельному потоку камери підшипника ГТД

The gas-liquid flow in the bearing chamber (BC) of the gas turbine engine is realized due to the interaction of the sealing air and oil supplied for lubrication and cooling of friction units. The complex nature of the flow movement is determined not only by the BC geometry but also by the presence of rotating elements and the manner of oil supply and flow removal. The most important result for the engineering practice of BC flow modelling is the determination of the heat transfer coefficient to the inner wall. The variety of influencing factors causes difficulties even at the stage of an integral mathematical model of the process formation, which makes it possible to determine this coefficient. As a result, significantly different approaches are used – from three-dimensional CFD modelling of a heterogeneous flow up to the use of a criterion equation that formally considers the effect of geometric and regime parameters on a heat transfer. The first approach requires significant computational resources, and certain difficulties arise in setting the initial and boundary conditions, especially in terms of droplet parameters. Homogeneous models somewhat simplify the problem, including when formulating the boundary conditions. However, all the effects of the influence of the droplet diameter are levelled. In both cases, the computation time is long, and the results of CFD simulations require selective experimental verification. Therefore, it is problematic to use this approach solely for engineering problems, as well as when generalizing experimental data in several regime parameters. When using the criterion equation, the calculation procedures are as simple as possible. However, dimensionless complexes do not ensue from the basic equations of heterogeneous media mechanics and do not consider the effects of interfacial interaction during heat transfer in the BC. Therefore, the possibility of the application of the proposed correlations for other BC geometries and oil and air supplying conditions also needs to be experimentally confirmed. This makes it problematic to use such correlations at the design stage of the BC and the oil system as a whole. The preferred approach for engineering practice is when the thermohydraulic processes in the BC are described on the basis of the proven equations of heterogeneous media mechanics with the transition to a two-dimensional problem by averaging the phase parameters along the axis. This averaging is justified by the fact that the main heat carrier from the core to the inner wall of the BC is the radial flow of droplets. In view of the low volume fraction of droplets, the Lagrange approach is used to simulate a two-phase flow in the BC core. The droplet parameters along the trajectory are calculated considering the interfacial interaction with the air. In this regard, the air velocity field is determined by considering the geometry of the BC, the flow through the seals and the shaft speed. Here, it is possible to consider not only the droplet polydispersity but also the effects of primary droplet reflection, the formation, and movement of secondary droplets during the formation of a near-wall oil film, the thermal resistance of which directly affects the value of the internal heat transfer coefficient.

Flow and heat transfer studies of multijet impingement cooling for different configurations: A review

AbstractIncreasing the gas turbine engine's turbine intake temperature has long been a potential strategy for increasing the specific work output of the engine. However, the melting temperature of the turbine blades and vane material limits the maximum intake temperature. As a result, internal and external cooling techniques are commonly used to maintain the vane material in a safe condition. This study provided an overview of internal impingement cooling to highlight the significance of geometrical variations, such as flat plate, curve plate, and actual vanes. It was observed that flat and curved plate impingement heat transfer studies were reported extensively, whereas limited studies were found on the conjugate effects on airfoil surfaces. The importance of conjugate heat transfer studies and their impact has recently been described in the literature. In most of the literature, a wide range of instruments, such as Laser Doppler Velocimeter, Particle Image Velocimeter, liquid crystal sheets, and so forth, were used for experimental investigations. According to most studies, the local value of internal surface temperature and heat transfer coefficient are vital factors of local flow behavior. Jet‐to‐jet spacing, jet‐to‐plate spacing, jet hole diameter, and jet Reynolds numbers played a crucial role in both numerical and experimental analyses. Different geometric variations strongly influence flow behavior. Therefore, the usual method for determining interior temperature distributions and heat transfer coefficients by considering generalized geometries like the flat and curved plate may not produce accurate conjugate solutions. Most of the computational studies on the flat and curved plate indicate the usage of κ−ω shear stress transport and κ–ε realizable model to predict the heat transfer coefficient.

Convective radiative moving fin with temperature-dependent thermal conductivity, internal heat generation and heat transfer coefficient

Convective radiative moving fin with temperature-dependent thermal conductivity, internal heat generation and heat transfer coefficient

Theoretical Considerations for Scaling Convection in Overall Effectiveness Experiments

Abstract An increasingly common experimental technique allows measurement of overall effectiveness by matching the Biot number between experimental and engine conditions. While much work has been devoted to determining the appropriate flow conditions necessary to scale adiabatic effectiveness, little attention has been paid to subtleties beyond matching the Biot number that arises when performing overall effectiveness experiments. Notably, the ratio of the internal and external heat transfer coefficients must be matched. The density ratio and the specific heat ratio have been shown to play important roles in scaling adiabatic effectiveness; however, now we demonstrate the requirements for the coolant and freestream flow conditions required to conduct an appropriately scaled overall effectiveness experiment. Since the viscosity and thermal conductivity of the fluids influence heat transfer coefficient behavior, this gives rise to an additional nondimensional parameter that should be matched to properly perform an overall effectiveness experiment. In this paper, we demonstrate that this new nondimensional parameter will be matched provided that Pr∞, Prc, and Rec are matched in addition to Re∞ and the advective capacity ratio. We demonstrate the validity of this requirement through computational fluid dynamics simulations, which are well-suited for this since over-constrained requirements can be overcome by altering gas properties. Simulations of an internally cooled wall exposed to a hot freestream were performed with various gases to show the sensitivity of the overall effectiveness to these previously ignored requirements. An additional set of simulations on a film-cooled plate reveals additional complexities when coolant mixes with the freestream gas.